Linking MBT with AD for greater flexibility

Developments in biogas utilisation mean that the combination of MBT with anaerobic digestion offers a flexible and efficient solution when treating residual waste. Alex Marshall explains

Mechanical biological treatment coupled with anaerobic digestion (MBT-AD) is a flexible solution for the treatment of mixed municipal and commercial waste streams. MBT-AD has been widely applied in continental Europe with facilities in Germany, Spain and Malta and now there are plans for similar facilities in the UK.

The process typically consists of three stages - mechanical pre-treatment, wet pre-treatment and wet anaerobic digestion. The mechanical pre-treatment removes elements such as metals to be sent for recycling and refuse-derived fuels (RDF) for energy recovery. It also removes non-useful or problematic items for landfill.

The wet pre-treatment takes the fines from the mechanical stage, pulps the organic material into a pumpable organic solution and goes on to remove fine contaminants such as sand, grit and floating material such as plastics. Finally, wet anaerobic digestion facilitates the production of biogas, a renewable fuel, from the organic matter and stabilised digestate. The digestate can be used in land remediation, for thermal recovery as a biomass-rich RDF, or alternatively landfilled as a stable form of carbon.

A wide range of bolt-on technologies can be added to the core MBT-AD plant. Biogas can be utilised within combined heat and power (CHP) gas engines to provide renewable energy, both in the form of electricity and heat, which can be used within the plant or exported to other users. Alternatively, the biogas can be processed utilising biogas-upgrading technology to concentrate and clean the gas to biomethane standard. Biomethane can be input directly into the gas grid or alternatively utilised in transportation.

The use of wastewater
Complimentary wastewater treatment technologies can include reverse osmosis with ultra-filtration (RO/UF) or sequencing batch reactors (SBR). SBR units are widely employed for landfill leachate treatment, they have a relatively high initial capital expenditure but the microbes within them can be notoriously difficult to control.

RO/UF is a high-efficiency technology that is able to provide clean process water suitable for reuse within the plant. It has a lower capital cost than SBRs, but greater operational requirements. The system also produces a concentrate that must be disposed of suitably.

For exhaust air treatment there are three key add-on technologies - biofilters, biofilter towers and regenerative thermal oxidisers (RTOs). Biofilters are a simple low-cost solution where wood chippings are used as a medium for odour-neutralising bacteria. These systems have a relatively high footprint.

Biofilter towers originate from the chemicals industry and are essentially an engineered low footprint biofilter where the operator has a greater degree of process control. Finally, there is the RTO - this system has the greatest efficiency of the three solutions at odour-neutralisation, it also has higher energy requirements for operation and is a more expensive overall solution.

Influencing factors
The technology employed for the drying of digestate will depend upon the end use of the material. Typically centrifuges and drum-driers can be employed to reduce the level of dry solids to 50% utilising the waste heat from the gas engines. With the addition of further energy, dry solids can be reduced further to ~80%. Alternative dewatering technologies include screw presses and belt driers.

Currently there are four operating plants utilising Haase MBT-AD technology in Europe. In Germany there are facilities located near Luebeck, Schwarze Elster and Goettingen. The Luebeck and Schwarze Elster facilities began construction in 2005. The Goettingen facility has been fully operational since mid-2008. The plant at Leon, Spain began construction in 2002. 2009 will see the start of operations at the plant in Malta and will be the integral treatment technology for the island's residual wastes.

MBT for Manchester
Haase is now working with Clarke Energy to develop three MBT-AD plants with Viridor Laing and Costain for the Greater Manchester Waste PFI project. The facilities incorporate high-efficiency GE Jenbacher CHP engines for biogas that can achieve 41% electrical efficiency. These engines will provide electricity in excess of the plant's requirements that will be sent to the National Grid.

The heat that is recovered from the engines will be utilised to heat the process and to dry the digestate. Drying the digestate is beneficial in order to reduce transport costs and associated carbon emissions while increasing the calorific value of the material. RDF will be sent to a major CHP energy facility at the Ineos Chlor chemical works in Runcorn.

The Manchester MBT-AD facilities also incorporate reverse osmosis and ultra-filtration units. As Manchester is an urban area, it is particularly more sensitive to odour problems associated with the processing of waste. The planned MBT-AD facilities incorporate the highest standards in air treatment utilising regenerative thermal oxidisers for the processing of exhaust air from facility areas.

Clarke-Haase is also working with Biffa to develop the residual waste treatment technology for the West Sussex Material Resource Manage-ment Contract. This will be the largest contract ever awarded by the council. Here a 300,000tpa MBT-AD facility is planned adjacent to the existing Brookhurst Wood landfill site.

This facility will treat not only the residual household waste for the whole of West Sussex but, will process household waste recycling centre waste from the region. The RDF produced by the facility will be either used in cement kilns or used as a substitute for fossil fuel.

The facility will initially incorporate GE Jenbacher gas engines capable of achieving 42% electrical efficiency from the biogas that will be produced. These engines will also use waste heat for process heating and digestate drying. In future years the council is also looking to develop biogas-upgrading at the facility.

Flexible solution
Here, excess biogas will be further processed utilising organic liquids to remove the carbon dioxide from the gas, whilst concentrating the methane to natural gas standards. This biomethane will be utilised either as a transport fuel for waste collection vehicles or alternatively sent to the grid for utilisation elsewhere.

In summary, MBT-AD offers a flexible approach to the treatment of residual wastes. The system is adaptable and can form the core technology for the treatment of a region's waste streams. The developments in biogas utilisation and CHP mean that the process will continue to be adapted as new complimentary technologies are available on the market.

Alex Marshall is product manager at Clarke Energy

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