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4. Reprocessing technologies for tyres

4.1 Introduction

A key challenge facing many central and local governments around the world is the recovery, re-use and final disposal of used tyres. Many countries have implemented legislative provisions requiring the treatment of waste tyres prior to final disposal to landfill, while Member States of the European Union are required to transpose national legislation to enforce a ban on the landfilling of whole tyres by 2003 (Amirkhanian, 2001).

The increased costs associated with the management of used tyres has, in many cases, resulted in illegal dumping and stockpiling, which have inherent costs for local authorities. In addition, discarded tyres present a range of other environmental and public health risks. For example, they provide a suitable habitat for disease carrying vectors, such as mosquitoes and create a significant fire risk both at illegal dumps and regulated tyre stockpiles (Wright & Williams, 1999).

4.2 The Tyre Recycling Process

Managing used tyres falls into three generic categories: collection, reprocessing and end use.

  • Collection

    This presents the initial challenge when formulating a tyre recovery programme and is primarily the responsibility of the tyre companies and dealers with private registered haulers used to transport discarded tyres to crumb rubber producers or other facilities for end use or disposal. A well managed and promoted collection system is required in order to minimise the proportion of tyres illegally dumped. Local governments may introduce a tyre amnesty where a one off collection day is introduced or a bring system where a person per tyre limit is set at a central drop-off location.

  • Reprocessing

    If a local authority implements a tyre recovery programme it will invariably involve collaboration with the private sector to establish infrastructure for crumbing or shredding. The shredding process involves inspection, to remove contaminants such as rocks, organics, bolts and other metals), cleaning, debeading (pulling the steel bead from around the rim of the tyre), and shredding. The extent of the shredding process depends upon the desired shred size for the particular end use. The production of crumb rubber requires an additional step of granulating the small shreds to the required end use size.

  • End Uses / Markets

    The economic viability and development of end use markets for used tyres varies considerably. However, there are currently three primary end use markets that are used for recovered tyres. These are energy recovery, crumb rubber products and materials replacement, however, while alternative options do exist, such as civil engineering fill, these are currently underdeveloped and require additional research.

    However, while all three factors are intrinsically linked this investigation will primarily focus on the development of existing, new and emerging technologies that may be used to develop a reprocessing sector for the designated waste streams.

4.3 Reprocessing Technologies

4.3.1 Energy Recovery

Waste tyres are a high grade energy source. On average a tyre comprises 85% compound rubber (natural and synthetic rubber plus carbon black) which in turn is 89% carbon, 5% cord and approximately 10% steel. Substituting up to 20% of the kiln fuel requirements with tyres can be managed with no production or pollution problems created.

The energy recovered from used tyres in the U.K is approximately 27%, which is low when compared with other European countries, such as Finland, Germany, Austria and Sweden, where between 50% and 80% of used tyres contribute to energy recovery. When comparing the calorific value of tyres to alternate fuel sources in Figure 1, it can be seen that tyres have a higher energy value when compared to traditional coal fuels. Therefore the potential exists to increase the energy yield recovered from used tyres, while reducing the consumption of alternate natural resources (Wright & Williams, 1999).

Figure 4.1: Average calorific value of varying fuels including tyres (Department of Trade and Industry, 1997)

See figure at its full size (including text description).

1. Alternative Fuels

Tyres can be used in various forms as tyre derived fuel (TDF). They may be burned whole, or shredded and burnt, or converted to combustible gas via pyrolysis. Their energy content varies and is estimated to be between 26 GJ/tonne to 33 GJ/tonne. The main disadvantage of used tyres as an alternative fuel is that they are widely dispersed and require collection and transport. The transport costs of whole tyres can be expensive as their bilk density is low at 0.16 t/m3, however this can be increased to 0.50 t/m3 if the tyres are shredded before transport. Shredding is energy intensive and costly and an alternative is to slice tyres in half and nestle the halves together for transport. It is estimated that this may increase the density to around 0.3 t/m3 (Cement Industry Energy management Association (CIEMA), 1995)

Whole and shredded tyres are being used as an alternative fuel source in five cement kilns in the UK including Cauldon in Staffordshire, Hope in Derbyshire and Wesbury in Wiltshire. However, this is not unusual as other countries around the world that also use tyres as a fuel source in cement kilns include Canada, USA, Australia, Sweden, Denmark, Switzerland, Belgium, Holland and Japan, some of which have extremely high environmental standards and stringent legislative provisions (ENDS, 2002).

