From autoclaving to plasma gasification - advanced waste treatment options

How we deal with our waste is becoming an increasingly pressing problem as resource use increases and landfill runs out. Here are some of the techniques currently at the forefront of waste management and waste-to-energy processes. None are without their drawbacks.

Material Recycling Facility (MRF): These are the main 'engines' of most processing facilities. MRFs handle massive volumes of municipal materials and commercial and industrial material. A combination of advanced mechanical sorting techniques and manual quality control is used to separate the materials in readiness for recycling.

Mechanical Biological Treatment (MBT): This process combines a sorting facility to remove recyclable elements (such as metals, plastics, glass and paper) from a mixed waste stream with one or other forms of biological treatment (such as composting or anaerobic digestion) used for the residue. MBT plants usually process mixed household waste as well as industrial or commercial waste. They can also be configured to process the waste to produce a high calorific fuel called Refuse Derived Fuel (RDF) consisting of biodegradable wastes and plastics: the RDF can be used in power plants or cement kilns. Some environmental groups oppose the MBT-RDF combination as being a worse option than localised recycling.

Autoclaving: Originally designed to sterilise hospital equipment, an autoclave applies pressurised steam and/or superheated water to equipment and supplies. Increasingly, industrial-sized autoclaves are being used in waste treatment. Unsorted black bags of household waste are fed into the system, which applies heat at 130 deg C or more. A combination of the steam, the pressure and the rotation of the vessels results in the organic contents being broken down into a fibrous biomass and the non-organics being sterilised. The non-organics (aluminium and steel cans, glass and plastics) are now clean and easily recycled. The organic biomass has several uses (as soil conditioner, biomass fuel, or fibreboard for the construction indsutry). 

Anaerobic digestion (AD): This process treats food or garden waste in an oxygen-free environment to produce biogas (comprising roughly 65 per cent methane and 35 per cent C02, which can be converted to energy) and a nutrient-rich residue which can be used as a fertiliser. AD also has the potential to produce combined heat and power (CHP). AD is a proven technology which attracts Government support in the UK through Renewable Obligation Certificates (ROCs). It’s also a ‘carbon positive’ technology, since the residue reduces the need for fertilisers made from fossil fuels. Drawbacks include the fact that small amounts of stray plastics or glass cause problems. AD works best near to large farms with dependable levels of slurry to feed it.

Advanced Anaerobic Digestion (AAD): Includes a pre-digestion phase which helps to optimise the key processing stages of AD by breaking down the organic materials beforehand. This can be done with thermal hydrolysis (the Cambi proces), which uses initial high temperatures combined with high pressures, or through enzymic hydrolysis (the Monsal process), which uses a more gradual temperature rise over a period of several days. The end result is the same – a far greater conversion of organic matter into biogas once the material reaches the AD process. Another side effect is an increase in the organic value of the biomass residue, and a reduction in its volume.

In-Vessel Composting (IVC): IVC treats green and/or food waste in an enclosed but oxygenated (ie normal) environment. It produces compost, but not energy.

Gasification & Pyrolysis: These techniques involve heating waste until it breaks down into a flammable mixture of hydrogen and carbon monoxide (known as syngas) and a residual slag or ash. The syngas can be used to power gas turbines to generate electricity or converted into other fuels such as methane, ethanol or synthetic diesel. Drawbacks include the fact that syngas from waste contains a tar residue that needs to be filtered out first – thus reducing the efficiency of the process. Opponents also argue that it’s a dirty process which releases emissions, particularly dioxins, into the atmosphere.

Plasma Gasification: This involves super-heating waste to temperatures of 1300 deg C or more using a high-voltage current, which breaks down complex molecules. The process produces syngas and a glassy slag which can safely go into landfill or even be used in the construction industry.  Drawbacks include huge capital costs, and the high running costs to generate the electricity to create the plasma arc. There are also concerns around metal pollutants created in the plasma-arc process which could get into the atmosphere.