Treated sewage effluent is the wrong solution to low river flow

DRiPS is opposed to Thames Water’s proposal to construct a sewage treatment plant in Sevenoaks discharging treated effluent into the River Darent. In the first of a series of articles, we explain the reasons behind our opposition.

The River Darent is currently one of the very few major rivers in the South East which does not receive either treated effluent or significant sewage overflow discharges. This alone makes the river very precious.

It’s not the only reason it’s precious. The Darent, which flows through the beautiful Darent Valley just outside London, is one of only 200 chalk streams in the world. Chalk streams are largely fed by water from underground aquifers. The cool, nutrient-rich water supports an exceptionally high number of species – so much so that chalk-stream habitats have been described as England’s rainforests.

Thames Water is proposing to place a small sewage works in this remaining relatively pristine and globally important river environment. We understand from Thames Water that the plant is not a solution to the shortage of sewage treatment capacity in this area. It will only treat a population equivalent of 7,000. At a public meeting in 2023 we learnt that this proposal is directed towards a different issue – the quantity of water in the river.

Abstraction by water companies is leaving too little water for the river
The underground aquifers which feed the river are also the source of the water used by Thames and other water companies to supply homes in Kent and London.

The river has long been described as grossly over-abstracted. For example, an independent report from 2021 showed that in dry years more water is taken from the underground wells than is returned through rainfall. The report concludes that sections of the river will inevitably dry out in future – which happened in the 70s and 80s.

Since DRIPS was set up 40 years ago, we have had considerable success in safeguarding the river’s future. Lobbying efforts that followed the dreadful sight of dry riverbeds last century helped secure a multi-million-pound action plan. This included reducing abstraction from some pumping stations in the Darent Valley. The river has not dried up since, but water levels in hot dry summers (such as the one we are experiencing) see river levels and flows drop perilously. Abstraction remains too high.

The agreed solution to low flows should be honoured
A solution to this problem has already been agreed. Two years ago, further abstraction reductions were written into water resource management plans by South East Water, Thames Water and SES Water. This commitment was achieved by dedicated and patient work from the Catchment Partnership, bringing together all the river’s stakeholders. It involves closing abstractions at Horton Kirby and Eynsford.

With the sewage plant proposal, Thames expects to return an equivalent volume of treated effluent into the river. In so doing, it avoids its responsibility to reduce the quantity of water it takes from the landscape. There are issues and threats associated with replacing natural river water with effluent which are particularly pertinent to chalk stream ecosystems. We will address these in future posts.

A sewage plant is not a solution for the long-term health of the River Darent. Water flow issues are best addressed by holding Thames to its agreement to reduce abstraction.