The pollution generated by a demand site is carried in the wastewater return flows to wastewater treatment plants and receiving bodies of water. Wastewater flows distributed from a given demand site to multiple destinations are assumed to have approximately the same concentrations. Therefore, the pollution streams flowing from a single source are proportional to the volume of flow. Thus, the amount of pollution that flows out of a demand site into a return flow link is a fraction of the pollution generated.
DSReturnLinkPollInflowDS,Dest,p = DSOutflowRoutingFractionDS,Dest x MonthlyPollGeneratedDS,p
For example, if the routing fraction from Demand Site North Agriculture to North Aquifer was 35%, and the routing fraction from Agriculture North to the Weaping River was 25% (with 40% of water consumed by the demand site), the fraction of Agriculture North's pollution that flow towards North Aquifer would be 0.35 / (0.35 + 0.25) = 0.58.
Stormwater runoff from catchments can also be routed to wastewater treatment plants, in order to model combined sewer overflow (CSO). In this case, non-point source pollution generated by the catchment is carried in the runoff to the treatment plant. The Soil Moisture Model determines the split between surface runoff and infiltration, with the non-point source pollution generated assumed to follow the same split.
CatchmentRunoffLinkPollInflowCatch,Dest,p = CatchmentSurfaceRunoffFractionCatch,Dest x MonthlyNonPointSourcePollutionCatch,p
Some of the pollutant might decay or otherwise be lost as it passes through the return flow or runoff links. The pollution that flows out of the return flow or runoff link is a fraction (entered as data-see Water Quality\Pollutant Decrease in Return Flows) of the inflow.
DSReturnLinkPollOutflowDS,Dest,p = (1- DSReturnLinkPollDecreaseRateDS,Dest,p ) x DSReturnLinkPollInflowDS,Dest,p
CatchmentRunoffLinkPollOutflowCatch,Dest,p = (1- CatchmentRunoffLinkPollDecreaseRateCatch,Dest,p ) x CatchmentRunoffLinkPollInflowCatch,Dest,p