The UK plants are owned by LaFarge (formerly Blue Circle Cement) who aim to have zero fuel costs by 2006 by burning a range of alternative fuels such as recycled liquid fuel, processed sewage sludge, packaging waste and meat and bone meal. The UK and European governments acknowledge that the use of scrap tyres in cement manufacture is a valuable recovery route and an economically attractive substitute for traditional fuels (Making Waste Work, DoE, 1995). The UK Environment Agency has highlighted that cement kilns suited to burning whole or used tyres due to:

  • High temperatures
  • Long residence time
  • Oxidising atmosphere
  • High thermal inertia
  • Akaline environment
  • No ash residue
  • Continuous fuel requirement

This ensures that there is:

  • Complete destruction of the rubber and cotton content of the tyre
  • No smoke or odour
  • Overall reduction in emissions
  • The metal is incorporated into the cement clinker

2. Pyrolysis

An additional technology that has recently been established in the UK is the patented process used to convert raw pyrolysed carbon rich char into a reinforcing black filler for blending with commercial grades of carbon black for use in automotive rubber parts. Following a successful production process test run of 2 tonnes per hour, Coalite Tyre Services commissioned their first plant in Bolsover, England in 2002. They have an annual capacity of 15,000 tonnes per annum and hope to expand to 60,000 by 2004 and reach a theoretical maximum of 90,000 tonnes thereafter. The advantage of using recovered tyres to produce the carbon black filler is that there is a constant supply of raw materials with no fluctuating prices, therefore long term stable prices can be assured resulting in the cost price being 20% cheaper than carbon black generated from other raw materials, such as oil (ENDS, 2001, ENDS, 2002, PRWeb, 2003 & Personal Communication2, 2003).

3. Gasification

The largest of these, at around 60,000 tonnes a year, is the proposed Four Ashes gasification plant in Staffordshire. Like the existing Sita Tyre Recycling plant the intention is to generate electricity (around 15.5 MW) from the used tyres. The capital cost of the plant is estimated to be £8 million for a 30,000 tonne capacity process with operating costs of $90 / tonne. Construction to operation is expected to take 2 years (McLanaghan, 2002).

4. Polymerisation

Environmental Waste International Inc. is installing a tyre polymerisation plant in the UK. The plant, which costs NZ $17 million, will take 18 months to construct and will have a maximum processing capacity of 3000 tyres per day. The process uses a microwave system to separate scrap tyres into steel, carbon black, oil and hydrocarbons that can be used to generate electricity to run the system with any excess power being sold to the national electricity grid (ISWA, 2003 & Environment Agency, 2003).

4.3.2 Paving Applications

A variety of State and local government agencies, including Los Angeles County and the California Department of Transportation (Caltrans), have shown that the use of crumb rubber to produce rubberised asphalt concrete (RAC) is a significant and viable end use for tyres.

Caltrans began using RAC in 1980, and between 1980 and 1998, Caltrans used a total of 2,458,930 tonnes throughout its regional districts, which based upon the formula developed by the Rubber Pavements Association, this translated to 4.5 million discarded tyres. Caltrans estimated that it is currently using RAC on 10 to 12 percent of its projects, while the Rubber Pavements Association estimates that Caltrans could use RAC on up to 40 percent of all paving projects.

Los Angeles County has been the leader among local governments in the use of RAC. The county established, with the assistance of the CIWMB, the Southern California Rubberized Asphalt Concrete Technology Center to promote the use of RAC. The use of tyres in RAC can produce significant cost savings and diversion potential for local paving and road maintenance operations. While the cost savings will vary based on the project, the Southern California Center has produced design examples with cost savings of $22,852 per mile for a simple asphalt overlay and savings of $170,776 per mile for roadway reconstruction. Based on LA County's use of rubberised pavement since 1993, RAC diverts approximately 2,000 tyres per lane per mile, with the use of rubberised asphalt providing a variety of benefits that include:

  • Longer lasting surface (50-100 percent).
  • Resistance to rutting and cracking.
  • Reduced road noise (50-80 percent).
  • Less buildup of road surface.
  • Reduced cost of project and/or ongoing maintenance expenses.

In addition to RAC, local governments may also consider the use of rubberized emulsion aggregate slurry (REAS) in street resurfacing projects. Although REAS is more expensive per lane mile, LA County's experience shows it can divert almost 80 tyres per lane mile. REAS also provides a number of other benefits, including increased performance and extending the roadway's lifespan.

While RAC/REAS provides a number of benefits, it may not be appropriate in every application. The use of RAC/REAS should be determined on a case-by-case basis taking into account the uses of the roadway and its initial condition for compatibility.

In order to promote the use of RAC throughout the districts administrative and contractual systems have been modified to ensure that RAC is considered. In addition, local governments can request that Caltrans use rubberized asphalt on projects within their area, while construction contractors are advised liase with tyre processors to ensure that the bids and material requirements are accurate.

The County of Santa Clara has established the use of rubberized paving materials as part of its open bidding process. Contractors are required to submit bids that contain options using tyre-derived paving materials. The county then can assess the up-front costs and performance projections, as well as any special factors that may effect the determination to use recycled materials.

The City of Thousand Oaks has used RAC to pave more than 130 miles of roadway since 1992 using 1.3 million tyres. Recent costs for RAC have averaged $49 per ton. The city found that the improvements of increased skid resistance, reduced road noise, improved riding qualities, and imperviousness to water have made the use of RAC cost-effective and desirable over traditional asphalt concrete.

4.3.3 Molded Rubber Products

At present this market is still in its initial development stage but it represents the greatest potential for value-added recycling. The CIWMB has issued grants to various private businesses and public agencies for pilot projects to fund the development and purchase of molded rubber products. Examples of crumb rubber applications include rail crossings, sound barriers, industrial flooring, sealant, shoe soles, carpet pads, playground mats, pond liners, conveyer belts, recycling bins, oil spill absorber, floating docks, wharf pilings and buffers, agricultural pipes, animal bedding and fencing.

