The fertilising potential of manure-based biogas fermentation residues: pelleted vs. liquid digestate

The liquid by-product of biogas production: characterisation and impact on soil fungi

ABSTRACTA liquid digestate (LD) obtained from the anaerobic digestion of mixed organic waste was characterised and tested on the fungi Pleurotus eryngii, Irpex lacteus and Trametes versicolor. Aqueous mixtures of LD at doses of 0.5, 1 and 2% (v/v) were tested directly or after interaction with 5% (w/v) biochar (BC-LD) and/or 100 mg L-1 soil humic acid (HA-BC-LD and HA-LD). Total luminescence (TL) analysis of LD showed the presence of fluorophores typical of scarcely aromatic matter, while the Fourier transform infrared (FTIR) spectrum evidenced absorption bands typical of labile and non-condensed material. Some spectroscopic variations of the LD sample were observed after its interaction with the other materials. All LD treatments markedly promoted hyphal extension of P. eryngii whose growth rate increased up to 38% at the highest LD dose. The LD alone had no influence on the other fungi, whose growth was stimulated by some combinations of LD with the other materials. In facts, the growth rate of I. lacteus increased in BC-LD 1 and BC-LD 2 (P ≤ 0.05) and in all treatments with HA (up to 6% in HA-LD 2, P ≤ 0.01), while the growth of T. versicolor was promoted by HA-BC-LD 2 treatment (P ≤ 0.05) and slightly inhibited by all BC-LD combinations (up to 7% by BC-LD 1, P ≤ 0.05). The overall results obtained encourage the addition of LD to the soil and suggest a possible use of this material as substrate ingredient for the cultivation of edible mushrooms such as P. eryngii.

Magnetic iron oxide nanoparticle enhanced microwave pretreatment for anaerobic digestion of meat industry sludge

Our study investigates the effects of iron oxide (Fe3O4) nanoparticles combined microwave pretreatment on the anaerobic digestibility and soluble chemical oxygen demand (SCOD) of meat industry sludge. One of our main objectives was to see whether the different microwave-based pretreatment procedures can enhance biogas production by improving the biological availability of organic compounds. Results demonstrated that combining microwave irradiation with magnetic iron oxide nanoparticles considerably increased SCOD (enhancement ratio was above 1.5), the rate of specific biogas production, and the total cumulative specific biogas volume (more than a threefold increment), while having no negative effect on the biomethane content. Furthermore, the assessment of the sludge samples' dielectric properties (dielectric constant and loss factor measured at the frequency of 500 MHz) showed a strong correlation with SCOD changes (r = 0.9942, R2 = 0.99), offering a novel method to evaluate pretreatment efficiency.

Effect of Biogas Slurry on the Soil Properties and Microbial Composition in an Annual Ryegrass-Silage Maize Rotation System over a Five-Year Period

Soil health is seriously threatened by the overuse of chemical fertilizers in agricultural management. Biogas slurry is often seen as an organic fertilizer resource that is rich in nutrients, and its use has the goal of lowering the amount of chemical fertilizers used while preserving crop yields and soil health. However, the application of continuous biogas slurry has not yet been studied for its long-term impact on soil nutrients and microbial communities in a rotation system of annual ryegrass-silage maize (Zea mays). This study aimed to investigate the impacts on the chemical properties and microbial community of farmland soils to which chemical fertilizer (NPK) (225 kg ha-1), biogas slurry (150 t ha-1), and a combination (49.5 t ha-1 biogas slurry + 150 kg ha-1 chemical fertilizer) were applied for five years. The results indicated that compared to the control group, the long-term application of biogas slurry significantly increased the SOC, TN, AP, and AK values by 45.93%, 39.52%, 174.73%, and 161.54%, respectively; it neutralized acidic soil and increased the soil pH. TN, SOC, pH, and AP are all important environmental factors that influence the structural composition of the soil's bacterial and fungal communities. Chemical fertilizer application significantly increased the diversity of the bacterial community. Variation was observed in the composition of soil bacterial and fungal communities among the different treatments. The structure and diversity of soil microbes are affected by different methods of fertilization; the application of biogas slurry not only increases the contents of soil nutrients but also regulates the soil's bacterial and fungal community structures. Therefore, biogas slurry can serve as a sustainable management measure and offers an alternative to the application of chemical fertilizers for sustainable intensification.

