Due to sewage's potentially ...
Published by Arnold Liu, CEO & Scientific Director at Quantum Dynamics, Inc.
Due to sewage's potentially varying composition (affecting instrument calibration) and varying depth of flow / void fraction , it will probably be best to utilize some sort of non-fouling velocimeter and (laser?) level gauging device. calculating the instantaneous wetted cross sectional area and multiplying by instantaneous flow velocity to approximate instantaneous volumetric flow. Primitive, yes, but one wouldn't have to worry about composition affecting instrument calibration, etc.
If one attempts to restrict the flow to conform to a full pipe flow there may be problems of sewage backup should anomalously high flow occurs -- as could be seen on multiple occasions recently during "100 year" storm events.