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Disinfection by-products (DBPs) are often the most frequently detected chemicals in recycled water. In particular, N-nitrosodimethylamine (NDMA) has been previously identified as a DBP of health concern. While NDMA concentrations in recycled water can be minimised by additional treatment, reduction of precursors in the secondary wastewater is also important. In this study we investigated the removal of NDMA precursors during wastewater treatment, and the potential impact of different chloramination or ozonation strategies on NDMA formation in recycled water. The two key routes for N-nitrosamine precursor removal were activated sludge treatment and reverse osmosis. Precursor removal during activated sludge treatment was related to ammonia removal, suggesting that nitrification is an important removal mechanism for NDMA precursors. Significant variability in NDMA precursors in primary wastewater requires further investigation, but analysis of process polymers used in treatment indicated they did not cause the variation observed. While there was no difference between NDMA formation in laboratory experiments using inline-formed monochloramine or preformed monochloramine at disinfectant doses below 3 mg/L Cl2, increasing the dose to 10 mg/L Cl2 resulted in significantly more NDMA formation from inline-formed monochloramination. It is therefore important to control the disinfectant dose at the treatment plant since a slight increase in dose could result in a significant increase in NDMA. Ozonation may have promise for NDMA precursor removal, without significant NDMA formation. However, ozone will produce its own suite of DBPs, and therefore it is important to test the formation of other DBP classes before implementation.

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