WASTE PROCESSING: THE STATUS OF MECHANICAL AND BIOLOGICAL TREATMENT

Transcription

1 WASTE PROCESSING: THE STATUS OF MECHANICAL AND BIOLOGICAL TREATMENT Monique Kallassy 1, Boris Efremenko 1, Martin Champel 1 1 Veolia Environmental Services- Technical and Investment Division, France ABSTRACT The term Mechanical and Biological (MBT) is commonly used to describe a hybrid process which combines Mechanical techniques used to sort municipal solid waste (MSW) and Biological techniques used to stabilise the organic fraction of it. Recently, these concepts have attracted considerable attention and have received support from public officials and environmental organisations. MBT is a step within the whole waste management plan. Today, there are over 150 MBT plants operating in Europe and are expected to treat more than 13 million tpy. In this paper, different Mechanical and Biological Treatment configurations that exist on the international market are described. INTRODUCTION The landfill directive is one of the most important environmental directives the European Parliament has dealt with in recent years. It marks the beginning of a major shift in waste management practice in Europe. This directive aims to reduce the negative environmental impacts of landfilling (Pan & Voulvoulis, 2007).This is mainly to be achieved by reducing the quantity of organic matter landfilled, through measures such as recycling of the organic waste stream or pretreatment of residual wastes before landfilling. Alongside incineration and other treatment processes, Mechanical Biological Treatment (MBT) is playing an increasing role. MECHANICAL BIOLOGICAL TREATMENT The term of Mechanical Biological Treatment or MBT is applied to several different combinations of mechanical, and biological processes (Muller et al., 2003). MBT utilizes well-known technologies (, composting, anaerobic digestion), brought together in order to recover as many recyclables as possible and divert the biodegradable fraction from landfill. MBT treats the residual fraction of the Municipal Solid Waste (MSW). It is important to note that the composition of the MSW is very different depending on the waste management politics and the economical situations (available population income and consumer behaviour) of each country. In Europe for example, in some countries like Germany (Raussen & Kern, 2005), Austria and Finland (Tolvanen & Hänninen, 2005), biowaste fraction is collected separatly from residual waste fraction. Other countries like France and Spain (Korz, 2005) do not have a very developped source separated organic fraction collection. The organic fraction ends up in the MSW. Oustide Europe, in many developing and emerging countries like Brazil (Münnich et al., 2006) and Egypt waste analyses show that the proportion of biowaste fraction (40-70%) is higher in MSW than in industrial nations (28-35%). Mechanical separation techniques Mechanical separation does not always take place before biological treatment. We can also have a Biological Mechanical Treatment (BMT) configuration. The mechanical treatment is a combination of one or several materials such as: -, crusher and mill: for the reduction of the material size. - Bag splitter: a more gentle shredder used to split plastic bags whilst leaving the majority of the waste intact. - and vibrating, star and disc screens: for separating and sizing materials. - Ballistic separator: density and elasticity separation for light plastics, paper and heavies (stones & glass).

2 - Magnet and eddy current separators: for removing ferrous and non ferrous metals. - Pelletiser: for improving the handling transportation and feeding of Refuse Derived Fuel (RDF). - Optic separation (NIR): for the of specific plastic polymers. - Water-based separation of differential material densities: light materials as plastics and heavies like stones and glass. - Wind shifter that removes light papers and plastics from the main stream. - Rotating drum: uses gravity to tumble, mix, and homogenize the wastes. Dense, abrasive items such as glass or metal will help break down the softer materials, resulting in considerable size reduction of paper and other biodegradable materials. Besides we can have a manual separation of plastics, contaminants and big size objects. Manual processes of separation are especially appropriate in many cases in developing countries, singularly or in combination with mechanical processing operations (CalRecovery, Inc. & IETC, 2005). The setting up of the equipments bases on the fact that the small MSW fraction have a high content of organic matter. Biological Pre-treatment BRS or DANO drum is an aerated turning drum where MSW spends 2 to 4 days. The combination of the mechanical rotation and the beginning of the biological degradation of the organic material allow a reduction of the organic fraction size and a better mecanichal separation of the organic fraction from the physical contaminants. Biological treatment processes The Biological Treatments utilize the activity of microorganisms to degrade and turn the Organic Matter (OM) into a stabilised output (composting) and/or biogas (Anaerobic digestion) or to dry the material (biodrying). Composting The composting process involved the aerobic degradation of the organic matter providing stabilisation and sanitation of the material which can be returned to the soil. The composting process can occur in: - an enclosed or in-vessel system such as containers, silos, agitated bays, tunnels, enclosed halls or rotating drums. These are high tech processes. - a static or aerated windrow which is rather cheaper technology. An MBT-Aerobic composting configuration aims at producing either a compost or a biostabilised output (known also as compost-like) to be landfilled. The combination of mechanical treatment equipments is different in both cases (see examples below). If the produced compost is in accordance with the requirements of local standards, it can be used as a soil improver. If not, the compostlike landfilling allows to avoid the emission of green house gases as the material is stabilised. The basic steps of a MBT-Aerobic stabilisation and an MBT-composting configurations include: Compost-like production Shredding Aerobic stabilisation RDF and recyclables Compost production Biological Pretreatment Sorting of recyclables, RDF& physical contaminants Composting Refining Figure 1 MBT-composting configurations Anaerobic Digestion The Anaerobic Digestion (AD) uses anaerobic bacteria to break down organic material without oxygen. During the anaerobic digestion 2/3 of the biodegradable organic matter (protein, starch, lipids, fatty acids, cellulose and hemicellulose) are transformed into biogas (composed mainly of methane and carbon dioxide). Non biodegraded molecules with lignin and humus precursors stay in the digestate. The digestate is not a stabilised OM. It needs a curing aerobic phase in order to transform residual OM molecules into stabilised molecular forms (Doublet et al. 2005). There are five main ways in which AD systems can be configured: Wet or dry, plug flow or fully mixed, mesophilic or thermophilic, single

