Difference between revisions of "Upflow roughing filter"

Revision as of 00:28, 19 September 2012

Upflow roughing filter.
Photo: WHO.

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.

Suitable conditions

Use: As a pre-treatment stage prior to slow sand filtering or other purification processes.

Filter Classification. Chart: SANDEC.

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.

Caretaker training

Comprehensive training of local staff is essential since technical installations must be run by operators. While most technical problems can be eliminated or reduced by appropriate design and construction, human aspects which might affect treatment plant operation are more difficult to control. Careful, formal training of caretakers and extensive support, guidance, and supervision in the first years of operation are necessary.

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.

Field experiences

African Killer Bees Supported Sustainability

The rapid sand filters of Salaga, a district centre located in West Africa, were never filled with filter media and the population of the town was therefore exposed to unsafe water. The old treatment plant had to be extended and replaced by an appropriate treatment scheme. Roughing and slow sand filters were thus tested in a pilot plant shaded by a large baobab tree. The location was chosen by the external experts suffering from the Ghanaian sun. The pilot plant was constructed by the local water authority, and the expatriates supervised the start of the field tests. However, African killer bees also chose the same large baobab tree as their habitat and were very much attracted by the white skin of the foreigners. Under remote supervision by the expatriates, the field tests were continued by the local staff which gained experience and confidence in a treatment scheme they wit/have to run in future at full scare and without foreign assistance. Thus, the African killer bees somehow contributed to developing local sustainability.

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.
• Surface water treatment by roughing filters - A design, Construction and Operation manual (1996). SANDEC.