# Septic Systems On-site wastewater systems (OWSs) are considered the cause of significant non-point source pollution to water bodies, especially in rural areas and suburban areas. Quantifying their effects on water quality is important (McCray et al., 2005). Among the several conceptual and (or) mathematical models, a biozone algorithm proposed by Siegrist et al. (2005) was adapted to current work due to the fact that the algorithm fully describes biozone processes, validated at watershed scale, and provides coefficients specifically developed for biozone processes that were calibrated to field scale experimental data. In this algorithm, septic tank effluent directly drains into subsurface soil layer (infiltrative surfaces) affecting soil moisture content and the percolation of soil water through the vadose zone. The amount of percolation is regulated by unsaturated hydraulic conductivity as a function of soil moisture content. Total suspended solid (TSS), biochemical oxygen demand (BOD) and nutrients in the septic tank effluent (STE) trigger complex bio-physical processes in the biozone which, in turn, affect the hydraulics of soil water. The accumulation of TSS and plaque (i.e., dead bodies of biomass) in the pore space causes clogging and eventually hydraulic failure of the onsite wastewater system. The concentration of nutrients, BOD, and Fecal Coliform is estimated by a first-order decay equation based on the reaction coefficients specifically developed for each constituent by Siegrist et al. (2005). The biozone is considered as a control volume either on top of soil layer 1 or wedged in between soil layer 1 and soil layer 2 depending on the soil thickness. This control volume will accept the OWS effluent as well as infiltration from above. It can output flow vertically below and can contribute to detention storage on the surface (if the biozone layer is fully saturated). --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://swatplus.gitbook.io/io-docs/theoretical-documentation/section-6-management-practices/septic-systems.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.