3 stage or multistage, batch or continuous (Juniper, 2007). An MBT-AD releases biogas which is usually burnt in a gas engine to produce energy. It is important to note that in a MBT-AD configuration, the non-organic fraction must be very efficiently separated prior to the digester. Unlike the Aerobic biodegradation, the AD cannot accept a significant contamination of the organic fraction. For this reason AD has suffered high failure rates when using MSW as feedstock. The basic steps of a MBT-Anaerobic digestion configuration includes: Biodrying Sorting of recyclables, RDF & physical contaminants AD Dewatering Composting or aerobic stabilisation Figure 2 MBT-Anaerobic digestion configuration Biodrying uses the aerobic bacteria activity that occurs at the beginning of a classic composting in order to heat up and to remove the water content of the waste in a small duration (generally 7 days). Besides the microbial activity a high airflow passes though waste in order to increase the water evaporation. This kind of process has been designed to reduce the weight of the waste and increase its calorific value. The aim of this process is not to stabilise the OM fraction but to have a good separation of RDF that can be used in industrial applications. A MBT-biodrying configuration is simple: Size reduction & homogenization Biological drying RDF and recyclables Figure 3 MBT-biodrying configuration Other new MBT configurations like MBT- Ethanol production and MBT-autoclaving are emerging in the market. They are not yet regarded as proven technologies. MBT outputs An MBT plant can be built in a variety of combination depending on the final product required: - Compost used in agriculture as soil improver. It is considered as a product and should have good quality and no environmental negative impact. In order to produce high grade compost, several key points have to be considered: a collection of hazardous and toxic waste by the community prior to treatment, no shredding step in the process, a succession of good equipment of providing the separation of the Organic Matter from the physical contaminants. - Refuse Derived Fuel (RDF) known also as solid recovered fuel or specified recovered fuel (SRF): are used for energy recovery in an industrial process (cement kilns and industrial boilers) as a substitute to fossil fuel. The Net Calorific Value of this kind of product in industrialised countries varies between MJ/kg and have a moisture content of % (CalRecovery, Inc. & IETC. 2005). RDF should have a low concentration of chlorides to avoid corrosion effects in stockers and boilers. The presence of small particles of metal and of glass fines (<0.125 cm) in RDF can be an issue in the combustion system (CalRecovery, Inc. & IETC. 2005). - Ferrous and non-ferrous metals can be recycled. - Biogas is valorised in a motor, boiler or a cogeneration unit (heat and electricity production) or purified to recover methane.

