How will waste water and sludge be treated at the relocated Cambridge WWTP?
The proposed WWTP will deal with waste water treatment for the catchment areas and for wet sludge tankered in from the local satellite facilities. The wet sludge from these satellite plants will be transported by tankers and deposited into the first stage of the Sludge Treatment Centre (STC) process. The sludge treatment process would produce nutrient rich biosolids cake for use as bio-fertiliser for spreading on agricultural land and produce energy via anaerobic digestion as biogas is produced as a by-product. This is no different to the operations ongoing at the existing Cambridge WWTP.
As part of its statutory function, Anglian Water operates the existing Cambridge WWTP. An overview of the treatment process proposed for waste water and sludge is explained in the text and on the image below.
- The existing Cambridge WWTP receives waste water from the Cambridge catchment either directly from the connected sewerage network or tankered to the plant from homes and businesses that are not connected.
- This waste water is then screened to remove solids such as grit from road run-off, and large nondegradable objects (such as nappies, face wipes and plastic bags).
- The screened waste water then flows to primary treatment where a large proportion of the solid organic matter is separated from the water by allowing it to gravitate to the base of the primary settling tanks. The settled solids are pumped to the sludge treatment centre for further treatment and a weir near the top of the tanks then transfers the flows to the secondary treatment stage.
- Secondary treatment is the biological treatment process which relies on bacteria to further break down the solids. For the proposed WWTP the current proposal is to utilise a modern membrane aerated bioreactor (MaBR) configuration to ensure low energy utilisation for maximum oxygen transfer, however other activated sludge process options remain a potential.
- Final treatment provides the finest grade of treatment to ensure the effluent complies with discharge consent limits.
- The existing Cambridge WWTP is an integrated WWTP, as would be the Proposed Development. Integrated WWTP incorporate a sludge treatment function, in the form of a STC, which treats the sludge derived from the waste water being treated, and the “wet sludge” produced by other satellite plants which do not have integrated STC. The sludge treatment process produces nutrient rich biosolids cake used as a bio-fertiliser for spreading on agricultural land as well as producing energy via anaerobic digestion as biogas is produced as a by-product.
- The treated effluent is discharged through an outfall to the nearby River Cam.
Alongside waste water treatment, all storm flows which are conveyed to the proposed WWTP following heavy rainfall would be partially treated, which may include elements of screening and settlement. The sludge treatment process would produce sludge for use as bio-fertiliser for spreading on agricultural land and produce energy via anaerobic digestion as biogas is produced as a by-product.
Depending on the condition and design of the existing sewer network, the capacity of the WWTP and the volume and timing/duration of rainfall, storm flows arriving at a WWTP following heavy rainfall are either stored until full treatment capacity is available or if storage capacity is used before full treatment capacity is available partially treated waste water is discharged via the storm overflow to the watercourse.
What happens after heavy rainfall?
Storm overflows play a vital role in our combined waste water network systems as they protect homes and businesses from sewer flooding during periods of extreme rainfall. The Environment Agency (EA) issues permits for our storm overflows.
There is an existing combined sewer overflow (CSO) to the River Cam approximately 2km upstream of the existing Cambridge WWTP, which operates as part of the existing Cambridge WWTP system. During storm events, when waste water infrastructure is operating at full capacity, the CSO may be used as an overflow to convey storm water and waste water to the River Cam, and prevent flooding of streets and homes. Analysis for the CSO shows that it has discharged at most four times per year between 2018 and 2020. As discharge from the CSO contains untreated waste water, each CSO overflow to the River Cam adversely impacts water quality downstream.
The existing Cambridge WWTP includes storm water storage which are used during storm events to store combined volumes of waste water and storm water. Once the storm tanks are full and preliminary treatment through settlement has occurred, the settled storm water is discharged to the River Cam.
The pumping infrastructure of the proposed WWTP has been designed to receive all flow conditions (including storm) without having a negative impact on the existing Cambridge sewer network. Should the level of flow ever exceed the facility’s ‘flow to full treatment’ capacity, storm pumps will start working to divert the excess incoming flows to the WWTP stormwater storage and treatment plant. This stormwater management solution will be in accordance with the agreement reached with the Environment Agency as part of the treatment facility permit.
Will the waste water and sludge treatment processes at the relocated WWTP be different to those at the existing WWTP?
In terms of process steps these will be the same as those used on the existing Cambridge WWTP, however the process units and technologies used to carry out the process steps will be more energy efficient, due to advances in technology.