Engineers are stacking gravity, microscreens, and foam to strip waste before it chokes biofilters in recirculating aquaculture systems. The configuration you pick determines how much TSS, COD, and fine organics make it downstream—and how big the biofilter, UV, and ozone units need to be.
Intensively fed RAS generate large amounts of particulate (faeces, uneaten feed) and dissolved organic waste, linked directly to feed input (sciencedirect.com). Rapid solids removal is critical: “the aim should be to remove the faeces…as quickly as possible” to avoid breakdown into finer particles that clog biofilters (fishfarmingexpert.com). In practice, engineers combine multiple stages—a coarse settler, a microscreen drum filter, and often a foam‑fractionator (protein skimmer with ozone)—each targeting different waste fractions.
Microscreen drum filters: size cutoffs and OPEX
Principle: continuous microscreen filtration via a rotating drum with filter cloth (typically 60–200 μm pore; μm is micrometre). Untreated water flows into the drum and is filtered by gravity and slight vacuum; solids accumulate on the cloth and are periodically backwashed into a sludge trap (fishfarmingexpert.com) (link.springer.com). Operators often use 60 μm cloth in freshwater RAS to retain most faecal pellets.
Coarse‑solids removal: a commercial drum filter removed ≥90% of particles >55 μm in testing (academic.oup.com). In salmonid RAS, a drum filter accounted for roughly 40–45% of the total TSS (total suspended solids) mass removal (researchgate.net).
Fine‑solids removal: efficiency drops sharply for fine particulates. The same study reported only 8–26% removal of particles <55 μm and an overall 19–44% of total TSS captured depending on configuration (academic.oup.com). Mechanical filters do not remove dissolved nutrients (ammonia, nitrate, DOC) except indirectly when bound in particulates.
Operations: drum filters require energy for pumping and backwashing; they may recirculate 0.5–1.0× system flow as backwash over 12–24 h. One report found a disc filter used 45% less backwash water and energy than a comparable drum filter for the same solids removed (fishfarmingexpert.com). Finer weaves capture more solids but foul faster and increase head loss (link.springer.com).
Design implication: specify adequate cloth area (or multiple drums) to handle expected solids loading and avoid excessive pressure. A drum filter typically reduces the solids load into the biofilter by roughly half, which can shrink biofilter size or reduce cleaning frequency; fine particulates still pass downstream.
Radial‑flow settlers: gravity workhorse
Principle: passive sedimentation in a large clarifier. Water enters slowly (often via a distribution tube) and flows radially over a large area; heavy and flocculated particulates settle into a conical hopper. A sweep arm or pump periodically removes sludge buildup. Settlers are sized by surface loading (flow per clarifier area). A radial‑flow settler corresponds to a clarifier in packaged water treatment terms.
Coarse‑ and fine‑solids removal: one radial‑vertical cell settler achieved 82% average TSS removal—reducing mean influent from ~11.4 mg/L to effluent ~2.1 mg/L (researchgate.net). In a full‑scale salmonid setup (4500 L/min flow), a radial‑flow settler removed 77.9%±1.6% of TSS, compared to 37.1%±3.3% by a simple swirl separator under the same conditions (researchgate.net). Settlers also substantially reduce particle size; mean particle diameter dropped ~33–48% across the clarifier (researchgate.net).
Hydraulic loading: cited studies used surface loadings around 0.0031 m³/s·m² (~4.6 gpm/ft²) (researchgate.net), and designers often target 0.003–0.005 m³/s·m² in RAS settlers. Higher flows or less area sharply reduce removal; inlet/outlet geometry and tank height also matter (engineers should consult empirical sizing guides or pilot‑test on their particulates).
Energy and maintenance: settlers operate by gravity, with minimal OPEX beyond the main circulation pump and periodic sludge removal—no backwash. The trade‑offs are footprint and head space: large tanks and ~3–5 m height for adequate HRT (hydraulic retention time).
Design implication: many RAS place a settler (or swirl separator) ahead of the drum filter. With ~80% of solids already removed, the drum sees much lower TSS, improving its efficiency and reducing sludge volume. If a settler alone is relied upon, the biofilter must handle unremoved fines and dissolved organics—shifting burden downstream.
Protein skimmers: fine organics and COD
Principle: foam fractionation removes organic matter and fine particulates via bubbles that adsorb hydrophobic organics and micro‑solids; a column skimmer collects foam (“foamate”) carrying concentrated waste. Skimmers excel in high‑salinity water but can be used in freshwater RAS when enhanced by ozone, salt addition or surfactants (orbit.dtu.dk).
Particle removal: skimmers are not primarily solids filters, so TSS removal is modest. In a low‑salinity marine finfish RAS trial, total solids removal ranged from ~6.5% to 38.5% depending on configuration (academic.oup.com). For comparison, the drum filter in the same trial removed 19–44% under similar conditions (academic.oup.com). In the best case (high head height, aeration), the skimmer removed ≈26.9% of solids <55 μm (academic.oup.com).