4 Besides these products, we can have other substrates such as: - Digestate that is often dewatered before being stabilised. Depending on the quality of the stabilised digestate it could be landfilled or used in agriculture. - Compost-like, heavies and other refuse are often landfilled. - Low calorific dry fraction that can be incinerated in a MSW waste to energy plant or landfilled. SOME EXAMPLES OF MBT PLANTS Veolia Environmental Services (VES) operates 124 biological treatment plants all over the world including 22 MSW treatment plants (around 700 kt treated MSW). Example 1: Lantic plant. The plant is located in Lantic (Western France). The configuration of the plant was improved in 2003 in order to produce a good compost quality. The total investment cost is 12 M. The treatment capacity of the plant is 14 kt/y of MSW with 4 kt/y of biowaste, 7 kt/y of green waste and 2 kt/y of seaweeds. in order to get good contaminants separation from the organic fraction. The separate collection of hazardous and toxic waste by the community is necessary in this configuration. In fact, MSW composting has longer suffered from a bad image due to the high content of pollutants. Today, mechanical separation has been improved as well as source separation of hazardous and toxic waste and we are able to produce high quality compost from residual MSW. Example 2: Montazah plant. The plant is located in Montazah in Alexandria, Egypt. The start-up of the composting plant with one treatment line was in Currently 2 additional lines were set-up (investment costs 700 k ). The plant capacity is now 250 ktpy. The quality of MSW in this country is very different comparing to European MSW composition with higher content of organic fraction. The manual of recyclables is very developed on MSW before collection. The MSW coming in the site have a low content of plastics, glass and papers. The process for MSW is composed of: 2 BRS Length 24m 30 mm Ballistic separator Flip-flop screen 10 Composting in windrows Figure 4 Lantic Flow Diagram Recyclables Sorting of bulking material 30 mm Manual Fe 60 mm Composting in windrows Figure 5 Montazah Flow Diagram The quantity of compost produced is 9000 tpy. The compost complies with French standards NFU and local label (Cerafel). The key of success of such a process is the absence of shredding in the process and the use of the BRS with a sufficient retention time The annual quantity of produced compost is around 75 kt/y. This compost complies with local regulation and VES quality standards.

5 Example 3: Rostock plant. The plant is located in Rostock (Germany). It was designed to treat 150 Kt/y of MSW mainly. The investment cost is around 20 M. The process is designed to produce RDF from MSW: AD Aerobic stabilisation Wind shifter Optic separation Figure 6 Rostock Flow Diagram In Germany, there is a very developed selective of recyclables and organic fraction. Landfilling of untreated MSW is banned since And it is forbidden to make MSW compost for agricultural use. This explains why Germany is a pioneer in terms of MBT with RDF & compost-like productions model and has developed the technology soon. The quantity of produced RDF in Rostock plant is 35% from the input MSW. RDF is used for energy recovery. Example 4: Dresden plant. The plant is located in Dresden(Germany). It was designed to treat 85 Kt/y of MSW mainly. The investment cost is around 21 M. Biodrying The quantity of produced RDF is 53% from the input MSW. RDF is used for energy recovery. Main barriers to the development of MBT Today there is still a lot of uncertainty about the outlets for MBT outputs. There is uncertain legal framework for the use of RDF. It is not yet considered as a product and can potentially be used in co-combustion applications (power stations, cement kilns) or can be directly incinerated in a fluidised bed (RDF combustion plants). Austria and Germany have a very strict thresholds for the landfilling of the compostlike material. This is not the case elsewhere. The cost of an MBT plant may appear very expensive. This can limit the development of such a waste treatment. CONCLUSION The term MBT found its origins in Germany and in Austria where the most processes are designed for the stabilisation of MSW before landfilling. Today MBT processes are used by various countries but aim to different objectives: producing RDF and/or compost, recovering materials and/or stabilising the OM before landfilling. MBT is an intermediate step between MSW collection and the final treatment (recycling, landfilling, compost using, thermal treatment). The MBT configuration choice depends on local regulation, economical context and opportunity besides available final treatments. Different configurations of MBT would result in very different quality of outputs (Pan & Voulvoulis, 2007) and there is no universal MBT solution. A close analysis of the local context is therefore necessary before any MBT designing. Fe and non Fe Pelletiser Screening Wind shifters Fe and non Fe Manual Fe RDF Refuse Figure 7 Dresden Flow Diagram

6 REFERENCES CalRecovery, Inc. and IETC Solid Waste Management: United Nations Environment Programme, ISBN: Doublet S., Berger S., Couturier C., Leclerc B La qualité agronomique des digestats. Echo MO n 51. Jan- Feb Juniper Anaerobic digestion Technology for biomass Projects. Prepared for Renewables East Korz D., Status and trends of the residual waste treatment option in selected EU member states: Spain. In proceedings of the conference The future of residual Waste Management in Europe Muller, W. Niesar, M., Turk, T Optimised Mechanical Treatment and Material Segregation though Ballistic Separation Within Mechanical Biological Waste Treatment; In Proceedings of the 9th International Waste Management And Landfilling Symposium, CISA: Cagliari, Sardinia 2003; Paper 100. Münnich K., Mahler C.F., Fricke K Pilot project of mechanical-biological treatment of waste in Brazi l. Swience Direct. Waste Management 26 (2006) Pan, J, Voulvoulis, N The role of MBT in reducing methane emissions from landfill Disposal of Municipal Solid Waste in the United Kingdom. Air & waste management, 57: Raussen T., Kern M Status and trends for Biomass use and consequences for waste management- the example of Germany. In proceedings of the conference The future of residual Waste Management in Europe Tolvanen O., Hänninen K Mechanical-Biological Waste treatment and associated occupational hygiene in Finland. Sciences Direct. Waste Management 26 (2006)