Biological denitrification of water in a two-chambered immobilized-cell bioreactor

Summary A double-chambered bioreactor based on a composite immobilized-cell gel layer/microporous membrane structure was applied to the continuous denitrification of high-nitrate water. Immobilized denitrifying bacteria (Pseudomonas denitrificans) were provided with separate flows of nitrate and carbon (C) nutrient, with no contamination of the treated water by cell leakage from the gel. Using acetate (7.5 mm) as a C source and a C/N ratio of 3 (mol/mol), specific denitrification rates ranging from 15 to 25 g NO_inf3^sup–· h^–1 · – cm^–2 membrane surface (50–85 g NO_inf3^sup–· h^–1 · cm^–3 gel) were obtained. The denitrifying activity remained stable for several months. At the flow rate used (10 cm³ · h^–1), the effluents contained noticeable amounts of NO_inf2^sup–ions but the treated water remained uncontaminated by the carbon nutrient. Most NO_inf2^sup–ions disappeared from the treated water in a second reactor connected in series. When fed with an unchlorinated sludge supernatant as C nutrient, immobilized bacteria performed efficient denitrification of water for only 3 weeks. Diffusion experiments showed that acetate ions diffused much less rapidly than NO_inf3^sup–or NO_inf2^sup–ions through the composite structure. Further developments of the system are considered.