Integrated waste management
A concept for sustainable green energy Rising energy prices and increasing worldwide commitment to reducing greenhouse gas emissions and landfill are driving the development of new approaches to the management of solid waste. Marc Van Craen and Karel Van Velthoven investigate More and more countries around the world are limiting or even banning landfill, driving alternative waste solutions towards combinations of maximum recycling and alternative energy generation. The concept of ‘integrated waste management‘ is now emerging as a mature strategy to cope with the ever-growing complexities of handling large volumes of solid waste. Many companies that have been particularly associated with one particular technological approach, such as recycling or waste-to-energy, are now developing a more integrated concept, responding to thedemands of evolving legislation and theneeds of local communities. This can be seen, for example, on the website of DublinCity Council in Ireland, which states that ‘Thermal treatment is used as an integral part of integrated waste management throughout Europe because it is a safe, clean technology superior to landfill and is compatible with high levels of recycling. Countries with highrecycling levels also have high levels of thermaltreatment’. What is an Integrated Waste Management Center (IWMC)? In an integrated waste management the concept of ‘waste’ is replaced by a concept of ‘resource’, combined with a well-organized and controlled waste stream. A modern integrated waste management policy is based on a combination of waste prevention and avoidance, maximized recycling of used goods, waste re-use, sorting and separate waste collection. Such a concept automatically results in minimized landfilling leaving only a final amount of municipal solid waste (MSW) for further treatment. Integrated waste management centres separate the municipal solid waste into very specific remainder fractions, allowing optimal recycling and/or energy recovery of each specific waste stream. The organic fraction of the waste in an integrated waste management centre is sent to an aerobic or anaerobic process for recycling through composting and energy capture via digestion to biogas. The non-organic fraction that cannot be recycled or used for energy production from composting or digestion is considered for heat and/or electricity generation through thermal production processes. This residual waste has an average heating value of about 15,000 kJ/kg and is called refuse-derived fuel (RDF). Other fractions such as inert steel, aluminium and ash residues are recycled from the municipal solid waste or re-used as sand or granulate for a multitude of construction purposes, as (non)-ferrous metals, as industrial salt, gypsum and many more. Dedicated technologies ensure that every last fraction of the waste can be re-used. In this way, in an integrated waste management centre, waste as a resource is not only converted into valuable electricity and heating. It’s a total and sustainable solution turning each waste fraction into the most valuable resource. Exploiting technological synergies The strength of an integrated waste management concept is the combination of several technologies in one installation. This combination of pre-processing, mechanical and organic recycling and power-generating technologies offers a synergy which would be unattainable if each technology was employed individually. This synergy leads to more energy and material recovery and maximum landfill diversion of up to 95%. At the heart of any integrated waste management concept is the pre-processing of the waste received by the waste management centre. Through this pre-processing step, the material most suitable for each specific process is selected and transferred. Therefore, each process is allowed to achieve the highest performance, getting the maximum value out of each specific waste fraction, be it in the form of green energy or recycled materials. Pre-processing for optimal recycling and energy capture The principal objectives and benefits of waste recovery are to maximize recyclable materials, energy and landfill diversion rate. Maximum recovery of materials Up to 30% of the incoming materials can be recycled through anintegrated waste management centre (IWMC). This recycling represents a significant amount of materials to be recycled intorecovery markets, thereby saving primary natural resources andpreventing the emission of large quantities of greenhouse gases(GHG). The types of material that will typically be recovered during the different treatment processes of an IWMC for distribution to recycling markets include: Paper, cardboard and plastics Ferrous and non-ferrous metals, both from pre-processing and bottom ash treatment Compost obtained from the anaerobic digestion process, which when maturated can be used as agricultural or forestry fertilizer Bottom ash from the thermal process which can be used as roadway base as well as aggregate material in concrete manufacturing processes Post-treated ash from thermal conversion which can be used as an aggregate material in the manufacturing of cement bricks and concrete paving blocks. To maximize material recovery, the emphasis of the IWMC design is towards the implementation of recovery processes and utilizing automatic sorting equipment that significantly increases the overall recycling rate, compared to traditional facilities. Energy recovery Specific material streams that cannot be recycled are used by the IWMC as fuel source. This residual fraction of the municipal solid waste is converted into so called refuse-derived fuel (RDF). RDF can be used for heat and/or electricity generation through advanced thermal production processes. To facilitate energy recovery, a fully integrated waste management centre incorporates both thermal and biological treatment. The thermal treatment is the waste-to-energy technology, which processes the RDF to produce high temperature flue gases, whose energy is extracted via steam, which is then used to generate electricity or heat. The biological process anaerobically digests the organic fraction of the municipal solid waste. This anaerobic digestion produces biogas that powers internal engines of the IWMC. ‘Green’ power generation replaces an equivalent amount of electricity which would otherwise have been produced from fossil fuels. And it makes optimal use of the residual municipal solid waste fraction (RDF), which would have otherwise been disposed of at a landfill. For each tonne of municipal solid waste received by the integrated waste management centre, up to 600 kWh of green electricity is generated. Each 1000 tonnes of municipal solid waste has the potential to generate enough electric power to supply a city of well over 30,000 inhabitants. Landfill diversion solution The landfill diversion rate resulting from an integrated waste management centre can be as high as 95%, reducing the residual landfill to only 5% after the integrated cycle of recycling and energy recovery. Additional benefits of IWMCs Integrated waste management centres are developed with a keen understanding of environmental concerns and as a result are linked with an overarching objective to reduce the contribution to greenhouse gases (GHG) by solid waste management. The design of the waste management centre, the choice of building materials and waste processing technologies can all contribute to GHG reduction. Figure 1. GHG emissions in kg CO2/ton MSW Click here to enlarge image Many studies have been done to get a better idea on GHG or equivalent CO2 emissions produced through the disposal of municipal solid waste. These studies compare the GHG produced through various municipal solid waste processing methodologies, as opposed to GHG produced from direct landfilling of the municipal solid waste. [Editor’s note: for more see the article from John Skinner on page 30.] above Qatar overview drawing with different elements numbered and named Click here to enlarge image In general, the conclusion reached is that the total amount of GHG generated as a result of material recycling with anaerobic digestion/composting of the organic fraction, and energy recovery from the residual fraction is considerably less than the amount of GHG generated from landfill disposal of the same tonnage of municipal solid waste — see figure 1. Minimal community impacts In addition to the above, the integrated wastemanagement concept specifically addresses concerns regarding potential odours, disease carriers such as rats and flies, litter derived from delivery vehicles and waste processing, and noise. The design of the concept is such that delivered waste is immediately covered and surrounded by enclosed buildings. As a result, negative visual impacts can be avoided by isolating the waste from the surrounding neighbourhood. The processing systems and buildings can be kept under negative air pressure with all facility process airbeing subjected to biologic filtration aimed at the elimination ofsubstances that produce odours such as mercaptans, amines andamides. Maintaining waste processes within closed buildings also minimizes facility noise to the site surroundings. A closer look at a typical integrated waste management centre Keppel Seghers is an international group and the environmental technology arm of Keppel Corporation Limited. The company is a provider of comprehensive environmental technology solutions and offers solutions for wastewater, process water, drinking water, sludge and solid waste. Its scope of activities ranges from EPC turnkey contractor, R&D, operations and maintenance to active investment schemes such as DBOO, BOT, PPPetc. It aims for all its environmental solutions to be fully integrated systems for maximum sustainability As such, Keppel Seghers’ technologies offer a view to a cleaner future for today’s children and tomorrow’s world. The overview below shows the different elements of a typical integrated waste management centre as offered by Keppel Seghers. Waste reception The waste enters the facility through the waste reception area and is stored in a bunker, sized to allow for adequate storage during peak delivery times. Waste is then transferred via overhead crane to be treated mechanically through several size and density sorting processes. Recycling The aim at this stage is to separate the waste into two principal waste flows: wet (organic) and dry. The wet flow is taken to the organic treatment area. The dry fraction undergoes further sorting processes (manual, optical and magnetic) designed to recover the highest amount of recyclable materials. Organic treatment During organic treatment, the biodegradable fraction of the MSW selected by the pre-processing system, is processed by means of an anaerobic digestion process, which results in the maturation of the dehydrated digestate into a clean marketable compost. The wet organic fraction is pre-processed in rotating horizontal bioreactors where paper and cardboard is transformed into organic fibres. The bioreactor output is classified in a subsequent processing system that separates the volatile biodegradable fraction, recovers some recyclable materials, and transfers theresidue to the thermal treatment section of the IWMC. Thanks to this