A novel algal biofilm photobioreactor for efficient hog manure wastewater utilization and treatment

A fast microbial nitrogen-assimilation technology enhances nitrogen migration and single-cell-protein production in high-ammonia piggery wastewater

Conventional methods, such as freshwater dilution and ammonia stripping, have been widely employed for microalgae-based piggery wastewater (PW) treatment, but they cause high freshwater consumption and intensive ammonia loss, respectively. This present work developed a novel fast microbial nitrogen-assimilation technology by integrating nitrogen starvation, zeolite-based adsorption, pH control, and co-culture of microalgae-yeast for the PW treatment. Among them, the nitrogen starvation accelerated the nitrogen removal and shortened the treatment period, but it could not improve the tolerance level of microalgal cells to ammonia toxicity based on oxidative stress. Therefore, zeolite was added to reduce the initial total ammonia-nitrogen concentration to around 300 mg/L by ammonia adsorption. Slowly releasing ammonia at the later phase maintained the total ammonia-nitrogen concentration in the PW. However, the pH increase could cause lots of ammonia loss air and pollution and inhibit the desorption of ammonia from zeolite and the growth and metabolism of microalgae during the microalgae cultivation. Thus, the highest biomass yield (3.25 g/L) and nitrogen recovery ratio (40.31%) were achieved when the pH of PW was controlled at 6.0. After combining the co-cultivation of microalgae-yeast, the carbon-nitrogen co-assimilation and the alleviation of pH fluctuation further enhanced the nutrient removal and nitrogen migration to high-protein biomass. Consequently, the fast microbial nitrogen-assimilation technology can help update the industrial system for high-ammonia wastewater treatment by improving the treatment and nitrogen recovery rates.

Comprehensive assessment of microalgal-based treatment processes for dairy wastewater

The dairy industry is becoming one of the biggest sectors within the global food industry, and these industries use almost 34% of the water. The amount of water used is governed by the production process and the technologies employed in the plants. Consequently, the dairy industries generate almost 0.2-10 L of wastewater per liter of processed milk, which must be treated before being discharged into water bodies. The cultivation of microalgae in a mixotrophic regime using dairy wastewater enhances biomass growth, productivity, and the accumulation of value-added product. The generated biomass can be converted into biofuels, thus limiting the dependence on petroleum-based crude oil. To fulfill the algal biorefinery model, it is important to utilize every waste stream in a cascade loop. Additionally, the harvested water generated from algal biomass production can be recycled for further microalgal growth. Economic and sustainable wastewater management, along with proper reclamation of nutrients from dairy wastewater, is a promising approach to mitigate the problem of water scarcity. A bibliometric study revealing limited work on dairy wastewater treatment using microalgae for biofuel production. And, limited work is reported on the pretreatment of dairy wastewater via physicochemical methods before microalgal-based treatment. There are still significant gaps remains in large-scale cultivation processes. It is also crucial to discover robust strains that are highly compatible with the specific concentration of contaminants, as this will lead to increased yields and productivity for the targeted bio-product. Finally, research on reutilization of culture media in photobioreactor is necessary to augument the productivity of the entire process. Therefore, the incorporation of the microalgal biorefinery with the wastewater treatment concept has great potential for promoting ecological sustainability.

Strategies to enhance biohydrogen production from microalgae: A comprehensive review

