Difference between revisions of "Chapter Seven: Water Treatment"

From Ministry of Water DCOM Manual
(Chapter Seven Water Treatment)
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==== 7.5.1.3.1 Design criteria ====
 
==== 7.5.1.3.1 Design criteria ====
 
Length of the channel L = 20 (maximum depth of flow)
 
Length of the channel L = 20 (maximum depth of flow)
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L/d = Vh/Vs
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Where,
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Vh = Horizontal flow velocity Vs = Vertical settling velocity
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L = Length of the grit channel
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D = Depth of flow in the channel
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There are three types of grit channels that can be designed, these include horizontal flow, Rotational flow and Vertical flow
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<gallery>
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<gallery>
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Example.jpg|Caption1
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Example.jpg|Caption2
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</gallery>
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<gallery>
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<gallery>
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Example.jpg|Caption1
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Example.jpg|Caption2
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</gallery>
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</gallery>
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</gallery>
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==== Sand Traps ====
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This pre-treatment unit is designed to trap sand after water has been guided into the intake chamber in order to reduce the potential for wear and tear as well as silting up the unit operations that are located downstream of the intake structure. The minimum diameter of washout pipes of such sand traps is 75 mm and the bigger the main intake pipe, the bigger is the flushing pipe for sand. Figures 7.15, 7.16 and 7.17 show the layout plan and two sections of such a sand trap for small streams intake.
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<gallery>
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<gallery>
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Example.jpg|Caption1
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Example.jpg|Caption2
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</gallery>
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<gallery>
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Example.jpg|Caption1
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Example.jpg|Caption2
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</gallery>
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</gallery>
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==== Pre-chlorination ====
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This is a unit operation that is used for purposes of controlling algae growth in raw water and the process of preparation of the chemical to facilitate dosing will include the standard preparation of the aqueous solution as done during disinfection. This is often added upon establishment of occurrence of algal blooms during certain periods of the year as confirmed by laboratory tests undertaken daily. The amount of the dose will be established daily in order to pre-determine the pre-chlorination dose that has to ensure no interference with downstream unit operations in case the flow sheet involves biological treatment processes like slow sand filter or others. The location of dosing of the chemical has to be secured against direct sunlight and has to ensure intense mixing. Where chlorine gas is used for pre-chlorination, the usual measures that are taken when dosing chlorine in gaseous form have to be taken including leakage detection. To minimise chlorine consumption, pre-chlorination has to be done downstream of the fine screens.
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Other design features can be seen in the Appendix H. of this DCOM Manual. It should be noted that pre-chlorination also controls growth of bacteria in pipes and tanks (BRITANICA, 2020). Lanfair et al (2020) cautions that if pre-chlorination is applied to water with a high concentration of natural organic matter (nom), the latter is suspected to react with chorine to form Disinfection by-products (DBPs) that include trihalomethanes or haloacetic acids that are suspected to contribute towards stomach and bladder cancer.
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==== Water pre-conditioning (pH adjustment) ====
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Water pre-conditioning can entail a number of pre-treatments undertaken prior to pre- chlorination which is often the final step in pre-treatment. This unit operation involves adjustment of the pH upstream in order to ensure the chemicals used during further treatment processes are dosed to water that has the correct pH range for maximum efficiency. A good example is when the treatment flow sheet includes coagulation with Ferrous Sulphate that requires an optimum pH range of 7 to 8.5. As a matter of fact,the application of this unit operation guarantees that the daily fluctuations in quality are also reflected in the chemical that is used for pH correction whether an acid or an alkali is used. It is usual to apply lime for increasing pH and use of acids like dilute sulphuric acid or hydrochloric acid. Other pre-conditioning measures include use of sodium carbonate (soda ash) to remove hardness (i.e. calcium carbonate).
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==== Primary Treatment ====
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Primary treatment of water removes material that will either float or readily settle out by gravity. This water treatment level includes the physical processes of screening, grit removal and sedimentation.
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==== Sedimentation ====
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In designing sedimentation tanks, the required detention time determines the dimensions of the tank. A rectangular tank is the simplest design to use. Detention time is calculated as Volume/Flow rate (Q). The detention times based on the average daily flows and usually range from about 45 minutes to 3 hours depending on water turbidity. The ideal inlet reduces the entrance velocity and distributes the water as uniformly as possible across the depth and width of the tank. Outlets are usually weirs which are sufficiently long to reduce the flow velocity, and so avoid the re-suspension  of the solids in the water. Plain sedimentation tanks should be designed for a surface loading in the range of 0.1 - 0.5 m3/m2/h. The exact surface loading to be adopted should be determined after carrying out settlement tests on samples of raw water, typical of all regimes of the water source. The settling properties of water will depend on the soil and vegetation conditions in the catchment area, and they will vary considerably between different locations and regimes of the water source. Figure 7.18 (a and b) shows a cross section through a circular sedimentation tank while figure Figure 7.19 (a) and (b) shows a cross section through a rectangular sedimentation  tank.
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<gallery>
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<gallery>
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Example.jpg|Caption1
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Example.jpg|Caption2
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</gallery>
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<gallery>
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<gallery>
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Example.jpg|Caption1
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Example.jpg|Caption2
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</gallery>
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</gallery>
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</gallery>
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==== Lamella Plate Settlers (Inclined plate settlers) ====
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Settling efficiency of a basin depends upon the design surface loading. Lamella plates and small diameter tubes having a large wetted perimeter, relative to wetted area providing laminar flow conditions and low surface loading rates have shown good results in terms of settling efficiency and economy in space as well as cost. Plate or tube configuration can be horizontal or steeply inclined. In inclined plates or tubes (55o
 +
– 60o) continuous gravity drainage of the settleable material onto the floor below can be achieved, without impairment of effluent quality. However, to work effectively an efficient flocculation stage is critical.
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 +
In purpose built lamella plate settlers the water enters at the base of the lamella plates and travels upwards between the lamellas. Each space between the lamella plates  tends to act as a semi -independent settling modules with the lamella plates extending from near the base of the tank to about 125 mm above the top water level. The clarified water is collected by saw-toothed notched launders running along each side of the plate. Unless sludge is removed mechanically by a scraper, sufficient depth beneath the plates is required for access during cleaning, although this can be aided by pressurised water. A typical arrangement is illustrated in Figure 7.20.
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<gallery>
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<gallery>
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Example.jpg|Caption1
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Example.jpg|Caption2
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</gallery>
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</gallery>
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Plates are made of stainless steel or plastics with a width of 1.25 to 1.5 m and a length of 2.5 to 3.25 m including the length above water. It is advisable to use standard lengths of metal sheets and plastic sheets available in the market. In Tanzania sheets are in dimensions of 120 cm x 240 cm. Plate thickness is usually about 0.7 mm for stainless steel whilst the horizontal spacing between plates is varied according to the nature of the raw water but within the range 50 – 80 mm.
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Total settled area is then:
 +
 +
A = (n-1) × L × W × cos θ (7.2)
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Where;
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n = number of plates
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L = plate length in water (m), less the transition length W = the plate width (mm), and
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θ = the angle of inclination of the plates to the horizontal (55<sup>o</sup> – 60<sup>o</sup>)
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A = Settled area (m<sup>2</sup>)
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Table 7.2: Important parameters for the design of lamella plate settler.
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{| class="wikitable"
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|-
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! S/No !! Parameter !! Units !! Range
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|-
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| 1 || Surface8 loading rate || m/h || 5 to 10 m/h<sup>9</sup>
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|-
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| 2 || Hydraulic Loading Ratio ||- || 0.25 -0.5
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|-
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| 3 || Angle of inclination of the plates, θ || - || 45-70<sup>o</sup> Typical: 60<sup>o</sup>
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|-
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| 4 || Plate spacing, w || mm|| Min 20
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|}
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<gallery>
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Example.jpg|Caption1
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Example.jpg|Caption2
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</gallery>
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==== Primary Filtration ====
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Primary filtration of water removes material through filtration whose particles size is greater than the opening size. The target particles are the ones which are not removed by sedimentation tanks.
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==== Slow Sand Filtration ====
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A Slow Sand Filter (SSF) is basically a large tank containing the sand bed. A distinguishing feature of slow sand filters is the presence of a thin layer, called the schmutzdecke, which forms on the surface of the sand bed and includes a large variety of biologically active micro-organisms.
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Water is introduced at the top and trickles down through the sand bed to the under- drains and goes to the storage tank. The impurities in the water are retained at the upper layers of the sand bed. In the process, the schmutzdecke consisting of bacteria and microscopic plants grow. The schmutzdecke removes the organic matter and most of the pathogenic micro-organisms in water which might be smaller than the pores of the sand.
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<gallery>
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Example.jpg|Caption1
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Example.jpg|Caption2
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</gallery>
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===== (a)Elements of a Slow Sand filter =====
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Figure 3.43(a) presents, in diagrammatic form the various elements that go to make up a Slow Sand Filter. Essentially the SSF elements consist of:
 +
(i)a supernatant (raw) water reservoir, the principal function of which is to maintain a constant head of water above the filter medium, this head providing the pressure that carries the water through the filter;<br>
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(ii)a bed of filter medium (nearly always sand), with in and upon which the various purification processes take place<br>
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(iii)an under-drainage system, which fills the dual purpose of supporting the filter medium while presenting the minimum possible obstruction to the treated water as if emerges from the underside of the filter-bed; and<br>
