Evaluating the Aqueous Phase From Hydrothermal Carbonization of Cow Manure Digestate as Possible Fertilizer Solution for Plant Growth

Effects of biogas slurry on hydrothermal carbonization of digestate: Synergistic valorization of hydrochars and aqueous phase

In this study, livestock manure digestate (LMD) was used as feedstock for hydrothermal carbonization (HTC) at different temperature (180-260 °C) and residence time (0-4 h). Nutrient flow and distribution during the HTC process were evaluated by comparing the effects of livestock manure biogas slurry (LBS) and ultrapure water (UW) to determine the optimal reaction conditions for the synergistic production and application of hydrochars (HC) and aqueous phases (AP). Compared with UW, the HC yields derived from LBS as solvent were increased by 27.05-38.24% under the same conditions. The C content, high heating value (HHV), and energy densification of HC obtained from LMD and UW were higher than those obtained from LMD and LBS, and the ash content was lower. While, LBS circumstance improved the porosity, N content and some trace elements e.g. Ca, Fe and Mg in HC that showed excellent fertility potential. In addition, the recovery rate of K, TOC, NH4+-N, and TN concentrations in AP were significantly higher in the LBS circumstance than in UW. The results show that the addition of UW is more favorable for fuel generation, and the HC obtained from LMD and UW at 220 °C has the potential to be used as a fuel. Whereas, the addition of LBS enhanced the potential of HC and AP for agricultural applications simultaneously. It is recommended to use HC and AP obtained from LMD and LBS at 240 °C for using as fertilizer.

Biowaste management by hydrothermal carbonization and anaerobic co-digestion: Synergistic effects and comparative metagenomic analysis

The feasibility of anaerobic co-digestion in semicontinuous mode of two major urban biowaste, food waste (FW) and garden and park waste (GPW) (75 % FW and 25 % GPW) as well as the co-digestion of FW with the process water originated from the hydrothermal carbonization of GPW (95 % FW and 5 % process water), both on a COD basis, has been assessed. The effect of varying organic loading rate (OLR) from 1.5 to 3.5 g COD/L·d on methane yield, gross energy recovery, and microbiome population was evaluated. For comparison, anaerobic digestion of FW was also conducted to determine the best strategy for sustainable biowaste management. This study showed an optimal OLR of 2.5 g COD/L·d. Acetic and propionic acid content increased as OLR raised for each condition studied, while methane yield decreased at the highest OLR tested indicating overloading of the system. The anaerobic co-digestion of FW and process water showed a 10 % increase on methane production compared to anaerobic digestion of FW (324 vs. 294 mL CH4 STP/L·d). Moreover, it enhances the process due to a greater abundance and diversity of hydrolytic and acidogenic bacteria belonging to Bacterioidota, Firmicutes, and Chloroflexi phyla, as well as promotes the hydrogenotrophic pathway under higher propionic concentrations which is not usually favoured for methane production. The integration of hydrothermal carbonization of GPW with the anaerobic co-digestion of 95 % FW and 5 % of process water results in the highest potential energy recovery and could be a good strategy for sustainable management of urban biowaste.

Wood Distillate Promotes the Tolerance of Lettuce in Extreme Salt Stress Conditions

Soil salinization is an adverse phenomenon in agriculture that severely affects crop growth and yield. The use of natural products, such as wood distillate (WD, derived from the pyrolysis of woody biomass), could be a sustainable approach to enhance the tolerance of plants cultivated in the saline soils. Hence, this study aimed to evaluate the potential of WD, a foliar sprayed at 0.2% (v/v), in lettuce plants subjected to grow under both moderate and high soil sodium chloride (NaCl) concentrations (ranging from 0 to 300 mM). The changes in the physiological and biochemical responses of these plants to the varying salt stress conditions allowed the identification of a maximum tolerance threshold (100 mM NaCl), specific to lettuce. Beyond this threshold, levels related to plant defense antioxidant power (antiradical activity) were lowered, while those indicative of oxidative stress (malondialdehyde content and electrolyte leakage) were raised, causing significant losses in leaf fresh biomass. On the other hand, WD significantly improved plant growth, enabling plants to survive high salt conditions >200 mM NaCl. Collectively, these observations highlight that treatments with WD could be of paramount importance in coping with current environmental challenges to have better yields under soil conditions of high salt concentrations.

