Optimization of algae mixotrophic culture for nutrients recycling and biomass/lipids production in anaerobically digested waste sludge by various organic acids addition

The effects of carbon nitrogen ratio and salinity on the treatment of swine digestion effluent simultaneously producing bioenergy by microalgae biofilm

In order to remove high concentrations of ammonia nitrogen (NH4+-N) and refractory sulfamethazine (SM2) from swine digestion effluent, different carbon/nitrogen (C/N) ratios and salinity were used to determine the effects of pollutants removal in the microalgae biofilm system. Microalgae biofilm treatment under optimal environmental conditions in synthetic swine digestion effluent were C/N ratio of 20 and salinity of 140 mM. In order to make the actual swine digestion effluent discharge up to the standard, three different two-cycle treatments (suspended microalgae, microalgae biofilm, microalgae biofilm under the optimal conditions) were studied. The results showed that after two-cycle treatment with microalgae biofilm under the optimal conditions, the actual swine digestion effluent levels of total nitrogen (TN), NH4+-N, total phosphorus (TP), chemical oxygen demand (COD), SM2 were 22.65, 9.32, 4.11, 367.28, and 0.99 mg L-1, respectively, which could satisfy the discharge standards for livestock and poultry wastewater in China. At the same time, first-order kinetic simulation equations suggested a degradation half-life of 4.85 d for SM2 under optimal conditions in microalgae biofilm, and microbial community analysis indicated that the dominant genus was Halomonas. Furthermore, 35.66% of lipid, 32.56% of protein and 18.44% of polysaccharides were harvested after two-cycle in microalgae biofilm treatment under optimal environmental conditions. These results indicated that the regulation of C/N and salinity in microalgae biofilm for the treatment of swine digestion effluent was a high-efficiency strategy to simultaneously achieve wastewater treatment and bioenergy production.

Wastewater treatment with algal based membrane bioreactor for the future: Removing emerging containments

The difficulty of developing pollutants in aquatic ecosystems and their potential effects on animals and plants have been raised. Sewage effluent can seriously harm a river's plant and animal life by reducing the water's oxygen content. Due to their increasing use and poor elimination in traditional municipal wastewater treatment plants (WWTPs), pharmaceuticals are one of the developing pollutants that have the potential to penetrate aquatic ecosystems. Due to undigested pharmaceuticals and their metabolites, which constitute a significant class of potentially hazardous aquatic pollutants. Using an algae-based membrane bioreactor (AMBR), the primary objective of this research was to eliminate emerging contaminants (ECs) identified in municipal wastewater. The first part of this research covers the basics of growing algae, an explanation of how they work, and how they remove ECs. Second, it develops the membrane in the wastewater, explains its workings, and uses the membrane to remove ECs. Finally, an algae-based membrane bioreactor for removing ECs is examined. As a result, daily algal production using AMBR technology might range from 50 to 100 mg/Liter. These kinds of machines are capable of nitrogen and phosphorus removal efficiencies of 30-97% and 46-93%, respectively.

Potential use of saline resources for biofuel production using halophytes and marine algae: prospects and pitfalls

There exists a global challenge of feeding the growing human population of the world and supplying its energy needs without exhausting global resources. This challenge includes the competition for biomass between food and fuel production. The aim of this paper is to review to what extent the biomass of plants growing under hostile conditions and on marginal lands could ease that competition. Biomass from salt-tolerant algae and halophytes has shown potential for bioenergy production on salt-affected soils. Halophytes and algae could provide a bio-based source for lignoceelusic biomass and fatty acids or an alternative for edible biomass currently produced using fresh water and agricultural lands. The present paper provides an overview of the opportunities and challenges in the development of alternative fuels from halophytes and algae. Halophytes grown on marginal and degraded lands using saline water offer an additional material for commercial-scale biofuel production, especially bioethanol. At the same time, suitable strains of microalgae cultured under saline conditions can be a particularly good source of biodiesel, although the efficiency of their mass-scale biomass production is still a concern in relation to environmental protection. This review summaries the pitfalls and precautions for producing biomass in a way that limits environmental hazards and harms for coastal ecosystems. Some new algal and halophytic species with great potential as sources of bioenergy are highlighted.

From waste-activated sludge to algae: a self-reliant cultivation process in photoreactors using saline conditions

In this study, microalgae-bacteria (MB) systems using saline conditions (3 and 5% salinity) were built in order to use waste-activated sludge (AS) as raw material for cultivating lipid-rich microalgae. Algae were observed to be flourishing in 60 days of operation, which totally used the N and P released from the sludge biomass. A prominent improvement of lipid content in MB consortia was obtained under algae growth and salinity stimulation, which occupied 119-136 mg/g-SS rather than a low content of 12.1 mg/g-SS in AS. Lipid enrichment also brought a 3.1-3.3 times total heat release (THR) in the MB biomass. The marine spherical algae Porphyridium, as well as filamentous Geitlerinema, Nodularia, Leptolyngbya were found to be the main lipid producers and self-flocculated to 23.0% (R1) and 33.5% (R2) volume under the effect of residue EPS. This study had a big meaning in not only waste sludge reduction but also in manufacturing useful bioenergy products.

