Mechanical Biological Treatment
What is Mechanical Biological Treatment (MBT)?
Mechanical Biological Treatment is a term used to describe a group of technologies that deal with residual municipal waste – that is waste which has not been collected for recycling or composting and has been left in wheelie bins or black bags. When waste goes to an MBT facility, any further recyclable or re-usable material will be extracted from it by a mechanical process, with the waste then subjected to further biological treatment to produce a more biologically stable material. MBTs operate as part of a wider integrated approach involving additional treatment stages, such as energy recovery.
How does the MBT process work?
As the name suggests, there are two main stages – a mechanical stage and a biological stage. The mechanical stage has two main roles, breaking down the waste into smaller parts (e.g. by shredding) and removing some recyclable material. During the biological stage, the waste will either be composted or digested, usually in an enclosed system. If an anaerobic digestion system is used, it can produce methane which can provide energy for the plant.
Are there many examples in the UK or beyond of MBT being used as an effective waste management solution?
There are currently more than 70 MBT plants operating in Europe. Most MBT technologies have been developed in Germany, but Austria, Switzerland and the Netherlands are also developing markets. Some approaches have been developed in the UK, such as the composting-based MBT plant in Dorset, which opened in 2006. The facility is designed to take 50,000 tonnes per year of mixed waste, and produces around 9,000 tonnes of compost-like output per year. There is also a facility in Leicester.
What is wrong with the current method of using landfill to deal with municipal residual waste – would this not be less expensive than building MBT plants?
In the past, almost all residual municipal waste has been sent to landfill. Historically, landfill has been cheaper but this will not be the case for much longer. The European Landfill Directive has set very stringent targets to reduce the amount of waste we send to landfill and failure to comply could potentially lead to European infraction fines which, in a worst case scenario, could be as much as £500,000 per day for Northern Ireland. As well as the financial consequences, landfill is a major contributor to greenhouse gas emissions due to the methane released by decomposing waste.
What is the average size of an MBT plant?
The capacity of MBT plants can range from 10,000 tonnes per annum (tpa) to facilities of 250,000tpa. The NWRWMG’s plan is to install plant/s to give a capacity of about 140,000tpa.
Will an MBT plant have any adverse effects on traffic and transport in the surrounding area?
MBT plants will most likely be served by large numbers of heavy goods vehicles (HGVs) so the site layout and road configuration will need to be suitable to accept a range of light and heavy vehicles. The traffic movements anticipated for a 50,000tpa plant would be 20-30 refuse collection vehicles a day.
Will an MBT plant produce any emissions which will be harmful to the local population or staff working at the facility?
Studies have found no increase in cancer or asthma in populations close to MBT facilities (Source: www.defra.gov.uk). Emissions and potential risks to health can be readily controlled in an MBT facility.
Will there be any problems of unpleasant odour or dust generated by an MBT plant?
MBT facilities are usually located within an enclosed building so potential odour emissions can therefore be controlled through the building ventilation system. Biofiltration systems can be used to control odours in air extracted from working areas if required.
Will an MBT facility attract flies, vermin or birds?
The enclosed nature of MBT operations will limit the potential to attract vermin and birds. Effective housekeeping and on-site management of tipping and storage areas will minimise the risk that flies could accumulate during hot weather.
Will an MBT plant create a lot of unwelcome noise for residents living in the area?
The facility will be inside a building and will be screened so as not to cause any significant nuisance to the local community.
Will there be any litter problems produced by an MBT facility?
With MBT, litter problems are minimised by adhering to good working practices, such as vehicles using covers and reception and processing being undertaken indoors.
Will an MBT plant have any significant impact on local water resources?
Common to any new waste treatment process, the enclosed nature of MBT operations significantly reduces the potential for impacts on the water environment. Pollution of water is unlikely due to MBT facilities being under cover and rainfall is unlikely to come into contact with the process.
What is energy recovery?
Energy recovery is a waste management method used to recover energy from residual municipal waste – the waste you are left with after recycling and composting. Energy recovery disposes of waste in such a way that energy is produced, in the form of electricity and/or heat.
How does the energy recovery process work?
There are a number of energy recovery technologies, including incineration, gasification, pyrolysis and use of cement kilns. The North West Joint Committee has decided that incineration is not an acceptable technology in the North West Region.
