In this study, two scenarios of a municipal wastewater treatment plant (WWTP) are presented, which include the integration of the hydrothermal carbonization (HTC) process into the sludge line as a post-treatment of the anaerobic digestion (AD) process. The objective of the simulation is to investigate the performances of AD + HTC treatment to reduce sludge production and improve nutrient and energy recovery. For this purpose, the scheme of an under-construction WWTP was considered, named Trento 3 (Trento, Italy) and with a treatment capacity of 300,000 PE. In the first scenario, the HTC process was fed with thickened sludge from the Trento 3 WWTP, while in the second scenario, dewatered sludge from other local WWTPs was also used as feedstock for the HTC process. Both scenarios allowed to obtain a considerable sludge reduction ranging from 70 to 75% with a notably increase in the biogas production up to 47%, due to the recycling of HTC liquor (HTCL) to the anaerobic digester. Considering nutrients recovery, all the phosphorus and nitrogen present in the HTCL could be used for struvite precipitation with an average yearly gain of 1 million euros. Moreover, the introduction of HTC in the Trento 3 WWTP could allow a reduction in the sludge management costs of up to 2 M€/year. Graphical Abstract

An increasing number of industrial plants integrate the anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW) with a subsequent composting phase. To improve the plant productivity, a fraction of OFMSW digestate can be converted into a carbonaceous material, called hydrocar (HC), through Hydrothermal Carbonization (HTC), and then composted together with the OFMSW digestate itself, to produce “hydrochar co-compost”. The aim of this paper is to present the design and assembly of batch bioreactors, built in-house to investigate the co-composting process of OFMSW digestate and its HC, and to provide some preliminary results. The OFMSW digestate from an industrial plant was carbonized at 200 °C for 3 h in a 2 L HTC reactor, to produce wet HC after filtration. The ratio of OFMSW digestate and green waste (1:1) used as bulking medium was reproduced in four bioreactors with an increasing percentage of HC substituting the OFMSW digestate (0, 25, 50, 75%). The bioreactors managed to effectively compost the solid wet biomasses in a wet environment with temperature and oxygen control, while measuring online the oxygen consumption and thus the dynamic respirometric index (DRI). The DRI_24,max measured with AIR-nl solid respirometer (standardized offline measurement) started from values above 800 mg O₂ kg_VS⁻¹ h⁻¹ before composting and dropped at the end of the process to values in the range 124–340 mg O₂ kg_VS⁻¹ h⁻¹ for the four mixes, well below the recommended limit of 500 mg O₂ kg_VS⁻¹ h⁻¹ for high-quality compost stability. These offline DRI values were confirmed by the online DRI measurements. This research is part of the international C2Land Project funded by the European Institute of Innovation and Technology Climate Knowledge and Innovation Community (EIT Climate-KIC), which is greatly acknowledged.

The book aims to analyse and discuss the conventional and emerging treatments of digestate generated in the anaerobic treatment of organic waste. Thus, in the circular economy framework, the book will address up-to-date strategies for the treatment and resource recovery from anaerobic digestate. Anaerobic digestion is an expanding technology nowadays, especially when considering the thrust in ongoing research on the value-added products and energy recovery from biomass wastes, i.e., sludges, agro-industrial waste, animal waste, food waste and organic fraction of municipal solid waste, etc. Anaerobic digestate is a leftover and waste management authorities across the world do not have so many options for digestate management except using the digestate for land application as a fertilizer or composting. However, researchers and field engineers are still looking for robust options for digestate management. Thus, a timely book on digestate management will be an invaluable addition to this domain. The key features of the book include: the broad range of biomass waste covered, discussion of conventional to advanced technological options, and the inclusion of successful case studies. ISBN: 9781789062748 (paperback) ISBN: 9781789062755 (eBook) ISBN: 9781789062762 (ePub)