Co-composting: An Opportunity to Produce Compost with Designated Tailor-Made Properties

Co-composting is a technique that allows the aerobic degradation of organic waste mixtures, primarily aiming at obtaining compost that can be used as fertiliser or soil amendment. As compared to the typical composting activity, the main difference is not merely the use of more than one feedstock to start and sustain the biodegradation process, but also the possibility of combining various kinds of waste to obtain ‘tailored’ products with designed properties, or to reclaim and valorise natural resources, such as degraded soils or polluted soils and sediments. Set up of appropriate co-composting protocols can be a way to optimise the management of waste produced by different sectors of agriculture and industry and also from human settlements. Different formulations can not only optimise the biodegradation process through the adjustment of nutrient ratios, but also lead to the formation of products with innovative properties. Moreover, co-composting can be a technique of choice for the reclamation of soils degraded by intensive agriculture or contaminated soils and sediments. In fact, an appropriate mix of organic waste and soils can restore the soil structure and induce fertility in nutrient-depleted soils, and also remediate polluted soils and sediments through degradation of organic pollutants and stabilisation of heavy metals. While the selection of different mixes of organic waste may lead to the design of composts with specific properties and the potential valorisation of selected waste materials, there are still several factors that hamper the development of co-composting platforms, mainly insufficient knowledge of some chemical and microbiological processes, but also some legislative aspects. This chapter illustrates the progress achieved in co-composting technology worldwide, some key legislative aspects related to the co-composting process, the main scientific and technical aspects that deserve research attention to further develop co-composting technology, and successful applications of co-composting for the reclamation of soils and sediments, allowing their use for cultivation or as growing media in plant nurseries. A specific case study of the production of fertile plant-growing media from sediment co-composting with green waste is also illustrated.