The dynamic features and microbial mechanism of nitrogen transformation for hydrothermal aqueous phase as fertilizer in dryland soil

Hydrothermal aqueous phase (HAP) contains abundant organics and nutrients, which have potential to partially replace chemical fertilizers for enhancing plant growth and soil quality. However, the underlying reasons for low available nitrogen (N) and high N loss in dryland soil remain unclear. A cultivation experiment was conducted using HAP or urea to supply 160 mg N kg-1 in dryland soil. The dynamic changes of soil organic matters (SOMs), pH, N forms, and N cycling genes were investigated. Results showed that SOMs from HAP stimulated urease activity and ureC, which enhanced ammonification in turn. The high-molecular-weight SOMs relatively increased during 5-30 d and then biodegraded during 30-90 d, which SUV254 changed from 0.51 to 1.47 to 0.29 L-1 m-1. This affected ureC that changed from 5.58 to 5.34 to 5.75 lg copies g-1. Relative to urea, addition HAP enhanced ON mineralization by 8.40 times during 30-90 d due to higher ureC. It decreased NO3-N by 65.35%-77.32% but increased AOB and AOA by 0.25 and 0.90 lg copies g-1 at 5 d and 90 d, respectively. It little affected nirK and increased nosZ by 0.41 lg copies g-1 at 90 d. It increased N loss by 4.59 times. The soil pH for HAP was higher than that for urea after 11 d. The comprehensive effects of high SOMs and pH, including ammonification enhancement and nitrification activity inhibition, were the primary causes of high N loss. The core idea for developing high-efficiency HAP fertilizer is to moderately inhibit ammonification and promote nitrification.

Optimization of two-phase synthesis of Fe-hydrochar for arsenic removal from drinking water: Effect of temperature and Fe concentration

Arsenic-contaminated water is a global concern that demands the development of cost-effective treatments to ensure a safe drinking water supply for people worldwide. In this paper, we report the optimization of a two-phase synthesis for producing a hydrochar core from olive pomace to serve as support for the deposition of Fe-hydroxide, which is the active component in As(V) removal. The operating conditions considered were the initial concentration of Fe in solution in the hydrothermal treatment (phase I) and the temperature of Fe precipitation (phase II). The obtained samples were characterized for their elemental composition, solid yield, mineral content (Fe and K), phenol release, As(V) sorption capacity, and sorbent stability. Correlation analysis revealed that higher Fe concentrations (26.8 g/L) ensured better carbonization during hydrothermal treatment, increased arsenic removal, reduced concentrations of phenols in the final liquid, and improved stability of the sorbent composite. On the other hand, the temperature during Fe precipitation (phase II) can be maintained at lower levels (25-80 °C) since higher temperatures yielded lower adsorption capacity. Regression analysis demonstrated the significance of the main effects of the parameters on sorption capacity and provided a model for selecting operating conditions (Fe concentration and phase II temperature) to obtain composite sorbents with tailored sorption properties.

