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Anaerobic digestion of organic wastes is clearly encouraged by current regulations in Europe. In complement to the energy supply it represents, this biological treatment process also allows the recycling of organic matter and nutrients contained in biodegradable wastes. Indeed the digestion residue can be further promoted as soil improver or fertilizer. The sustainability of anaerobic digestion plants partly depends on the management of these digestion residues. Digestates present particular characteristics that can reduce their direct agricultural valorization and minimize by the way the benefit of such a biological treatment. Thus the first part of this two part paper aims at reviewing and discussing a wide range of biochemical, biological and physical indicators used to assess the agronomic quality of organic products and the feasibility of an aerobic treatment by composting. The definition of agronomic quality is very complex and no single parameter can be picked out to assess the quality of solid digestates coming from different sources. A relevant choice of these parameters will lead to state on digestates agricultural use, whether they can be directly used on soil after digestion or if they need a composting post-treatment before utilization. The second part of this two part paper will choose indicators to characterize several digestates in order to assess their future agricultural use.

The aim of this research was to characterize six organic solid digestates in order to discuss their potential direct agricultural utilisation or their further valorization through an aerobic post-treatment. The studied digestates came from four different sources: waste water treatment sludge, food-processing wastes, agricultural solid wastes and source-selected organic fraction of municipal solid wastes. Physico-chemical, biochemical and biological characteristics showed that a direct use of digestates and application on land as soil amendment or fertilizer is not recommended according to current standards. A posttreatment appeared thus to be necessary in order to obtain commercial amendments or fertilizers. A composting posttreatment of these digestates would assure their full biological stabilization as these organic residues still contain residual biodegradability and present sometimes phytotoxic risks. The prediction of operational parameters showed that the composting post-treatment of these digestates will probably lead to a limited rise in temperature, require moderate aeration supply and last less than a typical composting treatment of organic wastes.

Management of municipal solid waste (MSW) is a challenge faced by urban centres worldwide, including the European Union (EU) and Canada, as a result of urbanization and economic prosperity accelerating the generation of municipal solid wastes (MSW) along with its organic fraction. Within a concept of waste recovery, source separation and on-site treatment of urban organic waste (UOW) can resolve major economic issues and faced by urban centres along with environmental and social issues associated with landfilling. In this context and as compared to the traditional landfilling practice (Base Sce), this paper examines on-site UOW composting strategies using a combination of centralized composting facilities (CCF), community composting centres (CCC) and home composting (HC) (Sce 1, 2 and 3). This study consists of a feasibility and economic study based on available data and waste management costs. This study indicates that on-site treatment of UOW using practices such as home and community composting can lower management costs by 50, 37 and 34 % for the rich European countries (annual GDP over $25000 US), the poorer European countries (annual GDP under $25000 US), and Canada, respectively. Furthermore, on-site composting can reduce greenhouse gas (GHG) emissions by 40 % for Europe and Canada, despite gas capture practices on landfill sites.However, the performance of home composters and the quality of the compost products are issues to be further addressed for the successful implementation of UOW on-site composting.

This study aimed to experimentally acquire evolution profiles between depth, bulk density, Free Air Space (FAS), air permeability and thermal conductivity in initial composting materials. The impact of two different moisture content, two particle size and two types of bulking agent on these four parameters was also evaluated. Bulk density and thermal conductivity both increased with depth while FAS and air permeability both decreased with it. Moreover, depth and moisture content had a significant impact on almost all the four physical parameters contrary to particle size and the type of bulking agent.

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