Upflow roughing filter
Roughing filters are often used to pretreat water by removing suspended solids from the water that could rapidly clog a slow sand filter. Roughing filters can also considerably reduce the number of pathogens in the water, as well as the amount of iron and manganese. There are many types of roughing filters with different flow directions (downflow, upflow and horizontal flow filters), and with different types of filter medium (e.g. sand, gravel, coconut husk fibre). Upflow roughing filters are relatively cheap and easier to clean than downflow or horizontal flow filters.
An upflow filter box can be made of bricks, concrete or ferrocement. It can have a round or rectangular shape, with vertical or partially inclined walls, and it is usually about 1.5 m deep. Water flows in through an underdrain system on the bottom, usually a perforated PVC pipe, which also permits rapid abstraction during cleaning when the flow direction is reversed (backwashing).
For backwashing, a special drainage valve is installed which can be opened quickly. The underdrains are covered with a layer of coarse gravel, on top of which lie several layers of finer gravel and coarse sand. The filter layers are covered with a 0.1 m-deep layer of boulders, to avoid exposing the outflow directly to sunlight; this helps to prevent algal growth. The outflow is stored in an outlet structure. In some cases, the outflow of one roughing filter is fed to another roughing filter with finer material for further cleaning.
Filtration rate: Approximately 0.6 m/h.
Performance: If raw water with a turbidity below 50 NTU is used as the source for a roughing sand filter, the outflow has a turbidity below 12 NTU. Approximately 84–98% of suspended solids are removed. Better results are obtained with two or three filters in series.
Contents
Suitable conditions
Use: As a pre-treatment stage prior to slow sand filtering or other purification processes.
Construction, operations and maintenance
The filters should preferably be operated on a continuous basis. Operation consists of regulating the water flow and checking the turbidity of the effluent. Flow, turbidity and maintenance data are written in a logbook. If the turbidity gets too high, the filter may become clogged. In such cases, the filter should be cleaned about once a month, while leading the effluent to outlet. The inlet and outlet boxes are then cleaned, and backwashing and refilling are done twice. The monthly cleaning is performed by the caretaker and takes about half a day. No special assistance from users is required to clean the filters. Every two months, all valves should be completely opened and closed, to keep them from becoming stuck.
After a year or more (depending on the turbidity of the raw water), hydraulic cleaning alone is no longer adequate, and the different filter layers have to be removed and cleaned, which requires several people. The filter should be cleaned before the turbidity of the raw water reaches a maximum (e.g. before the rainy season starts). Occasionally, the valves need to be repaired or replaced, and if a steel weir is used this may need to be painted or replaced. New caretakers can be trained by experienced technicians.
Potential problems
- High loads of organic and other suspended material in the raw water clog the filter and reduce the hydraulic cleaning capacity.
- Roughing filters only remove some of the solids and pathogens in the water, and additional treatment is needed.
Costs
Initial cost: Reported construction costs are US$ 20–40 per m3 of water per day, for a structure designed to be in operation for 24 hours a day.
Acknowledgements
- Brikke, François, and Bredero, Maarten. Linking technology choice with operation and maintenance in the context of community water supply and sanitation: A reference document for planners and project staff. World Health Organization and IRC Water and Sanitation Centre. Geneva, Switzerland 2003.