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(iv)a system of control valves to regulate the velocity of flow through the bed, to prevent the level in the raw water reservoir from dropping below a predetermined minimum during operation, and to permit water levels to be adjusted and backfilling to take place when the filter is put back into operation after cleaning<br>
 +
(v)It comprises approximately 1.2 m depth of fine sand supported on two or three gravel layers. The effective size of the sand used in slow sand filters is about 0.2 mm, but may range between 0.15 mm and 0.35 mm, and with a coefficient of uniformity of between 1.5 and 3.0.<br>
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(vi)It is a very simple and effective technique for purifying surface water. It will remove practically all of the turbidity from the water as well as most of the pathogens without the addition of chemicals. Slow sand filters can frequently be constructed largely from locally-available materials.<br>
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In a SSF the water is purified by slow percolation through a bed of fine sand. Pre- treatment is necessary with raw waters having an average turbidity of 25 NTU or more, but should be considered also for less turbid raw waters 5 to 25 NTU) to improve effluent quality and reduce frequency of cleaning. The SSF is also useful for treating groundwater containing solids in suspension, e.g. ferric and manganese compounds converted by aeration from the soluble state of the salts. <br>
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Table 7.3 below shows the typical performance of the Slow Sand Filter.
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{| class="wikitable"
 +
|-
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! Parameter of Water Quality !! Purification Effect of Slow Sand Filtration
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|-
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| Colour || 30% to 100% reduction
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|-
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| Turbidity || Turbidity is generally reduced to less than 1 NTU
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|-
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| Faecal Coliforms || 95% to 100%, and often 99% to 100%, reduction in the level of faecal coliforms
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|-
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| Cercariae|| Virtual removal of cercariae of schistosomes, cysts
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and ova
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|-
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| Viruses|| Virtually complete removal
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|-
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| Organic matter|| 60% to 75% reduction in COD
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|-
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| Iron and manganese || Largely removed
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|-
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| Heavy metals || 30% to 95% reduction
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|}
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(Source: 3rd Design manual for water supply and waste water disposal, 2009)
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===== (b)Design Considerations of Slow Sand Filter (SSF) =====
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* Raw water quality and necessity for pre—treatment and/or aeration;
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* Necessity for chlorination room and clear water storage, pumping and distribution;
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* Site location, foundation conditions, space for expansion and pre-treatment;
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* Availability of source of filter media, and construction materials;
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* Fencing and security
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* The filter sand must be free from any clay or silt content and preferably of a well- rounded quartz material. Organic matter should be avoided.
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===== (c)Design Criteria; =====
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* When choosing the filter sand, the grain size distribution should meet the effective size 0.15 to 0.35 mm and the coefficient of uniformity should be less than 3 ,
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* In order to calculate for the total area of filter beds, a working rate of 0.1 - 0.2, m3/m2/hr is recommended. When one filter is not operational, the working rate of the remaining filter should not exceed 0.2 m3/m2/h.
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* The turbidity in the incoming water should not exceed an average of 5 - 10 NTU. In cases of higher turbidity, preliminary treatment such as roughing filters is necessary
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* The inlet structure should be designed in such a way that the raw water is equally distributed over the filter bed area.
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* To achieve this, the inlet velocity should be around 0.1 m/s and the width of the inlet structure should be at least (0.05 x Q) metres, where Q is the design flow in m3/h.
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* The minimum size of a filter unit should be 15 to 20 m2;
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* The height of the supernatant water should be 1 to 1.5 m,
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* The oxygen content of the water after filtration should not be less than 3 mg/l.
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* Calculate filter surface area = flow capacity (m3) / rate of filtration
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===== (d)Main Water Under-drain =====
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* Calculate Diameter of the under drain = (22 × d2) / (28 × r)
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* Area of holes or slots to be 1.5% of area of the filter
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* As a check, the oxygen content of the water after filtration should not be less than 3 mg/l.
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* Freeboard above water level = 0.2 - 0.3 m
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* Height of walls above ground surface= > 0.8 m
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* Gravel filter support = 0.3 - 0.5 m
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* Depth of under—drainage system = 0.3 - 0.5m
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* Area (A) per filter bed = 10 - 100 m2
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===== Rapid Gravity Sand Filtration =====
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This is a process in which water flows onto the top of the filter media and is driven through it by gravity. In passing through the small spaces between the filter's sand grains, impurities are removed. The water continues its way through the support gravel, enters the under-drain system, and then flows to the reservoir. It is the filter media which actually removes the particles from the water. The filter media is routinely cleaned by means of a backwashing process.
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Rapid Sand Filtration (RSF) is a technique commonly used for treating large quantities of drinking water. It is a relatively sophisticated process usually requiring power- operated pumps for backwashing or cleaning the filter bed, and some designs require flow control of the filter outlet. A continuously operating filter will usually require backwashing about every two days or so when raw water is of relatively low turbidity and at least daily during periods of high turbidity. Because of the higher filtration rates, the area requirement for a rapid gravity filtration plant is about 20% of that required  for slow sand filters.
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Surface loading should be between 4 and 7 m3/h.m2, and the filter structure should be designed with a minimum height between the top of the filter media and the bottom of the wash water channel of at least 30% of the height of the filter media as this expands during backwashing. It may be necessary to include for air-scour as well as backwashing, or for the two combined in a single operation.
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Normally a sufficient distribution of the wash water will be achieved if:
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* Ratio of area of orifice to area of bed served (1.5 to 5) × (103) : 1
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* Ratio of area of main to laterals served (1.5 to 3) : 1
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* Diameter of orifices 5 - 20 mm
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* Spacing of orifices 100 - 300 mm centre to centre
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* Spacing of laterals approximating to the spacing of orifices.
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The filter bed should be approximately 1.0 m thick and preferably consist of well- rounded quartz sand with an effect size of 0.7 - 1.0 mm and uniformity coefficient in the range of 1.3 - 1. 5. The available hydraulic head above the top of the filter bed should be 1.3 - 1.5 m. The following stratification of the sub structure should be used  to support a filter of an effective grain size as suggested earlier (finest strata at the top).
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* 0.15 m of grain size 2 - 2.8 mm
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* 0.10 m of grain size 5.6 - 8 mm
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* 0.10 m of grain size 10 - 20 mm
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* 0.10 m of grain size 20 - 40 mm
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* 0.10 m of grain size 40 - 60 mm
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The washing velocity should be in the range of 35 – 55 m3/m2/hour, however care must be taken to ensure that sand carry over into the wash-water channel does not occur and the actual wash-water rate adjusted accordingly.
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In order to achieve proper washing of the filter a storage volume sufficient for continuous washing for an 8 to 10 minute period should be available.
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===== (b)Design Steps =====
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The following design steps should be followed;
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(i)Filter Units
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* Rate of filtration should be 4 - 6 m3/m2/hr
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* Determine flow capacity in the filter (m3/day. demand)
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* Calculate filter surface area = flow capacity (m3) / rate of filtration<br>
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(ii)Main Water Under-drain
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* Select flow capacity and flow velocity
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* Calculate area required = (low capacity)/ (low velocity) (m2)
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* Calculate diameter of the under drain, d= Square Root of (4A/Π)
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* Area of holes = 0.2 - 0.4 % of the area of filter
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* Use five layers of sand particles in the filter as indicated above:
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* Water depth 1 – 1.2 m
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* Sand depth 1 – 1.2 m
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===== (c)Filter Backwashing =====
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A major cause of poor performance by rapid gravity filters is a result of either inadequate or excessive backwashing rates. Backwashing is sometimes carried out by water alone but more often by air and water usually applied one after the other by reverse flow to the filter bed. The first operation however is to allow the filter to drain down until the water lies a few centimetres above the top of the bed. Air is then introduced through the collector system at a rate of about 6.5 to 7.5 mm/s.
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Where air and water is applied separately, air scour normally lasts about 3 – 4 minutes and the water wash about 4 – 6 minutes. Where applied concurrently, air is first introduced and after about 1.5 – 2 minutes when it is fully established water is introduced and the combined backwash last for about 6 – 8 minutes. Air is stopped first and the water run for several more minutes to rinse the bed. Generally, total water consumption per wash amounts to about 2.5 bed volumes, but should normally not exceed 2% of the treated water output in well run plants.
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'''Comparison between Slow Sand Filters and Rapid Sand Filters'''
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a)Base material: In SSF it varies from 3 to 65 mm in size and 30 to 75 cm in depth while in RSF it varies from 3 to 40 mm in size and its depth is slightly more, i.e. about 60 to 90 cm.<br>
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b)Filter sand: In SSF the effective size ranges between 0.2 to 0.4 mm and uniformity coefficient between 1.8 to 2.5 or 3.0. In RSF the effective size ranges between 0.35 to 0.55 and uniformity coefficient between 1.2 to 1.8.<br>
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c)Rate of filtration: In SSF it is small, such as 100 to 200 L/h/sq.m. of filter area while in RSF it is large, such as 3000 to 6000 L/h/sq.m. of filter area.<br>
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d)Flexibility: SSF are not flexible for meeting variation in demand whereas RSF are quite flexible for meeting reasonable variations in demand.<br>
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e)Post treatment required: Almost pure water is obtained from SSF. However, water may be disinfected slightly to make it completely safe. Disinfection is a must after RSF.<br>
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f)Method of cleaning: Scrapping and removing of the top 1.5 to 3 cm thick layer is done to clean SSF. To clean RSF, sand is agitated and backwashed with or without compressed air.<br>
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g)Loss of head: In case of SSF approx. 10 cm is the initial loss, and 0.8 to 1.2m is the final limit when cleaning is required. For RSF 0.3m is the initial loss, and 2.5 to 3.5m is the final limit when cleaning is required.
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==== Other Types of Filters ====
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===== (a)Pressure Filters =====
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Pressure filters are circular pressurised vessels containing the filter media and usually designed for vertical flow. They work on the same principle as rapid gravity filters differing in that the filter medium is enclosed in a steel vessel and the water is forced through it under pressure.
