Liquid Fraction Of Digestate Research Articles

Digestate in replacement of synthetic fertilisers: A comparative 3–year field study of the crop performance and soil residual nitrates in West-Flanders

Nitrogen (N) is an essential macronutrient for plant growth. As a widespread source of plant-available N, ammonia synthesis via the Haber-Bosch process has proven an extremely valuable commodity in farming systems since the middle of the twentieth century. However, its heavy reliance on ever-shrinking fossil fuel reserves and its sizeable carbon footprint have fostered the exploration of alternative, more sustainable, fertilising prospects. Through the recycling and reuse of nutrient byproducts, biobased fertilisers (BBF) can help reduce the European Union’s dependency on imported synthetic fertilisers. In this study, we examined digestate, the liquid fraction of digestate, pig slurry and pig urine as potential substitutes for synthetic fertilisers. In a full-scale field approach using a different crop each year (maize, spinach, potatoes), the agronomic performance of the treatments (defined as the crop N uptake and the crop yield) and the environmental performance (taken as the residual soil nitrates after harvest) of the BBF treatments were compared with those of a synthetic fertiliser benchmark (calcium ammonium nitrate) at three N regimes. As regards short-term fertilising capability, results showed that yields obtained from BBFs were not statistically different (p > 0.05) than those obtained with synthetic fertilisers. Likewise, for soil residual nitrates (0–90 cm), measured in October–November of each year, no difference (p > 0.05) was detected between the BBFs and the synthetic fertiliser reference treatments. However, the non-superiority test showed that some BBFs tended to perform better in terms of residual nitrates than the synthetic regimes. Generally, results pointed to a fast N release ability of the BBFs, indicated by the presence of nitrates at different soil depths. Hence, as with the mineral fertiliser, BBFs were prone to leaching which calls for adequate N management strategies. The N content of some BBFs were shown to vary over time, hence adequate and timely nutrient characterisations must be carried out prior to field application to ensure a more accurate N accountancy and reduce risks of over-fertilisation (or under-fertilisation).

Purification of Liquid Fraction of Digestates from Different Origins-Comparison of Polymeric and Ceramic Ultrafiltration Membranes Used for This Purpose.

Circular economy, clean technologies, and renewable energy are key to climate protection and modern environmental technology. Recovering water and valuable minerals from the liquid fraction of digestate is in line with this strategy. Digestate, a byproduct of anaerobic methane fermentation in biogas plants, is a potential source of water, minerals for fertilizers, and energy rather than waste. This study examined digestate from municipal and agricultural biogas plants and highlights the need for research on both due to their differences. The use of membrane techniques for water recovery from liquid digestate offers an innovative alternative to conventional methods. This study used standalone membrane filtration and an integrated system to produce water suitable for agricultural use. Ceramic membranes with cut-offs of 1, 5, 15, and 50 kDa and polymeric membranes of polyethersulfone and regenerated cellulose with cut-offs of 10 and 30 kDa were tested. The results showed that the membrane material significantly affects the transport and separation properties. Higher cut-off values increased permeate flux across all membranes. Ceramic membranes were more susceptible to fouling in standalone ultrafiltration, but were more effective in purifying digestate than polymeric membranes. The best results were obtained with a ceramic membrane with a 1 kDa cut-off (for example, for the integrated process and the municipal digestate, the retention rates of COD, BOD5 and DOC were 69%, 62%, and 75%, respectively).

Bio-physical pre-treatments in anaerobic digestion of organic fraction of municipal solid waste to optimize biogas production and digestate quality for agricultural use

