Thinking in circles: changing the way you look at water treatment

Linear thinking about wastewater treatment wastes resources and energy. Professor Bruce Jefferson says the sector could and should move to a model that uses circular economy thinking.

Now we have celebrated the centenary of the activated sludge process it is perhaps timely to reflect on what we would like to look back on in another 100 years. Unquestionably the use of aerobic biological processes (activated sludge and biofilm systems) has been a success in protecting human health and the environment. However, is the approach still fit for purpose or do we need to re-evaluate what success looks like?

Wastewater enters the treatment works and we add chemicals and energy to treat the water. But as well as using high levels of chemicals, we also produce greenhouse gas emissions and sludge. In fact, around 2 per cent of the UK’s total energy consumption is down to wastewater treatment. We also use about 1 million tonnes of chemicals (water and wastewater) but only recycle 0.1 per cent of those chemicals. This is a linear economy approach and raises the question: should we accept such an approach in protecting ourselves and the environment?

Many would now say no. Wastewater is a resource rich in carbon, nutrients, energy and, of course, water. As such we can re­imagine a sewage treatment works as an effluent factory where energy is generated, nutrients recovered and water produced suitable for a range of end purposes. Such thinking aligns strongly to the general discussions around the potential for switching to a circular economy and highlights the potential of the water sector to make this journey.

Significant uptake is already apparent. Recent advances in sludge processing such as advanced digestion and downstream gasification and pyrolysis have transformed the energy landscape associated with sewage over the past ten years. Sludge processing sites are now net energy exporters, albeit through the inclusion of sludge imports from smaller sewage works. The future looks as though such market opportunities will become more open, as seen through the current consultations on sludge trading.

Delivery of the Water Framework Directive requires many treatment works to reduce nutrient levels to very low levels. A number of relatively recent technologies have emerged that are effective in meeting these discharge targets. However, they use energy and chemicals to remove nutrients and thus represent continued linear economy thinking.

Reflecting a desire for solutions more aligned to the circular economy concept, a number of innovative alternative technologies are being developed that produce added value through recovery of nutrients, use of waste materials or opportunities to cascade. For instance, commercially available nanoparticle embedded media enables selective removal and recovery of phosphate. Contact columns using the media are able to meet extremely low discharge levels of phosphorous. However, as with all sorbents, the implementation challenge is the management of regeneration liquids. In this case the liquid is a caustic solution that needs to be used every few days, so the regeneration process must be conducted on site. The key innovations are with local clean-up of the regeneration liquid so that it can be reused with minimal top-up of fresh caustic.

The by-product of this is calcium phosphate at a purity level higher than its mined equivalent. Similar activities are underway with ammonia, offering a future where it will be possible to grow crops fertilised entirely with recycled nutrients. While total crop production uses more fertiliser than could possibly be recovered through this route, it does offer an alternative that in the future may be viewed in a similar way to organic produce.

Other options, such as the use of algae or steel slag, are able to meet the challenges associated with more restrictive phosphorus discharges. Each option delivers different added value and has different strengths and weaknesses. We need to understand the true trade-offs to establish the risk-reward balance when considering the implementation of these innovative solutions.

As with anything in the circular economy, the question is always asked: if this is so good, why are we not already doing it? The truth is that sometimes the best option from a social and environmental perspective may incur greater risk from an economic perspective. The challenge then becomes about the management of the uncertainty that is always associated with innovation. The risk is that we shelve the best options instead of understanding what needs to change to make them fit for purpose.

From my perspective as a university researcher, the appetite of the water sector for innovations that deliver step changes is clear. The desire to implement and do so quickly is clear. This increases the need to reassess how we manage the innovation process. A move away from traditional linear approaches is crucial. We need greater iteration between the different stakeholders involved throughout the development path. It is fitting that delivery of a circular economy vision of wastewater treatment is best achieved through more circular interaction during the development of the technologies.

Professor Bruce Jefferson, professor of water engineering, Cranfield University