Keith Hayward looks across the water utility energy landscape at some recent developments and technologies.
The motivations behind the efforts of an increasing number of water utilities to drive down their energy use vary across the world, covering cuts in overall energy use and in reliance on fossil fuels. One driver is simply the cost of energy, which is coming under increasing scrutiny as utilities look to a greater extent at the whole life costs associated with operating their assets. There are other drivers too.
One of the most ambitious current moves in Europe is being taken by municipal company Aarhus Vand, which serves some 300,000 customers in the Danish city of Aarhus. It is aiming for its Egaa wastewater plant to produce 50% more energy than it uses, as part of a goal for the utility of being energy positive and having a carbon footprint of zero by 2030. The utility set itself this goal after the city as a whole set a target of getting its carbon footprint in balance by 2030. (June 2016, p13)
Meanwhile, in the US, municipal wastewater treatment plants are now referred to as water resource recovery facilities (WRRFs), reflecting a shift in outlook on their role. The Water Environment Federation states on its website that WRRFs can account for 30-40% of total energy consumption for local governments, adding: ‘Many WRRFs have begun steering away from their dependency on foreign oil by researching wind, solar, and hydroelectric power, as sustainable solutions are needed quickly.’
Whatever the motivation, it is abundantly clear that there are already huge opportunities for driving down this energy use, with greater opportunities on the way.
Renewable energy sources
Renewables are transforming the energy supply landscape. Like all energy users, water utilities will increasingly have the opportunity to turn to green energy suppliers. They can also exploit renewable technologies directly themselves, as many utilities already do, with the prospect of benefitting from the higher efficiencies and lower costs that are coming thanks to progress with the technologies and larger volume production of components.
Not only do water utilities need power, they also tend to have space available, making solar photovoltaic electricity generation an appealing option. Impressive roof-mounted installations have for some time been a feature at a number of utilities in the US, for example. Developments in the technology continue to progress, including the emergence of use of new materials such as graphene.
One solar option that has taken off to a considerable extent in Japan, and which is starting to be picked up elsewhere, is the use of floating solar systems. Deployed on a reservoir, these can for example be used to meet the energy needs for pumping. (February 2017, p14-15) Recent projects in Europe have included the 12,000 solar PV panel system for the UK’s United Utilities and the 23,000 panel system for Thames Water on the Queen Elizabeth II reservoir near London. The latter is a 6338 kWp (kilowatt-peak) installation. These were provided by Floating Solar UK, a venture involving French company Ciel et Terre, supplier of the Hydrelio system for deploying solar panels.
Japan’s lead on the use of floating solar is reflected in the fact that most of Ciel et Terre’s installations are located there. A 13,744 kWp installation is due to open at Japan’s Yamakura dam in spring next year. Growing use elsewhere includes a large installation due to be completed this year in Pei County, China.
The potential of solar power is perhaps most prominent in the emergence of solar desalination. Sisyan LLC, working in Baja California Sur, Mexico, notes on its website: ‘In the last six years, solar panel prices have dropped 75% to around $0.50 / watt. They are projected by SunEdison to fall under $0.25 / watt by 2030.’ The company says that to date its pilot projects have desalinated some 22 million gallons.
Utilities can also make direct use of other green power sources. This includes use of wind turbines and of various forms of hydropower. Together, they can deliver substantial reductions in carbon emissions.
As an example, the Ebswien utility operating the 680,000m3/day wastewater plant serving Vienna, Austria is currently undertaking an ambitious overhaul of the plant. (February 2016, p19-21) This follows on from a renewable energy project begun in 2006. Measures in this renewables project included installing a Kaplan turbine at the outlet of the treatment plant, producing up to 1.3 million kWh a year. A hydrodynamic screw was added upstream of this, adding around 0.5 million kWh a year. Solar, solar thermal, and wind power systems were added. Together the renewables replaced the traditional sources for 11% of the plant’s 63GWh/year power needs.
Renewables can also be used on the water supply side, including exploiting hydropower opportunities in supply networks. For example, where water utilities need to manage the pressure in their supply networks, they can do so by using pressure reducing valves. These work by dissipating energy. An alternative approach is to use technologies that reduce the pressure and generate electricity at the same time, such as the Difgen system offered by Zeropex.