This study optimized the anaerobic digestion (AD) of separated collected organic fractions of municipal solid waste (OFMSW) to produce energy and digestate as biofertilizer. Due to OFMSW’s partial recalcitrance to degradation, enzymatic (UPP2, MCPS, USC4, USE2, A. niger) and physical (mechanical blending, heating, hydrodynamic cavitation) pre-treatments were tested. Experimental and modeling approaches were used to compare AD performance regarding energy sustainability and digestate quality. Digestate was separated into solid and liquid fractions, and then chemically and physically characterized by investigating the nutrient release mechanisms. Principal Component Analysis was applied, equally weighing energy and digestate productions. Unlike previous studies focusing only on biogas, this study evaluated the effects of pre-treatments on both biogas and digestate production, viewing AD as a biorefinery process for urban waste valorization. Results showed that all pre-treatments were energetically sustainable, but enzymatic pre-treatments yielded digestates richer in nutrients (increase of 80% N, 200% P and 150% K as compared to OFMSW) and with greater organic matter degradation compared to physical pre-treatments. The liquid fraction of digestate from enzymatic pre-treatments had higher nutrient concentrations, while those from physical pre-treatments had more balanced nutrient content, making them more suitable for fertigation.

Solid and Liquid Fraction of Digestate as an Alternative Mineral Nitrogen Source: Two-Year Field Research in Croatia

Solid and Liquid Fraction of Digestate as an Alternative Mineral Nitrogen Source: Two-Year Field Research in Croatia

An Integrated Process of Struvite Precipitation/Membrane Filtration Using Flat Ceramic Membranes Is an Effective Method for the Treatment of Liquid Fraction Digestate from a Municipal Biogas Plant

One method of processing municipal waste biogas plant digestate is to separate it into solid and liquid fractions. Since the digestate can be a potential source of water, it must undergo the appropriate treatment. Pressurised membrane processes preceded by struvite precipitation can be particularly useful in this regard. Experiments were conducted to determine the effectiveness of treating the digestate liquid fraction from a municipal waste biogas plant using an integrated process that combines struvite precipitation with membrane filtration, employing flat ceramic membranes with different cut-off values. The results confirm that this integrated process is effective for digestate treatment. A significantly increased improvement in the final quality of the test solution and a reduction in membrane fouling intensity were observed compared to those of these processes conducted separately. It is noteworthy that the purest solution was obtained when struvite precipitation and filtration through a flat ceramic membrane with a cut-off of 1 kDa were combined. This approach enabled the precipitation of struvite, a valuable fertiliser; the protection of the membranes from fouling; and a high degree of organic compound removal. The recovered water from the digestate (after dilution or removal of excess salts) can be used in agriculture or horticulture.

A comparative analysis of various nanocomposites for the remediation of heavy metals from biomass liquid digestate using multi-criteria decision methods

In the past, biomass-based waste sources have demonstrated their potential for remediating heavy metals in contaminated soils and water. Nanoparticles have been extensively studied for their ability to absorb and recover heavy metals from the liquid portion of biomass digestate, contributing to a circular economy in biomass plants. By combining nanocomposites with the liquid fraction of biomass digestate, the efficiency and effectiveness of heavy metal remediation using the Multi-Criteria Decision Making (MCDM) technique can be enhanced. The objective of this study is to evaluate and compare different biomass-based nanocomposites for heavy metal remediation using the Analytical Hierarchy Process (AHP) and Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) methods. The AHP method prioritizes and ranks alternatives based on criteria such as heavy metal removal efficiency, production cost, and potential environmental impacts. The investigation reveals that porosity value is the most important criterion (48 %) compared to other parameters, while surface area has the lowest importance (6 %). The TOPSIS method uses distance measures to rank alternatives based on their proximity to the ideal solution. Among the tested nanocomposites, zinc oxide demonstrates optimum performance with removal rates of 92 % for copper, 95 % for lead, and 88 % for nickel. Additionally, an economic analysis of the system reveals a return on investment of approximately 5.9 %. In conclusion, this study presents a sustainable, cost-effective, environmentally friendly, and efficient solution for mitigating heavy metal pollution.

Removal of metals and emergent contaminants from liquid digestates in constructed wetlands for agricultural reuse.

