Aquaculture Filtration: Mechanical, Biological and Disinfection Explained

Filtration is what makes intensive fish farming possible: it removes what the fish produce so the water can carry them again. In a recirculating system it is the largest single block of equipment; in flow-through and pond systems it is smaller but still decides discharge compliance. This guide explains each stage, what sizes it, and how to read a filtration quote.

1. Everything is sized from feed, not from water volume

Feed is the input that becomes solids, ammonia and oxygen demand. Kilograms of feed per day at peak — not tank volume, not farm hectares — sets the size of every filtration stage. A design presented in tank cubic metres with no feed load stated cannot be checked. Establish peak feed with the biomass calculator and the fish growth calculator, then size stages with the filtration calculator.

2. Mechanical filtration: get solids out fast

Faeces and uneaten feed must leave the loop before they break apart, because dissolved organics load the biofilter and consume oxygen. Rotating drum filters with 40-90 micron screens are the commercial standard; swirl separators and settlement basins handle coarser duty. Sizing depends on flow, solids load and screen mesh, and backwash water volume is a real operating cost that belongs in the quote.

3. Biological filtration: ammonia to nitrate

Bacteria on media convert toxic ammonia to nitrite and then to comparatively harmless nitrate. Moving-bed reactors and fixed-film systems are sized on the total ammonia nitrogen produced per day — derived directly from feed load and protein content — and on the removal rate the media can sustain at the design temperature. Cold water means slower bacteria and more media. A biofilter quoted by volume without a stated TAN removal capacity at your temperature is not comparable.

4. Degassing: carbon dioxide is the hidden constraint

High-reuse systems accumulate dissolved carbon dioxide, which suppresses growth and appetite long before it kills. Stripping columns or cascade degassers with forced ventilation solve it, and they need air flow, not just structure. This is the most commonly undersized stage in cheap RAS packages.

5. Oxygenation and the alkalinity link

Oxygen is added back after treatment, typically as pure oxygen through cones or low-head oxygenators. Nitrification consumes alkalinity, so buffer dosing is part of the same water chemistry system. Both belong in the treatment scope, not in a separate afterthought. Size demand with the oxygen demand calculator and read dissolved oxygen management.

6. Disinfection: UV and ozone

UV sterilisers reduce pathogen load in the recirculating loop and on incoming water; dose is expressed in mJ/cm² at end-of-lamp-life and at the design transmittance of your actual water — a UV quoted in watts alone tells you nothing. Ozone additionally oxidises fine organics and improves clarity, but needs contact time, off-gas destruction and careful residual control.

7. Denitrification, discharge and reuse rate

Nitrate accumulates in high-reuse systems and is normally managed by water exchange, or by a denitrification reactor where make-up water is scarce or discharge is regulated. Reuse rate drives all of it — model it with the RAS water turnover calculator. Sludge dewatering and effluent treatment are frequently left out of quotes and later become permit conditions.

8. Energy: the lifetime cost of a filtration choice

Pumping through the treatment train, blowers for the biofilter and degasser, UV lamps and oxygen generation run continuously. Two filtration designs with the same purchase price can differ substantially in kWh per kilogram of fish. Price it against the local tariff with the energy cost calculator and see energy efficiency in RAS and pond systems.

9. What a comparable filtration quote must state

Design flow and total head; peak feed load and TAN load the system is designed for; drum filter screen size, throughput and backwash volume; biofilter media type, volume, specific surface area and guaranteed removal at design temperature; degasser air-to-water ratio; oxygen transfer efficiency; UV dose at end of lamp life; installed kW per stage; instrumentation included; and the interface boundary with civil works. Anything missing is a change order waiting to happen — see RAS cost overruns and scope gaps.

10. What buyers actually ask us

The enquiries we receive are rarely phrased in engineering terms. "Why is my ammonia high?", "how much filtration do I need for 100 tonnes?", "is a drum filter necessary or can I use settlement?", "how much does a RAS filtration system cost?", "can I buy the biofilter separately and save money?" and "which brand of drum filter is best?" are the real questions. The answers all trace back to peak daily feed load and to whether the quote states removal capacity at your design temperature — which is why brand comparison is the last step, not the first.

11. Keyword cluster: the searches this guide answers

This page covers aquaculture filtration systems, RAS filtration, fish farm water treatment, drum filter for fish farm, biofilter sizing aquaculture, moving bed bioreactor aquaculture, UV steriliser for fish farm, ozone in aquaculture, CO2 degasser RAS, aquaculture water recirculation equipment, and fish farm effluent treatment. Each of those searches is a stage in the same treatment train, and the train is sized as one system — buying stages independently is the most common cause of a RAS that never reaches its rated biomass.

12. From specification to suppliers

Once the treatment train is written down in these terms, offers can be normalised stage by stage instead of compared as bottom-line prices. FishMatch Group reviews the specification manually before approaching matched filtration manufacturers and integrators; supplier identities stay confidential until introductions are made. Start with a human-reviewed RFQ or read water quality management for commercial aquaculture. Calculator outputs are indicative planning figures, not engineering design.