Simulating Active and Failing Systems

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.

Figure 6:4-3 Schematic of computations in the biozone algorithm. An active system becomes failing due to clogging in the biozone, then gets repaired to a new active system in several months of time period