Anaerobic digestates grown oleaginous microalgae for pollutants removal and lipids production

Anaerobic digestates were potential mediums for cultivating oleaginous microalgae, but their various components brought uncertainties for aglal growth and lipids production. In this study, three microalgae strains were tested to grow on four typical anaerobic digestates. The results showed that anaerobic food wastewater was an optimal medium for C. pyrenoidosa and S. obliquus culture (N. oleoabundanst cannot survive), achieving the highest biomass (2.15-2.32 g L^-1) and lipids production (20.6-32.5 mg L^-1·d^-1). In contrast, three microalgae strains could grow suboptimally in anaerobic municipal (0.79-0.95 g L^-1) and toilet (0.92-1.40 g L^-1) wastewater, but showed poor performances in anaerobic swine wastewater. The growth of microalgae removed 40.9-63.4% of TOC, 83.7-96.3% of NH₄⁺-N and 70.3-89.4% of TP in the three ADs. In addition, it was unfortunately found that the lipids content and saturation degree in fatty acids significantly decreased in ADs with sufficient nutrients. It suggests that some measures should be taken to balance biomass, lipids production and quality for cultivating microalgae in anaerobic digestates.

Self-regulation mechanism difference of Chlorella vulgaris and Scenedesmus obliquus in toxic sludge extract caused by hydroquinone biodegradation

Chlorella vulgaris (C. vulgaris) and Scenedesmus obliquus (S. obliquus) were compared to remove toxicity under conditions of sludge extract cultivation for 30 days. The toxicity of sludge extract, the growth characteristics, photosynthetic pigment, superoxide dismutase (SOD) enzyme and catalase (CAT) enzyme activities of the two microalgae were studied by contrast. The results showed that small molecular organic matter (<500 Da) was more easily utilized by microalgae. The toxicity in the toxic group of C. vulgaris and S. obliquus on the 30^th day decreased to 56.8 ± 1.2% and 60.7 ± 2.8%, respectively. In the toxic group, the maximal SOD enzyme activity of C. vulgaris and S. obliquus were 2.02 U/mg proteins and 8.21 U/mg proteins, respectively, demonstrating that toxicity caused more oxidative damage to S. obliquus than to C. vulgaris. Proteomics analysis revealed that C. vulgaris mainly regulates energy synthesis and distribution primarily through sugar metabolism, and biomass synthesis primarily through carbon metabolism, whereas S. obliquus mainly regulates energy synthesis and distribution primarily through sugar metabolism and oxidative phosphorylation, resulting in sludge toxicity stress regulation.

Integrated role of algae in the closed-loop circular economy of anaerobic digestion

Following the surging demand for sustainable biofuels, biogas production via anaerobic digestion (AD) presented itself as a solution for energy security, waste management, and greenhouse gas mitigation. Algal-based biorefinery platform serves an important role in the AD-based closed-loop circular economy. Other than using whole biomass of micro- and macroalgae as feedstock for biogas production, the integration of AD with other bio- or thermochemical conversion techniques can achieve complete valorization of biomass residue after processing or valuable compounds extraction. On the other hand, anaerobic digestate, the byproduct of AD processes can be used for microalgal cultivation for lipid and pigments accumulation, closing the loop of resource flow. Furthermore, algae and its consortium with bacteria or fungi can be employed for combined biogas upgrading and wastewater treatment. Innovative strategies have been developed to enhance biogas upgrading and pollutant removal performance as well as minimize O₂ and N₂ content in the upgraded biomethane.

Comparative Life-Cycle assessment of microalgal biomass production in conventional growth media versus newly developed nanoemulsion media

Microalgae biomass is a potential feedstock for biodiesel, animal feed, biofertilizer, and other products such as bioactive compounds. Most of the reported studies describe microalgae as a green process, however, the impacts associated with its growth media and cultivation have seldom been studied. With an aim to analyze the environmental impacts, the present study compares the life-cycle assessment of microalgal cultivation in two growth media. The data used was obtained from the experimental sets where microalgaeC. pyrenoidosawas cultivated in BG11 (control or SC-1) and silicone oil nanoemulsion (previously developed medium or SC-2) on a lab scale. The environmental impacts were evaluated using the ReCiPe midpoint and endpoint method using Sima Pro 9.0 software based on a "cradle-to-gate" approach. The total environmental score for 1 kg microalgal biomass production was 99.25Pt in SC-1, and 53.39Ptin SC-2, concluding greater environmental burden by SC-1. The photobioreactor construction material along with the operation led to maximum emissions, human toxicity, and resource depletion. In summary, the newly developed nanoemulsion medium was found to be eco-friendly that has the potential to minimize the usage of conventional nutrients and resources.

