The comparison between vibration and aeration on the membrane performance in algae harvesting

CO2 bio-mitigation using genetically modified algae and biofuel production towards a carbon net-zero society

CO₂ sequestration carried by biological methodologies shows enhanced potential and has the advantage that biomass produced from the captured CO₂ can be used for different applications. Bio-mitigation of carbons through a micro-algal system addresses a promising and feasible option. This review mostly focused on the role of algae, particular mechanisms, bioreactors in algae cultivation, and genetically modified algae in CO₂ fixation and energy generation systems. A combination of CO₂ bio-mitigation and biofuel production might deliver an extremely promising alternative to current CO₂ mitigation systems. Bio mitigation in which the excess carbon is captured and bio fixation which the carbon is captured by algae or autotrophs and used for producing biofuel. This review revealed that steps for biofuel production from algae include factors affecting, harvesting techniques, oil extraction and transesterification. This review helps environmentalists and researchers to assess the effect of algae-based biorefinery on the green environment.

Optimization of Air Flotation and the Combination of Air Flotation and Membrane Filtration in Microalgae Harvesting

On account of its small size and poor sedimentation performance, microalgae harvesting is restricted from a wider application. Air flotation is an efficient and fast solid–liquid separation technology, which has the potential to overcome the impediments of microalgae harvesting. In this study, factors influencing microalgae harvesting by air flotation were investigated. The results illustrated that bound extracellular organic matter (bEOM) had a greater effect on microalgae harvesting by air flotation, compared with dissolved extracellular organic matter (dEOM). Microalgae harvesting by air flotation in different growth stages proceeded, and the effect of air flotation in the heterotrophic stage was better than the autotrophic stage. The molecular weight distributions demonstrated that after air flotation, the proportion of high MW substance increased, while the proportion of low MW substance decreased, regardless of whether dEOM or bEOM. Membrane filtration was carried out for the algal solutions before and after air flotation. The membrane of pre-flotation algal solution had a higher critical flux of 51 L/m2·h than that of no-pre-flotation (24 L/m2·h), and, thus, pre-flotation had an active effect on membrane filtration in microalgae harvesting. Moreover, the combination of air flotation and membrane filtration provided an efficient technology for microalgae harvesting.

Efficient Bioflocculation of Chlorella vulgaris with a Chitosan and Walnut Protein Extract

Simple Summary

With the increase in population size, global climate changes, and the improvement of living standards, the fossil fuel resources may run out in the future. Microalgae have been considered the next generation of sustainable and renewable feedstock to produce biofuel and a large spectrum of high-value products, such as healthy oils, carotenoids, and proteins. Unlike terrestrial plants, the production of added-value chemicals from microalgal species is not seasonal; they can be grown under climate-independent conditions in bioreactors; can use wastewater as a source of nutrients, contributing to wastewater treatment; and can convert CO₂ into organic compounds more efficiently. However, the utilization of microalgal biomass is heavily dependent on microalgal biomass harvesting and concentration technology. Flocculation represents a relatively low-cost and efficient approach for the harvesting of microalgal biomass at a large scale. However, in traditional flocculation, most of the chemical flocculants covalently bind to the microalgal surfaces, contaminating the final product, which significantly limits their application. This study aims to develop an efficient and convenient bioflocculation technique to harvest microalgae.

Abstract

Bioflocculation represents an attractive technology for harvesting microalgae with the potential additive effect of flocculants on the production of added-value chemicals. Chitosan, as a cationic polyelectrolyte, is widely used as a non-toxic, biodegradable bioflocculant for many algal species. The high cost of chitosan makes its large-scale application economically challenging, which triggered research on reducing its amount using co-flocculation with other components. In our study, chitosan alone at a concentration 10 mg/L showed up to an 89% flocculation efficiency for Chlorella vulgaris. Walnut protein extract (WPE) alone showed a modest level (up to 40%) of flocculation efficiency. The presence of WPE increased chitosan’s flocculation efficiency up to 98% at a reduced concentration of chitosan (6 mg/L). Assessment of co-flocculation efficiency at a broad region of pH showed the maximum harvesting efficiency at a neutral pH. Fourier transform infrared spectroscopy, floc size analysis, and microscopy suggested that the dual flocculation with chitosan and walnut protein is a result of the chemical interaction between the components that form a web-like structure, enhancing the bridging and sweeping ability of chitosan. Co-flocculation of chitosan with walnut protein extract, a low-value leftover from walnut oil production, represents an efficient and relatively cheap system for microalgal harvesting.

