Thames Water is exploring the idea of building a small sewage plant for Sevenoaks in the medium term.

Graeme Kasselman, London system planning lead at Thames Water explained the thinking behind the idea at our AGM.

Drainage and Wastewater Management Plan (DWMP)

Currently, sewage from the Sevenoaks area is treated at the Long Reach works near Dartford and treated water discharged into the tidal Thames. It takes about eight hours for sewage from Sevenoaks to reach the treatment station.

Following a proposal by the National Infrastructure Commission, water companies have been asked to produce plans describing the impact and mitigation required to accommodate population growth and climate change on sewage and sewage treatment works – similar to requirements in the five yearly Water Resource Management Plans. Thames Water published their first iteration in May 2023. It will be repeated/refreshed every five years. Full details, including the Long Reach catchment strategic plan, are available from this page.

The Darent Valley

Initially there were no plans for sewage treatment in the Darent area. However, the Thames delivery team wondered whether this missed an opportunity. This is because there is no integrated water system in the Darent. Abstracted water, once used by customers, is treated and discharged to the tidal Thames. In effect, water is taken from an ecosystem that needs it and is returned elsewhere to flow out to sea. It raises the question as to whether some of the water could be restored locally to improve the environment.

Sewage Treatment for Sevenoaks

Thames is looking at the possibility of constructing a new sewage works north of Sevenoaks for about 7,000 population equivalent. Advanced treatment technology would be used to achieve high quality treated water and nature-based solutions such as wetlands would be incorporated to increase biodiversity. The treated water would discharge into the Darent.

The introduction of a local sewage plant is linked to reversing the plan to close abstractions from the river. Graeme described the advantage of this proposal in Sevenoaks as being the environmental benefit. There may well be an economic benefit too. Closure of the Horton Kirby and Eynsford abstractions will require Thames Water to identify a new water source contributing around 7Ml/d to domestic supply. The proposed sewage works would return about the same volume of water into the river per day. For Thames Water this might be a more cost-effective option than connecting the local water system to the London ring main.

Questions raised

Audience members asked about raised nutrient levels in the effluent water. Graeme said that technology can reduce Nitrogen and Phosphorus content but that levels would still be higher than currently in the river. Would that be a price worth paying due to the increased flow, he asked? He also mentioned the plan to include a very large wetland/nature reserve through which the water would be released. He was asked if contaminants would build up the mud and sediment in the river. He said they might but it was necessary to explore solubility to identify whether contaminants would settle out.

Another question referred to pharmaceuticals present in waste water. He explained that currently there is little technology available for removing pharmaceutical compounds, adding that pharmaceuticals were a challenge and microplastics too. He highlighted the fact that rivers have been contaminated with other substances over time – such as compounds banned from use in farms in the 1970s.

Many questions remain

Would the Darent be at threat from future contamination if a treatment works were situated in the vicinity?

The Darent is in the fortunate position of being unaffected by the sewage outflows that are so regularly in the news, polluting rivers and coasts. Would the Darent be at threat from future contamination if a treatment works were situated in the vicinity?

It is true that technological advancements in sewage treatment are delivering cleaner water to be returned to the environment. However, this will not replicate the water that characterises chalk streams and which contributes to the unique ecological characteristics. There are concerns about what can and cannot be stripped out of sewerage, even allowing for the additional safeguard of the treated water flowing through a wetland before entering the river.

We are tantalisingly close to seeing abstraction further limited on the river – something that we have long campaigned for. The sewage plant would overturn these plans, replacing natural flow with treated outflow. While a base flow level for the river would be guaranteed with a sewage treatment plant, risks are introduced with regard to water quality.

On the other hand, a reduction in abstraction would give us “natural” water in the river but we do not know how much, and the river would continue to be at the mercy of weather patterns. Would the reduction in abstraction result in increased flows in the river anything near that from the treatment works? Would there be any mitigation measures if the aquifer levels dropped due to a prolonged drought resulting in diminishing flows in the river?

We will be pursuing answers to questions such as these in order to reach an opinion as to what is in the best long-term interest of the river and its natural environment.