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The removal of solids from the effluent of aquaculture systems is one of the key operations in the effective application of these processes. Typically, the solids represent the insoluble waste materials derived from the fish and the feed. In intensive systems, if the suspended solids are not effectively removed they will accumulate rapidly to cause significant chemical and particulate contamination - this causes increased stress on the fish and reduces their overall performance. If the solids remain within the system, even for only a short period, they will contribute to a loss of hygiene as they start to decompose and solubilise. As a consequence the solids will contribute an additional chemical loading of the water and increase the demands made on the biofiltration process and additional solids formation within the system.
The precise nature of the suspended solids depends on a range of intervening factors;
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- the type of feed formulation and feeding regime,
- water composition, the species of fish in aquaculture,
- design and operation of the water flow path within the system and
- prevailing climatic conditions.
Generally, the majority of the particles are in the range of 20-400 microns and are derived from feed and faecal materials that are composed of agglomerated microbes and undigested vegetable materials. Typically they have a slightly higher density than the process water.
 The primary particulate recovery process is, at present, based on the use of microsieve filters for the removal of very dilute particles from the water; these filtered wastes typically contain only 10-500 mg of dry matter per litre of water. Further thickening processes (secondary) of the waste sludge that can rapidly separate and concentrate the solids are required, allowing a greater fraction of clean water to be returned to the system.
At the moment, a number of operations are being considered to this purpose, including
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- belt filters,
- gravity settlers
- and centrifugal (swirl) settlers
with the addition of appropriate coagulation and flocculation additives to aid separation.
The dewatered sludge volume is significantly decreased by these processes, allowing more effective sludge handling and further thickening in wetlands and other suitable treatments, so as to allow the possibility of an almost complete recycling of the water.
A further challenge to the sludge thickening process is the appropriate use of additives to aid sludge thickening without prejudicing the future uses and applications of either the thickened sludge or the recovered water.
prepared by Vincenzo Zonno and Bob Lowitt
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