Power from poo: Rethinking energy recovery from wastewater

February 15th, 2016

Sarah E. Cotterill, School of Civil Engineering & Geosciences, Newcastle University

Times have changed considerably since ‘The Great Stink” of 1858, where the smell of accumulating waste in the Thames, from an outdated and inadequate sewer system, was exacerbated by a particularly hot summer.1 Waste then was viewed as a problem – provoking fear of its possible effects – so the solution was to get rid of it.

A century and a half later, Heidrich2 estimated the calorific value of domestic wastewater to be in the region of 8 kJ/L. A considerable sum, when multiplied by circa 12 billion litres treated every day in the UK. Furthermore, the research itself acknowledges the value of our waste: a 180 degree turn from the mind-set of the 1850s. Yet if this progress is compared with that made in the communications or transport industries over a similar time frame – how do waste and energy fare? Is this change in mind-set being realised in practical technologies?

This article showcases collaborative results from Newcastle University and Northumbrian Water to better understand the intertwined relationship between wastewater and energy. Northumbrian Water, a water and wastewater utility in the North East of England, operate 418 wastewater treatment plants and 765 pumping stations, serving 2.7 million customers. Since 2012, all of their sewage sludge has been processed through advanced anaerobic digestion3, supplying power for 70% of their wastewater treatment and 20% of their total energy use across the business.

Whilst these advances are admirable, this article challenges the way we view energy recovery from wastewater, asking whether we can recover more, earlier and reduce our energy input, as opposed to solely achieving net gains.

Issue

Wastewater treatment is acknowledged to be energy intensive, using around 3% of the UK’s total electricity use4, predominantly due to the aeration involved in the activated sludge process5. Current methods rely on a ‘net gain’ approach, tagging an energy recovery process on the end of the existing treatment chain. The issue here is that the energy recovered is an ‘add-on’ and a significant amount of energy has already been consumed upstream. It may be possible to recover energy earlier, and in doing so, reduce the amount of energy applied in wastewater treatment.

This article outlines a case study of three pilot scale trials of a novel wastewater treatment technology called a microbial electrolysis cell [MEC]. MECs function like a battery, with two electrodes. One of the electrodes is covered in electrochemically-active bacteria that consume the organic material contaminating the wastewater, releasing electrons (and protons) in the process. The electrons travel in a circuit producing an electrical current, which can be used to produce electricity, or (in combination with the protons released) higher value products such as hydrogen. This process allows energy generation to happen side-by-side with wastewater treatment. As it’s fully anaerobic, it functions without the use of energy-intensive aeration. This implies that energy input can be reduced, alongside energy generation, promoting a more favourable energy balance.

Despite a decade of research into MEC, much has been on a small scale with less than 10 pilots in the 100-1000L range globally. Compounding this, many still operate with synthetic wastes such as acetate. The issue with this, is that results from synthetic studies are often far more predictable than those operated with real wastes6. If few researchers venture out of the lab to test the technology under plausible conditions (temperature, scale, waste etc.) then it is extremely hard to determine the potential of MEC as a wastewater asset.

Case study

The collaboration between Newcastle University and Northumbrian Water has produced three pilot plants aiming to provide a proof of concept and test the technology’s viability across the spectrum of treatment works’ in the company. The first pilot7,8, was installed at Howdon sewage treatment works (STW) – which, with a 1 million population equivalent (PE), is one of Northumbrian Water’s largest coastal sites. The trial served as a proof of concept, generating 0.6 L/d of 99% pure hydrogen gas, throughout 12 months of operation, with real wastewater at temperatures as low as 1 ?C7,8.

Fishburn STW, near Sedgefield in County Durham, was chosen as the site for the second pilot, as a polar opposite to the Howdon trial. With a 2,600 PE and flows of 10-20 L/s, the aim was to test the applicability of the technology across different scales. Gas production was improved, with up to 3L/d produced initially. Yet there were issues with the stability of the technology – with some cells failing during operation – resulting in a drop in gas production to 1L/d after 6 months.

