Is combined sewer overflow spill frequency/volume a good indicator of receiving water quality impact?

It is often assumed that the frequency or volume of combined sewer overflow (CSO) spill is a good indicator of receiving water pollution impact. Whilst this assumption would appear to be true, recently there have been challenges to its veracity. To test this basic premise, an integrated model (SYNOPSIS) has been applied to the urban wastewater system of a semi-hypothetical catchment. By increasing the storage volume at a single downstream tank in the drainage system, the CSO spill frequency and volume was reduced. River water quality criteria, based on UPM standards, were calculated and related to spill frequency and volume over a series of long-term simulation runs. It was found that, up to certain storage volume levels, decreasing overflow frequency improved river DO and BOD and total ammonia. Beyond these volumes, however, there was no further improvement in DO/BOD and an increase in total ammonia. It is concluded that overflow frequency/volume can be used as a performance indicator for receiving water quality, provided its significant limitations are understood.