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close this bookSurface Water Treatment by Roughing Filters - A Design, Construction and Operation Manual (SKAT, 1996)
close this folderPart 2: Design, construction and operation of roughing filters
close this folder16. Design examples
View the document(introduction...)
View the document16.1. Treatment of an upland river
View the document16.2 Treatment of a lowland stream
View the document16.3 Treatment of reservoir water
View the document16.4 Rehabilitation of a slow sand filter plant
View the document16.5 Standard designs for compact water treatment plants

16.5 Standard designs for compact water treatment plants

Water treatment plant projects can be implemented on a standard design basis to reduce design inputs as well as construction time and costs. This approach is especially appropriate in rural water supply programmes for the construction of several treatment plants having to treat raw water of similar quality. In such situations, routine construction procedures can be developed to reduce construction periods. Furthermore, compact designs and careful supervision of the construction will lower investment costs and enable the use of economic construction procedures such as the ferrocement technique. Standard design modules often cover a range of different design capacities. They may be implemented successively in different construction phases to meet the actual water demand of the community. Every water treatment project will nevertheless have to be carefully adapted to the local situation and, therefore, calls for a critical evaluation of the prevailing conditions.

A standard design example is illustrated in Annex 6/6 . Upflow roughing filter(s), slow sand filter(s) and a reservoir are integrated in one structure. This example uses a circular design, often applied in reservoir construction, and takes advantage of locally available construction techniques. A circular ring, placed around the reservoir located in the centre, provides space for two treatment lines comprising upflow roughing filters and slow sand filters. Design capacity of the illustrated example amounts to 30 m³/d. Hence, two such standard design units are required to cover the water demand of the village in question. These two units may be located in different places to treat different raw water sources, and may improve the reliability of a water supply system. Depending on the raw water quality, the illustrated structure could be used to host alternative treatment systems such as upflow roughing filters installed in the outer ring and two slow sand filters placed in the centre tank. Such a layout would require a separate reservoir.

The construction of small standard design units also enables a phased increase of the treatment plant design capacity, satisfying the future water demand development. A further advantage of a phased implementation is the integration of the operational experience in the extension design. Filters can frequently be operated at higher filtration rates without affecting the treated water quality or without substantially reducing filter running periods. The filtration rate of slow sand filters may for instance be increased from 0.1 to 0.2 m/h (recommended range in the literature). With an efficient pretreatment and use of sand coarser than 0.15 - 0.35 mm (recommended range in the literature for the specific sand size d10%), it may be increased to 0.3 and exceptionally to 0.4 m/h.

The filter units of a compact water treatment plant are dimensioned as follows:

· treatment plant capacity

30

m³/d

30 . 24=

1.25

m³/h

· upflow roughing filters in layers



filtration rate

0.3

m/h

total filter area required

4.2


· slow sand filters



filtration rate

0.125

m/h

total filter area required

10