Biogas from organic household waste
by the CADDET Danish National Team

A new waste plan announced by the Danish government aims to improve waste management and optimise the exploitation of the energy resource in the waste. Source separation is central to this plan, and organic household waste is to be collected separately for treatment in biogas plants. The goal is to treat 100,000 tonnes/year of this waste by 2004.

The Grindsted biogas plant  Photograph: Grindsted Kommune

Biogas and fertiliser

Anaerobic digestion is the preferred treatment of organic household waste in Denmark. In an optimal operating plant, about 80% of the waste is transformed into energy and fertiliser. The biogas potential from organic household waste is 120–170 Nm3/tonne. The rate of digestion depends on the bacteria population and their growth conditions, and it can be increased by processing at thermophilic temperatures (50–60°C). With regards to pathogens, household waste is considered a higher risk than manure and most organic industrial wastes. Efficient reduction of these pathogens can be achieved by thermophilic digestion for a certain period or by separate hygienisation at 70°C for at least one hour. It is also important to obtain a clean fertiliser from which non-degradable material, such as glass, plastic and metal, has been removed. The content of heavy metals must be below the permitted limit and pathogenic micro-organisms must be removed.

Technological challenge

Special technologies have had to be developed to improve the economic competitiveness of using household waste in biogas plants. This is because handling household waste is technically more complicated than handling manure and organic industrial wastes. Experience has shown that it is the plastic content and other inert material which cause the problems. Before digestion, special pre-treatment of the waste is required to remove plastics. However, even after thorough pre-treatment there are remnants of plastic and other items at the outlet of the digester which must be removed in order to produce a clean fertiliser.

De-waster at the Vaarst-Fjellerad biogas plant  Photograph: PlanEnergi Nordjylland

Biogas plants in operation

Currently, there are four biogas plants in Denmark treating source-separated municipal solid waste. At the Studsgaard and Sinding biogas plants in Herning, a total of 11,000 tonnes/year of organic household waste are co-digested with manure and organic industrial wastes. Digestion of organic household waste started in a small-scale pilot project retrofitted to an existing biogas plant in Sinding. The project was extended to a full-scale ‘second generation’ plant in 1994 and, in 1996, the Studsgaard plant became operational. The waste is collected in plastic bags, which are then pre-processed in a closed-off, fully automatic plant. The bags are torn open, emptied and sorted for plastic. After digestion, a separator removes any remaining plastic and other particles.

At the biogas plant in Grindsted, 1,200 tonnes/year of organic household waste are co-digested with wastewater treatment sludge and organic industrial wastes. Here, the household waste is collected in paper bags that can be processed along with the waste to avoid the problems of removing plastic. The quality of source separation is high – only 1% of organic household waste is rejected. The biogas plant pays about DKK 0.50/paper bag (where DKK is the Danish krone), which is more than the cost of a plastic bag. However, this extra expense is expected to be compensated for by the advantages of fewer operational stops, extra biogas production and a clean fertiliser.

In Aarhus, the second largest city in Denmark, source-separated household waste is being collected on a test basis. However, the municipality has decided to collect this waste from all households from the year 2000 onwards. The biogas plant, which at the moment can treat 4,000 tonnes/year, will be expanded to receive 17,000 tonnes/year by January 2001. This ‘green waste’ will be collected in green-coloured plastic bags and other waste in black plastic bags. The bags will be optically identified and separated in a central receiving plant. This method enables existing waste containers to be used. Thus, waste collection costs will not increase but some of the biomass will be lost because it will stick to the plastic. The use of plastic bags is necessary to withstand the collection process and to allow the optical identification.

At the Vaarst-Fjellerad biogas plant, a separate line for organic household waste is currently processing about 365 tonnes/year, but has a capacity of about 4,000 tonnes/year. The waste is collected in plastic bags and transported to a receiving tank at the biogas plant. The plastic is then removed by a de-waster, on a test basis, before the waste is led into the digestion tank. The de-waster has been developed from a machine normally used for de-boning fish and chicken. The waste is forced, under high pressure, through a sieve, which retains plastic and other inert material but allows the biodegradable fraction to pass through. To date, the results have been promising but long-term reliability still has to be demonstrated. Slurry and various organic industrial wastes are also treated at the plant.

Danish municipal solid waste biogas projects

Biogas plant

Method of collection

Organic household waste consumption (tonnes/year)

Total organic waste consumption (tonnes/year)

Co-digestion of organic household waste with:

Herning (two plants)

Plastic bags

11,000

181,000

manure
industrial organic waste

Grindsted

Paper bags

1,200
4,000 (capacity)

32,500

wastewater sludge
industrial organic waste

Vaarst-Fjellerad

Plastic bags

365 (running-in period)
4,000 (capacity)

55,000

manure
industrial organic waste

Aarhus

Plastic bags

4,000 (capacity)
17,000 (by 2001)

135,000

manure
industrial organic waste

Plastic removal

Although it appears that collection in paper bags causes fewer operational problems and results in lower treatment costs, these savings are partly neutralised by the extra cost of buying the bags. However, municipalities using paper bags have found that their residents have a better understanding of the source-separation system. In Grindsted, only 1% of the waste is missorted, while this figure can rise to 33% in other municipalities.

Removal of plastic from the waste is still the biggest challenge and the pre-treatment process requires further development. The results of the de-waster tests in Vaarst-Fjellerad are eagerly awaited. If it is successful, this technology could replace the expensive pre-treatment process in the biogas plants processing household waste collected in plastic bags.

Conclusion

The Danish experience of biogasifying source-separated organic household waste co-digested with other organic wastes shows that, with the proper technology, it is possible to operate biogas plants profitably. However, the thoroughness of the source separation is crucial. With the proper technology, it should be possible to increase the amount of waste processed within the next few years to about 20,000–25,000 tonnes/year, making it possible to fulfil the goal of 100,000 tonnes/year by 2004.

For more information contact the CADDET Danish National Team in Tollose.

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

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