Given the increasing pressure on water bodies, it is imperative to explore sustainable methodologies for wastewater treatment and reuse. The simultaneous presence of multiples contaminants in complex wastewater, such as the liquid effluents from biogas plants, can compromise biological treatment effectiveness for reclaiming water. Vertical subsurface flow constructed wetlands were established as low-cost decentralized wastewater treatment technologies to treat the liquid fraction of digestate from municipal organic waste with metals, antibiotics, and antibiotic resistance genes, to allow its reuse in irrigation. Twelve lab-scale planted constructed wetlands were assembled with gravel, light expanded clay aggregate and sand, testing four different treating conditions (liquid digestate spiked with oxytetracycline, sulfadiazine, or ofloxacin, at 100 μg/ L, or without dosing) during 3 months. Physicochemical parameters (pH, chemical oxygen demand (COD), nutrients, metals, and antibiotics), the microbial communities dynamics (through 16S high-throughput sequencing) and antibiotic resistance genes removal (qPCR) were monitored in influents and effluents. Systems removed 85.8%-96.9% of organic matter (as COD), over 98.1% of ammonium and phosphate ions, and 69.3%-99.4% of nitrate and nitrite ions, with no significant differences between the presence or absence of antibiotics. Removal of Fe, Mn, Zn, Cu, Pb and Cr exceeded 82% in all treatment cycles. The treatment also removed oxytetracycline, sulfadiazine and ofloxacin over 99%, and decreased intl1, tetA, tetW, sul1 and qnrS gene copies. Nonetheless, after 3 months of ofloxacin dosing, qnrS gene started being detected. Removal processes relied on high HRT (14 days) and various mechanisms including sorption, biodegradation, and precipitation. Microbial community diversity in liquid digestate changed significantly after treatment in constructed wetlands with a decrease in the initial Firmicutes dominance, but with no clear effect of antibiotics on the microbial community structure. Removals above 85% and 94% were observed for Streptococcus and Clostridium, respectively. Results suggest that vertical subsurface flow constructed wetlands were a suitable technology for treating the liquid digestate to reuse it in irrigation agricultural systems, contributing to the circular bioeconomy concept. However, a more profound understanding of effective wastewater treatment strategies is needed to avoid antibiotic resistance genes dissemination.

The Effects of Liquid Biogas Digestate on Yield and Mineral Nutrition of Cucumber Growing in Greenhouse

This study aimed to investigate the effects of a liquid fraction of digestate obtained from different biogas plants on the growth and mineral nutrition of cucumber plants under greenhouse conditions. For this purpose, Liquid Biogas Digestates (LBD) obtained from two different plants (A-B) with different properties were applied to pots with 10 kg of soil in 5 different doses (0, 20, 40, 60, and 80 t ha-1) and the effects of the treatments were observed. As a result of the research, the highest yields increased 24.6% for digestate A in A5 (80 t ha-1) and 29% for digestate B in B3 (40 t ha-1) compared to control. While LBD contributed to the increase of N, Ca, Zn, Cu, and Mn concentrations in the leaf samples, it was observed that the dose increase did not have a linear effect on N, Ca, Zn, Cu, and Mn concentrations in the leaf samples. It is thought that liquid biogas wastes produced in biogas plants have positive effects on fruit yield, agricultural practices can be taken as the basis for the disposal of these wastes and the use of liquid biogas residues in soils by eliminating potential risks can provide significant benefits.

Enhanced hydrolysis and acidification of corn straw via liquid fraction of digestate: Environmental adaptability and microbial mechanisms

Anaerobic digestion (AD) of abandoned straws is an eco-friendly approach to realize its recycling. However, the low hydrolysis rate of lignocellulose poses a hindrance to achieving efficient AD. In this study, the enhanced hydrolysis and acidification of corn straw (CS) utilizing the liquid fraction of digestate (LFD) was investigated across varying solid-to-liquid inoculum ratio (SIR), pH levels, and temperature conditions. The succession patterns of the microbial community, driven by different parameters, were simultaneously identified. The findings revealed that the optimal yield of volatile fatty acids (VFAs) was observed under conditions with an SIR of 4, pH of 10, and a temperature of 45℃, predominantly dominated by acetic acid. And both pH and temperature were not conducive to VFAs accumulation in extreme environments. Comparatively, changes in pH had a greater impact on the acid production of the LFD-enhanced system and was also the primary environmental factor leading to differentiation in microbial community structure. Although Firmicutes was an absolutely dominant bacterium at the phylum level, the abundances of Caproiciproducens and Clostridum_sensu_stricto_1 at the genus level, mainly transforming complex organic substances into simpler metabolic byproducts, were greatly sensitive to the variations in SIR, pH, and temperature. This study could have practical implications for guiding improvements in large-scale AD systems with lignocellulosic straws.

