Application of submerged membrane bioreactor for aquaculture effluent reuse

Discharging the nutrient rich aquaculture effluents into inland water bodies and oceans is becoming a serious concern due to the adverse effect that brings in the form of eutrophication and subsequent damages to those waters. A laboratory scale biological reactor consisting of a denitrifying compartment followed by a submerged membrane bioreactor (SMBR) compartment was used to treat 40 L d−1 of aquaculture effluent with − an average concentration of 74 mg L−1 nitrate (NO3 ). Sugar was added to the aquaculture effluent in order that to enter into the denitrifying compartment at a carbon: nitrogen ratio (C:N) of 2:1 and 4:1. A hollow fibre membrane with a pore size of 0.4 µm and a filtration area of 0.20 m2 was used in the SMBR and was operated at an average flux of 0.20 m3 m−2 d−1. An intermittent suction period of 12 min followed by a relaxation period of 3 min was maintained in the SMBR throughout the experiment. Different aeration rates of 1, 3, 5 and 10 Lpm were applied to the SMBR to determine the rate of membrane fouling and 5 Lpm aeration rate was found to be optimum with respect to the rate of fouling of membrane at a C:N ratio of 4:1. The average rate of fouling at 1, 3, 5 and 10 Lpm were 1.17, 0.70, 0.48 and 0.52 kPa d−1, respectively. The increase in the rate of fouling when the aeration was increased from 5 to 10 Lpm may be due to the breakage of suspended particles into finer particles which could have increased the fouling of membrane. It was also found that increasing the C:N ratio from 2:1 to 4:1 resulted in more cake being formed on the membrane surface as well as an increase in the reduction of NO − from 64% to 78%. Preliminary calculations show that 2.4 to 3.2 g of suspended solids 3 could be accumulated per square meter of membrane surface before physical cleaning of membrane is required (at a transmembrane pressure of 20 kPa).

Recirculated aquaculture systems require efficient treatment to remove suspended solids, ammonia, nitrate and nitrite from the aquaculture effluent in order to reuse the treated effluent. Aquaculture effluent with average TN and NO3 concentrations of 74 and 19 mg L−1 was treated by a submerged membrane bioreactor attached to the back of an anoxic reactor; the average flow rate was 40 L d−1. The treatment system was able to remove 70% of the total nitrogen and 78% nitrate when sugar is used as carbon source for denitrification at a C:N ratio of 4:1. The SMBR produced an effluent with turbidity less than 0.5 NTU consistently for more than 200 days. An air flow rate of 5 Lpm is required to operate the membrane and at this air flow rate the TMP reached 14 kPa after a month of operation. A power-law equation correlated the cake resistance at the end of a treatment cycle with the cake density on the membrane surface.