Improving river abstraction efficiency

Improving river abstraction efficiency

Although there are undeniable environmental concerns over river abstraction, it remains a critical method to ensure an adequate and reliable water supply. ABB’s Martin Richardson outlines how using the latest variable speed drive technology when operating pumps can lower energy costs, reduce abstraction demand and improve the environment.

River abstraction is a necessity for supplying water for drinking, farm irrigation, industry and power generation. Yet, abstraction comes with an environmental cost. By changing the natural flow pattern of lakes and rivers, abstraction can alter the amount of water in the environment, which in turn affects wildlife. Improving the efficiency of abstraction can allow water companies to maintain their green credentials, while still ensuring a healthy water supply.

With concerns over the sustainability of current abstraction levels and the licences required, utilities are under increasing pressure to minimise the amount of water they remove. Over-abstraction can have a devasting impact on the environment, as witnessed in the Sonoran Desert in Arizona. Here water is taken for irrigation and urbanisation, which has resulted in land subsidence, water scarcity and loss of wildlife and habitat.

In the UK, bodies such as the Environment Agency (EA) and Ofwat issue licenses and schemes to help control the abstraction rate. An example is Ofwat’s Abstraction Incentive Mechanism (AIM), which encourages water companies to help protect environmentally sensitive sites by restricting abstraction when surface water flows are low. Applying to all water companies from April 2020, AIM has sought to improve the sustainability and resilience of water supply networks.

MAKING PUMPING MORE EFFICIENT

Although pumping systems are used every day by water utilities, they are not always used efficiently. Water is often removed using submersible pumps, which need vast amounts of energy, resulting in a large production of carbon.

The main problem is that many pumps are in situ for years and are used either ‘off’ or ‘on’, with their flow rates controlled by partially throttled valves. This is an inefficient way of controlling flow, as the physical restriction causes increased head loss across the partially closed delivery valve, making the pump work harder and using far more energy than necessary.

INSTALLING VARIABLE SPEED DRIVES

A better method is to fully open the delivery valve and use a variable speed drive (VSD) to control the pump motor’s speed and hence flow. Pump energy use can be cut by up to 50%, while even more energy can be saved using the latest high-efficiency motor technology: such as the synchronous reluctance motor (SynRM).

Although pumping systems are used every day by water utilities, they are not always used efficiently.

For example, Southern Water used a SynRM on a low lift pump that extracts around 25 million litres of water per day from the local river. The highly energyefficient SynRM cut maintenance demands of the pump while improving reliability. The existing 75kW DC motor required maintenance to its brushes every three months, and a recent failure had made it critical to find a more reliable solution. A major consideration for Southern Water was cutting operational and downtime costs as part of the total expenditure (TOTEX) approach adopted by Ofwat’s Asset Management Period. Besides improved reliability, the ABB drive is also expected to produce a 4% increase in motor efficiency.

Ten years ago, the EA’s Shropshire Groundwater Scheme replaced fixed speed pump control with VSDs across seven pump sets, cutting daily energy use by over 990kWh. As well as improving reliability, the pumping costs were reduced by £85 per day, with carbon dioxide emissions falling by half-a-tonne per day.

TACKLING LEAKAGE

As well as making abstraction more energy efficient, there is also scope to use VSDs to reduce the amount of water that needs to be abstracted. This can be achieved by cutting leakage in UK water networks, increasing the security of the country’s supply while cutting the cost and carbon footprint associated with taking water from the environment.

Water companies across the country are making sterling efforts to detect and fix leaks, yet the public often has the impression that nothing can be done on a widespread scale to solve this problem on a long-term basis.

The main culprit in causing leaks is pressure. Keep this at the right level, and many leaks can be prevented. Again, VSDs and their ability to control motor speed come into play. Many pumping stations run the pumps at a steady, high speed until the pressure gets to the desired level. The pumps will then turn off and turn on again when the pressure drops below this set point.

This causes frequent starting and stopping, meaning that more non-return valves need to be activated, resulting in pressure transients that can damage pipe walls and joints. Flow rates can be lowered a lot of the time, and using VSDs to do this can go a long way towards keeping pressure in check. Even a small reduction in pressure of around 20% can cut leakage rates by up to 50%.

Getting the best combination of flow and pressure can be achieved by setting the VSDs to run the pumps at the right speed and ramping up the speed slowly to the desired level. This reduces pressures, avoiding forcing water through existing gaps. A steady speed also cuts the need for frequent starts, and when starts and stops are needed, they are more gentle and less damaging.

Although the public might have a perception that water utilities merely respond to leaks and fix them when they occur, the industry does, in fact, have the means to prevent many of them from happening, cutting costs and making the industry both more resilient and environmentally responsible.

THE FUTURE

Ofwat’s interventions on abstraction are a welcome development, while Defra will also consult about the change in abstraction regulation planned to come into effect in 2023. Yet, as the population grows and the weather becomes increasingly unpredictable, abstraction from lakes, rivers, and other watercourses remains necessary to meet burgeoning demand.

This means that abstraction is here to stay, and therefore so is the challenge of making it as sustainable as possible. Well established technological solutions can help ensure that abstraction is carried out sustainably when water flows are healthy, while also helping to improve the resilience of supply and mitigate against risks such as extreme weather events.

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