The biozone algorithm was integrated into the latest version of (SWAT+ version 2009 and the GIS interface version ArcSWAT+ 2009). Each unit of onsite septic systems is represented by a septic HRU. A septic HRU represents either one OWS or a hypothetical system that aggregates several OWSs that have similar HRU properties such as soil type, landuse and slope. In the latter case, septic HRUs in a subbasin are not spatially referenced and only lumped simulation is available. Septic HRUs are generated during the GIS interface processing based on the septic land use type defined by the users. Biozone and other properties of a septic system specific to a HRU is listed in septic input files (*.sep files). The septic water quality database developed based on literature lists STE loading rate and pollutants concentration in STE of the conventional system 25 kinds of advanced systems and an untreated effluent system (septwq.dat). Pollutant characteristics included in the water quality database are per capita septic effluent flow rate, Bio-chemical Oxygen Demand (BOD), Total Suspended Solid (TSS), Total Nitrogen (TN), ammonium, nitrite, nitrate, organic nitrogen, Total Phosphorus (TP), phosphate, organic P, and Fecal Coliform. A generic type for conventional system and advanced system are listed as the first and the second in the water quality database list, so the user can select these types with limited information. The hydrologic linkage to SWAT+ allows daily STE directly to be introduced to the biozone layer by increasing the soil moisture content in the subroutine percmain. The movement of soil water through percolation in and out of the biozne layer is then simulated by a SWAT+ soil water model in percmicro. The movement of nutrients through soil profile from the biozone layer is simulated by a combination of nutrient modules in SWAT+ and biozone. SWAT+ septic input parameters are presented in the updated SWAT+ Input/Output File Documentation (Neitsch et al., 2010).

In SWAT+ modeling, active systems are septic systems that are in operation or functioning as per guidelines and failing systems are septic systems that are subject to hydraulic failure and the effluent discharged differ from the standards. A typical service life span of an OWS ranges from 10 to 25 years depending on maintenance, pollutant loading rate, soil conditions and other factors. A septic HRU starting as an active system becomes a failing system as the biozone gets clogged by TSS and plaque of biomass. In the SWAT+ biozone module, biozone clogging, or hydraulic failure, is the main cause of system failure. There are other types of failing, but they are difficult to model in the SWAT+ structure and thus hydraulic failure was modeled as the only type of failing.

The schematic of the SWAT+ biozone algorithm is described in Figure 6:4-3. The biozone processes in septic HRUs are simulated on a daily basis as a subbasin-level. The biozone subroutine is called within the HRU iteration loop whenever the current HRU is septic. Each septic HRU is simulated based on a maintenance plan in which a failing system is assumed to be fixed and re-activated in two to three months time. In an active system, saturated water content gets lower as plaque fills in the soil pore space and field capacity increases as the amount of live bacteria biomass grows as filamentous biomass soaks up soil water. System failure occurs when the soil achieves saturated water content and field capacity. The model starts counting the number of days as the system fails and remains as a failing system. STE migrates to upper soil layers as water does not percolate through the bottom of the biozone layer. Depending on the thickness of soil above the biozone layer, it may take a few days to months until failure creates STE surface ponding. As STE migrates to upper soil layers, the nutrients in the STE are transported along with the STE. The amount of nutrients that transports to the upper soil layers is estimated based on the nutrient concentration in STE and the amount of water that migrates to the upper layer. There are no special treatment processes that apply to the nutrients in failing septic systems. If the number of days a septic system remained as failing exceeds the designed time, the failing system is updated to an active system and related properties are reinitialized as a fresh active system. An active system simulates soil water hydraulics and pollutants decay by executing the biozone equations described above.