Database of Waste Management Technologies Life

Mechanical Biological Treatment (MBT) Processes

 

Generic information

MBT is a generic term for an integration of several processes commonly found in other waste management technologies such as Materials Recovery Facilities (MRFs), sorting and composting or anaerobic digestion plant.

MBT plant can incorporate a number of different processes in a variety of combinations. Additionally, an MBT plant can be built for a range of purposes.

MBT is a residual waste treatment process that involves both mechanical and biological treatment processes. The first MBT plants were developed with the aim of reducing the environmental impact of landfilling residual waste. MBT therefore compliments, but does not replace, other waste management technologies such as recycling and composting as part of an integrated waste management system.

A key advantage of MBT is that it can be configured to achieve several different aims. In line with the EU Landfill Directive and recycling targets, some typical aims of MBT plants include the:

  • Pre-treatment of waste going to landfill;
  • Diversion of non-biodegradable and biodegradable MSW going to landfill through the mechanical sorting of MSW into materials for recycling and/or energy recovery as refuse derived fuel (RDF);
  • Diversion of biodegradable MSW (BMSW) going to landfill by:
    • Reducing the dry mass of BMW prior to landfill;
    • Reducing the biodegradability of BMW prior to landfill;
  • Stabilisation of the organic fraction of MSW into a compost-like output (CLO) for use on land (Compost-like Output (CLO) is also sometimes referred to as ‘stabilised biowaste’ or a soil conditioner; it is not the same as a source segregated waste derived ‘compost’ or ‘soil improver’ that will contain much less contamination and has a wider range of end uses);
  • Conversion of the organic fraction of MSW into a combustible biogas for energy recovery; and/or
  • Drying materials to produce a high calorific organic rich fraction for use as SRF.

The Figure below illustrates configurations for MBT and highlights the components within each.

Figure 1

Waste Preparation

Residual waste requires preparation before biological treatment or sorting of materials can be achieved. Initial waste preparation may take the form of simple removal of contrary objects, such as mattresses, carpets or other bulky wastes, which could cause problems with processing equipment downstream.

Further mechanical waste preparation techniques may be used which aim to prepare the materials for subsequent separation stages. The objective of these techniques may be to split open refuse bags, thereby liberating the materials inside; or to shred and homogenise the waste into smaller particle sizes suitable for a variety of separation processes, or subsequent biological treatment depending on the MBT process employed.

A summary of the different techniques used for waste preparation is provided in the Table below:

Table 1: Τechniques used for waste preparation in MBT plants.

Table 1

Sorting

A common aspect of many MBT plants used for MSW management is the sorting of mixed waste into different fractions using mechanical means. As shown in the Figure above, the sorting of material may be achieved before or after biological treatment (BT). No sorting is required if the objective of the MBT process is to pre-treat all the residual MSW to produce a stabilised output for disposal to landfill.

A variety of different techniques can be employed, and most MBT facilities use a series of several different techniques in combination to achieve specific end use requirements for different materials. Separation technologies exploit varying properties of the different materials in the waste. These properties include the size and shape of different objects, their density, weight, magnetism, and electrical conductivity. A summary of the different options for waste separation is shown in Table 2.

Table 2: Techniques used for waste separation in MBT plants

Table 2

Biological Treatment (BT)

The biological element of an MBT process can take place prior to or after mechanical sorting of the waste, as illustrated in the previous Figure. In some processes all the residual MSW is biologically treated to produce a stabilized output for disposal to landfill and no sorting is required. The biological processes used are either:

  • Aerobic Bio-drying
  • Aerobic In-vessel composting
  • Anaerobic digestion

In Greece and other EU countries, at present, the market or outlet for many of the outputs from MBT is still under development. Plants being specified today will need to provide materials into as yet undeveloped markets. It is prudent to install or at least maintain the option of installing for flexibility in the degree and types of separation of materials that any proposed plant can achieve.

MBT Configurations

Simple MBT Plant

Sometimes, the design of MBT is configured in a simple form, that bio-stabilises the entire waste stream over an extended time period of around 8-10 weeks, to achieve maximum reduction in the biodegradability of the waste, with the resulting organic product (following mechanical segregation of recyclable materials) sent direct to landfill or used as a soil improver. A simple schematic diagram of the process is presented below.

Figure 4

Simple MBT plant configuration

Whilst this represents the simplest form of MBT, this configuration does not provide an optimal approach in terms of biological efficiency, plant footprint and cost. Alternative approaches are usually employed that are presented in the relative tabs. Such configurations mainly aim at sorting the organic fraction from the rest of the waste stream and to treat (biological treatment) the sorted organic fraction, instead of the entire waste stream.

This alternative approach offers a number of advantages as follows:

  • The mechanical separation step segregates the biological and non-biological elements so that only the moisture laden and biologically active waste elements are treated;
  • By concentrating the organics fraction, it is possible to achieve enhanced degradation efficiencies in the biological process;
  • Along with lower tonnages this results in a reduced footprint for the biological treatment step which in turn leads to overall plant footprint savings and lower capital costs; and
  • Enhanced degradation efficiencies lead to greater mass loss in the biological treatment step and hence reduced output tonnages.
  • Sorting the waste allows an MBT process to separate different materials which are suitable for different end uses. Potential end uses include material recycling, biological treatment, energy recovery through the production of RDF, and landfill.