Anaerobic energy

Anaerobic digestion is nothing new. But the time has come for a re-evaluation of its application to industrial waste streams

All the best financial investors know that timing is everything, and there is a right time to enter the market and a wrong one. This advice would not be wasted on manufacturers of equipment for the treatment of wastewater, as there is a litany of promising technologies brought to the marketplace only to fail as they were the right product at the wrong time.

Identifying the right time to enter the market is not easy but the signals emerging at the moment give a clear message. Consider that: climate change is headline news.

And every severe weather event (such as the storms that battered England in mid-January) is identified as a harbinger of the future under global warming. Energy costs are rising and the UK is no longer in charge of its own energy supplies. Ofwat has made it clear that the next periodic review (PRO9) will require a much greater commitment to sustainability on behalf of the Water plcs. The pressures on sludge disposal to land continue to increase. And, finally, the space available for landfilling of solid waste is diminishing and the use of the space that is available is increasingly restricted by European directives. Reading these signals could not be simpler: the future is anaerobic.

Anaerobic digestion is not a new technology, and indeed it is the de facto approach used by water companies to stabilise sludge as the first stage in the process of generating biosolids intended for recycling to agricultural land. It is perhaps the application in this role that has provided our existing perception of the technology and restricted its application to other waste streams. It is recognised that anaerobic treatment has a lower energy requirement and generates less sludge than aerobic processes.

But sewage sludge is actually a difficult substrate to treat anaerobically. It requires a long hydraulic retention time (upwards of 12 days) in heated digesters, which in turn leads to a capital-intensive process. But research led initiatives over the last decade have led to improved reactor designs, operating conditions and specialised microbial consortia as start-up seeds.

This in turn has enhanced the performance of anaerobic digesters and created an impetus to re-evaluate the anaerobic treatability of many industrial waste streams, which can generally be treated at much higher organic loading rates than sewage sludge. As a result the range of waste streams that can be treated anaerobically has steadily increased.

Increasingly, however, anaerobic digestion is seen not as a process for stabilising sludge, but as an opportunity to recover the energy embedded in the substrate, traditionally in the form of methane and more recently as clean and green biohydrogen. For instance, it has been estimated that conventional anaerobic digestion of the UK's municipal solid waste could generate 1.4GW electricity, or 1.9% of the nation's demand. If these calculations also included potential energy generation from industrial solid and high-strength liquid wastes, then anaerobic digestion would start to be a significant player in the renewable-energy market.

Where energy recovery is the goal, there are considerable gains to be achieved by application of two-stage anaerobic processes compared with the conventional single-stage mesophilic option. The rate-limiting stage of a conventional mesophilic anaerobic process is the breakdown of large molecular weight materials, and in particular biopolymers such as proteins, carbohydrates and fats, into their component monomers.

The subsequent stages involving the catabolism of these monomers to volatile acids and then methane are by contrast quite rapid. Consequently, two-stage processes aim to speed up the reaction of hydrolysis in the first stage, thus permitting both faster and greater acidogenesis and methanogenesis in subsequent stages.

The effectiveness of the first stage of a two-stage process can be measured by its ability to increase the amount of soluble organic material with a concomitant reduction in volatile organic solids. But there are a bewildering range of alternatives to achieve this. It might involve the input of energy in the form of: sonication, temperature and pressure, or alternatively provide a high-rate first stage such as acid phase digestion or enzymic hydrolysis, which delivers the optimum growth conditions for the proliferation of hydrolytic and acidogenic microorganisms. It might also include a chemical process such as alkaline pre-treatment to weaken the microbial cell walls followed by disruption using, for instance, homogenisation.

When comparing the relative efficacy of two-stage and single-stage anaerobic processes, the whole-life costing can be complex and involve a large number of variables. Two-stage processes have clearly identified costs such as the additional capital and operating costs associated with the first-stage process. But to offset these are a range of benefits that will be waste specific. These include: increased methane generation (an additional 30% methane yield is typical), enhanced solids destruction (up to 50% less sludge produced), higher solids loadings to the second stage (which can reduce the second-stage reactor volume by as much as half) and finally an improved solids dewatering of the resultant solids (which can increase cake dry solids from 20% to 30%).

Currently, two-stage technologies have been restricted largely to municipal sludge.

But timing is everything, and the time is now right to extend this technology to a wider range of waste streams and in particular those wastes generated by the industrial sector.

It is relatively straightforward to assess the potential of a waste stream for anaerobic digestion, as this is based on simple tests such as chemical oxygen demand, volatile organic solids, and total Kjeldahl nitrogen.

Where waste streams are found to be deficient in key components, there is always the potential for co-digestion with an alternative waste in order to create an admixture with the ideal properties.

But bench studies will always be needed, not only to ensure the waste stream is amenable to anaerobic digestion but also to provide the important quantitative data to undertake a meaningful cost benefit analysis.

While such trials are not complex, some patience is necessary as a period of acclimatisation will generally be required, and it is usually 16 to 20 weeks before this is apparent. The potential rewards from application of anaerobic digestion are high, not only for the client but also for the environment. Anaerobic digestion has waited a long time but it is one technology whose time has finally arrived.

To learn more about the potential applications of two-stage anaerobic digestion, Aqua Enviro is holding a one-day course called The potential of new generation anaerobic digestion processes for the treatment of industrial liquid and solids wastes. Further details are available at www.aqua-enviro.net or email franceseldon@aquaenviro.co.uk

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