4.3.4 Civil Engineering Applications

The use of shredded tyres in civil engineering applications is a major potential market for waste tyres. For example, used tyres have been used as an accepted beneficial application in landfill engineering as leachate drainage layers. In the UK, used tyres will continue to be used for this purpose after the ban on the landfilling of tyres is introduced as they replace virgin aggregate that would otherwise be used to form the drainage blanket. The future use of tyres in landfill engineering applications is difficult to gauge. However, it is certain that this use will decline over time as the number of landfill sites reduces.

Since 1998, there has been an experimental artificial tyre reef structure in Poole Bay, to which the Engineering and Physical Sciences Research Council (EPRSC) provided funding. The tyre modules are still in their original position and have colonised successfully with marine life. The University of Southampton's School of Ocean and Earth Sciences is responsible for this reef, which has been put in place to study the biological and chemical effects of tyres in the sea. If the results of the continuing study are favourable, it may open up new applications for tyres, not only as artificial reefs but also for coastal defence uses, harbour walls, etc.

Road surfaces containing rubber crumb were laid in various locations in the UK to trial the performance over more traditional surfaces. A two-year trial in Surrey, England was found to be performing well after the surface experienced both summer and one winter seasons. The Highways Agency subsequently evaluated all other roads containing rubber crumb and subsequently certified the material for general use. Recovered tyres have been used for the first time in the Scottish Highlands as a sub-base for a road reinstatement scheme. The road was realigned using waste tyre blocks to raise the road above the surrounding peak bog.

In the US the use of used tyres in civil engineering application is still only in the demonstration phase in many states. In 2001, the CIWMB sponsored a project in the San Francisco Bay area at a new interchange on Interstate 880. Six hundred thousand shredded tyres were used as lightweight fill for a highway on-ramp built on unstable bay mud. It is considered that shredded tyres have an enormous potential to be used as lightweight fill in civil engineering applications, and they can replace other conventional lightweight fill such as expanded foam. Besides providing a major end use of tyres, tyres used as fill provide improved permeability and greater insulating properties than traditional fill materials.

Civil engineering fill has been limited to a few pilot projects in California (Humboldt County and Chico, in Butte County); however, the CIWMB is strongly supporting the development of this market. The State of Maine has been a major user of tyres for civil engineering fill, making it the predominant use for its abatement piles. This market can have a significant impact on discarded tyre use as individual projects can use several hundred thousand tyres.

Civil engineering applications require that tyres are shredded, and minor adjustments to project designs may need to be made. The performance of the material can exceed current options available and can substantially reduce costs associated with lightweight fill. Examples of civil engineering projects include the following:

  • Overpass fill.
  • Levee slurry wall (mix with concrete).
  • Retaining wall fill.
  • Roadway base fill.
  • Bridge abatement fill.

4.3.5 Playground Equipment

Recent State and federal laws in the USA have required schools and public agencies to renovate playground equipment in order to meet new safety and accessibility standards. In an effort to encourage the development of new uses for tyres, the California Legislature passed the Playground Safety and Recycling Act, which established a $2 million matching grants program to replace and upgrade public playground equipment. The act requires 50% of the funds be used to purchase equipment made from recycled materials such as tyres.

The City of Torrance took part in a public/private partnership program with Sears, Roebuck and Co. In 1997, the city received 10,000 pounds of recycled rubber resurfacing products for several local schools as part of the store's R.O.T.A.T.E. (Recycling Old Tyres Aids The Environment) program. The program includes special tyre collection events. A local processor turns the tyres into crumb rubber, and the R.O.T.A.T.E. program supplies products at no cost to the locality where the tyres were collected. Los Angles County helped to coordinate the program and supplied education material on the recovery and reprocessing of tyres to more than 500 primary students.

4.4 Tyre Reprocessing Overseas

4.4.1 UK and Europe

The European Union (EU) Landfill Directive (EC Directive on the Landfill of Waste 1993/31/EC) was transposed into the domestic legislation of each Member State by 2001. The legislation must make provisions for the banning of tyres to landfill by 2003 or 2006 for those countries taking advantage of the four-year derogation period (allocated to countries that rely on landfill for excess of 80% of their total waste disposal capacity). The phased ban on the landfill of tyres becomes effectual for whole tyres two years proceeding the Directive coming into force, while a ban on the landfill of shredded tyres commences up to five years following the Directive coming into force (Parpworth, 1999 & ENDS, 2002).

The table below is compiled from information provided by the UK Used Tyre Working Group (2000) and gives a breakdown for 1998.

Table 4.1: Comparison of European recovery rates for tyres

  Tyre arisings (tonnes) Weight of Tyre/person /year (kg) Overall recovery rate (%) Reuse (%) Retreading (%) Materials recycling (%) Energy recovery (%) Export
(%)

Belgium

45,000

5

94

  

22

11

33

28

Finland

30,000

6

80

  

6

60

2.5

11.5

France

370,000

6

39

  

20

9

7

3

Germany

596,000

7

92

2

14

15

45*

16

Netherlands (car only)

45,000

3

100

16

29

8

47

  

Spain

241,000

6

19

  

13.5

0.5

3.5

1.5

Sweden

58,000

7

98

19

8.5

6.5

54

10

UK

468,000

8

70

16

18.5

10.5

18

7.5

* (Capacity not actual usage)

The data in the above table should be viewed as indicative only. It can be seen that the tyre arisings for each country broadly fit the estimated tyre circulation of 6 to 9 kg of tyre per person per year, with the exception of Belgium and the Netherlands. This may be due to increased reliance on public transport and that the tyre arisings for the Netherlands include car tyres only.

