Waste treatment options
There are many different ways of treating waste leftover after recycling and composting has taken place. Treating waste reduces what is landfilled and can have other benefits such as producing energy.
Derbyshire councils are working towards a recycling and composting rate of 55 per cent or more by 2020.
Reducing waste and increasing recycling and composting is a top priority. However, there will be always be a proportion of waste which cannot be recycled or composted which will need managing in other ways to stop it going to landfill.
Modern ways of treating waste are being used extensively in Europe and now in the UK. They turn waste into a valuable resource instead of sending it to landfill. Materials can be recovered for recycling, with the rest of the waste being used to generate heat and power instead of it just going into the ground.
The councils of Derbyshire and Resource Recovery Solutions (Derbyshire) Ltd (our main waste contractor) are looking in to ways to deal with Derbyshire's waste that can't be recycled or composted and currently goes to landfill. A treatment plant is currently being built in Sinfin, Derby which will reduce the amount of waste ending up in landfill.
What kind of treatment options are available?
There are a number of treatment options which are used in Europe and increasingly in the UK. They include:
Anaerobic digestion (AD)
AD can only be used to treat biodegradable material, such as garden and food waste. The material is put into a closed container without oxygen. Bacteria break down the waste creating a biogas that can be used as fuel or burned to create heat or electricity. A fibre is produced which can be used as a nutrient rich soil conditioner. A liquid is also produced which can be used as a fertiliser.
Mechanical biological treatment (MBT)
The term MBT can cover a number of different combinations of mechanical sorting and biological processes. The waste can either be sorted, recyclables removed and then treated biologically by AD with any non-biodegradable waste sent to landfill. Or the waste can be dried and then sorted into recyclables going for recycling. The waste leftover can either be processed further as fuel, used as a low grade soil conditioner or sent to landfill.
Energy from waste (EfW) creating electricity/heat
EfW burns waste in large containers to over 850 degrees Celsius with enough oxygen that the waste is fully burnt. The heat created is put through a boiler to generate steam. The steam is then used to drive a turbine which makes electricity or is used for heating. 'Bottom ash' is produced which can be recycled into aggregate. The residues from the air pollution control systems 'fly ash' which are produced are hazardous and require disposal at special landfill sites.
Advanced thermal treatment (ATT)
There are two main forms of ATT.
Pyrolysis heats the waste to between 300-850 degrees Celsius without oxygen. A 'syngas' is produced which can be combusted. The hot gases are fed through a heat exchanger where steam is produced and fed through a steam turbine to generate energy. Or the syngas is refined to a high quality and used in a gas engine to produce electricity. A solid waste is also produced called 'char' which requires specialist disposal.
Gasification is a similar process to pyrolysis but heats the waste at higher temperatures, above 650 degrees Celsius, but with some oxygen. The amounts of oxygen are not sufficient for the waste to be fully burnt. A 'syngas' is produced that can generate electricity and heat in the same way as for pyrolysis. There is a potential market to recycle the ash into aggregate for the construction industry.