Sand Filters - Tech Talk 45
Sand filters may not be practical when filtering water containing organic material (fish waste, both dissolved and solid, is organic) because bacteria grow on the sand grains, causing them to stick together. The bacteria will adhere so strongly that the sand bed will not expand when put in a backflush mode. The backflush water will "channel" in one place only, leaving a hard clump or "cake" composed of a mixture of sand, waste material, and bacteria on the top of the sand bed. This can take as little as one week to form in a new sand filter and must then either be broken up or the sand must be replaced.
If backflushing can be done as much as three times per day (either manually or automatically) to avoid the caking problem, then a sand filter may indeed be a good choice for your fishkeeping situation. The bacteria floc that forms on the sand can provide additional biofiltration.
Note: Discharge a minimum of two tank volumes of water with each backflush cycle to achieve 90% waste removal.
This chart shows head loss (a measure of flow resistance) for clean AES filters as a function of flow rate through them. Refer to this chart when sizing pumps as follows: Using the design flow rate and filter, find your head loss on the vertical axis. Realize that, as your filter collects material, the head loss will increase and your flow rate will diminish. Add the filter head loss to the lift and other flow resistances to compute your Total Dynamic Head (TDH). Select a pump that will provide the design flow at the TDH.
Do not exceed manufacturer's rated maximum filter pressure.