More than anything else, water utilities are in the business of moving water around, whether that be clean or dirty. That puts pumping right at the heart of what they do, meaning it deserves a special mention in relation to efforts of water utilities to reduce the energy they use.
Recent responses from major pump manufacturers in this respect include Xylem’s launch last year (June 2016, p24) of its Flygt Concertor intelligent wastewater pumping system and its Flygt FGC400 wastewater pump controller. The Concertor operates in 2.2 to 7.3 kW pump range and the company claims that savings of up to 70% compared to conventional pumping systems can be achieved. The company also announced last month that the first North American installation of the Concerto intelligent wastewater pumping system had been installed for DC Water in the US.
This gives a hint as to the potential in this area. The launch from Xylem came not long after it published a report on the gains that could be achieved from implementing improved pumping and other mechanical systems. The report, ‘Powering the wastewater renaissance: energy efficiency and emissions reduction in wastewater treatment’, looked at the gains that could be achieved from using technologies such as high efficiency pumping, variable speed pumping, variable speed blowers, high efficiency mixing, and optimised control systems. The research suggested that emissions associated with electricity use in wastewater treatment could be cut by around half by using readily available high efficiency technologies, and that 95% of these reductions could be achieved at zero or even negative costs. Looked at from a global perspective, the report found this could deliver a total yield of $40 billion in net present yield. (February 2016, p17-18)
Water supply and energy
The preceding aspects of water and energy, covering use of renewables and efficient pumping, are relevant to utility water supply activities. There are other issues of interest too for this side of the sector, not least the very fundamental point that energy use relates directly to water use. In other words, reduction in water use can contribute to a reduction in energy use. The ever-growing pressures on water resources means that water utilities are increasingly turning to demand management measures to reduce the demand for water. So these measures can at the same time contribute to a reduction in the utility’s energy needs, and in fact it feeds through also to the wastewater side of the sector in terms of the volume of wastewater arriving at treatment plants. And it is not just about reducing the demand of customers. The high levels of leakage in most water supply networks represent an energy burden in terms of treating and pumping water that does not reach its desired destination. Fixing leaks therefore feeds back to a utility’s energy balance.
Another aspect of this picture is that not all of a utility’s greenhouse gas emissions stem directly from fossil fuel use. For example, groundwater can contain methane, which is released to the atmosphere following abstraction before the water is supplied. Methane is a more potent greenhouse gas than carbon dioxide, in fact having around 25 times the global warming potential. Dutch water utility Vitens operates one of Europe’s largest groundwater treatment plants – Spannenburg. Working with what is now Royal HaskoningDHV, it developed and implemented a methane recovery scheme to reduce its carbon footprint by recovering and partially reusing some 1000 tonnes of methane annually, saving energy equivalent to 1250 homes.
Energy requirements clearly also depend upon the demands of whatever treatment process is used. It is very much energy costs that ultimately determine the viability of deploying a particular technology. This has been one of the key issues around the use of membranes, not least their use in desalination.
The solar desalination system of Sisyan, for example, makes use of solar power, but at the same time it brings together energy efficient components on the process side. According to the company, it combines: membrane-based ultrafiltration pretreatment; variable frequency drives; axial piston pumps; axial piston motors; and reverse osmosis.
Research into this water sector quest for energy-efficient desalination continues. One of the latest efforts is the MIDES (microbial desalination for low energy drinking water) research project being funded under the EU’s Horizon 2020 programme. Partners of the 48-month project, running from 2016 to 2020, include Aqualia and Fujifilm. It aims to develop the world’s largest demonstration of microbial desalination cells (MDCs) as a pretreatment for reverse osmosis, with demonstration sites planned for Chile, Spain and Tunisia. According to the project website, all current reverse osmosis desalination units consume electric energy of at least 3kWh/m3. MDCs treat wastewater and generate electricity for desalination at the same time. The site says MDCs can produce approximately 1.8kWh of bioelectricity for each cubic metre of wastewater treated. This can remove the salt in seawater without additional energy input, or reduce the salinity to lower the amount of energy required to complete the desalination process.