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===== (b)Upward Flow Filters =====
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Upward flow filters are theoretically more efficient than gravity filters where the water to be filtered flows upwards through the naturally desegregated, progressively finer and finer media so that coarser particles are trapped first in the coarser bottom layers. This tends to extend the period between backwashing. Several sand grades, getting progressively finer upwards have also been used in which case some restraining means such as a grid is required often located about 0.1 m below the surface where upward backwashing is used.
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The major reason why RSF is a preferred option of water filtration to SSF is rate of filtration where RSF is higher per filter area flexibility, where SSF are not flexible for meeting variation in demand, method of cleaning where in the case of RSF, no sand is lost. Also RSF requires minimal land requirement (Source: Ugandan Water Supply Design Manual (2013).
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===== (c)Roughing Filters =====
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Roughing filters have their place as a form of primary treatment, especially for turbid or highly changeable river water. This technique of primary treatment has been greatly under-utilised in Tanzania in the past. Although much research has been undertaken since late 1970s (Mbwette & Wegelin, 1984). It is used primarily to remove solids from high turbidity source waters prior to treatment with such unit operations as slow sand filters.
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In a typical roughing filter, there are series of tanks which are filled with progressively smaller diameter media in the direction of the flow which can either be horizontal or vertical. With respect to the vertical flow direction, up-flow or down-flow roughing filters can be designed. The media in the tank which may include gravel, rice husks or any other suitable local material, plays an important role of reducing the vertical settling distance of the particles to a distance of a few millimetres
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===== (i)Advantages of Roughing filters =====
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* Can considerably reduce the number of pathogens in the water, as well as the amount of iron and manganese.
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* Can be considered a major pre-treatment process for turbid surface water since they efficiently separate fine solid particles over prolonged periods.
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* Long filters (10 m) at low filtration rates (0.5 m/h) are capable of reducing high suspended solids concentrations (1000 mg/l TSS down to less than 3 mg/l).
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* Capable of reducing peak turbidities by 80 to 90 percent and faecal coliforms by 77 to 89 percent.
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* They are placed at the treatment plant site and operated in combination with other pre-treatment units such as dynamic filters or sedimentation tanks.
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NB: For detailed information, designers are recommended to download the comprehensive SANDTEC design report entitled ‗Surface Water Treatment by Roughing Filters, A Design, Construction and Operation Manual‘ available at http//sandec.ch/WaterTreatment/Documents/Surface%20Water%20Treatment.pdf Features of Roughing Filters (RF).
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The main part of the filter is the section containing the filter material. However, a Roughing filter comprises the following six elements:-
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# Inlet Flow Control
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# Raw Water Distribution
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# Actual Filter
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# Treated Water Collection
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# Outlet Flow Control
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# Drainage System
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===== (ii)Parameters for Design of Roughing Filter =====
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# Filter media size
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# Filtration Rates
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# Filter Length
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# Filter media materials
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===== (iii)Filter Media Size =====
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*The size of filter media decreases successively in the direction of water flow and ideally the uniformity of filter media fractions is maximized to increase filter pore space (storage capacity) and aid in filter cleaning (Boller, 1993).
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*The use of multiple grades of filter media in a roughing filter promotes the penetration of particles throughout the filter bed and takes advantage of the large storage capacities offered by larger media and high removal efficiencies offered by small media
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*The effect of surface porosity and roughness of filter media on particle removal efficiency in roughing filtration is insignificant compared to the size and shape of macro-pores in the filter (Wegelin, 1986)
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*Media types commonly used in roughing filtration are quartz, sand and gravels but can be replaced by any clean, insoluble and mechanically resistant material (Graham, 1988)
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*Common grades of media used in roughing filters are provided in the Table 7.4
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Table 7.4: Different sizes of media in a Roughing filter
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{| class="wikitable"
 +
|-
 +
! Roughing Filter description !! 1st compartment !! 2nd Compartment !! 3rd Compartment
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|-
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| Coarse || 24-16|| 18-12 || 12=8
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|-
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| Normal || 18-12|| 12-8 || 8-4
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|-
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| Fine ||12-8 || 8-4 || 4-2
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|}
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(Source: Onyeka Nkwonta* and George Ochieng (1996))
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===== (iv)Filtration Rates =====
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* Filtration rate has a significant influence on the treatment removal.
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* Good removal in roughing filters are best achieved with low filtration rate (Boller, 1993), because low filtration rates are critical to retain particles that are gravitationally deposited to the surface of the media.
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* While used as pretreatments for iron and manganese removal, it can operate at filtration rates of 1.5 - 3 m/h (Hatva, 1988). It is reported that horizontal flow roughing filter is capable of removing metals like iron, manganese, turbidity and color at a filtration rate of 1.8 m/h (Dastanaie, 2007)
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* At increased filtration rates (2 m/h), coarse particles penetrate deeper into the bed and they cause decrease in filter efficiency (Wegelin et al. (1986). Whereas at 1  m/h there is good distribution of solids loading throughout the bed. Hendricks (1991) also suggested that normal filtration rate of horizontal roughing filters is between 0.3 and 1.5 m/h.
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 +
===== (v)Filter Media Length =====
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* Improved cumulative removal efficiencies are typically correlated to longer filter lengths (Collins, 1994; Wegelin, 1986).
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* Incremental removal efficiencies decreases with increasing filter length due to the preferential removal of larger particles early in the filter (Wegelin, 1996).
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* The rate of decline is dependent on filter design variables and the size and nature of particles in suspension. The  use of different media sizes  may  allow for treatment targets to be met by a shorter filter with multiple media sizes compared to with long filter packed with one media size.
 +
 +
===== (vi)Filter Media Materials =====
 +
The following material could therefore be used as filter media:
 +
a)Gravel from a river bed or from the ground.<br>
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b)Broken stones or rocks from a quarry.<br>
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c)Broken burnt clay bricks.<br>
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d)Plastic material either as chips or modules (e.g. used for trickling filters) may be used if the material is locally available.<br>
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e)Burnt charcoal, although there is a risk of disintegration  when cleaning  the  filter material, should only be considered in special cases (e.g. for removal of dissolved organic matter).<br>
 +
f)Coconut fibre, however, due to the risk of flavoring the water during long filter operation, should be used with care.<br>
 +
g)Broken burnt bricks and improved agricultural  waste (e.g. charcoal, maize  cobs), can also be effectively used as pretreatment media (Ochieng (2006) and therefore could serve as alternatives where natural gravel is not readily  available.
 +
 +
===== (vii)Types of Roughing Filters =====
 +
There are many types of roughing filters with different flow directions and with different types of filter medium (e.g. sand, gravel, coconut husk fibre). However the common types are:
 +
i.Horizontal Flow Filters (HRF)<br>
 +
ii.Vertical (VRF)<br>
 +
iii.Dynamic (DRF)
 +
 +
==== (d)Bank Filtration (BF) ====
 +
Bank filtration (BF) is the infiltration of surface water, mostly from a river system into a groundwater system induced by water abstraction close to the surface water (e.g. river bank). This water abstraction is commonly done by operating wells.
 +
 +
As the water flows through the soil, it is filtered and hence its quality is improved. In the context of developing or newly-industrialised countries, bank filtration may contribute to a more sustainable water cycle by recharging stressed  groundwater bodies with filtered surface water. (Sharma & Amy 2009; Huelshoff et al.)
 +
 +
Bank filtration is a water treatment technology that consists of extracting water from rivers by pumping wells located in the adjacent alluvial aquifer. During the underground passage, a series of physical, chemical, and biological processes take place, improving the quality of the surface water, substituting or reducing conventional drinking water treatment.
 +
 +
Bank filtration works by pumping pressure in the alluvial aquifer adjacent to the river that forces the water to percolate from the river into the aquifer. In this path, a series of physical and biogeochemical processes take place, including physical filtration, adsorption, absorption, biodegradation, and dilution. Thus, riverbank-filtrate often shows better quality than river water, making its treatment for human consumption easier and less expensive.<br>
 +
 +
The removal of sediment, organic and inorganic compounds, and pathogens takes place during the first metres from the river in what is known as the hyporheic zone, which usually presents reducing conditions due to the high microbial activity which consumes the oxygen in the water. Within this zone there are important biochemical processes and Redox reactions that affect groundwater quality.<br>
 +
 +
The efficiency of BF depends on the local conditions including the hydrology and hydrogeology of the site, the geochemistry of water (from both the river and the aquifer), the geochemistry of microbial populations, and associated metabolic activity. This is the reason as to why it is difficult to define general procedures for identifying appropriate sites to implement the BF technique, as well as the expected efficiency of the process.
 +
One limitation on the efficiency of BF is the clogging of the bed and the banks of the river, which decreases the hydraulic conductivity in the hyporheic zone. This clogging can be caused by infiltration of the fine sediments, gas entrapment, bio-film formation related to microbiological activity, or the precipitation and co-precipitation of inorganic compounds, being the first of these the most influential factor in clogging formation.<br>
 +
 +
'''Siting and Design Parameters'''
 +
# The most important parameters for success during BF are the flow path length, the thickness of the aquifer, and the infiltration area in the river (Grischek et al. (2002).
 +
# The siting and design of a BF system depend on hydrogeological, technical, economical, regulatory, and land-use factors.
 +
# Riverbank filtration wells can be designed either vertically (as the most common practice especially for the extraction of low water quantities) or horizontally (for higher extraction rates).
 +
# Local factors such as river hydrology, hydrogeological site conditions (i.e., aquifer thickness and hydraulic conductivity), and the aims of water withdrawal are used to determine the capacity of the wells, the travel time of the bank filtrate, distance between the river and the well.
 +
# Horizontal wells (sometimes with a radial pattern), also known as collector wells, are directed toward the river and extract water from beneath the riverbed, whereas vertical wells extract water along the riverbed.
 +
# The BF wells can also be distributed parallel to the riverbank in galleries or groups
 +
 +
==== Floatation ====
 +
Floatation may be defined as the transfer of a suspended phase from the bulk of a dispersion medium to the atmosphere/liquid interface by means of bubble attachment. There are three basic processes involved which are: bubble generation, bubble attachment and solids separation.
 +
 +
Floatation is described as a gravity separation process, in which gas bubbles attach to solid particles to cause the apparent density of the bubble-solid agglomerates to be less than that of the water thereby allowing the agglomerates to float to the surface. The different methods of producing the gas bubbles give rise to different types of Floatation processes which are Dissolved-air Floatation, Electrolytic Floatation and Dispersed-air Floatation.
 +
 +
==== Dissolved-Air Floatation ====
 +
This is a relatively new solution for the clarification of surface and ground waters. The process as shown in Fig 7.24 is relatively simple and can be very effective. After flocculation, the produced floc attaches to micro-bubbles and rises to the water's surface. The floated solids are periodically evacuated either hydraulically or mechanically, depending on the sludge concentration required.

