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E is for energy

Rising energy prices, growing demand for water, Malcolm Brandt and Roger Middleton of the engineering consultancy Black & Veatch ask where utilities can get the best efficiencies

At Glencorse Water Treatment Works a hydro turbine will produce up to one-third of the work’s onsite power requirementsAt Glencorse Water Treatment Works a hydro turbine will produce up to one-third of the work’s onsite power requirements

After manpower, energy is the second highest operating cost for most water and wastewater companies. Population growth, urban migration and increased affluence all intensify the demand for drinking water and on effluent disposal facilities.

Furthermore, new technologies have been adopted to meet increasingly strict drinking water and effluent quality standards. As a consequence the water sector’s energy consumption has increased considerably during the last decade. The result is that high energy consumption, which is inextricably linked to the issue of carbon emissions, will affect the water industry worldwide for the foreseeable future. It is important, therefore, to minimise the use of energy by optimising efficiency across the water cycle.

This article builds upon some of the principal findings of an UK Water Industry Research (UKWIR) managed project to develop a compendium of best practice and technologies for the energy efficient design and operation of water industry assets. The project, supported by Global Water Research Coalition (GWRC) partners worldwide, looked at the whole water cycle from abstraction to discharge. The report also reviewed energy recovery technologies incorporated in water company assets.

Energy requirement

For European countries the energy required for water treatment and supply – plus wastewater carriage and treatment – accounts for 1%-3% of national energy consumption. For the UK the figure is 2% of national energy consumption.
In the USA the percentage of national energy consumption used in the provision of water services is greater, 3%-4%, as a result of higher levels of demand for water. For example, domestic consumption per capita for England and Wales (2005) was 151l per capita per day (lcd); compared with South California (2004) 550 lcd, or New Jersey (2004) 284 lcd.

An important, implicit conclusion that can be drawn from these figures is that reductions in water demand will have a direct and substantial impact upon the energy consumed by the water sector. Any reduction in demand will reduce the volumes to be processed in the whole water cycle and thereby energy demand.

Recognising which activities within the water cycle are the most energy intensive is important when identifying how energy consumption can be best reduced. Figure 1 shows the pattern of energy use in the water cycle in the UK, based upon data from water companies’ 2009 annual return to Ofwat. The data reflects water companies’ expenditure on energy for various elements of the water cycle.

Cost of pumping

The information Figure 1 provides is aligned closely to energy consumption data from other sources across the world, which shows that pumping represents upwards of 80% of drinking water energy demand and at least 30% for wastewater. For wastewater services, the major energy demand is from aeration, which accounts for up to 60% or more of consumption.

In addition to illustrating energy consumption, Figure 1 also shows the potential energy savings for each element of the water cycle through interventions such as more efficient pumping and aeration equipment or management.

The graph also suggests which activities have the most potential to act as a source of renewable energy. Here is a further summary of the energy efficiency opportunities evidenced in the report – the reductions are shown as percentages:

  • Conservation/water loss reduction: 5% - 10%. The potential for reduction is greater where utilities are resource constrained
  • Existing pumps – 5-10%
  • Pump technology improvements: 3-7%
  • Clean water processes – up to 20%, but low usage
  • Activated sludge process – up to 25%
  • Building services – up to 15%
  • Renewable energy: combined heat and power (CHP) engines from biogas can contribute significantly to the net energy demand of the water industry
  • Utilities that only abstract, treat and distribute drinking water have limited opportunities to generate renewable energy – hydro turbines

As they seek measures that will help reduce energy consumption water companies will also face challenges that will, potentially, have the opposite effect. As has already been shown cutting demand will have a very positive impact upon energy consumption.

Demand

There are, however, factors that point to increased rather than reduced demand. The world’s population is growing and with this growth comes the need for more water.
The pressure on the water sector’s energy requirement is twofold: more drinking water is used, and more wastewater requires treatment. In addition consumption per head is rising, both across the developed world and in areas of rapidly increasing personal affluence such as India and China.

