Aquaculture can reuse 90–99% of its water, but solids management is the swing factor. Fine-tuning filter backwash and squeezing more water from sludge routinely returns 70–85% of flows that were once discharged.
Aquaculture is water‑intensive, and the stakes are rising. Recirculating Aquaculture Systems (RAS; closed loops that recirculate and treat water) can cut freshwater needs dramatically: complex RAS designs can reduce required water to ~10% of a flow‑through system, and in practice often achieve 90–99% reuse of water (i.e., only 1–10% exchange) (www.fao.org).
In Indonesia — a leading shrimp/fish producer with >610,000 t crustaceans, ~10% of world production (www.sciencedirect.com) — better solids capture and reuse is central to sustainable expansion. Effective solids management recovers nutrients and water while cutting disposal costs (unbscholar.lib.unb.ca; www.globalseafood.org). In practice, that means smarter backwash cycles and efficient dewatering — the often‑ignored levers inside every solids line and primary separation train.
Backwash volume and recovery potential
Filter backwash — periodic reverse flushing of filters to remove trapped solids — is a quiet water sink. In municipal drinking‑water filtration, backwash typically uses 2–8% of processed water (www.mdpi.com). In RAS, capturing and reusing backwash can reclaim a similar share. Using advanced treatment, facilities have recovered ~85–96% of filter backwash water (www.watertechonline.com).
Solids‑removal filters in RAS (e.g., drum, disc, sand) need backwashing, but when that cycle runs matters. Best practice is to trigger backwash only when needed — using differential pressure (DP; the pressure drop across a filter) or turbidity thresholds — rather than fixed intervals. Seasonally adjusting sand filter backwash intensity/interval saved nearly 10% of backwash water annually in one study (www.mdpi.com). Splitting large filters into parallel zones and rotating backwash operations can also avoid halving flows and cutting peak demand.
In RAS specifically, backwash timing should match solids load: flushing too early wastes water, too late lets solids bleed through. On drum screens or automatic screens, adding DP sensors and flow meters to fine‑tune start/stop can help; delaying the trigger by 10–20% (while still meeting effluent TSS, total suspended solids, targets) can cut events by a similar fraction. Data from water treatment plants show smart scheduling (e.g., monthly tuning) can save up to 15–20% of backwash volume in certain months (www.mdpi.com). Translated to RAS, if filters normally waste 5% of flow, a 10% saving equals a 0.5% net water recovery — thousands of cubic meters annually at moderate scale. For screening duties, many farms pair drum units with an automatic screen for continuous debris removal.
Collection, clarification, and polishing trains
Rather than discharge, backwash should be collected in a sump or holding tank. Coarse solids settle or are skimmed (e.g., via swirl clarifiers or settling basins), and the supernatant is treated before reuse. Coagulation/flocculation — adding alum, ferric chloride, or polymers — binds fine particles; ferric sulfate significantly improved backwash solids settling in trials (effects vary by salinity) (www.mdpi.com; unbscholar.lib.unb.ca). In practice, a compact clarifier following a dosed sump, fed by a metered dosing pump, is a common setup.
Advanced filtration can polish the clarified stream. Ultrafiltration (UF; membrane filtration that removes colloids and fine particles) or ceramic microfiltration units can remove ≥98% of 1–5 µm particles; a Saudi Aramco pilot achieved ~85% recovery of the backwash stream with >98% removal of 5 µm solids (www.watertechonline.com). Farms often deploy skid UF as pretreatment or add a final cartridge filter in a corrosion‑resistant stainless cartridge housing. Energy costs and fouling must be managed in membrane units.
Biological treatment can help, too. In RAS, some backwash water — still rich in ammonia/nutrients — can be recirculated through the biofilter or even an attached aquaponic loop; a hybrid Chinese RAS used biofilm reactors to scrub backwash (including organic nitrogen) and achieved water reuse in connected shrimp ponds (www.researchgate.net). While biological systems take time to mature, they buffer spikes and recover nutrients; packaged biological digestion systems are often integrated upstream of polishing.
The goal is to clean backwash water enough to safely reintroduce it to fish culture. The Saudi project implies only 15% of backwash needed disposal (www.watertechonline.com). Geotextile bag demonstrations show drained water pumped to a “supernatant sump,” requiring only minor disinfection before reuse (www.globalseafood.org); many farms use low‑operating‑cost ultraviolet disinfection for that step. In short, >70–80% of the backwash effluent volume can typically be returned with modest treatment.
