Anaerobic Digestion Technology Feature: The ArrowBio Process
Introduction
The ArrowBio process is a Mechanical Biological Treatment (MBT) system that uses water as a medium for the gravitational separation of inorganic and organic fractions of Municipal Solid Waste (i.e. ‘black bag’ waste). It recovers recyclates and produces renewable energy while also helping to reduce carbon emissions.
The ArrowBio process has been developed over the last 15 years through a series of pilot and demonstration plants to a level of significant maturity. There is a full scale commercial plant in operation in Tel Aviv, Israel processing 70 000 tonnes MSW per annum. A second plant is under construction in Sydney, Australia and will process 100 000 tonnes MSW per annum.
The ArrowBio process is a flexible one and can be integrated with other synergistic technologies. For example, in the ‘Sustainable Energy Park’ ArrowBio is combined with the ThermoFuel process, the latter converting waste plastics into synthetic diesel. ArrowBio provides energy and a clean waste plastic stream to ThermoFuel which in turn uses the energy to convert the waste plastics into synthetic diesel.
The Process
The process consists of two distinct yet integrated components: a “front-end” wet Materials Recovery Facility (MRF) and a “back-end” advanced anaerobic digestion system.
The wet MRF combines standard solid waste handling technology in combination with liquid and slurry pumping. Water based processing facilitates easier conveyance of material around the facility while simultaneously suppressing dust and odour.
Within the wet MRF recyclable material is cleaned, recovered and further separated by type (ferrous metals, non-ferrous metals and plastics) and can be sold directly to the recycling market.
As the ArrowBio process recovers the recyclable elements of the waste stream, it should be favoured by environmental pressure groups who tend to object to alternative processes such as incineration on the basis that incineration reduces the necessity to separate and recycle.
Two Stage Anaerobic Digestor
The process has at its core an Upflow Anaerobic Sludge Blanket (UASB) Digestion system which optimises the bio gas yield. Extracting the maximum possible gas yield is a primary aim of the ArrowBio process. In addition to being good environmental policy, the gas/electricity produced is approximately five times the parastic load of the plant. This surplus energy can be sold to external parties, the revenues from which contributing to the upkeep of the plant.
ArrowBio’s anaerobic digestion is carried out in two stages:
- Acetogenic – microbial communities reduce complex organic material into simpler molecules
- Methanogenic – microbial communities convert the simpler molecules into biogas, water and digestate
In this instance and as compared to single tank AD, the biomass achieves maximum efficiency due to the optimisation of microbial action as affected by pH levels, temperature and residence times. It also follows that this optimisation ensures that the maximum possible energy in the waste is converted into renewable energy.
The ArrowBio process also includes a “hydrocrusher” which macerates the biodegradable waste with high pressure water jets. Food and paper are torn apart, greatly increasing the surface area available to biological action while simultaneously suspending the organic chemicals in a watery solution. The supplier suggests that the resulting solution is able to produce higher gas yields than would have normally been possible from a thicker sludge.
Products
In addition to the recyclable material, the ‘front end’ also produces residues that are sent to Landfill. Residues is that fraction of the waste stream that is not recoverable (e.g. grit and cullet) and typically comprises < 10% of the input waste.
In addition to biogas, the ‘back end’ of the system also produces water and digestate. Some of the water is re circulated back into the system with the remainder available for alternative uses (e.g. drinking water, irrigation, discharge to sewer etc.). As for the digestate, it can be used as a soil improver, a landfill cap or as aggregate material in construction.
Due to the ArrowBio design (i.e. lack of high pressures and temperatures coupled with the use of water) the process does not produce any harmful air, noise, odour, ground or water pollution.
E-mail: mail@oaktech-environmental.com
Web: www.oaktech-environmental.com
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