OPINION: Ben Brown MSc, Director of WRM explains why he considers Dry AD offers advantages and opportunities for the waste sector in the UK.
Anaerobic digestion (AD) facilities are well established throughout the UK although just four of the 113 commercial and industrial waste fed facilities are examples of the ‘dry’ or ‘high solids’ anaerobic digestion technology.
There are a variety of reasons why the technology has been developed to a lesser extent than its wet AD counterpart and the treatment of co-mingled organic wastes (typically garden and food waste) has instead been served by in-vessel composting capacity.
But we only have to look more broadly at Europe, where installed dry AD capacity increased by 50%1 in the five years to 2015, to see that Wet AD is not the only effective option. In fact, Dry AD presents varied and numerous advantages for the waste sector at a time when many IVC plants are reaching the end of their design lives, and with significant policy reform expected in the Resources and Waste Strategy.
What is Dry AD?
Conceptually, dry AD is similar to wet (or conventional) AD in that the process receives waste feedstock which is broken down by a naturally occurring microbial community to produce a methane rich biogas. As the name suggests, the feedstock tolerance of ‘dry AD systems’ allows for the processing of solid or stackable organic wastes, typically with dry matter in the range of 20-45%, which may not be suitable for conventional wet AD without significant dilution which typically operates at dry matter contents of <15%. This higher solid input is also reflected in a higher solid digestate output which, after conditioning, is comparable to a compost product.
Another notable difference with Dry AD is the approach to feeding, mixing and process management
Another notable difference with Dry AD is the approach to feeding, mixing and process management. The high solids content of the feedstock and substrate precludes maceration and pumping through the system, and two variants of dry AD have been developed to address this challenge. Horizontal plug-flow systems operate in a semi-continuous manner and used mechanical flails to agitate and move substrate along the digester. Tunnel digestion systems are comparable to IVC systems and work on a batch basis with material being loaded, mixed and discharged by mobile plant. To achieve optimum digestion conditions, both systems utilise a percolate circulation system which sprays the waste mass with a microbe rich liquid which seeds fresh feedstock into process substrate.
Operationally, the two approaches have their respective merits and numerous factors such as gas yield, processing capacity requirements, operational control and costs, the required site footprint and digestate quality will all determine the type of dry AD which suits individual process needs.
But a key benefit of both forms of dry AD technology is that it can receive and process a wider range of feedstocks such as co-mingled garden and food wastes whilst also having the capacity to service food-only recycling collections.
Size of the opportunity
According to recent industry research, there are approximately 1.4 million tonnes per annum of co-mingled organic household waste that could be treated by Dry AD in the UK, allowing greater value to be extracted from the food waste portion whilst also expanding gas production to putrescible garden wastes.
Given the far-reaching changes in the biowaste market heralded by the publication of Defra’s Resources and Waste Strategy, the adoption of new technologies and Dry AD specifically is something that all local authorities should now be considering as part of their biowaste strategy.
And, in a matter of days now, we can expect the publication of a second industry consultation document on consistency in household waste collections which includes the provisions on organic wastes. Following the responses to the first industry consultation, which did not acknowledge the range of collection and treatment options which might be better suited to individual authority regions, the updated drafting is very likely, following the announcement of recent exemptions, to include a shift towards allowing comingling certain waste collections such as food and garden wastes. It is a shift that will enable local authorities to deliver a service that provides best value against collection costs while still achieving the environmental gains that underpin the Strategy’s objectives of diverting organics from the residual waste stream.
Furthermore, it looks increasingly likely that the strategy will refrain from self-selecting a preferred treatment technology, allowing the market the freedom to innovate and provide solutions that best fit with the needs of each local authority. With this increase in flexibility in local decision making, it is inevitable that the months ahead will see rising interest in and potentially the development of alternative treatment technologies and facilities while the implementation of comingled garden and food waste collections presents a significant opportunity for local authorities to take advantage of the benefits Dry AD affords.
By necessity, as local authorities explore the procurement routes, market operators will need to undertake feasibility assessment to explore the potential for new waste treatment infrastructure. The feasibility of Dry AD technology specifically for processing co-mingled garden and food wastes was recently the key focus of a project undertaken by WRM on behalf of the York and North Yorkshire Local Enterprise Partnership, sponsored by the low carbon growth fund.
This major study found favourable conditions for the development of Dry AD facilities and illustrated significant opportunities for comingled waste streams to be managed at a higher level of the waste hierarchy.
The project also clearly highlighted the possibility Dry AD offered for biogas capture as well as high quality compost and digestate. In addition, a significant opportunity was revealed by the study, for gas capture from the putrescible fraction of garden waste as well as green waste.
WRM’s work included an evaluation of both the plug-flow and tunnel systems, examining the technological and economic merits of both collection and treatment for dry AD. This indicated that the approach has significant potential in areas with lower population density, or in area areas with a comparatively high proportion of dwellings with gardens.
A key consideration for any new biowaste strategy
Following a decade of policy vacuum, the Resources and Waste Strategy is undoubtedly a driver for positive change. In fact, it represents the biggest opportunity for advances in the waste sector in many years. The timescale in which the necessary preparations must be made if we are to realise these advances however is challenging and the requirement to plan for change is immediate.
Understanding options, gearing up for long term biowaste procurement contracts and preparing for mandatory food waste collections must therefore be a major focus in the months ahead.
Dry AD is an option that is aligned with the objectives of the Resources and Waste Strategy, which local authorities should now be considering as part of their biowaste strategy, taking into account the difference in collection costs associated with comingled food and garden waste.
Ultimately, the adoption of Dry AD offers a real opportunity for some local authorities to make the shift towards more cost-effective waste collections, while still delivering environmental gains through renewable energy production.
To find out more about developing an effective biowaste strategy, a guide is available from WRM.