Database of Waste Management Technologies Life

Composting, Anaerobic Digestion and MBT "products": considerations for their use

Recyclables

Recyclables derived from the various MBT processes are typically of a lower quality than those derived from a separate household recyclate collection system and therefore have a lower potential for high value markets. The types of materials recovered from MBT processes almost always include metals (ferrous and non-ferrous) and for many systems this is the only recyclate extracted.

However, these plants can help enhance overall recycling levels and enable recovery of certain constituent items that may not otherwise be collected in household systems (e.g. batteries, steel coat hangers, etc.).

Other materials which may be extracted from MBT processes include glass, textiles, paper / card, and plastics. The most common of these is glass, which may be segregated with other inert materials such as stones and ceramics. These materials are typically segregated and arise as the "dense" fraction from air classifiers or ballistic separation (see Table 1 on mechanical waste preparation technologies). This dense fraction could find application for use as a low grade aggregate; however this would be subject to achieving a suitable quality material. This mixed material from some processes has found application as Alternative Daily Cover (ADC) at landfill sites, though this would not count towards recycling performance or diversion from landfill.

Segregating glass for recycling from residual waste or a mixed waste arising from an MBT plant would require material-specific sorting techniques if recycling into high-value products is to be achieved. Examples of this approach can be found both in MBT plant as well as more traditional MRF processes. In these examples manual sorting of glass has been applied to segregate the material. However, labour costs should be considered, because if they are very high then this approach becomes uneconomic. There are also significant issues with respect to worker Health and Safety, and the handling of broken glass objects from mixed waste streams.

Textiles, paper and plastics, if extracted, are unlikely to receive an income as a recyclate and in some instances may not yield a positive value. Most of these plants can experience problems with the heavier textiles such as carpets. Clearly none are likely to separate textiles into different types of fibre.

Although less common, paper can potentially be separated for recycling but often it is combined with textiles and plastics; recycling markets or outlets for the material may be limited. Manual sorting or more sophisticated mechanical sorting can be undertaken on this waste stream. The quality of the paper will be lower than if source segregated and the markets available will be fewer and of lower value. With the improving performance of kerbside recycling schemes there has been an increase in the quantity of paper separately collected for recycling. This paper will be able to secure a market more easily than paper separated in an MBT facility.

Consequently, few MBT processes attempt to segregate paper for recycling, preferring instead to utilise it as a high calorific value Refuse Derived Fuel (RDF), which is easily achieved using conventional mechanical sorting techniques.

Any plastics separated from these processes will almost always be mixed plastics. The use of high-tech optical sorting technology, such as Near Infra-Red (NIR), offers the potential to recover high value material-specific waste streams, such as segregated plastic by polymer type. Application of such techniques is currently not so common in MBT processes. The capital costs associated with installing such technologies are high, and cost/benefits of adopting them would be significantly influenced by the effectiveness of any recycling achieved upstream through kerbside collection systems serving to limit the quantity of recyclable materials present in residual waste.

Compost and Compost-Like Outputs (CLO) / Digestate

Compost/Digestate

In Greece, specifications for the use of "compost" in agriculture exist in CMD 114218/1997. Quality of compost is not defined into different classes and the "origin" of compost is not considered (whether it is from mechanically separated organics, from source separated organics, biowaste, sludge, e.t.c.). Specifications in comparison to current EU legislation are in the following table.

Table 1: Limit values in CMD 114218/1997 - comparison with EU legislation

Parameter Soil Improvers / Growing media, (Decisions 2006/799 and. 2007/64 for Ecolabel)[1] Biowaste Directive (2nd draft, not adopted)
(values normalized to organic mater content 30%)
CMD 114218/1997
Class 1 Class 2 Stabilised Biowaste
Cd (mg/kg dm) 1 0,7 1,5 5 10
Cr (mg/kg dm) 100 100 150 600 510
Cu (mg/kg dm) 100 100 150 600 500
Hg (mg/kg dm) 1 0,5 1 5 5
Ni (mg/kg dm) 50 50 75 150 200
Pb (mg/kg dm) 100 100 150 500 500
Zn (mg/kg dm) 300 200 400 1500 2000
As (mg/kg dm) 10 15
Mo (mg/kg dm) 2
Se (mg/kg dm) 1,5
F (mg/kg dm) 200
Salmonella Absent in 25g
Helminth Ova Absent in 1,5g
E. Coli (MPN) <1000/g 0
PCB's (mg/kg) 0,4
PAHs (mg/kg) 3
Physical contaminants > 2mm (d.w.) <0,5% * <0,5% <3%
Stone & gravel > 5mm <5% <5%
Plastic (% d.w) <0,3
Glass (% d.w) <0,5
Moisture (%) <75 <40
Organic matter (% d.w.) ≥20
Total N ( %) ≤3 (organic N ≥ 80%)
Particle size (for the 90% w.w. of the product) <10 mm
Electrical Conductivity, dS/m <1,5 (applies to growing media)

