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Increasing requirements for landfill diversion and recovery of energy and recyclables from waste have put waste conversion technologies in focus for future waste management development. The market is full of proven and unproven technologies, sometimes marketed as the one solution. In practice, a network/combination of technologies adopted to the local situation is often advantageous by means of sustainability and economics.

Mechanical-biological treatment (MBT) is a proven solution that can recover solid refuse-derived fuel (RDF), biogas and recyclables from waste and provides a comparatively high flexibility to variations in the amount and composition of input as well as the treatment targets.
 

Core elements

Mechanical-biological waste treatment is a material-specific process. Mixed residual waste is separated into various fractions, each of which is treated and, if possible, recycled in a way that is customized to its properties.

The core elements of MBT are mechanical or physical separation technologies and the biological treatment of biodegradable waste components unless they are diverted to recycling.
 

Most MBT plants divide their input into a fines fraction for biological treatment and a coarse high-calorific fraction that undergoes extended mechanical treatment. Mechanical-biological stabilization plants (MBS) deviate from this concept as their entire input or the mechanically separated, high-calorific fraction undergoes biological drying.

The objectives of mechanical-biological waste treatment vary depending on the location, waste flows and legal and economical situation, and they can thus be weighted differently:

• minimizing climate-relevant methane emissions from landfills;
• decreasing landfill leachate contamination;
• reducing landfill void consumption;
• minimizing the amount of thermal waste treatment;
• recovery of recyclable materials;
• producing a high-calorific secondary fuel;
• producing biogas; and
• in certain cases producing liquid and/or solid fertilizers.
  
   

MBT types

MBT plants are grouped into the following types based upon the main technology used in the biological stage:

• MBT with a major landfill fraction
• aerobic processing;
• MBT with dry anaerobic treat-ment; and
• MBT with wet anaerobic treatment
• MBT (MBS) for solid recovered fuel/refuse-derived fuel (SRF/RDF) production
• Short aerobic drying process (BD) and efficient material separation after drying for use in combustion and recycling
   

Anaerobic technologies yield both solid output streams and biogas (methane) that can be used as a source of energy. Anaerobic stages are always followed by an aerobic treatment phase. Installations with digestion stages can operate as full-stream or partial-stream fermenters (in relation to the input to biological treatment).

Basic elements of most MBT plants:

• input control/selection;
• extraction of material with high energy content (high calorific value) by sieving [diameter greater than 60-150 millimeters (3 to 6 inches)] or other technologies;
• metal separation with a magnetic separator for ferrous metals (always available) and eddy current separator for nonferrous metals (available at many plants);
• biological treatment of fines;
• if necessary, further mechanical treatment of the biologically treated group for the withdrawal of calorific constituents by sieving or air classification; and
• if necessary, further processing of the high-calorific fraction.

In biological drying plants, usually the total input is shredded and fed to the biological drying process. Separation is easier after the drying process.
 

Making the distinction

The biological steps of the mechanical-biological residual waste treatment process are widely identical to those employed for the composting and anaerobic digestion of separately collected organic waste.

MBT has tougher requirements with regards to mechanical treatment and some biological treatment machinery due to its broader input spectrum and more heterogeneous feedstock. MBT also necessitates more mechanical effort to extract a significant amount of material that does not endure biological treatment, for example the high-calorific coarse fraction and ferrous and nonferrous metals.

Residual waste also normally tends to have a higher potential risk from spots of contamination and a higher level of contaminants than separately collected organic waste. Hence, biologically treated residual waste is not always suitable for agricultural application.

The table below shows, for example, the average breakdown of solid material flows at German MBT plants handling residual waste. A distinction is made between MBT (upstream of a landfill) and MBS technology (with the primary objective of producing alternative fuels or biological drying).

Benefits

The benefits of MBT over bioreactor landfill include:

• full control and prevention of gaseous emissions in enclosed systems;
• industrial process in which the total waste is involved and no dry (unaffected) zones such as exists in a landfill;
• leaves more stabilized material in the landfill (aerobic degradation is more effective on poorly biodegradable substances than a landfill’s anaerobic processes);
• higher gas yield and capture (intensive treatment and no loss of open installation areas or leaks in a landfill);
• valuable resources (i.e., metals, wood, plastics and paper) are recycled and not lost in the landfill;
• producing a high-calorific solid fuel; and
• less land consumption and leaves no burden for future generations.
   

The benefits of MBT over solid waste incineration include:

• often cost-effective (capital and operating costs);
• economic operation of smaller units is possible;
• less sensitive to fluctuations in waste composition and waste production, thereby significantly lower economic risk;
• no burning of water and stones, only appropriate, high-calorific value waste going into energy recovery;
• less potential of highly toxic emissions, as the high temperature thermal processes can be avoided (except for the combustion of the RDF); and
• usually less resistance from the population.
  
   

A proven element

MBT is a widespread technology in Europe. In Germany, untreated municipal solid waste (MSW) has been effectively banned from landfilling since June 2005. About 5 million tons of mixed MSW are annually treated in Germany by MBT/MBS.

In 2005 and 2006 when plants began using biological treatment steps, they often experienced technical problems. The German Environmental Protection Agency (Umweltbundesamt, UBA) wanted to get an overview about the real situation and gave the order for an individual evaluation of all German MBT plants to Wasteconsult International in 2007.

The initial problems of the first sophisticated full-scale MBT plants have since been resolved. Now nearly a decade of operational experience with modern-type MBT plants in Germany and many other European countries exists. When choosing the right technology and supplier, MBT can be a reliable, proven element of a modern waste management system.

Shrinking natural resources, fast growth of the world population and increasing prosperity in emerging and developing countries requires consequently resource optimized acting in general and especially in waste management.

A massive increase of the share of materials recovered from waste is necessary. This would enhance material supply and save energy as well as reduce carbon dioxide emissions, because producing raw materials from recyclables consumes significantly less energy than from fresh natural resources. Resource recovery means climate protection. Enhanced MBTs and sensor-based waste sorting plants must become the heart of a sustainable, material-specific waste management system. Current MBTs are the first step on this very promising way. MBT could be incorporated into a MRF operation with integrated biological treatment or with pure material separation.

Incineration alone does not meet the requirements of a sustainable, resource-optimized waste management concept because the energy that was spent for the production of the materials that are used as fuels is completely lost in the incineration process. Furthermore, precious waste components like nonferrous metals are often irrecoverably lost in the incinerator ash.

Dr. Matthias Kuehle-Weidemeier is CEO of Wasteconsult International, based in Hanover, Germany, and can be reached at [email protected].