Effect of organic amendments obtained from different pretreatment technologies on soil microbial community

The application of organic amendments (OAs) obtained from biological treatment technologies is a common agricultural practice to increase soil functionality and fertility. OAs and their respective pretreatment processes have been extensively studied. However, comparing the properties of OAs obtained from different pretreatment processes remains challenging. In most cases, the organic residues used to produce OAs exhibit intrinsic variability and differ in origin and composition. In addition, few studies have focused on comparing OAs from different pretreatment processes in the soil microbiome, and the extent to which OAs affect the soil microbial community remains unclear. This limits the design and implementation of effective pretreatments aimed at reusing organic residues and facilitating sustainable agricultural practices. In this study, we used the same model residues to produce OAs to enable meaningful comparisons among compost, digestate, and ferment. These three OAs contained different microbial communities. Compost had higher bacterial but lower fungal alpha diversity than ferment and digestate. Compost-associated microbes were more prevalent in the soil than ferment- and digestate-associated microbes. More than 80% of the bacterial ASVs and fungal OTUs from the compost were detected 3 months after incorporation into the soil. However, the addition of compost had less influence on the resulting soil microbial biomass and community composition than the addition of ferment or digestate. Specific native soil microbes, members from Chloroflexi, Acidobacteria, and Mortierellomycota, were absent after ferment and digestate application. The addition of OAs increased the soil pH, particularly in the compost-amended soil, whereas the addition of digestate enhanced the concentrations of dissolved organic carbon (DOC) and available nutrients (such as ammonium and potassium). These physicochemical variables were key factors that influenced soil microbial communities. This study furthers our understanding of the effective recycling of organic resources for the development of sustainable soils.

Balance of total mass and nitrogen fluxes through consecutive digestate processing steps: Two application cases

The high water content and low nutrient concentration of digestate complicate its storage, transportation, and utilization. Subsequent digestate processing can effectively remove water and influence nutrient partitioning among digestate fractions and final products. The current study was carried out to evaluate the performance of two typical digestate processing chains, solid and liquid ones, respectively, and to give practical recommendations for optimization. Two fully operating biogas plants with advanced heat utilization were considered as data sources. The digestate mass flow balance of dry matter (DM), water, total N (TN), and ammonium-N mass flows was performed and the efficiency of the examined processing units was calculated. It was found that solid-liquid separation of raw digestate shifted 73-87% of TN and 60-93% of NH4-N to the liquid phase. Subsequent drying of separated solid fraction removed about 6% of the initial water and required 84% less thermal energy per kg N recovered than the processing of separated liquid. The final product, pellets, contained 14% of initial TN, but only 2% of initial NH4-N as a result of microbial conversion of inorganic N during drying. Vacuum evaporation of separated liquid fraction removed 34% of the initial water and left a DM-rich concentrate. At the same time, an ammonium sulfate solution (ASS) containing 21% of initial TN and 34% of initial NH4-N was produced. Both evaluated processing chains showed specific advantages and challenges. Solid products were characterized by a high share of recalcitrant organic compounds and could serve as a soil improver. Liquid processing concentrated plant-available N in ASS, which could be used as valuable inorganic fertilizer.

A critical review on retaining antibiotics in liquid digestate: Potential risk and removal technologies

Substantial levels of antibiotics remain in liquid digestate, posing a significant threat to human safety and the environment. A comprehensive assessment of residual antibiotics in liquid digestate and related removal technologies is required. To this end, this review first evaluates the potential risks of the residual antibiotics in liquid digestate by describing various anaerobic digestion processes and their half-lives in the environment. Next, emerging technologies for removing antibiotics in liquid digestate are summarized and discussed, including membrane separation, adsorption, and advanced oxidation processes. Finally, this study comprehensively and critically discusses these emerging technologies' prospects and challenges, including techno-economic feasibility and environmental impacts.

