Novel bioaugmentation strategy boosted with biochar to alleviate ammonia toxicity in continuous biomethanation

Physicochemical properties of carbon-based materials enhance in situ biomethanation performances under organic overload

Carbon-based materials gained attention for their potential to improve anaerobic digestion (AD) performance. Meanwhile, in situ biomethanation, where external H2 is injected into the AD process to enhance the CH4 content in biogas, is more subjected to process inhibition than AD while facing sudden changes in operational parameters. This study explored the effects of carbon-based materials on a semi-continuous in situ biomethanation process performance and stability. Two biochars and one granular activated carbon were tested at a concentration of 10 g·L-1. The experiment was conducted in three phases: a one-week start-up phase in AD conditions, an 8-week phase of in situ biomethanation, reaching a steady state, and a 2-week overload phase performed to create instability during the in situ biomethanation process. All additives significantly mitigated process failure under overload conditions, with CH4 production reaching 117 ± 16 vs 160 ± 16 NmL CH4·d-1 on the first week of organic overload (control vs average of all supplemented conditions). Specifically, the use of GAC-BC, with the highest surface area, pore volume, and diameter, led to a tenfold increase in CH4 production compared to the control in the overload phase. This improvement was associated with higher archaeal diversity and dominance of the Bacteroidales class. Conversely, the biochars, with lower surface properties, did not enhance microbial growth or improve final VFA consumption, resulting in final VFA concentrations similar to the control (11gCOD·L-1). These findings highlight the importance of surface properties in additives for mitigating VFA accumulation under stressed conditions during in situ biomethanation.

Applicability analysis of algae biochar for anaerobic membrane bioreactors in wastewater treatment: A review from a sustainability assessment perspective

The incorporation of biochar can significantly enhance the performance of anaerobic membrane bioreactors (AnMBRs), achieving up to a 95 % increase in pollutant removal efficiency and an 86 % improvement in methane production. Algae biochar, in particular, shows great promise as an effective additive in AnMBR systems because of its low cost (approximately $0.470/kg) and the abundance of raw material sources. This paper presents a comprehensive applicability analysis of algae biochar-AnMBRs from a sustainability assessment perspective, addressing technical, environmental, economic, and social dimensions. Key technical benefits include a reduction in membrane fouling by 92.1 % and an enhancement of energy recovery by 58.7 % compared to conventional AnMBRs. Following this, the paper evaluates algae biochar-AnMBRs from environmental, economic, and social viewpoints to emphasize the practical applicability and potential of this process. Finally, this review addresses the limitations related to the full-scale implementation of this technology and proposes strategic approaches to overcome these challenges. Overall, the review provides valuable insights into the practical application of algae biochar-AnMBR systems, with a strong focus on sustainability.

A critical review on emerging industrial applications of chars from thermal treatment of biosolids

Thermochemical treatment is rapidly emerging as an alternative method for the management of stabilised sewage sludges (biosolids) to effectively reduce waste volume, degrade contaminants, and generate valuable products, particularly biochar and hydrochar. Biosolids-derived char has a relatively high concentration of heavy metals compared with agricultural chars but is still applied to land due to its beneficial properties and ability to retain metals. However, non-agricultural applications can provide additional economic and environmental benefits, promote sustainability and support a circular economy. This review identifies extensive non-agricultural opportunity for biosolids biochar, including adsorption, catalysis, energy storage systems, biological process enhancement, and as additives for rubber compounding and construction. Biosolids chars have received limited attention vs agricultural char, and we draw on both areas of literature, as well as evaluating differences between agricultural and biosolids chars. A key opportunity for biosolids biochar in comparison with other materials and agricultural chars is its sustainable and low-cost nature, relatively high metals content, improving catalyst properties, and ability to modify in various stages to tune it to specific applications. The specific opportunities for hydrochar have only received limited attention. Research needs to include better understanding of the benefits and limitations for specific applications, as well as adjacent drivers, including society, regulation, and market and economics.

