In many countries, solid waste is recycled into fish meal plants or treated along with the municipal waste, whereas liquid waste is disposed of through the municipal sewage system or directly into a waterbody. In the latter case, care must be exercised to ensure that the receiving waterbody can degrade the biological and chemical constituents of the waste in a manner that is not detrimental to the aquatic fauna and flora.
Designing appropriate measures to dispose of liquid waste from fish processing operations requires assessment. This is done through the evaluation of various physicochemical and biological parameters of which the most important ones are: solid content, pH, temperature, odor, organic matter, biochemical oxygen demand or BOD, chemical oxygen demand or COD, oil and grease content, nitrogen and phosphorous content.
Primary and secondary waste treatments
Depending on the effluent polluting capacity and nature, one or several treatments can be considered. These treatments are classified into primary and secondary treatments.
Primary treatments include operations designed to remove floatable and settling solids. They include screening, sedimentation, and flotation to remove oil and grease and other suspended solids.
Secondary treatments comprise biological and physicochemical treatments. In biological treatments, the organic polluting matter is degraded by micro-organisms, which metabolise it into energy and biomass. These microorganisms can be aerobic or anaerobic. The most used aerobic processes are activated sludge system, aerated lagoons, trickling filters or bacterial beds and the rotating biological contractors. In anaerobic processes, the anaerobic microorganisms digest the organic matter in tanks to produce gases (mainly methane and CO2) and biomass. Anaerobic digesters are sometimes heated, using part of the methane produced, to maintain a temperature of 30 to 35°C.
In the physicochemical treatments, also called coagulation-flocculation, a chemical substance is added to the effluent to reduce the surface charges responsible for particle repulsions in a colloidal suspension, thus reducing the forces that keep its particles apart. This reduction in charge causes flocculation (agglomeration) and particles of larger sizes are settled and clarified effluent is obtained.
The sludge produced by primary and secondary treatments is further processed in digesting tanks through anaerobic processes or sprayed over land as a fertilizer. In the latter case, care must be exercised to ensure that the sludge is freed of its pathogens.