RAS Solids Removal: Settlers, Screens, Bead Filters and Foam Fractionation
Solids classes in recirculating aquaculture, how settlers, swirl separators, radial-flow settlers, microscreens, bead filters and foam fractionators work, and why clarification precedes biofiltration.
Clarification is one of the five core processes of a recirculating aquaculture system (RAS). According to SRAC Publication 453, about half of the feed consumed by fish is excreted as solids, and these solids quickly break down into finer particles. SRAC 451 explains that, once decomposed by bacteria, faecal solids and uneaten feed consume dissolved oxygen and release ammonia, so they should leave the system as quickly as possible.
Classes of solids
- Settleable solids — particles that generally settle within one hour in still water (SRAC 451); SRAC 453 places them above about 100 µm. They are the easiest class to remove.
- Suspended solids — roughly 1–100 µm (SRAC 453). They do not settle in the culture tank or in conventional basins, can limit how much fish the system holds, and irritate gills (SRAC 451).
- Fine and colloidal solids — colloids are 0.1–1 µm (SRAC 453). SRAC 451 reports that fine suspended solids below 30 µm can make up more than half of total suspended solids and add to oxygen demand.
- Dissolved solids — dissolved organics such as protein raise oxygen demand and become a problem in systems with very little water exchange (SRAC 451).
The organic fraction of suspended solids, measured as volatile suspended solids (VSS), encourages bacterial growth linked to water quality and odour problems (SRAC 453).
Why solids removal comes before the biofilter
SRAC 453 explains that a clarifier is traditionally placed upstream of the biofilter to strip organically rich VSS, which lowers the organic load and lets a nitrifier-rich biofilm develop. The FAO guide lists the benefits of screening tank effluent: lower organic load on the biofilter, clearer water, better nitrification because the biofilter does not clog, and more stable biofiltration overall. FAO also advises placing pumps after mechanical filtration so that solids leaving the tanks are not broken up into finer, harder-to-catch particles.
Gravity settling
Settling basins use the density difference between particles and water. SRAC 453 recommends a minimum water depth of about 1.2 m, a hydraulic retention time of 15–30 minutes and, for rectangular basins, a length-to-width ratio of 4:1 to 8:1; site-specific settling times can be estimated with Imhoff cone tests. Basins are sized by overflow rate (surface area relative to flow), so high recirculation rates demand large, heated floor areas, and they remove fine solids poorly. For these reasons they are better suited to lightly loaded systems and to treating facility discharge than to the main recirculating flow. SRAC 451 notes that inclined tube settlers can improve sedimentation by reducing turbulence and spreading flow evenly.
Swirl separators and radial-flow settlers
In dual-drain culture tanks, settleable solids are concentrated into a small bottom-drain flow and sent to a compact settling unit. In a swirl separator, water enters a conical vessel tangentially and spins around the centre; capture is still mainly driven by gravity, and faeces and feed fines settle slowly because their specific gravity is only slightly above that of water. A radial-flow settler feeds water into a central turbulence-damping cylinder, from which it flows outward to perimeter overflow launders at steadily falling velocity.
Davidson and Summerfelt (Aquacultural Engineering, 2005) compared both devices on a 150 m³ Cornell-type dual-drain tank where 7–8% of the flow left through the bottom drain, at a surface-loading rate of 0.0031 m³/s per m². Mean TSS removal efficiency was 37.1% for the swirl separator and 77.9% for the radial-flow settler, and the settler's performance was less variable. The swirl separator accounted for about 23% of the solids mass removed daily, the radial-flow settler about 48%, and a microscreen drum filter on the full recirculating flow removed about 40–45% in either configuration.
Screen filters (microscreens)
FAO describes microscreen filtration as the practical standard for tank effluent, typically with filter cloth of 40–100 µm; SRAC 453 gives 20–60 µm as typical, enough to catch all settleable solids and larger suspended solids. In a rotating drum microscreen, water enters the drum and passes through the screen panels, driven by the water-level difference inside and outside the drum. Captured solids are lifted to a backwash zone as the drum rotates, rinse nozzles spray them off the panels from the outside into a sludge tray, and the sludge flows out by gravity for external treatment. Drum microscreens need little floor space and operate at very low head, but they are limited for fine particles and can produce considerable backwash water; dual-drain tanks with swirl separators and foam fractionators are used to lengthen wash intervals (SRAC 453).
Granular media and floating bead filters
SRAC 451 lists granular media (sand or pelleted plastic) alongside screens as the main mechanical options for suspended solids. In a floating bead filter, water passes through a 30–90 cm bed of plastic beads; when the bed fills it is backwashed mechanically (propeller) or pneumatically (air), the solids are allowed to settle and concentrated sludge is drawn off. SRAC 453 reports single-pass capture down to about 30 µm and, over multiple passes, removal of essentially all suspended particles. Bead filters also support nitrification and are widely used where water is reused for 30–100 days.
Foam fractionation
Foam fractionation (protein skimming) injects air bubbles at the bottom of a closed water column; surface-active compounds and fine particles attach to the bubbles and form a foam that is channelled to waste (SRAC 451). It removes fine solids and some dissolved organics that sedimentation and screening cannot. SRAC 453 notes that it works reliably in saltwater, while in freshwater performance is often erratic because surfactants are scarce and fish oil in feeds suppresses foaming. The FAO guide adds that ozone causes microscopic particles to flocculate into larger ones that filters can then capture.