The desirability of power cuts

The power utilities use to meet treatment standards is growing, as is political pressure to reduce energy consumption. Chris Webb looks into the problem and some of the solutions

Higher treatment standards required by an intimidating array of EU directives and the imperative of meeting national river quality objectives have forced water companies to invest heavily in new treatment plant.

The CCL costs Severn Trent, Northumbrian and Southern alone an estimated £8.5M. Wessex Water has gone as far as citing the levy as a primary reason for its falling profits. The trouble is, water companies are unable to pass the levy costs on to customers at least until the next price review in 2004.

It is a catch-22 situation in which the water companies argue the levy has siphoned money from their budgets which might otherwise have been spent on measures to improve energy efficiency.

As a result, water companies are increasingly focusing on their energy consumption and potential to generate renewable power. However, they face a significant challenge if they are to offset their soaring energy use.

According to some estimates it takes .5kWh of electricity to deliver every m3 of drinking water. In total, some calculate, the water industry consumes more than 6,000GWh/pa. On top of this, the increasing volume of sewage treated means greater sludge generation, with an estimated 36% increase

forecast between 2000 and 2005. There is pressure to treat sludge disposal in a holistic way. But, while digestion can generate useful amounts of methane biogas – classed as renewable energy under the renewables obligation – incineration, drying or transport of liquid sludges are very energy intensive.

In recent years, even the preferred option of sludge disposal to agricultural land close to sludge treatment centres, arguably the most favourable option in terms of energy efficiency, has conflicted with plans to revise the EU directive on sludge disposal which is likely to severely restrict this route. Moreover, tighter regulations on metals in soils receiving sludges may increase the need to transport sludge long distances, making the practice more energy intensive and less economic.

The majority of water companies are now unwilling to go down the sludge-to-land route and are looking to expand their options, even if it means greater expense.

Anglian Water, for example, has had to fit secondary treatment and UV disinfection to a large number of its coastal discharge facilities. The result has been a steep rise in energy use at those sites, which shows no signs of slowing.

Many other water companies are experiencing similar problems. Meanwhile the Department for the Environment, Food and Rural Affairs (DEFRA) maintains that full treatment is necessary for all of what it calls ‘significant’ marine discharges. But, what are the main areas in which water companies could achieve energy savings?

Pumping, it is generally acknowledged, accounts for up to 90% of water companies’ energy use, and here there is an interesting anomaly. Wastewater pumps are less efficient than clean water ones because they need higher tolerances to cope with solids. However, WwTWs commonly use wastewater pumps for tasks other than pumping wastewater.

To reduce pumping costs water companies are attempting to make the most of the high level of connectivity between sources, often using computer-based modelling to optimise the costs of transportation, selecting the lowest cost solution week-by-week. Yorkshire Water, which operates a Water Resources Allocation Plan (WRAP), is a case in point. Anglian Water is another company to have trialled the use of WRAP.

Improving and maintaining the efficiency of pumps is a key starting point for companies’ energy efficiency initiatives. When pump motors are replaced, installing high-efficiency models may achieve up to a 5% saving in energy use, according to manufacturers.

Different regions of the UK have contrasting characteristics, which can be exploited improve energy and economic efficiency. The flat nature of Anglian’s region requires different energy management strategies to the hilly terrain encountered by Yorkshire Water. Anglian achieves savings through pump optimisation and ‘off-peak’ pumping rather than maximising the use of gravity feed from the high Pennine resources used, when available, in Yorkshire.

Anglian also seeks to save energy by using water sources that require less complex treatment. For example the company uses chalk groundwater, where possible, in preference to raw water pumped storage reservoirs at Covenham, near the north Lincolnshire seaside resort of Cleethorpes, and in Alton, near Ipswich, east Suffolk. But even these ‘conjunctive-use’ systems are limited by the investment needed in the alternative resources.

Membrane treatment processes are very useful in addressing the latest water treatment challenges, for example in the eradication of Cryptosporidium and nitrate. However, they are also characterised by high energy consumption. The two applications use very different types of membrane. For Crypto barrier applications microfiltration or ultrafiltration are used. These are low pressure systems operating at between 0.5-2bar. Running costs are similar to other filtration processes.