Impacts of Biogas Slurry Fertilization on Arbuscular Mycorrhizal Fungal Communities in the Rhizospheric Soil of Poplar Plantations

The majority of terrestrial plants are symbiotic with arbuscular mycorrhizal fungi (AMF). Plants supply carbohydrates to microbes, whereas AMF provide plants with water and other necessary nutrients-most typically, phosphorus. Understanding the response of the AMF community structure to biogas slurry (BS) fertilization is of great significance for sustainable forest management. This study aimed to look into the effects of BS fertilization at different concentrations on AMF community structures in rhizospheric soil in poplar plantations. We found that different fertilization concentrations dramatically affected the diversity of AMF in the rhizospheric soil of the poplar plantations, and the treatment with a high BS concentration showed the highest Shannon diversity of AMF and OTU richness (Chao1). Further analyses revealed that Glomerales, as the predominant order, accounted for 36.2-42.7% of the AMF communities, and the relative abundance of Glomerales exhibited negligible changes with different BS fertilization concentrations, whereas the order Paraglomerales increased significantly in both the low- and high-concentration treatments in comparison with the control. Furthermore, the addition of BS drastically enhanced the relative abundance of the dominant genera, Glomus and Paraglomus. The application of BS could also distinguish the AMF community composition in the rhizospheric soil well. An RDA analysis indicated that the dominant genus Glomus was significantly positively correlated with nitrate reductase activity, while Paraglomus showed a significant positive correlation with available P. Overall, the findings suggest that adding BS fertilizer to poplar plantations can elevate the diversity of AMF communities in rhizospheric soil and the relative abundance of some critical genera that affect plant nutrient uptake.

Anaerobic co-digestion of Chinese cabbage waste and cow manure at mesophilic and thermophilic temperatures: Digestion performance, microbial community, and biogas slurry fertility

The objective of this study was to investigate the effects of mixing ratios of Chinese cabbage waste (CCW) and cow manure (CM) on methane yields, microbial community, and biogas slurry fertility during anaerobic co-digestion. Batch experiments were conducted at mesophilic and thermophilic temperatures with five different CCW/CM mixing ratios. Methane yields at mesophilic and thermophilic temperatures were 4.2-184.4 mL g^-1 Volatile solids_added (mL g^-1 VS_added) and 11.8-321.7 mL g^-1 VS_added, respectively. The richness and diversity of bacteria and archaea at mesophilic temperatures were higher than those at thermophilic temperatures. Compared with the unfertilized control, the dry weight of corn seedlings with the follow-up application of mesophilic or thermophilic biogas slurry increased by 12.3 %-73.4 % or 16.8 %-43.3 %, respectively. This study demonstrates that thermophilic temperatures are conducive to increasing methane yields, but mesophilic temperatures are conducive to improving the biogas slurry fertility.

Impact of market constraints on the development of small-scale biogas technology in Sub-Saharan Africa: a systematic review

The sustainable production and use of small-scale biogas energy are required to ensure clean household energy access in developing countries, including the Sub-Saharan Africa (SSA) region. This is influenced by market risks, which can be identified as political, economic, social, technical, legal, and environmental (PESTLE). This study examines peer-reviewed and grey literature for the period from 2000 to 2020 to identify the PESTLE constraints and assess their impact on the sustainable development of the technology in the SSA region. The production of biogas with small-scale plants is commonly done by rural and peri-urban households. Results show that economic constraints are the most dominant and reducing at a slow pace. This is followed by political constraints, which have received much attention in the last two decades. Despite the policy improvements, broader national bioenergy policies and interventions are still to make significant gains, especially in the Central African region. In order of significance, the Southern, East, and West Africa regions have made greater progress in reducing the constraints. To achieve the sustainable development of the technology, there is a need to further address the PESTLE constraints at national and regional levels. This study partly deduces that the unsustainable production, use, and inadequate regulation of the small-scale biogas sector are delaying its transition in the SSA region.

