More and more municipalities in Europe are requiring residents to collect their vegetable, fruit, and garden waste separately. This growing stream of bio-waste needs to be processed somewhere, and this involves a choice that receives relatively little attention: opting for one large, central processing plant or for smaller facilities close to the source. On July 24, researchers from the French institute SayFood published a study that provides quantitative substantiation for this trade-off for the first time.
Bio-waste can be processed in various ways. In composting, microorganisms break down the waste into compost, which can serve as fertilizer. In anaerobic digestion, something similar occurs without oxygen, producing biogas in addition to digestate, which can be used as an energy source. Both are considered more sustainable than the third option in the study: local waste incineration.
Transport distance outweighs economies of scale
The researchers developed a combined technical-economic and life cycle model in which costs and environmental impact are weighed simultaneously. In addition to the processing itself, they also included transport to the installation, using life cycle data from heavy trucks powered by electricity, gas, and diesel. In total, 156 combinations of installation size and transport distance were calculated and compared with local incineration.
This implies a non-linear relationship between scale and distance. Composting without any transport distance scored best on the combination of costs and environmental impact. Of all the configurations calculated, 45 proved to be both cheaper and more environmentally friendly than incineration, and another 111 remained more environmentally advantageous despite higher costs. According to the study, as transport distance and plant size increase, the advantage of scale diminishes more rapidly than is often assumed.
For Dutch municipalities and waste processors, who regularly face the choice between regional bundling and local processing of organic waste, the study provides a quantitative substantiation for this assessment. According to the authors, the threshold values for transport distance and scale have hitherto been insufficiently characterized, even though these directly determine whether investments in processing infrastructure pay off in terms of costs and CO2 reduction.
Source: arxiv.org
Photo: Edward Howell on Unsplash
