Mechanical Filtration

Mechanical filtration is the physical removal of suspended and settleable solids from the culture water by straining, settling or hydrocyclonic separation. It is the first engineered treatment stage after the culture tank, and its output is two streams: clarified water to the rest of the loop, and a concentrated sludge stream that leaves the system.

Organic solids do not stay solid. Suspended in warm, aerated, biologically active water, they hydrolyse and are consumed by heterotrophic bacteria, which converts particulate waste into dissolved organic carbon, ammonia and an oxygen demand distributed throughout the loop.

• Every particle removed early is a load the biofilter never has to carry.
• Heterotrophic bacteria feeding on dissolved organics compete with nitrifiers for space and oxygen on the biofilter media.
• Suspended solids scatter ultraviolet light, so disinfection performance depends directly on clarification upstream.
• Fine solids irritate gill tissue, which affects fish health independently of any chemical parameter.
• Solids left in the loop consume oxygen continuously, and that consumption is paid for at the oxygenation stage.

Three mechanisms are in common use, and most farms use more than one. Settling uses gravity and requires quiet water and time. Hydrocyclonic separation uses induced rotation to throw dense particles outward and downward, in a small footprint with no moving parts. Screening physically intercepts anything larger than the aperture, which is the only method that captures non-settleable particles.

• Capture early and gently. Every pump, every sharp bend and every aeration stage upstream of the filter shears particles into fractions that will not be caught.
• Mesh aperture is a choice, not a maximum. A finer screen captures more but blinds faster, uses more backwash water and imposes more head loss.
• Backwash water is a real loss. It leaves the loop carrying heat and treated water, and it must be counted in the water and energy balance.
• Separation performance is a function of retention and quiescence in settling devices, and of rotation and residence in hydrocyclonic ones — both are degraded by excess flow.
• The filter is a hydraulic component as well as a treatment one. Its head loss varies continuously with fouling and must be accommodated in the pump selection.

• Place the filter as close to the culture tank as the layout allows, with the minimum of pumping and turbulence between them.
• Size for peak solids production, which is peak feeding, and confirm behaviour at low load where flow may be too high for good separation.
• Provide isolation and bypass so the filter can be serviced without stopping the loop, with a clear procedure for what happens to solids during the bypass.
• Specify backwash supply and drainage properly. An undersized backwash supply produces a filter that cannot clean itself at peak load.
• Choose materials and mesh for the water chemistry, including any ozone or disinfection residual that may reach them.
• Allow physical access for mesh panel replacement, which is a routine task.

• Trend backwash frequency. It is the most sensitive available indicator of the system's solids load, and it responds to feeding changes before water chemistry does.
• Inspect the backwash discharge visually every day; a clear discharge on a heavily fed system means solids are going somewhere else.
• Watch the level differential across the screen. A rising differential at constant load indicates blinding, fouling or a failing spray bar.
• Remove sludge from site on a schedule, not when storage is full.
• Record filter condition alongside water quality, so a biofilter excursion can be correlated with the solids load that preceded it.

• Clean or replace spray nozzles routinely; blocked nozzles are the most common cause of a filter that appears overloaded.
• Inspect mesh panels and seals for damage and bypass at every opportunity.
• Chemically clean the screen when mechanical backwashing no longer restores the differential, following the manufacturer's method and isolating the loop from the cleaning agent.
• Service drive motors, bearings and level switches on schedule.
• Clean and inspect separator underflow valves, which block silently.

• Pumping raw tank water through a centrifugal pump before filtration, shearing the solids the filter was bought to catch.
• Selecting the finest available mesh without accounting for blinding, backwash water use and head loss.
• Storing sludge on site indefinitely and allowing its leachate back into the loop.
• Treating rising backwash frequency as a filter fault rather than as information about the solids load.
• Providing no bypass, so the filter is serviced only when the loop can be stopped.
• Judging performance from water clarity alone rather than from what leaves in the backwash.

Fine and colloidal solids are the fraction mechanical filtration does not reach. Protein skimming, or foam fractionation, removes surface-active dissolved and colloidal material by adsorbing it onto fine bubbles and stripping the resulting foam. It is most effective in saline water, where bubble behaviour supports stable foam, and its role is complementary to screening rather than a substitute for it.

Sludge itself is increasingly treated as a process stream rather than a waste. Thickening, dewatering and anaerobic treatment change both the disposal cost and the nutrient recovery available, and they belong in the system design rather than in a later contract.

Mechanical filtration exists to remove waste before biology has to. Capture early, capture gently, get the concentrate off site, and every downstream process — biofiltration, disinfection, oxygenation — works closer to its design. Neglect it and each of those processes quietly inherits the difference.