A solid link

Increasing requirements for landfill diversion and recovery of energy and recyclables from waste have put waste conversion technologies in focus. The market is full of proven and unproven technologies, sometimes marketed as the one solution. However, in practice, a combination of technologies adapted to the local situation often is 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 provide comparatively high flexibility in regard to variations in the amount and composition of input as well as the treatment targets. 

Core elements

MBT is a material-specific process. Mixed residual waste is separated into various fractions, each of which is treated and, if possible, recycled in a manner that is customised to the fractions’ 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.

The objectives of MBT vary depending on location, waste flows and legal and economic situation and include:

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

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 treatment;
• MBT with wet anaerobic treatment
• MBT (mechanical-biological stabilisation, MBS) for solid recovered fuel/refuse-derived fuel (SRF/RDF) production; and
• short aerobic drying process and efficient material separation after drying for use in combustion and recycling.
   

Anaerobic technologies yield solid output streams and biogas 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).

Basic elements of most MBT plants include:

• input control/selection;
• extraction of material with high energy content (high calorific value) by sieving or other technologies;
• magnetic separation of ferrous metals (always available) and eddy-current separation of 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 has been completed.
 

Making the distinction

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

MBT has tougher requirements with regard to mechanical treatment and some biological treatment machinery in light of 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 contamination and a higher level of contaminants than separately collected organic waste. Hence, biologically treated residual waste may not always suitable for agricultural application.

A table available online 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;
• higher gas yield and capture;
• valuable resources are recycled;
• produces a high-calorific solid fuel;
• leaves more stabilised material in the landfill; and
• less land is consumed and no burden is left for future generations.
   

The benefits of MBT over solid waste incineration include:

• economic operation of smaller units is possible;
• less sensitive to fluctuations in waste composition/production;
• only appropriate, high-calorific-value waste goes into energy recovery;
• less potential of highly toxic emissions; and
• less resistance from the population.
  
   

A proven element

MBT is widespread in Europe. In Germany, untreated municipal solid waste (MSW) has been effectively banned from landfill since June 2005. About 4.55 million tonnes of MSW are treated yearly in Germany using MBT/MBS.

When plants began using biological treatment, they often experienced technical problems. The German Environmental Protection Agency wanted an overview of the situation and in 2007 asked for evaluations of all MBT plants in the country.

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An elevated cab is one of several features improving operational efficiency at the Macon County Solid Waste Management agency in North Carolina. When it comes to waste management, efficiency, safety and reliability are priorities driving decisions from day one, according to staff members of the Macon County Solid Waste Management Department in western North Carolina. The agency operates a recycling plant in a facility originally designed to bale incoming materials. More recently, the building has undergone significant transformations centered around one machine: a SENNEBOGEN telehandler (telescopic handler).

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SENNEBOGEN 340G telehandler improves the view in Macon County, NC

An elevated cab is one of several features improving operational efficiency at the Macon County Solid Waste Management agency in North Carolina. When it comes to waste management, efficiency, safety and reliability are priorities driving decisions from day one, according to staff members of the Macon County Solid Waste Management Department in western North Carolina. The agency operates a recycling plant in a facility originally designed to bale incoming materials. More recently, the building has undergone significant transformations centered around one machine: a SENNEBOGEN telehandler (telescopic handler).

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SENNEBOGEN 340G telehandler improves the view in Macon County, NC

An elevated cab is one of several features improving operational efficiency at the Macon County Solid Waste Management agency in North Carolina. When it comes to waste management, efficiency, safety and reliability are priorities driving decisions from day one, according to staff members of the Macon County Solid Waste Management Department in western North Carolina. The agency operates a recycling plant in a facility originally designed to bale incoming materials. More recently, the building has undergone significant transformations centered around one machine: a SENNEBOGEN telehandler (telescopic handler).

Initial problems of the first full-scale MBT plants have since been resolved, and many have now established nearly a decade of operational experience.

Shrinking natural resources, fast growth of the world population and increasing prosperity in emerging and developing countries requires optimised resource recovery.

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. Resource recovery also 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, and the technology could be incorporated into material recovery facility operations with integrated biological treatment or with pure material separation.

Incineration alone does not meet the requirements of a sustainable, resource-optimised waste management concept because energy that was spent to produce the fuel materials is lost in incineration. Furthermore, precious waste components such as nonferrous metals are often irrecoverably lost as well.

Dr. Matthias Kuehle-Weidemeier is CEO of Wasteconsult International of Hanover, Germany, and can be reached at info@wasteconsult.de or on the Web at www.wasteconsult.de.