High-carbohydrate wastewater treatment by IAL-CHS with immobilized Candida tropicalis

A novel immobilized-cell bioreactor was developed and utilized for treatment of high-carbohydrate wastewater and production of single-cell protein (SCP) simultaneously. The system is the internal airlift loop reactor with ceramic honeycomb support for biofilm (IAL-CHS), and the microorganisms tested was Candida tropicalis, which has a high protein yield. The optimum temperature for C. tropicalis was 35-40 °C, and bioreactor experiments were performed at 35 °C. The internal airlift loop reactor with ceramic honeycomb supports allowed a large and dense accumulation of biomass, but also generated significant SCP. Mass-transport resistance did not limit the overall COD-removal kinetics and oxygen mass transfer may have been accelerated to the biofilm. Fermentation of carbohydrates to organic acids lowered the pH below the optimum range of 4.5-6.5, and pH below 4.5 retarded or stopped the oxidation reactions for the organic acids. Since organic-acid oxidation is necessary for attaining COD removal from the wastewater, pH control may be essential for effective treatment in some cases with high-carbohydrate wastewaters. The addition of 0.35 g/L urea could increase the COD removal rate. Continuous wastewater was carried out and the organic loading reached 15 kg COD/(m3 day).