There is also little sign that the trend towards more exacting water quality standards will cease. Technology continues to expand our ability to analyse water quality; and experience suggests more detailed analysis results in more proscriptive standards. Meeting these standards requires more treatment steps and more energy-intensive technologies.

If regulation has the side effect of increasing energy consumption stakeholders need to ensure that the standards being imposed offer substantive benefits. Regulators should be able to demonstrate that the public health or environmental improvements they seek are proportionate to any adverse environmental impact resulting from increased use of energy and greater greenhouse gas emissions.

The use of orthophosphate to manage plumbosolvency provides an example of the tensions that can arise. Orthophosphate dosing has helped reduce levels of plumbosolvency. However, this treatment step also increases the level of phosphorus that needs to be removed during the treatment of wastewater.

Affordability

The affordability of energy will drive efficiency improvements in the future. For asset planners, the design priorities should be linked not with current energy prices but with estimates of future energy costs. An asset with a 50-year life should be assessed against energy cost estimates for, say, 25 years ahead.

Capital costs are less than 10% and energy costs are already more than 80% of the whole life cost of plant; with higher energy costs this balance should be a major consideration for investment. Higher future costs will also raise the operational profile to maintain plant at best efficiency condition.

The relationship between water and energy demands is linear. Consequently, as has been previously stated, one of the best ways of reducing the water industry’s energy requirements is to reduce the amount of water put into supply.

Conservation will reduce all components of the water and energy cycle. Demand reduction, however, cannot be achieved by water companies in isolation.

There are many other energy conservation measures, however, over which water companies have a much greater level of influence. Increased use of natural pre-treatment is seen as a way of reducing the energy consumption of the overall water treatment process. Allowing a greater level of material to settle out of the raw water before it enters the main treatment process cuts the energy requirements of subsequent elements of the treatment train.

Optimising existing treatment processes is another area which has demonstrated significant potential to reduce energy demands. The changes need not require major capital investment and often focus on operations-led adjustments to process management.

“Through energy recovery, the water cycle offers water companies significant potential to reduce the amount of energy they procure from suppliers”

Two case studies in the compendium demonstrate this:

  • Fixed-speed borehole submersible, duty/standby pumps were changed to small variable-speed units, running in parallel resulting in reduced borehole drawdown compared with the single fixed-speed pump arrangement. The payback period was 33 months; energy was saved and the carbon footprint reduced.
  • In secondary aeration process lanes, the inlet penstock controls were re-calibrated to ensure equal flow split and ammonium control was used to regulate the dissolved oxygen input in the last pocket of each lane delivering a 30% reduction in energy use.

Through energy recovery, the water cycle offers water companies significant potential to reduce the amount of energy they procure from third-party suppliers. Energy recovered from advanced sludge treatment, for example, can be used to feed the treatment process using CHP engines. This has been achieved at a number of locations including Anglian Water’s Cotton Valley sludge treatment centre.

The opportunity to install hydro turbines exists at any point in the hydraulic gradient where energy has to be dissipated for operational reasons. For example at the head of a treatment process to reduce raw water transmission main or wastewater collection system terminal pressures; within distribution for pressure management; and at the end of effluent discharge pipes.

At Scottish Water’s new Glencorse Water Treatment Works the energy of raw water flows is being harnessed by a hydro turbine to produce up to one-third of the works’ onsite power requirements.

Although there are many steps water companies can take to reduce their energy consumption, their ability to bring about meaningful change needs to be viewed in context. International estimates of the energy used for domestic hot water are reported to be between 10 and 20% of nation’s energy consumption compared with between 1% and 4% for the municipal water cycle.

It has also been suggested that reducing the domestic hot water temperature by 20°C will have more impact on carbon emissions than can be delivered by energy efficiency measures. Both are relevant. Conservation and energy reduction are future imperatives within the whole water cycle whatever the commercial or stakeholder pressures.

Topic: Energy/Water Nexus , Sustainability & social value
Tags: water treatment , effluent

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