How will the waste water treatment process at the proposed WWTP be regulated with respect to protecting the River Cam?
The Environment Agency regulates WWTP by setting compliance limits (via a discharge permit) for the quality of the effluent discharged to a watercourse. The proposed WWTP is designed and will be operated to meet the requirements of the Environment Agency discharge permit. The level of treatment and monitoring required is based on the population the WWTP serves and where the waste water is discharged.
As a result of the permit pre application process, the Environment Agency has proposed the indicative limits for the proposed WWTP which form an element of the assumed criteria for assessment of impacts on water quality in the River Cam. The proposed requirements would replace those in the permit for the existing Cambridge WWTP. The existing Cambridge WWTP outfall for treated effluent and stormflows entering the river is on the same stretch of the River Cam and within 100 metres upstream of the proposed relocated WWTP outfall.
The Cambridge sewer network collects water entering surface drains. It is not economically proportionate to provide full treatment capacity for these larger diluted ‘storm’ flows at the WWTP. Therefore, once the rate of flow into the pumping station at the entrance into the WWTP exceeds a set level the flows cannot be fully treated. Storm pumps divert the excess incoming flows to be stored temporarily, most commonly this is done using large concrete uncovered tanks. The Environment Agency issue and monitor permits for storm overflows and set the rates at which waste water must be fully treated.
This stormwater management solution will be in accordance with the agreement reached with the Environment Agency. The solution proposed will utilise the new transfer tunnel to deliver an element of the required storm storage. This will mean there will be storm storage underground as well as in onsite storm storage tanks.
In the event of a prolonged or particularly heavy storm the stormwater storage will reach capacity. In this eventuality, the screened and partially settled stormwater excess will enter overflow pipework and be transferred to the River Cam. The circumstances under which these transfers of partly treated storm water to the Cam occur are also regulated by the Environment Agency.