Enhancing nutrient recovery from food waste anaerobic digestate

The study synthesised the raw liquid fraction of digestate into a nutrient rich solid digestate through acidification whilst preventing nitrogen loss through ammonium volatilisation during evaporation. To stabilise ammonium in the digestate, it was acidified with sulphuric, nitric, and phosphoric acid to produce solid digestate with ammonium sulphate, ammonium nitrate and ammonium phosphate, respectively. These treatments were compared against urea ammonium nitrate, raw digestate, and unacidified solid digestate. To evaluate the effect of these transformed digestate products in soil, a plant growth experiment (Kikuyu; Cenchrus clandestinus) was conducted, and characterised, plant growth, soil chemistry, and rhizosphere bacterial communities. Plant growth was enhanced by all digestate treatments compared to control and urea ammonium nitrate. Ammonium phosphate solid digestate plant growth was significantly higher than all other acidified treatments due to the high P content. Moreover, digestate-amended soil had elevated Proteobacteria and putative denitrification genes.

Pollutants abatement in aqueous solutions with geopolymer catalysts: A photo fenton case

Environmental pollution is a serious threat to human health and the natural environment, and it has aroused widespread concern. One of the most effective processes in the removal of pollutants from wastewater is the Fenton reaction. This process is based on the production of highly reactive •OH radicals due to the rapid reaction between Iron ions and hydrogen peroxide under acidic conditions.The hydroxyl radical has a high oxidation potential of E°(•OH/H2O) = 2.8 V/SHE at acidic pH, so they are extremely reactive and non-selective oxidizing agent towards organic contaminants in wastewater.In order to avoid the drawbacks of a standard Fenton reaction, a photo Fenton reaction has been tested working at neutral pH in water in the removal of refractory pollutants. For the first time, a heterogeneous system was experimented, impregnating porous metakaolin-based geopolymers, obtained by using hydrogen peroxide and vegetable oil in different ratios, as foaming agents, with iron working as photocatalyst. The dirty wastewater as scrubber water (SCRW) and liquid fraction of digestate (LFD) were tested obtaining 40–90% abatement of Total Carbon Content.

Ultrafiltration of digestate liquid fraction by hollow-fiber membranes: Influence of digestate pre-treatment on hydraulic capacity and nutrient removal efficiency

With the growing interest in anaerobic digestion as a processing method of agro-food waste, proper disposal of digestate has become a prominent problem. In the study, ultrafiltration of the sugar beet pulp digestate was conducted using hollow-fiber membranes with a cut-off of 150 kDa operating in an outside-in system. The effect of different pre-treatment strategies of digestate i.e. centrifugation alone or in combination with coagulation with polyaluminium chloride, biocoagulation with chitosan and flocculation with polyacrylamide on mitigation of membrane fouling and permeate quality was investigated. High ultrafiltration performance was obtained using centrifugation followed by flocculation with 416 mg/dm3 of polyacrylamide as digestate pre-treatment. Removal efficiencies of chemical oxygen demand (COD), total solids, phosphorus and nitrogen were 89.80, 90.08, 88.37 and 64.12%, respectively. Permeate flux and total membrane resistance averaged 22.33 dm3/m2/h and 5.32 • 1011 m−1. Hydraulic parameters and removal rates determined during ultrafiltration preceded by centrifugation and biocoagulation with 100 mg/dm3 of chitosan did not differ significantly from the values mentioned above for the pre-treatment consisting of centrifugation and flocculation with polyacrylamide. The results indicate the possibility of filtering the liquid fraction obtained by centrifugation, coagulation or flocculation of the digestate, and using natural auxiliary agents during its pre-treatment.

