The Environmental Biorefinery: Using Microalgae to Remediate Wastewater, a Win-Win Paradigm

Large-Scale Waste Bio-Remediation Using Microalgae Cultivation as a Platform

Municipal and agricultural waste treatment is one of the key elements of reducing environmental impact with direct effects on the economy and society. Algal technology has been tested to enable effective recycling and valorisation of wastewater nutrients including carbon, nitrogen and phosphorus. An integrated evaluation and optimisation of the sustainability of an algal bio-refinery, including mass and energy balances, carbon, water and nutrient use and impact analysis, was assessed. A bio-refinery approach of waste remediation using algal cultivation was developed at Swansea University, focusing on nutrient recovery via algal biomass exploitation in pilot facilities. Mass cultivation (up to 1.5 m3) was developed with 99% of nitrogen and phosphorus uptake by microalgal cultures. Nannochloropsis oceanica was used as a biological model and grown on three waste sources. The compounds obtained from the biomass were evaluated for animal feed and as a potential source of energy. The bioremediation through algal biotechnology was examined and compared to alternative nutrient recovery passive and active methods in order to know the most efficient way of excess nutrient management. Conclusions emphasise the high potential of algal biotechnology for waste remediation and nutrients recovery, despite the need for further development and scalable applications of this new technology.

The effect of primary treatment of wastewater in high rate algal pond systems: Biomass and bioenergy recovery

The aim of this study was to assess the effect of primary treatment on the performance of two pilot-scale high rate algal ponds (HRAPs) treating urban wastewater, considering their treatment efficiency, biomass productivity, characteristics and biogas production potential. Results indicated that the primary treatment did not significantly affect the wastewater treatment efficiency (NH₄⁺-N removal of 93 and 91% and COD removal of 62 and 65% in HRAP with and without primary treatment, respectively). The HRAP without primary treatment had higher biodiversity and productivity (20 vs. 15 g VSS/m²d). Biomass from both systems presented good settling capacity. Results of biochemical methane potential test showed that co-digesting microalgae and primary sludge led to higher methane yields (238-258 mL CH₄/g VS) compared with microalgae mono-digestion (189-225 mL CH₄/g VS). Overall, HRAPs with and without primary treatment seem to be appropriate alternatives for combining wastewater treatment and bioenergy recovery.

Biosorption and Biodegradation of the Environmental Hormone Nonylphenol By Four Marine Microalgae

Microalgae are the most abundant microorganisms in aquatic environments, and many possess the ability to remove organic contaminants. The presence of endocrine disruption compounds (EDCs) in many coastal marine systems and their associated risks have elicited great concern, especially in the case of nonylphenol (NP), which is classified as a priority contaminate by the U.S. EPA. In this context, batch experiments were conducted to investigate the intracellular absorption, extracellular adsorption and biodegradation of NP by four species of marine microalgae: Phaeocystis globosa, Nannochloropsis oculata, Dunaliella salina and Platymonas subcordiformis. The results showed a sharp reduction of NP in medium containing the four microalgal species during the first 24 h of incubation, and the four species exhibited the greatest capacity for NP adsorption and absorption within 24 h of culture. However, the amount of NP absorbed and adsorbed by all four microalgae decreased with increasing time in culture, and intracellular absorption was greater than extracellular adsorption. After 120 h of exposure to NP, the four species could biodegrade most of the NP in the medium, with efficiencies ranging from 43.43 to 90.94%. In sum, we found that the four microalgae have high biodegradation percentages and can thus improve the bioremediation of NP-contaminated water.

Nutrient removal in an algal membrane photobioreactor: effects of wastewater composition and light/dark cycle

Graesiella emersonii was cultivated in an osmotic membrane photobioreactor (OMPBR) for nutrients removal from synthetic wastewater in continuous mode. At 1.5 days of hydraulic retention time and under continuous illumination, the microalgae removed nitrogen (N) completely at influent NH₄⁺-N concentrations of 4-16 mg/L, with removal rates of 3.03-12.1 mg/L-day. Phosphorus (P) removal in the OMPBR was through biological assimilation as well as membrane rejection, but PO₄^3--P assimilation by microalgae could be improved at higher NH₄⁺-N concentrations. Microalgae biomass composition was affected by N/P ratio in wastewater, and a higher N/P ratio resulted in higher P accumulation in the biomass. The OMPBR accumulated about 0.35 g/L biomass after 12 days of operation under continuous illumination. However, OMPBR operation under 12 h light/12 h dark cycle lowered biomass productivity by 60%, which resulted in 20% decrease in NH₄⁺-N removal and nearly threefold increase in PO₄^3--P accumulation in the OMPBR. Prolonged dark phase also affected carbohydrate accumulation in biomass, although its effects on lipid and protein accumulation were negligible. The microalgae also exhibited high tendency to aggregate and settle, which could be attributed to reduction in cell surface charge and enrichment of soluble algal products in the OMPBR. Due to a relatively shorter operating period, membrane biofouling and salt accumulation did not influence the permeate flux significantly. These results improve the understanding of the effects of N/P ratio and light/dark cycle on biomass accumulation and nutrients removal in the OMPBR.

Sustainability Analysis of Microalgae Production Systems: A Review on Resource with Unexploited High-Value Reserves

Sustainability, at present, is a prominent component in the development of production systems that aim to provide the future energy and material resources. Microalgae are a promising feedstock; however, the sustainability of algae-based production systems is still under debate. Commercial market volumes of algae-derived products are still narrow. The extraction and conversion of primary metabolites to biofuels requires cultivation at large scales; cost-effective methods are therefore highly desirable. This work presents a complete and up to date review on sustainability analysis of various microalgae production scenarios, including techno-economic, environmental, and social impacts, both in large-scale plants for bioenergy production and in medium-scale cultivars intended for the production of high added-value chemicals. The results show that further efforts in algal-based research should be directed to improving the productivity, the development of multi product scenarios, a better valorization of coproducts, the integration with current industrial facilities to provide sustainable nutrient resources from waste streams, and the integration of renewable technologies such as wind energy in algae cultivars.

