Solids and TSS

Total suspended solids, TSS, is the mass of particulate material held in a volume of water, measured by filtration and drying. In a recirculating system that material is predominantly fish faeces, uneaten feed, sloughed biofilm and bacterial floc.

TSS is a single number describing a population of particles with very different behaviours. Settleable solids fall out of quiet water and can be removed by gravity or rotation. Suspended solids stay in the water column and require screening. Fine and colloidal solids pass most conventional equipment and are the fraction that accumulates.

• Solids left in suspension mineralise, adding ammonia and dissolved organic carbon to the loop.
• Organic loading on the biofilter favours heterotrophic bacteria that displace nitrifiers.
• Suspended solids scatter and absorb ultraviolet light, reducing the delivered disinfection dose.
• Decomposing solids exert a continuous oxygen demand that must be met by the oxygenation system.
• Fine particles irritate gills, contributing to stress and secondary infection independently of chemistry.
• Solids accumulating in pipework and dead zones go anaerobic, producing hydrogen sulphide.

• Minimise shear between production and capture. Every pump and every sharp fitting upstream of the filter reduces the capturable fraction.
• Capture concentrated streams concentrated. The tank bottom drain carries most of the settleable load in a small flow and should be treated as such.
• Removal efficiency at a given aperture is meaningless without knowing the particle size distribution arriving at it.
• Uneaten feed is the most expensive solid in the system: it is paid for as feed and then paid for again as treatment.
• Solids removed from the water but retained on the farm are only partially removed.

• Design the tank for solids transport first, since nothing downstream can capture what never left the tank.
• Route the bottom drain to a dedicated separation step rather than combining it with the clean side flow.
• Place mechanical filtration before any pump where the layout permits it.
• Size sludge thickening, storage and removal for peak production, and provide a route off site.
• Provide a means of sampling solids at several points in the loop, so capture performance can be measured rather than assumed.
• Eliminate dead legs and low points in pipework where solids can settle and go anaerobic.

• Feed to appetite and observe. Uneaten feed is both a production loss and the largest avoidable solids input.
• Trend backwash frequency and sludge volume as proxies for solids load; they respond faster than laboratory TSS.
• Inspect tank floors and pipework low points routinely for accumulation.
• Remove sludge from site on schedule, and keep sludge handling physically separated from clean areas.
• Correlate any biofilter or UV performance change with the solids record before investigating those units in isolation.

• Flush low points and dead legs on a schedule rather than when odour appears.
• Clean separator underflows and sludge lines, which block progressively and silently.
• Calibrate feeders and verify delivered quantity against intended quantity.
• Inspect and maintain sludge thickening and dewatering equipment as production equipment, not as ancillary plant.
• Sample and record TSS periodically to anchor the operational proxies against a real measurement.

• Judging solids management by water clarity, which reflects only the fine fraction.
• Pumping raw tank water before filtration and destroying the capturable particle size.
• Overfeeding, then treating the resulting solids and ammonia as an equipment shortfall.
• Storing sludge on site and allowing its leachate back into the loop.
• Ignoring settled solids in pipework until hydrogen sulphide announces them.
• Measuring TSS without ever considering particle size distribution.

Fine and colloidal solids are the unresolved fraction. Foam fractionation removes surface-active colloidal material and is most effective in saline water; ozone can alter particle surface properties to improve their removal, at the cost of introducing a residual that must be controlled. Both are additions to screening rather than replacements for it.

Sludge is also a resource question. Thickening and stabilisation determine disposal cost, and anaerobic treatment or nutrient recovery can change the economics of the waste stream. Those options constrain the design of the collection system, so they belong in the original design rather than in a later retrofit.

Solids are the upstream cause of most downstream problems. Their mass comes from feed digestibility and feeding accuracy; their treatability comes from particle size, which is destroyed by shear. Move them out of the tank quickly, capture them before they are broken up, and remove them from the site — then biofiltration, disinfection and oxygenation all work closer to design.