Source separation, selective collection and in reactor digestion of biowaste

Abstract

Biowaste or the organic fraction of domestic waste, for instance kitchen, fruit and garden waste, is collected selectively in several European communities. The complementary fraction is called the dry or non recyclable fraction. A Dutch study reported that 92% of the participants that have a weekly collection service of both fractions (biowaste and non recyclable fraction) and 80% of the participants in the alternating collection program (one week biowaste and the next week non recyclable fraction) are pleased with separate collection of biowaste. Dominating problems that arise in case of alternating collection are a repulsive odor and an infestation with flies and maggots. By expanding the definition of biowaste to include non recyclable or soiled paper like dirty newspapers, table napkins and paper handkerchiefs, most of these problems can be overcome without changing the way compostable waste is collected and processed. The expanded definition of biowaste was used in this paper. Over a 12 month period a quality survey of the collected biowaste was conducted by the composting facility Intercompost, Hoeselt, Belgium. A special aspect was the fact that in one participating community baby diapers were included in the soiled paper fraction; this is called “biowaste+”. The biowaste+ had a 10% non recyclable paper fraction opposed to only 1–2% of non recyclable paper present in the conventional biowaste. Baby diapers were a rather notable part (more than 80%) of this non recyclable paper fraction of biowaste+ and as a consequence might contribute to a large extent to improve the collection and treatment of biowaste. It was demonstrated that rural districts yielded about 35% more biowaste than more urban districts; resp. ± 122 kg biowaste/capita.year versus ±90 kg biowaste/capita.year. In Hoeselt the biowaste+ yield was about 130 kg/capita.year. Biowaste+ is also separately collected in another Belgium community, namely Brecht. The purity level of the biowaste+ and the amount of non recyclable paper (including diapers) were comparable with the results of Hoeselt. In Hoeselt the biowaste+ fraction is composted aerobically. In Brecht on the other hand, the biowaste+ is processed using the Dry Anaerobic Composting process (DRANCO). The latter process is discussed in more detail. The biological start-up of the dry anaerobic composting installation at Brecht, Belgium, is reported. The reactor has a total volume of 808 m³ and a design capacity of 730 m³. After 2 months of start up, the fermentor was at full loading rate, i.e. 8 kg bVS/m³ _reactor .day and the installation was working at full capacity, i.e. 40 ton/day. After 1 year of full-scale operation, the installation can handle peak loads of 60 ton/day. The average biogas production is 107 m³/ton with an average methane content of 55%. This corresponds to an annual gross energy production of approximately 600 toe (ton oil equivalent). The average biogas production rate is 4.0 m³/m³ _reactor .day. The end product of the aerobic process in Hoeselt and the end product in Brecht, the Humotex, are of similar composition. Both are of high quality and are sold as a soil conditioner or as a potting substrate. However, the double processed compost has a garanteed hygienic quality which makes it a product suitable for export.