MECHANICAL BIOLOGICAL TREATMENT
MBT relates to a group of solid waste treatment systems. These systems enable the recovery of materials contained within, and the stabilisation of, the biodegradable component of the material. The plant’s sorting component resembles a materials recovery facility. This component is either configured to recover individual and/or recyclable elements of the waste (such as metals, plastics, glass or paper), or produce a solid recovered fuel (SRF) that can be used for the generation of power either immediately, or stored for future use. The process typically involves factory style conveyors, industrial magnets, eddy current separators, trommels, shredders and other tailor made systems. Alternatively, the sorting is done by hand.
MBT is already well established in Italy, Germany and France and has become a cornerstone of West Sussex, Lancashire County Council’s and North London Waste Authority’s (NLWA) waste management strategies.
AD is a series of processes in which micro-organisms break down biodegradable material in the absence of oxygen. It is widely used to treat waste water sludges and organic wastes because it provides volume and mass reduction of the input material. As part of an integrated waste management system, AD also reduces the emission of landfill gas into the atmosphere and comprises a renewable energy source. This is due to the process producing a methane and carbon dioxide rich biogas suitable for energy production, helping replace fossil fuels. Also, the nutrient-rich solids left after digestion can be used as fertilizer. Combined MBT/AD plants can have capacities from 50,000 to 250,000 tonnes of waste per annum. However, because of its flexibility, smaller scale plants dispersed over a wider area reduce the transportation costs and overall carbon footprint.
Denmark plans to increase its AD waste disposal capacity by 10-20 times over the next 10 years. Its use is being expanded widely elsewhere, and it is currently used to dispose of all Tel Aviv’s waste.
The advantages of the above two alternatives over incineration are:
- Much lower effect on climate and environment.
- Quicker to build and operate, with less planning obstacles.
- Cheaper to build and operate. WRAP have found that MBT has a median cost of £53 per tonne compared to £80 per tonne for modern incinerators.
- MBT could avoid burning plastics altogether.
- Both are likely to generate higher employment levels.
Plasma waste treatment
Plasma waste treatment has over the past decade become a more prominent technology because of the increasing problems with waste disposal and because of the realization of opportunities to generate valuable co-products.
Plasma vitrification of hazardous slags has been a commercial technology for several years, and volume reduction of hazardous wastes using plasma processes is increasingly being used. Plasma gasification of wastes with low negative values has attracted interest as a source of energy and spawned process developments for treatment of even municipal solid wastes.
Numerous technologies and approaches exist for plasma treatment of wastes. This review summarizes the approaches that have been developed, presents some of the basic physical principles, provides details of some specific processes and considers the advantages and disadvantages of thermal plasmas in waste treatment applications