Using the Aqueous Phase Produced from Hydrothermal Carbonization Process of Brown Seaweed to Improve the Growth of Phaseolus vulgaris

Seaweeds are considered a biomass for third-generation biofuel, and hydrothermal carbonization (HTC) is a valuable process for efficiently disposing of the excess of macroalgae biomass for conversion into multiple value-added products. However, the HTC process produces a liquid phase to be disposed of. The present study aims to investigate the effects of seed-priming treatment with three HTC-discarded liquid phases (namely AHL₁₈₀, AHL₂₄₀, and AHL₃₀₀), obtained from different experimental procedures, on seed germination and plant growth and productivity of Phaseolus vulgaris L. To disentangle the osmotic effects from the use of AHL, isotonic solutions of polyethylene glycol (PEG) 6000 have also been tested. Seed germination was not affected by AHL seed-priming treatment. In contrast, PEG-treated samples showed significantly lower seed germination success. AHL-treated samples showed changes in plant biomass: higher shoot biomass was recorded especially in AHL₁₈₀ samples. Conversely, AHL₂₄₀ and AHL₃₀₀ samples showed higher root biomass. The higher plant biomass values recorded in AHL-treated samples were the consequence of higher values of photosynthesis rate and water use efficiency, which, in turn, were related to higher stomatal density. Recorded data strongly support the hypothesis of the AHL solution reuse in agriculture in the framework of resource management and circular green economy.

Impact of starch-based bioplastic on growth and biochemical parameters of basil plants

The recent use of bioplastics in agriculture is considered an ecological choice, aimed at limiting the environmental impact of plastics, in line with the Sustainable Development Goals of the United Nations. However, the impact of bioplastic residues on the environment is unclear as knowledge is lacking. This is the first study investigating the effect of a starch-based bioplastic on the growth and biochemical parameters of basil. Bioplastic was experimentally prepared and added to the soil at 2.5 % (w/w), corresponding to twice the concentration of plastic mulch film residues currently found in cultivated soils, in view of the increasing agricultural use of bioplastics. Basil plants were grown without (controls) and with bioplastic addition for 35 days, under controlled experimental conditions. Compared to the control, plants exposed to bioplastic showed stunted growth (in terms of shoot fresh weight, height, and number of leaves). Significant reductions in the content of chlorophyll, protein, ascorbic acid, and glucose were also observed. Finally, the treatment caused oxidative stress, as evidenced by the increased content of malondialdehyde in the shoots. The addition of bioplastic increased the electrical conductivity and reduced the cation exchange capacity of the cultivation soil. These results suggest that bioplastic in soil may promote the onset of stressful conditions for plant growth in a similar manner to plastic. They will be complemented by further investigations to unravel the mechanisms underlying these responses, involving different doses and types of bioplastics and other crop species.

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.

Phosphorus retention and agronomic efficiency of refined manure-based digestate—A review

Digestate, a by-product from anaerobic digestion of organic materials such as animal manure, is considered a suitable plant fertilizer. However, due to its bulkiness and low economic value, it is costly to transport over long distances and store for long periods. Refinement processes to valorize digestate and facilitate its handling as a fertilizer include precipitation of phosphorus-rich mineral compounds, such as struvite and calcium phosphates, membrane filtration methods that concentrate plant nutrients in organic products, and carbonization processes. However, phosphorus retention efficiency in output products from these processes can vary considerably depending on technological settings and characteristics of the digestate feedstock. The effects of phosphorus in plant fertilizers (including those analogous or comparable to refined digestate products) on agronomic productivity have been evaluated in multiple experiments. In this review, we synthesized knowledge about different refinement methods for manure-based digestate as a means to produce phosphorus fertilizers, thereby providing the potential to increase phosphorus retention in the food production chain, by combining information about phosphorus flows in digestate refinement studies and agronomic fertilizer studies. It was also sought to identify the range, uncertainty, and potential retention efficiency by agricultural crops of the original phosphorus amount in manure-based digestate. Refinement chains with solid/wet phase separation followed by struvite or calcium phosphate precipitation or membrane filtration of the wet phase and carbonization treatments of the solid phase were included. Several methods with high potential to extract phosphorus from manure-based wet phase digestate in such a way that it could be used as an efficient plant fertilizer were identified, with struvite precipitation being the most promising method. Synthesis of results from digestate refinement studies and agronomic fertilizer experiments did not support the hypothesis that solid/wet separation followed by struvite precipitation, or any other refinement combination, results in higher phosphorus retention than found for unrefined digestate. Further studies are needed on the use of the phosphorus in the solid phase digestate, primarily on phosphorus-rich soils representative of animal-dense regions, to increase understanding of the role of digestate refinement (particularly struvite precipitation) in phosphorus recycling in agricultural systems.

