Coupled process of in-situ sludge fermentation and riboflavin-mediated nitrogen removal for low carbon wastewater treatment

Application of exogenous electron mediator in fermentation to enhance the production of value-added products

Electron transfer is essential for the production efficiency of value-added products in anaerobic fermentation, such as butanol and ethanol as biofuels, and short-chain fatty acids (SCFAs) including butyric acid and acetic acid as platform chemicals. Electron mediators (EMs), also known as electron shuttles, can facilitate electron transfer to counter irreversible or slow redox reactions that limit fermentation. The addition of EMs has been shown to be an effective strategy to promote fermentation by various bacteria, particularly Clostridium species, for these valuable product syntheses. This paper reviews recent advancements in the application of exogenous electron mediators (EEMs) across various scenarios. Common EEM types, their characteristics, and mechanisms are summarized, and different application scenarios are discussed to elucidate the effect of EEMs. Key technical challenges and future directions for EEM application are also explored.

Mixotrophic denitrification using thiocyanate as an electron donor: Role of thiocyanate under different C/N conditions

Thiocyanate (SCN-) is a highly toxic reducing inorganic compound commonly found in various nitrogen-rich wastewater and is also a promising electron donor for mixotrophic denitrification. However, its extent of involvement in mixotrophic denitrification under conditions of carbon limitation or excess remains unclear. In this study, five reactors were constructed to investigate the participation and microbial mechanisms of SCN- in mixotrophic denitrification under high C/N and low C/N conditions. The results showed that under low C/N conditions, SCN⁻ synergizes with organic electron donors to enhance denitrification, with mixotrophic systems exhibiting higher electron utilization efficiency and achieving 1.12 times the TN removal rate of autotrophic and heterotrophic systems combined. Under high C/N conditions, an ample supply of electron donors achieve 100% nitrate removal; however, SCN⁻ also participates in denitrification, competing with organic electron donors for NO3--N, which leads to the waste of organic matter. Additionally, high-throughput sequencing revealed that under low C/N mixotrophic conditions, SCN⁻ effectively promoted the growth of SCN⁻-degrading microorganisms such as Thiobacillus, with its abundance increasing from 0% to 8.86%, approximately 1.85 times higher than under autotrophic conditions. This enhancement strengthened the sulfur and nitrogen metabolic capabilities of the microbial community, enabling the system to utilize SCN- more fully for denitrification. This study provides novel insights for reducing the addition of external organic matter to nitrogen-rich wastewater containing SCN-, offering theoretical and technical support for energy-saving and emission reduction in denitrification processes of actual industrial wastewater treatment.

Role of biogeochemical and hydrodynamic characteristics in simulating nitrogen dynamics in river confluence

The confluence area is the link of different river systems, whose specific hydrodynamic characteristics can significantly influence mass transport and distribution, which can further make a difference to microorganism growth and biogeochemical processes. However, the specific influences of hydrodynamic characteristics in confluence on formation processes of microbial communities and the biogeochemical processes remain unclear. To this end, the present study established an indoor self-circulation confluence flume and conducted 28-day culture experiment to thoroughly investigate the characteristics of microbial communities and nitrogen dynamics in sediment of confluence area. Results illustrated that the initial homogenous microbial communities gradually emerged differences among varied hydrodynamic zones with experiment going on. Concentrations of nitrogenous materials also changed at different experiment period, NO3- concentrations peaked at day 14, and then exhibited significant downtrend. The mean NO3- concentrations decreased the most in flow separation zone, with a 62 % decrease from day 14 to day 28. A numerical model was further established following the thermodynamics of enzyme catalysis reactions to simulate nitrogen transformation rates based on abundances of associated functional genes (gene-centric model). The average relative deviation between simulated and measured N2 production rates was 32 %. To further investigate the influence of hydrodynamic characteristics on nitrogen dynamics, DamKöhler numbers were calculated as the ratio of characteristic residence time to reaction time. DamKöhler numbers were better fitted with measured N2 production rates than simulated results of gene-centric model, signifying the importance of hydrodynamic characteristics in simulating nitrogen dynamics in confluence area.

