Disposal Routes

This section details the different technologies that are available to treat waste that we produce. Some of these technologies are yet to reach a commercial scale within the UK but are approved within other parts of Europe. More often then not our waste is landfilled, but as sites across the UK are becoming limited in capacity we have to find new ways of disposing or utilising our waste.

Advanced Thermal Treatment

There are two main types of advanced thermal treatment, pyrolysis and gasification.

Pyrolysis is a medium temperature thermal treatment process where organic derived materials in the waste are broken down under the action of heat and in the absence of oxygen. Only carbon based materials can by pyrolysed.

Gasification operates at much higher temperatures, where air or oxygen is used to partially combust the waste.

Both processes produce gas known as syngas and a solid residue known as char. Both of which will require disposal.

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Anaerobic Digestion

Anaerobic digestion takes biodegradable waste such as garden and kitchen waste, and converts this in a digestate (containing biosolids and a liquid) and biogas. This is a biological process where the waste material is decomposed by bacteria in the absence of air. This is the key differentiation from composting.

The process takes place within an enclosed vessel system and under controlled conditions at elevated temperatures. The decomposition process leads to the production of methane, a biogas, which can be harvested and used as fuel to produce electricity.

Following the decomposition process the digestate containing biosolids and liquids can be used as a biofertiliser subject to quality and market availability.

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Autoclaving

Autoclaving is seen as a pre-treatment process prior to waste disposal. This technology has been used to sterilise hospital waste for many years using a steam process. Waste is placed into a pressurised steel drum and processed under the action of steam. Waste is reduced to a flock like material.

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Composting

This is a biological process similar to anaerobic digestion but it uses air and the action of micro-organisms. To ensure composting occurs at its optimum, five factors need to be taken into consideration:

• temperature;
• moisture content;
• oxygen concentration;
• material porosity; and
• the relative amounts of carbon and nitrogen within the material.

There are two types of composting process, windrow for green or organic garden derived wastes and in-vessel for garden and kitchen waste. The former type uses shredded green or garden waste and piles these into elongated rows known as windrows. These are aerated through turning or by forcing air through the material.

The latter embraces a variety of techniques to compost garden and kitchen waste in an enclosed vessel or tunnel. These systems benefit from a greater degree of control of the composting process and can be designed to achieve and maintain specified temperatures to facilitate optimum bacterial decomposition.

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Energy from Waste

Energy from waste (EfW) facilities combust waste under controlled conditions for three reasons:

• to reduce the quantity
• to eliminate any hazardous properties
• to generate electricity and / or heat.

EfW plants have stringent regulatory targets placed upon their operational performance to ensure emissions and flue gases are controlled. There are two residues from the combustion process a solid bottom ash and flue gas treatment residues. The bottom ash can be reused within the construction industry however the flue gas treatment residues may harbour hazardous substances that require specialist disposal.

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Landfill

Landfill is a waste disposal system whereby waste is deposited within the ground on a permanent basis. Landfill sites are regulated by the Scottish Environment Protection Agency to ensure they function in accordance with specific environmental guidelines. Landfill sites are required to be lined with an impermeable membrane to prevent loss of leachate (a toxic liquid by-product of waste decomposition) and covered to prevent wind blown litter and attraction of vermin. Methane is also produced from the decomposition process, to ensure this does not accumulate within the landfill site this gas has to be flared.

By disposing of waste to landfill the value of that waste material cannot be recovered.

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Mechanical Biological Treatment

Mechanical Biological Treatment (MBT) is a process of pre-treating waste prior to disposal or reprocessing. The purpose of MBT is to separate a mixed waste stream into several component parts to give further options for recycling and recovery.

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Recycling

Recycling involves taking a waste item and reprocessing this material into a usable item either in the same form, as the original product, or into a different product.

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