Chicken manure and pig fat are found abundantly around the globe, and there is a challenge to get rid of them. This waste has considerable energy potential to be recovered into fuel, but extracting this energy from some by-products, especially fat, isn’t an easy task. When anaerobic digestion technology stepped to the level of anaerobic co-digestion, the utilisation of hardly degradable waste became feasible. Our research was conducted on anaerobic co-digestion of chicken manure as the primary substrate with pig fat as a fat reach supplement in a semi-continuous mode at different organic load rates. The influence of fat waste on the process of biogas production from chicken manure and the composition of the obtained products was determined using an organic load rate of 3.0–4.5 kg VS·(m³·day)⁻¹. A sturdy and continuously growing biogas production was observed at all organic load rates, implying the synergetic effect on chicken manure and pig fat co-digestion. The highest specific methane yield, 441.3 ± 7.6 L·kg VS⁻¹, was observed at an organic load rate of 4.5 kg VS·(m³·day)⁻¹. The research results showed that co-digestion of chicken manure with pig fat is an appropriate measure for fat utilisation and contributes to the increase in biogas yield, methane concentration, and overall methane yield at investigated organic load rates.

Biogas production from manure and other organic matter, or combinations thereof, is part of the circular economy, and the use of the digestate from biogas production for plant nutrition and thus for soil fertility restoration and organic carbon (C) sequestration completes the circular economy cycle. The use of digestate from biogas production in agriculture is one of the sustainable ways to manage manure or organic waste, an alternative to conventional chemical fertilizers, and a means to achieve the objectives of the European Green Deal. To this end, two two-factor pot experiments were carried out in 2019–2020. Factor A—1. Control (without fertilization), 2. Liquid cow manure 170 kg ha⁻¹ N (N₁₇₀), 3. Digested manure 170 kg ha⁻¹ N (N₁₇₀), 4. Digested manure 140 kg ha⁻¹ N (N₁₄₀), 5. Digested manure 110 kg ha⁻¹ N (N₁₁₀); Factor B—1. Plants not cultivated, 2. Plants cultivated. The experiments determined the effect of digestate on the changes in soil nitrate (NO₃-N), ammonium (NH₄-N) and mineral (NO₃-N + NH₄-N) nitrogen, and available phosphorus (P₂O₅) and potassium (K₂O) in the soil without plants as well as with plants cultivated and evaluated the risk of migration of macronutrients into deeper soil layers. The results showed that the application of the highest allowed rates under the Nitrates Directive (N₁₇₀) and N₁₄₀ digestate is accompanied by the addition of high levels of ammonium (NH₄-N) nitrogen, which alters the balance of ammonium (NH₄-N) and nitrate (NO₃-N) nitrogen in the soil, and the extent of their migration to the deeper layers. These results suggest that the application of digestate to agricultural land enhances the migration of nitrogen and, in phosphorus-rich soils, of phosphorus (PO₄-P) compounds to deeper layers (>25 cm). In order to achieve environmental objectives, digestate rates should be based on the agrochemical properties of the soil and the needs of the plants and should not exceed 65% of the nitrogen needed by the plants from mineral fertilizers.

