Evaluation of a Granular Sludge USB Process for Denitrification of Groundwaters with Low Hardness Including Polluted Field Samples

Research was conducted to further evaluate a denitrifying upflow sludge-blanket (USB) unit process for treatment of soft groundwater amended to a high pH as previously shown to be essential for effective granulation. From kinetic studies, a saturation constant of 15 mg-N/l was determined reflecting a high affinity of the granular sludge for the ethanol-nitrate substrate. The influence of carbon loading during the course of continuous treatment runs demonstrated that nitrite persisted in the effluent at carbon to nitrogen ratios below 1.1. Groundwater samples polluted with nitrate were obtained from two towns in Kumamoto Prefecture and used in continuous treatment experiments. Influent nitrate as high as 29 mg-N/l was completely removed at a HRT of only 30 min. The characteristics of these groundwaters without chemical additions (other than pH adjustment) were well suited for subsistence of granular sludge with good retention characteristics. Discussion was offered to establish protocol for progressing to actual treatment in the field, which should be possible for groundwaters of low hardness common to Japan with only minimal chemical adjustments (e.g., pH) at the treatment site.