Nitrification rates of algal-bacterial biofilms in wastewater stabilization ponds under light and dark conditions

The objective of this study was to investigate nitrification rates in algal-bacterial biofilms of waste stabilization ponds (WSP) under different conditions of light, oxygen and pH. Biofilms were grown on wooden plates of 6.0 cm by 8.0 cm by 0.4 cm in a PVC tray continuously fed with synthetic wastewater with initial NH4-N and Chemical Oxygen Demand (COD) concentrations of 40 mg l−1 and 100 mg l−1, respectively, under light intensity of 85-95 μE m−2 s−1. Batch activity tests were carried out by exposure of the plates to light conditions as above (to simulate day time), dim light of 1.8-2.2 μE m−2 s−1 (to simulate reduced light as in deeper locations in WSP) and dark conditions (to simulate night time). Dissolved oxygen (DO) concentration and pH were controlled. At some experiments, both parameters were kept constant, and at others they were left to vary as in WSP. Results show biofilm nitrification rates of 945-1817 mg-N m−2 d−1 and 1124-1615 mg-N m−2 d−1 for light and dark experiments. When the minimum DO was 4.1 mg l−1, the biofilm nitrification rates under light and dark conditions did not differ significantly at 95% confidence. When the minimum DO in the dim light experiment was 3.2 mg l−1, the nitrification rates under light and dim light conditions were 945 mg-N m−2 d−1 and 563 mg-N m−2 d−1 and these significantly differed. Further decrease of DO to 1.1 mg l−1 under dark conditions resulted in more decrease of the nitrification rates to 156 mg-N m−2 d−1. It therefore seems that under these experimental conditions, biofilm nitrification rates are significantly reduced at a certain point when bulk water DO is between 3.2 mg l−1 and 4.1 mg l−1. As long as bulk water DO under dark is high, light is not important in influencing the process of nitrification.