* Plastic, glass and metal, d.w

No effort has been made so far in Greece to use compost/digestate in farms. It is assumed that the ministry of Agriculture and the local authorities will have to issue a license in case of farm application, similarly to the one required for treated sludge (as required by CMD 80568/4225/1991 - Dir. 86/278/EC). In any case, authorities related to agricultural activities (central and local) express their opinion during the EIA procedure of MSW facilities.

Under the EU policy framework and specifically according to the Waste Framework Directive (WFD) 98/2008/EC, compost and digestate may be considered as "products", if certain End of Waste (EoW) criteria are met. The purpose of EoW criteria is to further encourage recycling in the EU by creating legal certainty and a level playing field as well as removing unnecessary administrative burdens. Preconditions for EoW criteria for certain specified waste are laid down in Article 6 of the Waste Framework Directive (WFD).

In general the material shall cease to be a waste, if it has undergone a recovery, including recycling, operation and complies with specific criteria to be developed in accordance with the following conditions:

  1. the substance or object is commonly used for specific purposes;
  2. a market or demand exists for such a substance or object;
  3. the substance or object fulfils the technical requirements for the specific purposes and meets the existing legislation and standards applicable to products; and
  4. the use of the substance or object will not lead to overall adverse environmental or human health impacts.

The criteria shall include limit values for pollutants where necessary and shall take into account any possible adverse environmental effects of the substance or object.

The first technical working group (TWG) meeting on end-of-waste (EoW) criteria for compost and digestate of the European Commission took place at the Joint Research Centre "Institute for Prospective Technological Studies (JRC-IPTS)" on the 2 March 2011 in Seville. 47 participants of the Commission, the EU Member States and stakeholder organisations attended the workshop. The workshop was part of the activities of the TWG for supporting the development of technical proposals on end-of-waste criteria for biodegradable waste subject to biological treatment.

In the first draft report of the JRC-IPTS (JRC-IPTS Final Report 2008), which was presented at the workshop in Seville, these general EoW requirements have been checked for compost and digestate. It is stated that composting and digestion are recovery (recycling) operations within the definition of Article 6 of WFD.

Their products are used for specific purpose (a):

  • compost is used as soil improver,
  • organic fertiliser and as mixing compound in growing media;
  • digestate is primarily used as organic fertiliser.

A market or demand (b) for compost and also for digestate exists due to their nutrient level and agronomic value.

Standards and technical requirements (c) exist for compost. The European Compost Network has recently established a voluntary Quality Assurance Scheme (ECN-QAS) including product and process requirements for compost, which should be extended for digestate in 2012. Currently some Quality Assurance Schemes for digestate exist on national basis (UK, DE, BE, SE) which need to be harmonised on European level. Sampling and testing methods for compost were developed in the CEN Technical Committee (TC) for soil improvers and growing media as well as in the CEN TC 400 for harmonizing methods in the field of sludge, soil and treated bio-waste. These methods have to been approved for analysing digestate.

Composting and digestion have direct and indirect influences on exhaust and greenhouse gas emissions, soil quality and human health (d). Composting and digestion compared to other Municipal Solid Waste (MSW) management options show the least negative impacts and the most benefits.

The draft report (JRC-IPTS Final Report 2008) contains specific criteria for compost (see Table below). Still under discussion is, if a "positive" or "negative" list for sources should be set up to fulfill the requirement that only clean biodegradable wastes are allowed to be used as sources for the production of end-of-waste compost and digestate. The most contentious sources are sewage sludge and mixed municipal waste.

At least the producer of compost and digestate have to demonstrate by external independent testing, that there is a sufficient high probability that any consignment of compost delivered to a customer complies with the minimum quality requirements and is as good as the properties declared.

