Fish pond biofilters avoid using sand filters
filters, sand, water, flow, pressure, recirculation systems, particles, backflushing, clean, flow rate, flushing, cost, common, applications, design.
Summary:
Sand filters are used for the mechanical filtration of solids, not to be confused with fluidised sand filters, which are used for biological filtration. They are bad for fish ponds.
Although the size of particle that the sand filters remove is dependant on factors such as the size of the sand particles, the depth of the bed of sand and the flow rate through the bed, sand filters are usually regarded to filter water to a nominal 10 microns.
The water flow-rate to cross sectional area ratio of such filters is very low, and the filters tend to rapidly block in the first few centimetres, with the rest of the filter staying clean.
Such filters are only of use in applications where the use of pressure filters is impossible or the water is generally very clean and there are only a few particles that need removing.
Very large filters of this design are very difficult to clean effectively, usually resulting in the bed being periodically dug out and replaced by fresh sand.
Pressurised sand filters are very expensive to use for high flows due to the cost of pumping the water through them.
They are however used extensively in hatcheries and also some recirculation systems, where they are either plumbed in for all the water or as a side stream, where only a percentage of the water flows through.
A way round this is to add ozone or other disinfectant chemicals to the water when back flushing (the advantage of ozone here is that any residual amounts after backflushing will quickly be neutralised by the organic compounds in the water).
Now that self cleaning mechanical screen filters are available with screens of less than 10 microns, the use of sand filters is becoming less common.
