Minimizing brine discharge in a combined biophysical system for nitrate removal from inland groundwater

•Nitrate removal from groundwater using ion exchange and regenerant recycling.•Brine discharge and chloride addition to product water were minimized.•Regenerant quality significantly improved using ozonation after denitrification.•Denitrification of brine showed low ethanol demand and minimal sludge production. Health concerns regarding microbial contamination on the one hand and brine disposal regulations on the other limits the application of both biological and physico-chemical techniques for nitrate removal from polluted inland groundwater. In the current work, a combined system consisting of ion exchange (IX) and advanced biophysical treatment of the recirculated regenerant was investigated to achieve a safe and reliable process for nitrate removal from groundwater with minimal brine discharge and chloride addition to the product water. Using a feed water composing 26mgNO3−–N/L, Cl− 243mg/L and 16mgSO4−2–S/L, optimal IX operation was found to be at a service cycle length of 380 bed volumes with ‘full treatment’ of the water source rather than a ‘split treatment’ option with shorter service cycle and final product water blending. ‘Full treatment’ resulted in nitrate concentrations meeting regulations while minimizing both Cl− addition to the treated water (1.11meqCl− added per meqNO3−–N removed) and waste brine production (0.25% of the water volume treated). In the product water, the DOC was 0.5mg/L lower than the feed (tap) water and before disinfection the bacterial count was 10–700cfu/mL. Spent regenerant was first treated in a sequential batch denitrification bioreactor followed by ozonation for polishing. The SBR unit achieved complete nitrate removal in 8h with nitrate removal rates of 2.6±0.4gN/Lreactor/d and a low average ethanol to nitrate mass ratio of 1.68±0.18. An ozone dose of 3–5mg/L brine allowed for efficient recycling of the denitrified regenerant by removing suspended solids by foam fractionation. In spite of the low brine blow-down, DOC in the recycled regenerant brine after more than a year of continuous operation was maintained at relatively low levels of 61.0±11.6mg/L.