Anaerobic Digestion and Opportunities for International Technology Transfer

by P Lusk, Anaerobic Digestion Activity Leader within the IEA Bioenergy Programme

Introduction

Around the world, pollution of the air and water from municipal, industrial and agricultural operations continues to grow. Governments and industries are constantly on the lookout for technologies that will allow for more efficient and cost-effective waste treatment. One technology that can successfully treat the organic fraction of wastes is anaerobic digestion (AD). When used in a fully-engineered system, AD not only provides pollution prevention, but also allows for sustainable energy, compost and nutrient recovery. Thus, AD can convert a disposal problem into a profit centre. As the technology continues to mature, AD is becoming a key method for both waste reduction and recovery of a renewable fuel and other valuable co-products.

By spreading solid residues from the anaerobic digestion process, nutrients are returned to the soil.

Technology Application

Worldwide, there are more than 115 AD plants operating or under construction, using municipal solid waste or organic industrial waste as their principal feedstock. The total annual capacity is almost five million tonnes. Another 40 AD plants are in the planning phase with an annual capacity of nearly two million tonnes. More than 50 prime technology licence holders have a proven system operating at the pilot- or full-scale level (at least 150 tonnes per year). Operators have found that AD provides environmental benefits allowing their waste disposal facilities to meet increasingly stringent regulations.

 The use of AD for treating industrial wastewaters has grown tremendously during the past decade to the point where there are now more than 1,000 vendor-supplied systems in operation or under construction throughout the world. Over 30 types of industries have been identified with having wastewaters suitable for AD treatment, including processors of beverages, chemicals, food, meat, milk, pulp and paper, and pharmaceuticals. Many of these industries use AD as a pretreatment step to lower sludge disposal costs, control odours, and to reduce final treatment costs at a municipal waste water treatment facility.

Today, farm-based manure facilities are perhaps the most common use of AD technology. Six to eight million family-sized, low-technology digesters are used in the Far East with varying degrees of success. There are also over 600 farm-based digesters operating in Europe and North America. Farmers often cite odour control, pathogen control, and containment of nutrient run-off as the drivers influencing their investment decision.

Energy from AD

Biogas can be used in all applications designed for natural gas. Today, biogas is commonly burned in an internal combustion engine to generate electricity. When producing electricity, there is the added potential for heating water from the engine's cooling and exhaust systems. Biogas is also burned in boilers to produce hot water and steam. A promising near-term application for electricity generation is the use of gas turbines. In the future, fuel cells and the Stirling engine may be able to use biogas to produce electricity cost-effectively and recove r process heat.

 Biogas is successfully compressed for use as an alternative transport fuel. Carbon dioxide, hydrogen sulphide and water are separated from the biogas to obtain usable methane. After this upgrading procedure, the fueling technique is the same as that used for compressed natural gas (CNG) vehicles. Worldwide, around one million vehicles are now using CNG as fuel. Because CNG burns very cleanly, many fleet operators have reported savings of 40­50% in vehicle maintenance costs.

Environment and AD

Using residues from the AD process can also have very beneficial environmental impacts. The amount, quality and nature of these products will depend on the quality of the feedstock, the method of digestion and the extent of the post-treatment refining processes.

 The main product of all AD processes is a solid digestate which can be matured into a compost product and used as a soil improver or growing media. Small quantities of surplus liquor are also available. These liquors can be spread directly onto farmland or de-watered to provide separate liquid fertiliser and solid products.

AD mitigates a number of other environmental concerns. Emissions of volatile solids and volatile fatty acids are directly related to odour strength. With urban encroachment into rural areas, an increasing number of landfills use a digester for the purpose of odour control. The AD process also destroys pathogens.

Economics

Like most renewable energy options, AD's economic merit relies on a variety of factors. Discussion of the economics of AD is complex due the wide range of parameters that affect the costs and the number of accrued 'external' benefits. In addition, each country has different circumstances, infrastructure and fiscal arrangements that affect the relative and absolute costs of various waste management options. Even within a single country these costs vary considerably.

It should also be noted that reference prices generally reflect a 'turn-key' AD facility built in Europe, where investments in components such as plant machinery, land, and infrastructure are significantly higher than for other locations. This is especially true for Asia and South America.

Recent trends in treatment costs for the anaerobic digestion of municipal solid waste and source-separated wastes.

International Technology Transfer

 Beyond technology nuts-and-bolts, there is an effort conducted by the International Energy Agency (IEA) Bioenergy AD Activity to provide reliable information on AD cost-effectiveness, markets for biogas and the other co-products, advanced technologies for biogas utilisation, environmental benefits, and institutional barriers.

 The Activity's principal objectives are to accelerate exchange of information and practical experience, identify barriers to the deployment of AD technology, encourage the use of AD technology and, where relevant, assist and encourage national pilot and demonstration programmes. The goal is to increase the deployment of AD technologies and to transfer the 'lessons learned' from past experience.

Summary

AD systems are now widely used throughout the world. The factor most strongly influencing the economic merit of an AD facility is maximising the sales of all usable co-products. Advanced technology end-use applications can increase the economic value of biogas, but only after sufficient production scale has been achieved to reduce significantly the unit cost of ownership.

The u se of more sophisticated AD processes for industrial waste treatment will increase. AD can decompose some organic toxic and hazardous materials in co-digestion schemes and this potential will be realised.

For the future, the driving forces for the use of AD will probably drift away from energy production. Organic stabilisation, pathogen reduction, and the production of a high-quality soil improver will be important reasons to use AD in developing countries. Operational savings and minimal sludge production from AD versus aerobic treatment will become more important in energy and landfill deficient areas.

For more information contact Philip D Lusk, Resource Development Associates, 240 Ninth Street NE, Washington, DC 20002-6110 USA.

Tel: +1 202 546 6283; Fax: +1 202 546 3518; e-mail: plusk@pipeline.com

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