The DWMP blog – Episode 3. What is the drainage and wastewater system?

This is the third in a series of blogs.?If you haven’t already seen Episode 1 and 2, I suggest that you start there (https://tinyurl.com/DWMP-blog).

What do we mean by the drainage and wastewater system??What is in scope for a Drainage and Wastewater Management Plan.

The drainage and wastewater system performs two key functions; it takes away surface water from rainfall to minimise the risk of flooding properties and other areas and it conveys wastewater to a treatment works to minimise the impact on the environment.

In areas built in the last 60 years the two functions may be provided by completely separate pipe networks, but in many areas, they are provided by one combined network. ?A combined network will have overflows to discharge excess flow to watercourses or the sea in heavy rainfall.

How does the published guidance and the actual practice of DWMPs cope with the complexity of functions, systems and responsibilities?

Flood risk

The system to manage flooding can consist of four components:

• Sustainable drainage components such as infiltration and storage features
• Pipe networks to take flow to a watercourse (either directly or through a treatment works)
• For extreme events, the temporary use of landscape features such as roads, footpaths and parks to convey or store floodwater.
• Watercourses to convey the flow to the sea.

Ownership of the different features is varied, with property owners, local authorities or sewerage undertakers responsible for sustainable drainage, sewerage undertakers responsible for most pipe networks, local authorities responsible for the landscape features and riparian owners responsible for watercourses.?Overall responsibility for coordination is with the Lead Local Flood Authority.?How should DWMPs plan across these responsibilities.?

Drainage area planning started out focussed on combined sewer networks that delivered both functions, as these were the oldest and most in need of improvement.?As the methodologies matured it was realised that surface water sewers delivering the flood management function also needed to be assessed and planned and these were included in studies.

Unfortunately, the DWMP guidance seems to take a step backwards on surface water sewers.?Its definition of sewerage excludes surface water sewers; although it does say that they should be considered alongside what I imagine might be considered “real” sewers.?In practice, if previous studies had included surface water sewers, then they were included, otherwise they were ignored.

Lead local flood authorities and the environmental regulators have already carried out a lot of work to understand flood risk and this should be incorporated into DWMPs.?

DWMPs should explicitly review maps of the flood risk from surface water for each catchment and consider how the drainage and wastewater system could avoid making this risk worse or even improve it.?The plans should also consider how the landscape could be modified to convey flood water from sewers in extreme storms without causing impact.?Implementation of these changes would be a local authority responsibility rather than the sewerage undertaker.

Environmental impact

The system to manage wastewater consists of:

• A pipe network to collect and convey the wastewater.
• A treatment works to treat the wastewater.
• A watercourse or the sea to dilute the treated wastewater and complete the treatment process.

Integrated planning between the sewerage network and the wastewater treatment works was an explicit aim of the DWMP guidance.?However, in practice this did not always happen.?

The existing mature planning methodology under the Urban Pollution Management programme does take an integrated view of discharges from overflows and from treatment works and their impact on the receiving waters.?However, this tends to be focussed on the impact of overflows with solutions being to reduce overflow discharges.?There was an opportunity with DWMPs to take a more strategic view of wastewater and effluent discharges – akin to the view taken in Water Resource Management Plans.?This could consider which treatment works discharges were unsustainable in the long term because the watercourse provided low dilution or was sensitive.?A long-term strategy of relocating discharges or prioritising effluent for reuse rather than discharge could then be developed.

A key metric for wastewater discharges has always been the dilution ratio.?This is not the dilution provided to an actual discharge but is rather the ratio of the dry weather wastewater component of the discharge to the low summer flow in the river.?This is built into EA permitting policy and the Storm Overflow Assessment Framework (SOAF).?Generally, a dilution ration of more than 8 indicates that a discharge should be acceptable with some limitations.?A dilution ration of less than 2 indicates that the discharge is likely to cause an impact.?The DWMPs should assess the dilution ratio for every discharge and potential discharge and develop a strategy for long term actions.?This can give a strategic view similar to that of WRMPs by considering relocation of discharges to suit the environment.?

At present the DWMP guidance states that they should rely on environmental regulatory planning in River Basin Management Plans and the National Environment Programme.?However, these generally look only 6 years ahead and often react to changes that have already occurred in the water environment.?They therefore do not allow long term planning of the impact of changing population and climate change. ?The sewerage undertakers need to take a lead on these studies to take control of their long-term planning.?

These long-term plans need to take into account the changing social and political climate around discharges to the water environment.?The existing UPM approach is based on maintaining a healthy water environment, for coastal discharges it also considers bacteriological standards for safe swimming and shellfish harvesting.?It can be extended to similar safe swimming conditions in rivers.?However, it does not consider the views of the YISMIDs (Yuk it’s sewage, make it disappear.) who want no wastewater discharges to the water environment as a point of principle.?Some companies started to consider these views in DWMPs by considering reducing spill frequencies from overflows; but what is the acceptable and affordable frequency.

Summary

In summary DWMPs need to integrate better with other studies and take the lead on long term planning to ensure that they really deliver what is required and best value.

I have found this edition of the DWMP blog the most difficult to write; trying to condense a lot of challenging topics into a five-minute read.?I welcome comments and will incorporate them into an improved version.