2.2 Solids Filtration

Effective solids filtration is a key component to a well-functioning system and potentially the most important aspect as it influences the efficiency of all other processes. Solids are mostly produced from uneaten feed, fish waste, and bacteria biofilms (classified as suspended solids) (Timmons and Ebeling 2013). If waste is not removed, it can settle on plant roots (preventing uptake of nutrients), collect in areas of low water flow (resulting in poor water quality), cause the build-up of noxious gas, and clog pipes (preventing sufficient water flow) (Somerville et al. 2014).

The solids filtration utilized depends on the quality and quantity of feed entering the system, with all designs coming directly from RAS technology. The two main categories of solids filtration are sedimentation and mechanical filtration (Lennard 2012).

Sedimentation: Sedimentation refers to solids settling from the water column via gravity, which occurs in the clarifier. Clarifier, or (solids removal) designs include baffles, radial flow filters, and swirl separators (Figure 3a, b, c). Radial flow separators are most commonly used and have been shown to be more effective at removing settleable solids than a swirl filter in RAS (Davidson and Summerfelt 2005). Baffle and swirl clarifiers are similar in solids removal efficiency (Danaher et al. 2013). Recommendations for construction material follow that of fish tanks mentioned above.

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).

Mechanical filters have a high price tag, often making them prohibitive for small-scale practitioners. In addition, they require more advanced knowledge to operate and are difficult to obtain in developing countries. This type of filtration would be appropriate for a large, decoupled aquaponic system or those that focus the majority of their operation on fish production.

Source: Janelle Hager, Leigh Ann Bright, Josh Dusci, James Tidwell. 2021. Kentucky State University. Aquaponics Production Manual: A Practical Handbook for Growers.