Biological nitrate removal in household slow sand filters using mixed culture and carbon source loading

Abstract

The objective of this work was to evaluate the use of pond bacteria paired with acetate and sawdust as viable inoculums for facilitating heterotrophic denitrification in household slow sand filters. This was achieved through culturing pond sediment bacteria and pairing it with both carbon sources in nitrate contaminated batch and filter column experiments. The inoculum culture tested positive for denitrifying bacteria including those of the Bacillus, Paraclostridium and Clostridium genera. Batch experiments conducted over 17 h with the acetate carbon source achieved complete denitrification for the C/N ratios of 2 and 5 at concentrations of 200 and 400 mg/L of nitrate. Incomplete denitrification was attained at C/N ratio of 0.5. Sawdust batches loaded with 1 g sawdust under similar concentration, volume, and time loading achieved incomplete denitrification with nitrite accumulation of up to 100 mg/L. In inoculated filter column experiments fed with 2 L of contaminated feed daily, the acetate carbon source fed filters achieved complete denitrification at feed concentrations of 200 and 400 mg/L nitrate, while sand/sawdust mixed columns only achieved complete denitrification for the 200 mg/L nitrate feed. 400 mg/L nitrate feed was reduced to 237 mg/L. No significant nitrite accumulation was observed in the filters. TOC readings in the control, sawdust and acetate filters were 7.072, 13.702 and 11.764 mg/L carbon at the 200 mg/L nitrate feed and 5.984, 6.630, and 7.142 mg/L carbon at the 400 mg/L nitrate feed.