The table shows a significant variation with regards to the way in which used tyres are managed across Europe. Belgium, Finland and Sweden operate highly managed tyre recovery systems supported by set fees, while Sweden exports significant volumes of used tyres to the Baltic states. The Netherlands has also taken a managed approach supported by a set fee structure with specific targets introduced for particular recovery sectors. The level of tyre arisings in France and Germany is most directly comparable to the UK. Germany exported around 100,000 tonnes of used tyres in 1998 where the cost of disposal at the retail point can be as much as £3 per car tyre. In France, cement kiln capacity is actually reducing, however, there are moves to increase civil engineering applications and the take up of tyres in energy recovery facilities. The majority of tyres in Italy and Spain are landfilled, and both countries have a very substantial distance to move to meet the landfill ban (UsedTyre Working Group, 2000).

The main concern in relation to the landfilling of tyres is their potential to burn. Tyre fires in landfills are often difficult to control, and as a result conditions often exist for uncontrolled pyrolysis, resulting in the generation of complex chemical mixtures. A landfill in Knighton, Powys (Wales) that contains 10 million tyres has been burning for some 9 years (1998). Large tyre fires often pollute varying mediums resulting in atmospheric pollution by black smoke, dioxins, and polycyclic aromatic hydrocarbons, metal contaminated soil and vegetation and deleterious effects on water quality. This problem is particularly acute where a tyre fire has been extinguished with wash water which subsequently enters a watercourse by means of runoff or via infiltration of the soil. In addition to the potential fire risk associated with disposing of large quantities of tyres to landfill, there also remains the problem of instability which results from tyres rising to the surface and subsequently creating settlement problems, thus affecting the sites potential for future land use and reclamation (Environment Agency, 2003).

The number of vehicle road tyres in use in the U.K in 1996 was in excess of 121 million, with 87% disproportionately distributed on cars, 9% on light vehicles, 3% on heavy goods vehicles and the remainder on buses. However, due to their size, heavy goods vehicles account for 20% of tyres by weight. It is estimated that in 1996 the U.K generated 37 million used car and lorry tyres weighing approximately 380,000 tonnes.

Significant advances have been made in the UK with regards to the way that used tyres are managed. In 1989/90, 70% of used tyres were landfilled or stockpiled, with 5% used for energy recovery. However, in 1996 only 26% (97,800 tonnes) were landfilled or stockpiled, while 31% (117,200 tonnes) were retreaded, 27% (102,000 tonnes) were used for energy recovery and 16% were used for material recovery (in the form of 11% (41,000 tonnes) as granulate and 5% (20,000 tonnes) for physical reuse (Environment Agency, 2003).

The ban on the landfill of whole and shredded tyres in the draft Landfill Directive will have a significant impact on the way in which used tyres are managed in the U.K. It is evident that the current waste management systems and procedures in operation to deal with the increasing quantity of used tyres are insufficient and thus additional research and development is required.

The successful reuse of tyre rubber has been demonstrated in the UK with the use of both tyre crumb in road construction and as granulate for playground surfaces. However due to the inherent cost of granulating rubber and the comparatively cheap availability of alternate aggregate materials, reuse at present is limited to approximately 11% of the total quantity of used tyres generated in the U.K (ENDS, 2001 & ENDS, 2002).

The Scrap Tyre Working Group recommended that more should be done to improve the public perception of retreads. Further more it suggested that the Government should adopt a positive policy by introducing fiscal incentives, such as reduced VAT on retreaded car tyres, to encourage wider use, (Environment Agency, 2003).

In Switzerland, new regulations are being introduced to mitigate the illegal dumping of tyres. The Swiss Automobile Union (AGVS) estimates that nearly 20% of tyres discarded each year are illegally dumped and they are advocating taxation of NZ $5 for car tyres and up to NZ $10 for vans and four-wheel drive vehicles. Under the new legislation authorised tyre disposers will also have to prove they have sufficient storage facilities and account for the tyres they have disposed (ISWA, 2003).

4.4.2 Los Angeles County - USA

Los Angeles County in the USA has a population of 9.9 million and a significant tyre problem producing more than 30 million used tyres per annum and importing an additional 3 million from nearby states. Due to the large numbers of tyres generated, an extensive amount of research has been undertaken to find alternative uses, and of the estimated total 34 million tyres, 72% are recovered and diverted from landfill to end uses such as reuse, retread, crumb rubber and energy recovery. The recovered tyres have been used by local authority infrastructure departments for rubberised asphalt in local road construction, as tyre shreds and other rubber products in additional civil engineering applications, with crumb rubber products also used in playground renovations.

