Nature’s way of tackling wastewater spills

More frequent and more intense discharges from combined sewer overflows have made headlines in Britain over the past year. Emma Wren and Tony Smithson believe a holistic approach can bring about a solution.

Combined sewer overflows (CSOs) have historically been an important feature of wastewater systems in many parts of the world, providing a critical pressure release when sewer networks or treatment works reach capacity to prevent sewage backing up in buildings and on streets and open spaces. Designed to spill wastewater and rainwater runoff into rivers, estuaries or the sea only in emergencies, they’re use is becoming more and more frequent, polluting watercourses and closing beaches.

The UK government reports that 90% of storm overflows discharged at least once in 2021, with 5% discharging more than 100 times, including in high priority nature sites such as Sites of Special Scientific Interest. The US Environmental Protection Agency says CSOs are among the major sources responsible for beach closings and restrictions on fishing for shellfish, and for other water body impairments.

Population growth is one reason why overflows in many urban areas are discharging more wastewater, more frequently. London’s sewer network, which includes 57 CSOs in the lower Thames, was designed for 4M residents, not the near 9M now, and the largely Victorian system cannot cope. It is why £3.8bn is being invested to build Thames Tideway, a new 25km tunnel that will intercept existing outfall sewers, store combined sewage and rainwater runoff, and transfer it to a treatment plant. Urban creep, including covering green spaces with impermeable surfaces, poorly regulated development and using the sewer network to discard items like wet wipes and cooking oils that can block the system are exacerbating the problem, and putting further strain on ageing infrastructure.

Climate change will make the situation worse as storms and heavy rain are likely to become more common as temperatures rise. Unless tackled, overflows designed to operate infrequently will discharge contaminants and pathogens directly into the water on a more regular basis, restricting our use of rivers, bathing waters and beaches, and making popular post-pandemic activities like wild swimming a health hazard.

Can we engineer our way to a more holistic approach?

The answer is yes, but not every city needs a Thames Tideway tunnel or has the financial resources to refurbish the existing sewer network to separate wastewater from clean rainwater. In the right place and under the right circumstances, alternatives to a conventionally engineered solution might be more cost-effective and more beneficial. That often means looking at the bigger picture and not focusing on solving a problem solely where it is occurring. As Ofwat’s senior director for strategy, finance and infrastructure told a wastewater conference in January, water companies must think as creatively and as innovatively as possible and fully embrace blue (eg wetlands and ponds) and green (eg trees and land-use planning) solutions to deliver more for customers and the watercourse requires many small-scale measures where they can have a cumulative positive impact. Upstream and rural measures include:

• planting trees across slopes and along river corridors improves water infiltration, and reduces and slows run-off on farmland
• restoring bends in watercourses increases retention capacity by allowing rivers to naturally expand their banks and flood onto floodplains
• creating shallow ponds and wetlands of different depths to store water
• managing farmland so soil is in excellent condition and able to let water soak through into the ground

In urban areas, sustainable drainage systems (SuDS) help to manage rain close to where it falls. Green roofs, rain gardens, infiltration trenches, tree pits and permeable paving are examples of SuDS and they all mimic natural drainage, so water naturally soaks into the soil and the ground rather than entering down the nearest drain.

These blue and green interventions form pieces of a jigsaw that can be used individually or together across a catchment or to complement existing, upgraded or new engineered solutions. They will help to mitigate flooding and the risk of water pollution – and reduce our over reliance on CSOs when heavy rain falls, and our water networks become inundated. Importantly, blue and green infrastructure can also deliver a host of benefits beyond the intended aims. Trees store carbon, reduce sediment transfer and soil erosion, and improve landscape diversity and wildlife habitat, for example. Green spaces, such as pocket gardens and green roofs, meanwhile, provide recreational and health benefits, as well as helping to build climate resilience.

Greater use of catchment-wide interventions in the UK will require changes to the permitting regime. Existing legislation and compliance are focused on single point discharges and water quality at that location or the number and volume of spills. This means action and investment is typically centred on reducing the number of spills and improving performance statistics at one site. It’s a missed opportunity. A holistic view would assess the sensitivity of a watercourse to spills, identify discharges likely to cause the largest environmental damage and pose the biggest threat to human health as well as explore the opportunities to achieve wider benefits alongside improving water quality. It would also identify what single or series of measures could be taken to limit future occurrences.

Until we innovate and explore how blue and green solutions can be integrated into our catchments or to supplement traditional interventions, we will keep on falling back on ever larger engineered solutions. It is time to give nature a chance, and let it play a leading or supporting role in tackling discharges from combined sewer overflows.