Composting
Composting has been practiced across the world for centuries. Composting only deals with the organic part (kitchen and garden waste) which makes up about a quarter of the household bin. Other technologies are needed to deal with the rest. However, after the outbreak of foot and mouth disease, new controls mean that composting kitchen waste cannot take place in the open air. There are different types of composting technologies but all work on the same principle of micro-organisms breaking down the organic waste to produce compost. The production of compost is speeded up by controlling the amount of heat, air and water. The basic compost is then left to mature before it can be used. Composting is used extensively across Europe.
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Advanced Thermal Treatment (ATT) Advanced Thermal Treatment (ATT) is an alternative thermal approach to incineration. The two types of ATT are Pyrolysis and Gasification. Both work on the principle that the waste is heated to produce a gas which is then burnt to generate electricity. Gasification produces more gas and only leaves a solid material similar to charcoal. Pyrolysis produces less gas but also produces useful chemicals as well as a solid material. Gasification is not a new technology - it was used to produce town gas from coal for over a century - but its use for household waste is much newer. Unlike incineration, the waste treated in ATT requires some pre-treatment (e.g. in a mechanical biological treatment plant.) ATT is proven to deal with single types of waste eg. Tyres and clinical waste. It is not yet widely used to deal with household waste, but some large plants are now operating particularly in Japan and Germany. |
Anaerobic Digestion Anaerobic Digestion is a biological process in which organic waste (e.g. fruit and vegetable peelings) is broken down by micro-organisms This is done inside containers without any oxygen present. As organic waste only makes up about a quarter of the household bin other technologies are needed to deal with the rest. The process produces a gas called 'biogas' as well as liquids and solids called a 'digestate'. The biogas can be either burnt on the site to produce electricity and heat, or cleaned and used as a vehicle fuel (e.g. in a bus fleet.) The digestate can be used as a fertiliser or compost. As with composting, following the Foot and Mouth outbreak, special operating procedures must be met. Anaerobic digestion is a proven technology in the UK for dealing with sewage sludge and farm waste. Plants treating both separated and residual household waste are in use in France, Germany and Spain. |
Mechanical Biological Treatment (MBT)
Mechanical Biological Treatment (MBT) is a general term used to describe a range of processes applied to residual household waste. In its simplest form MBT uses machines to sort residual household waste into different types of materials for recycling and one of two further treatmevents: 1 Composting the organic part (kitchen and garden waste). 2 Drying the rest of the waste so that it can be used as a refuse derived fuel (RDF). This RDF can then be used as a fuel for energy production, for example burnt with coal in power stations. MBTs are used in Europe, particularly in Germany and Italy. MBT plants to treat almost 1 million tonnes of waste are now planned for the UK. |
Autoclaving
Autoclaving works like a pressure cooker using high temperature steam to 'cook' residual household waste. This sterilises it, kills off harmful germs and cleans things like cans and bottles which can then be sorted out and recycled. The remaining fibre-like material can be further treated either in an ATT plant, used as a form of refuse derived fuel (RDF), or used in the manufacture of various building materials (e.g. roofing tiles.) There are currently no operational autoclaving plants that treat household waste, but there are some small scale demonstrator units in Europe. However the process has been used on a smaller scale for many years to treat clinical waste from hospitals.
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Incineration
Incineration works like a power station but burns waste instead of coal. It produces energy which is used to generate electricity and heat. It is not necessary to sort or pre-treat residual household waste before it is incinerated. Roughly25% of the waste entering the plant leaves as ash from which metals can be removed for recycling. This so-called "bottom ash" can be used as an aggregate or turned into building materials. Incineration plants have to comply with the strictest European pollution control standards. The filtering of gases from these plants to meet these standards produces a toxic ash (weighing equivalent to 5% of the waste that went in). This so called 'fly ash' is kept separate and has to be disposed of in a special landfill site. There are approaching 20 household waste incineration plants currently operational in the UK with a number of others either under construction or for which planning permission has been granted. The technology is widely used across Europe, Scandinavia and North America for managing household waste.
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What Goes In... Organic Waste (Kitchen And Garden Waste) What Comes Out... Compost |
What Goes In... Treated Household Waste What Comes Out... Gas, Liquids And Solids Which Can Be Turned Into Fuel, Products And Electricity |
What Goes In... Household Waste What Comes Out... Compost. Biogas: Which Can Be Used To Make Electricity. Liquids: That Can Be Used As Fertilzer |
What Goes In... Household Waste What Comes Out... Recyclable Materials And Rdf Or Compost |
What Goes In... Household Waste What Comes Out... Recyclable Materials And RDF |
What Goes In... Household Waste What Comes Out... Heat: Which Can Produce Electricity, Ash: Some Of Which Can Be Recycled |
