Dilution of sewage: Is it, after all, really worth the bother?

Abstract

Dilution of sewage caused by extraneous water is believed to have a negative effect on the daily operation of the wastewater infrastructure. Ecological consequences arise due to an increased activity of combined sewer overflows (CSO) and to a decreasing efficiency of treatment processes, while the elevated pumping activity induces a proportional increase in operational cost. This work aims at putting these drawbacks into perspective by an in-depth investigation of several catchments for which detailed sewer and treatment plant models were set up to verify the effect of a virtual removal of the existing sewage dilution. Results showed that extraneous water is responsible for 5–35% higher CSO volume. Effects at the treatment plant seem less significant with small gains in treatment efficiency (2–4%). Energy savings (up to 5%) only marginally exceed the rise in sludge production cost assuming this extraneous water would be eliminated. Finally, a pragmatic approach towards strategies of removing dilution is suggested by the use of a decision tree.