Preparing new ground: How to put stabilized biowaste to proper use in landfill restoration

Recently there has been renewed interest in composting the organic fraction of MSW for restoring the soil on former landfills. There are obvious health and safety concerns associated with such use, and stakeholders exploring this option will have to become familiarized with current knowledge and a set of proposed guidelines.

by Jim Baird, MacKenzie Hutton, Anne L. Savage, Andrew Hipkin, Paulo Cruz and Iain MacLeod

Many municipalities around the world face onerous targets for pre-treating waste and diverting biodegradable municipal waste (biowaste) from landfill. They thus seek ways to dispose of biowaste effectively and economically. One option attracting growing interest is to use stabilized biowaste on former landfill sites. When evaluating this option, it is necessary to consider environmental protection, landfill site characteristics, planning, quality assurance and liability. Below we guide biowaste producers and local authorities through these issues, helping them to meet landfill diversion targets by using biowaste in an environmentally safe manner.

The article is concerned primarily with municipal solid waste (MSW) subjected to mechanical-biological treatment (MBT) in a mixed waste composting facility, and is partly based on recent research carried out by Remade Scotland for the Scottish Executive.1

Trends in MSW compost generation

The first mechanized treatment plants composting MSW were established some 50 years ago in Scotland, the Netherlands, Switzerland, Germany, Austria and the US. The mid-1980s saw a significant growth in MSW composting with Germany, the Netherlands, France, Spain, Italy, Denmark, Belgium, Portugal and Austria leading the way. But by the mid-1990s, production had declined as municipalities developed compost schemes using source-segregated feedstock (green waste) rather than MSW and as concerns about quality remained unresolved. In 1994, composting in the US received a setback when the Supreme Court dismissed the flow control law guaranteeing the input of waste to composting facilities and waste was diverted to the cheaper option of landfills.

Municipal waste feedstock before the inorganic fraction is separatedClick here to enlarge image

However, there has been renewed interest in recent years in composting MSW. New facilities have been constructed in the US where 11 plants were operating in 2001-2002 with more planned. MSW composting is practised widely in the Middle East where compost is in demand in desert areas, though environmental and health standards remain a concern. In Asia, Thailand has a history of MSW composting, and municipalities in Indonesia and China are becoming increasingly interested. Australia currently has six biowaste plants operating, with three further plants awaiting contracts.

Specifying quality

Development of a generally accepted specification for biostabilized waste has proved difficult, mainly due to a lack of consensus on the classification of material produced by MBT plants and the environmental risk this material might pose. Most specifications governing compost produced from biowaste deal with three main issues:

• levels of potentially toxic elements (PTEs), such as heavy metals
• sampling of the material
• stability of the compost product.

Local regulations may also apply. In England, for example, the composting process must comply with the Animal By-Products Regulations 20052 to ensure the absence of Salmonella and Enterobacteriaceae (Scotland, Wales and Northern Ireland have their own versions of the ABPR).

Proposed guidelines for biowaste

PTEs and other contaminants concentrate within the feedstock as organic matter is degraded. Before deciding what to with the treated waste, it is thus important to ensure that biodegradation is complete and that PTEs have reached maximum levels (i.e. the material is stable).

Table 1 shows proposed standards by Remade Scotland derived from a variety of guidelines and statutory requirements, including an indicative standard developed by the Scottish Environment Protection Agency (SEPA).3

Devising a sampling protocol

Biostabilized waste is a heterogeneous material that is not easy to sample. So that it can be analysed and compared with a recognized standard, producers need to know how to take samples that are representative of the entire batch.

Effective sampling requires an unambiguous protocol that allows material from different batches and processing plants to be compared. Such a protocol should state:

• the minimum number of samples to be taken and how often
• when the samples are to be collected (i.e. when the biodegradation process is deemed complete)
• how the samples are to be collected
• who is to collect the samples and what training they should receive.

A representative ‘composite’ sample for analysis consists of numerous point samples extracted from random locations within a batch and mixed thoroughly. This composite is deemed representative of the entire batch. Statistical methods can be used to show with a high degree of confidence that most of the biowaste is below the regulatory threshold for PTEs most of the time.