Recent progress on the phytotoxic effects of hydrochars and toxicity reduction approaches

Hydrothermal carbonization of wet biomasses has been known to produce added-value materials for a wide range of applications. From catalyst substrates, to biofuels and soil amendments, hydrochars have distinct advantages to offer compared to conventional materials. With respect to the agricultural application of hydrochars, both positive and negative results have been reported. The presence of N, P and K in certain hydrochars is appealing and may contribute to the reduction of chemical fertilizer application. However, regardless of biomass, hydrothermal carbonization results in the production of phytotoxic organic compounds. Additionally, hydrochars from sewage sludge often contain heavy metal concentrations which exceed the regulatory limits set for agricultural use. This review critically discusses the phytotoxic aspects of hydrochar and provides an account of the substances commonly responsible for these. Furthermore, phytotoxicity reduction approaches are proposed and compared with each other, in view of field-scale applications.

Combustion kinetics of hydrochar from cow-manure digestate via thermogravimetric analysis and peak deconvolution

Hydrothermal carbonization (HTC) can convert wet biomass into hydrochar (HC), a solid carbonaceous material exploitable as fuel. In this study, HTC was applied to anaerobic digestate from cow manure. HCs obtained at three HTC temperatures (180, 220, 250 °C) were characterized in detail and their combustion behavior was investigated by thermogravimetric analysis (TGA) coupled with peak deconvolution. Increasing HTC temperatures increased the fixed carbon content (17.9-20.7%), the ash content (27.2-32.5%) and the calorific value (14.3-18.2 MJ/kg), while decreased the hydrogen (5.01-4.54%) and oxygen content (24.09-12.35%) of HCs. DTG profiles peak deconvolution unveils the presence of five major components in the HCs. HCs combustion kinetics were studied applying the KAS method. Average apparent activation energy values of 100, 88, 67 kJ mol^-1 were obtained for HC180, HC220, HC250, respectively. HTC at 250 °C produced the HC with the best fuel characteristics.

Phenotyping of Different Italian Durum Wheat Varieties in Early Growth Stage With the Addition of Pure or Digestate-Activated Biochars

This study aims to highlight the major effects of biochar incorporation into potting soil substrate on plant growth and performance in early growth stages of five elite Italian varieties of durum wheat (Triticum durum). The biochars used were obtained from two contrasting feedstocks, namely wood chips and wheat straw, by gasification under high temperature conditions, and were applied in a greenhouse experiment either as pure or as nutrient-activated biochar obtained by incubation with digestate. The results of the experiment showed that specific genotypes as well as different treatments with biochar have significant effects on plant response when looking at shoot traits related to growth. The evaluated genotypes could be clustered in two main distinct groups presenting, respectively, significantly increasing (Duilio, Iride, and Saragolla varieties) and decreasing (Marco Aurelio and Grecale varieties) values of projected shoot system area (PSSA), fresh weight (FW), dry weight (DW), and plant water loss by evapotranspiration (ET). All these traits were correlated with Pearson correlation coefficients ranging from 0.74 to 0.98. Concerning the treatment effect, a significant alteration of the mentioned plant traits was observed when applying biochar from wheat straw, characterized by very high electrical conductivity (EC), resulting in a reduction of 34.6% PSSA, 43.2% FW, 66.9% DW, and 36.0% ET, when compared to the control. Interestingly, the application of the same biochar after nutrient spiking with digestate determined about a 15-30% relief from the abovementioned reduction induced by the application of the sole pure wheat straw biochar. Our results reinforce the current basic knowledge available on biological soil amendments as biochar and digestate.