Slow Sand Filter
The slow sand filter is the oldest type of large-scale filter. In the slow sand filter, water passes first through about 36 inches of sand, then through a layer of gravel, before entering the under drain. The sand removes particles from the water through adsorption and straining.
Unlike other filters, slow sand filters also remove a great deal of turbidity from water using biological action. A layer of dirt, debris, and microorganisms builds up on the top of the sand. This layer is known as schmutzdecke, which is German for “dirty skin.” The schmutzdecke breaks down organic particles in the water biologically, and is also very effective in straining out even very small inorganic particles from water.
Maintenance of a slow sand filter consists of raking the sand periodically and cleaning the filter by removing the top two inches of sand from the filter surface. After a few cleanings, new sand must be added to replace the removed sand.
Cleaning the filter removes the schmutzdecke layer, without which the filter does not produce potable water. After a cleaning the filter must be operated for two weeks, with the filtered water sent to waste, to allow the schmutzdecke layer to rebuild. As a result, a treatment plant must have two slow sand filters for continuous operation.
Advantages:
- As they require little or no mechanical power, chemicals or replaceable parts, and they require minimal operator training and only periodic maintenance, they are often an appropriate technology for poor and isolated areas.
Slow sand filters are recognized by the World Health Organization, Oxfam, United Nations and the United States Environmental Protection Agency as being superior technology for the treatment of surface water sources. According to the World Health Organization, “Under suitable circumstances, slow sand filtration may be not only the cheapest and simplest but also the most efficient method of water treatment.”
Slow sand filters are very reliable filters which do not usually require coagulation/flocculation before filtration. However, water passes through the slow sand filter very slowly, and the rate is slowed yet further by the schmutzdecke layer. As a result, large land areas must be devoted to filters when slow sand filters are part of a treatment plant. Only a few slow sand filters are operating in the United States although this type of filter is more widely used in Europe.
Number of slow sand filters operating in each state as of 1991.