Revision as of 12:57, 3 June 2020

1 Chapter Seven Water Treatment

1.1 Introduction

In this chapter, different categories of the unit operations that are utilized to achieve different water treatment levels are described. It is followed by description of the recommended approach of design of treatment plant components. Emphasis should be given to potential water sources that have undergone investigations on the variability of both the quality and quantity for at least two years. The data gathered should be used for selection of appropriate treatment flow sheets and designing it.

Classification of the qualities of water sources found in Tanzania according to the complexity of its treatment
Water treatment is any process that improves the quality of water to make it more acceptable for human consumption. Production of drinking water treatment involves the removal of contaminants from raw water to produce water that is pure enough for human consumption without any short term or long term risk of any adverse health effect.

The processes involved in removing the contaminants from water include physical processes such as settling and filtration, chemical processes such as disinfection and coagulation and biological processes such as slow sand filtration. Table 7.1 and Figures – 7.8 presents the recommended flow Sheets for the Most Common Water Sources in Tanzania. Water contaminants removal processes are best represented in water treatment flow sheets (Figures 7.1 – 7.8).

1.1.1 Table 7.1: Recommended water treatment flow sheets for the most common water sources in Tanzania
S/No Nature of the Water Source Recommended Flow Sheet(Minimum) Remarks
1 Rainwater harvesting Periodic disinfection Assuming design allows flushing of first rains
2 Deep well with no minerals Residual chlorination Assuming water is supplied through a distribution line and storage tank.
3 Shallow wells with no pollution Periodic chlorination Well used by many families
4 Highland stream with no soil erosion Screens, grit chamber or sand trap, sedimentation with lamella plates, Slow Sand Filtration (SSF),chlorination Water supplied via distribution pipes and storage tanks.
5 Deep well with Iron and Manganese Aeration,sedimentation, SSF or RSF, chlorination Assume water is supplied via distribution pipe and storage tanks
6 Deep well with Fluoride Sedimentation tank, Bone char, chlorination or Sedimentation, UF/MF, NF, disinfection. Energy for pumps needed & chlorine not used to avoid corrosion of Polyamide fibres.
7 Surface water with low pH Screens, Lime, sedimentation with lamella plates, RSF, disinfection
8 Surface water with Nitrate & Sulphate Screens, Coagulation, Flocculation, Sedimentation with lamella plates,RSF,disinfection. Or Screens,UF/MF, NF, disinfection
9 Surface water with Cadmium, Selenium, Arsenic, Mercury, Lead, Copper,Uranium, Chromium,Cyanide, Nickel Screens, Coagulation, Flocculation, Clarifier, RSF, UF/MF, NF. Areas near mining sites with metal recovery possibility.
10 Surface water with pesticides Screens, Lamella settlers, RSF, UF/MF, NF Areas with intense large scale agriculture

Floculation ← Coagulation ← Grit Chamber/Sand trap ← Screens ← ↓ Clarifier → RSF → Disinfection

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Classification of Unit Operations to achieve water treatment levels
Categories for water treatment levels are; pre-treatment, primary treatment, secondary treatment and tertiary treatment.

  • Pre-treatment includes units like Scum and floating matters removal, Screening (fine and coarse), Sand trap, Grit removal, Pre-chlorination, Water conditioning (pH correction).
  • Primary treatment comprises of Sedimentation, Primary filtration, Floatation, Aeration.
  • Secondary treatment includes Coagulation, Flocculation, Clarification, Filtration, Softening, Reverse Osmosis, Capacitive De-Ionisation (CDI), Ion Exchanger, Adsorption, Constructed wetlands.
  • Tertiary treatment includes Disinfection, Softening, Water conditioning, Water polishing.