Dissolved organics: skimmers can partially reduce dissolved organic load. Ozone‑augmented skimmers show significant COD (chemical oxygen demand) reductions: 25–53% COD drop under increasing ozone dose over 8 days, with ~15% transparency improvement (sciencedirect.com). Kovács et al. observed that adding ozone in tandem with skimming significantly reduced microparticles and bacterial activity in a commercial RAS, boosting ORP (oxidation‑reduction potential) and clarity (orbit.dtu.dk). Skimmers do not remove inorganic nitrogen species—ammonia and nitrite remain in water and must be handled by biofilters or denitrification.
Energy and outputs: skimmers require electrical energy for air or venturi pumps (and for ozone generation if used). They produce a foamate effluent of very high organic concentration (often >1–2 g/L solids) but at low volume. Skimmers also provide some aeration and CO₂ stripping, but less predictably than dedicated degassing units.
Design implication: adding a protein skimmer (often with an ozone unit upstream) can substantially improve overall water quality in high‑loaded systems. A design might place the skimmer after the biofilter or as a side stream, depending on strategy (to avoid stripping needed nitrates too early). Because skimmers only remove a fraction of fine solids, they are best used as a supplement; engineers must allocate space for the skimmer column and ozone loop, and provide disposal for the foamate.
Downstream impacts: biofilters, UV, O₂/CO₂
Biofilter size and performance: high solids load leads to frequent biofilter clogging or loss of nitrification capacity. Emparanza notes accumulated solids “clog” filter media, reducing specific surface area for nitrifiers and causing bacteria to die off (researchgate.net). In contrast, systems that remove ~80% of solids with a settler (and another ~40% with a drum) see much less clogging. With lower organic debris, heterotrophic competition declines and nitrifiers can thrive; a well‑polished RAS may size its biofilter to the expected ammonia load, while a poorly filtered system must oversize the biofilter (or use staged/moving‑bed designs) to cope with extra BOD (biochemical oxygen demand).
Oxygen and CO₂ demand: unremoved organics increase respiration by heterotrophic microbes, raising O₂ demand and CO₂ production. If a protein skimmer removes a portion of amino‑nitrogen before the biofilter, that saves oxygen that would otherwise be used by nitrifiers or heterotrophs.
Disinfection and degassing: high turbidity impairs UV disinfection as bacteria hide in particulates. UV chambers achieve ~90% bacterial kill only at turbidity <10 NTU (nephelometric turbidity units) (researchgate.net). In practice, drum filters and settlers reduce turbidity to a few NTU, allowing UV doses to be effective; a dedicated ultraviolet system can then run at lower operating cost. Ozonation is less efficient when organics are high because ozone is consumed by COD, requiring higher dose or contact time (sciencedirect.com).
Sludge form and handling choices
Drum filters produce relatively dry, easily dewatered sludge (often 5–10% solids). Settlers yield dilute slurry (often <5% solids) requiring thickening. Protein skimmers produce an even more concentrated foamate (high COD, low volume). Engineers must plan the sludge‑handling train accordingly (e.g., settling tank or filter press after a drum filter; belt press or sludge dryer for settler sludge; small discharge line for skimmer foam).
Sizing cues and selection scenarios
In coldwater RAS, a radial‑flow clarifier had mean influent TSS ≈11.4 mg/L and effluent ≈2.1 mg/L (82% removal) (researchgate.net). A microscreen drum filter in that system captured ~40–45% of the TSS mass (researchgate.net). Under optimal conditions, a foam fractionator removed up to ~26.9% of the <55 μm fraction, and overall skimmer removal never exceeded ~40% in that trial (academic.oup.com).
Energy/water use: disc‑style filters can cut backwash water ~45% and energy ~45% compared to drums for the same solids load (fishfarmingexpert.com).
Ozone effect: bench‑scale RAS ozonation studies saw COD drop by 25–53% over 8 days (for low→high ozone dosing) and improved transparency by 15% (sciencedirect.com).
Design implication: engineers should size and select solids‑removal tech based on feed load and species. E.g., a large tilapia farm in Indonesia (high feed rate) might justify a large radial settler plus drum filters, whereas a smaller recirculating shrimp hatchery might use drum filters and ozone skimmers only. In all cases, the expected % solids removal informs the required biofilter nitrification capacity. As a rule of thumb, assume ~50–80% of SS can be removed by mechanical clarification (settler+drum) (researchgate.net) (researchgate.net); remaining dissolved and colloidal waste must then be handled biologically or chemically.
Designers must also ensure compliance with discharge or recirculation standards (e.g. Indonesian regulations often specify maximum SS/BOD in effluent). By quantifying each stage’s removal (as above), one can balance capital vs.