Concept for Biomass and Organic Waste Refinery Plants Based on the Locally Available Organic Materials in Rural Areas of Poland

The importance of developing efficient and environmentally friendly means of biomass conversion into bioenergy, biofuels, and valuable products is currently high in Poland. Accordingly, herein, two new energy and biofuel units are proposed, namely, POLpec and POLbp, which are used as reference sources for comparing energy consumption and biofuel production in other countries or regions in the world. One POLpec equals 4400 PJ (195.1 Mtoe), reflecting the annual primary energy consumption of Poland in 2020. Meanwhile, one POLbp equals 42 PJ (1.0 Mtoe), referring to the annual production of biofuels in Poland in 2020. Additionally, a new import–export coefficient β is proposed in the current study, which indicates the relationship between the import and export of an energy carrier. More specifically, the potential of biomass and organic waste to be converted into energy, biofuels, and valuable products has been analysed for the rural areas of Poland. Results show that the annual biomass and organic waste potential is approximately 245 PJ (5.9 Mtoe). Finally, the concept of a biomass and organic waste refinery plant is proposed based on the locally available organic materials in rural areas. In particular, two models of biomass refinery plants are defined, namely, the Input/Output and Modular models. A four-module model is presented as a concept for building a refinery plant at the Institute of Technology and Life Sciences—National Research Institute in Poznan, Poland. The four modules include anaerobic digestion, gasification, transesterification, and alcoholic fermentation. The primary reason for combining different biomass conversion technologies is to reduce the cost of biomass products, which, currently, are more expensive than those obtained from oil and natural gas.

The challenges and perspectives for anaerobic digestion of animal waste and fertilizer application of the digestate

The increase in livestock production creates a serious problem of managing animal waste and by-products. Among the wide range of waste valorization methods available, anaerobic digestion is very promising. It is a form of material recycling that also produces renewable energy in the form of biogas, which is reminiscent of energy recycling. The effluent and digestate from the anaerobic digestion process need to be processed further. These materials are widely used in agriculture due to their composition. Both the liquid and solid fractions of digestate are high in nitrogen, making them a valuable source for plants. Before soil or foliar application, conditioning (e.g., with inorganic acids) and neutralization (e.g., with potassium hydroxide) is required to eliminate odorous compounds and microorganisms. Various methods of conducting the process by anaerobic digestion (use of additives increasing activity of microorganisms, co-digestion, multiple techniques of substrate preparation) and the possibility of controlling process parameters such as optimal C/N ratio (15-30), optimal temperature (psychrophilic (<20 °C), mesophilic (35-37 °C) and thermophilic (55 °C) for microorganism activity ensure high efficiency of the process. Literature data describing tests of various digestates on different plants prove high efficiency, determined by yield increase (even by 28%), nitrogen uptake (by 20%) or phosphorus recovery rate (by 43%) or increase of biometric parameters (e.g., leaf area).

Anaerobic Digestate from Biogas Plants—Nuisance Waste or Valuable Product?

Biogas production in waste-to-energy plants will support the decarbonization of the energy sector and enhance the EU’s energy transformation efforts. Digestates (DG) formed during the anaerobic digestion of organic wastes contain large amounts of nutrients. Their use for plant fertilization allows for diversifying and increasing the economic efficiency of farming activities. However, to avoid regional production surpluses, processing technologies allowing the acquisition of products that can be transported over long distances are required. This study therefore aimed at determining the effect of applied methods of DG treatment on the chemical composition of the resulting products and their effect on the yields and chemical composition of plants. The following digestate-based products (DGBPs) were tested: two different digestates (DGs), their liquid (LF) and solid fractions (SF) and pellets from DGs (PDG), and pellets form SFs (PSF). Results from the experiment show that during SF/LF separation of DGs, >80% of nitrogen and 87% of potassium flows to LFs, whereas >60% of phosphorus and 70% of magnesium flows to SFs. The highest yields were obtained using untreated DGs and LFs. The application of DGs and LFs was not associated with a leaching of nutrients to the environment (apparent nutrients recovery from these products exceeded 100%). Pelletized DG and SF forms can be used as slow-release fertilizer, although their production leads to significant nitrogen losses (>95%) by ammonia volatilization.

