Effects of pre-treatment technologies on quantity and quality of source-sorted municipal organic waste for biogas recovery

Source-sorted municipal organic waste collected from different dwelling types in five Danish cities and pre-treated at three different plants was sampled and characterized several times during one year to investigate the origin of any differences in composition of the pre-treated waste introduced by city, pre-treatment technology, dwelling type or annual season. The investigated pre-treatment technologieswere screw press, disc screen and shredder + magnet. The average quantity of pre-treated organic waste (biomass)produced from the incoming waste varied between the investigated pre-treatmenttechnologies: 59 percent, 66 percent and 98 percent wet weight, respectively (41 percent, 34 percent and 2 percent reject, respectively). The pre-treatment technologies showed differences with respect to distribution of the chemical components in the waste between the biomass and the rejected material (reject), especially for dry matter, ash, collection bag material (plastic or paper)and easily degradable organic matter. Furthermore, the particle size of the biomass was related to the pre-treatment technology. The content of plastic in the biomass depended both on the actual collection bag material used in the system and the pre-treatment technology. The sampled reject consisted mostly of organic matter. For cities using plastic bags for the source-separated organic waste, the expected content of plastic in the reject was up to 10 percent wet weight (in some cases up to 20 percent). Batch tests for methane potential of the biomass samples showed only minor variations caused by the factors city, pre-treatment technology, dwelling type and season when based on the VS content of the waste (overall average 459 STP m3/t VS). The amount of methane generated from 1 t of collected waste was therefore mainly determined by the efficiency of the chosen pre-treatment technology described by the mass distribution of the incoming waste between biomass and reject.