Kombineret bioforgasning og kompostering af kildesorteret organisk dagrenovation i batch-anlæg

Summary and conclusions

This report presents results from the development, establishment and evaluation of a full-scale plant for anaerobic digestion and composting of solid biodegradable waste, including source-separated biodegradable municipal solid waste (biowaste). The project was carried out by Solum A/S and Aalborg University, Department of Life Science, with support from the Danish Environmental Protection Agency.

The biological process used in the full-scale evaluation was based on a two-phase digestion process, followed by composting. The principle behind the digestion process is to separate the procedure into two phases: a hydrolysation process and a methane-producing process. Hydrolysation was carried out in concrete silos, and the methane was produced in a hermetically-sealed reactor tank, which doubled as a gas storage tank.

To compile mass balances for energy and plant nutrients, samples were taken of the process's raw materials (biowaste and structure material in the form of shredded branches), as well as composted biomass and products from the process (compost, plastic fractions and structure material). In addition, the process's biogas production was evaluated at laboratory, pilot and full-scale levels.

At the full-scale level, the project showed that it is possible to undertake combined anaerobic digestion and composting of biowaste without screening out impurities prior to the biological process. This means that pre-treatment costs are reduced, and the production of waste with a high content of biodegradable matter can be avoided. The products of the biological process are energy, in the form of biogas, and compost.

The laboratory, pilot and full-scale evaluations of gas yield resulted in 70-80 Nm³ of methane per tonne of biowaste with structure material (mix ratio: 0.5 tonne shredded branches to 1 tonne biowaste). For the full-scale evaluation, the process resulted in a net energy yield of 4241 MJ per tonne of treated biowaste with structure material. Additionally, 341 kg compost and 198 kg structure material were produced. The structure material can be reused in the process. Of the 6.1 kg nitrogen and 0.93 kg phosphorous that were added during the process, most was found present in the compost and structure material (81% and 90% respectively), and this could be reintroduced into the biological cycle.

The energy yield from the process can be increased substantially if production of compost is given low priority. Using theoretic scenarios, it has been shown that the absolute highest energy yield can be achieved by producing biogas, then drying the un-screened digestate in order to prepare it for incineration in a waste incineration plant. Potentially, this method can produce an energy yield of 8850 MJ for 1 tonne of biowaste and 0.5 tonne of structure material. However, the advantages of producing compost for agricultural purposes instead of incineration fractions are obvious: no slag or flue gas residues, production of organic fertilizer and organic matter for agriculture, preservation of natural resources (mining phosphorous ores, peat wetland), as well as a consciousness of resource use and recycling.

The process proved itself to be very robust at the full-scale evaluation level. In the project period (1 February 2003 to 31 December 2004) 14,000 tonnes of biowaste were treated at the plant.

The results from a working-environment (human health and safety) evaluation at the full-scale facility are also presented in the report. Additionally, emissions from the plant's biofilter were measured during the composting process. The result of this showed an efficient biofilter that gave rise to an impact of approximately 5 odour-units in a radius of up to 140 m from the filter.

The estimate for the cost of the plant, as well as running costs and administration expenses associated with the treatment solution, are also presented in the report. The calculations of the estimated cost show that a plant designed for 25,000 tonnes of biowaste yearly requires an investment of DKK 35 million (2004). Running costs are estimated at DKK 145–210 per tonne of biowaste and administration expenses are estimated at DKK 80–150 per tonne of biowaste. It is estimated that a plant projected to treat 25,000 tonnes of biowaste per year can be run at an all-inclusive operating cost of DKK 500 per tonne of biowaste when run at full capacity.

 



Version 1.0 Maj 2005, © Miljøstyrelsen.