Recommended overall design approach for treatment plant components
The design of the main conveyance units for the treatment plants including pipes and channels should be designed for a period of 20 years (design life). On other hand, the individual unit operations should be designed for a design period of 10 years in an approach that allows for phased implementation. Intentionally, to allow for adoption of the latest technologies and to avoid tying substantial capital in the treatment plants. It is recommended that a potential water source should be closely and intensely monitored for a period ranging from 2 to 3 years. This period can include the time when the feasibility study for the water supply project is being undertaken including the environmental impact assessments. Water source quality and quantity data should be used for determination of the most suitable treatment flow sheet.

Documents and websites consulted and those which are hyper-linked to the DCOM Manual

The following documents were consulted for purposes of making reference for the design of the treatment plants in Tanzania:

  • URT, 2009. Design Manual for Water Supply and Wastewater Disposal.
  • Ministry of Drinking Water and Sanitation, May 2013. Operation and maintenance manual for rural water supplies. India.
  • The Republic of Uganda, Ministry of Water and Environment, 2013. Water Supply Design Manual, 2nd edition.
  • World Bank Phillipines, February 2012. Water Partnership Program. Rural Water Supply Vol.I Design Manual.
  • Washington State Dept. of Health USA, October 2019. Water System Design Manual.
  • URT, July 1997. Design Manual for Water Supply and Wastewater Disposal.

Throughout this manual, on a number of occasions the designers are referred to the websites of the Ministry of Water (https://www.maji.go.tz) or RUWASA (https://www.ruwasa.go.tz) vide the various hyperlinks inserted in order to access the standard drawings for various appurtenances.

Water treatment design considerations
The manual has made reference to a number of design guidelines that are relevant for the design of various unit operations and wherever necessary the full guidelines have been hyper-linked to the manual. For each unit operation, a few critical design criteria have been provided.

Before proposing or designing any treatment plant for any planned water supply project, water quality of the anticipated water sources to be treated has to be known to the designer. Knowing the historical and current water quality trends of the sources will help in designing a treatment plant that can address the localized water quality challenges of concern in the given area apart from the general water quality parameters.

The sizing and selection of the treatment technology and different units to be installed should always aim at meeting the established national and international water quality standards and associated health criteria which are often updated from time to time. There are different criteria and standards across the world, however in Tanzania, the most recent standards of Tanzania Bureau of Standards and the World Health Organization (WHO) guidelines are the guiding documents recommended to be referred when designing a water supply project.

1.2 Water treatment levels and units

1.2.1 Pre-treatment

1.2.1.1 Scum and Floating Materials Skimmer

This is the unit operation that enables the manual or automated removal of the scum and floating matter ahead of the screening units. These are designed to skim the entire width of the approach area ahead of the screens. In view of the variability of flow of water from the sources, skimmers ought to be designed such that they can be adjusted up or down depending on the quantity variation that is established during the feasibility study. The width of the channel or any open conduit delivering the raw water will determine its design. Figure 7.9 shows the design of such a skimmer.

Figure 7.9: A Typical Skimmer (source;https:www.jmsequipment.com>skimming systems/vis.06/01/2020)


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1.2.1.2 Screening or straining

This unit operation consists of fine screens and coarse screens which perform the task of removal of all fine and coarse matters that may block the screen or damage downstream appurtenances or machines. This is a physical, pre-treatment process used to remove weeds, grass, twigs, bilharzial snails and other freshwater crustacea as well as coarser particles including plastics, tins and others so that they do not enter the pumping, treatment, or supply system. Screens are placed at the entrance to the intake of a water supply project.

The design considerations for surface water screens are;

  • They should be easily accessible, at least during medium and low flows and inclined downstream of the river or stream as well as during cleaning (if manually done) as indicated in Fig.7.10.
  • Distance between bars should be between 10 and 30 cm. for coarse screens and between 0.5 and 5 cm. for fine screens. The shape of the screen bars is either round or rectangular.
  • Approach velocity entering the screen (Va) from upstream should not exceed 0.3 to 0.5 m/sec. to limit sedimentation.
  • Velocity through the screens (Vs) should not exceed 0.7 to 1.0 m/sec. to prevent soft deformable materials from being forced through the screens.
  • The ratio of the width of the screens (Ø) and the space between the bars (b) determines the ratio between the two velocities (Va) and (Vs).
  • Small screens are made removable for cleaning, medium sized can be hand raked in-situ whilst large screens will need in-situ mechanical or electrical operated rakes.

Figure 7.10 presents the formula for calculation of the head loss through the screens as well as the screen bars coefficient (ƥ).

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1.2.1.3 Grit Removal

Grit consists of the heavy inorganic fraction of sewage solids that includes road grit, sand, egg shells, broken glass, coconut shells and metal pieces. The purposes of including grit channels in the design are as follows:

  • To protect pumps and other mechanical parts from excessive wear and tear,
  • To avoid undue clogging/filling up of subsequent unit operations,
  • To differentially remove grit but not the organic particulates in water.

The average specific gravity of grit is 2.5 with an average settling velocity S = 30 mm/sec. In comparison, while sand grit has an average solids density ƥs = 2650 kg/m3organics have a density ƥo ranging from 1020 to 1200 kg/m3.

1.2.1.4 Design approach

To exploit the differential sedimentation rates of the particles by providing channels that ensure removal of grit rather than any other lighter particles and to maintain the horizontal flow velocity Vh has to be maintained at about 0.3 m/sec. Provision of a parabolic or near parabolic cross section of the channel guarantees that the constant velocity is maintained at all flows. In practise, due to the difficulty of construction of parabolic sections, trapezoidal sections are used.

1.2.1.5 7.5.1.3.1 Design criteria

Length of the channel L = 20 (maximum depth of flow)

L/d = Vh/Vs Where, Vh = Horizontal flow velocity Vs = Vertical settling velocity L = Length of the grit channel D = Depth of flow in the channel

There are three types of grit channels that can be designed, these include horizontal flow, Rotational flow and Vertical flow

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1.2.1.6 Sand Traps

This pre-treatment unit is designed to trap sand after water has been guided into the intake chamber in order to reduce the potential for wear and tear as well as silting up the unit operations that are located downstream of the intake structure. The minimum diameter of washout pipes of such sand traps is 75 mm and the bigger the main intake pipe, the bigger is the flushing pipe for sand. Figures 7.15, 7.16 and 7.17 show the layout plan and two sections of such a sand trap for small streams intake.

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1.2.1.7 Pre-chlorination

This is a unit operation that is used for purposes of controlling algae growth in raw water and the process of preparation of the chemical to facilitate dosing will include the standard preparation of the aqueous solution as done during disinfection. This is often added upon establishment of occurrence of algal blooms during certain periods of the year as confirmed by laboratory tests undertaken daily. The amount of the dose will be established daily in order to pre-determine the pre-chlorination dose that has to ensure no interference with downstream unit operations in case the flow sheet involves biological treatment processes like slow sand filter or others. The location of dosing of the chemical has to be secured against direct sunlight and has to ensure intense mixing. Where chlorine gas is used for pre-chlorination, the usual measures that are taken when dosing chlorine in gaseous form have to be taken including leakage detection. To minimise chlorine consumption, pre-chlorination has to be done downstream of the fine screens.

Other design features can be seen in the Appendix H. of this DCOM Manual. It should be noted that pre-chlorination also controls growth of bacteria in pipes and tanks (BRITANICA, 2020). Lanfair et al (2020) cautions that if pre-chlorination is applied to water with a high concentration of natural organic matter (nom), the latter is suspected to react with chorine to form Disinfection by-products (DBPs) that include trihalomethanes or haloacetic acids that are suspected to contribute towards stomach and bladder cancer.

1.2.1.8 Water pre-conditioning (pH adjustment)

Water pre-conditioning can entail a number of pre-treatments undertaken prior to pre- chlorination which is often the final step in pre-treatment. This unit operation involves adjustment of the pH upstream in order to ensure the chemicals used during further treatment processes are dosed to water that has the correct pH range for maximum efficiency. A good example is when the treatment flow sheet includes coagulation with Ferrous Sulphate that requires an optimum pH range of 7 to 8.5. As a matter of fact,the application of this unit operation guarantees that the daily fluctuations in quality are also reflected in the chemical that is used for pH correction whether an acid or an alkali is used. It is usual to apply lime for increasing pH and use of acids like dilute sulphuric acid or hydrochloric acid. Other pre-conditioning measures include use of sodium carbonate (soda ash) to remove hardness (i.e. calcium carbonate).