Recovering phosphorous from biogas fermentation residues indicates promising economic results

The economics of producing energy-valuable gases by fermenting phytomass is deteriorated by the costs associated with waste management of highly diluted (typically 95% water) fermentation residues (FR). Previously, no better solution was known than to plough FR into the arable land and claim that it is an irrigation with soil improving and fertilizing effect. However, farmers soon realized that FR organic matter is of little agronomic value and nutrients are at agronomically insignificant levels. As FR watering has proved economically irrational in many countries the practice of separating water from the FR and using the solid fraction for energy purposes (such as charcoal) has dominated. However, most nutrients are lost in this way. For the first time it is proposed to activate the charred FR via calcium chloride (whose price is insignificant as it would be used for fertilization purposes anyway) and using the resulting sorbent to capture phosphorus (P) out of the FR's liquid fraction. It is reported for the first time that the activated char is capable of capturing 37.5 ± 4.7 kg P t^-1 whereas the P availability for plant nutrition outperforms FR as well as struvite. In addition, the char demonstrates the potential to improve soil characteristics and the metabolism of soil biota. The cost breakdown and subsequent market analysis indicates that the novel fertilizer shows signs of competitiveness.

Microbiome signature and diversity regulates the level of energy production under anaerobic condition

The microbiome of the anaerobic digester (AD) regulates the level of energy production. To assess the microbiome diversity and composition in different stages of anaerobic digestion, we collected 16 samples from the AD of cow dung (CD) origin. The samples were categorized into four groups (Group-I, Group-II, Group-III and Group-IV) based on the level of energy production (CH₄%), and sequenced through whole metagenome sequencing (WMS). Group-I (n = 2) belonged to initial time of energy production whereas Group-II (n = 5), Group-III (n = 5), and Group-IV (n = 4) had 21-34%, 47-58% and 71-74% of CH₄, respectively. The physicochemical analysis revealed that level of energy production (CH₄%) had significant positive correlation with digester pH (r = 0.92, p < 0.001), O₂ level (%) (r = 0.54, p < 0.05), and environmental temperature (°C) (r = 0.57, p < 0.05). The WMS data mapped to 2800 distinct bacterial, archaeal and viral genomes through PathoScope (PS) and MG-RAST (MR) analyses. We detected 768, 1421, 1819 and 1774 bacterial strains in Group-I, Group-II, Group-III and Group-IV, respectively through PS analysis which were represented by Firmicutes, Bacteroidetes, Proteobacteria, Actinobacteria, Spirochaetes and Fibrobacteres phyla (> 93.0% of the total abundances). Simultaneously, 343 archaeal strains were detected, of which 95.90% strains shared across four metagenomes. We identified 43 dominant species including 31 bacterial and 12 archaeal species in AD microbiomes, of which only archaea showed positive correlation with digester pH, CH₄ concentration, pressure and temperature (Spearman correlation; r > 0.6, p < 0.01). The indicator species analysis showed that the species Methanosarcina vacuolate, Dehalococcoides mccartyi, Methanosarcina sp. Kolksee and Methanosarcina barkeri were highly specific for energy production. The correlation network analysis showed that different strains of Euryarcheota and Firmicutes phyla exhibited significant correlation (p = 0.021, Kruskal-Wallis test; with a cutoff of 1.0) with the highest level (74.1%) of energy production (Group-IV). In addition, top CH₄ producing microbiomes showed increased genomic functional activities related to one carbon and biotin metabolism, oxidative stress, proteolytic pathways, membrane-type-1-matrix-metalloproteinase (MT1-MMP) pericellular network, acetyl-CoA production, motility and chemotaxis. Importantly, the physicochemical properties of the AD including pH, CH₄ concentration (%), pressure, temperature and environmental temperature were found to be positively correlated with these genomic functional potentials and distribution of ARGs and metal resistance pathways (Spearman correlation; r > 0.5, p < 0.01). This study reveals distinct changes in composition and diversity of the AD microbiomes including different indicator species, and their genomic features that are highly specific for energy production.

