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Given the growth in the number of biogas power plants and the increase in the generation of waste from energy production, it is relevant to study the sustainable nature of this waste. Digestate is a product of the anaerobic digestion process, and is a valuable bio-fertilizer containing organic matter and nutrients necessary for agricultural plants’ growth. The study showed that different rates of liquid and solid phases of anaerobic digestate influenced the contents of carbon and nitrogen in genetically young soil in alluvial deposits—Fluvisol. The application of solid digestate (SD) considerably increased soil organic carbon content (SOC) in the 0–10 cm soil layer; however, SOC did not reach the 20–30 cm layer. Liquid digestate (LD) significantly increased SOC in the deeper layers. The levels of mineral nitrogen (Nmin) and water extractable organic carbon (WEOC) increased in the 0–10 cm soil layer soon after fertilization with LD and SD. The mobile components of the soil (Nmin and WEOC) were characterized by high variability during the growing season. Within the 2-month period, their concentrations decreased drastically and were close to those of unfertilized soil. The research indicates that anaerobic digestate had a greater effect on mobile forms of carbon and nitrogen in the soil than on their total amounts.

The digestate is a prospective biofertilizer and potential source of income for many biogas plants worldwide. However, its actual impact on the soil properties and biomass yield is still unexploited. The different digestates from eight agricultural biogas plants were researched in terms of their chemical composition and the fertilizing potential. The results obtained from digestate chemical analysis indicate that the digestate biomass had large amount of nitrogen (up to 73 g kg−1 fresh mass) and potassium (up to 25 g kg−1 fresh mass). The value of the digestate was estimated in the range of 2.88–7.89 EUR Mg−1 for liquid digestate and 7.62–13.61 EUR Mg−1 for solid digestate based on the commercial fertilizer market price of nitrogen, potassium phosphorus, organic carbon, Cu, Zn, Fe and Mg. The digestate produced at the 1 MW biogas plant is worth EUR 941–2095 per day in addition to energy sales income. The application of digestate on low-fertility land in areas close to the biogas plant allows the production of up to three-fold more biomass suitable for biogas production. The digestate’s application on semi-natural grass biomass production in the low-fertility soils near the biogas plants could be an alternative strategy for the biogas plant feedstock portfolio diversification.

The shift from a fossil economy to a biological economy will stimulate the demand for bio-based energy generation. One promising strategy to meet the growing demand for biomass for energy production is to re-cultivate abandoned arable land. The digestate fertilisation approach for the sake of increased biomass yields is the key strategy towards more sustainable biogas generation in many regions. To minimise digestate utilisation costs, biogas plant operators seek to spread the liquid digestate as close as possible to its storage location. The present work is focused on the evaluation of the digestate impact on the eroded agricultural soil, the improved biomass yield of semi-natural grassland biomass and its biogas potential. Three-year field experiment results indicate the positive effect of solid and liquid digestate on soil fertility and quality. The amount of dissolved organic carbon in eroded loamy Retisol increased 3.7% and 15% in soil 0–10 cm layer with fertilisation rate of 170 kg ha−1 N of liquid and solid digestate, respectively. In terms of total nitrogen, 11.6% and 20% increases were achieved using 170 kg ha−1 N of liquid and solid digestate compared to unfertilised. Furthermore, the grass fresh biomass yield was up to 3 times higher compared to the unfertilised yield. The biomethane yield of biomass from digestate-fertilised fields was higher for every fertilisation rate applied. The highest biomethane yield was for 170 kg N ha−1 liquid digestate applied, yielding 19% more biogas compared to untreated field biomass due to both increased biogas yield and increased methane content.

The objective of this work was to determine the potential of using of solid and liquid digestate, as biofertilizer and soil amendment, and to address adverse unfertile soil erosion problems after analysing areas of potential use. We found a high concentration of organic carbon in a solid fraction of the digestate, which indicates a significant potential for returning carbon to the soil. The influence of digestate fertilization on soil fertility as a function of the amount of available phosphorus (P2O5) and potassium (K2O) in the 0–40 cm layer was found to be very important. The soil fertility category was changed from “high” (200–300 mg kg−1) to “very high” (>300 mg kg−1) according P2O5, using only 170 kg ha−1 of nitrogen in solid and liquid digestate rate. Fertilization with all phases and rates of digestate tend to increase the amount of K2O. The solid digestate fertilization using the 170 kg ha−1 N rate showed a significant increase (5 times) of mineral nitrogen content in the 0–40 cm soil layer. The advantage of solid digestate as a soil improver has been highlighted because it has a positive effect on soil fertility, quality, sustainability and durability. The amount of mobile humic acids (MHA) in eroded loamy Retisol increased 1.6 times in soil 0–40 cm layer with a fertilization rate of 170 kg ha−1 and 1.5 times – of 85 kg ha−1 of solid digestate compared to the untreated. Similar trends of MHA but in lower amount were identified when liquid digestate was applied.

The objective of this work was to determine the potential of using chicken manure (CM) as a sole feedstock for renewable biogas production under anaerobic conditions before spreading the digested chicken manure biomass on the fields as an organic biofertilizer. The chicken manure samples from four different locations of Lithuania were analysed and compared in order to better understand the chemical composition of the locally generated biomass resource. The annual quantities of chicken manure produced were investigated allowing estimation of the biogas potential if such biomass was treated anaerobically. Laboratory biomethane potential (BMP) tests performed indicated the biogas potential of methane – 508 mL CH4 g−1 VS added and this result was compared to biogas yields obtained in full scale anaerobic digestion plant using manure as a core substrate. Fermentation process data analysis of anaerobic digestion performance indicates that the system is able to adapt even if the same operational conditions, such as organic loading rate and retention time are applied. Once adapted, the performance of biogas plant fermenter was able to deliver 93% of chicken manure biogas potential with an OLR of 3.14 kg VS L−1d−1 and TAN concentration of 5.5 g L−1. The results indicated that it is possible to digest chicken manure as a stand-alone feedstock if ammonium concentration was carefully controlled via optimized fresh water dilution process.