Wastewater and energy
As with water supply, the use of renewables and of efficient pumping are both relevant to utility wastewater treatment activities. The wider energy opportunities for wastewater are based very much on the emergence of new treatment technologies and on the heat and energy content of the wastewater itself. This latter aspect is hardly new – the potential to use anaerobic digestion to produce biogas from sewage has a long history in the industry. But there is a minor revolution underway, with a growing number of utilities looking to make their wastewater operations energy neutral or energy positive, and even to use this as a foundation for energy neutrality for the utility as a whole.
The wastewater carbon footprint
For utilities looking at their wastewater activities and energy use as part of their carbon footprint, the picture is more complex than driving down energy use and shifting sources. Much of the carbon within the wastewater is ultimately returned to the atmosphere as carbon dioxide, due for example to the respiration of microorganisms during secondary wastewater treatment. This carbon would have originated from human waste, which in turn originated from food products. The short-term nature of this carbon cycle means this can be seen as a carbon neutral aspect of the footprint. It does nonetheless represent a release of carbon dioxide.
Of greater significance are the releases of nitrous oxide and methane that can occur. Nitrous oxide can be released during biological nutrient removal and has a global warming potential of 300 carbon dioxide equivalents. Methane, which is actively captured during biogas production, is also produced in the sewer and stripped into the atmosphere as the wastewater reaches the treatment plant. Methane has a global warming potential of 25 carbon dioxide equivalents.
These are just some of the complexities around wastewater treatment, which extend right through processing and management of the sewage sludge stream and the resulting sludge biosolids.
There is growing interest in the opportunities for decentralised wastewater treatment. This can be at the property level, or can be at a localised level potentially under the control of a utility. From the perspective of a utility’s centralised wastewater plant, decentralised treatment can help mitigate the growing demands placed on the plant’s treatment capacity. It also helps in turn to mitigate the energy demands of the centralised plant.
At the same time, part of the growing interest in decentralised treatment systems is based on the emergence of technologies that offer high energy efficiency at small scale, including exploiting the heat and energy in the wastewater.
Sewage and sewer heat recovery
Linked in part to the growing interest in decentralised wastewater treatment, there is also now greater realisation as to the potential of exploiting the heat in wastewater. This can be at the individual property level, recovering heat before the wastewater enters the sewer. But it can also be downstream of that, involving the utility to a greater or lesser extent depending on whether, for example, sewer heat is recovered for use in a district heating scheme, or whether heat recovery is used directly by the utility.
Such recovery of heat does need to be linked to the overall performance of the treatment plant, since temperature affects plant performance. The more heat is extracted, the lower the wastewater temperature will be, potentially slowing microbiological activity or increasing heating needs. Research in this area, such as that undertaken by Belgian utility Aquafin, has confirmed there can be an overall net benefit, but it also highlighted a need for better understanding of the impact the temperature change has on nitrous oxide emissions, with their far greater global warming potential.
Wastewater treatment is at the heart of the new thinking on water utility energy requirements. On the one hand this means looking to new technologies and to opportunities for far greater efficiency in areas such as pumping and aeration in order to reduce energy needs. It also means looking at the energy that can be extracted from the wastewater, especially through biogas production or from the sludge biosolids.
Energy extraction is not new. Indeed, a 2013 estimate for the UK water industry put the generation of electrical energy from sewage sludge at around 800 GWh/year. What is changing is the shift in expectation as to what can be achieved at wastewater treatment plants. Perhaps above all, it can be seen as a rethink of the use of the activated sludge process, now over 100 years old. This features aeration to drive the microbiological degradation of organic material, sometimes even using stages in which carbon is added to support nitrogen removal. The shift in thinking is to instead optimise the amount of carbon that is retained for biogas production.
There are a growing number of utilities and treatment plants that have made this type of switch. For example, Hamburg Wasser in Germany achieved energy self-suffiency at its main treatment plant in 2011. Meanwhile, VCS Denmark, which serves the Danish city of Odense, achieved energy neutrality by end of 2014.
