Phosphorus flocculation - combining phosphorus removal with denitrification

Phosphorus waste in fish culture systems is primarily found in the solid fraction and is removed through mechanical filters etc. 

Fish fecal matter is the primary solids source, with production around 170-200 kg total solids per ton feed used. Volume and characteristics of solids depends on applied collection systems and practices, but usually solids content is below 2 %.

Solids thickening and simultaneous phosphorus removal are aided by coagulation and flocculation processes, which are standard technology in waste water treatment and are sometimes used to treat effluent from freshwater recirculating aquaculture systems as well.

By using coagulation/flocculation followed by filtration/sedimentation, phosphorus reductions of 70-80 % are achievable in freshwater recirculating aquaculture systems, however untested in saltwater and knowledge of optimal conditions for brackish water recirculating aquaculture systems is very sparse.

A drawback of sludge hydrolysis and the concomitant decrease in pH is the resulting solubilization of calcium phosphate and the challenge for a balanced, low-pollution recirculating aquaculture systems is therefore the simultaneous removal of P and N. 

Synthetic polymers are most commonly used in flocculation, but their use is linked to residual unreacted monomers such as acrylamide, unreacted chemicals used to produce the monomer units such as formaldehyde and reaction byproducts. 

Bio-based flocculants such as modified starches could avoid these potential environmental risks and bio-based flocculants might also add additional digestible carbon for the single-sludge denitrification process. BONUS CLENAQ will test the applicability of this theory.
27 OCTOBER 2021