Effect of CO 2 addition on biomass energy yield in wastewater treatment high rate algal mesocosms

Microalgal co-cultivation -recent methods, trends in omic-studies, applications, and future challenges

The burgeoning human population has resulted in an augmented demand for raw materials and energy sources, which in turn has led to a deleterious environmental impact marked by elevated greenhouse gas (GHG) emissions, acidification of water bodies, and escalating global temperatures. Therefore, it is imperative that modern society develop sustainable technologies to avert future environmental degradation and generate alternative bioproduct-producing technologies. A promising approach to tackling this challenge involves utilizing natural microbial consortia or designing synthetic communities of microorganisms as a foundation to develop diverse and sustainable applications for bioproduct production, wastewater treatment, GHG emission reduction, energy crisis alleviation, and soil fertility enhancement. Microalgae, which are photosynthetic microorganisms that inhabit aquatic environments and exhibit a high capacity for CO2 fixation, are particularly appealing in this context. They can convert light energy and atmospheric CO2 or industrial flue gases into valuable biomass and organic chemicals, thereby contributing to GHG emission reduction. To date, most microalgae cultivation studies have focused on monoculture systems. However, maintaining a microalgae monoculture system can be challenging due to contamination by other microorganisms (e.g., yeasts, fungi, bacteria, and other microalgae species), which can lead to low productivity, culture collapse, and low-quality biomass. Co-culture systems, which produce robust microorganism consortia or communities, present a compelling strategy for addressing contamination problems. In recent years, research and development of innovative co-cultivation techniques have substantially increased. Nevertheless, many microalgae co-culturing technologies remain in the developmental phase and have yet to be scaled and commercialized. Accordingly, this review presents a thorough literature review of research conducted in the last few decades, exploring the advantages and disadvantages of microalgae co-cultivation systems that involve microalgae-bacteria, microalgae-fungi, and microalgae-microalgae/algae systems. The manuscript also addresses diverse uses of co-culture systems, and growing methods, and includes one of the most exciting research areas in co-culturing systems, which are omic studies that elucidate different interaction mechanisms among microbial communities. Finally, the manuscript discusses the economic viability, future challenges, and prospects of microalgal co-cultivation methods.

Study from microcosms and mesocosms reveals Escherichia coli removal in high rate algae ponds during domestic wastewater treatment is primarily caused by dark decay

While high rate algal ponds (HRAPs) can provide efficient pathogen removal from wastewater, the mechanisms involved remain unclear. To address this knowledge gap, the mechanisms potentially causing Escherichia coli (E. coli) removal during microalgae-based wastewater treatment were successively assessed using laboratory microcosms designed to isolate known mechanisms, and bench scale assays performed in real HRAP broth. During laboratory assays, E. coli decay was only significantly increased by alkaline pH (above temperature-dependent thresholds) due to pH induced toxicity, and direct sunlight exposure via UV-B damage and/or endogenous photo-oxidation. Bench assays confirmed alkaline pH toxicity caused significant decay but sunlight-mediated decay was not significant, likely due to light attenuation in the HRAP broth. Bench assays also evidenced the existence of uncharacterized ‘dark’ decay mechanism(s) not observed in laboratory microcosms. To numerically evaluate the contribution of each mechanism and the uncertainty associated, E. coli decay was modelled assuming dark decay, alkaline pH induced toxicity, and direct sunlight-mediated decay were independent mechanisms. The simulations confirmed E. coli decay was mainly caused by dark decay during bench assays (48.2–89.5% estimated contribution to overall decay at the 95% confidence level), followed by alkaline-pH induced toxicity (8.3–46.5%), and sunlight-mediated decay (0.0–21.9%).

Determination of Microalgal Lipid Content and Fatty Acid for Biofuel Production

Biofuels produced from microalgal biomass have received growing worldwide recognition as promising alternatives to conventional petroleum-derived fuels. Among the processes involved, the downstream refinement process for the extraction of lipids from biomass greatly influences the sustainability and efficiency of the entire biofuel system. This review summarizes and compares the current techniques for the extraction and measurement of microalgal lipids, including the gravimetric methods using organic solvents, CO₂-based solvents, ionic liquids and switchable solvents, Nile red lipid visualization method, sulfo-phospho-vanillin method, and the thin-layer chromatography method. Each method has its own competitive advantages and disadvantages. For example, the organic solvents-based gravimetric method is mostly used and frequently employed as a reference standard to validate other methods, but it requires large amounts of samples and is time-consuming and expensive to recover solvents also with low selectivity towards desired products. The pretreatment approaches which aimed to disrupt cells and support subsequent lipid extraction through bead beating, microwave, ultrasonication, chemical methods, and enzymatic disruption are also introduced. Moreover, the principles and procedures for the production and quantification of fatty acids are finally described in detail, involving the preparation of fatty acid methyl esters and their quantification and composition analysis by gas chromatography.

Growth of Chlorella vulgaris and nutrient removal in the wastewater in response to intermittent carbon dioxide

In this study, Chlorella vulgaris (C. vulgaris) were cultured in cell culture flask supplied with intermittent CO₂ enriched gas. The impact of CO₂ concentration (from 1% to 20% v/v) on the growth of C. vulgaris cultured in domestic wastewater was exploited in various perspectives which include biomass, specific growth rate, culture pH, carbon consumption, and the removal of nitrogen and phosphorus compounds. The results showed that the maximum microalgal biomass concentration, 1.12 g L^-1, was achieved with 10% CO₂ as a feed gas. At 20% CO₂ the growth of C. vulgaris suffered from inhibition during initial 1.5 d, but acclimated to low pH (6.3 in average) with relatively higher specific growth rate (0.3-0.5 d^-1) during subsequent culture period. After the rapid consumption of ammonium in the wastewater, an obvious decline in the nitrate concentration was observed, indicating that C. vulgaris prefer ammonium as a primary nitrogen source. The total nitrogen and phosphorus decreased from 44.0 mg L^-1 to 2.1-5.4 mg L^-1 and from 5.2 mg L^-1 to 0-0.6 mg L^-1 within 6.5 d under the aeration of 1-20% CO₂, respectively, but no significant difference in consumed nitrogen versus phosphorus ratio was observed among different CO₂ concentration. The kinetics of nutrients removal were also determined through the application of pseudo first order kinetic model. 5-10% CO₂ aeration was optimal for the growth of C. vulgaris in the domestic wastewater, based on the coupling of carbon consumption, microalgal biomass, the nutrients removal and kinetics constants.