Thermophilic anaerobic digestion of olive mill wastewater in an upflow packed bed reactor: Evaluation of 16S rRNA amplicon sequencing for microbial analysis

Lighting promotes sulfate removal and improves microbial community stability in upflow anaerobic sludge bed reactors under low ratio of chemical oxygen demand to sulfate

The anaerobic treatment of sulfur-laden organic wastewater is common; however, competition between sulfate-reducing bacteria (SRB) and methanogenic archaea (MA) can result in low removal efficiencies and unstable systems. Photosynthetic bacteria, capable of oxidizing reduced sulfides, can alleviate sulfide toxicity to microorganisms, thereby enhancing sulfate removal. This study compared the performance of anaerobic reactors under identical organic loads but with varying light conditions and different carbon-to-sulfur (C/S) ratios. The illuminated reactors outperformed the non-illuminated ones, achieving sulfate removal rates exceeding 85% when the light wavelength was optimized. Sludge analysis revealed that the illuminated group had larger particle sizes and higher protein and polysaccharide contents compared to the non-illuminated group. These findings suggest that light exposure enhances the removal of sulfate and organic matter, mitigates competitive inhibition, and promotes synergistic interactions among microbial populations, offering valuable insights for treating sulfate-rich wastewater using photosynthetic bacteria.

Representatives of the Synergistaceae family, taxonomic description and genome sequence of Caenicola nitritireducens gen nov., sp. nov., a novel fermenting and amino-acid degrading bacterium isolated from a municipal anaerobic digester sludge

Members of the phylum Synergistota are important but understudied components of microbial communities during anaerobic digestion. In this study, their diversity was assessed in full-scale anaerobic digester sludge samples from Marrakesh wastewater treatment plant (Morocco), using 16S rRNA gene community profiling, as well as targeted isolation, physiological characterization, and genome sequencing of novel Synergistaceae isolates. The 16S rRNA gene analysis identified 23 operational taxonomic units (OTUs) belonging to the family of Synergistaceae, representing 8.8 % of the total microbial community. 17 of these OTUs belonged to previously uncultured taxa. A dominant OTU19, presumably a new representative of the family of Synergistaceae was isolated in pure culture (strain DS-S4T) and subjected to both culture- and genome-based characterizations. Phylogenetic analysis revealed that strain DZ-S4T was related to Cloacibacillus porcorum CL-84T and Cloacibacillus evryensis 158T but with low sequence similarity of 89.94 % and 88.60 %, respectively. Based on genome relatedness, including Average Nucleotide Identity (ANI) and Amino Acid Identity (AAI), strain DZ-S4T is considered to represent a novel genus for which the name Caenicola gen.nov is proposed. Moreover, several phenotypic and eco-physiological properties differentiated the novel isolate from its related species, indicating that the strain represents a new species for which the name Caenicola nitritireducens sp. nov. is proposed, with strain DZ-S4T (=DSM 104940T = JCM 31897T) being the type strain. Additionally, this study investigates the ecological role of strain DZ-S4T, specifically the protein degradation, the bioconversion of carbohydrates, and the nitrite reduction during anaerobic digestion.

A cyclic shift-temperature operation method to train microbial communities of mesophilic anaerobic digestion

Temperature is the critical factor affecting the efficiency and cost of anaerobic digestion (AD). The current work develops a shift-temperature AD (STAD) between 35 °C and 55 °C, intending to optimise microbial community and promote substrate conversion. The experimental results showed that severe inhibition of biogas production occurred when the temperature was firstly increased stepwise from 35 °C to 50 °C, whereas no inhibition was observed at the second warming cycle. When the organic load rate was increased to 6.37 g VS/L/d, the biogas yield of the STAD reached about 400 mL/g VS, nearly double that of the constant-temperature AD (CTAD). STAD promoted the proliferation of Methanosarcina (up to 57.32 %), while severely suppressed hydrogenophilic methanogens. However, when the temperature was shifted to 35 °C, most suppressed species recovered quickly and the excess propionic acid was quickly consumed. Metagenomic analysis showed that STAD also promoted gene enrichment related to pathways metabolism, membrane functions, and methyl-based methanogenesis.

Olive mill wastewater treatment strategies to obtain quality water for irrigation: A review

