Potentials and limitations of biomethane and phosphorus recovery from sludges of brackish/marine aquaculture recirculation systems: A review

• Causes for low CH4 production from the salty sludges are reviewed.
• Phosphorus content of the sludges is summarized.
• Potential recovery of phosphorus as struvite from the salty sludge is proposed.
• High salinity and non-adapted inocula are major causes for low methane production.
• Corresponding approaches to enhance specific methane production are proposed.

Brackish/marine recirculation aquaculture systems (RAS) produce a relatively small but concentrated waste stream. The produced waste is perceived as a constraint for sustainable development of brackish/marine RAS. Appropriate disposal of sludge or waste from brackish/marine RAS is of great importance for widespread acceptance and implementation. Anaerobic stabilization of RAS sludge is considered as a potential cost-effective methodology to achieve effective sludge reduction and biogas production. Therefore, this review presents an overview of studies conducted on anaerobic digestion of sludge from brackish/marine RAS. Several researchers have shown that specific methane yield (SMY) of anaerobic digestion of sludges from brackish/marine RAS is relatively low, mainly in the range of 0.001–0.184 m3 CH4 (STP)/kg COD of sludge added. The possible reasons for low SMY are reviewed in this work and can be mainly attributed to applied experimental set-ups, particularly improper inoculum, and high salinity, mainly resulting from high sodium cation levels. This review also evaluates the potentials and limitations for phosphorus recovery from the waste streams. Additionally, corresponding approaches to enhance specific methanogenic activities are proposed, particularly about the need for further thickening sludges from brackish/marine RAS in order to increase SMY from the wastes and downsize the anaerobic digestion units.