Gasification is one of the main forms of advanced thermal treatment of waste. It is used to recover energy from residual waste. Gasification uses small amounts of oxygen to dispose of waste in such a way that useful energy is produced, in the form of electricity and heat. Gasification involves the partial oxidation of a substance. This means that oxygen is added but the amounts are not sufficient to allow complete oxidisation or full combustion to occur. The temperatures employed are typically above 650 degrees Celsius and the main product is a syngas, which is used to drive turbines or gas engines to produce electricity. The other main product is a solid residue of non-combustible materials (ash) which contains relatively little carbon.
Pyrolysis is a similar process to gasification but uses no oxygen rather than restricted oxygen, as in gasification. In the cement kiln process, the use of fossil fuels is displaced and that energy saved.
Are there many examples of energy recovery being used as an effective waste management solution?
More than 100 advanced treatment plants have been built and are operating in Europe, North America and Japan, capable of treating over 4 million tonnes of waste per year (Source: Juniper Consultancy Services Ltd).
What is wrong with the current method of using landfill to deal with residual waste – would this not be less expensive than energy recovery?
In the past, almost all residual municipal waste in the UK was sent to landfill. Historically, landfill has been cheaper, but this will not be the case for very much longer. In the UK, landfill tax will soon be over £48 per tonne, so we must reduce the amount of waste we send to landfill. By treating waste to produce electricity, waste becomes a valuable resource.
Will an energy recovery facility create problems of odour, dust and/or litter?
All waste treatment facilities are strictly regulated and will have systems in place to limit odour. Energy recovery facilities are enclosed and fitted with ventilation and filter systems to prevent odour and dust from escaping. The Northern Ireland Environment Agency will strictly monitor the operation and good practice at any plant to make sure there are no problems with odour, dust and litter at the site.
Will an energy recovery facility produce a lot of unwelcome noise for residents living in the area?
The main noise coming from the facility will be produced from vehicle movements and from the mechanical processing of the waste and air ventilation systems. Hours of receiving waste will therefore be limited to times of the day that will not cause a nuisance to the local community.
How significant will the vehicle movements around an energy recovery facility be?
During the planning stages for the facility, the number of proposed vehicle movements will be taken into consideration and the access to the site and its impact on the community will also be considered. For an average-sized plant that would accept 50,000 tonnes of waste per year, it is estimated (by Defra) that up to 20 refuse collection vehicles per day would be anticipated.
Will an energy recovery facility produce emissions that will be harmful to the environment?
All energy recovery plants must comply with strict European and National regulations, which ensure that the emissions produced by the facility and released into the air are thoroughly cleaned and constantly monitored. The impact from any dust and bio-aerosols that could be produced by the facility is limited by the facility having enclosed collection areas and ventilation systems.
Will emissions from an energy recovery facility have any negative impacts on the health of either staff working there or residents living in the area?
An independent study on health and environmental impacts of waste management processes found no evidence of health effects linked to advanced thermal treatment facilities (Source: www.defra.gov.uk). The Health Protection Agency has looked at studies carried out and is not aware of any consistent or convincing evidence of a link with adverse health outcomes.
Will an energy recovery facility prove an eyesore or impinge on the physical appearance of the surrounding area?
Planning conditions suggest any new facility built should be in keeping with the surrounding area, whether it’s of an industrial or rural setting. Good design of facilities can help to limit their visual impact.
Are there any environmental benefits of introducing an energy recovery facility?
The environmental benefits of energy recovery are that it enables the production of renewable ‘green’ energy, displaces the use of fossil fuels, reducing greenhouse gas and Co2 emissions and helps address concern about global warming as it produces a positive environmental effect.
Will an energy recovery plant have any employment benefits for the local population?
Staffing levels, including technical competence, management and administrative resources, will be required and will vary, depending on the size of the facility. Typically, there will be 30-40 employees for plants of 60,000tpa+. The plant would operate on a shift system, to allow for 24-hour operations.
Will an energy recovery plant have any educational benefits?
A facility is likely to provide vocational training for staff. New facilities may also be built, with a visitor centre to enable local groups to view the facility and learn more about how it operates.
Is there anything I can do to help improve the management of waste?
You can make a difference and help your local council in its attempts to tackle waste management more efficiently. Try to recycle or compost even more of your household rubbish than you are currently doing.