Search

The management of digestate, the main by-product of the anaerobic digestion (AD) process, is one of the most serious environmental issues. Although digestate is used on arable land as a fertilizer, it can have a negative impact on the environment due to nitrate leaching into the groundwater and ammonia volatilization into the atmosphere, with high economic and environmental disposal costs. Therefore, hydrothermal carbonization (HTC), a thermochemical biomass conversion process, could represent a sustainable and efficient alternative for digestate management. Hydrochar, the solid product of the HTC process, has been recently proposed as a plant growing medium in soilless culture systems (SCS). Here, using cow manure digestate as feedstock, we investigated the influence of the HTC process reaction temperature (180, 220 and 250 °C) and residence time (1 and 3 h) on the physical-chemical properties (pH, electrical conductivity, and mineral element concentrations) of the resulting hydrochars. Furthermore, in order to fully valorize hydrochar as a growing medium, their possible phytotoxic effects and those of their water extracts (prepared at two different concentrations and at different pHs) were tested in germination tests with cress seeds (Lepidium sativum L.). Concentrations of nutrients, heavy metals, organic acids, sugars and furan compounds were determined in the water extracts. Characterization analysis of these hydrochars revealed that they can be distinguished from each other by their physical-chemical properties, which were significantly affected by the two process parameters. Specifically, the HTC temperature had a greater effect on the composition of hydrochars than the residence time. Germination tests found hydrochar water extracts to show significantly lower phytotoxicity than the hydrochars themselves. Notably, the phytotoxic effect of the extracts decreased with increasing extraction ratio and decreasing pH. The chromatographic characterization of extracts identified the presence of potential phytotoxins, such as furan compounds (i.e., hydroxymethylfurfural and furfural). However, before using hydrochars as potential and innovative growing media for plants, their phytotoxicity should be limited, for example through their dilution with other substrates. Overall, AD-HTC coupling could represent a valuable eco-sustainable expedient in the field of biomasses, green economy and waste conversion and, therefore, further investigations in this direction are necessary., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Household biowaste represent the organic fraction of municipal solid waste and are an underutilized resource. Although previous studies have performed pyrolysis of organic waste, the vast majority has been on specific presorted feedstock or conventional lignocellulosic streams. Therefore, there is a lack of pyrolysis applications on representative food waste as retrieved from households and this can be attributed primarily to their high water content and their degradability. But via the intermediate step of drying, long-term storage and thermal treatment have become possible. In the framework of this study, household biowaste were pyrolyzed for the production of carbonaceous materials with a main focus on the analysis of produced tar compounds. Tars can be corrosive or cause clogging and disrupt the operation of pyrolysis and gasification plants. Their analysis has faced several difficulties due to inconsistency in the methodologies that have been applied by various groups. The tar protocol has provided a solid framework for consistent analysis of tars but until now has been solely used for the case of gasification. This study aimed to apply the tar protocol for pyrolysis and to enhance the detectability of the method for a wider range of tars by means of elemental analysis, attenuated total reflectance (ATR) and gas chromatography-mass spectrometry (GC-MS). GC- MS was performed by means of a specific column for PAHs identification and calibration methods were developed for the proper quantification of naphthalene which is the dominant tar compound. The results of the analysis showed that naphthalene concentration increased from torrefaction to carbonization but then decreased significantly for high temperature pyrolysis at 860 °C., (Copyright © 2017 Elsevier Ltd. All rights reserved.)