Formation of disinfection by-products during sodium hypochlorite cleaning of fouled membranes from membrane bioreactors |
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Authors: | Hao Wang Defang Ma Weiye Shi Zhiyu Yang Yun Cai Baoyu Gao |
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Affiliation: | 1. Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Qingdao 266237, China2. No.1 Institute of Geology and Mineral Resources of Shandong Province, Jinan 250014, China3. Shandong Engineering Laboratory for High-Grade Fe Ores Exploration and Exploitation, Jinan 250014, China |
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Abstract: | ?HAAs was dominant among the DBPs of interest. ?Rising time, dose, temperature and pH raised TCM and HAAs but reduced HANs and HKs. ?Low time, dose and temperature and non-neutrality pH reduced toxic risks of DBPs. ?The presence of EPS decelerated the production of DBPs. ?EPS, particularly polysaccharides were highly resistant to chlorine. Periodic chemical cleaning with sodium hypochlorite (NaClO) is essential to restore the membrane permeability in a membrane bioreactor (MBR). However, the chlorination of membrane foulants results in the formation of disinfection by-products (DBPs), which will cause the deterioration of the MBR effluent and increase the antibiotic resistance in bacteria in the MBR tank. In this study, the formation of 14 DBPs during chemical cleaning of fouled MBR membrane modules was investigated. Together with the effects of biofilm extracellular polymeric substances (EPS), influences of reaction time, NaClO dosage, initial pH, and cleaning temperature on the DBP formation were investigated. Haloacetic acids (HAAs) and trichloromethane (TCM), composed over 90% of the DBPs, were increasingly accumulated as the NaClO cleaning time extended. By increasing the chlorine dosage, temperature, and pH, the yield of TCM and dichloroacetic acid (DCAA) was increased by up to a factor of 1?14, whereas the yields of haloacetonitriles (HANs) and haloketones (HKs) were decreased. Either decreasing in the chlorine dosage and cleaning temperature or adjusting the pH of cleaning reagents toward acidic or alkaline could effectively reduce the toxic risks caused by DBPs. After the EPS extraction pretreatment, the formation of DBPs was accelerated in the first 12 h due to the damage of biofilm structure. Confocal laser scanning microscopy (CLSM) images showed that EPS, particularly polysaccharides, were highly resistant to chlorine and might be able to protect the cells exposed to chlorination. |
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Keywords: | MBR Biofouling EPS Chemical cleaning DBPs CLSM |
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