Is this infiltration water or return process water entering the tank?
RAB: Possibly a return from sludge thickening, which was used in the 1957 system - but it was all slow-sand prior to 1957 ? I don’t recall seeing any plan of what or where though. Or just possibly washing of the slow sand filters at LSW, though ditto, I don’t recall any pipeline for it. I wouldn’t have thought infiltration would be encouraged as it could pick up drain leakage too; and this looks to be a fair old flow.