Urban Energy – Waste-Based Biogas, pre-project: Household waste to biogas through a turnkey pre-treatment: Value stream analysis. Results of WP1.

In Denmark, there is a growing interest in integrating the organic share of municipal household waste (MHW) as a feedstock for biogas plants. However, separating the organic fraction (organics) from the other fractions of MHW remains challenging. This study
investigates the performance of a turnkey mechanical treatment unit as a centralized solution to recover the organics from MHW. It consists of a hammer mill crushing the organics against a 15-mm screen, allowing to collect these as a liquid pulp (biopulp) that can be used for biogas production.
Through 8 test runs spread over a 1-y period, a full truckload of MHW from the test area (Nyborg municipality; 110 inhabitants km^-2) was fed to the treatment unit. Both mixed MHW and source-separated MHW were tested. In order to simulate anaerobic digestion, the produced biopulp was fed to continuously stirred-tank reactors (CSTR; 20-L). The concentration of key substance flows has been measured (before and after digestion), as well as operational parameters such as the processing capacity, water- and electricity consumption.
Results did not allow to compare mixed MHW and source-separated MHW given the high amount of garden waste in the source-separated MHW obtained. Results for mixed MHW showed that the produced biopulp (ca. 10% dry matter; DM, of which 75% are volatile solids; VS) allowed the production of a stable biogas (methane content above 60%), with a biochemical methane potential (BMP) competing with the one of energy crops and ranging between 480 – 560 NL CH4 kg-1 VS. The maximal operating flow capacity achieved was 4 t waste h^-1; ca. 4 times lower than specified by the manufacturer. The biopulp concentration in heavy metals and hazardous substances was below the limit levels prescribed in the Danish legislation for land application, both before and after digestion. However, the amount of visible contaminants (plastics, glass, etc. > 2mm) varied between 5 and 10 g
contaminant g^-1 DM in the biopulp, and went as high as 50 g g^-1 for one test. In e.g. Sweden, there is a limit of 0.5 g g^-1 DM in order to apply the material on land. This, combined with the low flow processing capacity of the treatment unit, hinders its current feasibility as a largescale strategy for MHW’ organics recovery. Results also highlighted the industrial wastewater received from rice processing, currently used as a process water (instead/on top of tap water), as a substrate with a high BMP (> 600 NL CH4 kg^-1 VS), a low pH (ca. 3.65), and therefore a potentially interesting organic acid (e.g. for manure acidification).