Decontaminating streams and rivers: the way ahead

Ancient Greek philosopher Heraclitus famously wrote that “No man ever steps in the same river twice,” back at a time when water sanitation was not really an issue. Nowadays, it is now more a question of whether it is safe to bathe in water occasionally... In addition to offering people living in large cities the chance to take a refreshing open-air swim during the summer and any heatwaves, rivers and streams do of course provide us with some vital services.

Ecological and economic functions of watercourses

Watercourses are of course essential for socio-economic development, and in meeting the need for food and comfort of the general population. According to WWF estimations, two billion of the world’s population obtain drinking water from watercourses, whilst the livelihood of some 60 million people relies upon the fishing capacities provided by rivers and streams. From an environmental point of view, watercourses are absolutely essential. They are reservoirs of biodiversity and provide natural refuges for wildlife and plants. Essential links in ecological continuity, they are also a vital factor in maintaining life in forests and wetlands, and provide a means of replenishing groundwater levels. Lastly, streams and rivers also contribute to enhancing the landscape, and the well-being of local residents.

As all these functions may be affected and diminished by pollution, restoring and/or maintaining watercourses in good condition is therefore of vital importance. Whilst watercourses possess a certain degree of resilience through a capacity for self-purification, which enables them to eliminate some substances that can adversely affect functioning, this capacity remains limited.

Multi-faceted pollution 

So, what kinds of pollution can affect water quality? Pollution tends to have either a microbiological or chemical dimension (regardless of whether it occurs naturally or, like climate change, is related to human activity). 

In the first instance, we are essentially talking about organic or bacteriological pollution, typically associated with pathogenic micro-organisms and stemming predominantly from human or animal excrement, household refuse and plant/vegetable waste.

Meanwhile, chemical contamination is caused by the introduction of various pollutants such as fertilisers, pesticides and other products used by industry, as well as medicines.

How can we adapt and develop infrastructure to restore bacteriological balance in rivers and streams? 

To avoid or limit bacteriological pollution in water, it is essential to reduce the substances – or their potential capacity to pollute – discharged, especially in the form of wastewater. In areas covered by municipal sanitation systems, wastewater is collected in mains sewerage drains, then transferred to and treated at wastewater treatment plants, before then being discharged

Group subsidiaries VINCI Construction Grands Projets and Sogea Environnement have a recognised expertise in this field.

However, sewers and wastewater treatment plants only have a limited capacity and can end up becoming overloaded during periods of heavy rainfall, sometimes leading to untreated wastewater overflowing into watercourses. To prevent the risk of this occurring, temporary water storage structures are built in towns and cities – especially in major cities.

Given their location in restricted environments or their sheer scale, certain structures are genuinely exceptional feats of civil engineering. For example:

• The Austerlitz underground reservoir in Paris: located next to Austerlitz train station, this exceptional structure (50m in diameter with a capacity of 46,000 cubic metres) was delivered just before the 2024 Paris Olympic and Paralympic Games. Built by a consortium including VINCI Construction, the basin is designed to improve the long-term water quality of the Seine. It will do this by temporarily storing surplus water produced by periods of heavy rainfall, before then redirecting it into the wastewater treatment system. 

• The Thames Tideway Tunnel in London:the British capital’s wastewater treatment system was designed 160 years ago to meet the needs of four million inhabitants... compared with a population that is now close to ten million. With periods of heavy rainfall frequently leading to saturation of the system, it was decided to build a 7.2m diameter tunnel in order to temporarily store surplus wastewater and rainwater, whenever necessary. The 25km-long tunnel, which is buried at a depth of between 30m and 65m under the Thames, was built (one of the three work packages) by a Joint Venture including VINCI Construction Grands Projets, Bachy Soletanche Ltd and Costain. All works on the tunnel are expected to be completed by 2025

The need to combat all forms of water pollution 

These are just some of the solutions deployed to limit the declining quality of watercourses. Others are implemented depending on the situation and the needs identified locally, such as reed bed filter treatment plants – a wastewater treatment system that uses the natural purification properties of reeds.

Lastly, bacteriological decontamination is clearly a major challenge, but it is by no means the only one. Plastic pollution is another huge water conservation issue.