Microalgae represent a promising renewable feedstock for the sustainable production of biohydrogen. Their high growth rates and ability to fix carbon utilizing just sunlight, water, and nutrients make them well-suited for this application. Recent advancements have focused on improving microalgal hydrogen yields and cultivation methods. This review aims to summarize recent developments in microalgal cultivation techniques and genetic engineering strategies for enhanced biohydrogen production. Specific areas of focus include novel microalgal species selection, immobilization methods, integrated hybrid systems, and metabolic engineering. Studies related to microalgal strain selection, cultivation methods, metabolic engineering, and genetic manipulations were compiled and analyzed. Promising microalgal species with high hydrogen production capabilities such as Synechocystis sp., Anabaena variabilis, and Chlamydomonas reinhardtii have been identified. Immobilization techniques like encapsulation in alginate and integration with dark fermentation have led to improved hydrogen yields. Metabolic engineering through modulation of hydrogenase activity and photosynthetic pathways shows potential for enhanced biohydrogen productivity. Considerable progress has been made in developing microalgal systems for biohydrogen. However, challenges around process optimization and scale-up remain. Future work involving metabolic modeling, photobioreactor design, and genetic engineering of electron transfer pathways could help realize the full potential of this renewable technology.

Microalgal-bacterial immobilized co-culture as living biofilters for nutrient recovery from synthetic wastewater and their potential as biofertilizers

This study investigates nutrient recovery from synthetic municipal wastewater using co-immobilized cultures of Chlorella vulgaris TISTR 8580 (CV) and plant growth-promoting bacteria, Bacillus subtilis TISTR 1415 (BS) as living biofilters for a subsequent biofertilizer activity. The optimal condition for nutrient recovery was at the 1:1 ratio of CV/BS using mixed guar gum/carrageenan (GG/CG) binders. After 7-day wastewater treatment, the living biofilters removed 86.7 ± 0.5% of ammonium and 99.3 ± 0.3% of phosphates and were tested subsequently as biofertilizers for 20 days to grow selected plants. The highest optimal biomass and chlorophyll a content was 2 ± 0.3 g (CV/BS 3:1) and 12.4 ± 0.7 µg/g (CV/BS 1:1) from cucumber respectively, however, the close-to-neutral pH (8.0 ± 0.3) was observed from sunflower using CV/BS 1:1 living biofilters. Conclusively, the designed living biofilters exhibit the potential to recover nutrients from wastewater and be used as biofertilizers for circular agriculture.

Eco-friendly cultivation of microalgae using a horizontal twin layer system for treatment of real solid waste leachate

Solid waste leachate (SWL) requires dilution with water to offset the negative effects of high nutrient concentration and organic compounds for its microalgae-based treatment. Among attached cultivation systems, twin layer is a technology in which limited information is available on treatment of high strength wastewater using microalgae. Moreover, widespread application of twin layer technology is limited due to cost of substrate and source layer used. In the present study, potential of Scenedesmus sp. for the treatment of SWL was assessed on horizontal twin layer system (HTLS). Novel and cost-effective substrate layers were tested as attachment material. Wetland treated municipal wastewater (WMW) was used to prepare SWL dilutions viz, 5%, 10%, 15%, 20% and 25% SWL. Recycled printing paper showed maximum biomass productivity of 5.19 g m-2 d-1. Among all the SWL dilutions, Scenedesmus sp. achieved maximum growth of 103.05 g m-2 in 5% SWL which was 16% higher than WMW alone. The maximum removal rate of NH4+ -N, TKN, and PO43- P was obtained in 20% SWL which was 1371, 1588 and 153 mg m-2 d-1 respectively. Varying concentrations of nutrients in different SWL dilutions significantly affected lipid biosynthesis, with enhanced productivity of 2.28 g m-2 d-1 achieved in 5% SWL compared to 0.97 g m-2 d-1 in 20% SWL. Hence, it can be concluded that 5% SWL dilution was good for biomass and lipid production, while the highest nutrient removal rates were obtained at 20% SWL mainly attributed to biotic and abiotic processes. Based on these results HTLS can be a promising technology for pilot scale to explore industrialized application of wastewater treatment and algal production.