Improving sustainability and mitigating environmental impacts of agro-biowaste compost fertilizer by pelletizing-drying

The use of agro-biowaste compost fertilizers in agriculture is beneficial from technical, financial, and environmental perspectives. Nevertheless, the physical, mechanical, and agronomical attributes of agro-biowaste compost fertilizers should be engineered to reduce their storage, handling, and utilization costs and environmental impacts. Pelletizing and drying are promising techniques to achieve these goals. In the present work, the effects of process parameters, including compost particle size/moisture content, pelletizing compression ratio, and drying air temperature/velocity, were investigated on the density, specific crushing energy, and moisture diffusion of agro-biowaste compost pellet. The Taguchi technique was applied to understand the effects of independent parameters on the output responses, while the optimal pellet properties were found using the iterative thresholding method. The soil and plant (sweet basil) response to the optimal biocompost pellet was experimentally evaluated. The farm application of the optimal pellet was also compared with the untreated agro-biowaste compost using the life cycle assessment approach to investigate the potential environmental impact mitigation of the pelletizing and drying processes. Generally, the compost moisture content was the most influential factor on the density and specific crushing energy of the dried pellet, while the moisture diffusion of the wet pellet during the drying process was significantly influenced by the pelletizing compression ratio. The density, specific crushing energy, and moisture diffusion of agro-biowaste compost pellet at the optimal conditions were 1242.49 kg/m³, 0.5054 MJ/t, and 8.2 × 10^-8 m²/s, respectively. The optimal biocompost pellet could release 80% of its nitrogen content evenly over 98 days, while this value was 28 days for the chemical urea fertilizer. Besides, the optimal pellet could significantly improve the agronomical attributes of the sweet basil plant compared with the untreated biocompost. The applied strategy could collectively mitigate the weighted environmental impact of farm application of the agro-biowaste compost by more than 63%. This reduction could be attributed to the fact that the pelletizing-drying processes could avoid methane emissions from the untreated agro-biowaste compost during the farm application. Overall, pelletizing-drying of the agro-biowaste compost could be regarded as a promising strategy to improve the environmental and agronomical performance of farm application of organic biofertilizers.

Indicative Marker Microbiome Structures Deduced from the Taxonomic Inventory of 67 Full-Scale Anaerobic Digesters of 49 Agricultural Biogas Plants

There are almost 9500 biogas plants in Germany, which are predominantly operated with energy crops and residues from livestock husbandry over the last two decades. In the future, biogas plants must be enabled to use a much broader range of input materials in a flexible and demand-oriented manner. Hence, the microbial communities will be exposed to frequently varying process conditions, while an overall stable process must be ensured. To accompany this transition, there is the need to better understand how biogas microbiomes respond to management measures and how these responses affect the process efficiency. Therefore, 67 microbiomes originating from 49 agricultural, full-scale biogas plants were taxonomically investigated by 16S rRNA gene amplicon sequencing. These microbiomes were separated into three distinct clusters and one group of outliers, which are characterized by a specific distribution of 253 indicative taxa and their relative abundances. These indicative taxa seem to be adapted to specific process conditions which result from a different biogas plant operation. Based on these results, it seems to be possible to deduce/assess the general process condition of a biogas digester based solely on the microbiome structure, in particular on the distribution of specific indicative taxa, and without knowing the corresponding operational and chemical process parameters. Perspectively, this could allow the development of detection systems and advanced process models considering the microbial diversity.

Supporting decision making to achieve circularity via a biodegradable waste-to-bioenergy and compost facility

Bioproducts, such as energy and fertilizers, are strongly interrelated with the biodegradable waste treatment processes, within a holistic management strategy. Although different forms of biological treatment technologies are available, anaerobic digestion represents a process of major importance in the overall management strategy of biodegradable waste. This paper presents a methodology to support decision making for efficient management of biodegradable waste. The decision support framework provides the background towards the selection and design of a biodegradable waste installation with emphasis on the recovery of energy and organic fertilizer. The discrete steps are analytically defined and illustrated to assist managers and policy makers to organize their decision making in the whole spectrum of procedures required to promote sustainable biodegradable waste management programs. The methodological approach developed can be generically applied by public authorities, producers and stakeholders following essential basic steps regarding safe and environmentally friendly production of high-quality final product. Moreover, a demonstration is performed for a real-case study for the Region of Serres, Greece. The proposed installation is expected to manage 3,285 t of biodegradable waste and generate approximately 160,000 m³/a of biogas, 400 MWh_el/a and 450 MWh_thermal/a. The final bioproduct exceeds 3 kt of digestate that will be valorized in arable land close to the installation. Crucial interactions and managerial insights are also highlighted. The decision support framework aims to assist the research community, the private sector and decision makers to produce affordable and sustainable compost/digestate recovered from waste, also supporting the transition to a low carbon future and sustainable -circular- development.