Agronomic and environmental performance of 6 energy crops in a close loop with digestate fertilization

ABSTRACT This study examines the growth performances, nutrient uptake and potential methane yield of six energy crops fertilized with the digestate liquid fraction (DLF) with or without arbuscular mycorrhizal fungi (AMF) inoculation. The 3-year-experiment involved the four perennial (Arundo donax L. Miscanthus × giganteus Greef et Deu, Heliantus tuberosus L. Lolium perenne L.) and two annual (Zea mays L. and Sorghum bicolor (L.) Moench × sudanense Stapf.) crops. The cumulative highest yield was observed for A. donax (128.1 ± 8.4 Mg ha−1), followed by M. × giganteus and S. bicolor (83.0 ± 8.0 Mg ha−1), H. tuberosus and Z. mays (66.2 ± 7.7 Mg ha−1) and L. perenne (31.4 ± 2.9 Mg ha−1). A. donax showed the highest estimated methane yield per hectare (11.030 Nm3 ha−1). After 3 years, the fertilization with DLF increased the initial soil Na+ content by 76.9%. During the experiment, only temporary and marginal effects were shown by AMF inoculation, which did not affect biomass production by any crop. Interestingly, AMF significantly increased the NO3-N concentration (+70%) while it reduced the NH4-N concentration in percolation water (−32.8%). DLF could be considered a viable organic fertilizer for biomass production, but the soil Na+ concentration should be carefully monitored.

Dynamics of soil nitrogen and N-cycling-related genes following the application of biobased fertilizers

Biobased fertilizers recovered from animal manure are sustainable substitutes for synthetic mineral nitrogen (N) fertilizers, showing high potential to minimize environmental pollution while maintaining nutrient supply. This study investigated the response of maize (Zea mays L.) and soil microbes to the application of biobased fertilizers, i.e., pig manure (PM), the liquid fraction of digestate (LFD), and evaporator concentrate (EVA) in replacement of calcium ammonium nitrate (CAN) in a 56-day pot experiment. Apart from maize plant growth and soil chemical properties, the abundance of N-cycling-related genes was determined by destructive sampling on days 8, 16, and 56 after fertilization. The detected gene copies of bacterial and archaeal amoA in the soil were significantly increased by N fertilization. Relatively high NH4+-N concentrations (37.5–62.5 mg kg−1 soil dry weight) applied in this experiment may have promoted gaseous N losses via nitrifier nitrification and nitrifier denitrification shortly (8–16 days) after fertilization. Consequently, net N loss was observed in all the fertilized treatments, however, biobased fertilizers resulted in lower N loss as compared to CAN. The presence of maize plants also reduced the N loss, probably driven by the continuous NO3−-N uptake which reduced the N source for denitrification. Overall, the application of PM and LFD revealed no significant difference with CAN regarding either plant growth or soil biochemical properties. Whereas the EVA application resulted in lower biomass and nutrient uptake in the young maize plant compared to other treatments, probably attributed to salt stress due to the imbalanced ratio of N and Na in this product.

Towards efficient N cycling in intensive maize: role of cover crops and application methods of digestate liquid fraction

