USING MECHANICAL - BIOLOGICAL TREATMENT FOR MSW IN EUROPE

BioCycle October 2003, Vol. 44, No. 10, p. 58

With the first target date for decreasing landfilling set for 2006, more waste managers are looking for systems, concepts and projects that offer organics recycling solutions.

Claudia Heermann

THE PRESSURE is building on European countries to find alternatives to landfilling MSW, as the waste hierarchy emphasizes reuse, recycling and composting. Incineration is rated “second best,” and increasing restrictions on residual waste permitted to landfill have led to growing interest in Mechanical - Biological Treatment (MBT). In countries such as Austria, Germany and Italy, these systems have been utilized for about ten years with increasing success; they are now established as processes for value recovery and landfill diversion.
Recently, these concepts have attracted considerable attention in other areas of the world as well and have received support from public officials and environmental organizations. Yet there is much confusion about what these systems can do, originating in part from the fact that there are different process concepts with inconsistent capabilities that are marketed as MBT. This article aims to address the confusion by describing the different MBT systems available.

Limits On Landfilling
As in North America, landfilling remains the predominant waste management option in many European countries. To change this, the European Landfill Directive sets progressively lower limits on the organic content of landfilled materials and introduces outright bans on landfilling specific wastes, such as tires, to reduce environmental and health risks associated with landfills. The Directive states that “all waste that is sent to landfill must be treated, unless the waste is inert, or treatment does not contribute to the objectives of the Directive by reducing the quantity of the waste or hazards to human health or the environment.” In particular, it sets the following targets: By 2006, reduce biodegradable municipal waste landfilled to 75 percent of that produced in 1995; By 2009, reduce biodegradable municipal waste landfilled to 50 percent of that produced in 1995; and By 2016, reduce biodegradable municipal waste landfilled to 35 percent of that produced in 1995.
Taking into account the differing status of waste treatment infrastructure in Europe, the EU has included a four-year derogation to the Directive target years, which have been offered to those Member States which were landfilling more than 80 percent of their municipal waste in the year 1995. These countries included the United Kingdom, Greece, Spain and Ireland, all of which lacked a comprehensive alternative waste treatment infrastructure at that time.
By contrast, the leading EU Member Countries (for waste infrastructure) have already set even higher targets or earlier implementation timeframes for their own environmental standards under the Directive. In order to achieve these standards, they have introduced a variety of measures ranging from outright landfill bans; to bans for certain types of putrescibles; to charges and taxes on waste going to landfill; complemented by additional targets and incentives for alternative waste recovery methods. Landfill bans have already been introduced in Denmark, Germany, the Netherlands, Sweden and Switzerland.
This trend to landfill diversion has produced a “new” secondary waste stream known as “residual or grey bag waste,” which in most European countries is then incinerated for energy recovery to extract further value. However, waste incineration is an unpopular treatment option and many projects have been delayed or even abandoned due to public and political pressures against the expansion of waste-to-energy capacity. Therefore, the quest for alternatives to traditional mass burn incineration prompted the development of mechanical biological treatment systems and anaerobic digestion processes as well as the exploration of novel thermal treatment technologies, such as gasification and pyrolysis.
In the following, we will briefly describe the role MBT currently plays in Europe, before we analyze the capabilities of individual MBT concepts in terms of recycling, recovery and landfill diversion.

MECHANICAL PREPARATION AND BIOLOGICAL TREATMENT
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. The process concept has evolved from the simple combination of mechanical preparation, material separation and composting, to an integrated system with three or more different waste fractions - which can be recycled, composted and from which energy can be recovered via digestion or subsequent combustion.
Today, there are over 100 sites operating in Europe using some form of mechanical biological treatment. This total includes several advanced composting plants, particularly in Italy, that have been upgraded with mechanical separation front-ends and could therefore be described as “Basic” MBT systems. Similar systems adapting anaerobic digestion for the treatment of MSW exist in Belgium, Switzerland and the Netherlands.
The capacity of MBT facilities ranges from very small plants treating less than 10,000 metric tons per annum to large-scale integrated facilities with the capability of handling more than 200,000 metric tons; some of these are modular systems which can be adapted to fluctuations in the waste stream and individual project requirements.
Many of the early MBT systems were designed in the context of separate collection and recycling systems for packaging waste in response to the European Packaging Directive. In several European countries, this has led to new achievements in material segregation at source. However, even with successful curbside material separation schemes, there are still significant quantities of materials which can be recovered from the residual waste stream. This potential has been recognized by MBT system developers, who aim to extract maximum value from a stream. However, this means that most MBT processes have been optimized for residual wastes streams, and not nonsegregated household waste.
There are many companies able to deliver MBT systems and components: suppliers range from equipment makers to project developers as well as turnkey suppliers delivering plants with an integrated energy recovery capability. Some companies have entered the MBT market with their main experience in the mechanical separation and sorting stages; while the majority of MBT suppliers have developed their systems from composting and anaerobic digestion technologies and have then 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 of combining both stages of the MBT process into one efficient process flow.

