ABB shows how – not least via energy audits – UK utilities are assessing aeration, pump and other motor-driven equipment performance to improve costs and efficiency as energy costs continue to rise.
When it comes to their impact on the environment, Britain’s water companies are most often in the spotlight for their performance in the area of water quality. Along with other areas of industry, however, the water operators are also major energy users and are making big efforts to keep energy consumption under control, says Tony Hoyle of ABB.
Southern Water was one of just 20 organisations this year to be recognised under the Energy Efficiency Accreditation Scheme, run by the National Energy Foundation. Other operators have much further to go, but everyone in the industry is acutely aware of growing pressure to offset the rising cost of energy.
A survey by the Engineering Employers’ Federation (EEF) in 2005 found that continuing fuel price escalation is posing a real threat to competitiveness of UK industry in general. According to the EEF, 93% of companies surveyed experienced an increase in energy prices over the previous 12 months, with average weighted increases in gas and electricity of 47% and 34% respectively. These rates are more than double the level of increases reported in 2004 – and the water industry is feeling the pinch, just like everyone else.
This has led to increased awareness of the importance of measuring and controlling energy consumption wherever possible. That requires assessing not just obvious areas such as pumping throughout the distribution network, but also efficiency of individual treatment processes, which – if managed effectively – can make significant contributions to overall energy savings.
Aerobic Exercise
A good starting point for energy savings is the aeration plant. Pumping air to aeration tanks can account for over 70% of the energy consumption on a typical sewage treatment plant. Regulated dissolved oxygen (DO) levels optimise the rate of tank aeration and reduce pumping requirements. They also provide a stable environment for microorganisms that process the wastewater, which reduces sludge production and chemical usage.
Aeration can be achieved by mechanical agitation or by use of diffusers to feed air bubbles into the mass of sewage. The object is to keep DO levels just above the critical 2 mg/L-l needed to keep microorganisms working at their peak. Raising oxygen levels beyond this wastes energy, so keeping a close check on these levels using DO analysis equipment is a vital part of any energy efficiency initiative. Information from this equipment can then be used to control variable speed drives (VSDs) on motors regulating the aeration tank air feed pumps or to control the speed of an agitator.
Where the oxygen source is compressed air, over-aeration can prove even more expensive. This is because compressors turn around 90% of the energy they use into waste heat, which effectively makes compressed air 10 times as expensive as electricity. Keeping track of compressed air usage is thus paramount to controlling energy consumption. One way to measure this consumption is to use thermal mass meters. These meters directly measure mass flow, making them cheaper and easier to use than volumetric alternatives. They also offer a turndown ratio of 150:1 for excellent measurement accuracy. Alternatively, where installation space is at a premium, swirl meters may provide the best solution, such as at United Utilities’ Biddulph water treatment works, where they helped to save up to £10,000 on installation costs of two new aeration lanes.
Raising Quality
Significant regional differences in water resources and treatment requirements mean some companies are finding it difficult to bring overall energy consumption down.
In the case of Wessex Water, for instance, energy consumption increased as the company has increased sewage treatment to meet its environmental obligations. Recent examples include introduction of mandatory secondary treatment at Avonmouth and disinfection at various coastal locations. Similarly, electricity consumption at Thames Water rose by 2.7% in 2004 to reach 1240 GWh. Again, the company attributes the increase to introduction of more energy-intensive treatment to meet higher quality standards.
But the trick all the water companies need to master is an ability to act simultaneously on both fronts, aiming for improvements in both water quality and operating efficiency. Though such improvements, by themselves, will often be incremental, when taken together they can actually widen the scope of energy cost savings.
One example is using UV monitoring equipment to control and regulate raw water nitrate levels. Nitrate reduction is primarily achieved by blending a high nitrate source of raw water with a low nitrate source to bring the overall nitrate concentration down to an acceptable level. Where a low nitrate source isn’t readily available, it may be necessary to instead use water treated either by reverse osmosis or ion exchange processes, both of which require large amounts of energy. Capable of providing much greater accuracy than conventional monitoring methods, UV monitors can help optimise the level of treatment and blend ratios required – minimising both treatment costs and energy consumption.
Driving Success
Possibly the biggest potential for energy savings throughout any water network lies in the drives that control pumping operations throughout the network. Typically, reducing speed of a pump from 100% to 80% can cut energy consumption by up to 50%. But every kW of energy consumption saved has a number of benefits, including operational cost, maintenance and environmental impacts. The best way to realise these potential savings is to use the latest VSD technology. Many of today’s working drives were installed some 15 years ago. A modern drive will cause substantially lower energy losses in a motor than an older unit, so the whole system needs to be considered.
For Severn Trent Water, replacing three, 37 kW AC drives on water process pumps at its works in Wanlip, Leicestershire, with the latest technology alternatives helped achieve a 65% energy saving and a payback of only seven months. In another case, the company saved thousands of pounds in energy and maintenance costs by replacing two drives from the 1980s with new models at its Ladywood sewage pumping station in Ironbridge, Shropshire. The combined savings mean the new drives will pay for themselves in well under two years.
Successes such as these have persuaded Severn Trent Water to set up an Energy Saving Project Group to look at replacing and upgrading existing drives and installing new ones throughout its network of sewage treatment plants. In addition, in 2004/2005 the company completed a three-year programme to test pumps and replace or refurbish those found to be inefficient. The company is following this up in 2005/2006 with a review of around 400 of its largest energy-consuming sites. Taken together, such initiatives played a significant part in helping Severn Trent to improve its energy efficiency by nearly 4% during 2004/05, cutting its total energy use from 1.139 million kWh to 1.083 million kWh in 2004/2005.
