Nitrogen removal from hydroponic culture wastewater by autotrophic denitrification using thiosulfate

Using the upflow biological filter reactor, sulfur denitrification using thiosulfate of hydroponic culture wastewater was examined. Start-up periods of the reactor were one to two weeks. About 90% of nitrogen removal ratio were achieved over 80 days, at 6.3 kg/m³·days of nitrogen loading. Shock loading among 0.56-2.8 kgN/m³· day did not affect the reactor performance. However, when temperature went below 15°C, the effluent characteristics became poor. Suitable S/N and IC/N ratios were calculated as 3.3 and 0.15, respectively. The activities of sulfur denitrification, heterotrophic denitrification and sulfur reduction were examined by the bath experiments under several conditions using biomass grown in the reactor. In the anoxic conditions, denitrification using thiosulfate was occurred stoichiometrically in the presence of thiosulfate. The denitrification activity was highest (17 mgN/gBiomass·hr). When the electron donor was not added to the substrate, denitrification occurred using sulfur granules accumulated in the biomass. Seventy mg of sulfur granule were accumulated in one g of biomass. The denitrification activity using sulfur granules was 2.9-5.0 mgN/gBiomass·hr. Heterotrophic denitrification occurred in the presence of organic matter. The activities were 1.4-5.4 mgN/gBiomass·hr. In the anaerobic conditions, the accumulated sulfur was reduced to sulfide at a rate of 1.4 mgS/gBiomass·hr. These results suggested that sulfur denitrification, heterotrophic denitrification and sulfur reduction bacteria coexisted in the biofilm and sulfur cycle was established in the reactor. Accumulated sulfur plays an important role in the sulfur denitrification.