Agro-industrial wastewater-grown microalgae: A techno-environmental assessment of open and closed systems

Integrating electro-Fenton and microalgae for the sustainable management of real food processing wastewater

The present study demonstrates, for the first time, the feasibility of a two-step process consisting of Electro-Fenton (EF) followed by microalgae to treat highly loaded real food processing wastewater along with resource recovery. In the first step, EF with a carbon felt cathode and Ti/RuO2-IrO2 anode was applied at different current densities (3.16 mA cm-2, 4.74 mA cm-2 and 6.32 mA cm-2) to decrease the amount of organic matter and turbidity and enhance biodegradability. In the second step, the EF effluents were submitted to microalgal treatment for 15 days using a mixed culture dominated by Scenedesmus sp., Chlorosarcinopsis sp., and Coelastrum sp. Results showed that current density impacted the amount of COD removed by EF, achieving the highest COD removal of 77.5% at 6.32 mA cm-2 with >95% and 74.3% of TSS and PO43- removal, respectively. With respect to microalgae, the highest COD removal of 85% was obtained by the culture in the EF effluent treated at 6.32 mA cm-2. Remarkably, not only 85% of the remaining organic matter was removed by microalgae, but also the totality of inorganic N and P compounds, as well as 65% of the Fe catalyst that was left after EF. The removal of inorganic species also demonstrates the high complementarity of both processes, since EF does not have the capacity to remove such compounds, while microalgae do not grow in the raw wastewater. Furthermore, a maximum of 0.8 g L-1 of biomass was produced after cultivation, with an accumulation of 32.2% of carbohydrates and 25.9% of lipids. The implementation of the two processes represents a promising sustainable approach for the management of industrial effluents, incorporating EF in a water and nutrient recycling system to produce biomass that could be valorized into clean fuels.

Scale-up of microalgal systems for decarbonization and bioproducts: Challenges and opportunities

The threat of global climate change presents a significant challenge for humanity. Microalgae-based carbon capture and utilization (CCU) technology has emerged as a promising solution to this global issue. This review aims to comprehensively evaluate the current advancements in scale-up of microalgae cultivation and its applications, specifically focusing on decarbonization from flue gases, organic wastewater remediation, and biogas upgrading. The study identifies critical challenges that need to be addressed during the scale-up process and evaluates the economic viability of microalgal CCU within the carbon market. Additionally, it analyzes the commercial status of microalgae-derived products and highlights those with high market demand. This review serves as a crucial resource for researchers, industry professionals, and policymakers to develop and implement innovative approaches to enhance the efficiency of microalgae-based CO2 utilization while addressing the challenges associated with the scale-up of microalgae technologies.

Enhancing microalgae biomass production: Exploring improved scraping frequency in a hybrid cultivation system

Recently, hybrid systems, such as those incorporating high-rate algal ponds (HRAPs) and biofilm reactors (BRs), have shown promise in treating domestic wastewater while cultivating microalgae. In this context, the objective of the present study was to determine an improved scraping frequency to maximize microalgae biomass productivity in a mix of industrial (fruit-based juice production) and domestic wastewater. The mix was set to balance the carbon/nitrogen ratio. The scraping strategy involved maintaining 1 cm wide stripes to retain an inoculum in the reactor. Three scraping frequencies (2, 4, and 6 days) were evaluated. The findings indicate that a scraping frequency of each 2 days provided the highest biomass productivity (18.75 g total volatile solids m-2 d-1). The species' behavior varied with frequency: Chlorella vulgaris was abundant at 6-day intervals, whereas Tetradesmus obliquus favored shorter intervals. Biomass from more frequent scraping demonstrated a higher lipid content (15.45%). Extrapolymeric substance production was also highest at the 2-day frequency. Concerning wastewater treatment, the system removed 93% of dissolved organic carbon and ∼100% of ammoniacal nitrogen. Combining industrial and domestic wastewater sources to balance the carbon/nitrogen ratio enhanced treatment efficiency and biomass yield. This study highlights the potential of adjusting scraping frequencies in hybrid systems for improved wastewater treatment and microalgae production.

Energy balance and life cycle assessments in producing microalgae biodiesel via a continuous microalgal-bacterial photobioreactor loaded with wastewater

Life cycle assessments of microalgal cultivation systems are often conducted to evaluate the sustainability and feasibility factors of the entire production chain. Unlike widely reported conventional microalgal cultivation systems, the present work adopted a microalgal-bacterial cultivation approach which was upscaled into a pilot-scale continuous photobioreactor for microalgal biomass production into biodiesel from wastewater resources. A multiple cradle-to-cradle system ranging from microalgal biomass-to-lipid-to-biodiesel was evaluated to provide insights into the energy demand of each processes making up the microalgae-to-biodiesel value chain system. Energy feasibility studies revealed positive NER values (4.95-8.38) for producing microalgal biomass but deficit values for microalgal-to-biodiesel (0.14-0.23), stemming from the high energy input requirements in the downstream processes for converting biomass into lipid and biodiesel accounting to 88-90% of the cumulative energy demand. Although the energy balance for microalgae-to-biodiesel is in the deficits, it is comparable with other reported biodiesel production case studies (0.12-0.40). Nevertheless, the approach to using microalgal-bacterial cultivation system has improved the overall energy efficiency especially in the upstream processes compared to conventional microalgal cultivation systems. Energy life cycle assessments with other microalgal based biofuel systems also proposed effective measures in increasing the energy feasibility either by utilizing the residual biomass and less energy demanding downstream extraction processes from microalgal biomass. The microalgal-bacterial cultivation system is anticipated to offer both environmental and economic prospects for upscaling by effectively exploiting the low-cost nutrients from wastewaters via bioconversion into valuable microalgal biomass and biodiesel.