The challenge remains to find end uses for the remaining 8 million used tyres that are currently being stockpiled, illegally dumped or shredded and landfilled. Efforts to reduce tyre disposal and increase recovery have been driven by two diversion mechanisms. These comprise the Integrated Waste Management Act 1989 and the Tyre Recycling Act 1989 with the latter initially imposing a US $0.25 per used tyre levy to fund the tyre recovery programme in California. However, this was subsequently increased to $1.00 per tyre in 2000 in order to expand the responsibilities of the California Integrated waste Management Board (CIWMB, 1999).

The vast majority of recovered tyres are processed using energy recovery. Approximately 5.2 million tyres were burnt in cement kilns, energy recovery facilities or other cogeneration facilities. Several cement kilns and two coal-fired cogeneration facilities in the state are permitted to burn tyres as a supplement to their coal fuel source, with one plant alone (Air Products facility in Stockton) burning in excess of 1 million shredded tyres per annum. However, one facility (Modesto Energy (MELP) had been burning 6 million whole tyres per annum has closed its operation due to a recent stockpiled tyre fire and the inability to compete economically under energy deregulation.

Retreading accounted for approximately 2.4 million tyres in Los Angeles County in 2000, which is comparable to the New Zealand rate of approximately 10%. This can be one of the most cost effective methods of diversion, however only certain tyres may be retreaded due to their initial construction or excessive wear, with truck or heavy equipment tyres are best suited to this method of recovery. The cost savings over virgin tyres make the operation profitable for both the retreader and the consumer.

Approximately seven million tyres were used in crumb rubber production in 2000, primarily for both paving and molded products, while recovered tyres are also used for children's playgrounds and civil engineering purposes.

Case Study 1: Use of Rubberized Asphalt

The County of Los Angeles began limited use of rubber in asphalt in the 1970s. In 1985 the county used a 1½-inch layer of RAC to resurface a roadway that is holding up exceptionally well to this day. Federal regulations that had required the use of recycled rubber in paving projects under the Intermodal Surface Transportation Efficiency Act of 1990 served as an incentive for LA County in the use of RAC.

Since 1992 the county has been using both RAC and rubberized emulsion aggregate slurry (REAS) in its highway and street resurfacing products respectively. RAC use by the county since 1993 has resulted in diversion of more than 1.2 million tyres, paving close to 600 lane miles (a diversion of 2,000 tyres per lane mile). The county now uses RAC on 75 percent of its highway resurfacing projects, using funds for road construction generated by gasoline taxes.

Blending crumb rubber with asphalt and aggregate under specific conditions produces RAC. A crumb rubber producer grinds the waste tyres into crumb rubber. The crumb rubber is then blended with the asphalt and aggregate in a preset formula at the asphalt plant under the "wet" process and shipped to the construction site for use. A blender unit is needed at the asphalt plant.

REAS is defined as crumb rubber blended into asphalt emulsion at ambient temperature and used as slurry on road surfaces. Los Angeles has used REAS since 1993, paving more than 1,330 lane miles. This has resulted in diversion of 104,000 tyres (at 78 tyres per lane mile). REAS projects have been similarly paid for using gasoline taxes.

To support the use of RAC, LA County and the CIWMB jointly created the Southern California Rubberized Asphalt Concrete Technology Center (SCRACTC). The center serves as a professional outreach operation and as a clearinghouse for information regarding crumb rubber pavement use.

Program Characteristics

The LA County Department of Public Works operates the RAC/REAS program as part of its ongoing road construction and maintenance operations.

LA County requires the use of RAC/REAS in paving projects, as appropriate. Staff members assess the projects that are suited for use of RAC/REAS materials, and contractors are asked to submit bids accordingly. Public works staff monitors both the blending and the application process to ensure project success.

This coordinated effort by the public works staff is the primary reason for LA County's success in using RAC/REAS to divert tyres from landfills. The SCRACTC assists local governments in making RAC use determinations. The center, on behalf of CIWMB, provides grants for both roadway deflection testing and quality assurance control monitoring. These grants are given first to jurisdictions that have not used RAC before, then to others.

Costs, Economics, and Benefits

LA County requires large amounts of paving material in ongoing public works projects. The county has saved initial construction funds by using RAC, and administrators project substantial long-term savings in maintenance costs. The county found that RAC has a number of advantages over traditional asphalt concrete:

  • Cost-effectiveness.
  • Longevity.
  • Increased skid resistance.
  • Decreases in road noise (50-80 percent).
  • Lower maintenance requirements.
  • Better color contrast for striping.

Although REAS is more expensive per lane mile, it also provides a number of benefits:

  • High skid resistance.
  • Long-lasting color contrast for striping.
  • Extension of the roadway's life span.

Los Angeles has funded the use of both RAC and REAS directly through the use of gasoline tax revenue.

Case Study 2: State of Maine's Alternative Fill Programs

The State of Maine first began to explore the use of tyres for transportation projects as a paving material in the early 1990s. Federal regulations targeting recycled tyre use drove the effort. Finding the costs at the time to be prohibitive and the potential for use limited, the state began to look for another option. The civil engineering department of the University of Maine collaborated with Maine Department of Transportation (DOT) and the Maine Department of Environmental Protection (MDEP).

