The power of organic wastes
by E de Spiegeleer, CADDET Belgian National Team

A new anaerobic treatment plant in Belgium could prove the technological and financial feasibility of anaerobic fermentation for the whole range of organic wastes, whether liquid or solid, pure or mixed with inorganics, delivered in bags or not.

Belgium's newest biogas plant.

Introduction

Belgium’s newest biogas plant, now one of the largest in Europe, has been constructed in the old dock area of Ghent and came on-line in May this year. Owned by MAV NV and completely financed by the private sector, the plant is equipped to handle and treat:

  • sewage sludge;
  • organic waste from industrial production (food industries);
  • pre-sorted biowaste, fruit and vegetables;
  • fat sludge and abattoir residues.

The plant has a capacity of 200,000 tonnes/year which is supplied under contract by industry and farms in the area surrounding Ghent. The anaerobic treatment process was developed by the German-based company GBU mbH. Total fermentation capacity is 12,000 m3, divided between four digesters. The installation of multiple, smaller digesters allows safe and reliable operation and offers flexibility regarding the treated substrates.

The process

Anaerobic fermentation requires a liquid medium with a dry matter content of 8–12%. Because biowastes have a dry matter content of 20–25%, they have to be suspended in water prior to fermentation. This is done in a special suspension unit where the pre-crushed organic material is mixed, heated to about 70°C, suspended and separated from inorganic and cellulose particles. The homogeneous suspension of water and organic material is then pumped to a buffer tank, where it can be mixed with other liquid wastes if desired, and fed to the digesters at a constant rate.

The digesters are single-stage fermenters, where the phases of the fermentation process are performed in the same reaction chamber. However, the digesters also contain a post-fermentation chamber. The use of two separate chambers prevents freshly-fed substrate from mixing with substrate already fermented, which can cause effluent odour problems. Moreover, a pressure increase in the main chamber and a sudden expansion of the biogas into the second fermenting chamber has the advantage of allowing the blending of the substrate when surface scum and sediment layers have built up. This is achieved without any moving parts or additional energy. The biogas produced (30,000–40,000 m3/day) is collected in a non-pressurised gas holder consisting of a standard silo with a polyethylene foil bag. The gas is finally burnt in a CHP unit consisting of a gas engine rated at 2.8 MWe and a synchronous generator. A gas flare has also been installed for safety reasons.

Using biogas, the electrical output of the gas engine is 2.2 MWe and the electrical efficiency reaches 38.5%. The overall efficiency is about 87%, ie 1 m3 of biogas (65–75% methane) with an average energy content of 25.2 MJ, will be converted into 2.7 kWh of electricity and 12.2 MJ of thermal energy. The recovered heat is used for heating the digesters to about 35°C and for steam production (about 1.5 tonnes/day). Surplus heat can also be used in an absorption chiller for cooling. The generator works automatically according to the gas level in the gas holder, and the electricity produced is fed into the public grid. Further treatment of the fermented substrate depends on the market for the final products.

After the liquid fraction of the waste has been decanted into the digesters, the solid fraction of the effluent can be composted. After digestion, the liquid fraction is acidified in a buffer tank and then concentrated to a dry matter content of 30% in a fluidised-bed evaporator, which uses hot steam for heating. The integration of an evaporation process has the advantage that waste water is not discharged into the environment. Water recovered by condensation is used as process water.

The resulting concentrate can be either composted with the solid fraction, or dried and granulated. In the latter case, the final product is a dry granulate (>90% dry matter) which can be stored, handled and applied like a mineral fertiliser. However, this process has a high energy requirement, thus limiting the energy that can be sold.

The anaerobic treatment system

Economics

The total cost of the biogas plant was e21 million (where e is the euro). The average processing price of the organic wastes is e24.8/tonne, on the basis of a 10% dry matter content. The company expects to produce 10 million m3/year of biogas with an equivalent primary energy potential of 250,000 GJ. The CHP unit will convert this energy into about 26,000 MWhe/year, of which 50% can be sold to the public grid at an average price of e0.0595/kWh. This price includes a premium of e0.0248/kWh as it applies to a (green) renewable energy source. Annual heat recovery will be about 120,000 GJ. The process will also produce about 15,000 tonnes/year of high-quality organic fertiliser.

Conclusion

The MAV anaerobic treatment plant offers an economically credible and environmentally safe method of organic waste treatment. Having no expenditure on wastewater treatment or additional energy supply, and with the marketing of an important quantity of electricity and a high quality fertilizer, the costs of this plant are much less than the common alternatives such as landfilling or incineration.

For more information contact Herman Herpelinck or Catherine Meyvaert, MAV, Hulsdonk 27, B-9042 Desteldonk (Ghent), Belgium.Tel: +32 9 3423160; Fax: +32 9 3423161; e-mail: meyvaert.mav@gbunet.de

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

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