Application of denitrification and aerobic membrane bioreactors to improve removal of high concentrations of organic matter and total nitrogen in a side stream of a wastewater treatment plant

Abstract

Anaerobic digester supernatant, having a higher concentration of nitrogen than normal wastewater, requires an additional treatment plant as a side stream process to remove the nitrogen in the wastewater treatment plant. The current process used for reject wastewater, the conventional activated sludge (CAS) process, has limitations in total nitrogen removal due to the absence of an anoxic tank. In this study, a two-step method of nitrification and denitrification was introduced as the biological treatment of total nitrogen in the reject water. The first step is the membrane bioreactor (MBR) process to overcome the low ammonia nitrogen removal rate of the CAS process; the second step is a denitrification batch test to remove NO3- N, the final product of nitrification in the effluent of the MBR process. The potential use of excrement supernatant was evaluated as a carbon source in this denitrification batch test. The MBR process exhibited high removal efficiencies of 76.5% for sCOD, 99.5% for NH4-N, and 20.1% for total nitrogen, excluding total nitrogen from sCOD and NH4-N, and showed more stable results compared to the CAS process. In the denitrification batch tests, denitrification rate constants were 3.26 mgN/gVSS∙hr at a 9:1 ratio, 2.705 mgN/gVSS∙hr at a 7:1 ratio, and 2.302 mgN/gVSS∙hr at a 5:1 ratio for the sCOD of excrement supernatant. This indicated the potential for denitrification as a carbon source by showing only a slight speed difference of 0.6 mgN/gVSS∙hr compared to sodium acetate, which is mainly used as an external carbon source. The results suggest that the MBR process achieves a higher efficiency for removing ammonia nitrogen by maintaining a high concentration of mixed liquor suspended solids (MLSS) compared to the extended aeration process, and also suggests the potential role of excrement supernatant contained in the reject water as a carbon source.