Quaternary ammonium compounds inhibited phosphorus removal performance and aggravated the spread of resistance genes in enhanced biological phosphorus removal systems

As a common surfactant, quaternary ammonium compounds (QACs) are often used as the main ingredient in disinfection products. Hence, the three most commonly used QACs were selected for this study, including alkyltrimethylammonium compounds (ATMACs), alkylbenzyldimethylammonium compounds (BACs) and dialkyldimethylammonium compounds (DADMACs). Typical QACs at low (0.2 mg/L), medium (1 mg/L) and high (5 mg/L) concentrations to the enhanced biological phosphorus removal (EBPR) systems were studied. The QACs exposure study was conducted for a total of 93 days using four sequential batch reactors. DADMAC-C12 had the greatest inhibitory effect on EBPR performance, followed by BAC-C12 and ATMAC-C12. On day 51, R1 (stimulated by ATMAC-C12) and R2 (stimulated by BAC-C12) recovered their EBPR performance, whereas the P removal efficiency of R3 (stimulated by DADMAC-C12) was still only 65.9 %. Furthermore, the dosage of both ATMAC-C12 and BAC-C12 promoted sludge granulation, whereas DADMAC-C12 caused slight breakdown of formed aerobic granular sludge. The median volumetric diameter in R3 initially increased to a maximum of 409.9 mu m and finally decreased to 352.6 mu m at the end of the operation. The stress of QACs resulted in an enrichment of pathogenic bacteria (e.g., Flavobacterium) and antibiotic resistant bacteria (e.g., Pseudomonas) in the systems. The abundance of Pseudomonas in R1 and R3 finally reached up to 9.40 % and 9.44 %, respectively. Additionally, QACs disrupted bacterial cell membranes and resulted in the release of intracellular resistent genes in sludge (si-RGs). Then, si-RGs converted to extracellular RGs in sludge and RGs in water. Zoogloea, Rhizobium, Turneriella and Aquimonas were the dominant potential hosts of RGs in EBPR systems dosed with QACs. This study can serve as an important reference for the control of RGs in EBPR processes.