Figure 1

To place compost and digestate on the market a clear product declaration is necessary. This declaration should include instructions on their safe use and application recommendations. Further quality assurance is requested to achieve confidence by the end-user. In general, the producer of compost and digestate shall have a quality management system in place which should be audited externally by the competent authorities or by accredited quality assurance organisation in the Member States.

The consultation phase of the first draft report of the JRC-IPTS ended on 25 April 2011. The work on the EoW criteria for compost is well advanced. This is because of the information available from the JRC-IPTS report on EoW 2008, the ECN report "Compost production and use in Europe" and the requirements which are laid down in the ECN Quality Assurance Scheme (ECN-QAS) for compost. The JRC-IPTS would like to receive more data on digestate for including specific criteria for digestate and invites anyone in supplying data.

The second draft report is expected for September 2011 just before the next TWG meeting in October. As a final result of the work the JRC-ITPS will produce, if feasible, a study with a comprehensive assessment and with technical proposals aimed at defining end of waste criteria for compost and digestate until spring 2012. Based on this study the European Commission may propose measures under the comitology procedure.

Compost Like Output - CLO

MBT processing of mechanically separated organics can produce partially/fully stabilised and sanitised CLO or partially stabilised digestate material. Digestate material is produced from an MBT process that uses anaerobic digestion as the biological process. A step of aerobic treatment of digestate is usually applied to increase stabilisation and improve the properties of digestate (in this case, the term CLO may be applied for the aerobically treated digestate).

The potential applications of these outputs are dependent upon their quality and legislative and market conditions. CLO and digestate has the potential to be used as a source of organic matter to improve certain low quality soils, e.g. in the restoration of brown field sites, or for landfill cap restoration.

It is generally assumed that CLO/digestate derived from mixed waste will be of lower quality and value compared to compost derived from source-segregated materials, largely due to higher contamination levels. Trials on mixed waste derived materials have reported large amounts of physical contaminants (e.g. glass) and levels of potentially toxic elements, such as heavy metals.

The quality of CLO produced will vary with different MBT technologies, the quality of raw waste inputs, and the method and intensity of waste preparation and separation prior to biological treatment, as well as the methods used to screen of the outputs.

Due to its low quality, opportunities to apply CLO or digestate produced from mixed MSW to land will be limited.

Various studies indicate that "composts" (otherwise CLO), from the mechanical separated organic fraction of MSW, can only be used on non-agricultural land and must be shown to be ecologically beneficial. A risk-based assessment is needed in relation to their contamination content, and the nature of the land to which they are to be applied. This is similar approach to regulations covering the use of sewage sludge in agriculture.

If an outlet cannot be found for the CLO then it may have to be disposed to landfill. This will incur a disposal cost and any biodegradability remaining will reduce the amount of BMW diverted (this is in relation to the targets set by the 99/31 Directive regarding the diversion of Biodegradable Municipal Waste from landfills).

Refuse or Solid Recovered Fuels

Where the MSW is sorted / treated to produce a high calorific value waste stream comprising significant proportions of the available combustible materials such as mixed paper, plastics and card, this stream may be known as Refuse Derived Fuel (RDF).

The common term used for a fuel produced from combustible waste is Refuse Derived Fuel (RDF). The types of technologies used to prepare or segregate a fuel fraction from MSW include the MBT processes described. Other terminology has also been introduced to the industry as various fuel compositions may be prepared from waste by different processes. Examples include 'Biodegradable Fuel Product' (BFP) and 'Refined Renewable Biomass Fuel' (RRBF).

A CEN Technical Committee (TC 343) is currently progressing standardisation work on fuels prepared from wastes, classifying a Solid Recovered Fuel (SRF). The work of CEN TC 343 is based on the work done on a mandate of the European Commission. The mandate is on producing all necessary standards on solid recovered fuels (SRF). While the mandate of CEN TC 335 covers all fuels purely derived from "biomass" as defined in the scope of the Waste Incineration Directive 2000/76/EC, the mandate of CEN TC 343 is on fuels derived from all other non hazardous solid wastes. In this sense, within the term "SRF" all types of fuels derived from Municipal Solid Waste are incorporated, regardless they result from a biodrying process or a "conventional" MBT.