1.2.1.9 Primary Treatment

Primary treatment of water removes material that will either float or readily settle out by gravity. This water treatment level includes the physical processes of screening, grit removal and sedimentation.

1.2.1.10 Sedimentation

In designing sedimentation tanks, the required detention time determines the dimensions of the tank. A rectangular tank is the simplest design to use. Detention time is calculated as Volume/Flow rate (Q). The detention times based on the average daily flows and usually range from about 45 minutes to 3 hours depending on water turbidity. The ideal inlet reduces the entrance velocity and distributes the water as uniformly as possible across the depth and width of the tank. Outlets are usually weirs which are sufficiently long to reduce the flow velocity, and so avoid the re-suspension of the solids in the water. Plain sedimentation tanks should be designed for a surface loading in the range of 0.1 - 0.5 m3/m2/h. The exact surface loading to be adopted should be determined after carrying out settlement tests on samples of raw water, typical of all regimes of the water source. The settling properties of water will depend on the soil and vegetation conditions in the catchment area, and they will vary considerably between different locations and regimes of the water source. Figure 7.18 (a and b) shows a cross section through a circular sedimentation tank while figure Figure 7.19 (a) and (b) shows a cross section through a rectangular sedimentation tank.


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1.2.1.11 Lamella Plate Settlers (Inclined plate settlers)

Settling efficiency of a basin depends upon the design surface loading. Lamella plates and small diameter tubes having a large wetted perimeter, relative to wetted area providing laminar flow conditions and low surface loading rates have shown good results in terms of settling efficiency and economy in space as well as cost. Plate or tube configuration can be horizontal or steeply inclined. In inclined plates or tubes (55o – 60o) continuous gravity drainage of the settleable material onto the floor below can be achieved, without impairment of effluent quality. However, to work effectively an efficient flocculation stage is critical.

In purpose built lamella plate settlers the water enters at the base of the lamella plates and travels upwards between the lamellas. Each space between the lamella plates tends to act as a semi -independent settling modules with the lamella plates extending from near the base of the tank to about 125 mm above the top water level. The clarified water is collected by saw-toothed notched launders running along each side of the plate. Unless sludge is removed mechanically by a scraper, sufficient depth beneath the plates is required for access during cleaning, although this can be aided by pressurised water. A typical arrangement is illustrated in Figure 7.20.

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Plates are made of stainless steel or plastics with a width of 1.25 to 1.5 m and a length of 2.5 to 3.25 m including the length above water. It is advisable to use standard lengths of metal sheets and plastic sheets available in the market. In Tanzania sheets are in dimensions of 120 cm x 240 cm. Plate thickness is usually about 0.7 mm for stainless steel whilst the horizontal spacing between plates is varied according to the nature of the raw water but within the range 50 – 80 mm.

Total settled area is then:

A = (n-1) × L × W × cos θ (7.2)

Where; n = number of plates L = plate length in water (m), less the transition length W = the plate width (mm), and θ = the angle of inclination of the plates to the horizontal (55o – 60o) A = Settled area (m2)

Table 7.2: Important parameters for the design of lamella plate settler.

S/No Parameter Units Range
1 Surface8 loading rate m/h 5 to 10 m/h9
2 Hydraulic Loading Ratio - 0.25 -0.5
3 Angle of inclination of the plates, θ - 45-70o Typical: 60o
4 Plate spacing, w mm Min 20
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1.2.1.12 Primary Filtration

Primary filtration of water removes material through filtration whose particles size is greater than the opening size. The target particles are the ones which are not removed by sedimentation tanks.

1.2.1.13 Slow Sand Filtration

A Slow Sand Filter (SSF) is basically a large tank containing the sand bed. A distinguishing feature of slow sand filters is the presence of a thin layer, called the schmutzdecke, which forms on the surface of the sand bed and includes a large variety of biologically active micro-organisms.

Water is introduced at the top and trickles down through the sand bed to the under- drains and goes to the storage tank. The impurities in the water are retained at the upper layers of the sand bed. In the process, the schmutzdecke consisting of bacteria and microscopic plants grow. The schmutzdecke removes the organic matter and most of the pathogenic micro-organisms in water which might be smaller than the pores of the sand.

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1.2.1.13.1 (a)Elements of a Slow Sand filter

Figure 3.43(a) presents, in diagrammatic form the various elements that go to make up a Slow Sand Filter. Essentially the SSF elements consist of: (i)a supernatant (raw) water reservoir, the principal function of which is to maintain a constant head of water above the filter medium, this head providing the pressure that carries the water through the filter;
(ii)a bed of filter medium (nearly always sand), with in and upon which the various purification processes take place
(iii)an under-drainage system, which fills the dual purpose of supporting the filter medium while presenting the minimum possible obstruction to the treated water as if emerges from the underside of the filter-bed; and
(iv)a system of control valves to regulate the velocity of flow through the bed, to prevent the level in the raw water reservoir from dropping below a predetermined minimum during operation, and to permit water levels to be adjusted and backfilling to take place when the filter is put back into operation after cleaning
(v)It comprises approximately 1.2 m depth of fine sand supported on two or three gravel layers. The effective size of the sand used in slow sand filters is about 0.2 mm, but may range between 0.15 mm and 0.35 mm, and with a coefficient of uniformity of between 1.5 and 3.0.
(vi)It is a very simple and effective technique for purifying surface water. It will remove practically all of the turbidity from the water as well as most of the pathogens without the addition of chemicals. Slow sand filters can frequently be constructed largely from locally-available materials.

In a SSF the water is purified by slow percolation through a bed of fine sand. Pre- treatment is necessary with raw waters having an average turbidity of 25 NTU or more, but should be considered also for less turbid raw waters 5 to 25 NTU) to improve effluent quality and reduce frequency of cleaning. The SSF is also useful for treating groundwater containing solids in suspension, e.g. ferric and manganese compounds converted by aeration from the soluble state of the salts.
Table 7.3 below shows the typical performance of the Slow Sand Filter.

Parameter of Water Quality Purification Effect of Slow Sand Filtration
Colour 30% to 100% reduction
Turbidity Turbidity is generally reduced to less than 1 NTU
Faecal Coliforms 95% to 100%, and often 99% to 100%, reduction in the level of faecal coliforms
Cercariae Virtual removal of cercariae of schistosomes, cysts

and ova

Viruses Virtually complete removal
Organic matter 60% to 75% reduction in COD
Iron and manganese Largely removed
Heavy metals 30% to 95% reduction

(Source: 3rd Design manual for water supply and waste water disposal, 2009)

1.2.1.13.2 (b)Design Considerations of Slow Sand Filter (SSF)
  • Raw water quality and necessity for pre—treatment and/or aeration;
  • Necessity for chlorination room and clear water storage, pumping and distribution;
  • Site location, foundation conditions, space for expansion and pre-treatment;
  • Availability of source of filter media, and construction materials;
  • Fencing and security
  • The filter sand must be free from any clay or silt content and preferably of a well- rounded quartz material. Organic matter should be avoided.
1.2.1.13.3 (c)Design Criteria;
  • When choosing the filter sand, the grain size distribution should meet the effective size 0.15 to 0.35 mm and the coefficient of uniformity should be less than 3 ,
  • In order to calculate for the total area of filter beds, a working rate of 0.1 - 0.2, m3/m2/hr is recommended. When one filter is not operational, the working rate of the remaining filter should not exceed 0.2 m3/m2/h.
  • The turbidity in the incoming water should not exceed an average of 5 - 10 NTU. In cases of higher turbidity, preliminary treatment such as roughing filters is necessary
  • The inlet structure should be designed in such a way that the raw water is equally distributed over the filter bed area.
  • To achieve this, the inlet velocity should be around 0.1 m/s and the width of the inlet structure should be at least (0.05 x Q) metres, where Q is the design flow in m3/h.
  • The minimum size of a filter unit should be 15 to 20 m2;
  • The height of the supernatant water should be 1 to 1.5 m,
  • The oxygen content of the water after filtration should not be less than 3 mg/l.
  • Calculate filter surface area = flow capacity (m3) / rate of filtration
1.2.1.13.4 (d)Main Water Under-drain
  • Calculate Diameter of the under drain = (22 × d2) / (28 × r)
  • Area of holes or slots to be 1.5% of area of the filter
  • As a check, the oxygen content of the water after filtration should not be less than 3 mg/l.
  • Freeboard above water level = 0.2 - 0.3 m
  • Height of walls above ground surface= > 0.8 m
  • Gravel filter support = 0.3 - 0.5 m
  • Depth of under—drainage system = 0.3 - 0.5m
  • Area (A) per filter bed = 10 - 100 m2
1.2.1.13.5 Rapid Gravity Sand Filtration

This is a process in which water flows onto the top of the filter media and is driven through it by gravity. In passing through the small spaces between the filter's sand grains, impurities are removed. The water continues its way through the support gravel, enters the under-drain system, and then flows to the reservoir. It is the filter media which actually removes the particles from the water. The filter media is routinely cleaned by means of a backwashing process.

