There is a wide range of waste treatment techniques and technologies available that significantly reduce the amount of residual waste for which landfill is the only other option.
Some of these technologies generate energy (heat and/or electricity) from the treatment process. These technologies are particularly helpful in allowing local councils to not only meet their landfill reduction targets but also offer secure, sustainable and cost-effective energy supplies to local businesses and households. Some facilities capture the steam generated in the treatment process and use it to heat local houses or businesses.
Some of the more popular residual waste treatment technologies available on the market today are:
Mechanical Biological Treatment (MBT) is a general term to describe waste treatment systems that involve both mechanical and biological processes. MBT is predominantly a volume-reducing process, recovering recyclable materials from residual waste that would otherwise have been destined for landfill and biologically treating the biodegradable component of that waste.
There are three key stages:
Mechanical Heat Treatment (sometimes called Autoclaving) is essentially a steam treatment process which has been used on a relatively small scale by hospitals for many years to sterilise clinical waste.
When used to treat normal household waste, the waste would normally be shredded by machines before going through the autoclaving process. The shredded waste is processed in a pressurised sealed drum under the action of high-temperature steam. After about an hour, materials such as metals, glass and plastics have been partially cleaned ready for recycling (labels and glue are stripped away from food cans and glass bottles). Any non-recyclable waste has turned into a fibre-like material.
The main drawback of Mechanical Heat Treatment is that while it cleans and improves the quality of most of the recyclable materials, some plastics are deformed by the heat of the steam making them difficult to recycle.
At the moment, uses for the fibre-like material are limited and it is usually sent to landfill or used as a secondary recovered fuel. Trials are taking place to see if this material could be recycled into, for example, building materials.
Energy Recovery is where waste is burnt at very high temperatures to reduce its volume (by up 90 per cent) and to produce electricity and/or heat.
When the waste is burnt, it produces high pressure steam which is converted into electricity by turbines and a generator. The electricity can then be fed into the National Grid or used locally.
In addition to electricity production, some energy from waste plants can also be used to generate high-pressure hot water or steam that can be used to heat local houses or businesses. These plants are known as ‘combined heat and power plants’ because of the two types of energy they produce.
Approximately 25-35 per cent of the waste put through the incineration process is non-flammable (e.g. metals, glass) so at the end of the process, some reside (known as ‘bottom ash’) remains. This bottom ash is recovered for use in road construction or building products.
The emissions from Energy Recovery Facilities are controlled by the introduction of materials such as lime and activated carbon. At the end of the process, these materials are separated, collected and, due to the lime content, are designated as ‘hazardous waste’. They represent approximately three per cent of the total amount of material entering an Energy from Waste facility. Once collected, it is either sent to a hazardous waste landfill site or is treated so that it is no longer hazardous and sent to a normal landfill site.
Energy Recovery Facilities are becoming increasingly common around the country as they not only help councils meet their landfill reduction targets but also offer the additional benefit of providing a sustainable and secure energy supply. In 2008, 11 per cent of municipal waste in England was handled by an Energy Recovery Facility. This is expected to rise to 25 per cent by the year 2020.
Despite this growth in the extraction of energy from waste, the UK still lags well behind its European neighbours. On average across Europe in 2008, 17 per cent of municipal waste was treated in an energy from waste plant. Denmark leads the way, recovering energy from 54 per cent of its municipal waste.
Advanced Thermal Treatment is a term used to describe two types of technologies: ‘Gasification’ and ‘Pyrolysis’. Both technologies focus on treating biodegradable waste (paper, some plastics, kitchen waste etc).
The waste is heated to extract energy from it but, unlike incineration, Advanced Thermal Treatment technologies require pre-processing to remove oversize items, non-combustible materials (such as metals and glass) and excess moisture. The waste is then shredded to ensure that it degrades more evenly during the treatment process.
• Pyrolysis – in contrast to combustion, pyrolysis heats up the waste in the absence of oxygen at relatively low temperatures of between 300-800°C. The process produces a solid residue of non-combustible materials and carbon (known as ‘char’) and a gas mixture known as ‘syn-gas’ (synthetic gas), a mixture of hydrogen and carbon monoxide. The char either has to be transported to a separate facility to be burnt or sent to landfill. The syn-gas can be burnt to generate steam and turn a turbine, which in turn creates electricity.
• Gassification – somewhere between incineration (which relies on the presence of lots of oxygen) and pyrolysis (which relies on there being little or no oxygen present) is Gasification. In this technology, oxygen is added but the amounts are not sufficient to allow the fuel to be completely oxidised and full combustion to occur. The main end product is syn-gas which can be burnt to generate electricity. A small amount of ash is created from the non-combustible materials.
Advanced Thermal Treatment plants tend to be smaller than other types of Energy from Waste plants, which means they can be located in more built up areas, making it easier to transfer the electricity produced to local houses and businesses.
In the UK the development of Pyrolysis and Gasification technologies is in its infancy, although large scale plants have been built and are in operation in Europe, North America and Japan.