pre-processing, the material that is sent to the anaerobic digesters is very homogeneous and contains the smallest amount of undesirable inert materials, which could cause flotation/sedimentation problems in the digester. The volatile biodegradable fraction is fed into the digester reactors where it remains for 21 days. Biogas is produced in this module and the digestate is dewatered. Part of the liquid is sent back to the bioreactors and the rest is sent to the water treatment installation for purification. The biogas generated in the digesters is cleaned by removing water and other pollutants, such as H2S. The cleaned biogas is then ready to be used as fuel for the engine generators to generate electricity to make the installation run independently of grid or outside energy sources. In the event of an equipment fault, a stack flare is available to burn biogas temporarily until the equipment can be brought back on line. The dewatered digestate (solid material) from the digesters can now be composted in windrows. Since this material is highly concentrated, it is necessary to mix it with structuring degradable material (woody green waste) and to turn it periodically until organic fraction stabilization and sterilization is achieved. To recover the woody green waste, the compost undergoes a screening process before it is sent back to the windrow composting unit. Finally, this screening process is followed by a density selection process from which the rejects are sent to the thermal conversion unit of the IWMC for energy generation. Advanced thermal treatment The remaining residue of the dry fraction is a combustible material that cannot be recycled. In an IWMC, this fraction is split into two different streams prior to continuing with the advanced thermal process. The waste fraction with the lowest energy content (calorific value) is sent to the advanced thermal recycling (waste-to-energy installation). This represents about 70% of the overall amount of dry fraction sent to the thermal processes. The one with the highest calorific value, the RDF, is sent to the thermal conversion unit for post-treatment. There are many reasons justifying the use of these two very different thermal technologies: The heat produced by both thermal conversion technologies is recovered in their respective boilers, where the thermal energy is transferred from flue gas to water to produce steam. The steam is sent to the turbine-generator to produce electricity Upon exiting their respective boilers, all flue gases from both lines will undergo the same flue gas cleaning process. This process comprises a succession of electrostatic filter, semi-dry absorber, active carbon injection, bag house filter and catalytic elimination of NOx using selective catalytic reduction (SCR). The gas flow is provided by an induced-draught fan that discharges the cleaned gas to the atmosphere through the stack All waste that cannot be recycled is thermally processed. Therefore, all inherent inert materials become a part of the bottom ash which, after being classified and maturated, will be marketed as construction, fill or road base material, thereby reducing, once again, the amount of residue to be disposed of inlandfill. The IWMC will adhere to a strict policy of maintaining the separation of waste streams. Therefore, both the non-marketable bottom ash and the residual fraction of the gas-cleaning by-products will be kept separate and will require disposal at a designated landfill.It should be noted that this integrated waste management concept is aimed at significantly reducing the amount of each of these fractions and as a result that overall landfill diversion is extremely high, up to 95%. Water and air control In order to accomplish an overall control of all air and water emissions, both wastewater and odour-treatment systems are an integrated part of the waste management centre. The wastewater treatment plant receives and treats the entire facility’s contaminated water and returns clean water to the various modules that require process water. As a result of the waste water treatment process, a small amount of left-over solids will be generated and require landfill disposal. It should be pointed out that an IWMC never discharges any wastewater into the surrounding sanitary sewer system or surface waters. Above Inking the 20-year contract for the 1st Integrated Waste Management Centre in the Middle East (Qatar) Click here to enlarge image To minimize odours, potentially contaminated air will be collected from the different process buildings and transferred to the odour treatment system. This system comprises an initial acid and moisturizing pre-treatment and a final biological purification in the bio-filters. This process results in highly-effective (> 99%) odour suppression. Conclusion Keppel Seghers is currently constructing the first IWMC in the MiddleEast, in Qatar. It is one of the few IWMCs in the world and whencompleted in 2009, 1550 tonnes of waste per day will be recycled, composted and turned into energy, resulting in 180,000 MWh/a electricity. An integrated waste management centre as presented above is by far the most sustainable solution when it comes to combining waste treatment and alternative, green energy production. Growing volumes of waste, soaring energy prices and the ever stronger global commitment to counter global warming make the integrated waste management concept a leading and total solution for a cleaner and less fossil-fuel-dependent future. Marc Van Craen, PhD Dr.Sc. is Business Manager, Waste-to-Energy at Keppel Seghers Belgium NVKarel Van Velthoven is Marketing Manager at Keppel Seghers Belgium NV