Phytotoxicity of hydrochars obtained by hydrothermal carbonization of manure-based digestate

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.

Correlations between the Composition of Liquid Fraction of Full-Scale Digestates and Process Conditions

Fast development of centralized agricultural biogas plants leads to high amounts of digestate production. The treatment and disposal of liquid fractions after on-site digestate solid–liquid separation remains problematic due to their high organic, nutrient and aromatic contents. This work aims to study the variability of the remaining compounds in the digestate liquid fractions in relation to substrate origin, process parameters and solid–liquid separation techniques. Twenty-nine digestates from full-scale codigestion biogas plants and one waste activated sludge (WAS) digestate were collected and characterized. This study highlighted the combined effect of the solid–liquid separation process and the anaerobic digestion feedstock on the characteristics of liquid fractions of digestates. Two major clusters were found: (1) liquid fractions from high efficiency separation process equipment (e.g., centrifuge and others with addition of coagulant, flocculent or polymer) and (2) liquid fractions from low efficiency separation processes (e.g., screw press, vibrating screen and rotary drum), in this latter case, the concentration of chemical oxygen demand (COD) was associated with the proportion of cow manure and energy crops at biogas plant input. Finally, SUVA254, an indicator for aromatic molecule content and the stabilization of organic matter, was associated with the hydraulic retention time (HRT).

Biogas Plant Exploitation in a Middle-Sized Dairy Farm in Poland: Energetic and Economic Aspects

Although cow manure is a valuable natural fertilizer, it is also a source of extreme greenhouse gas emissions, mainly methane. For this reason, this study aims to determine the impact of investments in a biogas plant on the energy and economic aspects of the operation of a dairy farm. A farm with a breeding size of 600 livestock units (LSU) was adopted for the analysis. In order to reach the paper’s aim, the analysis of two different scenarios of dairy farm functioning (conventional–only milk production, and modern–with biogas plant exploitation) was conducted. The analysis showed that the investment in biogas plant operations at a dairy farm and in using cow manure as one of the main substrates is a more profitable scenario compared to traditional dairy farming. Taking into account the actual Polish subsidies for electricity produced by small biogas plants, the scenario with a functioning biogas plant with a capacity of 500 kW brings €332,000/a more profit compared to the conventional scenario, even when taking into account additional costs, including the purchase of straw to ensure a continuous operation of the installation. Besides, in the traditional scenario, building a biogas plant allows for an almost complete reduction of greenhouse gas emissions during manure storage.

Microbiome Diversity and Community-Level Change Points within Manure-based small Biogas Plants

Efforts to integrate biogas plants into bioeconomy concepts will lead to an expansion of manure-based (small) biogas plants, while their operation is challenging due to critical characteristics of some types of livestock manure. For a better process understanding, in this study, three manure-based small biogas plants were investigated with emphasis on microbiome diversity. Due to varying digester types, feedstocks, and process conditions, 16S rRNA gene amplicon sequencing showed differences in the taxonomic composition. Dynamic variations of each investigated biogas plant microbiome over time were analyzed by terminal restriction fragment length polymorphism (TRFLP), whereby nonmetric multidimensional scaling (NMDS) revealed two well-running systems, one of them with a high share of chicken manure, and one unstable system. By using Threshold Indicator Taxa Analysis (TITAN), community-level change points at ammonium and ammonia concentrations of 2.25 g L^-1 and 193 mg L^-1 or volatile fatty acid concentrations of 0.75 g L^-1were reliably identified which are lower than the commonly reported thresholds for critical process stages based on chemical parameters. Although a change in the microbiome structure does not necessarily indicate an upcoming critical process stage, the recorded community-level change points might be a first indication to carefully observe the process.