Current sewage pollution issues could be the result of well-intended privatisation legislation

Graham Olsen is a retired water engineer, who worked on MEPAS, as well as held management roles with North West Water, Bechtel and MWH.

The issue is not a new one though with Thames Water last week fined £3.3M for a sewage pollution incident in 2017. In response, the government published a Storm Overflows Discharge Reduction Plan in August 2022.

However, what is occurring, more than 30 years after the introduction of legislation that created the privatised water companies, could be the result of that well-intended legislation. Nonetheless, the latest plans, to be customer funded, overlooks some fundamentals. Legislating without a full technical understanding of the consequences inevitably leads to failure.

In an ideal situation, foul flows and storm flows are conveyed in separate sewer systems. The foul flows always receive treatment, and there are no contaminated storm sewer discharges. But in reality, many of our Victorian systems mix foul and storm together in one system, meaning that surplus contaminated storm flows have to be released to rivers or beaches during storms. We even had a period in the 1980s when water authorities made a point of overriding former local authority legislation which had required developers to install separate systems (temporarily connected to the combined systems pending the construction of new trunk foul sewers).

In order to protect the natural environment and public health, UK civil engineers have, over a prolonged period, used their best endeavours to apply engineering judgement to capture, separate and treat wastewaters generated from domestic sewage and industrial processes.

While combined sewers were constructed to convey both raw domestic and industrial wastewaters plus surface water run-off flows generated during periods of rainfall, cost-effective improvements in sanitary engineering have included the installation of storm sewage overflow chambers to trap and capture the highly polluting “first flush” of raw sewage flow that is released at the start of every storm. This flow is like a tidal wave or bore in the system picking up much polluting matter that is deposited in sewers, which may include all sorts of debris, together with sediments that may settle out in older sewers that suffer from inadequate self-cleansing flow velocities. In recent years this well-known problem has been exacerbated by the increased use of wet wipes.

From the late 1950s a standard stilling pond overflow design was employed on combined sewers throughout the UK to concentrate denser materials contained within sewage flows and to pass these forward for biological treatment. At the same time the lighter particle fraction of flow was retained behind an internal baffle wall until a rainstorm event had subsided, when this part of the flow was also then passed forward to the treatment works. These chambers were much enhanced through university research for the 1980s Mersey Estuary Pollution Alleviation Scheme (MEPAS) project.

So what happened with the advent of the regional water authorities and the later creation of privatised water companies? In the associated Asset Management Plan (AMPs) water companies were charged with meeting a number of well-meaning standards. Once a year discharges of sewage to high amenity water courses containing particles measuring more than 6mm in two dimensions were not permitted. For moderate amenity waters, similar restrictions were limited to no more than 30 discharges per year, and in the case of low amenity waters the provision of good engineering design incorporating stilling ponds or similar measures were required. In other words, a cosmetic standard was being applied so that discharges might be less noticeable.

Even the large and hydraulically efficient stilling ponds incorporated into sewerage infrastructure during the late 1950s, that were further improved for MEPAS through the hydraulic testing of university laboratory scale models, could not guarantee that 6mm particulate matter contained within raw sewage would not be entrained within outfall flows. However, those individual stilling pond chambers specifically developed for MEPAS not only had enhanced particle separation performance but also had the distinct advantage of being of sufficient physical size and volume to contain the highly polluting first flush flow from each separate drainage catchment.

Unfortunately, UK water companies realised that by introducing 6mm mechanical screens into new smaller and more compact overflow structures, they might comply with legislation at a much lower capital cost. To satisfy the legislated particle size requirement, water companies rushed to co-operate with mechanical screen manufacturers in the design and fabrication of screens with the required 6mm spacing. These initiatives lowered both fabrication and construction costs but increased operating costs and failure risk due to the necessary reliance on electrical power systems. At the time of their development however, the long term reliability of these screening devices could not be satisfactorily predicted due to the limitations of performance data derived from the available testing procedures adopted. Fatally, the need to cope with the first flush at the start of a storm was not considered.

The cheaper and smaller overflow chambers are now widely adopted throughout the water industry. Hundreds of undersized overflow chambers around the UK now rely on 6mm mechanical/electrical screen devices and are of insufficient capacity to capture the highly polluting first flush flow from each and every rainfall event when all sorts of debris is swept from the drainage catchment into the sewers. Frequently, on hitting the mechanical screens the overwhelming quantity of first flush sewage debris knocks the screen out of action and flows thereafter flow over the top of the screen and into the watercourse totally untreated. This remains the situation until there is a manned maintenance visit. The problem now is that the water companies have elected to use an energy dependent device that appears to meet the required standards, yet invariably fails with the arrival of the first flush at the start of a storm.

While many previous designs were reliable, needing little maintenance, the new designs were totally electrical-energy dependent with inevitable repeated failures. Of course, the now rising costs of electrical energy will increase running costs which will be passed on to the customer. In reality, the well-designed chambers recommended for low amenity waters have a better overall performance that the 6mm mechanical screen chambers required for high amenity, so here is a great example of good intentions actually creating the opposite of the desired effect. Could this problem be addressed by making it a legal requirement to have a chamber adjacent to, or immediately upstream of any mechanical screen capable of containing the calculated first flush volume for the relevant catchment before the screen is engaged?

In my opinion it must be the case that these inadequate mechanically screened overflow chambers are now contributing to the high levels of pollution currently being reported in rivers throughout the UK. This all arises from well-intended legislation, followed by blinkered cost focussed research and testing that ignored well established technical requirements not defined in the legislation.

The MEPAS designs included a deep shaft CSO at Dingle, modelled and developed at Liverpool University. The shaft incorporated baffles and, like the other MEPAS CSO's a calibrated vortex controlled discharge. This design was specific to deep structures on confined sites with a lower level discharge, but it could easily be adapted to having a pumped return making it suitable for any site requiring first flush capture and protection of existing screens.

Could this design be adapted for retrofitting ahead of vulnerable 6mm screen CSOs? The necessity of considering the first flush volumes appears to be a major omission from the latest legislation.

• Graham Olsen is a retired water engineer, who worked on MEPAS, as well as held management roles with North West Water, Bechtel and MWH.

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