Department of water affairs and forestry technical guidelines for the development of water and sanitation infrastructure

9WATER TREATMENT WORKS

9.1General

As with dams, the design of the water treatment process is a specialist field of expertise and should be done by a suitably trained professional engineer.

Certain broad guidelines can however be given for planning purposes.

9.2Potable water quality

Potable water should comply with SANS 241.

9.3Selection of appropriate treatment process

The selection of an appropriate treatment process is essentially determined by:

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  The raw water quality (physical and chemical).
  
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  The prescribed final water quality.
  

9.4Recommended loading rates and design parameters for water treatment process units

The following loading rates and design parameters can be given as guidelines only and should be tested against actual circumstances:

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  Rapid mixing: at least 500mm head loss (mixing in G values, G = 2000^-5).
  
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  Floculation: 10 min. Retention with a total head loss of about 150mm and G = 50s^-5 (40 - 80)^-5
  
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  Horizontal flow settling tanks (in lieu of other flow types) are recommended for turbid raw waters > 200NTUs with manual sludge or hydraulic sludge withdrawal and a maximum loading rate of 1m³/m².h.
  
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  Direct gravity filtration as secondary solid/liquid phase separation step: filtration rate 5m/h with upstream flow control.
  

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  Upflow-downflow (series) filtration: only to be constructed when the raw water turbidity rarely exceeds 100NTU (say 5% of the time). Recommended filtration rates are:
  
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    Upflow: 5m/h
    
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    Downflow: 10m/h
    
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  Flotation: recirculation rate of 10% and a loading rate of 6m³/m².h.
  
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  Sludge and wash handling facilities: for design purposes assume that the sludge will thicken to a 10% concentration. The sludge dam should be large enough for a planning horizon of 6 to 8 years and space should be available for a second dam.
  
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  Production losses through water plant: allow 3% to 5%.
  
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  Chlorine contact time: 6% of flow capacity. 6% x 24 = 1,5 hours.
  
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  Clear water sump for high lift pumps: 2% of flow capacity.
  
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  Sludge disposal to sludge dams.
  
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  Design flow capacity of plant to be based on the average daily summer demand plus 5% for production losses.
  

The sludge from settling (sedimentation) tanks should at least flow to sludge lagoons from where only the supernatant flows to rivers. Reclamation can be considered where water is scarce and supernatant can be used.

9.5Automation

If a water treatment plant has more than 4 rapid gravity sand filters, the filter backwashing sequence may be automated. Other operations, for example chemical dosing and sludge withdrawal will only be automated in plants bigger than 50 000m3/d (579 litres per second).

Automation shall be motivated and not governed by hard and fast rules.

9.6Structural considerations

It is recommended that all reinforced concrete water retaining structures should be designed to a 0,2mm crack width using 30MPa concrete in accordance with BS8007.

9.7Accessibility

All units should be easily accessible, and easily removed for repairs. Walkways should be provided to give safe access to all points requiring inspection and to provide logical progression to operators doing inspections. Ease of handling chemicals should receive special attention.

9.8Specifications

The following specifications are relevant for the design, manufacture, supply, construction, and commissioning of water treatment plants.

Departmental Specification	Description
DWS 1930	Supply, installation and commissioning of water treatment plant equipment.
DWS 1940	Design, manufacture, supply, delivery, installation, and commissioning of package water treatment plant.
DWS 1950	Supply, installation, and commissioning of a reverse osmosis unit for the desalination of mineralised water.

These specifications are included in the Specifications Folder.

10PUMP STATIONS

10.1Introduction

Community water supply schemes are likely to involve the pumping of relatively small quantities of water. Pump stations are often be sited at small purification works or as booster pump stations along pipelines.

Normally the electrical power supply will be provided by Eskom or the local authority.

10.2Pump selection

The following steps should be followed to select the correct pump duty:

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  The system curve, relating to the hydraulic head lost in the system for different flows, is calculated and plotted as a graph above the required static head. Pump Station losses, including those of all valves in the pumpline, should be included in the calculation of the system curve.
  
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  The pump performance curves are then plotted on the same graph. If more than one pump is required then these are added either in series or in parallel, as required. The pump curves will intercept the system curve at the station duty point.
  
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  An ideal pump selection will result in each Pump Duty Point falling at or very near to the pump Best Efficiency Point (BEP).
  
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  When only one pump in a multi-pump arrangement is operating, the intercept with the system curve will be at a point of reduced head and increased flow with regard to the chosen Pump Duty Point. Care should be taken that the motor is not overloaded under the one pump condition, and that a margin of at least 15% in excess of what the pump will demand is ensured under the worst possible operating condition.