Biogeochemical Evolution of Domestic Waste Water in Septic Systems: 1. Conceptual Model

This paper presents a conceptual model, developed by synthesizing the results of many researchers, which describes the geochemical evolution of domestic waste water 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 waste water is driven by the microbially catalyzed redox reactions involving organic C and N in waste water and occurs in as many as three different redox zones. Anaerobic digestion of organic matter and production of CO 2 , CH 4 , and NH 4 + 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 O 2 for aerobic oxidation of organic C and NH 4 + and a consequent decrease in waste‐water alkalinity. The NO 3 − formed by NH 4 + 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 O 2 is not available in the drain field, aerobic digestion is incomplete, and the accumulation of organic matter may cause septic‐system failure. In the third redox zone, NO 3 is reduced to N 2 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.