Nutrient sequestration, biomass production by microalgae and phytoremediation of sewage water

Microalgae to bioenergy production: Recent advances, influencing parameters, utilization of wastewater - A critical review

Fossil fuel limitations and their influence on climate change through atmospheric greenhouse gas emissions have made the excessive use of fossil fuels widely recognized as unsustainable. The high lipid content, carbon-neutral nature and potential as a biofuel source have made microalgae a subject of global study. Microalgae are a promising supply of biomass for third-generation biofuels production since they are renewable. They have the potential to produce significant amounts of biofuel and are considered a sustainable alternative to non-renewable energy sources. Microalgae are currently incapable to synthesize algal biofuel on an extensive basis in a sustainable manner, despite their significance in the global production of biofuels. Wastewater contains nutrients (both organic and inorganic) which is essential for the development of microalgae. Microalgae and wastewater can be combined to remediate waste effectively. Wastewater of various kinds such as industrial, agricultural, domestic, and municipal can be used as a substrate for microalgal growth. This process helps reduce carbon dioxide emissions and makes the production of biofuels more cost-effective. This critical review provides a detailed analysis of the utilization of wastewater as a growth medium for microalgal - biofuel production. The review also highlights potential future strategies to improve the commercial production of biofuels from microalgae.

Synergistic promotion of microalgal growth and copper removal from synthetic wastewater by nanoscale zero-valent iron particles

The increasing concentrations of heavy metals in livestock wastewater pose a serious threat to the environmental safety and human health, limiting its resource utilisation. In the present study, microalgae and nanoscale zero-valent iron were selected to construct a coupled system for copper-containing wastewater treatment. The addition of 50 mg·L-1 nanoscale zero-valent iron (50 nm) was the optimal value for the experiment, which could significantly increase the biomass of microalgae. In addition, nanoscale zero-valent iron stimulated microalgal secretion of extracellular polymeric substances, increasing the contents of binding sites, organic ligands, and functional groups on the microalgal surfaces and ultimately promoting the settling of microalgae and binding of heavy metals. The coupled system could quickly adapt to copper-containing wastewater of 10 mg·L-1, and the copper removal rate reached 94.99%. Adsorption and uptake by organisms, together with the contribution of zero-valent iron nanoparticles, are the major copper removal pathways. Overall, this work offers a novel technical solution for enhanced treatment of copper-containing livestock wastewater, which will help improve the efficiency and quality of wastewater treatment.

Evaluation of Microalgal Bacterial Dynamics in Pig-Farming Biogas Digestate under Impacts of Light Intensity and Nutrient Using Physicochemical Parameters

Determination of the dynamics between microalgae and bacteria in pig farming biogas digestate is vital for a consistent and reliable application towards sustainable wastewater treatment and biofuel production. This study assesses the reliability of using physicochemical parameters as indicators for the rapid evaluation of microalgal bacterial dynamics in real digestate under impacts of light, nutrient loads, and N:P ratios. The relationship between variation profiles of nutrients, biomass and physicochemical properties in each experiment was analyzed. High light and high nutrient load enhanced biomass growth and nutrient removal rate. Ammonium addition (high N:P ratio) elevated NH3 level which inhibited the growth of microalgae, subsequently reducing the biomass growth and nutrient removal. Low N:P ratio triggered the accumulation of phosphorus and the growth of chlorophyll-a but exerted little influence on treatment. Variation profiles of dissolved oxygen, nutrient and biomass were highly consistent in every experiment allowing us to identify the shift from microalgal to bacterial predomination under unfavorable conditions including low light intensity and high N:P ratio. Strong linear correlation was also found between total nitrogen removal and electrical conductivity (R2 = 0.9754). The results show the great potential of rapid evaluation of microalgal bacterial dynamics for large scale system optimization and modelling.

Nutritional influences on biomass behaviour and metabolic products by Chlamydomonas reinhardtii

The recent works have shown the unicellular green alga Chlamydomonas reinhardtii is a relevant model for investigations of algal bioprocesses. In the current work, several media were evaluated in batch mode for a better understanding of C. reinhardtii metabolism. Nutrient-suppression using heterotrophic and mixotrophic conditions were performed. The findings showed C. reinhardtii metabolized lactose (from milk whey permeate) resulting in high biomass density (2.08 g/L) and total chlorophyll content (86.74 mg/m³). It was observed a specific growth rate of 0.023 h and 29 h for the doubling time. In sulfur-suppression, the algal growth (1.17 g/L) was reduced even though a carbon source (glucose) has been supplemented. Also, the specific growth rate (0.022 h) and the doubling time (31 h) was verified. The production of ethanol was slight and the acetic acid-suppression affected the C. reinhardtii performance providing slow cell growth (0.004 h) and high doubling time (154 h).

