How can a wastewater effluent diffuser be modeled?
A diffuser is best modeled with WaterCAD or WaterGEMS, as you can use minor loss coefficients on pressure pipes and model the diffusers either as emitter coefficients, or reservoirs set to the water surface elevation (and the flow through them will be controlled by the headloss through the diffuser pipes). See related forum discussions in the "See also" below
If you need to use SewerGEMS, it is recommended to try the GVF-Convex solver (or use SewerCAD). You should be able to achieve this with wet-wells as the storage tanks, pressure pipes, and outfalls with user defined tailwater as the diffuser end points. With pressure sub-networks, the GVF-Convex solver uses the same, EPANET solver as WaterCAD and WaterGEMS.
Forum Post - How to simulate a wastewater effluent diffuser in SewerCAD
Forum Post - Modeling Sewer Outfall Diffusers
Forum Post - Modeling of sub-sea outfall diffuser