Sedimentation
Tech Talk 48

1. Do whatever is possible to allow fish feces to drop intact into the waste collection area or self-cleaning bottom with minimal breakage. Minimize the use of pumps, aerators and air diffusers wherever feces is present.
2. Do not pump the waste prior to separation. Design for gravity flow (or siphon) into a sedimentation tank or basin. Splashing and turbulence can attach air bubbles and break apart solids. Feces and food particles smaller than 40 microns may not settle without chemical flocculent.
3. Always locate the biofilter after the solids removal system. Solids provide carbon for heterotrophic bacteria, which can foul a biofilter and reduce its performance.
4. Clean both the settling area and filters at least once a day, even if they contain little waste.
5. If further filtration is required after sedimentation, pump the water to an affinity bead clarifier, particulate filter, or foam fractionator.
   
   

Here is a good sedimentation basin design: Wide inlet (to reduce velocity), a surface area of .7 to 1.4 ft²  of basin per gpm flow (for feces with a specific gravity of 1.01 or greater), wide outlet weir (never a stand pipe), no baffles (which increase velocities), and a simple waste drain. A depth of just a few inches is enough for most designs.