Can food waste be sorted from residual waste and reused as biowaste?

Even though there is a great willingness to sort bio waste in Copenhagen, a significant amount still ends up in the residual waste stream each year. Therefore, a partnership project has been set up to explore the opportunities for utilization of the biowaste from the residual waste.

The purpose of the study was to

1. analyze size sorted waste to determine the proportion of the food waste that mechanically can be sorted from the residual waste,
2. check the stability and potential of biogas production from the biopulp in question.

Subsequently to investigate whether the degassed pulp will live up to current standards for the purpose of fertilizing farmland.

Collaboration between Bioman ApS, Gemidan A/S and Econet AS 

An amount of 20.760 kg residual waste was collected from Copenhagen households and transported to a temporary test facility at Gemidan’s in Ølstykke.

After the first processing the pulp was sent to Bioman ApS’ laboratory in Skanderborg for the purpose of biogas production. At Gemidan’s the waste bags were mechanically opened, and the waste sorted into three different fractions: A “large” fraction (>100 mm) and a “small” fraction (<60 mm) leaving an “in between” fraction (>60 mm - <100 mm). Each fraction has been analyzed for the waste composition.

The processing showed that the food waste mainly ends up in the small size category - in this case less than 60 mm.

Overall, the study finds that a mechanical opening of the waste bags, and subsequent size sorting, ensures that a large proportion of the food waste from the residual waste ends up in the smallest size category.

Biogas Production

In the case of biogas production, a test facility at Bioman ApS' laboratory was set up: An approx. 80 liters volume reactor with a 70 liters active volume and 10 liters gas volume. The biogas process was carried out at 42°C with a residence time of 30 days.

The trial was terminated after 12 weeks. However, after only 2 weeks of operation the biogas process became stable. The findings of the biogas production correspond with that of Bioman’s from the pulp of source-sorted food waste from private households.

Finally, the study shows that by several siftings of the pulp it is possible to remove impurities - especially after the biogas process. An analysis of the sifted pulp also shows that the pulp in question qualify for the current provisions for heavy metals, pollutants, and purity. It is therefore safe to say, that pulp from size-sorted residual waste easily can be used for biogas production with subsequent spreading on agricultural land without risk of contamination.

It is noted however, that Danish legislation does not allow spreading organic waste on farmland unless it is separately collected (i.e. not as a part of the residual waste).