Corncob PRB for on-site nitrate removal in groundwater

In order to obtain a new technology of on-site nitrate removal, a permeable reactive barrier (PRB) column was prepared by coupling the denitrifying bacteria with the use of agricultural waste. Nitrate removal in groundwater at low temperature (16 °C) and different hydraulic conditions was investigated in laboratory. Corncob was considered to be the most effective carbon source after the optimization experiments of nitrate removal. The PRB column was prepared by a 15-cm-high of corncob sandwiched between two layers of activated zeolite, where corncob was pre-mixed with denitrifying bacteria. The denitrification was studied at 16 °C and hydraulic retention time (HRT) ranged from 2.5 to 20.0 h, as well as the initial nitrate-N concentration in the range of 20–80 mg/L in groundwater. The values of NO3−-N, NO2−-N, NH4+-N, and chemical oxygen demand (COD) and the optical density at a wavelength of 600 nm (OD600) were measured as detection indexes. The results showed that the content of NO3−-N and COD continued to decrease, but the NO2−-N and NH4+-N increased first and then decreased quickly in the effluent. The removal efficiency of NO3−-N obtained 99.8%, 97.8%, 94.3%, and 85.9% at different initial nitrate-N of 20, 40, 60, and 80 mg/L and HTR of 24 h, respectively. The removal efficiency of NO3−-N and COD was 83.2% and 95.7% at HRT of 20 h and initial nitrate-N of 70 mg/L. Therefore, the corncob PRB column combined comprehensively utilization of agricultural residues with remediation of nitrate, which was effective for nitrate removal in groundwater at site temperature and HRT.