CEN TC 343 started in beginning of 2003. To speed up the work, the technical committee (TC) has decided to produce first technical specification (TS), which will after validation be transformed into standards (European Norms (EN)). The work of CEN TC 343 is organised via a Technical Committee (Chairman Martin Frankenhaeuser, Finland) and five Working Groups:

  1. WG1 Terminology and quality management (Convenor: Julio Calzoni, Italy)
  2. WG2 Fuel specification and classes (Convenor: Staffan Modig, Sweden)
  3. WG3 Sampling, sample reduction, determination of biodegradable fraction (Convenor: Timo Gerlagh, The Netherlands)
  4. WG4 Physical parameters (Convenor: Jorg Maier, Germany)
  5. WG5 Chemical parameters (Convenor: Giovanni Ciceri, Italy)

The Technical Specifications (TS), a kind of a pre-standard and Technical Reports (TR) are completed and published. For the classification of SRF a basic document has been used, which describes the actual situation in Europe for those parameters that are of special interest.

The upgrading phase to EN standard is running. The EN standards are expected to be ready in 2011 for Formal Vote by the national CEN members and will thereafter be published. The upgrading from TS to EN comprises also the validation of the TS's based on the outcome of a validation project called the "Quo Vadis" project[2] . The Quo Vadis project started in 2005 as a common project of the European Commission, universities, institutes and industry and was completed beginning of 2008.

The technical specifications classify the SRF by thermal value, chlorine content and mercury content. For example, the thermal value class will be based on the number of Megajoules one Kilogram of recovered fuel contains. In addition, there are many characteristics for which no specific values have been determined. Instead, they can be agreed upon between the producer and the purchaser of SRF.

It is anticipated that once standards are developed and become accepted by users, then SRF will become the terminology used by the waste management industry. European standards for SRF are important for the facilitation of trans-boundary shipments and access to permits for the use of recovered fuels. There may also be cost savings for co-incineration plants as a result of reduced measurements (e.g. for heavy metals) of incoming fuels. Standards will aid the rationalisation of design criteria for combustion units, and consequently cost savings for equipment manufacturers. Importantly standards will guarantee the quality of fuel for energy producers.

Biogas

An MBT plant that uses anaerobic digestion (AD) as its biological process will produce biogas. During AD, the biodegradable material is converted into methane (CH4) and carbon dioxide (together known as biogas), and water, through microbial fermentation in the absence of oxygen leaving a partially stabilised wet organic mixture known as a digestate.

The biogas can be used in a number of ways. It can be used as a natural gas substitute (distributed into the natural gas supply) or converted into fuel for use in vehicles. More commonly it is used to fuel boilers to produce heat (hot water and steam), or to fuel generators in combined heat and power (CHP) applications to generate electricity, as well as heat.

Biogas electricity production per tonne of waste can range from 75 to 225 kWh, varying according to the feedstock composition, biogas production rates and electrical generation equipment. Biogas is a source of renewable energy, with electricity generated from it being supporter by EU and National Legislation.

In most simple energy production applications, only a little biogas pretreatment is required. Biogas used in a boiler requires minimal treatment and compression, as boilers are much less sensitive to hydrogen sulfide and moisture levels, and can operate at a much lower input gas pressure.

Where biogas is used for onsite electricity generation, a generator similar to that used in landfill gas applications can be used, as these generators are designed to combust moist gas containing some hydrogen sulfide. Gas compression equipment may be required to boost the gas pressure to the level required by the generator.

Some electricity is used by the AD plant, but any excess electricity produced can be sold and exported via the local electricity distribution network. Excess heat can also be used locally in a district heating scheme, if there is an available user.

For high specification applications (e.g. vehicle fuel, natural gas substitute), or when using more sophisticated electricity generation equipment (e.g. turbines), biogas will require more pre-treatment to upgrade its quality. This includes the removal of hydrogen sulphide (a corrosive gas); moisture removal; pressurization to boost gas pressure; and removing carbon dioxide to increase the calorific value of the biogas. However, the cost of the equipment required to upgrade biogas can be prohibitive.

 


1 Products shall not contain sewage sludge. A product shall only be considered for the award of the Eco-label if it does not contain peat and its organic matter content is derived from the processing and/or re-use of waste

2 One important element of each standardisation process is the validation of the standards which prove their workability in the field of practice. To support the work of CEN TC 343 the European Commission, universities and institutes as well as industrial associations have committed themselves in a 2.5 million euros project called Quo Vadis (Quality Management, Organisation, Validation of standards, Developments and inquiries of SRF). A European database on SRF production according to the classification system and a validation of the TS on specification and classification has been drawn up. The project has been completed by early 2008.