Chapter Seven: Water Treatment

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)