The water company of the Danish city of Aarhus is another leading example of a utility making strong progress. Prompted in part by the fact that water and wastewater treatment processes account for some 25-40% of the municipality’s electricity bill, Aarhus Vand has already upgraded its Marselisborg plant. It has reported that for 2016 the plant produced 40% more electricity than it required, as well as 2.5 GW of heat for the local district heating system.
Aarhus Vand has now turned its attention to its Egaa plant, where a project is being implemented with the aim of shifting from a 2.9 GWh a year power requirement to the production of 50% more electricity than the plant uses. (June 2016, p13)
The overall aim for the Egaa plant is to optimise the carbon expended during treatment of the wastewater and the amount of carbon passed to the digestion stage for biogas production. The original plant did not have primary treatment. Instead, a microfiltration carbon harvesting step is being added, using filters from Norwegian company Salsnes Filter to remove approximately 60% of COD at the inlet. This will use polymer dosing. According to the utility, ferric chloride could have been used and removed an expected 70-80%, but this would leave insufficient organic matter to achieve denitrification for nitrogen removal in the second stage of treatment. The secondary treatment stage will feature the EssDe (Energy Self Sufficient) process of Demon, provided under licence by Sweco. This process uses anammox (anaerobic ammonia oxidation) bacteria for nitrogen removal, with the Egaa installation requiring it to operate as a cold anammox process in the Danish climate. Egaa will also include a new anaerobic digestion plant, with a CHP plant and Organic Rankine Cycle unit, and a Demon plant to treat digestate and feed back anammox bacteria.
This Danish expertise has been picked up in the US, where there is an established interest in wastewater plant energy efficiency. Danish companies AVK, Danfoss, DHI, Landia, LINAK, Nissen Energiteknik, Stjernholm and Grundfos have formed an alliance called Water Technology Alliance Chicago. This is leading a project to upgrade the Glenbard wastewater treatment plant in Chicago and will deliver a demonstration plant due to be completed in 2019. The project aims to combine improved wastewater treatment with substantial reductions in energy consumption. (December 2016, p21)
The Chicago project will also incorporate the use of an ESCO energy services company financing concept. This allows the utility to implement the upgrade and then pay the technology provider for it through the energy savings made. The potential of such an approach to accelerate deployment of energy efficiency measures was highlighted by Al Cho, Xylem VP Strategy and Business Development, speaking to Aqua Strategy about the water sector energy efficiency opportunity. (February 2016, p15-16)
The Austrian capital, Vienna, provides another leading European of utility wastewater energy ambitions. (February 2016, p19-21) Following on from the renewables project at its Simmering plant, utility Ebswien is part-way through a €250 million overhaul. The ‘E_OS – Energy_Optimisation Sludge Treatment’ scheme, which is due to be completed by the end of 2020, aims to reduce carbon dioxide emissions by 40,000 tonnes / year while providing biological nutrient removal at the 680,000m3/day dry weather flow plant. The existing primary sedimentation and first biological step are being replaced. A whole new sludge treatment system featuring six 30m high digesters is being constructed. A co-generation plant will use the 20 million cubic metres of methane produced a year to generate 78GWh of electricity and 82GWh of heat, generating all the energy the plant requires for wastewater treatment.
Many improved treatment process can potentially contribute to this transformation of the wastewater plant energy balance. These include technologies to improve the delivery of oxygen during secondary treatment, as is the case with membrane aerated biofilm reactors (MABR). This technology uses membranes to deliver oxygen by diffusion, cutting the energy requirements for oxygen delivery. For example, GE (Suez) states that its ZeeLung delivers ‘a four times reduction in energy compared to conventional fine bubble aeration systems in use today’. Other suppliers of MABR technology include Israeli company Emfcy and Irish company OxyMem, the latter claiming similar efficiency gains.
They include technologies that bring a more fundamental change in approach to treatment, such as the Nereda process, which is based on developing a fast settling granular sludge. Royal HaskoningDHV states that Nereda delivers a 50% saving on energy costs. Recent developments for Nereda include the announcements this month that Hach is to be a preferred technology supplier for the process, and that the first Nereda plant in Australia has opened.
