Mark Purvis                            WQR0020

Written evidence submitted by Mark Purvis

Comparison of actual capacity vs EA advised capacity for Upper Thames tributary Sewage Treatment Works

1. About the author

1.1. Mark Purvis is a resident of Ascott-under-Wychwood, a Cotswold village on the banks of the River Evenlode. He participates in the Evenlode and Windrush Catchment Partnerships and is Deputy River Secretary of the Cotswold Flyfishers, an angling club with waters on many of the region’s limestone streams. He regularly monitors water quality on the River Evenlode as part of local citizen science projects and has contributed to the chapter on sewage and flooding in the Ascott Parish Council’s Neighbourhood Plan.

2. Introduction

2.1. The limestone rivers of the Upper Thames catchment are suffering from ecological degradation due to excess phosphate. The major source of phosphate and other nutrients in these rivers is the poorly performing water treatment services operated by Thames Water Utilities Ltd. The rivers are subject to frequent discharges of untreated sewage as recorded through the event duration monitoring logs submitted annually to the Environment Agency. These frequent discharges are evidence that the small rural sewage treatment works that are typical in the region are unable to meet the growing capacity demands.

2.2. To assess the extent of this shortfall in capacity, a study has been made of the current capacities of ten sewage treatment works against the capacities that the works should have according to the Environment Agency’s guidance.

3. Environment Agency guidance on design capacity of sewage treatment works

3.1. According to the EA guidance document “Water companies: environmental permits for storm overflows and emergency overflows” sewage treatment works must be designed to a certain capacity, based on the infiltration rate (Imax), industrial effluent flow, per capita effluent flow and the population served.

3.2. The following tables show the values of treatment capacity (known as full flow to treatment, FFT) calculated using the EA formula for a number of sewage treatment works in the Upper Thames catchment. Working back from the current FFT, it is possible to calculate the original designed capacity for each works in terms of population. There are two columns for Witney as there is a target to increase the FFT to 291 l/s in the near future.

3.3. Note that because the maximum infiltration rate on a dry day already exceeds the current FFT, Bourton STW is beyond its capacity before any sewage effluent is received from its customers and hence it has a negative design population.

3.4. Only Burford, of the 10 treatment works assessed, currently meets the guidelines for sewage treatment capacity. Some of the others only have capacity for around 30 to 40% of the population they currently serve.

Table 1: STW capacity for Evenlode sewage treatment works

Table 2: STW capacity for sewage treatment works in the Coln, Windrush and Ampney Brook catchments

4. Storm tanks

4.1. The same guidance document advises on the size of storm tanks with which STWs should be equipped. Storm tanks are used to buffer untreated sewage before discharge to a watercourse when the rate of ingress to the works exceeds capacity (FFT).

4.2. Two different ways are permitted for this to be calculated, either using the FFT of the works or the size of population served.

4.3. The following table shows the current tank volumes at nine sewage treatment works. The second column shows the capacity required by the EA for a plant operating at its nominal flow rate (i.e. the current rate). The third row shows what capacity of storm tank would be required if the STW was operating at the correct flow rate as calculated in Tables 1 and 2. Finally, the fourth row shows the required tank size for the current population served by the works.

4.4. Note that, in some cases, the storm tank capacity is more than adequate but elsewhere the tank capacity is well below the required level meaning that the likelihood of storm tank overflow is greatly increased. This lack of capacity has the direct consequence of increasing the frequency and quantity of the untreated sewage discharges.

Table 3: Storm tank capacities at selected sewage treatment works

5. Conclusion

5.1. Throughout the Upper Thames catchment, the failure to invest in adequate sewage treatment capacity is leading to regular sewage pollution in the region’s rivers. In most cases the treatment works have insufficient capacity to serve their current populations. Rather than invest in increasing stw capacity or consolidating treatment in modern energy-efficient and clean state-of-the-art works, Thames Water chooses to manage over capacity by discharging untreated sewage directly into the small limestone streams, many of which run through the Cotswolds Area of Outstanding Natural Beauty.

6. Data sources

6.1. All input data for the calculations were obtained from Thames Water Utilities Ltd under Environmental Information Requests.

February 2021