Nitrate removal and softening requires reverse osmosis, a high-pressure technology operating at between 8-15bar. In the mid to late 1990s Anglian Water prided itself as a pioneer of the ultra-low pressure range of membranes now marketed by Koch. These membranes can operate at pressures as low as 5-6bar saving energy and, it is claimed, reduce operating costs.

The Generation game
The water companies are also showing an interest in generating their own low-carbon electricity, a move spurred by incentives under the Renewables Obligation (RO) and Enhanced Capital Allowances (ECAs). Wessex Water, for example, is refurbishing all of its digestion processes to maximise biogas generation. Currently the company generates some 10% of its annual power requirement from biogas. It has also recently installed 6MW of new biogas generation capacity based on modern Caterpillar engines, which are claimed to be 36% efficient, in calorific terms, compared to the old duel fuel engines which are only 29% efficient.

Earlier this year there were reports that Severn Trent was considering up to 15 new combined heat and power (CHP) plants in the 100kW to 7.5MW range, but so far these have not been confirmed by the company. Thames says it is considering new CHP plant but has made no firm decisions. The company is currently refurbishing its Long Reach biogas CHP plant after a fire.

Methane biogas from anaerobic sludge digestion is the principle source of renewable energy available to the water companies. Other sources are sludge incineration and, in the case of Northumbrian, hydroelectric power. Northumbrian Water is unusual in having some 6MW of hydroelectric capacity, this is now being recognised for its significant contribution to the company’s sustainability credentials.

Companies such as Thames and Severn Trent generate a significant proportion of their electricity needs in-house. In practice, however, it is often more commercially attractive for them to sell this on to electricity suppliers.

ECAs and exemption of CHP from the CCL have had a significant effect on the economics of such schemes. Water companies can secure Renewables Obligation Certificates (ROCs) worth 3p/kWh for biogas power, and can also claim enhanced capital allowances on the investment against Corporation Tax. Electricity sales from CHP plants are also exempt from the CCL. The impact of these changes is that new biogas CHP plant may pay back within two or three years, according to analysts.

One technology that holds some promise for the future is anaerobic wastewater treatment. This has been used to treat wastewaters from the food industry for some time. The process would be an additional stage in sewage treatment, but could produce large volumes of methane, which in turn could be burned for energy recovery. In his PhD thesis, with others, Paul Zakkour of Imperial College argued that anaerobic treatment could deliver significant energy savings. His models, in collaboration with Severn Trent, suggested the process could reduce CO2 emissions from sewage treatment by 16%. Water industry sources are more sceptical and see no huge benefits.

Another possible energy-saving technology showing early promise is ultrasonic treatment. With Purac’s Sonix system (WWT February, 2002), which has undergone trials at Wessex’s Avonmouth WwTW, sludge is passed through an ultrasonic reactor to break down cell walls and increase digestability. The process is particularly aimed at secondary sludges from activated sludge tanks. Sonix claims an increase in methane generation of up to 55% and an additional 33% breakdown of sludge solids.

Undoubtedly, water companies face an increasing challenge to cut down on the energy they consume. The extent the regulator, Ofwat, can intervene to ensure that they toe the line is debatable. The level to which Ofwat will let companies reap the benefits of their energy savings is equally uncertain. Ofwat says its approach to regulating the water industry is based on incentives to provide better service and reduce costs. In other words, when it sets price limits it assumes that companies can improve efficiency at least at the same rate as similar companies operating outside a monopoly environment. It expects that some of these efficiencies will arise from energy efficiency.

According to Ofwat spokesman Peter Mandich: “In a competitive environment these efficiencies are eventually passed onto customers as companies strive to retain their market share by reducing costs. The more innovative the efficiency measure the longer it will be kept. Similarly, in the water industry, companies that manage to outperform our price limit assumptions on efficiency get to keep such savings for five years before they are passed on to customers. In this way there is always a strong incentive to reduce energy costs.”

What this means for water companies is that they are allowed to retain any potential energy – and therefore cost – savings, for five years before they are passed back to custo-mers through new price limits. This arrangement, however, is under review.

In the end, however, the forthcoming Water Bill will impose a statutory duty on Ofwat to further promote sustainable development, and that just might give it the teeth it needs to make water companies more energy efficient


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