Treatment of mixed wastewater by vertical rotating microalgae-bacteria symbiotic biofilm reactor

A novel vertical rotating microalgae-bacteria symbiotic biofilm reactor was built to treat the mixed wastewater containing municipal and soybean soaking wastewater. The reactor was operated in both sequential batch and semi-continuous modes. Under the sequential batch operation mode, the maximum removal rates for Chemical Oxygen Demand (COD), Total Nitrogen (TN), Total Phosphorus (TP), and Ammonia Nitrogen (NH4+-N) of the mixed wastewater were 95.6 %, 96.1 %, 97.6 %, and 100 %, respectively. During the semi-continuous operation, the water discharge indices decreased gradually and eventually stabilized. At stabilization, the removal rates of COD, TN, and NH4+-N achieved 98 %, 95 %, and 99.9 %, respectively. The maximum biomass productivity of the biofilm was 2.69 g·m-2·d-1. Additionally, the carbohydrate, protein and lipid comprised approximately 22 %, 51 % and 10 % of the dry weight of Chlorella. This study demonstrates the great potential of the microalgae-bacteria symbiotic biofilm system to treat food and domestic wastewater while harvesting microalgal biomass.

Exploring the potential of Diplosphaera mucosa VSPA for the treatment of petroleum effluent with simultaneous lipid production

The discovery of unexplored, robust microalgal strains will assist in treating highly polluted industrial effluent, including petroleum effluent. In the current analysis, a newly isolated microalgal strain, Diplosphaera mucosa VSPA, was used to treat petroleum effluent in a lab-scale raceway bioreactor. Its treatment efficiency was compared with a well-known species, Chlorella pyrenoidosa. The D. mucosa VSPA strain proliferated in petroleum effluent at a high growth rate, with final biomass, and lipid concentrations reaching 6.93 g/L and 2.72 g/L, respectively. Treatment efficiency was calculated based on the final removal efficiency of ammonium nitrogen, phosphate phosphorus, and chemical oxygen demand, which was more than 90%. Control experiments suggested that the maximum removal of pollutants from petroleum effluent was due to microalgae growth. Some growth models, including the Gompertz, Logistic, Stannard, Richard, and Schnute, were used to simulate the experimental data, verifying the results. Good fitting of all models was obtained, with the R2 value reaching more than 0.90. The development of a suitable model can help in decreasing the efforts required for the scale-up of the process.

Advancement on mixed microalgal-bacterial cultivation systems for nitrogen and phosphorus recoveries from wastewater to promote sustainable bioeconomy

Biological wastewater treatment is a promising and environmentally friendly method that utilises living microorganisms to remediate water and enable recovery or conversion of contaminants into valuable products. For many decades, microalgae and cyanobacteria, photosynthetic living microorganisms, have been explored extensively for wastewater bioremediation. They can be used for recovering valuable nutrients such as nitrogen and phosphorous from secondary effluents and capable of transforming those nutrients into marketable products such as biofuels, biofertilisers, nutraceutical, and pigments for promoting a Bio-Circular Green economy. In recent years, there has been a shift towards mixing compatible microalgae with bacteria, which is inspired by their natural symbiotic relationships to increase nitrogen and phosphorus recoveries. With this enhanced bioremediation, recovery of polluted wastes can be intensified and higher biomass quality (with high nutrient density) can be achieved. This review focuses on the state-of-the-art of mixed microalgal-bacterial cultivating systems. A comprehensive comparison of existing studies that used Chlorella species as microalgae in various mixed microalgal-bacterial cultivating systems (suspension, biofilm, and immobilisation) for nitrogen and phosphorus recoveries from wastewater is conducted. Key technical challenges such as balancing microalgae and bacteria species, pH regulation, light distribution, biomass harvesting, and biomass conversion are also discussed. From the data comparisons among different cultivation systems, it has been suggested that immobilisation appears to require less amount of operational light compared to the suspended and biofilm-based systems for similar nitrogen and phosphorus removal efficiencies.

