Solid Waste & Recycling

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Mechanical Biological Treatment (MBT) — Part 1

In the face of dwindling landfill capacity reserves, waste authorities across Europe are increasingly turning to innovative waste treatment and volume reduction processes to preserve remaining landfil...


In the face of dwindling landfill capacity reserves, waste authorities across Europe are increasingly turning to innovative waste treatment and volume reduction processes to preserve remaining landfill capacity and to meet the European Union’s (EU) Landfill Directive targets (for recycling and the reduction of biodegradable waste going to landfill).

These targets, coupled with a ban on the disposal of untreated wastes going to landfill and universal public concern about incineration, mean that authorities and operators alike are commonly selecting Mechanical Biological Treatment (MBT) as the preferred option for dealing with municipal solid waste.

MBT is a generic term for a range of processes used to treat residual (i.e., post curbside collection of source-separated recyclables) solid waste using a combination of mechanical separation and biological treatment. This commonly comprises three stages: mechanical size reduction, biological drying (driving off moisture content) and material separation or “splitting” to segregate different output streams for different purposes.

The principal objectives for which MBT is currently utilised are considered as follows: reducing quantities (mass and volume) to landfill; stabilizing waste prior to landfilling; reducing the bio-chemical loading of the stabilized material going to landfill (in terms of liquid and gaseous emissions); and changing the mechanical behaviour of the stabilized material. This includes:

* Reducing landfill consolidation and settlement due to waste degradation and homogenization;

* Producing secondary recovered fuel (SRF) or refuse-derived fuel (RDF) for energy production;

* Harnessing biogas for energy production via anaerobic digestion (AD); and

* Composting the stabilized waste and/or AD digestate.

Although different technologies may be used — with varying levels of complexity and different outputs — they all have similar characteristics. They generally take residual municipal solid waste and, through various screening and conditioning processes, extract additional recyclable materials and reduce the biodegradability of the remaining waste. The processing of the incoming waste stream permits the extraction of fractions with end purposes in mind, both before and after biological processing of the residual waste.

Some systems have a high tolerance to variation in waste composition and are able to receive unsegregated waste, if preferred.

In the case of “thermal” MBT facilities, the organic fraction of the waste is dried to produce a relatively high-BTU-value SRF material which may be utilised for energy recovery and/or to produce a stabilized residue (suitable for landfill disposal). In the case of the less common “non-thermal” MBT facilities, the organic fraction of the waste may be anaerobically digested to produce biogas for energy recovery; potentially then maturing the resultant digestate via aerobic composting (for compost-type applications). In both cases, the goal of the treatment process is to divert untreated biodegradable waste away from landfill, effectively reducing the greenhouse gas “footprint” of the waste (by minimizing the potential for landfill methane generation).

Unfortunately, potential uses for low-grade compost or “compost-like outputs” (CLOs) are extremely limited, being restricted by the negative chemical, visual and pathogenic properties of the products. These CLOs do not, in most cases, meet the requisite quality requirements for compost products. As such, there is a concerted drive in Europe towards providing source-separated organics (SSO) facilities or, more frequently, leaf-and-yard waste composting facilities. These facilities are producing ever-increasing quantities of high quality compost products. In the face of this competition, CLOs are only likely to be viable where guaranteed long-term uses for the products can be identified (e.g., for daily cover, restoration purposes on an adjoining landfill, or non-arable uses such as controlled forestry or bio-fuel production).

MBT across Europe

As may be expected, those countries applying earlier adoption of waste treatment or pre-treatment processes are significantly further advanced in terms of providing MBT infrastructure capacity, centering on Italy (100 plants, 8 million tonnes per annum [Mtpa] capacity), Germany (60 plants, 6 Mtpa capacity) and Austria (17 plants, 0.8 Mtpa capacity). At the other end of the spectrum, recent additions to the European Union may have little or no current MBT infrastructure and are effectively starting from scratch. The current take-up of MBT technologies and the expected future distribution of MBT technologies across Europe are summarised on the map.

In terms of scale, the size of economically viable MBT facilities typically ranges from 20,000 tonnes per annum (tpa) to 300,000 tpa; with the optimal scale being between 60,000 and 200,000 tpa. More recently, however, there has been a notable up-sizing of capacity, with individual facilities acting as regional hubs for regional waste treatment activities (e.g., Rivenhall Airfield, UK, capacity 510,000 tpa).

In order to encourage the provision of new waste treatment capacity across Europe, the EU Landfill Directive incorporates a penalty for Member States failing to meet the set diversion targets by the specified deadline dates at a potential charge of C$330 (150) per tonne. A range of market distorters such as Renewable Obligation Certificates (ROCs), Climate Change Obligations (CCOs) and other tradable mechanisms are being used to promote changes in environmental policy and processes. In addition, where there is a continuing financial bias towards the cheap disposal of wastes to landfill, some member states (e.g., the UK) are seeking to redress the balance by way of increased taxes on landfill disposal. In the case of the UK, the landfill tax currently stands at C$53 (24) per tonne of biodegradable waste, escalating at a rate of C$18 (8) per year from 2008 to 2011. Coupled with the normal landfill disposal charge, this equates to a typical UK landfill gate price of some C$90 to C$110 (40 to 50) per tonne. By comparison, headline gate prices for processing of waste via MBT (i.e., excluding the fiscal benefits available to green power technologies) are currently in the order of C$130 to C$165 (60 to 75) per tonne. As such, it is anticipated that MBT treatment costs will reach parity or better with landfill disposal costs in the UK by 2010 or 2011.

It’s important to note that MBT is not quite the “zero waste” solution that’s sometimes claimed; there will always be a need for residual disposal facilities for the outputs from MBT and other waste treatment processes. MBT can, however, contribute towards recycling targets being met and the reduction of biodegradable waste sent to landfill. MBT can be part of an integrated waste management solution, provided it fits within the political agenda and affordability envelope of the local or regional waste authority.

The authors’ company is currently involved in a range of integrated waste management schemes across Europe and North America, including the provision of MBT and other waste treatment technologies. In the next issue they’ll discuss the applicability of MBT and stabilized landfill in the Canada.

Andy Wilson is the UK Waste Market Sector Leader at Golder Associates’ Maidenhead, UK office. Contact Andy at arwilson@golder.com Michael Cant is the Ontario Waste Sector Leader and a Senior Sold Waste Planner at Golder Associates’ Whitby, Ontario office. Contact Michael at mcant@golder.com


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