Model Assumptions

After STE effluent passes through the biozone, it is discharged into the soil layers below, where the constituents are subject to additional transport/fate processes that are expected to occur in natural soils. Siegrist et al. (2005) have made the following assumptions to represent the physical system of biozone algorithm development based on laboratory observations and available knowledge.

1. Typical biozone thickness is 2-5 cm.

2. The biozone receives a continuous daily loading of STE. Intermittent dosing over the course of one day is not considered.

3. Depending on the climatic region, bermuda grass or similar type of grass is the typical vegetation seen above drainfield.

4. No STE inflow occurs if soil temperature goes below the freezing point and the biozone processes become inactive.

5. After a hydraulic failure, the model starts counting the failure days once the STE saturates the upper soil layers completely. Therefore, the initial several days after the hydraulic failure are not counted as failing period.

6. A failing system automatically turns to a fresh new active system after a user specified failing period (typically 2-3 months).

7. Total solids and TDS concentrations of STE are the same.