Rapid Sand Filtration (RSF) is a technique commonly used for treating large quantities of drinking water. It is a relatively sophisticated process usually requiring power- operated pumps for backwashing or cleaning the filter bed, and some designs require flow control of the filter outlet. A continuously operating filter will usually require backwashing about every two days or so when raw water is of relatively low turbidity and at least daily during periods of high turbidity. Because of the higher filtration rates, the area requirement for a rapid gravity filtration plant is about 20% of that required for slow sand filters.

Surface loading should be between 4 and 7 m3/h.m2, and the filter structure should be designed with a minimum height between the top of the filter media and the bottom of the wash water channel of at least 30% of the height of the filter media as this expands during backwashing. It may be necessary to include for air-scour as well as backwashing, or for the two combined in a single operation.

Normally a sufficient distribution of the wash water will be achieved if:

  • Ratio of area of orifice to area of bed served (1.5 to 5) × (103) : 1
  • Ratio of area of main to laterals served (1.5 to 3) : 1
  • Diameter of orifices 5 - 20 mm
  • Spacing of orifices 100 - 300 mm centre to centre
  • Spacing of laterals approximating to the spacing of orifices.

The filter bed should be approximately 1.0 m thick and preferably consist of well- rounded quartz sand with an effect size of 0.7 - 1.0 mm and uniformity coefficient in the range of 1.3 - 1. 5. The available hydraulic head above the top of the filter bed should be 1.3 - 1.5 m. The following stratification of the sub structure should be used to support a filter of an effective grain size as suggested earlier (finest strata at the top).

  • 0.15 m of grain size 2 - 2.8 mm
  • 0.10 m of grain size 5.6 - 8 mm
  • 0.10 m of grain size 10 - 20 mm
  • 0.10 m of grain size 20 - 40 mm
  • 0.10 m of grain size 40 - 60 mm

The washing velocity should be in the range of 35 – 55 m3/m2/hour, however care must be taken to ensure that sand carry over into the wash-water channel does not occur and the actual wash-water rate adjusted accordingly.

In order to achieve proper washing of the filter a storage volume sufficient for continuous washing for an 8 to 10 minute period should be available.

1.2.1.13.6 (b)Design Steps

The following design steps should be followed;

(i)Filter Units

  • Rate of filtration should be 4 - 6 m3/m2/hr
  • Determine flow capacity in the filter (m3/day. demand)
  • Calculate filter surface area = flow capacity (m3) / rate of filtration

(ii)Main Water Under-drain

  • Select flow capacity and flow velocity
  • Calculate area required = (low capacity)/ (low velocity) (m2)
  • Calculate diameter of the under drain, d= Square Root of (4A/Π)
  • Area of holes = 0.2 - 0.4 % of the area of filter
  • Use five layers of sand particles in the filter as indicated above:
  • Water depth 1 – 1.2 m
  • Sand depth 1 – 1.2 m
1.2.1.13.7 (c)Filter Backwashing

A major cause of poor performance by rapid gravity filters is a result of either inadequate or excessive backwashing rates. Backwashing is sometimes carried out by water alone but more often by air and water usually applied one after the other by reverse flow to the filter bed. The first operation however is to allow the filter to drain down until the water lies a few centimetres above the top of the bed. Air is then introduced through the collector system at a rate of about 6.5 to 7.5 mm/s.

Where air and water is applied separately, air scour normally lasts about 3 – 4 minutes and the water wash about 4 – 6 minutes. Where applied concurrently, air is first introduced and after about 1.5 – 2 minutes when it is fully established water is introduced and the combined backwash last for about 6 – 8 minutes. Air is stopped first and the water run for several more minutes to rinse the bed. Generally, total water consumption per wash amounts to about 2.5 bed volumes, but should normally not exceed 2% of the treated water output in well run plants.

Comparison between Slow Sand Filters and Rapid Sand Filters a)Base material: In SSF it varies from 3 to 65 mm in size and 30 to 75 cm in depth while in RSF it varies from 3 to 40 mm in size and its depth is slightly more, i.e. about 60 to 90 cm.
b)Filter sand: In SSF the effective size ranges between 0.2 to 0.4 mm and uniformity coefficient between 1.8 to 2.5 or 3.0. In RSF the effective size ranges between 0.35 to 0.55 and uniformity coefficient between 1.2 to 1.8.
c)Rate of filtration: In SSF it is small, such as 100 to 200 L/h/sq.m. of filter area while in RSF it is large, such as 3000 to 6000 L/h/sq.m. of filter area.
d)Flexibility: SSF are not flexible for meeting variation in demand whereas RSF are quite flexible for meeting reasonable variations in demand.
e)Post treatment required: Almost pure water is obtained from SSF. However, water may be disinfected slightly to make it completely safe. Disinfection is a must after RSF.
f)Method of cleaning: Scrapping and removing of the top 1.5 to 3 cm thick layer is done to clean SSF. To clean RSF, sand is agitated and backwashed with or without compressed air.
g)Loss of head: In case of SSF approx. 10 cm is the initial loss, and 0.8 to 1.2m is the final limit when cleaning is required. For RSF 0.3m is the initial loss, and 2.5 to 3.5m is the final limit when cleaning is required.

1.2.1.14 Other Types of Filters

1.2.1.14.1 (a)Pressure Filters

Pressure filters are circular pressurised vessels containing the filter media and usually designed for vertical flow. They work on the same principle as rapid gravity filters differing in that the filter medium is enclosed in a steel vessel and the water is forced through it under pressure.

1.2.1.14.2 (b)Upward Flow Filters

Upward flow filters are theoretically more efficient than gravity filters where the water to be filtered flows upwards through the naturally desegregated, progressively finer and finer media so that coarser particles are trapped first in the coarser bottom layers. This tends to extend the period between backwashing. Several sand grades, getting progressively finer upwards have also been used in which case some restraining means such as a grid is required often located about 0.1 m below the surface where upward backwashing is used.

The major reason why RSF is a preferred option of water filtration to SSF is rate of filtration where RSF is higher per filter area flexibility, where SSF are not flexible for meeting variation in demand, method of cleaning where in the case of RSF, no sand is lost. Also RSF requires minimal land requirement (Source: Ugandan Water Supply Design Manual (2013).

1.2.1.14.3 (c)Roughing Filters

Roughing filters have their place as a form of primary treatment, especially for turbid or highly changeable river water. This technique of primary treatment has been greatly under-utilised in Tanzania in the past. Although much research has been undertaken since late 1970s (Mbwette & Wegelin, 1984). It is used primarily to remove solids from high turbidity source waters prior to treatment with such unit operations as slow sand filters.

In a typical roughing filter, there are series of tanks which are filled with progressively smaller diameter media in the direction of the flow which can either be horizontal or vertical. With respect to the vertical flow direction, up-flow or down-flow roughing filters can be designed. The media in the tank which may include gravel, rice husks or any other suitable local material, plays an important role of reducing the vertical settling distance of the particles to a distance of a few millimetres

1.2.1.14.4 (i)Advantages of Roughing filters
  • Can considerably reduce the number of pathogens in the water, as well as the amount of iron and manganese.
  • Can be considered a major pre-treatment process for turbid surface water since they efficiently separate fine solid particles over prolonged periods.
  • Long filters (10 m) at low filtration rates (0.5 m/h) are capable of reducing high suspended solids concentrations (1000 mg/l TSS down to less than 3 mg/l).
  • Capable of reducing peak turbidities by 80 to 90 percent and faecal coliforms by 77 to 89 percent.
  • They are placed at the treatment plant site and operated in combination with other pre-treatment units such as dynamic filters or sedimentation tanks.