Deterioration of biological pollutants removal induced by linear alkylbenzene sulphonates in sequencing batch reactors: Insight of sludge characteristics, microbial community and metabolic activity

Linear alkylbenzene sulfonates (LAS) are widely detected in wastewater, and pose potential risks to environment. The influences of LAS on the typical pollutants removal in sequencing batch reactors (SBRs) were evaluated. The results indicated that the removal efficiency of COD, NH₄⁺ and PO₄^3- was respectively reduced by 10.5-27.5%, 5.4-7.3% and11.6-28.9% with the exposure of 10-50 mg/L LAS. Mechanisms investigation found that LAS disrupted the sludge structure and reduced the biomass in reactors due to the saponification effects. Also, the presence of LAS altered the microbial community of activated sludge, and reduced the abundances of functional bacterial responsible for pollutants removal (i.e.Candidatus Accumulibacter, Nitrospira, Denitratisoma and etc.). Moreover, the LAS exhibited negative impacts on the microbial activity with increased LDH release but decreased ATP concentration. The genes expressions for microbial metabolism (i.e. carbohydrate metabolisms, energy metabolism) and typical pollutants removal (i.e. electron transport, phosphonate transport) were all downregulated in LAS-exposed SBRs.

Achieving efficient nitrite accumulation in glycerol-driven partial denitrification system: Insights of influencing factors, shift of microbial community and metabolic function

Partial denitrification (PD), which could provide sufficient nitrite for subsequent anaerobic ammonium oxidation, is a novel strategy for mainstream nitrogen removal. In this study, the performance of using glycerol as electron donor for nitrite accumulation in PD process was evaluated. Results showed that a C/N of 4.5 was effective for nitrite production (average nitrite accumulation rate: 34.32 mg N h^-1 gMLVSS^-1; average nitrate-to-nitrite transformation ratio (NTR): 91.1%) with pH ranging from 6.0 to 9.0. Also, a stable nitrite accumulation was achieved in long-term operation with the average NTR of 80.1%. Mechanism investigation found that the denitrifying bacteria Saccharibacteria (77.9%) was enriched in glycerol-driven reactors. Moreover, the enzymatic activity as well as the encoding genes (i.e. narG, narH and napA) involved in nitrate reduction were much higher than that for nitrite reduction (i.e. nirK), and this disparity was responsible for the efficient nitrite accumulation in glycerol-driven PD system.

Shifts of microbial community and metabolic function during food wastes and waste activated sludge co-fermentation in semi-continuous-flow reactors: Effects of fermentation substrate and zero-valent iron

The effects of food wastes (FW) composition and zero-valent iron (ZVI) on the volatile fatty acids (VFAs) generation, bacterial community succession and related metabolic functions during long-term FW and waste activated sludge (WAS) co-fermentation were investigated. The VFAs production in the carbohydrate-enriched reactor was approximately 3.0-folds of that in FW reactor. The ZVI contributed to the VFAs promotion by 3.6- and 6.7-folds in carbohydrate-enriched and FW reactors, respectively. Firmicutes (20.1-74.7%), Actinobacteria (0.9-26.3%), Bacteroidetes (3.4-65.7%), and Proteobacteria (9.1-28.5%) were the main bacteria in different fermentation reactors, and they were closely associated with the fermentation substrates and ZVI. Further analysis demonstrated that the key metabolic capacity (i.e. amino acid, carbohydrate and energy metabolism) and the genetic expressions of enzymes (i.e. fabA, fabZ, accA and accB) involved in VFAs generation were also related to FW composition, and were improved by the ZVI, which accounted for the significant VFAs promotion.

Intertidal wetland sediment as a novel inoculation source for developing aerobic granular sludge in membrane bioreactor treating high-salinity antibiotic manufacturing wastewater

This study proposed a novel approach of cultivating aerobic granular sludge (AGS) using intertidal wetland sediment (IWS) as inoculant in MBR for saline wastewater treatment. Granulation was observed in IWS-MBR during start-up, with increased sludge particle size (3.1-3.3 mm) and improved settling property (23.8 ml/g). The abundant inorganic particulates (acted as nuclei) and distinctive microbial community in IWS contributed to the granules formation. With the help of AGS, IWS-MBR system exhibited excellent TOC reduction of 90.3 ± 6.1% and significant TN reduction of 31.2 ± 5.0%, while the control MBR (Co-MBR) only showed 58.9 ± 7.2% and 10.4 ± 2.7%, respectively. Meanwhile, membrane fouling was mitigated in IWS-MBR, with a longer filtration cycle of 21.5 d, as compared with that of 8.9 d for Co-MBR. Microbial community analysis revealed that abundant functional bacteria associated with granulation and pollutants removal were enriched from IWS and set the basis for AGS formation and the superior treatment performance.