Every batch is different and producers must be confident that their procedures consistently produce acceptable batches, preferably without the expense of sampling and analysing every batch. Reliable feedstock screening technology that removes impurities is required to achieve a consistent and acceptable output.

Ensuring biowaste stability

Biowaste used in landfill restoration must be biologically stable such that organic matter will not be lost and heavy metals will not concentrate further. However, there is no clear agreement on how best to test and define stability. It is often described as ‘the point where biological activity is complete’ and is identified by monitoring the aerobic respiration rate, which is the amount (in mg) of oxygen consumed by a given quantity of biowaste within a given time period. Testing involves determining the production of carbon dioxide by the material or more usually its oxygen consumption.

A reduction in microbial activity does not always indicate that all available organic nutrients have been consumed and that biodegradation is complete. Other reasons why microbial activity may fall include:

• high or low process temperatures
• low moisture
• lack of free air space
• low oxygen content
• inappropriate pH
• lack of inorganic nutrients
• the presence of toxins.

A lack of sufficient moisture is a common problem in composting. Mass transport of water-soluble nutrients to the microbes is much lower in dry material, while dry conditions require far higher temperatures to kill pathogens.

Screening technology options

Screening systems not only affect PTE levels but have a significant role in removing glass, plastics and other non-compostable elements (NCEs) from the feedstock. The choice of appropriate equipment, plant or machinery to remove these impurities is critical in helping to meet quality standards. Such equipment can be incorporated at an acceptable cost into existing and new MBT plants with minimum impact on layout.

Screening is the first step for removing materials undesirable for composting from the feedstockClick here to enlarge image

Three main separation techniques are used with varying degrees of success to remove NCEs from the feedstock prior to composting:

• air classification - relies on differences in density, size and shape to separate mixed materials by placing them in a strong flow of air
• ballistic separation - uses the differences in trajectory attained by materials with differing air resistance, elasticity and inertia
• centrifugal separation - applies centrifugal forces to separate materials of different size and density.

Most plants employ more than one appliance using more than one of these techniques. Manufacturers often combine two of the above techniques into one machine, and many suppliers also advocate the use of appliances in series to deal with a distinct particle size distribution. Most equipment manufacturers/suppliers are happy to discuss the level of performance guarantee they are prepared to provide. Critical factors include:

• input material specification (determined by waste analysis)
• total tonnage to be processed
• rate of throughput.

Factors that influence both the equipment performance and cost include:

• location of the equipment
• proximity and relationship to other plant
• need for additional structural/civil works
• installation of bespoke feeding and discharge systems optimized to specified materials flow
• commissioning costs.

Use of stabilized biowaste in land restoration

A full assessment of the stabilized biowaste is essential to determine suitable application rates, maximize benefits and minimize harm to the environment.

Site characteristics and suitability

Careful assessment of the landfill site is required before restoration, as its design, topography and quality/type of capping system may affect the operation of machinery, restoration techniques and health/safety issues. Considerations that should be taken into account during restoration include:

• the potential for leakage of leachate and landfill gas through the landfill cap
• active or historic subsidence
• infrastructure for leachate treatment and gas extraction.

Soil characteristics

Soil for blending should be analysed for physical characteristics such as texture, structure, organic matter content and bulk density, and chemical characteristics such as pH and levels of various elements.

Part of the site should be designated for blending and storage facilities. Soil compaction prior to spreading the biowaste is a common cause of failure to provide a surface tilth that supports plant growth and should be avoided.

Environmental protection

A primary level of environmental protection will have been met if the stabilized biowaste meets minimum standards for PTEs, NCEs and hygiene. In practice, PTE levels may still be high, with some approaching the maximum permissible. This is because metals such as zinc, copper, cadmium and lead are present in a range of consumer products and not all these materials are removed during MBT.

Although the risk to human health and the environment may be low when biowaste is applied to closed landfill sites, concerns remain about the potential for leaching and pollution during restoration. This risk can be minimized by following appropriate guidelines for soil handling operations and pollution prevention.

