Performance of inorganic coagulants in treatment of backwash waters from a brackish aquaculture recirculation system and digestibility of salty sludge

In this work two inorganic coagulants, FeCl3 and polymeric aluminum sulfate (PAS) were tested for treating backwash waters from a drumfilter of a brackish RAS with a salinity level of 17.0 g/L. Their performances in terms of removal of total suspended solids (TSS), turbidity, total organic carbon (TOC), total phosphorus (TP) and reactive phosphorus (RP) were investigated. The results show that the removal efficiencies of total suspended solids (TSS), turbidity, total phosphorus (TP) and reactive phosphorus (RP) at the brackish conditions are higher than these in fresh condition with the same dosage of FeCl3 reported in literature. Moreover, dosing PAS caused a 9% decrease in biochemical oxygen demand (BOD5) of backwash waters, compared to that of the control group without adding PAS. The effects of FeCl3 and PAS as well as the effect of compatible solutes on biomethane potential (BMP) of the sludge from the RAS were examined. The results of BMP tests show that addition of glycine betaine (GB) 0.50 g/L, trehalose (T) 0.50 g/L and the combination 0.25 gGB/L plus 0.25 gT/L enhanced BMP of the sludge by 9.0%, 11.6% and 10.3%, respectively, compared with that of the control group without addition of compatible solutes. However, inorganic coagulants, FeCl3 and PAS, reduced BMP of the sludges from the sieve of the brackish RAS by 5.3% and 15.1%, respectively. Particularly, PAS (2.4 gAl/L) significantly lessened BMP. Therefore, PAS may not be a proper coagulant for concentrating sludges if anaerobic digestion is going to be adopted as a sludge post treatment approach to achieve sludge reduction and energy recovery. However, FeCl3 may be a potential coagulant to further coagulate and flocculate backwash waters from marine RAS without substantially affecting the anaerobic digestibility of sludge produced by addition of FeCl3.