The ubiquitous occurrence of micropollutants and their metabolites in the aquatic environment has posed threats to living organisms to a great extent. However, effective micropollutants removal normally requires longer hydraulic retention time (HRT) when using biological treatment systems. As an ideal and low-cost material for attached-growth microorganisms, polyurethane sponge has exhibited high potential to eliminate micropollutants. In this study, a sponge-based moving bed biofilm reactor (MBBR) was investigated at four different HRTs (24, 18, 12, 6 h), to better understanding of the effect of HRT on micropollutant removal. The MBBR as pretreatment to a membrane bioreactor (MBBR-MBR hybrid system) was also evaluated. Four groups of frequently detected micropollutants in wastewater (total 22 compounds) were selected, namely pharmaceuticals and personal care products (PPCPs), pesticides, hormones and industrial chemicals.
The MBBR alone showed stable and effective removals of TOC (92.6% - 95.8%), COD (93.0% - 96.1%) and NH4-H (73.6%-95.6%) at all HRTs while improving PO₄₋P removal at HRT of 18 h. The MBBR showed the highest performance efficiency for removing DOC, COD, NH₄₋N, PO₄₋P and TN at HRT of 18 h, which were 96.1±0.4%, 97.4±0.8%, 91.1±1.6%, 49.9±7.2%, and 72.3±6.9%, respectively. This could be explained by the food to microorganisms (F/M) ratio in the MBBR. In addition, higher NH₄₋N removal at HRT of 18 h could be attributed to the increased population of ammonium oxidation bacteria in the MBBR unit. Moreover, the use of phosphate for biomass growth and the phosphorus uptake by phosphate accumulating organisms (PAOs) could contribute to the high removal of PO₄₋P at HRT of 18 h. In terms of micropollutants removal, MBBR achieved comparable removal compared to other biological treatment such as activated sludge processes and membrane bioreactor. Although the micropollutants were subjected to biodegradation and sorption, the results indicated compound-specific variation in removal at all HRTs, ranging from 10.7% (carbamazepine) to 98.4% (ibuprofen). Among the selected micropollutants, most of them were biodegradable excluding carbamazepine, fenoprop and metronidazole. In addition, the micropollutants removal could remain constantly high even at lower HRTs with more consistent removal efficiency over the experimental period (except for carbamazepine, fenoprop, 17α-ethinylestradiol and 4-tert-octylphenol). Particularly, at HRT of 18 h, the removal of diclofenac was significantly improved by more than 30% and the removals of ketoprofen, gemifibrozil, acetaminiphen, bisphenol A, and pentachlorophenol were also better. Overall, HRT of 18 h was the optimum HRT for biological degradation of the micropollutants in the MBBR.
When using an MBBR as pretreatment to an MBR, the MBBR-MBR hybrid system achieved better removal efficiencies for selected micropollutants, such as metronidazole and carbamazepine. Both metronidazole and carbamazepine are nitrogen bearing compounds, where nitrogen is bound to the cyclic structure. The infinite SRT applied in this study could have facilitated the enhanced removal of the nitrogenous compounds. Even MBR can prevent the washout of slow-growing microorganisms like nitrifiers, the impact of MBR removal was minimal at all HRTs. This may probably due to the low MLSS concentration and the large pore size (0.2 μm; two orders of magnitude larger than the molecular sizes of micropollutants) of the MF membrane used in this study. In addition, a longer HRT (e.g. HRT of 24 h or 18 h) can significantly mitigate membrane fouling when compared with a relatively short HRT (e.g. HRT of 6 h). Especially, the TMP value maintained less than 15 kPa for 60 days (HRT of 18 h) and 68 days (HRT of 24 h). The level of EPS were similar at the beginning of all HRTs, then gradually increased to 15.24 mg/L, 16.43 mg/L, 19.88 mg/L and 22.93 mg/L at the end of operation for MBBR unit under HRT of 24 h, 18 h, 12 h, and 6 h, respectively. The SMP concentration varied for different HRTs but showed minor variation under the same HRT. The SMP concentration was lower at HRT of 24 h, while a significantly higher SMP concentration was observed at HRT of 6 h. As a whole, the MBBR-MBR hybrid system showed improvement in both micropollutants elimination and mitigation in membrane fouling.