The greater part of electricity used in industry, about two-thirds, is used by electric motors. This seems like a staggeringly high share, but the ubiquitous low voltage AC motor is fit into virtually anything that moves. In the UK alone, some 10 million motors are used, consuming £6.6 billion worth of electricity a year. Many of these run far less efficiently than they could, so there’s vast potential for savings.
With hundreds of motors used throughout the UK water network, taking steps to manage motor equipment performance is thus also essential in any attempts to save energy. One water company cut its motor electricity costs by 6% following introduction of a motor management policy. Motor usage accounts for a large proportion of the company’s yearly £25 million electricity bill. By replacing its existing motors with high efficiency units, the company expects an energy saving of about 24,000 kWh/year, with a return on investment of less than two years.
However efficient your motors, drives and pumps may be, it’s in the nature of the water business that the lion’s share of energy is used in simply pumping the water, wastewater and sewage around the network. It’s also universally true you can’t control what you can’t measure, so it follows that accurate flowmeters are a type of instrument water companies should be committing a lot more investment toward if they seriously intend to keep pumping costs down. After all, why waste energy pumping water out of leaky pipes? Leakage reduction and control initiatives are often only considered as water saving measures or as a response to public pressures, yet there’s a substantial energy saving to be made by reducing leakage. Even if networks are gravity fed, water will have been pumped somewhere in the cycle.
Pressure is being applied to water companies by Ofwat to more accurately identify and address leaks throughout their supply networks. This has already led some to employ new technologies providing more accurate monitoring and featuring additional capabilities, such as remote GSM communication, which enable leaks and other problems to be immediately traced and rectified. “Electronic” flowmeters that work on the magnetic, ultrasonic and other principles and incorporate electrical, rather than mechanical sensing equipment, have been around in various guises for some time. Yet they’ve generally been slow to be adopted by the water industry, largely because they cost more up front than mechanical metering equipment.
Still, in most cases, payback on the installation of an electronic flowmeter could actually be achieved in a matter of days. Furthermore, with no mechanical moving parts that can be subject to wear or that will need replacement, electronic flowmeters not only offer constant accuracy throughout their service life, but also eliminate the cost, time and disruption associated with maintenance.
Conclusion
Water and waste treatment operators face significant challenges. For some, increasing demands make it difficult to keep energy costs under control. As with the rest of industry, however, rising prices mean that it’s more important than ever to get to grips with energy.
With offices and facilities worldwide, ABB Ltd. is based in Zurich, Switzerland. Contact: www.abb.com/motors&drives
Energy Management
The starting point for a successful energy saving plan is to determine just where energy can be saved. Using instrumentation to monitor the efficiency of the processes outlined in this article is one way to do this. Where pumps, motors and drives are concerned, the best course of action is to carry out a regular energy audit. Typically, such audits should look at all aspects of operation, including the condition, age, efficiency, performance, usage habits and other parameters of installed equipment. This data can then be used to pinpoint potential areas for improvement.
Information from an energy audit also can be useful to assess the potential payback achievable by adopting the latest technology. This is particularly important as operators increasingly begin to embrace the idea of total cost of ownership, which takes into account all costs incurred throughout the life of a flowmeter instead of just its purchase price. One reason for this is a growing trend toward companies outsourcing responsibility for both supplying and running equipment to contractors, who must ensure they opt for the solutions offering best value for money over the long term. For example, replacing an average 1980s motor with a state-of-the-art high efficiency alternative could deliver a potential payback period due to lower energy consumption of 12 to 36 months, depending on factors such as annual operating hours, energy price and drive type being used.
An added incentive to investing in energy efficient equipment is availability of Enhanced Capital Allowances (ECAs). Introduced by the UK Treasury to encourage industry to find ways to cut energy consumption, ECAs are essentially a discount on energy efficient equipment. These enable companies to deduct the full cost of the investment against corporate taxes in the purchase year.
Compared to the usual capital allowances available, this has three advantages. First, companies can claim the full amount. Writing off equipment the traditional way won’t give full tax relief, as companies write off a percentage of a diminishing balance each year but never quite get down to zero. ECA saves capital cost by giving 100% tax relief. Second, it increases value of the tax benefit as this is calculated on this 100%. Third, it improves cash flow as the benefit is gained in the first year rather than spread over a number of years. This money stays in the recipients’ account and earns interest or is gainfully employed in their business. Depending on their tax rate and return on investment, the ECA is equivalent to a rebate of 6-15%. This will reduce or even eliminate the price differential to less efficient products and shorten the payback time.
The ECAs are applicable to a list of specific items that includes CHP (combined heat and power), boilers, motors, VSDs, lighting, refrigeration, pipe insulation materials and thermal screens, which meet the relevant energy efficiency criteria. Flowmetering equipment is also included on the Water Technology List, which is part of the ECA scheme and is aimed at promoting better control of water quality and consumption throughout industry: www.eca-water.gov.uk.
A site-wide energy audit may also assess fuel sources being used and propose alternatives where appropriate. Some audits even address the possibility of installing renewable energy technologies, such as solar panels and wind turbines, or the use of biomass, which can be especially relevant in sewage treatment plants.
Auditors also consider financial benefits in addition to actual energy savings. For example, the auditor may suggest operational changes that could potentially change the tariff paid by the company. This may be as simple as not using certain non-essential equipment during periods of peak demand. Depending on the energy pricing structure, implementing such changes can result in significant economic savings.
