Two microbial pathways are responsible for most of the methane produced during anaerobic digestion: acetoclastic methanogenesis (AM) and hydrogenotrophic methanogenesis (HM) coupled with syntrophic acetate oxidation (SAO). Identifying the dominant methanogenic pathway active in a system provides the information necessary to manage and optimize productivity, stability, process control, and gas quality in biogas reactors. In this study, a modified method is proposed to estimate methanogenic pathways in different biogas systems via short-term parallel incubations with methyl-labeled acetate (2-13C-acetate). Cavity ring-down spectroscopy was applied to measure the δ13C–CH4 and δ13C–CO2 isotopic signatures of produced biogas. Preliminary experiments demonstrated that longer incubation times led to significant variations in δ13C–CH4 and δ13C–CO2 and consequently interfered with the calculated fraction of CH4 produced from HM (fHM). This variability is likely caused by the dilution of 13CH4 and 13CO2 as 2-13C-acetate is consumed, along with potential changes in organic matter quality and quantity, microbial community composition, and environmental factors such as pH, volatile fatty acid content, and ammonia levels, during longer incubations. We applied this new approach to sludge from six full-scale reactors (three mesophilic and three thermophilic) and validated its potential with consistent estimates of fHM with minimal variation. Mesophilic reactors exhibited AM dominance, while HM was the dominant pathway in thermophilic reactors, aligning with reports in the literature.

Anaerobic digestion has been widely applied for waste treatment, renewable energy generation , biofertilizer production. The biogas potential in Brazil is sizable, but the state of Rio de Janeiro is largely dependent on fossil fuels , there is a lack of biogas potential assessments in the state. Thus, this study evaluated biomethane, electricity and biofertilizer potentials in the region. Three different scenarios of biomass supply were considered for four major biowaste streams: sewage sludge; cattle manure; sugarcane processing waste; and food waste. Biomethane generation from the assessed sources could reach 0.6-1.3 billion Nm(3) year(-1), corresponding to 1,768-3,961 GWh year(-1) of electricity , 1.6-3.3 million Mg year- 1 of biofertilizer. Cattle manure was responsible for 73-84% of the projected biomethane production, presenting an opportunity to reduce the sig-nificant emissions from livestock farming. The estimated biofertilizer production could meet the demands of the state , the produced electricity could offset up to 10% of the demand. The gas grid could facilitate the dis-tribution of upgraded biomethane, and 10-22% of the natural gas demand could be met. The findings of this work highlight the high potential for biogas generation in Rio de Janeiro, which is up to seven times larger than the current production.

During the production of nitrile rubber, significant amounts of nitrogen in the form of ammonium are generated in the wastewater. The discharge of this high-nitrogen wastewater can lead to serious environmental issues, including eutrophication, disruption of aquatic ecosystems, and groundwater contamination. To mitigate these impacts, this research explored the bioremediation capabilities of the macroalgae Ulva lactuca (Chlorophyta) for removing nitrogen from nitrile rubber production wastewater. The study employed single-phase and Michaelis-Menten decay models based on ammonium consumption, using various dilutions of wastewater to identify the optimal concentration for treatment. The physiological state of the macroalgae was monitored by measuring the photosynthetic capacity and specific growth rate during the experiments. In the presence of U. lactuca, ammonium concentrations decreased in all treatment groups, confirming that the ammonium kinetics conformed to both applied models. Our results show that U. lactuca effectively reduces ammonium concentrations, with an approximate removal rate of 0.020 µM·g−1·min−1 across different wastewater concentrations (70%, 80%, 90%, and 100%). Notably, the treatments with 70%, 80%, and 90% wastewater strength achieved about 67% reduction in ammonium, demonstrating the alga’s capacity to treat high-nitrogen wastewater. The photosynthetic performance of U. lactuca initially declined in control conditions but stabilized across all treatments, highlighting its adaptability. The kinetic analysis using the Michaelis-Menten model indicated a Vmax of 1342 μM·g−1·DMh−1, suggesting a robust capacity for ammonium uptake when fully saturated. Our study underscores the potential of Ulva lactuca as a cost-effective and efficient agent for wastewater bioremediation, particularly in settings with high nitrogen loads.

Purpose: This paper is aimed at the determination of digestate potential in a long-term carbon accumulation after its application as a biofertilizer.

