Mechanical Biological Treatment (MBT) systems have been under development in Germany for some ten years as an alternative to incineration.

Recently these concepts have attracted considerable attention in other areas of the world and have received support from NGOs and other environmental organisations. Yet there is much confusion about what these systems can do, originating in part from the fact that several different process concepts are described as MBT. This article aims to address this confusion by describing the different types of MBT systems available and put them into context of managing household waste.

At its simplest, MBT merely describes the mechanical waste preparation and biological treatment parts of a standard Integrated Approach to Waste Management. But the concept of MBT offers the opportunity for a more holistic approach of combining systems to recover the maximum potential from waste based on specific circumstances. This has evolved from the simple combination of mechanical preparation, material separation and composting, to an integrated system with three or more waste fractions, which can be recycled, composted and from which energy can be recovered. Some systems can offer a reasonably flexible approach to waste management due to their high tolerance of variation in waste composition. This means they can be adapted to take black bag waste, but can also accept the implementation of additional kerbside collections or green waste composting.

MBT was originally developed as a way of treating residual municipal wastes after source segregated recycling had taken place in order to recovery further value and to by-pass incineration of the residue stream. The process is basically a combination of:

• Waste preparation and separation
  
• Recovery of two or more waste streams for further utilisation or landfill
  
• Stabilisation of the bio-degradable fraction

In spite of originally being developed for residual MSW, some MBT systems have the ability to operate without extensive kerbside collection or source segregation, as is often the case in the UK. In fact the more advanced systems can also function without any additional collection infrastructure; they are also suited to the un-segregated household waste stream or “black bag” waste. For those authorities with high kerbside recycling participation MBT allows the authority to increase its recycling rate without significant additional effort which would normally be required to boost kerbside collection effectiveness. Low recycling authorities can improve their recycling performance without the need for costly source segregation infrastructure especially in areas where space or geography make kerbside collection impractical.

The Advantages offered by MBT systems

In the UK, MBT facilities, when used as part of an integrated approach to waste management, have many advantages for local authorities. These include:

Even with successful kerbside schemes there are still quantities of residual materials which could be recycled from the waste stream. Some of the systems are able to recover 15% to 20% from the residual waste for recycling if metals, inerts and plastics are removed. Metals are the easiest materials to remove using these systems and could boost local authority recycling rates by around 5% or more when used to manage residual waste from those authorities using kerbside collection. However individual recycling rates will depend on waste composition and the individual process technology used.

The capacity of MBT facilities ranges from very small plants treating 10,000 tonnes per annum or less to large scale integrated facilities with capacities of up to 150,000 tonnes and more. Some of them are modular systems which can be adapted to fluctuations in the waste stream and individual project requirements.

As independent technology analysts, Juniper has recently conducted a review of a number of MBT systems from around the world. We have found it helpful to group the systems that are commercially available into three main categories of MBT systems.

These can be classified as ranging from basic ‘grassroots’ MBT facilities to those based on more ‘high tech’ and innovative ideas. They all however combine material separation with stabilisation of the biodegradable fraction.

The Three Main Categories of MBT systems

Closed cycle

• Integrated system
• Optimised odour controls
• Energy recovery using AD
• Reduce residues

• Integrated innovative design
• Higher degree of waste separation and reduced residue stream
• Additional energy recovery options

With rising environmental standards and higher recycling requirements, integrated systems have been developed that combine the two technology stages as an integrated entity and include emissions and odour control facets within a closed cycle. Some of these systems propose Anaerobic Digestion for the biological treatment. These MBT systems are supplied by more specialised companies, which have developed the combined process in order to improve economic and environmental efficiency.

Innovative approaches have taken these concepts further and have developed systems with higher degrees of material separation and air pollution controls in order to minimise environmental impact. Some of these new systems produce an RDF type material, which can be used for energy recovery and fuel replacement purposes, to reduce further the environmental impacts of the waste. Some of these latest MBT systems have changed the order of separation and biological activity in order to improve the recovery of non-degradable materials and therefore reduce the amount of residues going to landfill. These systems use the biological properties of the waste for drying purpose to reduce contamination by clagging organics in the subsequent material separation. Others have improved the biological degradation by using special designs using semi-permeable membranes etc.

Several companies have also announced process concepts which further integrate the MBT system with thermal recovery of the residual fraction in either an adjacent combustion unit or a gasifier. None of these concepts has yet been implemented, but the idea of MBT and gasification has attracted a lot interest throughout the world.

Today numerous companies in Europe and elsewhere are able to deliver MBT systems and components. These suppliers range from equipment makers, supplying parts for the ‘simple’ combined shredding/separation and biological treatment operations, to project developers and turnkey suppliers delivering plants with an integrated energy recovery capability. Some companies have entered the MBT market with long-term knowledge concerning the mechanical separation and sorting stages, whereas the majority of MBT suppliers has developed their system from composting and anaerobic digestion technologies and have added the mechanical component through purchasing and licensing agreements with outside suppliers. However, the leading companies can be distinguished from the traditional component suppliers by their explicit knowledge of process integration, environmental management, plant operation and experience in combining both MBT stages into one efficient process flow.

Because the concept originated in Germany, the majority of technologies have been developed there. Today, there are over 25 MBT facilities operating today. Other established markets include Austria and Switzerland, closely followed by the Netherlands. Emerging markets include other European markets such as Italy and the UK, but also overseas countries such as Australia.