The recent IWA Specialist Group conference in Istanbul showcased the latest advances in use of anaerobic digestion. Kala Vairavamoorthy was struck by the growing potential of this process.
This June, in Istanbul, I joined 750 researchers and practitioners from around the world to share in the exchange of insights in their pivotal and rapidly evolving field of anaerobic digestion (AD). The historic venue united Europe and Asia, and – led by committee chairs Professors Hale Ozgun and Mustafa Evren Ersahin – the agenda deftly bridged the north-south divide. The 18th IWA AD Specialist Group conference provided a platform for the diffusion of ideas and know-how, a confluence of minds sharing experiments and experiences. For me, it brought home the huge potential of technologies in this field.
AD – more than a one trick pony
I arrived assuming that AD was a ‘one trick pony’, pioneered in 1859 in Mumbai and then evolving into a technology to stabilise sludge and produce methane gas. I arrived in Istanbul with this established in mind. Yet, by the time I departed, after absorbing a series of technical workshops and inspired keynotes, I could more fully grasp how this extraordinary technology has become a multifaceted field and is transforming the water sector in profound ways: from generating energy, to producing enriched and safe biosolids, recovering nutrients and high value chemicals, and helping decentralise the sanitation infrastructure.
Let me try to convey a fraction of the breadth of what was served in Istanbul.
Traditionally, AD had been seen as a way to reduce sludge volume, inhibit odour-causing compounds, and deter insects as vectors of disease. These real concerns are growing, and impose great needs on the water sector because of environmental impacts and operational costs. That’s why Istanbul rightly explored how to optimise the stabilisation process through high-rate digesters, which operate at higher temperature and loading rates, and thus more effectively break down organic matter and reduce sludge volume.
Releasing heat and energy
Those discussions only scratched the surface, however. The conference also revealed the latest thinking on how to convert liabilities into assets. For example, in previous decades, when AD produced the by-product of methane, that greenhouse gas was simply flared to make it go away. Today, AD’s most celebrated function is to transform organic ‘waste’ into methane as a means to generate clean power and recover biosolids, and to help utilities achieve ‘off the grid’ resilience.
Doing so opens up massive opportunities. Indeed, Istanbul made clear how much energy and heat could be generated, how important this dynamic process could be, and how many water systems throughout the global north and south could benefit from its adoption. By reducing your water sector’s reliance on fossil fuels, AD systems contribute to institutional and national energy security, and accelerate our transition to a low-carbon economy.
Many presenters showed how best to operate AD digesters to improve efficiency, reduce failures and produce high-quality biogas. Others showcased innovations in digital monitoring tools and advancements in understanding of the microbes in digesters that ensure reliable operations.
While use of AD processes dates back to the 19th century, few water professionals fully understand the complex interactions that take place within anaerobic digesters and septic tanks. Yet recent advances in what could be called ‘digital AD’ can now shed light on controlling this mysterious microscopic world. For example, some digital innovations provide real-time monitoring of key process parameters, such as pH, temperature and biogas composition. Others can predict the biomethane potential for sewage sludge. Such tools and techniques help operators prevent imbalances or gas leaks, support feedstock quality control, and optimise the performance of full-scale plant operations.
Rather than work in isolation, anaerobic digestion processes can be integrated within wider urban infrastructure. The Istanbul gathering showcased innovative AD projects that produce renewable energy while also providing heating for local communities and industries. For example, ‘trigeneration’ or ‘combined cooling heat and power’ systems maximise the energy value by using – and recycling – the outputs of electricity generation for applications such as district heating, industrial processes, and even cooling through absorption chillers.
The versatility of anaerobic digestion is exemplified by how AD systems cannot only be coupled with membrane systems, but also with algal systems and microbial fuel cells that take advantage of the synergy in wastewater treatment and energy recovery. Research on these integrated processes can speed their adoption – especially as utilities realised how AD can help them achieve net-zero ambitions.
Recovering precious resources – new things
For the water sector, scaling AD processes can hasten the circular economy. As we seek to convert ‘waste’ into ‘value’, AD ensures a more holistic process of resource recovery. Nutrients such as nitrogen, potassium and phosphorus can be recovered from ‘digestate’ (the residual material left after AD) and used as fertilisers. Doing so reduces the need for synthetic (petroleum-based) fertilisers and promotes sustainable agricultural practices. What’s more, AD systems can recover other valuable materials, such as metals and organic acids, further enhancing the economic and environmental value of the process.
“For the water sector, scaling anaerobic digestion processes can hasten the circular economy”
The discussions also covered how progress in microbial ecology is shedding light on the microscopic world behind AD. Newer studies in microbial ecology of anaerobic digesters allow water professionals to tailor the AD process to enhance efficiency and stability, and to produce not just methane, but also hydrogen, organic acids and other value-added chemicals. Presentations explored how ‘arrested methanogenesis’ – when the AD process stops or slows down because something isn’t right for the tiny organisms that produce methane gas – is being exploited by process engineers to convert organic waste into higher-valued volatile fatty acids. These are higher-value platform chemicals that can be used to produce biofuels, bioplastics and animal feed. Other developers are pioneering ways to enhance nutrient recovery from AD systems. Struvite precipitation, for example, helps recover phosphorus in the form of crystals; this process can yield a high-value fertiliser. Similarly, ammonia stripping and absorption can recover nitrogen from AD digestate.
Beyond yielding new and desirable outcomes, AD advances can also ensure protection of public health.