Mechanical dewatering and geotextile consolidation
After separation, sludge (a solids slurry) still holds large volumes of water. Efficient dewatering maximizes recovery. High‑speed centrifuges or belt presses can concentrate sludge to ~15–25% solids; in experiments, aquaculture sludge initially ~3–10% solids was concentrated by centrifugation to 19.3 ± 0.7% solids (unbscholar.lib.unb.ca). That removes roughly 80% of the sludge’s water mass in some cases; belt presses (often with polymer addition) can reach ~25% solids in practice. Each unit of sludge processed yields roughly 3–4× its solids weight as recovered water. The investment pays twice: waste weight (and fees proportional to weight) falls, and the recovered water — often needing only disinfection — returns to the system.
Geotextile dewatering bags (porous synthetic fabric tubes) are a low‑power alternative. Demonstrations show geotextile bags can raise sludge to ~30%–35% solids, with waste going from ~5–10% to ~30% in under a week (www.globalseafood.org). Mathematically, if initial sludge is 5% solids, concentrating to 30% means six times more solids per unit volume; water content drops from 95% to 70%, so ~74% of initial sludge water is removed. Filtrate is pumped to a recovery sump and reused after minor polishing. Bags scale easily but process more slowly; farms often combine mechanical units for quick reduction with bags or drying beds for final lift, supported by practical ancillary equipment.
Quantitatively, dewatering can often recover >70% of sludge water. Centrifuging sludge reduced water content from ~97% to ~81%, an ∼16% of total volume saved (i.e., 16 L per 100 L sludge) (unbscholar.lib.unb.ca). Geobagging from 5% to 30% solids removed roughly 74% of volume. If a farm generates 10 m³/day of sludge, these methods could reclaim 7–8 m³ of water for reuse. Disposal costs scale by sludge weight, so dewatering to ~20–30% solids can more than halve disposal costs per ton of solids removed (unbscholar.lib.unb.ca).
Integrated reuse impacts and savings
Stack the gains and the water math shifts fast. If backwash accounts for ~5% of flow, reusing 85% of that via treatment saves ≈4.25% of flow. Dewatering sludge might cut disposal volume/chemical oxygen demand (COD; a measure of organic load) by ~70%, with that same 70% volume returned. Together, such measures can save 5–10% or more of total water use. On a 100 m³/day RAS, recycling an extra 5% saves ~5 m³/day (≈1,800 m³/year). One water‑industry analysis estimated that reusing filter wash returns (instead of disposal) could reduce annual water fees by ~$150,000–250,000 for a midsized plant (www.mdpi.com); while far larger than a fish farm, proportionally similar gains apply as water or abstraction fees rise.
Key equipment choices matter to sustain reuse quality. Polishing backwash with UF skids or a final cartridge stage, settling with a compact clarifier, and disinfecting with UV units are common RAS configurations that align with the performance outcomes cited above.
Regulatory context and zero‑discharge drives
In Indonesia, no national aquaculture effluent standard explicitly mandates solids reuse, but environmental Baku Mutu (quality standards) limit TSS/BOD in discharge. Pressing water recovery to 85% and tightening filtration could help farms meet local limits for TSS (<50 mg/L) even while minimizing effluent volumes. Indonesian RAS initiatives (jurnal.untirta.ac.id) and shrimp projects (www.sciencedirect.com) increasingly focus on “zero discharge” and water reuse. This review shows that coupling optimized backwash with sludge dewatering could form the backbone of a water‑conservation strategy: it delivers quantifiable savings (multi‑tens to hundreds of cubic meters annually per facility) and aligns with sustainable aquaculture goals.
Key metrics and reuse ratios
- Backwash reuse: 80–96% recovery achievable (www.watertechonline.com), saving 2–8% of system water (www.mdpi.com).
- Dewatered sludge: 20–30% solids yield (unbscholar.lib.unb.ca; www.globalseafood.org), i.e., 70–80% of sludge volume reclaimed as water.
- Disposal shrinkage: from 3–10% raw solids down to ~20–30% means ~60–80% reduction in disposal mass (unbscholar.lib.unb.ca; www.globalseafood.org).
- Water‑intake reduction: integrated RAS often need only ~10% of single‑pass water (www.fao.org). Improved backwash/dewatering can push reuse ratios from, say, 90% to 95–99%.
Sources and citation notes
Data are drawn from aquaculture and water‑treatment studies and reports, including FAO guidelines and recent case studies. For example, Wolska et al. (2023) noted global backwash uses ~2–8% of treated water (www.mdpi.com); Mann (2023) found RAS sludge centrifuged to ~19.3% solids (unbscholar.lib.unb.ca); Ebeling et al. (2011) achieved ~30% solids in geobags (www.globalseafood.org); and a pilot reusing filter backwash reported ≥85% water recovery (www.watertechonline.com). These and other sources underpin the quantified outcomes above.
References: A list of all cited works (including publisher metadata) is provided below.