They also include technologies to improve sludge processing and biogas production, including the use of thermal hydrolysis, such as the Cambi Thermal Hydrolysis Process. Clearly the need for any additional treatment can impact energy demands, such as the use of advanced oxidation technologies for micropollutant removal. And the treatment energy equation extends through to thermal treatment for sludge solids processing.
Other contributions to energy efficiency
Possible actions to improve a water utility’s energy profile don’t end there. There are others, both in terms of the equipment and technologies that can be applied and the strategies and frameworks that can be used to approach the whole area of energy.
Data and control
The water sector is catching on to the potential of ‘big data’ to support smarter, data-driven operations and decision making. This links with progress in sensing, communications and modelling technologies, all of which stand to contribute to energy improvements.
As an example, utility Aqualia has been undertaking a pilot project at its Lleida wastewater plant in Spain. (June 2016, p25) Israel-based IBM Research has implemented machine learning algorithms at the plant as part of the IT giant’s First-of-a-Kind programme. Results released last year revealed a 13.5% cut in electricity use alongside a number of treatment improvements, through the system delivering recommendations every two hours to the plant operators.
This interest in energy is being reflected in the functionality that can now be incorporated into wastewater simulators. For example Canadian company EnviroSim Associates added energy use functionality and user enhancements to its BioWin software. (April 2016, p27) BioWin 5.0, launched in February of last year, allows users to calculate power requirements and track energy requirements, covering blowers, mixing, pumping, heating, and surface aeration. The new functions included the possibility to generate power / energy use plots automatically and to track the power required to pump various flows taking account of factors such as pipe material. It allows, for example, exploration of onsite CHP engine power generation and heat recovery options.
The interest in energy means management systems can take account of it in, for example, triple bottom line reporting or whole life cost investment planning. Management tools such as energy audits can be used, with developments in this area including the testing of a new European sewage plant energy audit method as part of the ENERWATER research project. (August 2016, p25) This area extends to the ISO 50001 Energy Management System.
The water sector has also been accumulating and sharing its experiences on energy management, especially in the US. For example, the results of a survey carried out by the Water Environment Research Foundation (now the Water Environment & Reuse Foundation) and funded by the New York State Energy Research and Development Authority highlighted the lessons drawn from five utilities, in Melbourne, Philadelphia, Los Angeles County, Johnson County, and Ithaca / Dryden, New York, who had each achieved 21-75% energy neutrality. (April 2016, p31)
These five had each taken a lead as energy neutrality champions in their wastewater treatment activities. Success requires the input of staff and managers, as well as external stakeholders such as governing boards. In terms of technical approach, the survey identified the benefits of using co-digestion. More broadly, it recommended that utilities seek to gain support from communities and politicians by spreading a message of ‘green renewable energy’. It also highlighted that the chances of success are greatly influenced by having a clear energy plan with goals, collaborating to get expertise, and being aware of costs and exploring funding opportunities.
Conclusion – the urgency for action on carbon emissions
One of the biggest challenges utilities face is the need to balance energy with other priorities. Wastewater treatment plants, for example, need to meet their water quality targets, or respond to other emerging needs, such as the potential for water reuse or nutrient recovery. There may also be long-term questions around the role that wastewater plants should play. For example, Denmark has made massive progress with renewables, especially wind power, which alone supplied over 40% of the country’s electricity production in 2014. As this transformation progresses, should the carbon in wastewater instead be used as a feedstock for bioplastics production? In the meantime, and for most countries, there is a pressing need to curb carbon emissions, and there is clearly scope for the water sector to makes its contribution in this respect.
- Australia, Austria, Chile, Denmark, Netherlands, Spain, Tunisia, UK, USA, Aqualia, AVK, Cambi, Ciel et Terre, Danfoss, Demon, DHI, Emfcy, EnviroSim Associates, Floating Solar UK, Fujifilm, Grundfos, IBM, Landia, LINAK, Nissen Energiteknik, OxyMem, Royal HaskoningDHV, Salsnes Filter, Sisyan, Stjernholm, Suez, Sweco, Xylem, Zeropex