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FIGURES 3a, b, c Clarifier designs for capturing settleable solids: a) baffles, b) radial flow filters, c) swirl separator

Proper sizing of clarifiers and appropriate water flow rate are essential for effective solids removal. If relying solely on a clarifier to remove settleable solids, a 30-minute retention time is required. This simply means that most solids that can settle via gravity will do so within 30 minutes. A water flow rate of 5 gallons per minute for small tanks and 25 gallons per minute for large tanks should be used to calculate the size of the filtration tank needed. Filtration that is under-sized (or a flow rate that is too fast) will not be adequate to remove fish solids, resulting in accumulation further down in the system. Likewise, oversizing the component is not ideal as it increases the upfront cost, requires a larger footprint in the facility, and results in a greater amount of water use through inefficient discharge.

Clarifiers will only remove the large solid particles in the water, leaving solids that are too small to settle out of the water (Summerfelt et al. 2001). These suspended solids must be removed. A practice made popular by the University of the Virgin Islands is directing water from the clarifier through tanks filled with orchard netting (Figure 4). Netting material traps fine solids, allowing clean water to be skimmed from the surface. Other options for removing suspended solids are fine mesh bags, women's stockings, filter pads, and others. These items may quickly become clogged if settleable solids are not effectively removed in the clarifiers.

Mechanical Separation: Mechanical separation is the active removal of solids via a screen or media (Lennard 2012). These filters are extremely efficient, removing solids larger than 50 microns, resulting in less time spent on cleaning and maintenance due to their convenient automatic backwash feature. Examples of these filters include drum filter (Figure 5a) and a pressurized bead filter (Figure 5b).