WATER TREATMENT: SLOW SAND FILTERS
Slow sand filters operate at slow flow rates, 0.1 - 0.3 metres per hour. The top layers of the sand become biologically active by the establishment of a microbial community on the top layer of the sand substrate. These microbes usually come from the source water and establish a community within a matter of a few days. The fine sand and slow filtration rate facilitate the establishment of this microbial community. The majority of the community are predatory bacteria who feed on water-borne microbes passing through the filter.
The microbial community forms a layer called the schumtzdecke and can develop up to 2cm thick before the filter requires cleaning. Once the schumtzdecke becomes too thick and the rate of filtration declines further it is scraped off, a process done every couple of months or so depending on the source water. Once this has been carried out, the slow sand filter will not be fully functional for another 3 to 4 days until a new schumtzdecke has developed, although this procedure can be speeded up by seeding the filter with bacteria from the removed schumtzdecke. Slow sand filtration is extremely good at removing microbial contamination and will usually have no indicator bacteria present at the outlet. Slow sand filters are also effective in removing protozoa and viruses. Slow sand filters require low influent turbidity, below 20TU and preferably below 10TU. This means that efficient pretreatment is required to ensure that the filters do not become overloaded. Slow sand filters can cope with shock turbidities of up to 50TU, but only for very short periods of time before they block. The sand used in slow sand filters is fine, thus high turbidities cause the bed to block rapidly and necessitates more frequent cleaning and therefore greater time out of action. Nevertheless, slow sand filters are still used in London and were relatively common in Western Europe until comparatively recently and are still common elsewhere in the world. The move away from slow sand filtration has largely been a function of rising land prices and labour costs which increased the cost of slow sand filter produced water, where this is not the case,slow sand filters still represent a cost-effective method of water treatment.