SYSTEM SUPPLIERS
These turnkey suppliers are actively marketing advanced MBT systems for different input streams which produce a range of end products to suit customer needs. There are a number of configurations employing different biological treatment options (such as composting and anaerobic digestion) as well as individual approaches to recycling and value recovery in the mechanical part of the process.
Each proprietary system is promoting its own individual concept for MBT using a combination of waste treatment technologies. Most suppliers offer a choice of technology combinations to suit different waste conditions, legal standards and customer requirements and it is this array of options that can be confusing for potential customers. However, not all systems provide fully integrated MBT processes and only a small number of configurations have gained sufficient operating experience to be called commercially proven.
In attempting to provide a comprehensive cost assessment of the different MBT categories, it is important to understand the details of individual technology concepts: In particular, to conduct a detailed analysis of the potential waste preparation and posttreatment requirements in order to determine the overall capital and operating costs and therefore to be able to provide a fair and meaningful cross-comparison between MBT systems.
The basic systems tend to be low-cost with little automation and are literally a materials recovery system with adjacent biological treatment often within the same building. The components of these systems are often ordered separately either directly by a public authority or through a project developer who is buying in the sorting and separation facility as well as the chosen biological treatment, mostly windrow or in-vessel composting.
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 odor control facets within a closed cycle. These MBT systems are supplied by more specialized companies, which have developed the combined process in order to improve economic and environmental efficiency using more advanced technologies.
Innovative approaches have taken these concepts further and have developed systems with higher degrees of material separation and air pollution controls in order to minimize 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 to improve the recovery of nondegradable materials and therefore reduce the amount of residues going to landfill. These systems use the biological properties of the waste for drying purposes to reduce contamination by clogging organics in the subsequent material separation. Others have improved the biological degradation by using special designs using semipermeable 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 linking MBT with gasification has attracted a lot of interest throughout the world.

RESIDUE REDUCTION PROCESSES
A lot of confusion about the capabilities of MBT systems arose from the claims made by some suppliers. In our experience, all MBT systems produce certain amounts of rejects from both the mechanical and the biological treatment(s) stages, as well as from air pollution and effluent cleaning procedures: these residues have to be disposed of somewhere. And therefore, like incineration, they do reduce the overall recovery rate. Thus MBT is more correctly characterized as a “Residue Reduction” process, enhancing value recovery from waste, rather than a “Zero Residue” process. In addition, these systems are often not complete solutions, as waste treated by MBT will nearly always require additional treatment or further disposal.
Modern MBT concepts produce a number of materials depending on the purpose of the process and the resulting quality of output material. In particular, the refuse derived fuel (RDF) produced by the advanced MBT systems has led to intensive discussions about the environmental impact of MBT residues, as the market conditions are not always favorable. The following potential outlets have different economic consequences ranging from additional costs for stabilization or landfill, to potential revenue streams from cocombustion: Landfill cover; Additional biological stabilization; Site remediation and horticultural applications; Homogenous fuel for novel and conventional thermal technologies; and cocombustion or cogasification with biomass and other fuels.
There is also an important political dimension; in some countries, environmental groups supporting MBT, such as Greenpeace, have stated that their preferred route for MBT residues is to landfill and not to energy recovery. This viewpoint has been criticized by others who point out that under EU policy, the maximum value should be recovered from waste. Many experts favor energy recovery using traditional grate systems or even novel processes such as gasification. The Calorific Value of those RDF type MBT residues can range between 12 and 16 MJ/kg depending on waste input and moisture levels. Many suppliers feel that energy recovery is the most sustainable option and have optimized their systems to produce high calorific fractions for this particular purpose. Whether or not this is the most beneficial outcome from an environmental and economic perspective varies on a case-by-case basis.
With the target dates for the European Landfill Directive drawing closer, the landfill diversion of waste in general, and residual MSW in particular is certainly a problem that could be addressed by Mechanical Biological Treatment. As MBT complements waste minimization and increased source segregation, recycling and composting activities, it seems to be an integrated waste management technique with huge potential, not only in the UK and continental Europe, but increasingly further afield. Hence, the high level of interest we have seen in Australia and some countries in Asia. The USA, Canada and Australia might not have Landfill Directives such as their European counterparts, but they have the same problem of dealing efficiently with their growing mountains of waste and they are searching for readily available, cost-effective solutions with low environmental impact - such as MBT.

Claudia Heermann is a senior consultant with Juniper Consulting Services, Ltd., which specializes in international analysis of trends within the waste management and renewable energy sectors. The company is based in Gloucestershire, United Kingdom. E-mail: info@juniper.co.uk or heermann@juniper.co.uk or website www.juniper.co.uk

Copyright 2003, The JG Press, Inc.