Ecological filter walls for efficient pollutant removal from urban surface water

Urban surface water pollution poses significant threats to aquatic ecosystems and human health. Conventional nitrogen removal technologies used in urban surface water exhibit drawbacks such as high consumption of carbon sources, high sludge production, and focus on dissolved oxygen (DO) concentration while neglecting the impact of DO gradients. Here, we show an ecological filter walls (EFW) that removes pollutants from urban surface water. We utilized a polymer-based three-dimensional matrix to enhance water permeability, and emergent plants were integrated into the EFW to facilitate biofilm formation. We observed that varying aeration intensities within the EFW's aerobic zone resulted in distinct DO gradients, with an optimal DO control at 3.19 ± 0.2 mg L-1 achieving superior nitrogen removal efficiencies. Specifically, the removal efficiencies of total organic carbon, total nitrogen, ammonia, and nitrate were 79.4%, 81.3%, 99.6%, and 79.1%, respectively. Microbial community analysis under a 3 mg L-1 DO condition revealed a shift in microbial composition and abundance, with genera such as Dechloromonas, Acinetobacter, unclassified_f__Comamonadaceae, SM1A02 and Pseudomonas playing pivotal roles in carbon and nitrogen elimination. Notably, the EFW facilitated shortcut nitrification-denitrification processes, predominantly contributing to nitrogen removal. Considering low manufacturing cost, flexible application, small artificial trace, and good pollutant removal ability, EFW has promising potential as an innovative approach to urban surface water treatment.

Riboflavin modified carbon cloth enhances anaerobic digestion treating food waste in a pilot-scale system

Previous laboratory-scale studies have consistently shown that carbon-based conductive materials can notably improve the anaerobic digestion of food waste, typically employing reactors with regular capacity of 1-20 L. Furthermore, incorporating riboflavin-loaded conductive materials can further address the imbalance between fermentation and methanogenesis in anaerobic systems. However, there have been few reports on pilot-scale investigation. In this study, a 10 m2 of riboflavin modified carbon cloth was incorporated into a pilot-scale (2 m3) food waste anaerobic reactor to improve its treatment efficiency. The study found that the addition of riboflavin-loaded carbon cloth can increase the maximum organic loading rate (OLR) by 40% of the pilot-scale reactor, compared to the system using carbon cloth without riboflavin loading, while ensuring efficient operation of the reaction system, effectively alleviating system acidification, sustaining methanogen activity, and increasing daily methane production by 25%. Analysis of the microbial community structure revealed that riboflavin-loaded carbon cloth enriched the methanogenic archaea in the genera of Methanothrix and Methanobacterium, which are capable of extracellular direct interspecies electron transfer (DIET). And metabolic pathway analysis identified the methane production pathway, highly enriched on the reduction of acetic acid and CO2 at riboflavin-loaded carbon cloth sample. The expression levels of genes related to methane production via DIET pathway were also significantly upregulated. These results can provide important guidance for the practical application of food waste anaerobic digestion engineering.

Redox mediator chlorophyll accelerates low-temperature biological denitrification with responses of extracellular polymers and changes in microbial community composition

Low temperatures limit the denitrification wastewater in activated sludge systems, but this can be mitigated by addition of redox mediators (RMs). Here, the effects of chlorophyll (Chl), 1,2-naphthoquinone-4-sulfonic acid (NQS), humic acid (HA), and riboflavin (RF), each tested at three concentrations, were compared for denitrification performance at low temperature, by monitoring the produced extracellular polymeric substances (EPS), and characterizing microbial communities and their metabolic potential. Chl increased the denitrification rate most, namely 4.12-fold compared to the control, followed by NQS (2.62-fold increase) and HA (1.35-fold increase), but RF had an inhibitory effect. Chl promoted the secretion of tryptophan-like and tyrosine-like proteins in the EPS and aided the conversion of protein from tightly bound EPS into loosely bound EPS, which improved the material transfer efficiency. NQS, HA, and RF also altered the EPS components. The four RMs affected the microbial community structure, whereby both conditionally abundant taxa (CAT) and conditionally rare or abundant taxa (CRAT) were key taxa. Among them, CRAT members interacted most with the other taxa. Chl promoted Flavobacterium enrichment in low-temperature activated sludge systems. In addition, Chl promoted the abundance of nitrate reduction genes narGHI and napAB and of nitrite reduction genes nirKS, norBC, and nosZ. Moreover, Chl increased abundance of genes involved in acetate metabolism and in the TCA cycle, thereby improving carbon source utilization. This study increases our understanding of the enhancement of low-temperature activated sludge by RMs, and demonstrates positive effects, in particular by Chl.

