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

Alleviation of propionic acid inhibition and enhancement of methane production in anaerobic digestion by micronano-bubble water supplementation

Micronano-bubble water (MNBW) was used to accelerate propionic acid (HPr) degradation and enhance methane production in anaerobic digestion (AD) in this study. Results showed that HPr degradation was accelerated by MNBW addition. Moreover, methane production was elevated by 13.5% to 18.4% under HPr inhibition by MNBW supplementation. Microbial analysis revealed that MNBW boosted the bacterial populations involved in protein and carbohydrate consumption. In addition, supplementation with MNBW improved the tolerance of archaea such as Methanosaetaceae to HPr inhibition, thus increasing methane production. MNBW also promoted alkalinity recovery, lowered intermediate alkalinity/partial alkalinity and volatile fatty acid/total alkalinity ratios, and enhanced buffering capacity. Enzyme activity increased by MNBW, including a 21.0% increase in coenzyme F420 and a 7.0% to 13.8% increase in extracellular hydrolase activity. This work highlights that MNBW is a promising approach to accelerate HPr degradation, optimize methanogenic pathways, and boost methane production.

Ammonia-stressed anaerobic digestion: Sensitivity dynamics of key syntrophic interactions and methanogenic pathways-A review

The problematic anaerobic digestion (AD) of protein-rich substrates owing to their high ammonia content continues to hinder optimum methanation despite their high potential for offsetting greenhouse gas (GHG) emissions. This review focuses on the analyses of the sensitivity dynamics of key AD processes as well as the microbial interactions and exchanges that occur with them. Aside from the apparent increased risk associated with thermophilic ammonia-rich substrate AD, the marginally higher energy generation compared to mesophilic systems is not commensurate to the energy requirement. Moreover, while comparable FAN thresholds have been confirmed, TAN thresholds are susceptible to physical chemistry and so vary greatly. Profiling of the metabolic capability of front-end AD microbiome revealed Bacteroidetes, Firmicutes, and Synergistetes as some of the ammonia-resilient bacteria groups while Proteobacteria and Actinobacteria were the most fragile taxa. Besides the predominance of incomplete propionate oxidizing bacteria under ammonia stress conditions, syntrophic propionate oxidation (SPO) is usually shifted from the methylmalonyl CoA to the dismutation pathway. Furthermore, besides their different recoverability potentials, distinct methanogenic groups are differentially impacted by different ammonia species. Prevailing literature evidence suggests that conductive material assisted bioaugmentation with SAO-HM consortia, and in-situ H2 supplementation are the most effective for expediting electron transfer and relieving ammonia stress. These valuable insights should inform the design of targeted ammonia inhibition mitigation strategies.

Synergism of floated paperboard sludge cake /sewage sludge for maximizing biomethane yield and biochar recovery from digestate: A step towards circular economy

Anaerobic digestion of floated paperboard sludge (PS) cake suffers from volatile fatty acids (VFAs) accumulation, nutrient unbalanced condition, and generation of digestate with a risk of secondary pollution. To overcome these drawbacks, sewage sludge (SS) was added to PS cake for biogas recovery improvement under a co-digestion process followed by the thermal treatment of solid fraction of digestate for biochar production. Batch experimental assays were conducted at different SS:PS mixing ratios of 70:30, 50:50, 30:70, and 20:80 (w/w), and their anaerobic co-digestion performances were compared to the mono-digestion systems at 35 ± 0.2 °C for 45 days. The highest methane yield (MY) of 241.68 ± 14.81 mL/g CODremoved was obtained at the optimum SS:PS ratio of 50:50 (w/w). This experimental condition was accompanied by protein, carbohydrate, and VFA conversion efficiencies of 47.3 ± 3.2%, 46.8 ± 3.2%, and 56.3 ± 3.8%, respectively. The synergistic effect of SS and PS cake encouraged the dominance of Bacteroidota (23.19%), Proteobacteria (49.65%), Patescibacteria (8.12%), and Acidovorax (12.60%) responsible for hydrolyzing the complex organic compounds and converting the VFAs into biomethane. Further, the solid fraction of digestate was subjected to thermal treatment at a temperature of 500 °C for 2.0 h, under an oxygen-limited condition. The obtained biochar had a yield of 0.48 g/g dry digestate, and its oxygen-to-carbon (O/C), carbon-to-nitrogen (C/N), and carbon-to-phosphorous (C/P) ratios were 0.55, 10.23, and 16.42, respectively. A combined anaerobic co-digestion/pyrolysis system (capacity 50 m3/d) was designed based on the COD mass balance experimental data and biogenic CO2 market price of 22 USD/ton. This project could earn profits from biogas (12,565 USD/yr), biochar (6641 USD/yr), carbon credit (8014 USD/yr), and COD shadow price (6932 USD/yr). The proposed project could maintain a payback period of 6.60 yr. However, further studies are required to determine the associated life cycle cost model that is useful to validate the batch experiment assumptions.

Up-flow anaerobic sludge blanket treatment of swine wastewater: Effect of heterologous and homologous inocula on anaerobic digestion performance and the microbial community

The effects of heterogenous (anaerobic sludge from treating distillery sewage, ASDS) and homologous (anaerobic sludge from treating swine wastewater, ASSW) inocula on anaerobic digestion and the microbial community in an up-flow anaerobic sludge blanket treating swine wastewater were compared. The highest chemical oxygen demand removal efficiencies with ASDS (84.8%) and ASSW (83.1%) were obtained with an organic loading rate of 15 kg COD/m3/d. For ASSW compared with ASDS, methane production efficiency was 15.3% higher and excess sludge production was 73.0% lower. The abundance of the cellulose hydrolyzing bacterium Clostridium sensu stricto_1 with ASDS (36.1%) was 1.5 times that with ASSW, while that of Methanosarcina with ASSW (22.9%) was > 100 times that with ASDS. ASDS reduced the content of pathogenic bacteria by 88.0%, while ASSW maintained a low level of pathogenic bacteria. ASSW greatly improved the methane production efficiency of wastewater and is more suitable for treating swine wastewater.

Insights into the Occurrence, Fate, Impacts, and Control of Food Additives in Food Waste Anaerobic Digestion: A Review

The recovery of biomass energy from food waste through anaerobic digestion as an alternative to fossil energy is of great significance for the development of environmental sustainability and the circular economy. However, a substantial number of food additives (e.g., salt, allicin, capsaicin, allyl isothiocyanate, monosodium glutamate, and nonnutritive sweeteners) are present in food waste, and their interactions with anaerobic digestion might affect energy recovery, which is typically overlooked. This work describes the current understanding of the occurrence and fate of food additives in anaerobic digestion of food waste. The biotransformation pathways of food additives during anaerobic digestion are well discussed. In addition, important discoveries in the effects and underlying mechanisms of food additives on anaerobic digestion are reviewed. The results showed that most of the food additives had negative effects on anaerobic digestion by deactivating functional enzymes, thus inhibiting methane production. By reviewing the response of microbial communities to food additives, we can further improve our understanding of the impact of food additives on anaerobic digestion. Intriguingly, the possibility that food additives may promote the spread of antibiotic resistance genes, and thus threaten ecology and public health, is highlighted. Furthermore, strategies for mitigating the effects of food additives on anaerobic digestion are outlined in terms of optimal operation conditions, effectiveness, and reaction mechanisms, among which chemical methods have been widely used and are effective in promoting the degradation of food additives and increasing methane production. This review aims to advance our understanding of the fate and impact of food additives in anaerobic digestion and to spark novel research ideas for optimizing anaerobic digestion of organic solid waste.