In Situ Batch Denitrification of Nitrate-Rich Groundwater Using Sawdust as a Carbon Source—Marydale, South Africa

Batch experiments were performed to denitrify groundwater using sawdust as a carbon source at Marydale, South Africa. Alkalinity, pH, electrical conductivity, nitrate, nitrite, ammonia, SO ₄ ²⁻ , heterotrophic plate count (HPC), dissolved organic carbon (DOC), potassium and chloride were monitored. Two soil depths, 75 to 100 and 165 to 200 cm, respectively, from the Marydale area were used as matrix material during denitrification based on contrasting chemical composition with respect to major ion composition and moisture to consider different denitrification rates for varying soil depths. Different N to C ratios were used to evaluate the denitrification efficiency and the least undesirable products, e.g., elevated SO ₄ ²⁻ , H₂S and other reduced compounds. DOC is directly proportional to the N to C ratio used. Nitrite was produced for most of the treatments as incomplete denitrification occurred. The incubation periods were 28 and 43 days, respectively. N to C ratios were 12.6:1, 24:1, 34:1 and 54:1. Longer incubation period and higher N to C ratio resulted in total removal of both nitrate and nitrite. The reaction was carbon-limited for lower N to C ratios. The denitrification rate was proportional to the carbon availability at any time during the experiment. There was no significant difference in denitrification using heterogeneous and homogeneous particle sizes for sawdust. Soil depth of 75-100 cm displayed a greater denitrification rate than 165-200-cm soil depth due to higher initial soil nitrate concentration. The method showed some specificity, as DOC, nitrite, nitrate, alkalinity and HPC were the only parameters that showed a change in concentration over the duration of the denitrification experiment under constant temperature and nitrogen gas atmosphere. DOC and HPC were unacceptable for domestic use, but methods such as boiling or chlorinating water can rid it of bacteria.