13.4 Drainage system

Roughing filters can be cleaned either hydraulically or manually. Drainage systems with a high hydraulic capacity and capable of abstracting the wash water evenly from the filter bed are necessary for hydraulic filter cleaning. Fig. 47 contains different drainage layouts. The installation of a false filter bottom is the best option for vertical-flow roughing filters. About 10-cm high concrete blocks support the perforated filter bottom made of roughly 5-cm thick concrete slabs and perforation holes of about 6 - 8 mm diameter. These slabs are usually installed with open joints of about 4 - 8 mm clear width. Perforated drainage pipes or perforated culverts have to be used in horizontal-flow roughing filters and could be a possible alternative to a false filter bottom in vertical-flow roughing filters. However, a false filter bottom cannot be used in horizontal-flow roughing filters as it would lead to water short circuits. Therefore, perforated drainage pipes and culverts will also have to be installed every 1 to 2 m perpendicular to normal flow direction. Since intake and dynamic filters are surface filters, the sludge which mainly accumulates on top of the filter bed is cleaned manually. Therefore, these filters do not require drainage installations with a high hydraulic capacity.

Pipes and shutoff devices are required for hydraulic filter cleaning and for complete dewatering of the filter box. Large pipe diameters of 150 to 250 mm are necessary for efficient hydraulic cleaning. The hydraulic capacity of these installations should allow an initial filter drainage velocity of 45 - 90 m/in. The outlet of drainage pipes should be located at the lowest possible level in order to make optimal use of the available hydraulic head. For cost reasons, these large diameter drainage pipes should be as short as possible and firmly fixed to the structures in order to withstand the considerable dynamic pressures generated by the flushing cycles. A manhole, as shown in Fig. 48, can be used as interconnection between filter bottom and drainage pipe to alternatively reduce the length of the hydraulic drainage pipes. In contrast to these large pipes, small tubes of 1 - 2 inches in diameter sealed by nipples will adequately dewater the different compartments (inlet, filter and outlet boxes) of a roughing filter. Small structures, however, can also be dewatered with the help of buckets or a tube used as a siphon.

Fig. 47 Layout of Different Drainage Systems for Roughing Filters

Fast opening devices are required to initiate a fast hydraulic cleaning cycle in order not to lose too much washwater during cleaning. These devices should be simple in design, sturdy and easy to operate. In the long run, they must be watertight and equipped with a shut-off device to interrupt the drainage process. Use of butterfly valves is the best but most expensive option. To reduce construction costs, different local designs of fast opening devices have been developed as illustrated in Fig. 34 on page IX-9. A good example of an appropriate technology is the modified milk can cover developed by CINARA in Colombia. ZHAS in China successfully uses a self-designed plug valve installed in a steel box, and Helvetas in Cameroon applies carefully shaped and firmly installed plugs held by a removable bar.

Fig. 48 Levelling of the Drainage System

Finally, structures for fast drainage and safe washwater disposal must be provided. The washwater is generally discharged into an open canal used to convey the hydraulic flush to a nearby surface water or to a small pond used for intermediate storage. Construction of a small lagoon is recommended to recover the solid matter washed out of the filter for use in agriculture. Direct discharge of the washwater in stagnant surface water may gradually silt up the reservoir and adversely affect its water quality.