Mechanical Biological Treatment - Biological Processing

Biological Processing

See also: composting and anaerobic digestion

The "biological" element refers to either:

• Anaerobic digestion
• Composting
• Biodrying

Anaerobic digestion harnesses anaerobic microorganisms to break down the biodegradable component of the waste to produce biogas and soil improver. The biogas can be used to generate electricity and heat.

Biological can also refer to a composting stage. Here the organic component is broken down by naturally occurring aerobic microorganisms. They breakdown the waste into carbon dioxide and compost. There is no green energy produced by systems employing only composting treatment for the biodegradable waste.

In the case of biodrying, the waste material undergoes a period of rapid heating through the action of aerobic microbes. During this partial composting stage the heat generated by the microbes result in rapid drying of the waste. These systems are often configured to produce a refuse-derived fuel where a dry, light material is advantageous for later transport combustion.

Some systems incorporate both anaerobic digestion and composting. This may either take the form of a full anaerobic digestion phase, followed by the maturation (composting) of the digestate. Alternatively a partial anaerobic digestion phase can be induced on water that is percolated through the raw waste, dissolving the readily available sugars, with the remaining material being sent to a windrow composting facility.

By processing the biodegradable waste either by anaerobic digestion or by composting MBT technologies help to reduce the contribution of greenhouse gases to global warming.

Usable wastes for this system:

• Municipal solid waste
• Commercial and industrial waste
• Sewage sludge

Possible products of this system:

• Renewable fuel (biogas) leading to renewable power
• Recovered recycable materials such as metals, paper, plastics, glass etc.
• Digestate - an organic fertiliser and soil improver
• Carbon credits – additional revenues
• High calorific fraction refuse derived fuel - Renewable fuel content dependent upon biological component
• Residual unusable materials prepared for their final safe treatment (e.g. incineration or gasification) and/or landfill

Further advantages:

• Small fraction of inert residual waste
• Reduction of the waste volume to be deposited to at least a half (density > 1.3 t/m³), thus the lifetime of the landfill is at least twice as long as usually
• Utilisation of the leachate in the process
• Landfill gas not problematic as biological component of waste has been stabilised
• Daily covering of landfill not necessary