Pilot-scale cultivation of water-net in secondary effluent using an open pond raceway for nutrient removal and bioethanol production

Photobioreactor configurations in cultivating microalgae biomass for biorefinery

Microalgae, highly prized for their protein, lipid, carbohydrate, phycocyanin, and carotenoid-rich biomass, have garnered significant industrial attention in the context of third-generation (3G) biorefineries, seeking sustainable alternatives to non-renewable resources. Two primarily cultivation methods, open ponds and closed photobioreactors systems, have emerged. Open ponds, favored for their cost-effectiveness in large-scale industrial production, although lacking precise environmental control, contrast with closed photobioreactors, offering controlled conditions and enhanced biomass production at the laboratory scale. However, their high operational costs challenge large-scale deployment. This review comprehensively examines the strength, weakness, and typical designs of both outdoor and indoor microalgae cultivation systems, with an emphasis on their application in terms of biorefinery concept. Additionally, it incorporates techno-economic analyses, providing insights into the financial aspects of microalgae biomass production. These multifaceted insights, encompassing both technological and economic dimensions, are important as the global interest in harnessing microalgae's valuable resources continue to grow.

Sustainable treatment of primary and secondary effluent by algal-bacterial flocculent biomass in raceway ponds

Two 5.5-L raceway open ponds were used to evaluate the removal of organic material and nutrients from wastewater. Algal-bacterial flocs were placed in the ponds to treat primary and secondary effluent. The organic loading rate ranged from 29 to 95 and 9 to 38 g sCOD m^-3 d^-1 for the reactor fed with primary and secondary effluent, respectively. The hydraulic retention time (HRT) gradually decreased in both reactors from 5.5 to 2.2 d during a period of 21 days, and after that, both reactors operated at an HRT of 1.1 d. A high biomass concentration of around 2.2 g L^-1 was sustained using primary and secondary effluent after 130 days. The biomass, developed with both substrates was very active and completely removed organic material and nutrients in less than 12 h. The algal-bacteria biomass had excellent settling properties and could settle in less than 10 min.

Field test of water-net based wastewater treatment for nutrient removal and bioethanol production

In this study, an open pond constructed in Myanmar, a region with tropical climate and favorable environmental conditions for algae growth, was considered to conduct field experiments on sewage inflow river water. The nutrient removal efficiency and productivity of Hydrodictyon reticulatum (H. reticulatum) were analyzed, and the maximum fermentation limit concentration for bioethanol production was determined. Three ponds were operated in batch mode to investigate the effect of light intensity. Photoinhibition was caused due to excessive light intensity in summer season in the region with tropical climate resulting in reduced facility efficiency in the absence of shade. For light blocking, a transparent film was found to be more effective than a translucent film. In the transparent film shading facility, the nitrogen and phosphorus removal efficiencies were maintained above 76% and 81%, respectively, and the productivity of H. reticulatum was 2.27 g m^-2 d^-1. For a raceway open pond facility shaded with transparent film, the performance was evaluated based on hydraulic retention time (HRT), and the productivity of algae was found to increase with increasing supply of nitrogen and phosphorous. Maximum biomass production of 3.21 g m^-2 d^-1 was observed with an HRT of 3 d, suggesting the possibility of long-term operation. As a result of evaluating the ethanol production based on the initial concentration of H. reticulatum, the yield of bioethanol at the initial reducing sugar content of 120 g L^-1 was 89.4%, but bioethanol production was only 8.9 g L^-1.

Challenges in Environmental Science/Engineering and fate and innovative treatment/remediation of emerging pollutants

Solid waste Management: There are two articles in this section. Shi et al. (2021) investigated the unbalanced status and multidimensional influences of municipal solid waste management in Africa. It was identified that economic growth, urbanization and geographical location are the most critical factors influencing the unbalanced statue of MSW management in Africa.

Biomass Potential for Producing Power via Green Hydrogen

Hydrogen (H2) has become an important energy vector for mitigating the effects of climate change since it can be obtained from renewable sources and can be fed to fuel cells for producing power. Bioethanol can become a green H2 source via Ethanol Steam Reforming (ESR) but several variables influence the power production in the fuel cell. Herein, we explored and optimized the main variables that affect this power production. The process includes biomass fermentation, bioethanol purification, H2 production via ESR, syngas cleaning by a CO-removal reactor, and power production in a high temperature proton exchange membrane fuel cell (HT-PEMFC). Among the explored variables, the steam-to-ethanol molar ratio (S/E) employed in the ESR has the strongest influence on power production, process efficiency, and energy consumption. This effect is followed by other variables such as the inlet ethanol concentration and the ESR temperature. Although the CO-removal reactor did not show a significant effect on power production, it is key to increase the voltage on the fuel cell and consequently the power production. Optimization was carried out by the response surface methodology (RSM) and showed a maximum power of 0.07 kWh kg−1 of bioethanol with an efficiency of 17%, when ESR temperature is 700 °C. These values can be reached from different bioethanol sources as the S/E and CO-removal temperature are changed accordingly with the inlet ethanol concentration. Because there is a linear correlation between S/E and ethanol concentration, it is possible to select a proper S/E and CO-removal temperature to maximize the power generation in the HT-PEMFC via ESR. This study serves as a starting point to diversify the sources for producing H2 and moving towards a H2-economy.

Converting nitrogen and phosphorus wastewater into bioenergy using microalgae-bacteria consortia: A critical review

Conventional wastewater treatment using activated sludge cannot efficiently eliminate nitrogen and phosphorus, thus engendering the risk of water eutrophication and ecosystem disruption. Fortunately, a new wastewater treatment process applying microalgae-bacteria consortia has attracted considerable interests due to its excellent performance of nutrients removal. Moreover, some bacteria facilitate the harvest of microalgal biomass through bio-flocculation. Additionally, while stimulating the functional bacteria, the improved biomass and enriched components also brighten bioenergy production from the perspective of practical applications. Thus, this review first summarizes the current development of nutrients removal and mutualistic interaction using microalgae-bacteria consortia. Then, advancements in bio-flocculation are completely described and the corresponding mechanisms are thoroughly revealed. Eventually, the recent advances of bioenergy production (i.e., biodiesel, biohydrogen, bioethanol, and bioelectricity) using microalgae-bacteria consortia are comprehensively discussed. Together, this review will provide the ongoing challenges and future developmental directions for better converting nitrogen and phosphorus wastewater into bioenergy using microalgae-bacteria consortia.