Anaerobic digestion of sludge from brackish aquaculture recirculation system

Abstract

The marine/ brackish recirculating aquaculture system (RAS) is a sustainable and environment-friendly cultivating system to meet the increasing demand of aquatic animal protein for the human consumption. However, the increasing amount of aquaculture waste becomes problematic for further application and demands much research attention on it. To solve this problem, a project “emission minimization of marine/brackish aquaculture recirculation systems “ was proposed, which aims to minimize the emission of brackish RAS and improve bio-energy yield as well as recovery phosphorus in the form of struvite. The current thesis working is a part of this whole project. The objectives involved here could be divided into four main parts : (1) to investigate the performances of CSTRs at different organic loading rates (OLR) and under different mixing regimes; (2) to investigate the effect of inorganic coagulants on methane potential of saline sludge and to investigate the effect of compatible solutes on methane potential of saline sludge, (3) to investigate effects of compatible solutes and shear forces from biogas recirculation on phosphatase activity (PA) (4) to investigate variation of particle size distribution of struvite at different Mg/P molar ratios, at different temperatures, or at different pH levels. To achieve goals described above, three similar CSTRs were setup in lab-scale and operated for almost one year. Reactor performances were monitored by weekly analysing of essential parameters, including TSS, VSS, tCOD, sCOD, TN, TP, etc. Some batch or jar tests were also carried out during experiments. (1) The steady state was obtained in all three CSTRs with inoculum adapted to high salinity already. A higher OLR could cause an increase in specific CH4 yield. The impeller-stirring enhanced the specific CH4 yield compared with biogas recirculation. (2) Methane potential of saline sludge was improved 5% at the presence of 0.5g/L trehalose, whereas it was inhibited 18% and 5% by addition of PAS at 2.4gAl3+/L and FeCl3 at 6.0gFe3+/L, respectively. (3) Addition of 0.5mM or 1mM trehalose enhanced acid PA in saline sludge during the digesting period of 0.25h-72h. A high level of shear stress from biogas recirculation could stimulate both acid and alkaline PA during the digesting period of 0.25h-12h. (4) The optimum conditions in synthetic brackish solution for harvesting more struvite with large sizes and obtaining a higher P-removal efficiency were that Mg/P molar ratio approached to 1, and pH was maintained at 8, and temperature was kept at 25 or 35°C. However, the conclusion referring to Mg/P was not applicable to the filtrate from R2, and more research would be required to verify the application of other conclusions to the filtrate of digestate.