First application of the novel anaerobic/aerobic/anoxic (AOA) process for advanced nutrient removal in a wastewater treatment plant

The stringent effluent quality standards in wastewater treatment plants (WWTPs) can effectively mitigate environmental issues such as eutrophication by reducing the discharge of nutrients into water environments. However, the current wastewater treatment process often struggles to achieve advanced nutrient removal while also saving energy and reducing carbon consumption. The first full-scale anaerobic/aerobic/anoxic (AOA) system was established with a wastewater treatment scale of 40,000 m3/d. Over one year of operation, the average TN and TP concentration in the effluent of 7.53 ± 0.81 and 0.37 ± 0.05 mg/L was achieved in low TN/COD (C/N) ratio (average 5) wastewater treatment. The post-anoxic zones fully utilized the internal carbon source stored in pre-anaerobic zones, removing 41.29 % of TN and 36.25 % of TP. Intracellular glycogen (Gly) and proteins in extracellular polymeric substances (EPS) served as potential drivers for post-anoxic denitrification and phosphorus uptake. The sludge fermentation process was enhanced by the long anoxic hydraulic retention time (HRT) of the AOA system. The relative abundance of fermentative bacteria was 31.66 - 55.83 %, and their fermentation metabolites can provide additional substrates and energy for nutrient removal. The development and utilization of internal carbon sources in the AOA system benefited from reducing excess sludge production, energy conservation, and advanced nutrient removal under carbon-limited. The successful full-scale validation of the AOA process provided a potentially transformative technology with wide applicability to WWTPs.

Henna plant biomass enhanced azo dye removal: Operating performance, microbial community and machine learning modeling

The bio-reduction of azo dyes is significantly dependent on the availability of electron donors and external redox mediators. In this study, the natural henna plant biomass was supplemented to promote the biological reduction of an azo dye of Acid Orange 7 (AO7). Besides, the machine learning (ML) approach was applied to decipher the intricate process of henna-assisted azo dye removal. The experimental results indicated that the hydrolysis and fermentation of henna plant biomass provided both electron donors such as volatile fatty acid (VFA) and redox mediator of lawsone to drive the bio-reduction of AO7 to sulfanilic acid (SA). The high henna dosage selectively enriched certain bacteria, such as Firmicutes phylum, Levilinea and Paludibacter genera, functioning in both the henna fermentation and AO7 reduction processes simultaneously. Among the three tested ML algorithms, eXtreme Gradient Boosting (XGBoost) presented exceptional accuracy and generalization ability in predicting the effluent AO7 concentrations with pH, oxidation-reduction potential (ORP), soluble chemical oxygen demand (SCOD), VFA, lawsone, henna dosage, and cumulative henna as input variables. The validating experiments with tailored optimal operating conditions and henna dosage (pH 7.5, henna dosage of 2 g/L, and cumulative henna of 14 g/L) confirmed that XGBoost was an effective ML model to predict the efficient AO7 removal (91.6%), with a negligible calculating error of 3.95%. Overall, henna plant biomass addition was a cost-effective and robust method to improve the bio-reduction of AO7, which had been demonstrated by long-term operation, ML modeling, and experimental validation.

