Use of Sequencing Batch Reactor for Biological Denitrification of High Nitrate-Containing Water

The efficiency of a sequencing batch reactor in denitrification of drinking water with relatively high nitrate concentrations (40–250 mg/L as N) was evaluated. Ethanol at a COD/N of 2.00 was found sufficient to reduce nitrate concentrations to acceptable levels (<10 mg/L as N). Within the first 6 min of reaction, nitrite accumulation in the range of 0.03–3.5 mg/L as N was observed increasing with the increase of initial nitrate concentrations. In the first hour, nitrate removal was significantly high in the range of 85.7–91.5%. Anoxic reaction times of 3, 5, and 7 h were required for nitrate concentrations of 40–160, 200, and 250 mg/L (as N) to achieve acceptable levels of nitrate and nitrite. Alkalinity of the denitrified water increased on average by 3.53 mg as CaCO₃ for each milligram of nitrate reduced and pH increased from 7.3 to the range of 8 to 9. Idle times between the operation cycles, in the range of 1–14 h, had an insignificant effect on denitrification. Residual COD concentrations in the range of 5–15 mg/L and sulfide concentrations (at initial nitrate concentrations ≥120 mg/L as N) in the range of 0.2–0.4 mg/L were recorded in the finished water. Elevated concentrations of COD in general are not advisable in drinking water, and specifically in this case, it could result in toxic sulfide formation in the treated water. There is a need to further study the optimization of the use of ethanol and polishing of the treated water. A sequencing batch reactor has the potential of being used as an alternative configuration for biological denitrification of drinking water.