In-vessel composting is mostly associated with the treatment of animal by-products and catering wastes. When the Composting Association published its first guide to in-vessel composting in 1999 – two years before the foot and mouth outbreak – the industry was anticipating the EU directive on the landfill of waste.

The advantage of in-vessel composting was seen as greater control over the composting process in terms of temperature and moisture to produce a more consistent process environment and output. It also gives better control over environmental emissions such as leachate, odours and bioaerosols.

After a rigorous risk analysis commissioned by DEFRA, the Animal By-Products Regulations 2003 were enacted, in order to allow once more the composting of animal by-products and catering waste. But this was under much stricter controls for hygiene and processing, and in-vessel composting became a statutory requirement.

In 1999, the Composting Association guide to in-vessel composting contained 25 entries from system suppliers, four from the UK, eight from Europe and 13 from the US and Canada. In 2004, a second edition of the guide was published.

Although there were a number of existing systems throughout Europe, and enclosed mushroom composting tunnels in the UK, many were unable to comply with the requirements of the UK’s national rules for composting catering waste – as a result, technological development was rapid and UK-based.

In-vessel systems have to ensure that composted catering waste reaches a temperature of 60ÚC for two days at a particle size of less than 40cm for the first of two stages of processing. The second stage must be repeated or may be carried out in open windrows, to reach a temperature of 60ÚC for eight days and being turned three times.

A unique situation

The requirements for treating catering waste in the UK are quite different from anywhere else in Europe, as a consequence of the foot and mouth outbreak, so none of the European systems had a known track record of being able to meet the UK conditions.

This situation opened up the UK domestic market to a wide range of technological innovation. Each site – and its system – is required to undergo a validation and approval process by the state veterinary service.

Innovation was required to meet the hygiene and sanitisation requirements and the condition that required two distinct stages of processing. This was met in a number of different ways, typically in double banks of batch tunnels, vertical composting units, and single vessels with internal mixing between two processing sections.

The animal by-products regulations have not been the only driver for the development of biological treatment. The non-fossil fuel obligation and the premium paid for electricity generated from renewable resources has also revitalised an interest in anaerobic digestion. Here organic wastes, including animal by-products and catering wastes, are digested by specific bacteria in an oxygen-reduced environment.

Energy from waste

Although a semi-liquid digestate is produced – which can be used or further treated as a soil-improver – the primary output is methane gas, which can be used to generate electricity for use in the process and as an input to the national grid. Other options are to use the produced heat either in the process or facility or for district heating projects.

The landfill allowance trading scheme (LATS) in England and the landfill allowances scheme in Northern Ireland, Scotland and Wales have put a responsibility on local authorities to divert biodegradable municipal waste from landfill to meet the requirements of the landfill directive.

Composting and anaerobic digestion technologies are now essential parts of mechanical-biological treatment (MBT) processes for residual waste treatment. The residual waste, rich in organic material, is what is left after the dry recyclables – such as paper, glass and plastics – have been removed, either in separate collections or by mechanical separation of mixed municipal waste such as in a materials recycling facility.

In a great many cases, the technology and the equipment used for source-separated organic wastes can be used directly before – or more usually after – the mechanical separation part of the process. A number of radically different designs have been used, for example vertical processes or large rotating continuous process systems.

At the other end of the scale, there is still a requirement for small local composting sites and systems. Composting technology has developed systems for in-vessel composting suitable for small-scale use. These include composting containers that fit on the back of a lorry and deliver compost right to the field for use, and systems based on plastic or membrane tubes.

Public awareness on the rise

Environmental protection requirements continue to tighten, and local communities become more informed and concerned about the impacts of composting. Composting technology has had to develop to take into account increasingly stringent legislation and also the need for an increasing number of both large centralised composting sites and small local sites. Local facilities are increasingly being sited on industrial estates or near centres of population.

Composting technology has had to adapt to an ever changing regulatory and economic environment, and the variety of systems described here is a snapshot of where the industry is today. But it must not be forgotten that technology alone cannot account for the expansion of the industry.

Hand in hand with the technological developments there have been developments in management, environmental awareness and environmental protection, along with the development and training of the companies and people who operate the facilities.

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