Mechanical Biological Treatment (MBT) is a new term and features more an more often on local authority planning applications. But many are still wonder what it is. Will it help if I say that it can capture and treat what’s left in your rubbish bag after you have recycled as much as you can from your waste before you put the left-over materials in your rubbish bin?
Mechanical Biological Treatment is used to describe not one specific process, but rather an amalgamation of different technologies brought together in an integrated process. MBT plants are designed to process mixed household waste as well as commercial and industrial wastes.
In MBT the first stage of treatment is mechanical. Waste is inspected for large objects which are unsuitable for treatment and then the heavier fraction and larger items are usually separated by passing them through a big inclined washing-machine-like (slowly rotating) drum. It’s called a trommel screen.
The point is that the whole MBT philosophy is really in its infancy. People are learning all the time how best to do this further separation of waste. In the past this rubbish would have gone straight into a landfill. So doing this extra separation is really quite new.
Some of the waste experts who were asked to come up with a separation process actually came up with a system which does it the other way around! It is called Biological Mechanical Treatment. This basically carries out the treatment the other way round, but very often such plants will still be called MBT Plants, because in essence they achieve the same ultimate aim. How confusing is that?
So, I hope that by now we have now established in your mind that Mechanical Biological Treatment (MBT) is a generic term for the integration of a number of waste management processes. Now lets start to use some more concepts which a waste management “savvy person” will need to know. MBT Plants have within them, sub-processes such as materials recovery facilities (MRF), often also refuse derived fuel (RDF) production and biogas producing Anaerobic Digestion Plants.
These are all part of the mechanical separation, sorting, composting fermenting and pasteurising that goes on in order to get rid of all that JUNK that you (the average person) put into the rubbish bin without hardly a thought about where it will be put without harming the earth!
Now these processes themselves have the potential to create a lot of pollution, and i n order to minimise environmental nuisance for odour, fly and noise nuisance, these facilities are required to be housed within a building and normally under negative air pressure.
When people object to plants like incinerators an MBT is an obvious alternative. Most MBT plants produce what is called RDF (refuse derived fuel) which contains the easily burnable part of the rubbish. RDF can be used in cement kilns or power plants and is generally made up from plastics and biodegradable organic waste.
In fact it is a common misconception that all MBT processes produce RDF. The best MBT Plants being built now ferment the organic biological rubbish portion in a digester plant (Anaerobic Digester), rather than enabling it to be burnt on another site. This produces methane biogas which is brilliant because it can be used for all sorts of things, but is most often fed into an internal combustion engine to produce electricity. This electricity is very sought after as it is green plus sustainable, and classified as renewable energy.
If there is no Anaerobic Digestion Plant in an MBT facility the organic fraction may be treated by composting, and the best compost is made from the source separated organic waste collected from households. Composting on a large metropolitan city scale of mixed municipal solid waste has the longest tradition in Germany relative to any other country.
In 1953, the first large-scale facilities in Baden-Baden and Blaubeuren started operating, followed later by plants in Heidelberg, Duisburg and other locations across the western part of Germany. Also, they stopped those ghastly smells coming out, and to this day the process gases are treated and the plants have very low odour emissions. Due to their success there are many more such plants being built.
These German plants showed that composting is also an economical method of drying materials, but just drying to put these materials in a landfill as reduced tonnage is pointless.
So, now we will round off this article by summarizing that Mechanical Biological Treatment is growing and likely coming to a place near you quite soon! However, because MBT is not a complete solution, its viability as an alternative to technologies such as direct incineration of residual waste will depend on how markets develop for the outputs of MBT. Research is also still awaited to tell the experts whether life cycle studies will show the use of MBT does actually assist in contributing to sustainable development objectives.
Steve Last is a resource and waste management professional. Find out more about MBT at the Mechanical Biological Treatment web site.
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