Chapter Thirteen: Treatment for Special Water Sources
Contents
1 CHAPTER THIRTEEN:TREATMENT FOR SPECIAL WATER SOURCES
Algae, arsenic, cyanobacteria, fluoride and natural organic matters are among water quality parameters that require special treatment to ensure the water produced meet the recommended conditions. Also, because iron as Fe2+ is found in some groundwater, its Operational and Maintenance for iron removal plants (IRPs) is presented in this chapter.
1.1 Algal Control
Algae are unicellular or multi-cellular chlorophyll bearing plants without any true root, stem or leaves. They may be microscopic unicellular colonial or dense mat-forming filamentous forms commonly inhabiting surface waters. Their growth is influenced by a number of factors, such as mineral nutrients, availability of sunlight, temperature and type of reservoir. During certain climatic conditions, there is an algal bloom which creates acute problems in treatment processes and production of potable water. The algae commonly encountered in water purification plants are diatoms, green algae, and blue green algae and algal flagellates. Algae may be seen floating (plankton) in the form of blooms. The problems caused by algae are as follows:
(a) Many species of algae produce objectionable taste and odour due to characteristic coil secretions. These also impart colour ranging from yellow-green to green, blue-green, red or brown;
(b) Profuse growth of algae interferes with chemical treatment of raw water by changing water PH and its hardness;
(c) Some algae act as inhibitors in process of coagulation carried out for water purification;
(d) Some algae clog filters and reduce filter run;
(e) Some algae produce toxin sand their growth in drinking water reservoirs is harmful for humans and livestock;
(f) Some algae provide shelter to a large number of bacteria, some of which may be pathogenic;
(g) Some algae corrode metal tanks, forming pits in their walls;
(h) Algae may also cause complete disintegration of concrete in contact with them;
(i) Prolific growth of algae increases organic content of water, which is an important factor for the development of other organisms.
1.1.1 Remedial Measures
(a) Preventive Measures
Preventive measures should, therefore, be based on control of those factors such as:
(i) Reduction of food supply,
(ii) Change of the environment or exclusion of sunlight though they are not always practicable,
(iii) Clear water reservoir, service reservoir s and wells may be covered to exclude sunlight, but such a remedy is obviously inapplicable in the case of large reservoir of raw water,
(iv) Turbid water prevents large penetration and thereby reduces algal population,
(v) Activated carbon reduces algal population by excluding sunlight but disappearance of activated carbon in the raw water may support algal growth again.
(b) Control Measures
Adequate records of number, kind and location of algae becomes handy for algal growth control. Algaecide dose used should be harmless to humans, have no effect on water quality, should be inexpensive and readily available and easy to apply. The most commonly used algaecides are copper, sulphate and chlorine/ bleaching powder.
Pre-Chlorination
Chlorine treatment is relatively cheap, readily available and provides prolonged disinfecting action. Though chlorine is generally used for disinfecting potable water it can also be used as an algaecide. Pre-chlorination has specific toxic effect and it causes death and disintegration of some of the algae. It also assists in removal of algae by coagulation and sedimentation. It prevents growth of algae on basin walls and destroys slime organisms on filter sand thus prolonging filter and facilitating filter washing.
Dosage: Effective chlorine dose should be such that sufficient chlorine is there to react with organic matter, ammonia, iron, manganese and other reducing substances in water and at the same time leave sufficient chlorine to act as algaecide. Dose required for this purpose may be over 5 mg/L. With chlorine treatment essential oils present in algae as well as organic matter of dead algae are liberated this may lead to development of odour and colour and taste. In such cases break point - chlorination is required. Post chlorination dose can be adjusted to obtain minimum 0.2 mg/ L residual chlorine in potable water at consumer end.
Method of Application: Chlorine is preferably applied as a strong solution of chlorine from chlorinator. Slurry of bleaching powder can also be used. For algal growth control, generally, chlorine is administered at the entry of raw water before coagulant feeder.
1.2 Iron Removal Plants (IRPs)
Two types of such plants are described below:
1.2.1 Compact Plant
The process involves spray aeration through a grid of pipes to flush out CO2, H2S and to improve pH level. Trickling of aerated water through a contact catalytic media viz., limestone of 20 mm size or a combination of MnO2 (Manganese dioxide) and lime; or hard coke, MnO2 and limestone. The relevant processes are:
(a) Sedimentation,
(b) Filtration through rapid gravity filter,
(c) Disinfection.
The structure consists of ordinary masonry or concrete. The aerator with contact media may be placed at the top of the sedimentation tank. Sedimentation tank may be rectangular with a length to breadth ratio of 3:1. The detention time may be around 3-5 hours. The surface loading may be around 25 m3/day/m². Filter media shall consist of sand with effective size 0.5-0.7 mm and a depth of 750-1,000 mm over a 450-600 mm deep gravel 3 to 50 mm size.
