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Composting
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Composting

Composting is a natural biological process in which organic material is broken down by the action of micro-organisms. Typical materials suitable for composting include, green waste and putrescible wastes with pre-sorting and screening to remove non-compostables, plus other enriched organic waste streams (sewage sludge, agricultural, food processing wastes).
Rural Compost Facility

The decomposition process takes place in the presence of air and results in elevated process temperatures, the production of carbon dioxide, water and a stabilised residue, known as humus. A high degree of stabilisation can generally be achieved in 3-6 weeks, however further 'curing' of the humus is normally carried out. For composting to occur in an optimum manner, five key factors need to be controlled; temperature, moisture, oxygen, material porosity and Carbon: Nitrogen ratio.

Composting processes for municipal waste management primarily fall into two categories: windrow composting, for green, or garden derived wastes and in-vessel composting for both garden and kitchen / catering derived organic wastes.

The following very accomplished video, which is based upon 3D computer modelled graphics, shows a composting plant system.

 

Windrow composting

Windrow composting is an established technology for dealing with green wastes in the UK, where the material is piled in elongated rows and aerated through either turning of the windrows or through air forced through the material. This may take place in buildings or externally.

Schematic of Inputs and Outputs of a typical Windrow Composting process

A typical Windrow Composting process schematic

Typical capacity: 50,000 tpa (range 2,000 to 100,000 tpa)

Land requirements: Approximately 2.5 to 5 Ha for 50,000 tpa, windrow management and additional space for curing and stockpiling. Less required for In-vessel composting and forced aeration systems.

Capital costs: Varies with size. For a more hi-tech 40,000 tpa plant the costs would range from (See full report). Lower technology facilities and those of a smaller scale may have capital costs of less than (See full report).

Operating costs: Direct treatment costs will be in the region of (See full report) per tonne for hi-tech, (See full report) for low tech facilities, costs varying with throughput and economies of scale. This excludes the cost of separation or disposal of residues or un-saleable material.

Staffing requirements: Staffing levels, including technical competence, management and administrative resources will vary depending on the size and the technology adopted.

In-vessel composting

In-vessel composting embraces a variety of techniques whereby the kitchen or garden derived wastes may be composted in an enclosed vessel or tunnel. The advantage of these processes is that they are more controlled and can be designed to achieve specified temperatures to facilitate bacteria destruction (in accordance with the requirements of the Animal By-products Regulation). These technologies have only had limited experience in the UK to date but are rapidly increasing in number due to recent legislation.

Schematic of Inputs and Outputs of a typical In-Vessel Composting process

A typical In Vessel Composting process schematic

Typical capacity: 50,000 tpa (range 2,000 to 100,000 tpa)

Land requirements: Less required for In-vessel composting than windrow systems, the larger scale (200,000tpa) may require 5 – 6Ha

Capital costs: Higher than Windrow facilities, a 40,000tpa plant may have capital costs of (See full report). Large systems (also composting with sewage sludge) ~ 200,000tpa have been quoted at (See full report).

Operating costs: Direct treatment costs will be in the region of (See full report) per tonne. This excludes any potential revenue from sale of compost

Staffing requirements: Staffing levels, including technical competence, management and administrative resources will vary depending on the size and the technology adopted.

Strengths and Weaknesses of Composting

Strengths

Weaknesses

Can reduce volume of organic waste fraction of MSW by 25-50%

Potential odour and leachate production, requires management

Stabilises organic fraction of MSW

Treats only the organic fraction of the waste stream (Green waste only for windrow, green & kitchen for in-vessel)

Potential useable product, although market development is necessary for higher value materials

Useable product specification and markets variable

Potential of co-composting operations with other waste streams e.g. paper, sewage sludge

Sensitive to cross contamination by glass and plastics, therefore requires careful source segregation or further post –treatment

May prove to be an important pre-treatment method under the EC landfill Directive

Animal By-products Regulation issues

Reduces organic wastes from landfill, which reduces the production of landfill gas and leachate.

If landfilled may still count as BMW and be subject to active Landfill Tax

Other Issues

Composting only deals with the organic (garden & kitchen waste) element of the municipal waste stream. Composting facilities may suffer from similar Not-In–My-Back-Yard (NIMBY) issues to other waste management facilities. There is a requirement for suitable applications for the compost and there is a British Standards Institution Publicly Available Standard (BSI PAS 100) for the compost quality in order to help standardise the market for compost materials. The increased regulation of composting, through the Animal By-Products legislation and other regulatory controls have increased the complexity and costs of composting operations in recent years.

The Dano Drum system is used in conjunction with composting.

For more information on Commercial Composting click here.

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