A comprehensive review on the advances of bioproducts from biomass towards meeting net zero carbon emissions (NZCE)

This review investigates the development of bioproducts from biomass and their contribution towards net zero carbon emissions. The promising future of biomasses conversion techniques to produce bioproducts was reviewed. The advances in anaerobic digestion as a biochemical conversion technique have been critically studied and contribute towards carbon emissions mitigation. Different applications of microalgae biomass towards carbon neutrality were comprehensively discussed, and several research findings have been tabulated in this review. The carbon footprints of wastewater treatment plants were studied, and bioenergy utilisation from sludge production was shown to mitigate carbon footprints. The carbon-sinking capability of microalgae has also been outlined. Furthermore, integrated conversion processes have shown to enhance bioproducts generation yield and quality. The anaerobic digestion/pyrolysis integrated process was promising, and potential substrates have been suggested for future research. Lastly, challenges and future perspectives of bioproducts were outlined for a contribution towards meeting carbon neutrality.

Innovative Coating-Etching Method of Biocarrier Fabrication for Treating Wastewater with a Low C/N Ratio

A novel method was used to fabricate the bio-carrier with both a high specific surface area and good compatibility. The results of monitoring the growth of biofilms at a low C/N ratio (0.83) showed that resulting carrier-PLA-cavity offered certain advantages for biofilm growth by providing an appropriate microenvironment for bacterial growth in wastewater treatment. The biofilm on carrier-PLA-cavity grew and updated faster than the naked-carrier. The biomass and thickness of biofilms growing on carrier-PLA-cavity were 10 kg/m³ and 500 μm, respectively. From the wastewater tests, 90% of the total nitrogen was removed via simultaneous nitrification and denitrification (SND) by the biofilm biomass attached to carrier-PLA-cavity, compared to 68% for the naked-carrier. The COD removal efficiency values of the carrier-PLA-cavity and naked-carrier were 94% and 86%, respectively. The microbial community analysis of carrier biofilms showed that Halomonas was the most abundant genus, and heterotrophic nitrification and denitrification were responsible for nitrogen removal in both reactors. Notably, this method does not require any complicated equipment or structural design. This novel method might be a promising strategy for fabricating biocarriers for treating wastewater with a low C/N ratio.

Emerging technologies for sustainable production of biohydrogen production from microalgae: A state-of-the-art review of upstream and downstream processes

Biohydrogen (BioH₂) is considered as one of the most environmentally friendly fuels and a strong candidate to meet the future demand for a sustainable source of energy. Presently, the production of BioH₂ from photosynthetic organisms has raised a lot of hopes in the fuel industry. Moreover, microalgal-based BioH₂ synthesis not only helps to combat current global warming by capturing greenhouse gases but also plays a key role in wastewater treatment. Hence, this manuscript provides a state-of-the-art review of the upstream and downstream BioH₂ production processes. Different metabolic routes such as direct and indirect photolysis, dark fermentation, photofermentation, and microbial electrolysis are covered in detail. Upstream processes (e.g. growth techniques, growth media) also have a great impact on BioH₂ productivity and economics, which is also explored. Technical and scientific obstacles of microalgae BioH₂ systems are finally addressed, allowing the technology to become more innovative and commercial.

Roadmap to sustainable carbon-neutral energy and environment: can we cross the barrier of biomass productivity?

The total number of inhabitants on the Earth is estimated to cross a record number of 9 × 10³ million by 2050 that present a unique challenge to provide energy and clean environment to every individual. The growth in population results in a change of land use, and greenhouse gas emission due to increased industrialization and transportation. Energy consumption affects the quality of the environment by adding carbon dioxide and other pollutants to the atmosphere. This leads to oceanic acidification and other environmental fluctuations due to global climate change. Concurrently, speedy utilization of known conventional fuel reservoirs causes a challenge to a sustainable supply of energy. Therefore, an alternate energy resource is required that can maintain the sustainability of energy and environment. Among different alternatives, energy production from high carbon dioxide capturing photosynthetic aquatic microbes is an emerging technology to clean environment and produce carbon-neutral energy from their hydrocarbon-rich biomass. However, economical challenges due to low biomass production still prevent the commercialization of bioenergy. In this work, we review the impact of fossil fuels burning, which is predominantly used to fulfill global energy demand, on the quality of the environment. We also assess the status of biofuel production and utilization and discuss its potential to clean the environment. The complications associated with biofuel manufacturing using photosynthetic microorganisms are discussed and directed evolution for targeted phenotypes and targeted delivery of nutrients are proposed as potential strategies to increase the biomass production.

