Over the past few decades, the frequent detection of micropollutants in the aquatic environment has raised particular health and environmental concerns. Wastewater treatment plants (WWTPs) serve as significant barriers to reduce the release of micropollutants. However, due to the diverse characteristics and low concentrations of micropollutants, WWTPs can only achieve variable and often inadequate removals, ranging from 12.5% to 100% for some frequently reported compounds.
This study investigated a sponge-based MBBR for its effectiveness in the elimination of various micropollutants, including pharmaceuticals and personal care products, steroid hormones, industrial chemicals and pesticides. A moving bed-submerged membrane bioreactor (MB-SMBR) was also evaluated in terms of micropollutant removal and membrane fouling.
During the batch experiments, non-acclimatized (virgin) sponge showed significant and rapid sorption of hydrophobic compounds. Acclimatized sponge could achieve much higher elimination of some acidic pharmaceutical compounds, such as acetaminophen, diclofenac, gemfibrozil, ibuprofen, ketoprofen, naproxen and salicylic acid. Carbamazepine, fenoprop and metronidazole were poorly removed during all the batch experiments.
The sponge-based MBBR was effective in removing organics and nutrients (except PO4-P). Most of the selected micropollutants (16 out of 22) showed removals of higher than 70%. The poorly or moderately removed compounds included carbamazepine (25.9%), fenoprop (31.0%), diclofenac (45.7%), metronidazole (54.8%), ketoprofen (58.2%), and gemifibrozil (62.4%). The low biodegradability and/or polar property were two causes for the insufficient elimination. Overall, the effectiveness of the MBBR for micropollutant removal was comparable with those of other biological treatment processes, including activated sludge and membrane bioreactors (MBRs). Biodegradation was the major removal mechanisms for most compounds during the MBBR treatment. Sorption was only significant for the refractory compounds, while the readily biodegradable compounds did not considerably accumulate on the biosolids.
With the incorporation of the SMBR, the whole system (MB-SMBR) could significantly reduce the effluent turbidity. However, the SMBR did not achieve much supplementary removal of organic, nutrients and micropollutants. The membrane fouling in the SMBR occurred to a minor extent during the first 84 days of operation, after which an abrupt TMP increase was observed. High EPS levels (16.24 mg/L) in the SMBR was a potential cause for the severe fouling. The overgrowth of filamentous bacteria could also been deemed a factor that accelerated the membrane fouling rate. The total membrane resistance was mainly attributed to the deposited cake layer (76.5%), followed by the pore blocking (12.0%), clean membrane resistance (10.5%) and irreversible fouling (1.0%).