Gasification chars and activated carbon: Systematic physico-chemical characterization and effect on biogas production

Gasification residues/chars (GR) and activated carbon (AC) are added to wastewater treatment processes mainly as a fourth purification stage, e.g., to adsorb heavy metals or pharmaceutical residues. However, the effects of GR or AC, which are transferred to the anaerobic digestion (AD) via the sludge, are not yet fully understood. Although, the positive effect of char addition on AD has been demonstrated in several investigations, systematic studies with chemically well described chars are still missing. Therefore, in this study, different chars were characterized in detail, subjected to AD in different concentrations, and their effect on methane production investigated. GR of a gasification plant with a floating fixed bed technology, carbon made by chemical impregnation with ZnCl2 from waste-wood, carbon produced by thermochemical activation with CO2 from GR and commercial powdered AC were used for the experiments. Among others, thermogravimetric analysis, physisorption, pH, and conductivity analysis were used to characterize the chars. Mesophilic AD batch tests with different concentrations (0.025, 0.05, 0.5, 1.0, 7.0, 14.0 gL-1) of all chars (GR and ACs, respectively) were performed with digester sludge from a wastewater treatment plant for a period of 47 d. Volatile fatty acids (VFA) as well as biogas production and CH4 concentrations were monitored. It could be shown, that concentrations below 1.0 g char L-1 did not result in significant effects on CH4 and/or VFA production, whereas high concentrations of GR and AC influenced both, the CH4 yield and kinetics. Depending on the production process and the characteristics of the chars, the effect on AD varied, whereby both, positive and negative effects on biogas yield and methane production were observed. This study provides the first systematic evaluation of char application to AD processes, and therefore allows for better predictions of char applicability and effect.

Comprehensive review on recent production trends and applications of biochar for greener environment

The suitability of biochar as a supplement for environmental restoration varies significantly based on the type of feedstocks used and the parameters of the pyrolysis process. This study comprehensively examines several aspects of biochar's potential benefits, its capacity to enhance crop yields, improve nutrient availability, support the co-composting, water restoration and enhance overall usage efficiency. The supporting mechanistic evidence for these claims is also evaluated. Additionally, the analysis identifies various gaps in research and proposes potential directions for further exploration to enhance the understanding of biochar application. As a mutually advantageous approach, the integration of biochar into agricultural contexts not only contributes to environmental restoration but also advances ecological sustainability. The in-depth review underscores the diverse suitability of biochar as a supplement for environmental restoration, contingent upon the specific feedstock sources and pyrolysis conditions used. However, concerns have been raised regarding potential impacts on human health within agricultural sectors.

Wood biochar enhances methanogenesis in the anaerobic digestion of chicken manure under ammonia inhibition conditions

The process of breaking down chicken manure through anaerobic digestion is an effective waste management technology. However, chicken manure can be a challenging feedstock, causing ammonia stress and digester instability. This study examined the impacts of adding wood biochar and acid-alkali-treated wood biochar to anaerobically digest chicken manure under conditions of ammonia inhibition. The results highlighted that only the addition of 5 % acid-alkali-treated wood biochar by volume can achieve cumulative methane production close to the typical methane potential range of chicken manure. The treated wood biochar also exhibited highest total ammonia nitrogen removal compared to the Control treatment. Scanning Electron Microscope revealed growing interactions between biochar and methanogens over time. Real-time polymerase chain reaction showed that treated wood biochar produced the highest number of bacterial biomass. In addition, 16S amplicon-based sequencing identified a more robust archaeal community from treated biochar addition. Overall, the acid-alkali treatment of biochar represents an effective method of modifying biochar to improve its performance in anaerobic digestion.

Syntrophic consortium with the aid of coconut shell-derived biochar enhances methane recovery from ammonia-inhibited anaerobic digestion

Anaerobic digestion (AD) of nitrogen-rich substrates often suffers from the issue of ammonia inhibition. Although bioaugmentation has been used to assist AD with high ammonia concentration, the combined effect of domesticated syntrophic consortium (MC) together with biochar on ammonia inhibited AD are still unknown. In the present study, MC was adapted and enriched by purposive domestication. As a novel strategy, coconut shell-derived biochar was used as a carrier to aid the MC. The results showed that the digestion system deteriorated completely without the assistance of MC and biochar when the TAN concentration exceeded 8.0 g L^-1. The combination of biochar and MC (B-MC treatment) could restore ammonia inhibition in 10 days and achieved a high methane yield of 357.5 mL g^-1 volatile solid, which was 7.5 % higher than that of MC treatment. Syntrophomonas, Syntrophobacter, and Methanoculleus in MC played a critical role in reducing propionic acid and butyric acid content and efficiently producing methane. Their abundances increased 12-fold, 10-fold, and 2-fold, respectively. With the assistance of biochar, MC had a better performance in relieving ammonia inhibition. This could be attributed to two aspects. First, biochar encouraged the growth or colonization of key microorganisms such as propionate and butyrate oxidizing bacteria and ammonia-tolerant archaea. Second, biochar induced the growth of conductive microorganisms such as Geobacter. From the perspective of enzyme genes, biochar increased the abundance of related enzyme genes in butyrate and propionate degradation, acetoclastic and hydrogenotrophic pathways. In conclusion, MC combined with biochar is a potential approach to alleviate ammonia nitrogen inhibition.