Algal consortia based metal detoxification of municipal wastewater: Implication on photosynthetic performance, lipid production, and defense responses

The present investigation was carried out to assess the competence of artificially engineered microalgal consortia i.e. consortia 1 (Scenedesmus vacuolatus + Chlorococcum humicola), consortia 2 (Tetradesmus sp. + Scenedesmus vacuolatus), and consortia 3 (Chlorococcum humicola + Scenedesmus vacuolatus + Tetradesmus sp.) for municipal wastewater treatment and lipid production under laboratory conditions. The purpose of the present study was to screen the competent microalgae consortia based on wastewater remediation, photosynthetic performance, and antioxidant defense responses. The outcome based on nutrient reutilization (78.98-98%), metal detoxification (50-94%), and biomass production (1.43-1.65 folds) reflected greater adaptability and tolerance of consortia 2 against different concentrations of wastewater. The photosynthetic performance parameters such as active photosystem II reaction centre, the quantum yield, and photosynthetic performance index were increased by 1.20-2.35 folds in consortia 2 after treatment with different concentrations of wastewater. Additionally, Fourier transform infrared spectroscopy peak showed at 1750 cm^-1 confirmed neutral lipid accumulation in consortia 2 at 100% concentration of wastewater. The measurement of oxidative stress markers such as thiobarbituric acid reactive species and hydrogen peroxide showed considerable decline in consortia 2 as compared to consortia 1 and 3. Interestingly, increased non-enzymatic (1.02-2.44 folds) and enzymatic antioxidant (1.05-4.14 folds) activity in consortia 2 reflected that oxidative stress was attenuated by the amplified activity of ascorbic acid, proline, cysteine, superoxide dismutase, catalase, and glutathione reductase. Overall, photosynthetic performance, lipid production, and antioxidants activity represented that the consortia 2 can be effectively used for sustainable wastewater treatment and lipid production. Thus, the synergistic association of two microalgae may be the superior and neoteric paradigm with multilevel benefits such as sustainable nutrient resource utilization, metal detoxification, and lipid production.

Comparison of the toxicity of waters containing initially sulfaquinoxaline after photocatalytic treatment by TiO2 and polyaniline/TiO2

This paper addresses the residual toxicity of waters after photocatalysis treatments. The initial waters contain 7 mg L^-1 of sulfaquinoxaline (SQX) which is a sulfonamide antibiotic generally recorded inside the water. The contaminated waters are treated by photocatalytic degradation process with bare titania and titania covered with polyaniline (PANI) conducting polymer. The degradation of SQX is conducted at different pH in order to find the optimal condition to obtain SQX concentration relatively equal to zero in the shortest amount of time. This occurs for PANI/TiO₂ at pH 12 and TiO₂ at pH 4. Toxicity assays (concentration of biomass, pigmentation tests, and cells counting) are undertaken on the microalgae Chlorella vulgaris in order to evaluate the residual toxicity of the 2 treated waters. The toxicity results highlight that the water treated by PANI/TiO₂ at pH 12 is the less toxic towards the algae cells. The water processed by bare titania at acidic pH displays unneglectable toxicity towards the algae cells which are larger than the toxicity of the original SQX solution.

Removal of total dissolved solids from wastewater using a revolving algal biofilm reactor

Total dissolved solids (TDS) comprising inorganic salts and organic matters are pollutants of concern to aquatic systems and water for human use. This work aimed to investigate the use of revolving algal biofilm (RAB) reactors as a sustainable and environmental friendly method to remove TDS from industrial effluents and municipal wastewaters. The wastewaters contained chloride, sodium, potassium, calcium, magnesium, and sulfate as the major components. The RAB reactors fed with synthetic industrial effluent with high TDS level demonstrated the best algal growth, with the highest TDS removal efficiency (27%) and removal rate (2,783 mg/L-day and 19,530 mg/m² -day). A suspended algal culture system only removed 3% TDS from the same wastewater. The TDS removal by the RAB reactors was considered due to several mechanisms such as absorption by the algae cells, adsorption by extracellular polymeric substance of the biofilm, and/or precipitation. Collectively, this research shows that the RAB reactors can serve as an efficient system in wastewater remediation for TDS removal. PRACTITIONER POINTS: Total dissolved solids (TDS) in wastewater are pollutants of concern. The RAB reactors can remove TDS from various types of wastewater. The RAB reactors removed TDS by adsorbing ions elements such as Cl, Na, K, Ca, Mg, and S. The algal biomass absorbs ions through extracellular polymeric substance.