AbstractDigestate, a by‐product of biogas production, is widely recognized as a promising renewable nitrogen (N) source with high potential to replace synthetic fertilizers. Yet, inefficient digestate use can lead to pollutant N losses as ammonia (NH3) volatilization, nitrous oxide (N2O) emissions and nitrate () leaching. Cover crops (CCs) may reduce some of these losses and recycle the N back into the soil after incorporation, but the effect on the N balance depends on the CC species. In a one‐year field study, we tested two application methods (i.e., surface broadcasting, BDC; and shallow injection, INJ) of the liquid fraction of separated co‐digested cattle slurry (digestate liquid fraction [DLF]), combined with different winter cover crop (CC) options (i.e., rye, white mustard or bare fallow), as starter fertilizer for maize. Later, side‐dressing with urea was required to fulfil maize N‐requirements. We tested treatment effects on yield, N‐uptake, N‐use efficiency parameters, and N‐losses in the form of N2O emissions and leaching. CC development and biomass production were strongly affected by their contrasting frost tolerance, with spring‐regrowth for rye, while mustard was winter killed. After the CCs, injection of DLF increased N2O emissions significantly compared with BDC (emission factor of 2.69% vs. 1.66%). Nitrous oxide emissions accounted for a small part (11%–13%) of the overall yield‐scaled N losses (0.46–0.97 kg N Mg grain−1). The adoption of CCs reduced fall leaching, being 51% and 64% lower for mustard and rye than under bare soil. In addition, rye reduced leaching during spring and summer after termination by promoting N immobilization, thus leading to −57% lower annual leaching losses compared with mustard. DLF application method modified N‐loss pathways, but not the cumulative yield‐scaled N losses. Overall, these insights contribute to inform an evidence‐based design of cropping systems in which nutrients are recycled more efficiently.

Insights into the microbial community structure in the biodegradation process of high-strength ammonia digestate liquid fraction in conventional activated sludge system

Biodegradation of digestate liquid fraction was performed in the activated sludge system with acetic acid, flume water, and molasses as external carbon sources. High-throughput sequencing was used to gain in-depth insight into the activated sludge microbial community. The type and amount of carbon source in influent (COD/TN ratio) significantly influenced microbial community structure, especially at the genus level, and thus the biodegradation performance of digestate liquid fraction. The highest total nitrogen and chemical oxygen demand removal efficiencies averaging 85.3% and 88.3%, respectively, were achieved in series with acetic acid and flume water and COD/TN ratio of 10.7 and 11.2, respectively. The microbial diversity in these series averaged at 3.08 and 3.65. The dominant bacteria at the phylum level in series with acetic acid were Proteobacteria and Bacteroidota, and at the genus level Azospira, while in series with flume water they were Bacteroidota and Firmicutes, and Macellibacteroides, respectively.

Phosphorus recovery as struvite and hydroxyapatite from the liquid fraction of municipal sewage sludge with limited magnesium addition.

Phosphorus (P) is an essential element to produce feed and fertilizers but also a nonrenewable resource. Both the predicted exhaustion of phosphatic rocks and the risk of eutrophication lead to an increasing necessity for P recovery methodologies to be applied in municipal wastewater treatment plants (WWTPs). One of the most promising solutions involves the precipitation of P-based minerals reusable as slow-release fertilizers. In this study, P recovery as struvite and hydroxyapatite from a municipal WWTP digestate liquid fraction (centrate) was investigated at varying pH (8-10), reagent typologies (MgCl2 , NaOH, Ca(OH)2 , and CaCl2 ), and concentrations under limiting magnesium doses through liquid- and solid-phase analyses and thermodynamical modeling. A maximum P recovery of 87.3% was achieved at pH 9 by adding NaOH and MgCl2 at a dose of 656mg/L (the higher tested). According to these data, it was estimated that 92.0 tons/year of struvite and 33.2 tons/year of hydroxyapatite could be recovered from the WWTP centrate with a cost for reagent consumption being almost 50% of the mean P market value. An increase in P precipitation was observed while comparing experiments with the same pH values but with a higher Mg2+ dose. Ca2+ addition led to extensive P precipitation but mainly as amorphous phases that interfere with struvite formation.