Leaching of PTEs from soil amended with stabilized biowaste

Concerns about spreading sewage sludge and other organic wastes on farmland have prompted many studies on PTEs in soils. These have focused on the solubility, availability to plants, and the extent to which PTEs are adsorbed or otherwise immobilized by mineral and/or organic matter present in soils.

Research on the leaching of PTEs from soils amended with stabilized biowaste has shown that:

• most PTEs are relatively insoluble and little leaching occurs at neutral and alkaline pH
• under field conditions, the mineral subsoil acts as a sink for metals, further reducing leaching.

Leaching of PTEs from stabilized biowaste is most likely to occur with heavy repeated applications over many years and on acidic sandy soils with low organic matter that receive high rainfall or irrigation.

The following should also be considered when restoring closed landfill sites with stabilized biowaste:

• PTE concentrations in biowaste are low apart from zinc, copper and lead. The leachability of most metals is reduced by binding to organic ligands and exchange sites.
• Some organic matter/metal complexes are more soluble than the mineral forms of the metal. This applies to zinc and nickel.
• The risk from leaching is reduced because there is only one application and the application rate is relatively low compared with those in most studies.
• The soil component of the blend is likely to be derived from subsoil horizons with a much lower PTE content than agricultural soils.
• Biowaste is often mixed with soils that have high clay content. Such soils will adsorb much of the PTE content not already immobilized by organic matter.
• Soil materials with high sand or organic matter content are less suitable for blending with biowaste, as they have insufficient structural stability or PTE-adsorbing capacity.

Liability and health and safety issues

Assuming adherence to guidelines and specifications, the risk of pollution from blending and restoration operations is considered low. The most likely incidents relate to potential pollution of nearby watercourses by solids during heavy rain and surface water run-off, and accidental damage of the landfill cap or pipework, resulting in leachate discharge and possibly landfill gas emission. If an incident occurs, the relevant environmental agency and planning authority should be informed and immediate remedial action taken.

Stabilized biowaste will have to undergo strict quality assurance before it can be used for soil redevelopmentClick here to enlarge image

Site owners/operators should ensure that restoration contractors are aware of all potential ‘danger’ areas such as cables, pipelines, wells and pumping equipment. If it is not possible to trace the owners or former operators of a closed landfill site in order to obtain proper records, a preliminary site survey is necessary to assess conditions prior to restoration.

Site end-use

The proposed end-use of the site is an important consideration. It is often not necessary or desirable to restore closed landfill sites to agricultural use, as this would increase the risk of contaminants entering the food chain and/or damaging the environment (even though low). Agricultural use could also damage the landfill cap and the infrastructure associated with landfill gas and leachate extraction.

Many sites could be restored for amenity or nature conservation uses. Sites in more remote areas could be restored to mimic the surrounding natural vegetation (such as heather or grass-dominated moorland).

Quality assurance

The object of quality control systems is to ensure that stabilized biowaste meets both statutory standards and end-user needs. A range of statutory and voluntary standards and controls exist in various countries. These provide a baseline for developing quality assurance programmes for stabilized biowaste.

In general, higher-level end-uses such as a soil amendment for land application are more likely to need a quality control system. The following should be considered when implementing a quality assurance programme:

• input materials and processes - includes waste arisings, handling and transportation, and MBT treatment. Each of these may be subject to statutory controls
• output materials - the specification of material outputs has an overriding constraint in terms of being ‘fit for purpose’ as specified by local regulations, directives and standards and as verified by agreed sampling protocols and analysis methodologies
• end-use applications - land use options for stabilized biowaste may be subject to control by local planning authorities and environment protection agencies.

Planning and environment regulation procedures

Stabilized biowaste is still regarded as waste. Therefore its disposal, including use in land restoration, may be subject to often complicated planning and environment procedures and regulations.

The determination of planning applications is influenced by factors such as:

• legal definition of biological, chemical and physical characteristics of treated waste
• availability of land, within specified categories and locations, which would benefit from the application of the material
• potential risk to human health and the environment
• regulatory and procedural requirements relevant to individual land use.

The route to obtaining planning permission will depend on whether a previously approved restoration scheme or plan is in place. An existing scheme could be amended to take into account the use of biowaste, but an environmental impact assessment (EIA) may be required or need to be modified. The relevant statutory body should be notified of any changes.

