Monitoring to manage wastewater pollution
Apr 14 2022
Author: Rosa Richards on behalf of SWIG
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Wastewater pollution from stormwater overflows or combined sewer overflows (CSOs) was in the news a lot in 2021, with Southern Water receiving a record fine of £90 million for unpermitted sewage discharges in July, and other water companies under scrutiny for polluting watercourses by environmental campaigners and the general public.
The Environment Agency is currently undertaking an investigation into more than 2,000 sewage treatment works to check whether they are being operated in compliance with their environmental permits. In response to public pressure, the finalised, post Brexit UK Environment Bill includes a statutory duty for water companies to achieve a progressive reduction in the negative impacts of CSOs, and a requirement to monitor river water quality upstream and downstream of each CSO. This is in addition to strategic priorities for Ofwat and the water companies it regulates, to significantly reduce the discharge of sewage from CSOs in the next pricing review (PR24) amongst other targets to drive performance from 2024-2029. Another related issue is property flooding from the sewer network, especially in cellars below ground level. So how can water companies reduce wastewater pollution from sewer networks? Using Yorkshire Water as an example what are some of the contributing factors? What measures are already in place to tackle wastewater pollution? Which extra technologies do water companies need to tackle leakage? And what innovations are on the horizon to tackle this issue?
Taking Yorkshire Water as an example, the scale of the challenge becomes obvious. The area served by Yorkshire Water has approximately five million customers in 2.6 million properties, 1,087 km of rivers, with 1 billion litres of sewage treated per day. Assets to operate and maintain include 12 long sea falls, 640 sewage treatment sites, 56,000 km of sewer, 2,700 pumping stations, 2,241 permitted storm overflows and more than 2 million manholes. Properties with cellars below ground level, typically Victorian terraces, are more prone to internal flooding. “The majority of flooding incidents are from lateral sewers serving Victorian terraces,” explains Thomas Ogden, innovation technical specialist at Yorkshire Water, “there are very few manholes on lateral sewers, so there’s a need for monitoring devices to go in gullies or soil stacks – we have 2,000 property level monitors.” Mr Ogden says that external property flooding is more of an issue in housing stock from the 1960s and 1970s onwards, as in this case there are generally no cellars, more manholes, a smaller network of sewers with narrow pipe diameter (150mm) and very little flow. Mr Ogden provides a wish list of technology that Yorkshire Water would find useful to address property flooding (Table 1).
In terms of wastewater pollution of watercourses, the majority of pollution incidents are from the sewer network, for example from rising mains, pumping stations, and from treatment works. A large proportion of the Yorkshire Water sewer network is within 50 metres of a watercourse, for example highway gullies, highway drains, and underground watercourses are often close to watercourses. There are currently 3,500 level monitors in the sewer network or about 1 per 15 km of sewer network, located to monitor CSOs and pollution hotspots in particular. There are 20 designated bathing waters (including one inland bathing water). Water companies are now monitoring discharges to watercourses, to comply with the 2021 Environment Bill. “It is no longer enough to know if spills are occurring,” explains Mr Ogden, “we want to know the volume of the spill and the make up of the spill, to understand the impact on the river water quality during heavy rainfall.” Mr Ogden says that analytics are needed to link the datasets for levels, flow and water quality together and provide a better understanding of the overall picture. You can only manage what you can measure, and various technologies could help improve understanding (Table 2).
In addition to the wish lists in Table 1 and 2, Mr Ogden suggests other technological solutions that would help including full pipe monitoring, power harvesting, ease of installation, low-cost flow monitoring and analytics to enable corrective action at the right time. Full pipe monitoring would help to identify blockages earlier than just point monitoring at a manhole which will only detect blockages once the sewage backs up the pipe (image 1). Power harvesting would help to extend asset life, enable an increase in the frequency of readings and the dial in frequency. Mr Ogden says that ideally monitors should be quick and easy to install; configuration should be possible through a web-based platform, with all installation data available online; and site information should be collected to enable a correct first time response (photos, access, traffic management details). Flow monitoring should be low cost, and ideally permanent online flow monitoring rather than the expensive flow surveys which are currently undertaken. Importantly analytics are needed to combine all the data with rainfall predictions, including long range weather forecasts to predict when a developing blockage will cause a problem and when to intervene. Analytics could also help to quantify environmental harm.
Various innovative technologies are available or in development to monitor water pollution, from a low-cost device which can monitor basic parameters up to a full-blown AI based sewer network control system. A brief overview of a selection is provided in Table 3. Some of these help to address the technology gaps identified above.
With the current drive to cut sewage spills, water companies will be seeking technological solutions to help them go beyond regulatory requirements and meet customer expectations.
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