As various projects were proposed, they were evaluated to determine if recovered tyres could be used as an alternative to clean fill. From 1993 to 1998, Maine DOT used 920,000 passenger tyes in five fill projects. The MTA used 1.2 million tyres in a single project in 1997-98 and plans to use a similar amount in a project in the summer of 2000. Lightweight fill is now the primary use for tyres removed from Maine's abatement piles; however, it is not the primary use for the state's current tyre flow. More than 5 million passenger tyre equivalents (PTE) are burned as fuel in three paper mills, making this the primary use. In addition, a total of 1.8 million tyres were used as the operations layer in two landfills from 1997 through 1999.

Program Characteristics

The management of Maine's tyre diversion program falls into two distinct parts. The collection and stockpile management is coordinated by the MDEP, which permits both haulers and the tyre chippers (shredders). Maine's tyre management program assesses a fee of $1.00 per tyre for all after-market tyres; these funds go into the general waste management account for various programs. Tyres are accepted at tyre dealers and transfer stations typically for an additional fee of between $1.00 and $5.00.

The tyre fee covers the cost associated with handling and transportation of the waste tyres to two tyre chippers, one in Maine and the other in Massachusetts. Maine does not permit the landfilling of whole or shredded tyres and exports only a small fraction to a cement kiln in the Canadian city of Montreal.

The end use of the tyre shreds in roadway applications falls under the direction of Maine's transportation agencies (MTA, MDOT, and municipalities) that incorporate the use of shreds as part of the bidding for the construction project. Shreds are specified for fill in a project, and the tyre chippers supply them to the site.

The chips are processed to meet specific requirements including chip size, minimization of crumb rubber, and the mitigation of steel belts and wires for the project. Due to the specific characteristics of tyre shreds, they have been used as fill in projects where they were desired for their lightweight, permeable, or insulating properties. The projects using tyre shreds include drainage layers under roadways, frost barriers, lightweight fill for embankments and retaining walls, and highway edge drains.

In most cases, a contractor supplied tyre shreds (as a subcontractor) to the contractor building the road. Local governments can request that contractors bidding on projects with fill requirements prepare a bid option using tyre shreds as applicable.

Having this as an option-but not a requirement-is desirable. The availability of tyre shreds, as well as the ability of the shredder to prepare the shreds to specification, may be an issue. This should be carefully and completely understood to avoid unnecessary and costly delays.

In cases where a locality has a tyre stockpile on hand, the acceptability of those tyres for the project is the primary consideration. Both the shredding contractor and the construction contractor need to assess the condition of the tyres for contamination. Transport and shredding will be the determining factors in the cost-effectiveness of tyre shreds for the project.

Costs, Economics, and Benefits

Funding for the State of Maine's tyre program come from a variety of sources. Ongoing collection of tyres is paid for by a combination of the tip fee charged by the retailer and the revenue for selling the final product. Non-state funds are involved.

Collection of tyres in abatement piles comes from MDEP's general waste management fund, special bonds issued specifically for tyre pile cleanup, and the party responsible for accumulating the tyre pile.

Funding for the assessment of tyre shred use, as well as the follow-up testing to monitor the quality of work, has been provided by MDEP and the transportation departments on a case-by-case basis. Funding for the actual use of the shreds comes from Maine's road construction funds. The use of shreds has actually resulted in savings for the fund. The exact amount, however, could not be determined.

The running average for costs associated with the purchase and placement of the shreds were approximately US $38 per ton or $27 per cubic yard. These costs have declined as both contractors and public works officials have become more familiar with tyre shred use. The cost for the tyre shreds transported to the sites ranged from $12 to $30 per ton and placement from $5 to $8 per ton.

The use of tyre shreds in civil engineering projects is economical. In most cases the cost of the tyre shreds was less than that of comparable materials available on the market. Tyres exceed the performance of the other available materials for most uses.

The primary expense for Maine has been in both the education of its civil engineering community and in the technical studies done on the projects. The primary benefit of the use of tyre shreds has been that Maine now has an end market for the remediated tyres from abatement piles. In addition to saving landfill space, Maine has improved the engineering performance of the projects that required clean lightweight fill. The state has also cut costs associated with the use of fill.

Challenges and Opportunities

The primary challenge was in determining whether or not tyres could be used in the role of fill. Issues included settlement of the fill material over time and ability to handle tyre shreds with standard equipment. Other factors included the performance of the shreds regarding frost penetration, exothermic reactions, and interaction with other construction materials.

Despite all of Maine's concerns, tyre shreds met or exceeded the standards. Maine closely studied each project and conducted exhaustive testing, gaining valuable insight into the use of shreds. With regard to exothermic reaction, it was noted that caution should be taken.

Guidelines to limit heating of tyre shred fills as given in ASTM D6270 "Standard Practice for Use of Scrap Tyres in Civil Engineering Application" should be followed. The tyre fill should be separated from the surrounding soil by a geomembrane. It should not be exposed to the surface (free oxygen flow), and both crumb rubber particles and excessive exposed steel should be kept to a minimum.

While traditional equipment can be used, the exposed steel in the tyre shreds caused flat tyres for the construction vehicles. Tracked or solid wheel vehicles are recommended for application.