Standard local planning procedures should be followed if no approval is in place. This is likely to require information about:

• the overall nature of the proposal
• the type, quantity and source of materials to be used
• drainage provision
• vehicle numbers and movements
• hours of operation
• time span of operational works
• soil treatment
• final seeding and planting.

In some areas, formal public notification and an EIA may be required.

The application of biowaste in land restoration is subject to local regulations governing pollution prevention, environmental protection and waste management licensing. Compliance with legislation may require a risk assessment that:

• characterizes the biowaste
• recognizes potential receptors (e.g. human health and ecosystem)
• identifies potential or introduced hazards
• evaluates the probability, consequences, significance and tolerability of risk.

Practical application: an emerging opportunity

Low-grade compost has been used worldwide for landfill restoration for many years. In some countries, land restoration projects represent a major market for this product.

• In the Netherlands, up to 164,000 tonnes (25% of MSW output) is used to landscape former landfills.
• In Canada, a plant at Newmarket in Ontario owned by Canadian Composting Inc. has an annual capacity of 150,000 tonnes of organic waste. When the compost does not meet prescribed standards, it is used for quarry restoration and other land rehabilitation projects.
• In Australia, a plant run by Southern Metropolitan Regional Council in Perth turns an eighth of the city’s household waste into compost. The compost has been used as a soil improver in various areas including grass verges alongside roads where it binds the sand and prevents it blowing on to roads, as well as on golf courses, vineyards and gardens.
• SITA UK uses compost in the restoration of landfills when they reach the end of their operational life.

The compost and biowaste products in these examples are derived from source-segregated material. However, different countries define MSW biowaste in different ways depending on the underlying waste collection schemes. For example, in the Netherlands and Germany, MSW biowaste comes from a source-segregated feedstock - i.e. it contains the organic waste from separate collection of MSW. In such cases, the organic residue collected from gardens and parks is defined as green waste. In other countries such as Spain and the UK, MSW biowaste is produced from a non source-segregated feedstock. This has an impact on the market for stabilized biowaste.

European interest in stabilized biowaste generation via MBT has been driven partly by the EU Landfill Directive, which sets targets for the diversion of untreated biodegradable waste from landfill. However, generation is not enough. Unless markets can be established, MBT plants will be used primarily as a means of pre-treatment prior to landfilling, rather than for compost production.

Conclusion

MBT processes can play a strategic role in meeting national targets for diverting MSW from landfill, but local authorities need guidance to help them utilize stabilized biowaste for this purpose. Guidance is particularly required on monitoring and sampling, technologies and processes, and planning and environmental regulations.

Jim Baird, MacKenzie Hutton, Anne L. Savage, Andrew Hipkin, Paulo Cruz and Iain MacLeod are all affiliated with Remade Scotland, UK.
e-mail: j.baird@gcal.ac.uk
web: www.remade.org.uk

Notes

1. Remade Scotland for the Scottish Executive, 2005 The Use of Stabilized Biowaste in the Restoration of Former Landfill Sites, Environment Group Research Report 2005/03. www.scotland.gov.uk/Resource/Doc/54357/0013017.pdf

2. www.opsi.gov.uk/si/si2005/20052347.htm

3. SEPA, 2004 Composting Position September 2004. www.sepa.org.uk/pdf/guidance/waste/sepa_comp_position_sep04.pdf

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BSI and ISO standards

British Standards Institution (BSI) standards currently apply only to source-separated ‘green’ wastes. The Publicly Acceptable Standard (PAS 100) is a minimum specification to guarantee an appropriate and safe product. Quality criteria include limits on stones, weed content, and physical and chemical content. These requirements also relate to toxicity to animals or plants, moisture and texture, and lack of odour. The traceability of materials after their production is also an important factor. In due course it may be possible to consider MBT wastes in a similar way, but final standards have yet to be specified.

The International Organization for Standardization (ISO) produces voluntary standards and guideline reference documents, including the ISO 14000 series on specific aspects of environmental management. For example, BS EN ISO 14001 is applicable to organizations, including waste processors, that wish to demonstrate compliance with environmental regulations and standards in areas over which they have control or influence.