Biological nitrogen removal from septic tank effluents

On-site wastewater treatment and disposal systems are used by approximately 25% of all housing units in the United States, with septic tank systems being most common. One of the environmental concerns associated with the use of septic tank systems is the potential for local and regional groundwater contamination. Nitrogen, which is typically not removed in conventional septic tank systems, is one of the most important contaminants associated with septic tank systems. Studies have directly linked nitrate-nitrogen with methemoglobinemia, a disease of concern to infants, and it may also be a carcinogen. This project examined a laboratory scale modified septic tank design which utilized an attached growth column for nitrification and an anoxic reactor for denitrification. A synthetic septic tank effluent was developed specifically for this project using respirometric methods. The septic tank design used in this research represents a relatively simple and potentially economic modification to traditional septic tank designs, yet has the potential to reduce nitrogen discharge to acceptable levels. The analysis of the experimental design results indicated that it was possible to remove significant amounts of nitrogen prior to discharge from the modified septic tank system. It was also determined that a hydraulic residence time of two days in the anoxic reactor and an operating mixer within the anoxic reactor produced the optimal result. A subsequent linear regression analysis of the nitrification efficiency determined that the initial BOD$\sb5$, initial NH$\sb3$-N, column depth, and an interaction variable of the column depth and the initial NH$\sb3$-N were the significant variables. Based on this model, three contour plots of the expected nitrification efficiency were developed for concentrations, volumetric loading rates, and surface loading rates. These contour plots can be used to further optimize the modified septic tank system used in this research. In addition, the contour plots can also be applied to other applications of biological nitrogen removal