Manure is considered a by-product or organic waste in cattle, pig, chicken or other animal breeding farms, which can be a valuable product as compost or feedstock for biogas production. The production of biomethane from biogas always copes with the formation of carbon dioxide (CO₂) as a by-product. This CO₂ may be recycled through the feedstock as a pretreatment to maximize homogeneity, and improve biogas yield and biogas quality. The CO₂-pretreatment process of cow manure (CoM), chicken manure (ChM) and pig manure (PM) was performed in the continuously fed agitated reactor at 25 °C temperature and ambient barometric pressure. Biogas yield and composition exploration were performed in an anaerobic continuous feeding digester with controlled mesophilic (37 °C) environmental conditions. The CO₂ pretreated PM, CoM and ChM yielded 234.62 ± 10.93 L/kg_VS, 82.01 ± 3.19 L/kg_VS and 374.53 ± 9.27 L/kg_VS biomethane from feedstock volatile solids, respectively. The biomethane yield from CO₂ pretreated CoM, ChM and PM achieved was higher over untreated manure by +33.78%, +28.76% and +21.78%, respectively. The anaerobic digestion process of tested feedstocks was stable, and the pH of the substrate was kept steady at a pH of CoM 7.77 ± 0.02, PM 8.07 ± 0.02 and ChM 8.09 ± 0.02 during all the experiment. The oxidation-reduction potential after pretreatment was within the optimal range (−255 ± 39.0 to −391 ± 16.8 mV) for anaerobic digestion. This process also had a positive effect on the energy generated from the feedstock, with ChM showing the greatest increase, from 2.38 MJ/kg to 3.06 MJ/kg.

Biomass, particularly agricultural residues and biomass rich in structural carbohydrates, offers significant potential for sustainable biogas production. Biological pretreatment using microorganisms, particularly Trichoderma species, is discussed as a cost-effective and environmentally sustainable approach to improving the decomposition of structural carbohydrates into fermentable sugars. This study aimed to assess the impact of employing a selective biological product (BP) on the biogas production process and biomethane potential using winter wheat straw (WWS) as a representative feedstock. The biological product, consisting of microorganisms of the Trichoderma spp. genus, was introduced to enhance microbial activity. The biogas potential results showed that WWS treated with the BP exhibited a remarkable improvement in biogas production. Specifically, biogas yield increased from 364.1 L/kg of mass in untreated straw to 439.9 L/kg in BP-treated straw, representing a substantial 20.8% increase. Furthermore, in continuous loading tests, the steady-state biogas yield from BP-treated straw ranged from 553.6 to 582.0 L/kg VS, which was notably higher compared to untreated straw with a yield of 490.0 L/kg VS. Overall, the results of this study demonstrated that the application of selective biological products significantly enhanced biogas production and biomethane potential from structural carbohydrates containing biomass sources.

Lignocellulose is a complex and abundant biomass source, and finding ways to efficiently break it down is essential for various applications, including bioenergy production and waste management. Biogas production can be significantly enhanced by adding rumen fluid to the anaerobic digestion process, which contains a variety of microorganisms with the enzyme activity necessary to breakdown complex lignocellulosic materials. This study examined the influence of rumen anaerobic bacteria inoculum on alfalfa biomass biogas yield and quality. Inoculation experiments were performed, and the higher biogas yield from organic matter was gained in experiment (A), with a rumen fluid addition of 340 ± 3.2 L/kg_VS, compared to the utilization of a digestate alone in (B), 238 ± 1.2 L/kg_VS. The results demonstrated that a pretreatment temperature of 37 °C (experiment D) yielded the highest biogas production, 381 ± 3.9 L/kg_VS, and maintained a high methane content of 63.9 ± 1.9%. Notably, pretreatment at 25 °C resulted in only a 3% increase over the raw sample and a pretreatment at 50 °C (respectively, experiments C and E) showed no significant changes, emphasizing the sensitivity of pretreatment efficiency to temperature variations.

The degradation of lignocellulose in biogas processes has been focused on the inoculant microorganisms involved, with a view to gaining a deeper understanding in order to improve lignocellulose degradation. The maximum volumetric biogas yield (12.17 L/L) was achieved with the inoculum used in experiment “B”, containing 400 g of digestate from the bioreactor along with 400 g of rumen fluid. The highest concentration of methane in biogas was obtained from the same inoculum composition (63.2 ± 1.5%). The second largest volumetric yield of 8.41 ± 0.45 L/L biogas was achieved in experiment “C,” where digestates were used as the main inoculum. Accordingly, in this case, the volumetric yield of biogas was 8.41 ± 0.45 L/L. The composition of rumen fluid and digestate increased biogas production from the same amount of alfalfa leaves by 30.9%.