Sustainable cultivation of Navicula incerta using cellulose-based scaffold incorporated with nanoparticles in air-liquid interface cultivation system

Microalgae cultivation using open cultivation systems requires large area and it is susceptible to contamination as well as weather changes. Meanwhile, the closed systems require large capital investment, and they are susceptible to the build-up of dissolved oxygen. Air-liquid interface culture systems with low water-footprint, but high packing density can be used for microalgae cultivation if low-cost culture scaffolds are available. In this study, cellulose-based scaffolds were synthesized using NaOH/urea aqueous solution as the solvent. Titanium dioxide (TiO₂), silica gel and polyethylene glycol 1000 (PEG 1000) nanoparticles were added into the membrane scaffolds to increase the hydrophilicity of nutrient absorbing to support the growth of microalgae. The membrane scaffolds were characterized by FTIR, SEM, contact angle, porosity and porometry. All three nanoparticles additives showed their ability in reducing the contact angle of membrane scaffolds from 63.4 ± 2.3° to a range of 52.6 ± 1.2° to 38.8 ± 1.5° due to the hydrophilic properties of the nanoparticles. The decreasing in pore size when nanoparticles were added did not affect the porosity of membrane scaffolds. Cellulose membrane scaffold with TiO₂ showed the highest percentage of microalgae Navicula incerta growth rate of 22.1% because of the antibacterial properties of TiO₂ in lowering the risk of cell contamination and enhancing the growth of N. incerta. The results exhibited that cellulose-based scaffold with TiO₂ added could be an effective support in plant cell culture field.

Wastewater based microalgal biorefinery for bioenergy production: Progress and challenges

Treatment of industrial and domestic wastewater is very important to protect downstream users from health risks and meet the freshwater demand of the ever-increasing world population. Different types of wastewater (textile, dairy, pharmaceutical, swine, municipal, etc.) vary in composition and require different treatment strategies. Wastewater management and treatment is an expensive process; hence, it is important to integrate relevant technology into this process to make it more feasible and cost-effective. Wastewater treatment using microalgae-based technology could be a global solution for resource recovery from wastewater and to provide affordable feedstock for bioenergy (biodiesel, biohydrogen, bio-alcohol, methane, and bioelectricity) production. Various microalgal cultivation systems (open or closed photobioreactors), turf scrubber, and hybrid systems have been developed. Although many algal biomass harvesting methods (physical, chemical, biological, and electromagnetic) have been reported, it is still an expensive process. In this review article, resource recovery from wastewater using algal cultivation, biomass harvesting, and various technologies applied in converting algal biomass into bioenergy, along with the various challenges that are encountered are discussed in brief.

Inclined algal biofilm photobioreactor (IABPBR) for cost-effective cultivation of lipid-rich microalgae and treatment of seawater-diluted anaerobically digested effluent from kitchen waste with the aid of phytohormones

Previous study has demonstrated that freshwater can be replaced with seawater for dilution of feed to algal production and wastewater treatment, but high harvest cost in suspended-growth systems is still a troublesome limitation for large-scale production. Therefore, a novel inclined algal biofilm photobioreactor (IABPBR) was constructed for algal bioproduct production and treatment of seawater-diluted anaerobically digested effluent (SA) in this study. Fluffy polyester was selected as the best carrier for the algal biofilm among ten discarded materials. With the help of phytohormones, the viability of SDEC-18 was clearly enhanced and an algal biomass productivity of 5.66 g/m²/d was achieved. The SDEC-18 biofilm provided removal capacities of 0.65, 0.25 and 3.31 g/m²/d for TN, TP and COD. Phytohormones clearly enhanced the lipid biosynthesis, with an extraordinary lipid productivity of 3.98 g/m²/d being achieved. Moreover, an automatic harvesting system was designed for the efficient harvesting process during large-scale production.