The effect of fertilization with microfiltered liquid digestate on the quality parameters of Citrus fruits

Nowadays, the adoption of sustainable agricultural practices, including the reduction of synthetic fertilizers, has become a challenge for the agriculture sector. In this experimental work, the effect of the liquid fraction of digestate (by-product of the anaerobic digestion process) as a fertilizer was evaluated. The aim of the research was to verify to which extent digestate can affect growth and quality parameters of orange fruits, comparing the results to those obtained for fruits grown on soil treated with conventional mineral fertilizers. To assess the effectiveness of the treatments, different qualitative and quantitative parameters of Citrus fruits were measured. In particular, the results showed slight differences between the two treatments, suggesting that digestate may be used for the production of high-quality fruits. Moreover, in some orchards, the Citrus fruits of the plants treated with digestate showed a higher concentration of health-promoting compounds, such as vitamin C, flavonoids, phenolic content, when compared to the control group. Thus, digestate can be considered an optimal source of plant nutrients and can be used as a crop growth promoter, since it represents an effective strategy for reducing the mineral fertilizers input.

Positive effect of Ca addition on the risk of Cu and Zn in digestate as biofertilizer

The addition of Ca in anaerobic digestion will passivate the heavy metals in digestate. The effects of CaCl2, Ca(OH)2 and CaCO3 addition on the anaerobic digestion process and the speciation changes of Cu and Zn in solid fraction of anaerobic digestate (SFD) after digestion were studied. Then the effects of Ca-containing solid fraction digestate (SFD-Ca) application on the speciation changes of Cu and Zn in soil and the effects of ryegrass on the enrichment of Cu and Zn were also explored through pot experiments. The results indicated that: The addition of Ca(OH)2 was beneficial to the transfer of Cu and Zn from the liquid fraction of anaerobic digestate (LFD) to SFD, and the transfer effect was the best. The transfer amount of Cu and Zn increased by about 10.0% and 4.2%, respectively, compared with the blank group. The SFD-Ca promoted the increase of biomass, inhibited the enrichment of Cu and Zn by ryegrass, and the effect of a high concentration of Ca(OH)2 was the best. Ryegrass biomass increased by 19.7%, and the enrichment of Cu and Zn decreased by 39.3% and 31.1%, respectively. High concentration of Ca(OH)2-containing solid fraction digestate (SFD-Ca(OH)2) had the best passivation effect on Cu and Zn in soil, the residual state of Cu and Zn in soil increased by 4.3% and 7.8%, respectively. Adding a high concentration of Ca(OH)2 had a positive effect on ryegrass biomass and reduced the risk of Cu and Zn when SFD was used as biofertilizer.

Digestate from Agricultural Biogas Plants as a Reservoir of Antimicrobials and Antibiotic Resistance Genes-Implications for the Environment.

Antimicrobials and antibiotic resistance genes (ARGs) in substrates processed during anaerobic digestion in agricultural biogas plants (BPs) can reach the digestate (D), which is used as fertilizer. Antimicrobials and ARGs can be transferred to agricultural land, which increases their concentrations in the environment. The concentrations of 13 antibiotics in digestate samples from biogas plants (BPs) were investigated in this study. The abundance of ARGs encoding resistance to beta-lactams, tetracyclines, sulfonamides, fluoroquinolones, macrolide-lincosamide-streptogramin antibiotics, and the integrase genes were determined in the analyzed samples. The presence of cadmium, lead, nickel, chromium, zinc, and mercury was also examined. Antimicrobials were not eliminated during anaerobic digestion. Their concentrations differed in digestates obtained from different substrates and in liquid and solid fractions (ranging from 62.8 ng/g clarithromycin in the solid fraction of sewage sludge digestate to 1555.9 ng/L doxycycline in the liquid fraction of cattle manure digestate). Digestates obtained from plant-based substrates were characterized by high concentrations of ARGs (ranging from 5.73 × 102 copies/gDcfxA to 2.98 × 109 copies/gDsul1). The samples also contained mercury (0.5 mg/kg dry mass (dm)) and zinc (830 mg/kg dm). The results confirmed that digestate is a reservoir of ARGs (5.73 × 102 to 8.89 × 1010 copies/gD) and heavy metals (HMs). In addition, high concentrations of integrase genes (105 to 107 copies/gD) in the samples indicate that mobile genetic elements may be involved in the spread of antibiotic resistance. The study suggested that the risk of soil contamination with antibiotics, HMs, and ARGs is high in farms where digestate is used as fertilizer.