Key internal factors leading to the variability in CO2 fixation efficiency of different sulfur-oxidizing bacteria during autotrophic cultivation

Variability in the apparent CO₂ fixation yield of four aerobic sulfur-oxidizing bacteria (Halothiobacillus neapolitanus DSM 15147, Thiobacillus thioparus DSM 505, Thiomonas intermedia DSM 18155, and Starkeya novella DSM 506) in autotrophic culturing was studied, and mutual effects of key intrinsic factors on CO₂ fixation were explored. DSM 15147 and DSM 505 exhibited much higher CO₂ fixation yields than DSM 18155 and DSM 506. The differences in CO₂ fixation yield were determined not only by cbb gene transcription, but also by cell synthesis rate, which was determined by rRNA gene copy number; the rRNA gene copy number had a more significant effect than cbb gene transcription on the apparent CO₂ fixation yield. Moreover, accumulation of EDOC was observed in all four strains during chemoautotrophic cultivation, and the proportion of EDOC accounting for total fixed organic carbon (TOC; EDOC/TOC ratio) was much higher in DSM 18155 and DSM 506 than in DSM 15147 and DSM 505. The accumulation of EDOC led to a significant decrease in the cbb gene transcription efficiency during cultivation, and a further feedback inhibitory effect on CO₂ fixation.

Intelligent mitigation of fouling by means of membrane vibration for algae separation: Dynamics model, comprehensive evaluation, and critical vibration frequency

Vibration membrane filtration has been confirmed as an effective method to improve algae separation from water. However, the fouling evolution process and the antifouling mechanism are not well understood. In this study, a novel hybrid method based on a dynamics model was proposed, a comprehensive evaluation was conducted, and the critical vibration frequency for accurate analysis and prediction of membrane fouling was developed. The dynamics model was studied with an improved collision-attachment model by considering all the concurrent and synergistic effects of the hydrodynamic interactions acting on algae. From the perspective of potential energy, the improved model systematically elucidated the reason why the antifouling performance was enhanced when the vibration frequency varied from 1 Hz to 5 Hz. In addition, the Technique for Order Preference by Similarity to Ideal Solution-grey relational analysis (TOPSIS-GRA) method with combined weights was incorporated for the first time to provide direct comprehensive evaluation evidence to determine the effect of the vibration frequency on membrane fouling. It was found that increasing the vibration frequency could not alleviate membrane fouling caused by extracellular organic matter. Moreover, the concept of a critical vibration frequency was proposed using genetic algorithm optimized back propagation neural network, and the energy consumption was analyzed. This combination could provide an effective means to choose the most appropriate vibration frequency, thereby improving the efficiency of the vibration membrane system in the algae separation process.

Efficient reduction of Cr(VI) and immobilization of Cr driven by an iron-air fuel cell: Reaction mechanisms and electricity generation

The iron-air fuel cell (IAFC) has been successfully employed for the oxidative removal of many pollutants, but its feasibility for reductive immobilization of Cr(VI) is still unknown. Herein, we developed an IAFC system consisting of an iron anode and an activated carbon-PTFE based air-cathode, and evaluated its performance for Cr(VI) removal and power generation. In this reaction system, cathodic reduction and Fe(II) reduction both contributed to the reductive removal of Cr(VI). It was found that the decrease of solution pH from 6.0 to 3.0 promoted the removal of Cr(VI) due to the enhanced yield of Fe(II) ions and cathodic reduction, accompanying the increased power generation from 1040 mW m^-2 to 2880 mW m^-2. Besides, the Cr(VI) removal and power generation could be also promoted by elevating Na₂SO₄ concentration from 0.01 M to 0.1 M. In the IAFC process, Cr(VI) was initially reduced to less soluble ionic Cr(III) homogeneously and heterogeneously and then Cr(III) was immobilized by adsorption and/or co-precipitation with the fresh Fe(III) (oxy)hydroxides. Generally, this study is of great interest for the engineering community to design the environmentally benign and cost-effective strategy for the treatment of wastewater in remote areas, where the electricity is not easily available.

Diclofenac inhibited the biological phosphorus removal: Performance and mechanism

This work aims to evaluate the effect of new contaminant diclofenac (DCF) in sewage on the performance of Enhanced Biological Phosphorus Removal (EBPR) and its mechanism. The results showed that low-level DCF had no significant effect on EBPR. However, when the concentration of DCF was 2.0 mg/L, the removal efficiencies of chemical oxygen demand (COD), NH₄⁺-N and soluble orthophosphate (SOP) decreased significantly to 71.2 ± 4.2%, 78.6 ± 2.9%, and 64.3 ± 4.2%, respectively. Mechanisms revealed that DCF promoted the ratio of protein to polysaccharide in activated sludge extracellular polymers and inhibited anaerobic phosphorus release and oxic phosphorus uptake. Intracellular polymer analysis showed that when the DCF content was 2.0 mg/L, the maximum content of polyhydroxyalkanoates (PHA) was only 2.5 ± 0.4 mmol-C/g VSS, which was significantly lower than that in the blank. Analysis of key enzyme activities indicated that the presence of DCF reduced the activities of exopolyphosphatase and polyphosphate kinase.