CRISPR/Cas technology promotes the various application of Dunaliella salina system

Dunaliella salina (D. salina) has been widely applied in various fields because of its inherent advantages, such as the study of halotolerant mechanism, wastewater treatment, recombinant proteins expression, biofuel production, preparation of natural materials, and others. However, owing to the existence of low yield or in the laboratory exploration stage, D. salina system has been greatly restricted for practical production of various components. In past decade, significant progresses have been achieved for research of D. salina in these fields. Among them, D. salina as a novel expression system demonstrated a bright prospect, especially for large-scale production of foreign proteins, like the vaccines, antibodies, and other therapeutic proteins. Due to the low efficiency, application of traditional regulation tools is also greatly limited for exploration of D. salina system. The emergence of the clustered regularly interspaced short palindromic repeats (CRISPR)/Cas system offers a precise editing tool to overcome the obstacles of D. salina system. This review not only comprehensively summarizes the recent progresses of D. salina in domain of gene engineering but also gives a deep analysis of problems and deficiencies in different fields of D. salina. Moreover, further prospects of CRISPR/Cas system and its significant challenges have been discussed in various aspects of D. salina. It provides a great referencing value for speeding up the maturity of D. salina system, and also supplies practical guiding significance to expand the new application fields for D. salina. KEY POINTS: • The review provides recent research progresses of various applications of D. salina. • The problems and deficiencies in different fields of D. salina were deeply analyzed. • The further prospects of CRISPR/Cas technology in D. salina system were predicted. • CRISPR/Cas system will promote the new application fields and maturity for D. salina.

Non-suspended microalgae cultivation for wastewater refinery and biomass production

Non-suspended microalgae cultivation technology coupled with wastewater purification has received more scientific attention in recent decades. Since the non-suspended microalgae cultivation is quite different from the suspended ones, the following issues are compared in this study such as advantages and disadvantages, pollutant removal mechanisms and regulations, influential factors, and microalgae biomass accumulation. The analysis aims to support the further application of this technology. The median removal rates of COD, TN, TP, NH₄⁺-N and NO₃^--N were 91.6%, 78.2%, 87.5%, 93.2% and 81.7%, respectively, by non-suspended microalgae under the TN & TP load rates up to 150 mg·L^-1·d^-1. The main pathway for TN & TP removal is microalgae cell absorbance. Light intensity, pollutant composition and microalgae metabolic types are the major factors that influence pollutant removal and the lipid content of microalgae. Meanwhile the mechanism concerning how macro-outer conditions influence the micro-environment and further growth of non-suspended microalgae requires more investigation.

Optimizing real swine wastewater treatment efficiency and carbohydrate productivity of newly microalga Chlamydomonas sp. QWY37 used for cell-displayed bioethanol production

This work aimed to study an newly isolated microalgal strain, Chlamydomonas sp. QWY37, that can achieve a maximum carbohydrate production of 944 mg/L·d, along with high pollutant removal efficiencies (chemical oxygen demand: 81%, total nitrogen: 96%, total phosphate: nearly 100%) by optimizing culture conditions and using an appropriate operation strategy. Through a cell-displayed technology that utilizes combined engineered system, a maximum microalgal bioethanol yield of 61 g/L was achieved. This is the first report demonstrating the highest microalgal carbohydrate productivity using swine wastewater without any pretreatments associated with direct high-density bioethanol production from the subsequent microalgal biomass. This work may represent a breakthrough in achieving feasible microalgal bioethanol conversion from real swine wastewater.