Moving bed biofilm reactor combined with an activated carbon filter for biological nitrate removal

A massive use of nitrogen based fertilizers in agriculture is worldwide one of the main causes for nitrate contamination of groundwater. Methods for removing nitrate from aquatic environment through physical and/or chemical processes often turn out to be not applicable because of unaffordable financial resource as well as essential infrastructure lack. On the other hand, biological processes seem to have potentiality to overcome these limitations since they are less expensive and easier to be performed. Accordingly, in the present work, a moving bed biofilm reactor (MBBR) filled with Kaldnes K1 as carrier media was used to remove nitrate from a synthetic groundwater at bench scale. Acetate was used as organic source. Different operational conditions were tested: influent nitrate concentrations of 30, 40, 50 and 60 mg/L; hydraulic retention times of 24, 18, 12 and 8 h; and COD/NO3-N mass ratios of 3.00 and 2.98. Experimental results showed that NO3-N = 60 mg L-1, HRT = 8 h and COD/NO3-N ratio = 2.98 were the optimal operating conditions that allowed achieving a NO3-N removal by 99 % and a COD removal by almost 100 %. Moreover, almost no NO2--N accumulation and null COD concentration were observed at the optimal operating conditions. An activated carbon filter was placed downstream to remove residual organic compounds prior to disinfection unit, thus avoiding the potential formation of harmful disinfection by-products (e.g. trihalomethanes (THMs)). The MBBR was able to show a rapid recovery whenever the operating conditions were defined as more severe, thus proving that the operating conditions can vary over a wider range. Furthermore, the results showed that the MBBR system can be used effectively as a biological process to remove nitrate from groundwater.