4.5 Tyre reprocessing in New Zealand

4.5.1 Case Study: The Lessons Learned from Hamilton

The recovery and re-use of used tyres is receiving increased attention across New Zealand following two recent tyre stockpiling incidents that occurred in Hamilton. A large number of, mostly truck tyres, were dumped in an illegal landfill, caught fire and subsequently burned for a week before being extinguished. In the clean up operation that ensued, Environment Waikato collected 3m3 of oil that had accumulated in a waterway beneath the ignited area. In a separate incident a stockpile of 80,000, mainly truck tyres, was created in Hamilton following the liquidation of Rubber Technology. This tyre reprocessing firm was contracted to collect and process end of life tyres from the New Zealand market. It seems that the reason for the failure of Rubber Technology was due to a lack of financial planning with regards to the cost of collection and processing, along with a lack of secure long-term markets for the product. The reprocessed material was supplied to a shoe sole manufacture, however this required a high degree of processing by a wide range of machinery to achieve the fine crumb required. This proved problematic to Rubber Technology as the machinery used was in a poor state of repair and required continual maintenance with breakdowns a frequent problem. It is estimated that the shortfall in capital and operational investment was between $250,000 and $1 Million. No subsidy was provided and a cost of $2.50 was charged per tyre for collection, however this proved insufficient to fund both the collection and processing system (Personal Communication3, 2003).

The stockpiled tyres remain at two locations in Hamilton and are now the responsibility of the landowner. Environment Waikato has taken responsibility for investigating the options available for managing the stockpile, with the least favourable environmental option being landfill disposal. It has been estimated that the disposal fee alone would be approximately $300,000. Alternative options include the use of a mono fill for the storage of tyres until a suitable application is identified at a later date. In the USA monofills have been designed using 3 foot recompacted liners and 5 foot soil caps, however the remaining issue is that of fire. Similarly the tyres could be evenly distributed along the base of a cell, however shredded or quartered tyres would leave exposed 'band wire' which has the potential to damage liner systems. Placing whole tyres in one area may also be used but the issue of differential settlement from the compaction of waste should be considered (Personal Communication3, 2003).

An additional option involves the procurement of the necessary shredding machinery to process the abandoned tyres prior to selling them to an end user or manufacturer. However, the tyres are primarily truck tyres and are therefore difficult and expensive to process. They would need to be quartered, shredded to 0.5 inches before being passed through an electromagnet to remove the high tensile metal fragments before being chipped. It has been estimated that the capital cost of the necessary machinery would be in excess of $200,000. It is anticipated that many tyre collectors across New Zealand are either stockpiling tyres on site or disposing of them to landfill (Personal Communication1, 2003).

To date the preferred technology and long term solution for Environment Waikato's used tyre waste stream is construct a pyrolysis plant. However, it is anticipated that the capital cost would prove to be prohibitively expensive with a pyrolysis plant estimated to cost in the region of $1M. An American company has shown interest in setting up such a plant and has declared that it would charge $6 per tyre for processing the tyre. It the meantime it appears that Environment Waikato are reluctant to transport the tyres to either JJ Laughton in Auckland or Tyre Disposal Limited in the Waipa District for fear of stockpiling (WasteMinz1, 2002).

4.5.2 Developing a Reprocessing Sector - What are the Options?

It appears that the general consensus of the tyre industry is that disposal of used tyres in New Zealand cannot be dealt with by market forces alone, and that a national coordinated approach should be adopted to ensure a long term solution to the recovery and reuse of tyres. Although the preference of the largest suppliers is for a mandatory levy on all tyres entering the New Zealand market, including those on new and used vehicles, incentives to recover tyres could take a number of forms including voluntary options (Personal Communication4, 2003).

An investigation carried out by No Throw, South Pacific Tyres and Bridgestone Firestone concluded that four main options were available for the processing of used tyres in New Zealand. However, while it may be appropriate for some processes and end use markets to be located in the Wellington region, the necessary volumes and economies of scale to make these options economically and technically viable is likely to require some level of national coordination. The main options available for reprocessing tyres in New Zealand are as follows:

1. Tyre Derived Fuel

It has been acknowledged by Holcim that used tyres could potentially be used as an alternative fuel source for the New Zealand cement industry and that this could account for up to 30,000 tonnes of used tyres per annum. No adverse environmental impacts or air quality deterioration are anticipated due to the extremely high temperatures associated with the cement kilns and product quality remains unaffected when using the alternative fuel source. However, investigations into the viability of using recovered tyres have concluded that insufficient volumes are currently being recovered across New Zealand. In addition, long wet kilns are used in New Zealand cement manufacturing, which would require modification and the installation of mid kiln technology to enable whole tyres to be dropped into the process. Alternatively, tyres would need to be pre-shredded so that they could be fed into the combustion zone of the kiln using the existing process (Personal Communication5, 2003).

Insufficient tyre volumes introduces technical difficulties during the cement making process as modifications and set up costs would be incurred each time the fuel source was changed. Capital investment would also be required to install the crumbing machinery needed to prepare the rubber before entering the kiln. Alternatively regional crumbing centres could be established across New Zealand as part of a national used tyre collection and processing system.

