Managing nutrients and system operations for biofuel production from freshwater macroalgae

Ensuring carbon neutrality via algae-based wastewater treatment systems: Progress and future perspectives

The emergence of algal biorefineries has garnered considerable attention to researchers owing to their potential to ensure carbon neutrality via mitigation of atmospheric greenhouse gases. Algae-derived biofuels, characterized by their carbon-neutral nature, stand poised to play a pivotal role in advancing sustainable development initiatives aimed at enhancing environmental and societal well-being. In this context, algae-based wastewater treatment systems are greatly appreciated for their efficacy in nutrient removal and simultaneous bioenergy generation. These systems leverage the growth of algae species on wastewater nutrients-including carbon, nitrogen, and phosphorus-alongside carbon dioxide, thus facilitating a multifaceted approach to pollution remediation. This review seeks to delve into the realization of carbon neutrality through algae-mediated wastewater treatment approaches. Through a comprehensive analysis, this review scrutinizes the trajectory of algae-based wastewater treatment via bibliometric analysis. It subsequently examines the case studies and empirical insights pertaining to algae cultivation, treatment performance analysis, cost and life cycle analyses, and the implementation of optimization methodologies rooted in artificial intelligence and machine learning algorithms for algae-based wastewater treatment systems. By synthesizing these diverse perspectives, this study aims to offer valuable insights for the development of future engineering applications predicated on an in-depth understanding of carbon neutrality within the framework of circular economy paradigms.

Modifying filamentous algae nutrient scrubbers for improved wastewater treatment and harvestability - comparison with microalgae

Algae have been well studied for their abilities to treat wastewater, and several types of treatment systems have been demonstrated at a range of scales. High Rate Algae Ponds (HRAP) are a microalgae-based system and Filamentous Algae Nutrient Scrubbers (FANS) a filamentous algae-based system. For FANS, nutrient removal rates are typically lower and more variable than HRAPs, while HRAPs have lower productivity and poor harvestability. This study investigated if modifying a FANS to mimic HRAPs (using high rate algae mesocosms HRAM), with respect to hydraulic retention time (HRT) and smaller footprint, overcomes FANS limitations, while increasing wastewater treatment and resource recovery compared to HRAPs. Biomass productivity on the FANS (10.5 ± 2.9 g m-2 d-1) and FANS with CO2 addition (19.0 ± 4.8 g m-2 d-1) were significantly higher (p < 0.01) compared to the HRAMs (6.7 ± 1.4 g m-2 d-1) and HRAMs with CO2 addition (8.1 ± 1.2 g m-2 d-1). Under phosphorus replete conditions, biomass production was significantly higher on FANS (44.8 ± 14.4 g m-2 d-1) than HRAMs (5.0 ± 0.6 g m-2 d-1). Effluent quality (nutrient removal) was significantly higher (p < 0.05) for FANS compared to HRAMS, regardless of treatment. For harvesting, FANS (2.9-41%) yielded significantly higher (p < 0.01) percentage solids with, and, without dewatering/gravity harvesting compared to the HRAM (0.04-0.11%). Modifying the operation of the FANS to mimic longer HRT of HRAMs resulted in higher areal biomass productivity and nutrient removal in the FANS than the HRAM, regardless of treatment. The use of filamentous algae on FANS greatly improved the percentage solids yield in the harvested biomass without the need for energy intensive harvesting techniques. Further investigations need to be undertaken to determine if benefits will be realised at fullscale.

Wastewater treatment using filamentous algae - A review

Wastewater treatment using algae is a promising approach for efficient removal of contaminating nutrients and their conversion into useful products. Monocultures of filamentous algae provide easier harvesting compared to microalgae, and better control of biomass quality than polyculture systems such as algal turf scrubbers. In this review, recent research into wastewater treatment using freshwater filamentous algae is compiled and critically analysed. Focus is given to filamentous algae monocultures, with key relevant findings from microalgae and polyculture systems discussed and compared. The application of monocultures of filamentous algae is an emerging area of research. Gaps are identified in our understanding of key aspects important to large-scale system design, including criteria for species selection, influence of nutrient type and loading, inorganic carbon supply, algae-bacteria interactions, and parameters such as pond depth, mixing and harvesting regimes. This technology has much promise, however future research is needed to maximise productivity and wastewater treatment efficiency.

