On our farm we require good quality water for jobs like cleaning in the dairy. We built a water filter that would be capable of treating a large volume of dam water so that it could be used to top up our rain water tank through dry periods. The design can be seen Low tech farm dam water filter.
In a slow media filter water passes slowly down through a filter medium. In our filter we used rockwool. The rockwool acts as a trap for sediment. Over time a layer of micro-organisms, mainly bacteria, builds up on top of the media. These trap and digest organic contaminants in the water. So it is a type of biological filter. Slow media filters have a simple design and have been used in many places mainly as a cheap and easy to make filter to improve drinking water. Studies have shown that they are effective in reducing turbidity and reducing bacteria and organic matter contamination in water.
A new filter needs to have water run through it for some time to condition the filter. This allows the biofilm to develop and for the filter to become effective.
Test results:
At the time of testing the dam water entering the filter was of reasonable quality. The turbidity was slightly elevated and fresh organic matter was in the low to moderate range. Coliform bacteria and total aerobic bacteria levels were elevated.
We tested before filter and after filter samples starting from day 2 after the filter was started. The tests were for coliform bacteria, total aerobic bacteria, turbidity, humic material by UV absorption and fresh or readily degradable organic matter by permanganate oxidation. Humic materials often give water from dams or creeks pale yellow or brown colours.
On day 2 before and after coliform and total aerobic bacteria counts were high and showed very little difference.
After operating for 8 days, filtered samples showed a 68% reduction in coliform bacteria.
After 18 days there was a 96% reduction in coliforms and 50% reduction in total aerobic bacteria.
At day 20 there was a 21% reduction in turbidity, 44% reduction in fresh organic matter and 15% reduction in UV absorbance.
Coliform bacteria are a large group of bacteria that are naturally present in water and soils. The group also includes some species that can cause illness. Therefore they are often used as indicators of water quality with higher than normal levels indicating possible contamination.
Maintenance: Our filter has now run for 6 weeks without any problems. We expect that at some later time the biofilm may build up and perhaps restrict the flow of water. There is a drain plug installed just above the biofilm layer which will allow some of the biofilm to be removed.
How the idea can be extended: If more filtered water is required then another filter with its own float valve and connection to the source water could be added. Both could then feed into the one collection barrel. A slow media filter could also be used to maintain the quality of water in a tank. In this case the filter would continuously take water from the tank, treat it then put the water back into the tank. The same type of filter could be installed in a gravity fed farm water supply. If the source water can be fed in by gravity and the treated water can be run off to below the filter then no pressurised water or pumps are needed.
A slow media filter is a low cost and low tech but effective way to improve the quality of surface water such as creek and dam water on farms.