Reaction engineering and kinetics of algae conversion to biofuels and chemicals via pyrolysis and hydrothermal liquefaction

A state-of-the-art review on pyrolysis and hydrothermal liquefaction of algae to fuels and chemicals with emphasis on reaction chemistry and kinetics.

Algae granulation for nutrients uptake and algae harvesting during wastewater treatment

To overcome the high separation cost of microalgae, natural microalgae granulation was performed in open sequencing batch reactors (SBRs) by treating synthetic wastewater. After operation for 60 days, easily settled algae granules were obtained with an average size of 0.61 mm, sludge volume index (SVI) of 125 ml/g and settling velocity of 12.2 m/h. More extracellular polymeric substances (EPS) (∼252 mg/g-VSS) were detected to excrete with a higher proteins/polysaccharides (PN/PS) ratio (∼7) for the algae granules on day 60, which are beneficial for granulation. Meanwhile, the algae granules were found to have a higher phosphorus (P) content (33.4 mg-P/g-TSS) with higher P bioavailability (91.8%) when compared to the seed algae (20.4 mg-P/g-TSS). The obtained algae granules possess great potential for P recovery and reuse.

Efficient bioremediation of tannery wastewater by monostrains and consortium of marine Chlorella sp. and Phormidium sp

This study evaluated the bioremediation potential of two marine microalgae Chlorella sp. and Phormidium sp., both individually and in consortium, to reduce various pollutants in tannery wastewater (TW). The microalgae were grown in hazardous 100% TW for 20 days, and the reductions in biochemical oxygen demand (BOD), chemical oxygen demand (COD), total nitrogen (TN), total phosphorous (TP), chromium (Cr) and total dissolved solids (TDS) of the wastewater monitored periodically. Both marine isolates reduced the BOD and COD by ≥90% in the consortium and by over 80% individually. Concentrations of TN and TP were reduced by 91.16% and 88%, respectively, by the consortium. Removal/biosorption efficiencies for chromium ranged from 90.17-94.45%. Notably, the TDS, the most difficult to deal with, were reduced by >50% within 20 days by the consortium. The novel consortium developed in this study reduced most of the ecologically harmful components in the TW to within the permissible limits of discharge in about 5 to 15 days of treatment. Thus, both the tested marine strains of Chlorella and Phormidium sp. are promising for bioremediating/detoxifying TW and adequately improve the water quality for safe discharge into open water bodies, in particular when used as a consortium.

Phycoremediation of municipal wastewater by microalgae to produce biofuel

Municipal wastewater (WW), if not properly remediated, poses a threat to the environment and human health by carrying significant loads of nutrients and pathogens. These contaminants pollute rivers, lakes, and natural reservoirs where they cause eutrophication and pathogen-mediated diseases. However, the high nutrient content of WW makes it an ideal environment for remediation with microalgae that require high nutrient concentrations for growth and are not susceptible to toxins and pathogens. Given that an appropriate algal strain is used for remediation, the incurred biomass can be refined for the production of biofuel. Four microalgal species (Chlamydomonas reinhardtii, Chlorella sp., Parachlorella kessleri-I, and Nannochloropsis gaditana) were screened for efficient phycoremediation of municipal WW and potential use for biodiesel production. Among the four strains tested, P. kessleri-I showed the highest growth rate and biomass production in 100% WW. It efficiently removed all major nutrients with a removal rate of up to 98% for phosphate after 10 days of growth in 100% municipal WW collected from Delhi. The growth of P. kessleri-I in WW resulted in a 50% increase of biomass and a 115% increase of lipid yield in comparison to growth in control media. The Fatty acid methyl ester (FAME), and fuel properties of lipids isolated from cells grown in WW complied with international standards. The present study provides evidence that the green alga P. kessleri-I effectively remediates municipal WW and can be used to produce biodiesel.

Evaluation of nutrients removal (NO3-N, NH3-N and PO4-P) with Chlorella vulgaris, Pseudomonas putida, Bacillus cereus and a consortium of these microorganisms in the treatment of wastewater effluents

In this research removal of NH₃-N, NO₃-N and PO₄-P nutrients from municipal wastewater was studied, using Chlorella vulgaris, Pseudomonas putida, Bacillus cereus and an artificial consortium of them. The objective is to analyze the performance of these microorganisms and their consortium, which has not been previously studied for nutrient removal in municipal wastewater. A model wastewater was prepared simulating the physicochemical characteristics found at the wastewater plant in Chapala, Mexico. Experiments were carried out without adding an external carbon source. Results indicate that nutrient removal with Chlorella vulgaris was the most efficient with a removal of 24.03% of NO₃-N, 80.62% of NH₃-N and 4.30% of PO₄-P. With Bacillus cereus the results were 8.40% of NO₃-N, 28.80% of NH₃-N and 3.80% of PO₄-P. The removals with Pseudomonas putida were 2.50% of NO₃-N, 41.80 of NH₃-N and 4.30% of PO₄-P. The consortium of Chlorella vulgaris-Bacillus cereus-Pseudomonas putida removed 29.40% of NO₃-N, 4.2% of NH₃-N and 8.4% of PO₄-P. The highest biomass production was with Bacillus cereus (450 mg/l) followed by Pseudomonas putida (444 mg/l), the consortium (205 mg/l) and Chlorella vulgaris (88.9 mg/l). This study highlights the utility of these microorganisms for nutrient removal in wastewater treatments.