1.2.1.1 Operation and Maintenance
The nozzles/orifices attached to the aeration pipe grid shall have their angles so adjusted as to ensure maximum aeration and to prevent loss of water. These nozzles/orifices shall require regular manual cleaning to remove incrusted iron.
(a) The residual iron deposits from inside the pipe grid shall be flushed out by opening end plugs or flanges. These operations should be repeated at least once in 2 months,
(b) The limestone and other contact media require manual cleaning and washing at least once in 45-60 days,
(c) The contact media bed should not remain exposed to sun for a long time to prevent hardening of bed by iron incrustation,
(d) The sedimentation tank inlet baffle wall opening shall be cleaned of iron slime at least once in 45-60 days,
(e) Sedimentation tank bed should be regularly scoured for removal of sludge,
(f) Floc forming aid (coagulant aid) may be used for better coalescing and agglomeration,
(g) The rapid gravity filter should have a water depth of about 1.2-1.5 m,
(h) Since iron deposits create incrustation of filtering media, at least 100-150 mm of tops and layer of sand shall be scrapped and replenished with fresh sand at least once on 60 days. The whole bed may require replacement once in 2 years or so,
(i) The characteristics of iron flocs are different from those of surface (river) water flocs. Due to the aeration process and contact of water with air, there may be incrustation of filter bed by residual oxidized deposits. To avoid this, common salt may be mixed with standing water and after 1-2 hours, the filter may be backwashed for better results and longevity of sand bed.
1.3 Package Type Iron Removal Plant (IRP)
The process incorporates the following steps:
(a) Dosing of sodium aluminates solution to the raw water pumping line, to raise pH up to the optimum level and to ensure subsequent coagulation, as it is an alkaline salt,
(b) Injection of compressed air for oxidation of dissolved iron,
(c) Thorough mixing of raw water, sodium aluminates and compressed air for proper dispersion in a mixing chamber of mild steel (MS) welded cylindrical shell equipped with one MS perforated plate fitted inside through which the mixture flows upward,
(d) Passing the mixture through an oxidation chamber of MS shell, in which a catalytically media of MnO2 (Manganese dioxide) is sandwiched between two MS perforated circular plates. (Through which the mixture flows),
(e) Passing the above mixture in to a MS welded cylindrical shell type of filter in which dual media comprising of Anthracite Coal or high graded bituminous coal, 3-6 mm size, is placed at the top and finer sand of 0.5-1.00 mm size with 98% silica content is placed at the bottom, over a gravel supported bed. At the bottom is the under drainage system. Backwashing is done by air agitation followed by backwash with water,
(f) Disinfection.
Operation and Maintenance
(a) Sodium aluminate should be so mixed as to raise the pH up to 8.5-9.5,
(b) The quantity of compressed air should be so regulated as to achieve the optimum oxygen level,
(c) The MnO2 (Manganese dioxide) may need replacement every 6-9 months,
(d) The inside of both the mixing chamber and oxidizing chamber should be coated with epoxy resin to avoid corrosion and incursion,
(e) The filtration rate should be controlled within a range of 100-125 lpm /m2,
(f) The inlet pipe at the top should be fitted with a cylindrical strainer to obviate the possibility of loss of anthracite coal during washing,
(g) After backwashing, rinsing of filtering media for at least 5 minutes has to be done to resettle the filtering media before normal functioning.
Where the iron content is very high the whole media like MnO2 (Manganese dioxide), anthracite coal, sand, gravel, strainers etc. require replacement and replenishment at least once a year for effective functioning and performance. The interior epoxy painting should also be done simultaneously.
Resources for O and M of Iron Removal Plant
(a) Unskilled labour required for re-sanding. Semi-skilled labour (caretakers) is required for plant operation. Skilled labour (supervising manger) is required for supervision,
(b) Materials and equipment include sand, basic tools, valve replacement and spares, flow indicator, turbidity apparatus, bacteriological testing equipment,
(c) Finances would typically be from the water organization revenue,
(d) The most widely used IRP in the rural area for removing excess iron from drinking water source is based on oxidation, sedimentation and filtration,
(e) Specific Treatment Technologies.
1.4 Brackishness Removal Plant
Membrane based desalination plants are mostly known as Reverse Osmosis (RO) plants. The RO design plant technology is dependent on parameters the manufacturer wants to address in a given area.
Based on the above process each of the manufacturers has designed the treatment units with variable components and design parameters. It is important that O&M manual is obtained from the manufacturer and a guide booklet for field level operators prepared with simple language for their easy understanding. In all such treatment plants the telephone number of the operator should be painted on the building/machinery for contacting them during breakdowns.
Common operational problems of reverse osmosis include fouling of the membrane if they are not sufficiently protected by the unit operations that are located upstream. It is not uncommon to have either ultrafiltration or microfiltration units upstream of reverse osmosis.
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