Digesting the facts

The rapid development and innovation in anaerobic digestion (AD) technology has been driven by a desire to capture an increased share in this expanding market. Current thinking is to ensure that the selected technology has the ability to cope with a wide range of feedstocks and at the same time provide the highest biogas yields. Feedstock inputs will vary from back of store food waste through a spectrum of commercial organic waste streams up to conventional animal manures and bio-fuel crops.

Jeremy Jacobs is the acting chief executive of the Assoiation for Organics Recycling, formerly The Composting Association

Site location

Site location is significant, both in terms of ensuring minimal carbon emissions associated with transporting the material to the site for processing, and also in respect of securing a sustainable location for its subsequent use.

The digestate is a low value material, relative to the cost of transporting it, and CO2-related benefits associated with conversion of AD's biogas into fossil fuel offsettable resources can become outweighed by CO2 emissions, which are associated with transporting the material any significant distance.

The location of AD facilites in relation to other buildings, that could potentially utilise the heat produced, is also important. It has been estimated that 60 % of the outputs from AD are in the form of low grade heat, a potentially valuable resource.

However, the costs of using such heat beyond the AD facility that produced it is relatively expensive compared to feeding renewable electricity into the national grid. The UK is in a good position to learn from other countries' experiences with combined heat and power systems and district heating schemes that are effective in terms of technology, location and investment.

Policy vision

Financing operations is something else to consider. Currently under the Renewables Obligation scheme, an AD facility can receive one Renewables Obligation Certificate (ROC) for each megawatt hour of electricity that it generates. As part of the scheme, generators receive certificates for electricity from renewable sources that can either be sold to other generators that have failed to hit their targets or to a special fund. Currently a ROC is worth approximately £45. However from next year, where a ‘new technology' has been adopted to generate renewable electricity, it will eligible for ‘Double ROCs'.

A similar scheme to that of ROCs is required for incentivising the use of renewable heat from AD. This is critical to promote further growth in the sector and it is hoped that the Department of Energy and Waste will incentivise renewable heat utilisation in the Energy Bill in the near future through the introduction of an amendment in the form of a ‘feed-in tariff'. Some EU Member States take a combined efficiencies approach in which subsidies are available only for AD schemes that use both the heat and energy produced. This encourages investment in the most efficient technologies.

Biogas upgrade

There are a number of possibilities for the biogas produced by AD and to improve the efficiency of the use and distribution of biogas, further treatment options are available. For example, it is possible for the gas to be pumped directly into the existing gas-grid as biomethane (this process requires additional cleaning to remove noxious gases such as hydrogen sulphide). This fuel source can then be passed to end users directly and used as a direct diesel replacement for transport fuel.

Biofuel producers who achieve this are currently supported under the Government's Renewable Transport Fuel Obligation to the tune of 35 pence per litre. This surely has to be the way to go in the future. The UK has already in place an extensive gas pipeline which could be used without significant additional cost.

Complimentary or adversary?

Currently opinion is divided over the newer technologies to emerge from the composting sector. When comparing AD and aerobic technologies such as in-vessel composting (IVC), there can be no competition as to which is the best technology because they work so differently.

Feedstock type and consistency is a key determining factor as to selecting the most appropriate treatment technology. Integration of AD with other biowaste treatment technologies such as composting provides a flexible approach, allowing sites to diversify their input types and increase their revenues accordingly.

AD processes require a high degree of competency to operate and given the interest in such systems to date, there needs to be a professional training mechanism in place to ensure that this growing sector of the industry has sufficient, suitably qualified and experienced operators.

PAS 110 and Quality Protocol for Anaerobic Digestion

At present a Quality Protocol and Publicly Available Specification (PAS 110) is being produced for the digestate which comes from AD. Due for publication soon, the Quality Protocol for digestates produced from source-segregated biodegradable materials and wastes will assist in building confidence in the use of digestates.

Such products have significant nutrient value and can be beneficially used as biofertilisers and soil conditioners in markets such as agriculture, field horticulture, forestry, and land restoration and remediation. However, it is important to establish a digestate marketing plan well in advance, particularly in respect of suitable timing and methods for its application to land.

In summary, we are entering an exciting and challenging phase of further development in biowaste management in the UK, and sites that are able to host both AD and composting offer great opportunities to manage biowastes in a flexible way and at the same time produce a range of useful products.