An integration of algae-mediated wastewater treatment and resource recovery through anaerobic digestion

Eutrophication is one of the major emerging challenges in aquatic environment. Industrial facilities, including food, textile, leather, and paper, generate a significant amount of wastewater during their manufacturing process. Discharge of nutrient-rich industrial effluent into aquatic systems causes eutrophication, eventually disturbs the aquatic system. On the other hand, algae provide a sustainable approach to treat wastewater, while the resultant biomass may be used to produce biofuel and other valuable products such as biofertilizers. This review aims to provide new insight into the application of algal bloom biomass for biogas and biofertilizer production. The literature review suggests that algae can treat all types of wastewater (high strength, low strength, and industrial). However, algal growth and remediation potential mainly depend on growth media composition and operation conditions such as light intensity, wavelength, light/dark cycle, temperature, pH, and mixing. Further, the open pond raceways are cost-effective compared to closed photobioreactors, thus commercially applied for biomass generation. Additionally, converting wastewater-grown algal biomass into methane-rich biogas through anaerobic digestion seems appealing. Environmental factors such as substrate, inoculum-to-substrate ratio, pH, temperature, organic loading rate, hydraulic retention time, and carbon/nitrogen ratio significantly impact the anaerobic digestion process and biogas production. Overall, further pilot-scale studies are required to warrant the real-world applicability of the closed-loop phycoremediation coupled biofuel production technology.

Recent advances in CO2 fixation by microalgae and its potential contribution to carbon neutrality

Many countries and regions have set their schedules to achieve the carbon neutrality between 2030 and 2070. Microalgae are capable of efficiently fixing CO₂ and simultaneously producing biomass for multiple applications, which is considered one of the most promising pathways for carbon capture and utilization. This work reviews the current research on microalgae CO₂ fixation technologies and the challenges faced by the related industries and government agencies. The technoeconomic analysis indicates that cultivation is the major cost factor. Use of waste resources such as wastewater and flue gas can significantly reduce the costs and carbon footprints. The life cycle assessment has identified fossil-based electricity use as the major contributor to the global warming potential of microalgae-based CO₂ fixation approach. Substantial efforts and investments are needed to identify and bridge the gaps among the microalgae strain development, cultivation conditions and systems, and use of renewable resources and energy.

Tertiary Wastewater Treatment Technologies: A Review of Technical, Economic, and Life Cycle Aspects

The activated sludge process is the most widespread sewage treatment method. It typically consists of a pretreatment step, followed by a primary settling tank, an aerobic degradation process, and, finally, a secondary settling tank. The secondary effluent is then usually chlorinated and discharged to a water body. Tertiary treatment aims at improving the characteristics of the secondary effluent to facilitate its reuse. In this work, through a literature review of the most prominent tertiary treatment methods, a benchmarking of their technical efficiency, economic feasibility, and environmental impact was carried out. The photo-Fenton method proved to be the most technically efficient process, significantly reducing the microbial load and pharmaceutical content (by 4.9 log and 84%, respectively) of the secondary effluent. Chlorination and UV irradiation exhibited the lowest treatment costs (0.004 EUR/m−3) and the lowest global warming potential (0.04 and 0.09 kg CO2eq. m−3, respectively). After all the data were aggregated, a decision-making tool was constructed in the form of a ternary diagram, which indicates the most appropriate tertiary treatment method according to the weight-per-process aspect (technical, economic, and environmental) selected by the user, with chlorination, UV irradiation, ozonation, microalgae cultivation, and constructed wetlands prevailing in the final results.

Sustainability and carbon neutralization trends in microalgae bioenergy production from wastewater treatment: A review

Reducing CO2 emissions using biomass is gaining popularity as an environmentally friendly strategy. Due to high growth rates, low production costs, and ability to withstand harsh conditions, microalgae have become quite popular. Microalgae may also undertake photosynthesis, converting CO2 and solar energy into sugar before becoming biomass, making them an excellent source of renewable and promising biofuels. CO2 sequestration and biofixation was utilized to compare the synthesis of biodiesel as a third-generation biofuel from various types of wastewater was also used as a source for the algal cultivation. This review article focuses on recent developments, research discoveries in the field of microalgal CO2 capture modification and the optimization of conversion efficiency. This review is intended to serve as a helpful and reference for the use of wastewater treatment with microalgae to collect CO2. The overarching objective of this study is to assist wastewater treatment systems in achieving carbon neutrality.