Management of source-separated organic household waste intended for anaerobic digestion

Driven by the Waste Management Directive and the Renewable Energy Directive, the biological treatment of organic household waste, such as food waste from kitchens, now needs to be undertaken by European Union member countries. Anaerobic digestion (AD), which allows for the utilisation of both energy (biogas production) and nutrients (through the agricultural use of digestion residue) is commonly suggested as the best way forward in this regard.

The common practice of acquiring organic waste from other waste involves introducing sorting guidelines to citizens, with the corresponding material fractions included in these procedures, followed by the separate collection of source-separated organic household waste (SSOHW). A main topic related to the implementation of this scheme on a large scale is feedstock characterisation.

This is important for system optimisation regarding both technical performance, e.g. by predicting methane production and the amount of residue, and also the environmental profile, e.g. by assessing the environmental value of impact contributions when substituting fossil energy and mineral fertilisers.

SSOHW is known as a highly heterogeneous waste stream, and thus its characterisation is not an easy task. SSOHW is also accompanied by non-biodegradable impurities in the collected waste fractions. This issue is usually addressed through the physical pre-treatment of SSOHW, at which stage it is desirable to reject the maximum amount of non-biodegradable impurities while minimising biodegradable matter loss.

Several well-established technologies, each with its own advantages and disadvantages, are known, and these sit alongside newly emerging solutions. To ensure the environmental sustainability of the waste management sector when implementing the AD of SSOHW, it is important that the process has a better environmental profile than the alternative treatment being displaced, in this case incineration.

When comparing AD to incineration, climate change effects indicated by global warming potential (GWP) from a life cycle assessment (LCA) perspective can be used as criteria. The overall aim of this PhD study is to provide background data for the environmental assessment of a wide range of AD of SSOHW implementations in Europe.

To achieve this aim, three specific objectives were formulated regarding waste characterisation, physical pre-treatment and European framework conditions: • Characterise individual material fractions present in Danish SSOHW pertinent to their biochemical methane potential and other parameters of importance for AD treatment. • Describe the technologies currently available for the physical pre-treatment of SSOHW prior to AD in Scandinavian countries, and provide the necessary data required to include them in LCA SSOHW management models. • Determine the framework conditions that will ensure the best AD of SSOHW performance when considering climate change.

Waste characterisations for all EU member states, as well as descriptions of all available pre-treatment technologies, were not possible to detail within the scope of the present PhD thesis. Therefore, waste characterisation was limited to Denmark, and only Scandinavian pre-treatment technologies were included, but it is assumed that the data, to some extent, can be used to describe more general European conditions if country-specific data are unavailable.

Regarding the first objective, hand-sorting of SSOHW in a Danish municipality (where the source separation of organic household waste has been implemented) was performed, desirable material fractions sampled and a range of laboratory investigations performed. The material fractions covered were: animal food waste (AFW), vegetable food waste (VF), kitchen tissue (KT), vegetation waste (VW), moulded fibres (MF), animal straw (AS), dirty paper (DP) and dirty cardboard (DC).

For the second objective, a thorough assessment of a new pre-treatment technology in Denmark was followed by making a comparison to alternative pre-treatment technologies in Scandinavian countries. For the technology assessment, the material flow analysis principle was used, in that the technology process was described and LCA inventory data were generated.

Amongst existing pre-treatment technologies, the screw press-, disc screen- and dispersion-based processes were represented by data from the literature. The last objective was addressed through two LCA studies. The first assessed climate change effects associated with the AD of SSOHW compared to incineration, by concentrating on individual material fractions, and the second assessed the climate change effects of optimising the AD of SSOHW at the pre-treatment stage of the life cycle.

Based on this work, the following results were achieved: • Using the GWP criterion only one material fraction – VFW – was always better for AD compared to incineration. For AFW, KT, VW and DP, performance with AD was better unless it was compared to a highly efficient incinerator. Material fractions such as MF and DC were attractive for AD, albeit only when AD with CHP and incineration with mainly heat production were compared.

AS was always better to incinerate. • In Denmark, food waste (both animal- and vegetable-derived) and kitchen tissue were the main material fractions allowing GWP mitigation with AD when it was compared to incineration, while the inclusion of other material fractions with SSOHW sorting guidelines was of less importance. • The new pre-treatment technology introduced in the present thesis is a promising solution for pre-treating SSOHW prior to AD, and it had advantages over the screw press-, disc screen- and dispersion-based pre-treatment technologies. • Any change in pre-treatment efficiency, such as ± 10% material recovered from the biomass, does not affect the net GWP of the AD of SSOHW significantly, meaning that other aspects, e.g. economy, practicality or other environmental aspects of relevance, might be used as guidance when selecting the technology for practical use.