The Welsh Water Capital Delivery Alliance has developed an exciting innovative solution to help to manage the longstanding challenges faced by the industry from combined sewer networks. Mike Taylor at Sweco, explains how Welsh Water’s innovative solution to address the increased volume of rainwater entering the network can form part of a catchment approach to improving the local environment.

The purpose of combined sewer overflows is to protect homes from sewer flooding during storm conditions. The UK’s legacy of a network of combined sewers means that the number of weather-related sewage spills from combined sewers has risen as the effect of climate change and urban impermeable areas and population growth have contributed to an increase in rainfall run off entering the network.

One area where this has become a particular concern is the sewerage catchments of Llanelli and Gowerton in South Wales.

The story so far

Welsh Water’s Wastewater Treatment Works at Llanelli and Gowerton both discharge treated effluent into a sensitive local water body. The area supports an important cockle industry and is a Special Area of Conservation under the European Union’s Habitats Directive.

Over the years, increasing urbanisation and more frequent, high intensity storms have put a lot of pressure on the combined sewer system, and the number of weather related spills into the estuary can exceed desirable frequencies.

To address this, Welsh Water has been delivering a catchment approach to reduce the overall number of spills in Llanelli and Gowerton designed to meet the outputs agreed with their regulator.

During AMP5, the company developed their RainScape approach which targeted the separation of surface water from the sewers and looked at how to use sustainable drainage systems to slow and manage flows at root cause in the upstream catchment area.

RainScape has had a significant impact, removing around 48Ha of impermeable area draining to the network and the volume of combined sewer overflow water spills has significantly reduced from 3.8 million cubic metres per year to around 1.2million cubic metres in the Llanelli Catchment alone. However, the final part of the catchment approach to meet quality requirements needed a different approach, as continuing with RainScape would be a significant expense, and take too long to complete.

In AMP6, the Welsh Water Capital Delivery Alliance continued to develop and deliver RainScape and smart network solutions, but an innovative solution was required to process all flows (including the weather influenced peak flows) arriving at the Llanelli and Gowerton works and to treat them to meet EU Final Effluent standards.

Traditionally, the required effluent quality would be achieved by installing large storm tanks which would store the peak flow water until it could be processed by the wastewater treatment works. However, achieving this would mean installing tanks with around 216,000 cubic metres of capacity at Llanelli, and 300,000 cubic metres at Gowerton – the combined equivalent of 206 Olympic swimming pools. High capital and operational costs, combined with the large land requirements, meant this approach was not only technically unfeasible and unmanageable; but completely unaffordable, so an alternative solution was required.

Peak Flow Equivalent Treatment

Welsh Water opted for a pioneering method using treatment technology which would only operate when flows exceeded levels that could be processed by the existing treatment works, an approach known as peak flow equivalent treatment (PFET). This approach will reduce the remaining 1.2million cubic meters down to 110,000 cubic meters spilling from combined sewer overflows.

Llanelli Wastewater Treatment Works receives two pumped flows which deliver a combined maximum of nearly 1,400 litres per second during the peak storm conditions.

The existing treatment works has capacity to treat at least 599 litres per second. The next 650 litres per second will, once the project is complete, go through the new PFET unit for treatment, and the potential remaining 150 litres per second at maximum flow will go into the existing storm tanks.

The two flows from the existing treatment works and the new PFET unit will then be blended together to ensure the combined effluents meet the permit standard for that site.

Finding the solution

The process of designing a solution was not without challenges.

The existing treatment works relies heavily on biological processing of the sewage but the intermittent nature of the flow through the PFET unit meant that this approach was not feasible, and the plant would need to rely on a purely physical method of treatment.

The requirement for any solution to sit inside the existing footprint of the wastewater treatment works presented a further challenge. The team carried out an assessment of all the available potential technologies, scoring them against criteria that included filtration performance, physical footprint requirement, operating costs and ongoing maintenance requirements.

Two of the most promising options identified by this analysis were then extensively tested in pilot plants at Gowerton Wastewater Treatment Works, using manufactured effluent to simulate multiple scenarios that may face the new unit from different levels of weather related flow and durations of operation.

Trials on the two selected options involved the analysis of effluent against a number of factors including those identified in the regulator’s consents for the treatment works. This included parameters such as total suspended solids, biochemical oxygen demand, chemical oxygen demand, and maximum and minimum pH, as well as total ammonia, nitrogen and phosphorus.

Extensive data modelling was used to allow the team to understand how the levels in the outflow would be affected by changes in the composition of the in-flow to the sites and to determine the levels of risk of Welsh Water permit requirements being exceeded over an extended period of time.

For this purpose, a Monte Carlo model was created for the system – a mathematical modelling tool that can calculate the probabilities of possible outcomes occurring in a complex system, based on a wide range of different variables; this allowed the team to calculate likely failure rates of the system.

Based on this work, a solution that uses compressible media filtration was selected and this, combined with UV treatment, will ensure that the blended effluent leaving the two treatment works will meet the regulator’s requirements, even in maximum-flow conditions.

Next steps

Having established the technology that will be used, the team is now in the process of creating a detailed design of the structures that will contain the new PFET unit – a process that involves extensive structural and dynamic modelling.

The new unit will be housed in a complex pre-cast concrete casing and be located on the sites alongside existing works. The early stages of construction are already underway on both sites, with the new process scheduled to go live in 2020.