Metabolites Re-programming and Physiological Changes Induced in Scenedesmus regularis under Nitrate Treatment

Nitrate is required to maintain the growth and metabolism of plant and animals. Nevertheless, in excess amount such as polluted water, its concentration can be harmful to living organisms such as microalgae. Recently, studies on microalgae response towards nutrient fluctuation are usually limited to lipid accumulation for the production of biofuels, disregarding the other potential of microalgae to be used in wastewater treatments and as source of important metabolites. Our study therefore captures the need to investigate overall metabolite changes via NMR spectroscopy approach coupled with multivariate data to understand the complex molecular process under high (4X) and low (1/4X) concentrations of nitrate (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\bf{NO}}}_{{\bf{3}}}^{{\boldsymbol{-}}}$$\end{document}NO3−). NMR spectra with the aid of chemometric analysis revealed contrasting metabolites makeup under abundance and limited nitrate treatment. By using NMR technique, 43 types of metabolites and 8 types of fatty acid chains were detected. Nevertheless, only 20 key changes were observed and 16 were down regulated in limited nitrate condition. This paper has demonstrated the feasibility of NMR-based metabolomics approach to study the physiological impact of changing environment such as pollution to the implications for growth and productivity of microalgae population.

Cultivation of lipid-producing microalgae in struvite-precipitated liquid digestate for biodiesel production

BACKGROUND

Using liquid digestate from the biogas industry as a medium to culture lipid-producing microalgae is considered mutually beneficial for digestate valorization and for reducing the cost of microalgal cultivation. However, the low transmittance and high ammonium (NH₄⁺-N) levels in liquid digestate negatively influence microalgae growth.

RESULTS

Struvite precipitation was used to pretreat liquid digestate. To obtain struvite-precipitated supernatant with an ideal transmittance, NH₄⁺-N concentration, salinity, and N:P ratio for microalgal growth, there should be a 1:1.2:1.2 NH₄⁺:Mg²⁺:PO₄^3- molar ratio in the liquid digestate, with KH₂PO₄ and MgCl₂ added through continuous stirring. The addition and stirring was subsequently stopped when the pH reached 8.5. Of the nine tested microalgae species, Dictyosphaerium ehrenbergianum exhibited the best growth in the supernatant. The biomass productivity and lipid content of D. ehrenbergianum cultured in the struvite-precipitated supernatant were 161.06 mg/l/days and 34.33%, respectively, which was higher than when cultured in the standard BG-11 medium. Moreover, the struvite-precipitated supernatant improved the accumulation of monounsaturated fatty acids and saturated fatty acids.

CONCLUSIONS

This study described a new way to combine liquid digestate treatment and microalgal biodiesel production. The struvite-pretreated liquid digestate can be used to culture D. ehrenbergianum for biodiesel production.

Full-scale photobioreactor for biotreatment of olive washing water: Structure and diversity of the microalgae-bacteria consortium

The performance of a full-scale photobioreactor (PBR) for the treatment of olive washing water (OWW) was evaluated under different HRTs (5-2days). The system was able to treat up to 3926L OWWday^-1, and consisted of an activated-carbon pretreatment column and a tubular PBR unit (80 tubes, 98.17L volume, 2-m height, 0.25m diameter). PBR was an effective and environmentally friendly method for the removal of phenols, COD, BOD₅, turbidity and color from OWW (average efficiencies 94.84±0.55%, 85.86±1.24%, 99.12±0.17%, 95.86±0.98% and 87.24±0.91%, respectively). The diversity of total bacteria and microalgae in the PBR was analyzed using Illumina-sequencing, evaluating the efficiency of two DNA extraction methods. A stable microalgae-bacteria consortium was developed throughout the whole experimentation period, regardless of changes in HRT, temperature or solar radiation. MDS analyses revealed that the interplay between green algae (Sphaeropleales), cyanobacteria (Hapalosiphon) and Proteobacteria (Rhodopseudomonas, Azotobacter) played important roles in OWW bioremediation.

Biotreatment of industrial olive washing water by synergetic association of microalgal-bacterial consortia in a photobioreactor

This study presents an effective technology for the olive processing industry to remediate olive washing water. A 14.5-L enclosed tubular photobioreactor was inoculated with a stable microalgal-bacterial consortium obtained by screening strains well adapted to olive washing water. The capacity of an enclosed tubular photobioreactor to remove toxic compounds was evaluated under photosynthesis conditions and without any external supply of oxygen. The results showed that the dominant green microalgae Scenedesmus obliquus, Chlorella vulgaris and the cyanobacteria Anabaena sp. and bacteria present in olive washing water (i.e. Pantoea agglomerans and Raoultella terrigena) formed a synergistic association that was resistant to toxic pollutants present in the effluent and during the initial biodegradation process, which resulted in the breakdown of the pollutant. Total phenolic compounds, COD, BOD₅, turbidity and colour removals of 90.3 ± 11.4, 80.7 ± 9.7, 97.8 ± 12.7, 82.9 ± 8.4 and 83.3 ± 10.4 %, respectively, were recorded in the photobioreactor at 3 days of hydraulic retention time. Graphical abstract Biotreatment of industrial olive washing water by synergetic association of microalgal-bacterial consortia in a photobioreactor.