Anaerobic digestate as a low-cost nutrient source for sustainable microalgae cultivation: A way forward through waste valorization approach

To suffice the escalating global energy demand, microalgae are deemed as high potential surrogate feedstocks for liquid fuels. The major encumbrance for the commercialization of microalgae cultivation is due to the high costs of nutrients such as carbon, phosphorous, and nitrogen. Meanwhile, the organic-rich anaerobic digestate which is difficult to be purified by conventional techniques is appropriate to be used as a low-cost nutrient source for the economic viability and sustainability of microalgae production. This option is also beneficial in terms of reutilize the organic fraction of solid waste instead of discarded as zero-value waste. Anaerobic digestate is the side product of biogas production during anaerobic digestion process, where optimum nutrients are needed to satisfy the physiological needs to grow microalgae. Besides, the turbidity, competing biological contaminants, ammonia and metal toxicity of the digestate are also potentially contributing to the inhibition of microalgae growth. Thus, this review is aimed to explicate the feasibility of utilizing the anaerobic digestate to cultivate microalgae by evaluating their potential challenges and solutions. The proposed potential solutions (digestate dilution and pre-treatment, microalgae strain selection, extra organics addition, nitrification and desulfurization) corresponding to the state-of-the-art challenges are applicable as future directions of the research.

Mariculture wastewater treatment with Bacterial-Algal Coupling System (BACS): Effect of light intensity on microalgal biomass production and nutrient removal

Mariculture wastewater generated from the mariculture industry has increased public concern due to its impact on the sustainability of aquatic environments and aquaculture practices. Herein, the Bacterial-Algal Coupling System was applied for mariculture wastewater treatment. Microalgae growth in heterotrophy and mixotrophy (2000-8000 lux) was first compared. The best microalgal growth and nutrient removal were obtained at 5000 lux, where biomass productivity of microalgae was 0.465 g L^-1 d^-1, and 98.1% of chemical oxygen demand, 70.7% of ammonia-nitrogen, and 90.0% of total phosphorus were removed. To further understand the nutrient removal through microalgae cultivation, the enzyme activities involved in the Calvin cycle and the Tricarboxylic Acid cycle at different light intensities were determined. Under mixotrophic cultivation, there was a coordination between photosynthesis and heterotrophic metabolism in the agal cell, which resulted in a high algal biomass production and removal efficiency of nutrients. This study provided a novel insight into the bioremediation of mariculture wastewater and microalgae cultivation.

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.

Mixotrophic Chlorella pyrenoidosa as cell factory for ultrahigh-efficient removal of ammonium from catalyzer wastewater with valuable algal biomass coproduction through short-time acclimation

To achieve ultrahigh-efficient ammonium removal and valuable biomass coproduction, Chlorella-mediated short-time acclimation was implemented in photo-fermentation. The results demonstrated short-time acclimation of mixotrophic Chlorella pyrenoidosa could significantly improve NH₄⁺ removal and biomass production in shake flasks. After acclimation through two batch cultures in 5-L photo-fermenter, the maximum NH₄⁺ removal rate (1,400 mg L^-1 d^-1) were achieved under high NH₄⁺ level (4,750 mg L^-1) in batch 3. In 50-L photo-fermenter, through one batch acclimated culture, the maximum NH₄⁺ removal rate (2,212 mg L^-1 d^-1) and biomass concentration (58.4 g L^-1) were achieved in batch 2, with the highest productivities of protein (5.56 g L^-1 d^-1) and total lipids (5.66 g L^-1 d^-1). The hypothetical pathway of nutrients assimilation in mixotrophic cells as cell factory was proposed with detailed discussion. This study provided a novel strategy for high-ammonium wastewater treatment without dilution, facilitating the algae-based "waste-to-treasure" bioconversion process for green manufacturing.

Strategy for Managing Industrial Anaerobic Sludge through the Heterotrophic Cultivation of Chlorella sorokiniana: Effect of Iron Addition on Biomass and Lipid Production

Microalgae provides an alternative for the valorization of industrial by-products, in which the nutritional content varies substantially and directly affects microalgae system performance. Herein, the heterotrophic cultivation of Chlorella sorokiniana was systematically studied, allowing us to detect a nutritional deficiency other than the carbon source through assessing the oxygen transfer rate for glucose or acetate fermentation. Consequently, a mathematical model of the iron co-limiting effect on heterotrophic microalgae was developed by exploring its ability to regulate the specific growth rate and yield. For instance, higher values of the specific growth rate (0.17 h^-1) compared with those reported for the heterotrophic culture of Chlorella were obtained due to iron supplementation. Therefore, anaerobic sludge from an industrial wastewater treatment plant (a baker's yeast company) was pretreated to obtain an extract as a media supplement for C. sorokiniana. According to the proposed model, the sludge extract allowed us to supplement iron values close to the growth activation concentration (K_Fe ~12 mg L^-1). Therefore, a fed-batch strategy was evaluated on nitrogen-deprived cultures supplemented with the sludge extract to promote biomass formation and fatty acid synthesis. Our findings reveal that nitrogen and iron in sludge extract can supplement heterotrophic cultures of Chlorella and provide an alternative for the valorization of industrial anaerobic sludge.