Abstract: This paper presents a conceptual model, developed by synthesizing the results of many researchers, which describes the geochemical evolution of domestic wastewater in conventional on-site septic systems as the result of the interactions of a few major constituents. As described by the model, the evolution of wastewater is driven by the microbially catalyzed redox reactions involving organic C and N in wastewater and occurs in as many as three different redox zones. Anaerobic digestion of organic matter and production of CO2, CH4, and NH4+ predominate in the first zone, which consists mainly of the septic tank. In the second zone, gaseous diffusion through the unsaturated sediments of the drain field supplies O2 for aerobic oxidation of organic C and NH4+ and the consequent decrease in wastewater alkalinity. The NO3- formed by NH4+ oxidation in this zone is the primary adverse impact of septic systems at most sites and is generally an unavoidable consequence of the proper functioning of conventional septic systems. If adequate O2 is not available in the drain field, aerobic digestions is incomplete, and the accumulation of organic matter may cause septic system failure. In the third redox zone, NO3- is reduced to N2 by the anaerobic process of denitrification. However, this setting is rarely found below septic systems due to a lack of labile organic C in the natural setting. Consideration of the changing redox and pH conditions in each zone aids our understanding of the fate of other constituents in septic systems.
Justification: While this is an old paper, the concepts haven’t changed much. I think it is a fantastic example of a simple manmade system that takes advantage of the microbes and nutrients but discusses the potential impacts if favorable conditions don’t exist. While it may not be biologically technical with fancy techniques for analyzing microbes, it is an interesting topic.