The third trial is currently in place at Chester-le-Street STW, a wastewater treatment plant with 25,000 PE and flows of 150-200 L/s. The reactor is fed settled waste, as opposed to the raw, screened waste used in the first two trials. Increased COD removal has been observed, with an average of 63% COD removed, compared to 33% and 44% in the previous trials. Furthermore, effluent from the third pilot has been below the industry discharge limit (of 125mg/L COD) 54% of the time (Figure 4), a feat that was never achieved in the pilot at Fishburn and scarcely achieved at Howdon. Gas production took longer to initiate (circa 3 months), probably due to a lower influent COD and colder start up temperatures.

Conclusions

Microbial electrolysis cells are able to produce energy whilst treating wastewater. However, this case study demonstrates that it is usually easier to recover energy than to guarantee satisfactory effluent standards. Yet the third trial rouses questions. Is the goal energy generation or low-energy wastewater treatment? It may be that the technology can be tailored for different applications based on the site suitability and requirements6.

Rising energy costs will likely drive industry to recover more energy, in order to offset their input. Yet is the ‘net gain’ approach the best option? The case study suggests that putting less energy in and recovering it earlier are both possibilities for the near future, but will the technology be able to cope with increasingly stringent effluent regulations.

Carrying out more large-scale, realistic studies is paramount to the progress, and potential uptake, of this technology as a commercial asset. Accepting failure – in the pursuit of long term success – is crucial. Pilot scale studies cost more, take longer and generally have poorer performance, but are essential to the process of technological development6.

The benefits of MEC have the potential to be enormous, but the technology is not yet ready for a rapid roll out. Wastewater treatment assets have fairly lengthy lifespans – typically renewed every 25 years – and therefore it’s more likely assets will be replaced incrementally, at the end of their design lives.

References:

Daunton, M. (2004) London’s ‘Great Stink’ and Victorian Urban Planning. Accessed online at www.bbc.co.uk/history/trail/victorian_britain/social_conditions/victorian_urban_planning_04.shtml on 19.01.2016 Heidrich ES, Curtis TP, Dolfing J. (2011) Determination of the internal chemical energy of wastewater. Environmental Science & Technology. 45(2): 827-832. Fox, N (2013) Northumbrian Water: serious about sewage. Accessed online at http://www.theguardian.com/sustainable-business/northumbrian-water-serious-about-sewage on 19.01.2016 Curtis, T.P. (2010) Chapter 13: Low-energy wastewater treatment: strategies and technologies. In Mitchell, R. and Gu, J. ed. Environmental Microbiology, 2. John Wiley & Sons. Olsson, G. (2012) Chapter 15. Energy and carbon footprint of water operations. Water and Energy: Threats and Opportunities. IWA Publishing, London. Cotterill, S., Heidrich, E.S., and Curtis, T. (2015) 9: Microbial electrolysis cells fro hydrogen production. In Scott, L and Yu, E. ed. Microbial electrochemical and fuel cells: fundamentals and applications. Woodhead Publishing. Heidrich, E.S., Dolfing, J., Scott, K., Edwards, S.R., Jones, C. and Curtis, T.P. (2013) ‘Production of hydrogen from domestic wastewater in a pilot-scale microbial electrolysis cell.’ Applied Microbiology and Biotechnology. 97(15): 6979-6989. Heidrich, E.S., Edwards, S.R., Dolfing, J., Cotterill, S.E., and Curtis, T.P. (2014) ‘Performance of a pilot scale microbial electrolysis cell fed on domestic wastewater at ambient temperatures for a 12 month period.’ Bioresource Technol. 173: 87-95

Sarah is an EngD research engineer on the STREAM programme (www.stream-idc.net) which is the industrial doctoral centre for the water sector. She is working with Prof. Thomas Curtis, Dr Elizabeth Heidrich and Dr Jan Dolfing at Newcastle University on the scale up and development of microbial electrolysis cells for wastewater treatment. Her work is funded jointly by the EPSRC and Northumbrian Water. More information can be obtained via Email: sarah.cotterill@ncl.ac.uk; Twitter: @powerfrompoo; Blog: www.powerfrompoo.wordpress.com.

The views expressed in this article belong to the individual authors and do not represent the views of the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here.