The Top 3 Technological Advances in Anaerobic Digestion in the Last 10 Years

The anaerobic digestion industry has grown and changed greatly over the last 10 years.  At the start of this period the only commercially significant Anaerobic Digestion plants in the UK were those which were working at sewage works for water companies. For them the main purpose was that of treating and sanitizing the sludge from waste water treatment works, and energy production in those days came as a low second priority. Compliance with requirements for spreading the residue on land was the main driver for research, and achieving the lowest whole-life cost for disposal of sludge was the aim.

There were some on-farm AD Plants, but most were small and very basic in their design. It is unsurprising then , that the first technological advance in this article comes from the waste water industries use of AD and is the development of hydrolysis methods to increase the proportion of bio-solids digested during the process and reduce the retention time and speeding the process up.

Hydrolysis and Anaerobic Digestion

Methods have been developed to essentially break down the bio-solids within the feedstock for AD plants, which enable the bacteria in the bio-reactor to utilize the nutrients better and therefore to produce more biogas more quickly.

The methods used in the “hydrolysis stage equipment” offered by AD technology providers these days, range from improved physical methods to “mash” and “crush” the incoming organic material into smaller particles, to clever techniques to ensure that as far as possible each cell of organic matter has been broken and the cell walls stripped open. The aim is that the VFA’s (volatile fatty acids held inside each cell) are released and dissolved in the liquid before they enter the digester.

At least three manufacturers of hydrolysis equipment for biogas plants now offer tried and tested hydrolysis stages to any AD plant designer, and the technologies on offer use one or more of the following methods to hydrolyse the feedstock (usually in addition to physical crushing and mashing):

•         Heat and pressure
•          Enzymes
•          Ultrasound.

Before we leave this subject though, we must stress the hydrolysis is best applied to manures and sludges, which are otherwise low biogas yielding and require long retention times in the digestion stage. MSW Food Waste, and bio-crop AD plants are not so much in need of a hydrolysis stage as their  feedstocks are much easier to digest by their nature, and therefore the additional costs of the hydrolysis stage may not be merited for such plants.

Dry AD Processes

The rise in the use of AD for purposes of digestion of organic feedstocks other than the treatment of liquids led designers to innovate by reducing the amount of added water to materials such as the organic fraction of municipal solid waste. The reasoning goes like this:

•  The most common AD method consists of adding water and slurrying-up the resulting mixture to be treated as if it is a liquid, and usually creates more liquid than is wanted (especially for a solid waste client who does not have a farm to spread liquid digestate) and the liquid may be d8ifficult and/or costly to dispose of
• So why not add a lot less water, or none at all, digest the feedstock material and handle it cheaply as a solid, avoiding any possibility of a plant that suffers blockages and grit build-up in the bottom of the digester?

This was seen as an opportunity, and at least four proven “high-solids” Anaerobic Digestion processes have now been developed by AD technology providers and are offered for the right feed materials as either batch digestion processes, or continuous plug-flow systems.

CHP (Combined Heat and Power also known as Co-generation(US))

Ten years ago, when digesters were built they were provided with gas engines as electricity generators and the spare electricity left-over when the demand for running the AD Plant equipment was provided would be sold into the local electricity grid. The hot water produced by the cooling system for the gas engine would simply run to the cooling radiator provided by the gas engine supplier and by wasted to the air around the plant.

Nowadays, renewable energy production has become a much higher requirement on the agenda for delivering cost-effective and truly sustainable anaerobic digestion facilities, and the cost benefit to the operator of not wasting this “free hot water” has also risen as fuel prices have soared. That means that CHP systems are now almost always a feature of all AD plant designs, and the hot water is used for:

• Heating the digesters themselves to maintain mesophilic or thermophilic digestion temperature requirements according to the requirements of the digester
•  Heating farm buildings and the farm itself - for on-farm digesters on -site
• Delivering space heating for factories, office units, homes and even as hot process water in some cases for factories, delivered via insulated pipelines which are used to take the hot water-off site to other premises.

That completes the three advances which I promised to explain at the start of this blog.

BUT, there are still more developments which are improving the viability of the AD process, which have been developed over the last two to three years, although there is no space to cover them in this article.

Add to this the fact that at the same time the regulatory framework within which the AD sector operates has been evolving  to make selling AD products more viable with for example, the PAS110 specification for the first time setting industry-wide standards by defining what comprises a quality digestate product.

Consider also, the UK government incentives now available with ROCs, the Feed-in-Tarrif, and the coming “Heat Initiative”, and also bear in mind the tax incentive which allows investment in renewable energy projects to be offset against tax at above the actual cost of the facility. Then also remember that the right projects will qualify for Carbon trust grants for EfW schemes, and the outlook for AD has never been more positive in the UK.

We hope that we have shown that the anaerobic digestion process is evolving through these advances, making it very much a renewable energy source for the future, which deserves the attention of all organic waste process designers.