Exploring the efficacy of wastewater-grown microalgal biomass as a biofertilizer for wheat

Microalgae possess the ability to grow and glean nutrients from wastewater; such wastewater-grown biomass can be used as a biofertilizer for crops. The present investigation was undertaken to evaluate two formulations (formulation with unicellular microalgae (MC1) and formulation with filamentous microalgae (MC2); T4 and T5, respectively), prepared using wastewater-grown microalgal biomass, as a biofertilizer (after mixing with vermiculite/compost as a carrier) in wheat crop (Triticum aestivum L. HD2967) under controlled conditions. The highest values of available nitrogen (N), phosphorus (P), and potassium (K) in soil and nitrogen-fixing potential were recorded in treatment T5 (75% N + full-dose PK + formulation with filamentous microalgae (MC2). Microbial biomass carbon was significantly enhanced by 31.8-67.0% in both the inoculated treatments over control (recommended dose of fertilizers), with highest values in T4 (75% N + full-dose PK + formulation with unicellular microalgae (MC1)). Both the microalgal formulations significantly increased the N, P, and K content of roots, shoots, and grains, and the highest total N content of 3.56% in grains was observed in treatment T5. At harvest stage, the treatments inoculated with microalgal formulations (T4 and T5) recorded a 7.4-33% increase in plant dry weight and up to 10% in spike weight. The values of 1000-grain weight showed an enhancement of 5.6-8.4%, compared with T1 (recommended doses of fertilizers). A positive correlation was observed between soil nutrient availability at mid crop stage and plant biometrical parameters at harvest stage. This study revealed the promise of such microalgal consortia as a biofertilizer for 25% N savings and improved yields of wheat crop.

Phycoremediation of Tannery Wastewater Using Microalgae Scenedesmus Species

A number of microalgae species are efficient in removing toxicants from wastewater. Many of these potential species are a promising, eco-friendly, and sustainable option for tertiary wastewater treatment with a possible advantage of improving the economics of microalgae cultivation for biofuel production. The present study deals with the phycoremediation of tannery wastewater (TWW) using Scenedesmus sp. isolated from a local habitat. The test species was grown in TWW under laboratory conditions and harvested on the 12th day. The results revealed that the algal biomass during the growth period not only reduced the pollution load of heavy metals (Cr-81.2-96%, Cu-73.2-98%, Pb-75-98% and Zn-65-98%) but also the nutrients (NO3 >44.3% and PO4 >95%). Fourier Transform Infrared (FTIR) spectrums of Scenedesmus sp. biomass revealed the involvement of hydroxyl amino, carboxylic and carbonyl groups. The scanning electron micrograph (SEM) and Energy Dispersive X-ray Spectroscopic analysis (EDS) revealed the surface texture, morphology and element distribution of the biosorbent. Furthermore, the wastewater generated during wet-blue tanning process can support dense population of Scenedesmus sp., making it a potential growth medium for biomass production of the test alga for phycoremediation of toxicants in tannery wastewaters.

Treating Urban Wastewater: Nutrient Removal by Using Immobilized Green Algae in Batch Cultures

Essential roles of microalgae during the tertiary treatment of municipal wastewater have been proven. In order to avoid wash out of the biomass and also modify the quality of the treated wastewater; some techniques such as cell immobilization have been developed. To do so, in this study four samples from two species of microalgae (Chlorella vulgaris and Chlamydomonas sp.) were determined and confirmed by taxonomic identification. The samples were immobilized in calcium alginate beads. Within 10 days the amounts of nitrate (No3(-)-N) and orthophosphate (Po4(3-)-P) were measured to calculate the removal efficacy for each individual sample. Based on the standard methods, the amount of nitrate and orthophosphate were determined every 3 days within 10 days. The results indicate that immobilized microalgae are able to remove the nutrients and reduce the amount of nitrate and orthophosphate, significantly. Furthermore, the C. vulgaris (YG02) was the best species in this experience with 72% and 99% of reduction in the amount of nitrate and orthophosphate, respectively.