The olive mill industry is a relevant sector in the economy of Mediterranean countries, while it involves high consumption of water and the production of effluents with high environmental impact. The efficient treatment of olive mill wastewater (OMW) is of high relevance, particularly for these countries. Climate changes are leading to increasing periods of droughts, and water recovery from polluted streams is essential to ensure the sustainability of this scarce resource. A combination of various technologies involving physical, chemical, and biological processes has been developed for OMW treatment. However, the treatments studied have limitations such as the operation costs, difficulty of industrial scale-up, and the fact that the vast majority do not lead to suitable treated water for discharge/reuse. As such, it is urgent to develop a solution capable of efficiently treating this effluent, overcoming the disadvantages of existing processes to convert OMW from a serious environmental problem into a valuable source of water and nutrients. In this review, several studies based on the OMW treatment are critically discussed, from conventional approaches such as the physical (e.g. centrifugation, filtration, and adsorption) and biological (anaerobic digestion and anaerobic co-digestion) processes, to the most recent technologies such as advanced membrane filtration, advanced oxidation processes (AOPs) and sulfate radical based AOPs (SR-AOPs). Due to the complexity of the effluent, OMW cannot be efficiently treated by a single process, requiring a sequence of technologies before reaching the required characteristics for discharge into water courses or use in crop irrigation. Reviewing the published results in this matter, it seems that the sequence of processes encompassing ozonation, anaerobic digestion, and SR-AOPs could be the ideal combination for this purpose. However, membrane technologies may be necessary in the final stage of treatment so that the effluent meets legal discharge or irrigation limits.

Novel lanthanum-iron oxide nanoparticles alleviate the inhibition of anaerobic digestion by carbamazepine through adsorption and bioaugmentation

Several reports have shown that pharmaceuticals and personal care products (PPCPs) have some negative effects on anaerobic digestion (AD), yet there are no convenient and efficient strategies for mitigating the adverse influences. The typical PPCPs of carbamazepine have a strong negative effect on lactic acid AD process. Therefore, in this work, novel lanthanum-iron oxide (LaFeO₃) nanoparticles (NPs) were used for adsorption and bioaugmentation to weak the negative effects of carbamazepine. The adsorption removal of carbamazepine increased from 0 to 44.30% as the dosage of LaFeO₃ NPs was increased from 0 to 200 mg/L, providing the necessary prerequisites for bioaugmentation. Adsorption reduced the probability of direct contact between carbamazepine and anaerobes, partly alleviating the inhibition of carbamazepine on microbes. The highest methane (CH₄) yield induced by LaFeO₃ NPs (25 mg/L) was 226.09 mL/g lactic acid, increasing by 30.06% compared to the control yield with a recovery to 89.09% of the normal CH₄ yield. Despite the ability of LaFeO₃ NPs to restore normal AD performance, the biodegradation rate of carbamazepine remained below 10% due to its anti-biodegradability. Bioaugmentation was primarily reflected in the enhanced bioavailability of dissolved organic matter, while the intracellular LaFeO₃ NPs promoted coenzyme F420 activity through binding to humic substances. Under the mediation of LaFeO₃, a direct interspecies electron transfer system with Longilinea and Methanosaeta as functional bacteria was successfully constructed and the corresponding electron transfer rate was accelerated from 0.021 s^-1 to 0.033 s^-1. LaFeO₃ NPs eventually recovered AD performance under carbamazepine stress in an adsorption and bioaugmentation manner.

Enhancing methane production in anaerobic co-digestion of sewage sludge and food waste by regulating organic loading rate

This study presented mechanistic insights into the long-term effects of stepwise-increasing organic loading rates (OLRs) on anaerobic co-digestion (AcoD) of sewage sludge and food waste. The maximum methane (CH₄) yield of 500.0 ± 10.5 mL CH₄/g VS_fed was achieved at medium OLR of 3.5 g VS/L/d. This excellent performance was associated with the high hydrolysis efficiency (78.4%), three-fold enhancement in the acidogenesis enzyme activity, and 87.0% enhanced methanogen activity. Soluble intermediates (carbohydrates and proteins) were largely degraded (>98.5%), especially tyrosine-like and tryptophan-like aromatic proteins. The particulates were effectively decomposed from macromolecules to micromolecules, and the crystallinity of cellulosic substances decreased by 24.5%. The newly-shaped combined syntrophic acetate oxidation-hydrogenotrophic methanogenesis pathway dominated enhanced CH₄ production. Energy balance analysis based on medium OLR demonstrated the high energy recovery potential in full-scale AcoD. These findings suggest the optimal medium OLR can facilitate the bioconversion of organics to CH₄ through a new metabolic pathway.

Proposed protocol for rate-limiting step determination during anaerobic digestion of complex substrates

Anaerobic digestion is a complex process, involving various microorganism groups and, consequently, several reactions. An easy-to-use protocol for the rate-limiting step determination of the process is proposed. The hydrogen production, acetate production, and acetate consumption rates can be calculated, according to a structured algorithm. During the rate limiting step determination, several compounds (biopolymer and monomer representatives, as well as sodium acetate) were used, combined or not with the substrate, to draw the corresponding conclusions. Three substrates were tested, characterized by specific organic compound groups (carbohydrates, proteins, and fats). All three substrates followed the acetate-consuming pathway for the organic matter conversion to methane. In this study, the rate-limiting step for the pathway of acetate consumption was acetate production. Determining the rate-limiting step through the proposed protocol can point to the appropriate actions needed to boost methane production, like substrate pretreatment, using an acidogenic reactor, or checking for the presence of inhibitors.