The performance, mechanism and greenhouse gas emission potential of nitrogen removal technology for low carbon source wastewater

Excessive nitrogen can lead to eutrophication of water bodies. However, the removal of nitrogen from low carbon source wastewater has always been challenging due to the limited availability of carbon sources as electron donors. Biological nitrogen removal technology can be classified into three categories: heterotrophic biological technology (HBT) that utilizes organic matter as electron donors, autotrophic biological technology (ABT) that relies on inorganic electrons as electron donors, and heterotrophic-autotrophic coupling technology (CBT) that combines multiple electron donors. This work reviews the research progress, microbial mechanism, greenhouse gas emission potential, and challenges of the three technologies. In summary, compared to HBT and ABT, CBT shows greater application potential, although pilot-scale implementation is yet to be achieved. The composition of nitrogen removal microorganisms is different, mainly driven by electron donors. ABT and CBT exhibit the lowest potential for greenhouse gas emissions compared to HBT. N2O, CH4, and CO2 emissions can be controlled by optimizing conditions and adding constructed wetlands. Furthermore, these technologies need further improvement to meet increasingly stringent emission standards and address emerging pollutants. Common measures include bioaugmentation in HBT, the development of novel materials to promote mass transfer efficiency of ABT, and the construction of BES-enhanced multi-electron donor systems to achieve pollutant prevention and removal. This work serves as a valuable reference for the development of clean and sustainable low carbon source wastewater treatment technology, as well as for addressing the challenges posed by global warming.

Towards advanced simultaneous nitrogen removal and phosphorus recovery from digestion effluent based on anammox-hydroxyapatite (HAP) process: Focusing on a solution perspective

In this paper, the state-of-the-art information on the anammox-HAP process is summarized. The mechanism of this process is systematically expounded, the enhancement of anammox retention by HAP precipitation and the upgrade of phosphorus recovery by anammox process are clarified. However, this process still faces several challenges, especially how to deal with the ∼ 11% nitrogen residues and to purify the recovered HAP. For the first time, an anaerobic fermentation (AF) combined with partial denitrification (PD) and anammox-HAP (AF-PD-Anammox-HAP) process is proposed to overcome the challenges. By AF of the organic impurities of the anammox-HAP granular sludge, organic acid is produced to be used as carbon source for PD to remove the nitrogen residues. Simultaneously, pH of the solution drops, which promotes the dissolution of some inorganic purities such as CaCO₃. In this way, not only the inorganic impurities are removed, but the inorganic carbon is supplied for anammox bacteria.

Denitrification performance and in-situ fermentation mechanism of the wastepaper-flora slow-release carbon source

Using wastepaper as external carbon sources is an optional way to achieve total nitrogen removal faced with low carbon to nitrogen ratio municipal sewage. Most of studies have primarily focused on using cellulose-rich wastes establishing the separate denitrification units to achieve in-situ fermentation, which can cause blockages and prolong the process chain. In response, a novel in-situ fermentation wastepaper-flora slow-release carbon source (IF-WF) was proposed using in the original denitrification unit. IF-WF could be efficiently utilized in situ and the denitrification rate increased with the increase of nitrate nitrogen. The fermentation products were highly available, but internal acidification of IF-WF inhibited fermentation. Moreover, IF-WF limited the growth of polysaccharides in the extracellular polymeric substances of denitrified sludge. IF-WF finally formed the structure dominated by nitrate-reduction bacteria outside and cellulose-degrading bacteria inside. These results provide guidance for understanding the mechanism of IF-WF for in-situ fermentation to promote nitrogen removal.

Design of biorefineries towards carbon neutrality: A critical review

The increase in worldwide demand for energy is driven by the rapid increase in population and exponential economic development. This resulted in the fast depletion of fossil fuel supplies and unprecedented levels of greenhouse gas in the atmosphere. To valorize biomass into different bioproducts, one of the popular and carbon-neutral alternatives is biorefineries. This system is an appropriate technology in the circular economy model. Various research highlighted the role of biorefineries as a centerpiece in the carbon-neutral ecosystem of technologies of the circular economy model. To fully realize this, various improvements and challenges need to be addressed. This paper presents a critical and timely review of the challenges and future direction of biorefineries as an alternative carbon-neutral energy source.