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Mechanical Biological Treatment

07 Aug, 2020 | Return|

According to the Knowledge Product (KP) studies commissioned by the Department of Environmental Affairs (DEA), mechanical biological treatment (MBT) combines both mechanical and biological treatment methods (open windrow composting, materials recycling facilities, anaerobic digestion and in-vessel composting). These are supported by a combination of pre-treatment and sorting techniques at the beginning of the process, and a selection of emissions control and quality control techniques at the end of the process. The mechanical and biological processes can be arranged in either order, with mechanical treatment preceding biological treatment or vice versa. Typical mechanical treatments will include a range of sorting technologies, from simple sieve / trommel separation techniques through to more advanced positive selection techniques like near infrared (NIR) segregation.

Configuration

Waste Streams Accepted	Mixed solid waste (MSW), commercial and industrial waste, wet type
Input capacity ranges	50k–500k tonnes per annum
Typical outputs	Energy, recyclate, fines, stabilised material
Purposes	Stabilise waste producing refuse derived fuel (RDF), useable recyclables and organic products in the process
Indicative capital cost	R852,5 million – R1162,5 million for a 100k tonnes per annum facility
Indicative operational cost	High depending on the scale of the operation. Each project has to be treated on its own merits.
Life Span	20–30 years
Skills requirements	High (microbiologists, chemical engineers and mechanical engineers)
Job creation opportunity	Variable depending on MRF

Technology restrictions

Not suitable for hazardous materials.
Not suitable for bulky or large waste streams.
The MRF component of the process will recover relatively low grade recyclables.

Main licence requirements [insert link] Advantages

Conserving resources and reducing emissions harmful to the environment.
Reduce volume and more rapid waste stabilisation.
Stabilisation of the waste reduces side-effects at the landfill site.
Hazardous waste contaminants will not reach municipal landfill sites due to the sorting of the waste prior to treatment.

Disadvantages

Potential for odour issues.
Green House Gases such as CH₄ and CO₂will be produced but CH₄ may be enriched to biomethane and be used in the same way as natural gas and CO₂ may also be recovered and refined to food grade level. The biomethane will release CO₂ when used as a fuel.
A variety of occupational health and safety issues.
Dry recyclables separated out during the process will be of poor quality.
Demands fixed tonnages of waste.

Case Studies

Mechanical Biological Plant in Athlone, Western Cape, RSA

Read more: (http://www.fge.co.za/project/a-large-waste-to-energy-plant/) In the Western Cape, landfill airspace comes at a premium and as a result landfill tipping fees are higher there than in the rest of the country. Gas prices are also higher in the Western Cape because it is so far removed from the natural gas fields of Mozambique and industrial gases have to be trucked in from Gauteng and further afield. These two factors made it feasible to develop a plant that both a) reduces waste to landfill and b) produces a valuable industrial gas. The New Horizon Energy plant which is part of Clean Energy Africa is designed to process 480 tons/day of municipal solid waste (MSW) using mechanical means to extract the organic fraction. The first stage of the process is the recovery of plastic, glass and cans in a dirty material recovery facility (dMRF). The organic fraction of MSW will be supplemented with 70 tons / day of pure organic waste and processed in an anaerobic digestion (AD) plant to produce approximately 1,250 Nm³ / h of biogas. After hydrogen sulphide (H₂S) and volatile organic compounds (VOC) removal and further upgrading, the biogas will be separated into its two main components – biomethane and CO₂. After compression to 250 bar the biomethane will be tankered off site as compressed natural gas (CNG) and the liquid CO₂ will be stored on site at -25°C, ready for dispatch to industrial clients. Specialised machinery and equipment is used to process the raw MSW to produce the following useful products:

Organic sludge for use in the AD plant.
Recyclables (paper, cardboard, plastics, metals, unbroken glass).
Refuse derived fuel (RDF) to be used as a furnace coal replacement.

As a result, only the crushed glass and minerals (grit, sand, etc.) from the original raw MSW stream will find their way back to landfill, thereby diverting as much as 90% by volume from landfill. After processing in the AD plant the organic digestate will be dewatered and processed into a nutrient-rich organic fertiliser, thereby ensuring that as little as possible goes to waste.