Inhibitory mechanisms on dry anaerobic digestion: Ammonia, hydrogen and propionic acid relationship

Inhibitory pathways in dry anaerobic digestion are still understudied and current knowledge on wet processes cannot be easily transferred. This study forced instability in pilot-scale digesters by operating at short retention times (40 and 33 days) in order to understand inhibition pathways over long term operation (145 days). The first sign of inhibition at elevated total ammonia concentrations (8 g/l) was a headspace hydrogen level over the thermodynamic limit for propionic degradation, causing propionic accumulation. The combined inhibitory effect of propionic and ammonia accumulation resulted in further increased hydrogen partial pressures and n-butyric accumulation. The relative abundance of Methanosarcina increased while that of Methanoculleus decreased as digestion deteriorated. It was hypothesized that high ammonia, total solids and organic loading rate inhibited syntrophic acetate oxidisers, increasing their doubling time and resulting in its wash out, which in turn inhibited hydrogenotrophic methanogenesis and shifted the predominant methanogenic pathway towards acetoclastic methanogenesis at free ammonia over 1.5 g/l. C/N increases to 25 and 29 reduced inhibitors accumulation but did not avoid inhibition or the washout of syntrophic acetate oxidising bacteria.

Osmoprotectants boost adaptation and protect methanogenic microbiome during ammonia toxicity events in continuous processes

Different osmoprotectants were used to counteract ammonia toxicity in continuous anaerobic reactors. The anaerobic microbiome osmoadaptation process and its role to the methanogenic recovery are also assessed. Three osmoprotectants (i.e., glycine betaine, MgCl2 and KCl) were respectively introduced in continuous reactors at high ammonia levels, namely RGB, RMg, RK, while a control reactor (RCtrl) was also used. After ammonia was introduced, the RGB, RMg, RK and RCtrl suffered 39.0%, 36.6%, 39.9% and 36.2% methane production loss, respectively. Osmoprotectants addition recovered significantly methane production by up to 68.9%, 54.3% and 32.2% for RGB, RMg and RK, respectively contrary to RCtrl, where production increased only by 13.6%. The recovered methane production was maintained in RGB and RMg for at least four HRTs, even after the addition of osmoprotectants was stopped, due to the formed methanogenic microbiota by osmoadaptation process, with Methanoculleus sp. as the dominant species.

Wood Biochar Enhances the Valorisation of the Anaerobic Digestion of Chicken Manure

In this study, the efficacy of biochar to mitigate ammonia stress and improve methane production is investigated. Chicken manure (CM) was subjected to high-solid mesophilic anaerobic digestion (15% total solid content) with wood biochar (BC). Wood biochar was further treated using HNO3 and NaOH to produce acid–alkali-treated wood biochar (TBC), with an improvement in its overall ammonium adsorption capacity and porosity. Three treatments were loaded in triplicate into the digesters, without biochar, with biochar and with acid–alkali-treated biochar and maintained at 37 °C for 110 days. The study found a significant improvement in CH4 formation kinetics via enhanced substrate degradation, leading to CH4 production of 74.7 mL g−1 VS and 70.1 mL g−1 VS by BC and TBC treatments, compared to 39.5 mL g−1 VS by control treatments on the 28th day, respectively. However, only the use of TBC was able to prolong methane production during the semi-inhibition phase. The use of TBC also resulted in the highest removal of total ammonia nitrogen (TAN) of 86.3%. In addition, the treatment with TBC preserved the highest microbial biomass at day 110. The presence of TBC also resulted in an increase in electrical conductivity, possibly promoting DIET-mediated methanogenesis. Overall, the acid–alkali treatment of biochar can be a novel approach to improve biochar’s existing characteristics for its utilisation as an additive in anaerobic digestion.