The Septage Impoundment Water and Materials balance began as a real project. The site is in a primarily rural county on the western slope of the Rocky Mountains in Colorado. Many of the remote residents use septic tank systems for sewage, and these require regular cleaning out, typically annually. The residents will contract a local sewage vacuum truck (aka "Honey Dipper") to empty the septic tank, which then hauls the liquid/solid waste to the local landfill where it is impounded, allowed to dry out (The Colorado Western slope is quite arid) , and disposed of on the landfill by "volunteer labor" provided by the County Department of Corrections (read prison labor).
The project required a water balance to determine the size of the three pond system - two ponds: one active to collect and impound solid/liquid waste and one inactive to dry previously impounded waste, and a third pond to evaporate decanted water from the active pond. This model was used to size the three ponds to ensure capacity for the future, that the evaporation pond had adequate evaporation potential to prevent long term accumulation, and that the drying out period was adequate to allow the first two ponds to switch roles.
The process simulated by the model is as follows:
Septage is discharged: A seasonally varying rate enters the system. This is steadily increased to reflect estimates of population growth.
Septage is impounded: at any time, one pond will be actively impounding waste. Solids will settle in the pond, displacing water, while the clear supernatant water is decanted into the evaporation pond.