What is it?

Like composting, anaerobic digestion reduces the bulk of organic waste by converting it into a relatively stable solid residue (digestate) similar to compost. Unlike composting , however, anaerobic digestion requires an oxygen-free environment (ie anaerobic conditions) for the specialised bacteria to function. The process produces a flammable gas consisting mainly of methane and carbon dioxide (biogas).

Anaerobic digestion is particularly suited to wet, organic wastes. As such it has been used in the water industry for sewage treatment since the end of the last century. Most large sewage treatment works use the process, recovering the biogas to meet on-site heat and power requirements.

The digestate usually requires stabilisation by composting before a saleable product can be produced. The biogas can be used to produce electricity, although up to one third of it may be needed to heat the digester itself, since the process requires warm conditions.

Household waste is made up of a range of materials including paper, card, plastics, textiles, glass, metal and putrescibles (organic material that is easily degraded such as fruit and foodstuffs). Of these, only putrescibles and paper are ideally suited to anaerobic digestion and the process will be easier to manage if these wastes comprise the only feed to the system. Garden waste may also be treated by anaerobic digestion, but the extent of degradation will vary according to the type of material. For example, grass cuttings are more easily treated than woody material. Isolation of the wastes that are more easily degraded may be achieved through separate collection but will necessarily have cost implications, and success will depend on how well householders exclude contaminants from the putrescible wastes. Alternatively an MRF could be used to separate the different components, feeding some to the anaerobic digester whilst recycling others such as glass and metal. This separation process would also involve significant cost, but could assist in integrating different waste management options more effectively.

The starting material is household waste rather than pre-segregated materials, which in the feed blending process is broken up into smaller pieces and mixed with other waste materials to facilitate degradation in the reactor. The sieve removes some of the unwanted materials such as metal and glass from the waste stream before passing to the mixing stage. In the mixing stage, the blended and sieved waste is mixed with steam and a starter innoculum to initiate microbial activity. The steam which can be generated using the biogas raises the temperature of the waste to increase the rate and extent of degradation within the reactor. Waste degradation is also made more effective by adding a bacterial innoculum. This innoculum is supplied from either the waste stream from the reactor or from the waste water produced during de-watering. The mixed waste is then fed into the reactor, in which degradation occurs, producing a relatively solid residue and biogas. The biogas can be used for energy generation directly, or can be used to generate steam. The solid waste that is produced is de-watered before further treatment or disposal.

Types of Anaerobic Digestion Plant

There are numerous designs and configurations of anaerobic digester. Some operate at warm temperatures (about 30-40°C - ‘mesophilic’ range). Generally speaking, the higher the temperature the faster the process, but thermophillic processes may be harder to control and will need more of the biogas for heating to keep them at the required temperature. Other variations include low or high volume systems, single or multi-stage digester vessels and continuous flow or batch processes. No clear ‘winner’ has yet emerged and advocates of proprietary processes are numerous. There are no plant currently operating in the UK which handle household waste, although facilities in Kent and Southampton are under consideration.

Digestate Applications

The fate of the digestate from anaerobic digestion depends on a number of factors, one of the most important being the degree of contamination. In a process which uses unsegregated household waste, the level of contamination may be such that it becomes difficult to find a beneficial use for the digestate. Under these circumstances, it will be disposed of to landfill, where because it has been pre-treated, it will present a lower risk of pollution than raw waste. The bulk will also have been reduced which will help to maximise the use of available landfill capacity. If limits of landfilling of organic wastes under the proposed EC landfill directive are implemented, such materials may require further treatment before disposal.

Digestate produced from pre-segregated waste may have application as a solid conditioner, but will often require further treatment (composting - ‘maturation’) to reduce its silage like odour. Public acceptance of the finished product will determine whether or not there is a market for anaerobically digested waste and therefore control over contamination is particularly important. All of the issues relating to marketing compost derived from waste also relate to digestate.

Anaerobic digestion is currently an expensive option, partly because it has yet to establish a track record in the UK, only treats part of the waste stream and required considerable capital investment. A clear understanding o the objectives of anaerobic digestion as part of a waste management strategy is required at the outset - ie is it to make landfill more acceptable or produce a saleable digestate. These considerations will determine the type of project and its associated costs.

Environmental Impacts

One of the concerns with any anaerobic digestion plant, irrespective of the material being treated, is odour.

The bioreactor and associated buildings, handling and composting facilities etc can be designed and managed to prevent any escape of odour from the system.

Odour impacts can be minimise by appropriate siting of the plant and effective site and plant management, including the use of covered areas with controlled airflow and efficient process management to ensure minimal outdoor storage of untreated materials.

The waste waster produced from de-watering of the solid digestate can contain relatively high concentrations of metals, dissolved nutrogen and organic material which may cause pollution if left untreated. In some cases treatment may be undertaken at a sewage works after discharge of the waste water to the sewer, but the levels may be so high that they breach limits imposed by the water companies. In this case, treatment on site will be required before discharge to sewer. Some of the waste water may also be recycled to the reactor where the dissolved organic material may be converted to biogas.

Impacts related to visual intrusion, noise and pest control will be similar to those associated with other waste treatment plant and with effective planning and management can be controlled to acceptable levels.

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