Quantitative and Qualitative Changes in the Genetic Diversity of Bacterial Communities in Anaerobic Bioreactors with the Diatomaceous Earth/Peat Cell Carrier

This paper analyses the impact of the diatomaceous earth/peat (DEP; 3:1) microbial carrier on changes in the bacterial microbiome and the development of biofilm in the anaerobic digestion (AD) of confectionery waste, combined with digested sewage sludge as inoculum. The physicochemical properties of the carrier material are presented, with particular focus on its morphological and dispersion characteristics, as well as adsorption and thermal properties. In this respect, the DEP system was found to be a suitable carrier for both mesophilic and thermophilic AD. The evaluation of quantitative and qualitative changes in the genetic diversity of bacterial communities, carried out using next-generation sequencing (NGS), showed that the material has a modifying effect on the bacterial microbiome. While Actinobacteria was the most abundant cluster in the WF-control sample (WF—waste wafers), Firmicutes was the dominant cluster in the digested samples without the carrier (WF-dig.; dig.—digested) and with the carrier (WF + DEP). The same was true for the count of Proteobacteria, which decreased twofold during biodegradation in favor of Synergistetes. The Syntrophomonas cluster was identified as the most abundant genus in the two samples, particularly in WF + DEP. This information was supplemented by observations of morphological features of microorganisms carried out using fluorescence microscopy. The biodegradation process itself had a significant impact on changes in the microbiome of samples taken from anaerobic bioreactors, reducing its biodiversity. As demonstrated by the results of this innovative method, namely the BioFlux microfluidic flow system, the decrease in the number of taxa in the digested samples and the addition of DEP contributed to the microbial adhesion in the microfluidic system and the formation of a stable biofilm.

Nitrification upon Nitrogen Starvation and Recovery: Effect of Stress Period, Substrate Concentration and pH on Ammonia Oxidizers’ Performance

Nitrification has been widely applied in wastewater treatment, however gaining more insight into the nitrifiers’ physiology and stress response is necessary for the optimization of nutrient removal and design of advanced processes. Since nitrification initiates with ammonia oxidation performed by ammonia-oxidizing bacteria (AOB), the purpose of this study was to investigate the effects of short-term ammonia starvation on nitrogen uptake and transformation efficiency, as well as the performance of starved nitrifiers under various initial substrate concentrations and pH values. Ammonium deprivation for 3 days resulted in fast ammonium/ammonia accumulation upon nitrogen availability, with a maximum uptake rate of 3.87 mmol gprotein−1 min−1. Furthermore, a delay in the production of nitrate was observed with increasing starvation periods, resulting in slower recovery and lower nitrification rate compared to non-starved cells. The maximum accumulation capacity observed was 8.51% (w/w) independently of the external nitrogen concentration, at a range of 250–750 mg N L−1, while pH significantly affected ammonia oxidizers’ response, with alkaline values enhancing nitrogen uptake. In total, ammonia accumulation after short-term starvation might serve as an important strategy that helps AOB restore their activity, while concurrently it could be applied in wastewater treatment for effective nitrogen removal and subsequent biomass utilization.

Thermophilic Dark Fermentation of Olive Mill Wastewater in Batch Reactors: Effect of pH and Organic Loading

In recent decades, olive oil consumption has almost tripled worldwide. Olive oil production is linked with the production of enormous amounts of olive mill wastewater, the main by-product derived from three-phase olive mills. Due to the environmental risks of olive mill wastewater disposal, the management and valorization of the specific waste stream is of great importance. This work focuses on the thermophilic dark fermentation of olive mill wastewater in batch reactors, targeting pH optimization and the organic loading effect. A series of experiments were performed, during which the organic load of the substrate remained at 40 g/L after dilution with tap water, and the pH was tested in the range of 4.5 to 7.5. The maximum yield in terms of produced hydrogen was obtained at pH 6.0, and the yields were 0.7 mol H2/mol glucose or 0.5 L H2/Lreactor. At the same conditions, a reduction of 62% of the waste’s phenols was achieved. However, concerning the effect of organic loading at the optimized pH value (6.0), a further increase in the organic load minimized the hydrogen production, and the overall process was strongly inhibited.

A review on biogas upgrading in anaerobic digestion systems treating organic solids and wastewaters via biogas recirculation

Biogas upgrading is an essential process for efficient and safe utilization of biogas produced from anaerobic digestion (AD), a cost-effective and environmentally friendly technology for bioenergy recovery from organic wastes. Biogas recirculation in AD reactors has been recently reported as a cost-effective and promising method to enhance methane content in biogas. This review aimed to summarize the state-of-the-art of biogas recirculation-based AD systems to better understand the possible mechanisms and main factors relating to in-situ biogas upgrading. It shows that biogas recirculation in the AD reactor can not only enhance methane content via both physicochemical and biological effects, but also help establish a robust AD system with high buffering capacity for highly efficient treatment of various organic wastes. More research works are demanding for a better understanding of the mechanisms and the optimization of the whole AD system, targeting its further development for high-calorie bioenergy production.