The original point of sewerage was to insulate humans from exposure to the noxious fumes, invisible bugs and chemical toxins in our waste. In recent years, the danger from complex ‘forever chemicals’ in our biosolids, or the transport of antibiotic-resistant bacteria and genes, has been elevated. While no panacea, the prospect of greater use of advanced AD to process waste streams, which may carry substances of concern, offers water treatment professionals a potential bioremediation option to manage organic wastes containing certain PFAS. Likewise, research suggests that the addition of, say, biochar or zero-valent iron to AD processes can help destroy or remove dangerous chemicals.
AD in the decentralisation puzzle
While sharing insights on ‘how’ AD works, the gathering also explored the ‘where’ of scaling use of the technology. For years, IWA has increasingly shown how distributed systems allow the water sector to become more agile, cost-effective, and efficient. The conference demonstrated how modular and scalable AD solutions can promote decentralised management levels, from state to city to neighbourhood to individual.
At the rural-urban interface, combined AD systems can co-digest agro-industrial waste with wastewater sludge. In urban wastewater management, practical solutions – such as the Hamburg Water Cycle in Germany, and Oceanhamnen Helsingborg and Hammarby Sjostad in Sweden – modular AD systems are integrated to co-digest sludge with food waste, and produce biosolids and energy.
Modular and decentralised AD transcends such use in Europe. As of 2014, China had 42 million household-scale biogas plants fed by livestock waste, domestic sewage, and agricultural residue. From 2017-2021, India has constructed more than 90,000 family-size biogas digesters. Innovative solar septic systems can heat water to maintain a high temperature in the tanks to ensure better anaerobic process efficiency.
Our increasingly decentralised world is competing to ‘reinvent the toilet’. As it does this, several breakthrough technologies have an AD component that shifts waste management to the household level. One toilet generates and then combusts biogas in a heat exchanger system that pasteurises the effluent – yielding a pathogen-free effluent safe for backyard discharge. Another toilet innovation removes pathogens, recovers nutrients, cleans water, and generates energy through an internal anaerobic membrane bioreactor.
Addressing fugitive gases
There is a potential concern for use of engineered systems to produce and capture methane – if the methane, a potent greenhouse gas, escapes, methane has a global warming potential more than 28 times that of carbon dioxide. There’s little data on how much pollution all these small-scale digesters may cause. Some estimate methane leaks could be as high as 40%, indicating a need for mitigation.
So, from Canada to Brazil and South Africa to Indonesia, urban systems must increasingly comply with domestic or international pressure to stem water pollution. They must also monitor, report and slash release of methane and nitrogen oxides. The rise of high-density living, coupled with significant underground sewage conveyance and treatment facilities, magnifies our sector’s focus on noxious emissions.
Alongside this, we know that AD processes – and the resulting production of methane – occur not only in digesters, but also throughout engineered infrastructure, such as septic tanks, sewers, or primary clarifiers, where anaerobic environments are present. We know also that anaerobic processes are more pronounced in warmer climates because of the higher temperatures. This meant there was a great deal of attention given at the conference to better understanding this downside of AD and the opportunities to respond by turning a potential environmental liability into a valuable resource.
In Paris, for example – one city with piped gas infrastructure – a 2021 study indicated that sewers contribute 33% of the detectable emissions from the ground, suggesting the scale of the issue.
Potential responses include use of collection systems (extraction wells and pipes), especially if combined with small, decentralised units at key points in the sewer network, which can capture methane before it is released into the atmosphere. Similarly, AD processes in sewers can promote microbial corrosion through production of hydrogen sulphide – another field of study.
Methane is of concern at treatment plants, where dissolved methane gets stripped off during the aeration process in the treatment plant, resulting in further fugitive emissions. Here we see, for example, that wider advances in microbial ecology seek to address concerns around methane release. This includes the cutting-edge knowledge from work by scientists at the University of Queensland, Australia. This applies bacteria to dissolved methane in the incoming sewage to perform nitrate/nitrite-dependent anaerobic methane oxidation for denitrification, thereby achieving simultaneous reduction of methane emissions and nitrate/nitrite pollution.
“we have to apply our understanding through a more integrative perspective”
Advances in microbial ecology approaches allow us to disentangle the metabolic processes that are occurring in the digesters, tailor the microbial community to meet the needs of the operation, ensure stability of the processes to prevent digester upsets, and improve efficiency of the digesters.
The lesson is that we have to apply our understanding of AD through a more integrative or ‘holistic’ systems perspective, where we look at the entire flow of wastes from generation to treatment.
Irreversible path forward
Despite AD’s numerous and significant benefits, Istanbul underscored the ongoing need for transparency, accountability and innovation. Well maintained and closely monitored systems help keep them viable, prevent the escape of methane and maintain public trust. Innovations spread as ideas are debated, tested and adapted. Much of the sharing of information to support this can happen online, of course, and IWA networks offer webinars, articles, and research papers to accelerate it. Yet there is no substitute for the give and take of live, in-person interactions, as face-to-face discussions generate more light than heat, and question and answer sessions yield new ideas.
In Istanbul, conversations advanced cutting-edge technologies and approaches that push the envelope of how much anaerobic digestion can achieve. By working together, showcasing best practices, and learning from each other, the AD community demonstrated how we can and will accelerate the adoption of this transformative technology and drive positive change in the water sector and beyond. From capturing methane and reducing sludge volume to recovering valuable resources and generating renewable energy, AD offers a multifaceted marvel and a solution to many of our most pressing challenges.
Of course, AD adoption will only come to the extent that experts and stakeholders in the water sector break out of their inner circles and convey its larger benefits to society, communicating these to the wider water community, including by me. •
The author: Kala Vairavamoorthy is CEO of the International Water Association
