Disinfection by-products management in high quality recycled water

During the production of high quality recycled water (HQRW) by means of reverse osmosis (RO) membrane filtration, chloramines (used to avoid biofouling) can generate disinfection by-products (DBPs) which could ultimately end up in the product water. This paper compares the fate of DBPs and organic halogen content generated during the production of HQRW under two different disinfection strategies employed prior to RO filtration: (i) in line-formed chloramines with a short contact time between secondary effluent and disinfectant versus (ii) pre-formed chloramines with a long contact time between secondary effluent and disinfectant. In general, pre-formed chloramines formed less trihalomethanes (THMs) in comparison to in line-formed chloramines even after a longer contact time. However, pre-formed chloramines still generated iodo-THMs (I-THMs) which are of concern since they are suspected to be more toxic than their chlorinated and brominated analogues. I-THMs were very effectively reduced by the advanced oxidation process placed after the membrane process. Chloramines also form other halogenated by-products whose identity is unknown. In this study, more than 95% of total organic halogen formed by chloramines was rejected by the RO membranes although the reduction of targeted low molecular weight DBPs was only around 50%. Specific rejections for individual DBPs vary, ranging from low for haloacetonitriles and high for haloketones.