Benefits and Limitations of Using Hydrochars from Organic Residues as Replacement for Peat on Growing Media

Greenhouse gas emissions from bio-based growing media: A life-cycle assessment

In this study, using an LCA approach we explored how bio-based peat alternatives (wood fiber, compost, and hydrochar based on willow and degassed fiber from agricultural waste) and their mixtures (75 % peat with 25 % peat alternative) as growing media (GM) for plant production in Denmark may provide benefits for reducing greenhouse gas emissions compared to peat. To perform this, foreground data (collected via personal communication and literature) was used together with background data from Ecoinvent V3.8. The chosen functional unit was 1 m3 of GM and the system boundary was from cradle to use as GM. The global warming potential of all the peat alternatives showed significant reduction, varying between 89 and 109 % compared to peat. When incorporating 25 % of each alternative with peat, the climate footprint was reduced by 16 to 33 % compared to pure peat. Thus, there are large climate prospects in replacing peat with bio-based alternatives, and the results underlines the relevance of being able to increase the proportion of the bio-based components in their mixtures with peat beyond the 25 % and towards 100 % replacement. The effectiveness of peat substitutes in term of reducing the CO2 emissions is affected by choice of the feedstock, their processing method and emissions of their end-use.

Effects of process water obtained from hydrothermal carbonization of poultry litter on soil microbial community, nitrogen transformation, and plant nitrogen uptake

Process water (PW) obtained from hydrothermal carbonization of nitrogen-rich (N-rich) biowaste is proposed to be a renewable resource utilized as a liquid N fertilizer. However, its effects on soil microbial community, N transformation, and plant N uptake are unclear or controversial. In this study, fertilizers were prepared with different percentages of PW (poultry litter, 220 °C 1 or 8 h, PW-S or -L) and urea to supply 160 mg kg^-1 total N in a barren alkali soil. Results showed that the addition of PW relative to pure urea decreased organic N mineralization by low bio-accessibility, increased N loss by high soil pH, and decreased NO₃^--N by low nitrification substrate. It supported the lettuce in health but decreased plant N uptake by low NO₃^--N. It significantly increased the gram-positive bacteria that responded to resistant organic matter, changed the bacterial community to enhance decomposition, detoxification, ureolysis, and denitrification, and to decrease nitrification. Its inhibition effect on nitrification activity was stronger than that on nitrifiers growth. Different from PW-S, the addition of PW-L seriously and significantly decreased seed germination index and fungal biomass that responded to N retaining capacity, respectively. The best fertilizer was 50% urea +50% PW-S that supported the seed germination and seedling growth, and mildly affected microbial community.

A cascade biorefinery for grape marc: Recovery of materials and energy through thermochemical and biochemical processes

The agro-industrial sector makes a high contribution to greenhouse gas emissions; therefore, proper waste management is crucial to reduce the carbon footprint of the food chain. Hydrothermal carbonization (HTC) is a promising and flexible thermochemical process for converting organic materials into energy and added-value products that can be used in different applications. In this work, grape marc residues before and after an extraction process for recovering polyphenols were hydrothermally treated at 220 °C for 1 h. The resulting hydrochar and process water were investigated to test an innovative cascade approach aimed at a multiple product and energy recovery based on the integration of HTC with anaerobic digestion. The results show that this biorefinery approach applied to grape marc could allow to diversify and integrate its potential valorisation options. The produced hydrochars possess an increased fixed carbon content compared to the feedstock (up to +70 %) and, therefore, can be used in soil, immobilizing carbon in a stable form and partially replacing peat in growing media (up to 5 % in case of hydrochar from grape marc after extraction), saving the consumption of this natural substrate. In addition, energy can be recovered from both hydrochar by combustion and from process water through anaerobic digestion to produce biogas. Hydrochars show good properties as solid fuel similar to lignite, with an energy content of around 27 MJ kg^-1 (+30 % compared to the feedstock). The anaerobic digestion of the process water allowed obtaining up to 137 mL of biomethane per gram of fed COD. Finally, while HTC process waters are suitable for biological treatment, attention must be paid to the presence of inhibiting compounds that induce acute toxic effects in aerobic conditions. The proposed approach is consistent with the principles of circular economy and could increase the overall sustainability and resilience of the agro-industrial sector.