Method: Literature survey of > 1000 papers was conducted and resulted in a selection of 21 papers that involved data of soil C accumulation after digestate addition for at least a period of 12 months. Meta-analysis was used for data analysis and interpretation of a large database. The results of incorporation of total organic carbon in the soil after digestate biofertilization were measured by one-way analysis of variance (ANOVA).

Results: A comprehensive literature review showed trends for carbon increase in the soil for different experiment periods up to 84 months and initial content of carbon in the soil. It was demonstrated that application of digestate, a byproduct of anaerobic digestion, to agricultural soils resulted in an increase of soil carbon content for a period of up to 8 years. Specifically, digestate derived from cow and pig manure had the highest potential to enhance soil carbon accumulation compared to digestate from other organic residues including food waste and sewage sludge, highlighting the need for a proper choice of the waste substrate used in anaerobic digestion. Soil carbon accumulation is notably more pronounced when digestate is applied to soils with low organic matter content, particularly sandy and loam soils. Conclusion: While digestate application to soils is typically used to substitute mineral fertilizers, it also leads to an overall increase in soil carbon content.

Research Highlights

• Among different types of manure cow and pig manure showed the highest results in the terms of long term (> 12 months) carbon accumulation in the soil after biofertilization.
• A general tendency of lower carbon increase in the soil for the period of 12 months and maximum increase at 36 months of experiment was found.
• Digestate can be very successfully used to help reforestation efforts, as they usually have lower soil C content than natural areas.
• Obtained results for the period of more than 12 months showed the highest soil carbon increase (more than 2% per month) for sandy soil.
• Wheat and maize were found to be the best crops in terms of the potential for carbon accumulation in the soil after digestate biofertilization.

Proper pretreatment of organic residues prior to anaerobic digestion (AD) can maximize global biogas production from varying sources without increasing the amount of digestate, contributing to global decarbonization goals. However, the efficiency of pretreatments applied on varying organic streams is poorly assessed. Thus, we performed a meta-analysis on AD studies to evaluate the efficiencies of pretreatments with respect to biogas production measured as methane yield. Based on 1374 observations our analysis shows that pretreatment efficiency is dependent on substrate chemical dominance. Grouping substrates by chemical composition e.g., lignocellulosic-, protein- and lipid-rich dominance helps to highlight the appropriate choice of pretreatment that supports maximum substrate degradation and more efficient conversion to biogas. Methane yield can undergo an impactful increase compared to untreated controls if proper pretreatment of substrates of a given chemical dominance is applied. Non-significant or even adverse effects on AD are, however, observed when the substrate chemical dominance is disregarded.

Society is largely moving into electric mobility to achieve net -zero emissions, with the choice of electrification as the sole viable option for decarbonizing personal road transport. While this perspective has some merits, it overlooks the potential of biomethane produced through anaerobic digestion (AD) as a carbon -negative solution. Biomethane from AD offers not only carbon -neutrality but the possibility of being carbon -negative, with estimates suggesting it could provide 10 % of the world 's primary energy consumption by 2050. AD provides socio-environmental advantages, including improved quality of life and employment opportunities, a particularly relevant topic in developing countries. The technology is mature, cost-effective, and applicable across various sectors and therefore it is imperative that it is considered as an alternative or complementation to electrification of road transport.

Digestate processing is a strategy to improve the management of digestate from biogas plants. Solid-liquid separation is usually the primary step and can be followed by advanced treatments of the fractions. The knowledge about the performance of the separators and the quality of the fractions is scattered because of many available techniques and large variability in digestate characteristics. We performed a systematic review and found 175 observations of full-scale solid-liquid separation of digestate. We identified 4 separator groups, 4 digestate classes based on substrate, and distinguished whether chemical conditioners were used. We confirmed the hypothesis that the dominant substrate can affect the efficiency of the digestate separation. Furthermore, the results showed that centrifuges separated significantly more dry matter and total P than screw presses. Use of chemical conditioners in combination with a centrifuge lowered the dry matter concentration in the liquid fraction by 30%. Screw presses consumed 4.5 times less energy than centrifuges and delivered 3.3 tonne ammonium N in the liquid fraction and 0.3 tonne total P in the solid fraction using 1 MWh. The results can provide data for systems analyses of biogas solutions and can support practitioners when choosing among full-scale separator techniques depending on the digestate type. In a broader perspective, this work contributes to the continuous improvement of biogas plants operations and to their role as nutrients recovery sites.