by Siu Cheng
The growing amount of municipal solid waste (MSW) and its disposal in landfills is currently one of our biggest ecological problems. Unmanaged landfills can pollute ground water, place pressure on land supplies and emit vast quantities of methane to the atmosphere, contributing to global climate change.
In the UK, landfills account for 41% of the country’s methane emissions,1 which is caused by the degradation of buried biodegradable organic waste (biowaste). Despite the associated environmental problems, landfill remains the most popular waste management solution across the world. In recognition of environmental problems they cause, and their unsustainable nature, the past few decades have seen an increase in pre-treatment processes/technologies to reduce the amount of biowaste sent to landfill such as incinerators, biological treatments, resource recovery and recycling.
Figure 1. ArrowBio process schematic Click here to enlarge image
Despite increased restrictions, legislative pressures and efforts to implement ‘pre-treatment’ or ‘alternative technologies’, landfill continues to be the dominant waste management option in many countries. With European countries facing increasing EU legislative pressure to reduce biodegradable MSW sent to landfill, it is important these countries implement suitable and effective waste treatment infrastructure.
The problem with biowaste
One of the difficulties of treating MSW is the ‘biowaste’ fraction from food and paper waste etc. It is the moisture content or ‘wetness’ of the biowaste which poses a challenge to many pre-treatment options wet biowaste sticks to recovered recyclates diminishing their resale value, or contaminating the load. The moisture content of biowaste also negatively impacts the combustion process of mass burn technologies. Some technologies therefore require some source separation of the biowaste, or the implementation of energy-intensive drying processes to reduce the moisture content before the waste can be processed. In some countries biowaste can account for up to 50% of MSW, so making a system which can effectively handle both recyclable and organic waste fractions essential to any waste management infrastructure.
ArrowBio plant, Sydney, Australia developed by ArrowBio Ecology Click here to enlarge image
Traditionally, biowaste has been aerobically composted, but in recent years anaerobic digestion (AD) has become an increasingly popular treatment option. AD has the benefit of being able to capture the methane produced during the decomposition process, as well as being able to encase odour and dust emissions.
Material reclamation facilities (MRFs) are widely used in many countries to recover recyclates from mixed waste. However, these are predominantly dry facilities which experience problems when the wet and sticky biowaste contaminates the recovered recyclates. Therefore, the amalgamation of AD and materials recovery into one single waste treatment facility such as a mechanical biological treatment (MBT) facility is an obvious single-site solution for the treatment of municipal waste.
Integration is essential
Using the simple principle that different materials will either float or sink in water, Israel-based Arrow Ecology has developed a water-based treatment system which eliminates the need for pre-sorting; working with the wetness of biowaste rather than against it. ArrowBio can sort a mixed waste input into different recyclate fractions for resale and recycling, and produce a renewable biogas from the organic material. The consistent use of water throughout the process serves several purposes. These include: the transportation of materials around the system, cleansing the recovered recyclates and ensuring maximum recovery and retention of organic material. Therefore, the use of water is key to the successful integration of the front-end MRF with the back-end AD system, resulting in a synergistic and harmonious operation within both systems. This successful amalgamation of the water-based MRF with AD is where ArrowBio’s strengths lie; providing one of the highest materials recovery rates and diversion rates of biodegradable MSW from landfill, as well as the greatest biogas yields compared to other MBT technologies around.2
The ArrowBio system has two distinct technology elements. The first is the water-based MRF or ‘hydromechanical sorting unit’, and the second is the AD technology. When waste enters the facility, it is received on the reception hall floor where it undergoes a dry pre-sort stage to isolate any oversized items e.g. mattresses, fencing etc. The remaining waste then goes on to the hydro-mechanical separation unit.
After passing through a series of trommels to open up any sealed bags, the waste then goes into a water-filled vat. Here, in the hydro-mechanical separation stage inorganic materials, such as metals and glass, have a specific weight/density which is greater than water and will sink, while plastics and biowaste will float closer to the surface. The use of water as a separation medium also allows for the removal of potentially hazardous materials, such as batteries and electronics, with minimal risk of breaking or crushing. This significantly reduces the risk of chemical contamination of the MSW.
The heavy materials which sink to the bottom are removed via underwater conveyor belts. They are further separated by magnets, eddy current separators and trommels into ferrous metals, non-ferrous metals, and inert materials. The lighter materials, consisting mainly of plastics, will float closer to the surface of the water and are skimmed off. After going through a further series of trommels and screens, plastics are sorted into individual fractions via air classification and manual or optical separation. However, plastics can be kept in a mixed format if appropriate.
Biodegradable material such as food and paper waste will remain in the water, become saturated and begin to break down. After going through an intense maceration phase to liquefy the biodegradable elements into the water, the organic solution enters a filtering system. Here, any residual contaminants such as sand, grit, and glass are screened out using a settling vat and cyclone. The resulting rich organic solution is pumped into the second stage of the process: biological AD reactors.
ArrowBio uses high efficiency Upflow Anaerobic Sludge Blanket (UASB) AD technology. UASB is widely used in the waste water industry and the use of this technology within ArrowBio allows for the efficient and maximum breakdown of the organic material, with the additional benefit of generating high quality biogas, typically with a methane content of 70%-80%. Upon entering the biological reactors the organic fluid undergoes two processes, both of which are orchestrated by naturally occurring microbial communities. In the first bioreactor tank, acidogenic fermentation transforms complex organic material into simpler organic acids and fatty acids. This acid-rich organic matter is then heated up to 36°C-40°C and transported to the methanogenic fermentation reactor for anaerobic degradation and generation of methane-rich biogas. The biogas can be used for generating renewable electricity at up to five times the power requirement of the plant, and for heating the AD tanks. It can also be upgraded in quality for use as a transport fuel or a natural gas substitute. The output water is cleaned, with a portion being recirculated for reuse in the hydromechanical sorting unit. The digestate material has potential for use as landfill cover or as brownfield remediation material after undergoing a pasteurization phase to fulfil EU animal by-product regulation requirements.
ArrowBio’s hydro-mechanical materials recovery facility uses a simple and effective segregation process for the maximum recovery of recyclable materials. The unique use of a liquid medium is able to reduce noise and suppress odour and dust emissions. The system can divert a significant amount of waste from landfill with minimal environmental impact, while simultaneously helping to contribute to local authority recycling efforts. It also eliminates the need for waste producers to carry out source segregation. ArrowBio can be considered a self-regulating and environmentally benign treatment option; able to generate its own power, heat and water requirement, whilst generating income through the sale of surplus renewable electricity and recovered recyclable material.
The ArrowBio facility at Jack’s Gully, Sydney (pictured on page 83) is part of WSN Environmental’s Macarthur Resource Recovery Park. Construction of the ArrowBio plant was completed in July 2008 and it is in the final stages of commissioning. The ArrowBio facility is designed to process 272 tonnes/day (300 US tons/day) of mixed waste.
Arrow Ecology is also pleased to announce that a second ArrowBio facility for Australia is currently under development at another site in Sydney, with development pending additional commitment of waste to the project.
Siu Cheng is the Business Development Assistant at Oaktech Environmental.
With thanks and acknowledgement to Ori Boulogne, Arrow Ecology & Over Sees Engineering (1999) Ltd.
1. DEFRA, 2007, e-Digest of Statistics. Available from: www.defra.gov.uk/environment/statistics/globatmos/gagccukem.htm [accessed 15th July 2009]
2. Juniper Consultancy Services, 2005, MBT: A guide for decision makers- processes, policies and markets’ report. Available from: www.juniper.co.uk/Publications/downloads.html
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