Metallic Organic Framework-Derived Fe, N, S co-doped Carbon as a Robust Catalyst for the Oxygen Reduction Reaction in Microbial Fuel Cells

Oxygen reduction reaction (ORR) provides a vital role for microbial fuel cells (MFCs) due to its slow reaction kinetics compared with the anodic oxidation reaction. How to develop new materials with low cost, high efficacy, and eco-friendliness which could replace platinum-based electrocatalysis is a challenge that we have to resolve. In this work, we accomplished this successfully by means of a facile strategy to synthesize a metallic organic framework-derived Fe, N, S co-doped carbon with FeS as the main phase. The Fe/S@N/C-0.5 catalyst demonstrated outstandingly enhanced ORR activity in neutral PBS and alkaline media, compared to that of commercial 20% Pt-C catalyst. Here, we started-up and operated two parallel single-chamber microbial fuel cells of an air cathode, and those cathode catalysts were Fe/S@N/C-0.5 and commercial Pt-C (20% Pt), respectively. Scanning electron microscopy (SEM) elaborated that the Fe/S@N/C-0.5 composite did not change the polyhedron morphology of ZIF-8. According to X-ray diffractometry(XRD) curves, the main crystal phase of the resulted Fe/S@N/C-0.5 was FeS. The chemical environment of N, S, and Fe which are anticipated to be the high-efficiency active sites of ORR for MFCs were investigated by X-ray photoelectron spectroscopic(XPS). Nitrogen adsorption/desorption techniques were used to calculate the pore diameter distribution. In brief, the obtained Fe/S@N/C-0.5 material exhibited a pronounced reduction potential at 0.861 V (versus Reversible Hydrogen Electrode(RHE)) in 0.1M KOH solution and –0.03 V (vs. SCE) in the PBS solution, which both outperform the benchmark platinum-based catalysts. Fe/S@N/C-0.5-MFC had a higher Open Circuit Voltage(OCV) (0.71 V), stronger maximum power density (1196 mW/m2), and larger output voltage (0.47 V) than the Pt/C-MFC under the same conditions.

Cultivation of the Marine Macroalgae Chaetomorpha linum in Municipal Wastewater for Nutrient Recovery and Biomass Production

Compared to microalgae, macroalgae are larger in size, thereby imposing lower separation and drying costs. This study demonstrates the feasibility of cultivating macroalgae Chaetomorpha linum in different types of municipal wastewaters, their ability to remove nutrient and their biomass composition for downstream biofuel production. Screening experiments indicated that C. linum grew well on primary (PW) and secondary wastewaters (SW), as well as centrate wastewater (CW) diluted to less than 20%. In a subsequent experiment, a step feeding approach was found to significantly increase biomass productivity to 10.7 ± 0.2 g AFDW·m^-2·d^-1 (p < 0.001), a 26.5% improvement in comparison to the control with single feeding, when grown on 10-CW; meanwhile, nitrogen and phosphorus removal efficiencies rose to 86.8 ± 1.1% (p < 0.001) and 92.6 ± 0.2% (p < 0.001), respectively. The CO₂-supplemented SW cultures (10.1 ± 0.4 g AFDW·m^-2·d^-1) were 1.20 times more productive than the corresponding controls without CO₂ supplementation (p < 0.001); however, similar improvements were not observed in PW (p = 0.07) and 10-CW cultures (p = 0.07). Moreover, wastewater type and nutrient concentration influenced biomass composition (protein, carbohydrate and lipid). These findings indicate that the application of the macroalgae C. linum could represent an effective wastewater treatment alternative that could also provide a feedstock for downstream processing to biofuels.