In relation to the paper and pulp industry Materials Processing Limited have investigated the possibility of a waste to energy market mixing tyres with wood waste equating to a 2% tyre mix using the total of New Zealand's tyre waste. They have had discussions with the Kinleith and Kawerau mills and have collected emission data from overseas processes. All parties concerned have expressed a keen interest in pursuing the matter further and negotiations with Bidgestone Firestone have commenced. However the limiting factor is the cost per tonne of the processed material as it has to be competitively priced with coal that has an approximate value of $50 - 60 per tonne. The collection and processing costs of used tyres must therefore be comparable to more conventional fuel sources for companies to convert to using an alternative.

2. Building and Construction

It has been suggested that shredded tyres could be used in the foundation material in the construction of new houses, however as no evidence has been found with regards to this use in other countries it is envisaged that a research programme would need to be undertaken to determine its viability. An additional suggestion is the use of 'handy planks' in the construction of new houses. This involves removing the rubber strip from a car tyre and encasing it in plastic, however this application is only suitable for car tyres and not for large passenger or industrial tyres.

3. Material Replacement

The primary uses for used tyres as a replacement for virgin materials are in roading and leachate drainage. While crumb rubber is now being used extensively in the manufacture of rubberised asphalt in other countries it is not widely used in New Zealand (Boyle & Khati, 1998). Transit New Zealand will promote the use of crumb rubber in asphaltic concrete if the 3 year trials currently being undertaken by Opus Central Laboratories concludes that this application is a suitable alternative to the materials currently being utilised. The main concern with regards to the use of used tyres as an aggregate replacement in roads is that countries that have a hot climate have experienced fires in roads constructed using rubber crumb. An investigation is also being undertaken by JJ Laughton into the applicability of its use in New Zealand roads.

The concern associated with using landfill engineering as the primary end market for the used tyres is that as the number of landfills decreases it is anticipated that within a time frame of 3 years an alternative use will need to be identified.

4. Land Banking

Land banking is a generic term for the use of used tyres in civil engineering applications. It is considered that shredded tyres have an enormous potential to be used as lightweight fill in civil engineering applications. However, it is still only in the demonstration phase in many states of the US and UK and greater investigation would be needed to determine its suitability in a New Zealand context.

4.5.3 Alternative Options for the Wellington Region

An additional reprocessing opportunity being developed that may play a role in providing a long term solution to the processing of recovered tyres across New Zealand. Solvent Rescue Ltd. of Christchurch has developed a process for treating used tyres using a pyrolysis plant that converts hydrocarbon solids into oil products. The pilot plant is currently processing 2kg per minute and trials investigating a range of waste streams has operated successfully thus far.

One firm (JJ Laughton) believes that in order to implement appropriate collection and reprocessing infrastructure in the Wellington region a public / private partnership would be required in order to share the capital costs of establishing collection and processing infrastructure. A landfill ban or levy of $2 per tyre would be required in order to instigate behavioral change and increase the volumes currently diverted from landfill. Due to the limited population size and available waste stream in order to make the user pays collection system economically viable only one licensed collector could be registered in an attempt to capture 80% of the recovered material. Under this arrangement the firm believes that the $500,000 capital investment estimated as being required to commission a processing plant in the Wellington region would be an economically viable proposition. However, if an 80% capture rate could not be established then an increased levy of $4 for example, would be necessary. Alternatively, the collection could be extended to include the wider region of Hutt and Porirua City's for example, however additional transport costs would be incurred (Personal Communication6, 2003).

It is likely that whatever the processing technology employed, a coordinated collection system would be needed for a successful and economically viable tyre reprocessing or reuse operation in the Wellington region. A similarly coordinated approach in the Auckland region, and/or nationally, might produce sufficient volumes to supply an alternative fuel for energy recovery or as a feedstock for a national reprocessing plant.

4.6 Summary of the Options Available for the Reprocessing of Used Tyres in the Wellington Region

A template has been formulated to summarise and assess the options available for the reprocessing of used tyres in a given region across New Zealand. The template, which is included in Appendix A, has been used to assess the development of a reprocessing sector for used tyres in the Wellington region and is included overleaf. The shaded area depicts information on various technologies which applies everywhere in New Zealand. Conversely, the non shaded area (Barriers to Regional Suitability), varies across New Zealand and has to be determined for each study region taking into account factors such as distance to reprocessors, existing regional reprocessing infrastructure, transport costs and volumes, for example.

The template for the Wellington Region includes information relating to the capital investment required to procure the various technology options and the minimal viable scale needed to operate each facility or process. Where this data has not been available specific information relating to a actual, planned or commissioned plant has been used. The end products of each option are included and whether the technology is under development or available as a proven process applied in New Zealand and / or overseas. An assessment and generic rating system of low, medium and high has been applied to the individual Economic Sensitivities and Risks, with low (white circle) indicating minimal cost or risk, whereas high (black circle) indicates significant cost or risk. The categories considered under Economic Sensitivities and Risks include:

  • Entry Cost - capital or other investment required to establish each process
  • Transport Costs - cost of transporting the plastic waste to the reprocessor. As plastic is light and often un-baled, transportation costs are generally considered high.
  • Product Demand - actual demand or potential market for the product
  • Feedstock Quality - the degree of polymer sorting required for each technology
  • Technology Development - small scale trials or widely used proven technology
  • Volume Dependent - minimum process throughput required for operation
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