The main particulate waste found in aquacultural water recirculation systems includes feces, uneaten food, and mucus. Fish have been particularly studied, and it is indicated that for salmon, for every 1 kg of food, between 0.3 and 0.4 kg of solids are produced as feces, and in the case of catfish between 0.2 and 0.7 kg of solids as feces.
In aquaculture it has been reported that the densities of the solids that are produced particularly in salmon farming have an average density between 1050 and 1190 kg/m3 (with a specific gravity of 1.19). These suspended particles vary greatly in size (from cm to µm) and shape.
The mean sedimentation rate for SS found in rainbow trout farming facilities was 1.7 cm/s and for feces extracted manually by massage a mean value of 0.7 cm/s was determined. In the case of salmon faeces, it has been described that the sedimentation rate is between 4 and 6 cm/s. Salmon feed has a higher sedimentation rate than faeces with values between 15 and 33 cm/s. The sedimentation rate of extruded diets for sea bream and sea bass is between 3.9 and 10.6 cm/s.
In relation to the size of the solids, it has been described for fish farm effluents for salmon ranging from 8.4 to 155 μm. In recirculating systems, sizes between 0.4 and 12 μm have been reported, with most particles in the range between 0.4 and 2 μm. It is generally known that 95% of suspended solids in recirculating aquaculture systems are less than 20 μm and that they represent between 40 and 70% of the total mass of SS.
The distribution of sedimentation rates and particle sizes are bioengineering data required for the design of gravity solids removal devices and for the selection of solids removal devices by size. This presentation will address the fundamentals of solids control by sedimentation and screening technological devices.