Mechanical-biological stabilisation/treatment (MBS/MBT) plants are used to treat household and commercial waste in order to gain an organically stable fraction that can be disposed of safely.
On 1 June 2005, German waste management laws changed and since then it is no longer allowed to dispose of untreated waste in landfill sites. Currently, there are approximately 60 MBS/MBT plants in operation in Germany with an overall capacity of 5.7 million tonnes per year.
The main aim of an MBS/MBT plant is to stabilise and treat waste in such a way, that potential damage to the environment caused by greenhouse gas emissions and highly contaminated landfill leachate is minimized. In addition, it is the aim to recover valuable secondary raw materials such as metals and to produce refuse-derived fuel (RDF) of high calorific value. Hence, MBS/MBT plants not only reduce the volume of the landfill fraction and greenhouse gas emissions but they also produce refuse derived fuels (RDF) that can be used as a substitute for fossil fuels.
Nehlsen developed, built and started operating its first MBS/MBT plant in 1998. Today, three modern MBS/MBT plants have resulted from this first pilot project and operate with an annual capacity of 163,000 tonnes.
The main treatment steps of a typical MBS/MBT plant consist of a mechanical pre-treatment, one or more biological treatment steps and usually another mechanical step to improve the fractions that were treated biologically. It is important to note that each MBT/MBS plant is based on an individual concept, designed and built to meet customers’ requirements and taking into account the waste input composition, as well as the desired output fractions and their qualities (e.g. RDF)
The general MBT process, for a plant focusing on the production of refuse derived fuel (RDF), is described below:
1. Mechanical pre-treatment
Firstly, the waste is delivered to a fully enclosed plant where rejects are extracted before the waste is fed into a mechanical shredder. Then the reduced and homogenised waste is separated into two fractions: the mainly organic fraction is treated biologically and the fraction containing most of the high calorific material is turned directly into RDF, which can be used in waste-to-energy (WtE) plants or cement kilns for the production of steam, heat or electricity.
2. Biological stabilisation/intensive rotting process
In this step, the organic fraction is aerated in a controlled manner in an in-vessel system. The conditions provided (high water content in organic material and sufficient oxygen) allow for a high level of biological activity within the organic waste: this decomposes the organic material and produces heat. As a consequence, water is evaporated which is transported out of the vessel system by the air flow.
The intense biological activity within the system lasts for several days and reduces the weight of the treated fraction by approximately 20 to 25% (organic matter loss and evaporation of water).
Several technically different in-vessel systems have been developed for this treatment step, such as containers or tunnels, which need to be chosen depending on the individual local conditions. After the drying process, further high-calorific material is mechanically separated from the material and also turned into RDF.
The remaining waste fraction mainly consists of residues destined for landfill, however, it still contains a certain amount of organic matter.
3. Enclosed biological post-rotting and open post-rotting process
In order to further reduce the organic matter content and to achieve a biologically stable landfill fraction, Nehlsen uses a second in-vessel rotting process, where warm humidified air is added to the already dried biological waste fraction. This induces another wave of microbiological activity inside the waste fraction, although the process is slower and less intense compared to the first rotting process.
In order to break down the organic material more quickly, the post-rotting process is optimised by controlling the amount of air and water added during this two to four weeks process.
Depending on the local environmental requirements, the material then finally undergoes a two - eight week open post-rotting process before being deposited on a landfill site.
4. Refuse-derived fuel production
Nearly half of the original input material leaves the plant as refuse-derived fuel (RDF), a substitute for fossil fuel that can be utilised in combined heat and power plants in order to produce electricity and steam, or in cement kilns to substitute heavy oil or coal.
This high-calorific material undergoes various mechanical treatment steps such as cutting, sorting, classifying and compacting, in order to achieve the various qualities our customers require. For more information on RDF-CHP plants please click here.
5. Air cleaning technology
One of the advantages of the fully automated in-vessel intensive and post rotting system is that dust and odour are collected and can be cleaned in the waste gas purification system.
To comply with the required legal limits, the waste gas from the mechanical and biological processes is treated and cleaned in a waste gas purification system. Highly polluted waste gas is cleaned by using regenerative thermal oxidation plants (RTO), while the less polluted waste air is treated using biofilters. With the combination of both waste air treatment systems, emissions fall well below the strict German legal limits while at the same time maintenance and operation expenses are kept low.