NB: For detailed information, designers are recommended to download the comprehensive SANDTEC design report entitled ‗Surface Water Treatment by Roughing Filters, A Design, Construction and Operation Manual‘ available at http//sandec.ch/WaterTreatment/Documents/Surface%20Water%20Treatment.pdf Features of Roughing Filters (RF).

The main part of the filter is the section containing the filter material. However, a Roughing filter comprises the following six elements:-

  1. Inlet Flow Control
  2. Raw Water Distribution
  3. Actual Filter
  4. Treated Water Collection
  5. Outlet Flow Control
  6. Drainage System
1.2.1.14.5 (ii)Parameters for Design of Roughing Filter
  1. Filter media size
  2. Filtration Rates
  3. Filter Length
  4. Filter media materials
1.2.1.14.6 (iii)Filter Media Size
  • The size of filter media decreases successively in the direction of water flow and ideally the uniformity of filter media fractions is maximized to increase filter pore space (storage capacity) and aid in filter cleaning (Boller, 1993).
  • The use of multiple grades of filter media in a roughing filter promotes the penetration of particles throughout the filter bed and takes advantage of the large storage capacities offered by larger media and high removal efficiencies offered by small media
  • The effect of surface porosity and roughness of filter media on particle removal efficiency in roughing filtration is insignificant compared to the size and shape of macro-pores in the filter (Wegelin, 1986)
  • Media types commonly used in roughing filtration are quartz, sand and gravels but can be replaced by any clean, insoluble and mechanically resistant material (Graham, 1988)
  • Common grades of media used in roughing filters are provided in the Table 7.4

Table 7.4: Different sizes of media in a Roughing filter

Roughing Filter description 1st compartment 2nd Compartment 3rd Compartment
Coarse 24-16 18-12 12=8
Normal 18-12 12-8 8-4
Fine 12-8 8-4 4-2

(Source: Onyeka Nkwonta* and George Ochieng (1996))

1.2.1.14.7 (iv)Filtration Rates
  • Filtration rate has a significant influence on the treatment removal.
  • Good removal in roughing filters are best achieved with low filtration rate (Boller, 1993), because low filtration rates are critical to retain particles that are gravitationally deposited to the surface of the media.
  • While used as pretreatments for iron and manganese removal, it can operate at filtration rates of 1.5 - 3 m/h (Hatva, 1988). It is reported that horizontal flow roughing filter is capable of removing metals like iron, manganese, turbidity and color at a filtration rate of 1.8 m/h (Dastanaie, 2007)
  • At increased filtration rates (2 m/h), coarse particles penetrate deeper into the bed and they cause decrease in filter efficiency (Wegelin et al. (1986). Whereas at 1 m/h there is good distribution of solids loading throughout the bed. Hendricks (1991) also suggested that normal filtration rate of horizontal roughing filters is between 0.3 and 1.5 m/h.
1.2.1.14.8 (v)Filter Media Length
  • Improved cumulative removal efficiencies are typically correlated to longer filter lengths (Collins, 1994; Wegelin, 1986).
  • Incremental removal efficiencies decreases with increasing filter length due to the preferential removal of larger particles early in the filter (Wegelin, 1996).
  • The rate of decline is dependent on filter design variables and the size and nature of particles in suspension. The use of different media sizes may allow for treatment targets to be met by a shorter filter with multiple media sizes compared to with long filter packed with one media size.
1.2.1.14.9 (vi)Filter Media Materials

The following material could therefore be used as filter media: a)Gravel from a river bed or from the ground.
b)Broken stones or rocks from a quarry.
c)Broken burnt clay bricks.
d)Plastic material either as chips or modules (e.g. used for trickling filters) may be used if the material is locally available.
e)Burnt charcoal, although there is a risk of disintegration when cleaning the filter material, should only be considered in special cases (e.g. for removal of dissolved organic matter).
f)Coconut fibre, however, due to the risk of flavoring the water during long filter operation, should be used with care.
g)Broken burnt bricks and improved agricultural waste (e.g. charcoal, maize cobs), can also be effectively used as pretreatment media (Ochieng (2006) and therefore could serve as alternatives where natural gravel is not readily available.

1.2.1.14.10 (vii)Types of Roughing Filters

There are many types of roughing filters with different flow directions and with different types of filter medium (e.g. sand, gravel, coconut husk fibre). However the common types are: i.Horizontal Flow Filters (HRF)
ii.Vertical (VRF)
iii.Dynamic (DRF)

1.2.1.15 (d)Bank Filtration (BF)

Bank filtration (BF) is the infiltration of surface water, mostly from a river system into a groundwater system induced by water abstraction close to the surface water (e.g. river bank). This water abstraction is commonly done by operating wells.

As the water flows through the soil, it is filtered and hence its quality is improved. In the context of developing or newly-industrialised countries, bank filtration may contribute to a more sustainable water cycle by recharging stressed groundwater bodies with filtered surface water. (Sharma & Amy 2009; Huelshoff et al.)

Bank filtration is a water treatment technology that consists of extracting water from rivers by pumping wells located in the adjacent alluvial aquifer. During the underground passage, a series of physical, chemical, and biological processes take place, improving the quality of the surface water, substituting or reducing conventional drinking water treatment.

Bank filtration works by pumping pressure in the alluvial aquifer adjacent to the river that forces the water to percolate from the river into the aquifer. In this path, a series of physical and biogeochemical processes take place, including physical filtration, adsorption, absorption, biodegradation, and dilution. Thus, riverbank-filtrate often shows better quality than river water, making its treatment for human consumption easier and less expensive.

The removal of sediment, organic and inorganic compounds, and pathogens takes place during the first metres from the river in what is known as the hyporheic zone, which usually presents reducing conditions due to the high microbial activity which consumes the oxygen in the water. Within this zone there are important biochemical processes and Redox reactions that affect groundwater quality.

The efficiency of BF depends on the local conditions including the hydrology and hydrogeology of the site, the geochemistry of water (from both the river and the aquifer), the geochemistry of microbial populations, and associated metabolic activity. This is the reason as to why it is difficult to define general procedures for identifying appropriate sites to implement the BF technique, as well as the expected efficiency of the process. One limitation on the efficiency of BF is the clogging of the bed and the banks of the river, which decreases the hydraulic conductivity in the hyporheic zone. This clogging can be caused by infiltration of the fine sediments, gas entrapment, bio-film formation related to microbiological activity, or the precipitation and co-precipitation of inorganic compounds, being the first of these the most influential factor in clogging formation.

Siting and Design Parameters

  1. The most important parameters for success during BF are the flow path length, the thickness of the aquifer, and the infiltration area in the river (Grischek et al. (2002).
  2. The siting and design of a BF system depend on hydrogeological, technical, economical, regulatory, and land-use factors.
  3. Riverbank filtration wells can be designed either vertically (as the most common practice especially for the extraction of low water quantities) or horizontally (for higher extraction rates).
  4. Local factors such as river hydrology, hydrogeological site conditions (i.e., aquifer thickness and hydraulic conductivity), and the aims of water withdrawal are used to determine the capacity of the wells, the travel time of the bank filtrate, distance between the river and the well.
  5. Horizontal wells (sometimes with a radial pattern), also known as collector wells, are directed toward the river and extract water from beneath the riverbed, whereas vertical wells extract water along the riverbed.
  6. The BF wells can also be distributed parallel to the riverbank in galleries or groups

1.2.1.16 Floatation

Floatation may be defined as the transfer of a suspended phase from the bulk of a dispersion medium to the atmosphere/liquid interface by means of bubble attachment. There are three basic processes involved which are: bubble generation, bubble attachment and solids separation.

Floatation is described as a gravity separation process, in which gas bubbles attach to solid particles to cause the apparent density of the bubble-solid agglomerates to be less than that of the water thereby allowing the agglomerates to float to the surface. The different methods of producing the gas bubbles give rise to different types of Floatation processes which are Dissolved-air Floatation, Electrolytic Floatation and Dispersed-air Floatation.

1.2.1.17 Dissolved-Air Floatation

This is a relatively new solution for the clarification of surface and ground waters. The process as shown in Fig 7.24 is relatively simple and can be very effective. After flocculation, the produced floc attaches to micro-bubbles and rises to the water's surface. The floated solids are periodically evacuated either hydraulically or mechanically, depending on the sludge concentration required.

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