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Balancing the benefits

During the next decade, there are ambitious plans to construct upwards of 1,000 anaerobic digesters to meet Government targets for waste diversion from landfill, greenhouse gas reduction and generation of renewable energy. This will cost around £3bn.

In light of this, it is essential that these digesters deliver on the expectations, which require that all of the pertinent issues are considered when taking a decision to employ this technology. Based on recent publicity, it would be easy to believe that anaerobic digestion (AD) offers nothing but good news. But anaerobic digesters are very much a curate’s egg, for which the bad parts must be fully understood before a realistic appraisal of its suitability for a given application can be made.

This feature aims to cover some of the more negative issues behind AD. It is written with the knowledge that, in the right place, there is no better process, but in the wrong place it will fail. There are many wrong places in the UK and, without foresight, the next decade may leave a legacy of underperforming digesters and disappointed clients.

The process breaks down the biodegradable organic fraction of a waste and converts it to methane, a source of renewable energy. It recovers valuable nutrients and produces an end-product, the digestate, that is suitable for recycling to agricultural land. But it will not deliver a significant reduction in the volume of waste. Digesters are typically fed at 10% dry solids and so 90% of the feed is water. This volume does not change during the digestion process.

Hydrolysis, the first stage of AD, involves the breakdown of complex polymeric organics into smaller monomers. It can be slow, and it is the speed of this reaction that will determine reactor size and process economics. A waste should have the right balance of carbon and nitrogen, ideally 12-20: 1. Adequate carbon is necessary to generate methane while nitrogen is required to produce ammonia, which serves to buffer the pH. Where the carbon to nitrogen ration is too low, the buffering capacity is compromised and the pH drops; too high and the methane yield is reduced.

Different wastes can be balanced to optimise the carbon to nitrogen ratio. For example, food waste has a very high ratio whereas farmyard manure has a very low ratio. Co-digesting the two in the correct proportions can bring a lot of benefits.

The potential energy yield of a waste is generally expressed as cubic metres of methane per kilogram of volatile solids, with food waste having a yield of around 0.45 and low yield wastes such as farmyard manure having values as low as 0.2. But it must be realised that this is a potential yield measured under optimal conditions, which may not be achieved in practice. Upfront testing to determine the likely yield is essential for design and costing purposes.

Equally important is security of feedstock supply. Digesters are designed to handle a certain waste throughput in tonnes per day and gas storage facilities, boiler and CHP capacity to handle the biogas produced. If the supply of feedstock is reduced, then there is inadequate gas to drive the downstream processes and energy production may cease.

AD is complex and cannot be treated as a ‘black box’ bolted to the end of a waste sorting operation. It is a consortium of temperamental micro-organisms which only flourish within a narrow band of operating parameters, in particular the temperature, pH, carbon to nitrogen ratio, feed dry solids concentration and available alkalinity.

They show their annoyance at process upsets by reducing or ceasing production of methane. This may then lead to inadequate gas production to drive the boilers or CHP system and lead to a reduction in digester temperature, unless a supplementary energy source has been provided. Digester failure scenarios will not happen where they are operated by trained personnel with adequate time and resources. Failure will almost certainly happen if the digester is left alone to look after itself.

Biogas, when converted to electricity via a CHP system, attracts subsidies in the form of Renewables Obligation Certificates (ROCs), but this brings its own problems. The most sensible option for CHP systems is a dual-fuel process, such that in the event of process upsets or feedstock interruptions, digester heating can continue. However, a dual-fuel CHP is not eligible for ROCs, which leaves a plant designer with the option of either incorporating an additional natural gas-fed boiler or risking digester failure.

On paper the energy recovery through CHP is impressive but only slightly more than 50% of this energy is in the form of heat. This can be used to provide digester heat, but without alternative applications, it is wasted energy. In such situations, a better alternative may be to consider using the biogas either for liquefied petroleum gas or for direct injection to the grid after appropriate gas clean up.

Digestate contains valuable nutrients in the form of nitrogen, phosphorus and other elements essential for the healthy growth of crops and, consequently, it is often referred to as a biofertiliser. There is also a national quality standard, BSI PAS 110, and a Quality Protocol which allow the end product to be classified as a product (not waste), to give potential purchasers an assurance of quality.

This is a welcome development but there are two hurdles to overcome before the benefits of biofertilisers can be enjoyed. The first of these is storage and transport. Digestate is a dilute material, typically 4-6% dry solids. Transport costs are high and likely to exceed the value of the product for distances over 20 miles. Because digestate is high in nitrogen, application rates are limited if recycled within a nitrate vulnerable zone (NVZ) - around 65% of available land in England is within an NVZ.

Furthermore, farmers only require biofertilisers at certain times of the year, so storage capacity is required on-site outside of these periods. Although digestate can be dewatered to produce a ‘cake’ of up to 30% dry solids that is easy to transport and recycle, this still leaves a problem with the dewatering liquid that contains much of the nutrient and must itself be treated to recover this.

A second problem with recycling is the absence of a defined market for the sale of biofertiliser. The Waste & Resources Action Programme is currently seeking to tackle this particular issue but, at present, it is difficult to recycle digestate profitably.

Undoubtedly AD is an attractive option to deliver energy from organic waste, achieve recycling of the treated end-product to land and help to meet landfill diversion targets. But it is not the only technology that can achieve this and, in many situations, it is not the best option either.

Ensuring an economically and environmentally successful digester means that many factors must be considered:

  • It is essential that there is a suitable, secure and local supply of feedstock and a guaranteed route for recycling the digestate within an economic distance.
  • Generating an income stream from recycling should be considered a benefit and not one on which the process economics relies.
  • Adequate resources should be available for digester operation and, above all, it should be recognised that the digester itself is likely to be the most complex plant on site. There will be planning issues, an odour potential and health and safety considerations.

If these limitations are recognised, then the future of digestion is bright. If not, then we are likely to see many failed digesters during the next decade.
Nigel Horan is a reader in public health engineering at the University of Leeds and Matthew Smyth is operations manager at Aqua Enviro

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