3.3. Performance of combined system
The AnMBR-CANON MBR combined system was continuously operated in 82 d for treating membrane RO concentrate of synthetic wastewater. The overall performance of the combined system treating membrane concentrate is summarized in Table 1, which showed that the COD and total SS removal performances were excellent. Apart from membrane retention and good conversion performance during AD, an average COD removal efficiency of 96.7% was associated with the carbon source demand for both aerobic decomposition and heterotrophic denitrification in CANON MBR. Compared with AnMBR, effluent COD of the combined system was lower than 48 mg/L, which may be due to the consumption for heterotrophic denitrification. However, nitrogen removal mainly relied on CANON MBR. Almost no nitrogen removal occurred in the AnMBR, which achieved total nitrogen (TN) removal of 10.6%, whereas the CANON MBR reduced its influent TN by an additional 70.8%, thereby providing an overall TN removal of 81.4% for the combined system. Meanwhile, removal of total phosphorus was not expected. This finding suggested that the concentrate was needed to remove phosphorous if the effluent was to be reclaimed. This approach can be also beneficial if the effluent is to be used for agriculture or irrigation purposes [31]. Phosphorus of wastewater had the potential to generate precipitation of P as magnesium ammonium phosphate (MAP, NH4MgPO4•6H2O) when concentrations of magnesium, phosphate, and ammonium approached an equimolar ratio of 1:1:1 [32]. In the combined system, the equimolar ratio between the effluent phosphorus (10.7 mg/L, 0.35 mmol/L) and ammonium (12 mg/L, 0.86 mmol/L) was about 0.4 facilitating recovery of the concentrated fertilizer under excess nitrogen.
Throughout the operation of 147 d, washout of the biomass was successfully avoided. The membrane retained all of the biological bacteria, and the VSS concentration improved significantly. The MLSS in the AnMBR and CANON MBR increased from 8000 and 4000 mg/L to 11,240 and 6160 mg/L, respectively. The average methane yield of 1672 mL/d was obtained under ambient temperature during the combined system stable operation stage. Accordingly, this value was equal to 223 mL CH4/g COD which was lower than that of the start-up process, accounting for 63.7% of the theoretical value. The relatively sharp drop of methane yield was in part due to accumulation of metabolic intermediate products in the reactor under unstable conditions [33]. Another probable reason is the methane’s increased solubility under ambient temperatures [29]. Moreover, biogas sparging in the current experiment created equilibrium between the gas and liquid phases, which resulted in the reduction of dissolved methane in effluent [34]. However, membrane effluent discharged by the vacuum pump may increase dissolved methane concentration of permeate to the point of oversaturation in this study.