Preventing excessive sewage discharges requires long-term decision-making, according to a new ICE insight paper.
Sewage discharges into the UK’s rivers and onto beaches have attracted significant attention recently.
They have increased in many areas due to urban expansion, changing weather patterns, and ageing infrastructure.
They occur because large parts of the UK have a combined sewage system. Rainwater and wastewater run in the same pipes to treatment works.
During heavy rainfall, combined sewer overflows (CSOs) act as a safety valve to prevent excess water from flooding homes and businesses.
But the discharges also create risks to public health, wildlife, and the environment.
Fixing the problem is likely to take decades. The public faces the prospect of ongoing sewage discharges and higher bills to pay for the solutions.
The ICE has published a new insights paper looking at the potential solutions.
Here are three key insights.
1. Engineering solutions should be combined with wider environmental interventions
There are two main solutions that focus on the sewer overflow itself:
- Storing storm flows in large tanks. When the storm has passed the tank can be emptied back into the sewer.
- Separating the foul and surface water. The surface water can then be discharged in a way that does not harm the environment or result in unacceptable flooding.
Storage is the most widely used approach.
However, this can’t be used at the scale required and constructing the concrete tanks has a high carbon impact.
Separation can avoid environmental damage and help recharge aquifers (permeable rock).
But it’s challenging to do in existing urban areas and takes a long time to deliver.
More powers and better guidance are also needed to help water companies, engineers and regulators implement separation on a large scale.
Developing nature-based solutions through blue-green infrastructure and sustainable drainage systems will also help.
2. Eliminating storm overflows altogether is neither economically nor environmentally viable
In 2022, a UK government-led taskforce estimated that eliminating storm overflows could cost between £350 billion and £600 billion.
Care must be taken when applying the estimates. But, if correct, household bills would rise between £569 and £999 per year.
Eliminating or limiting discharges also carries a significant cost in embodied carbon through construction.
Water companies would need an additional 118.43 million m3 of storage to achieve zero spills. This is equivalent to 40,000 Olympic-sized swimming pools.
A more realistic approach would involve retaining storm overflows in some capacity but reducing spill frequency.
Estimated costs here range from £5 billion to £280 billion and would have a lower impact on household bills.
For example, limiting spills to 40 on average per year, and 10 spills in sensitive catchments, could cost between £18 billion and £110 billion.
To reduce surface water flooding, some CSO investment could be co-designed with this objective in mind, according to the National Infrastructure Commission. This would lower the total investment needed.
3. Quick fixes could cause greater environmental damage
Reducing discharges from sewer overflows on the scale required, in an environmentally sensitive way, is a huge challenge.
The UK government is requiring water companies to spend an additional £56 billion between 2025 and 2050 to do so.
By 2025, the aim is to reduce overflow discharge from 2020 levels by 25%.
Water UK, the membership body for the water industry, will soon publish a plan setting out how each water company aims to improve its performance.
However, the investment assumes the main solution will be large storage tanks – with their associated carbon emissions.
Interventions must also avoid inadvertently increasing the risk of flooding.
Monitors are being installed in CSOs to produce more accurate data about the frequency and impact of their use.
This will be completed by the end of 2023 and will help develop smarter solutions.
For example, it’s important to resist starting work to resolve discharges that are doing no harm.
The industry must also look for whole-catchment, nature-based solutions wherever possible and make the most of the planned investment.
The way forward
Sewage overflows are one of many challenges the UK’s water and wastewater infrastructure system will face over the coming decades.
Civil engineers have a key role in ensuring that investment delivers the required improvements.
The public is rightly concerned about storm overflows.
However, there’s a need to remove emotion from the debate.
For example, CSOs are not the only cause of river pollution. Run-off from agriculture and highways is a major problem that also needs addressing.
Solutions should be based on sound principles and an understanding of the consequences of decisions.
Above all, it’s vital that all parties collaborate to find the answers which are economically and environmentally acceptable to the UK public.
Civil engineering insights into combined sewer
Content type: Insights paper