Utilization of centrate from wastewater treatment for the outdoor production of Nannochloropsis gaditana biomass at pilot-scale

Sidestream characteristics in water resource recovery facilities: A critical review

This review compiles information on sidestream characteristics that result from anaerobic digestion dewatering (conventional and preceded by a thermal hydrolysis process), biological and primary sludge thickening. The objective is to define a range of concentrations for the different characteristics found in literature and to confront them with the optimal operating conditions of sidestream processes for nutrient treatment or recovery. Each characteristic of sidestream (TSS, VSS, COD, N, P, Al³⁺, Ca²⁺, Cl^-, Fe^2+/3+, Mg²⁺, K⁺, Na⁺, SO₄^2-, heavy metals, micro-pollutants and pathogens) is discussed according to the water resource recovery facility configuration, wastewater characteristics and implications for the recovery of nitrogen and phosphorus based on current published knowledge on the processes implemented at full-scale. The thorough analysis of sidestream characteristics shows that anaerobic digestion sidestreams have the highest ammonium content compared to biological and primary sludge sidestreams. Phosphate content in anaerobic digestion sidestreams depends on the type of applied phosphorus treatment but is also highly dependent on precipitation reactions within the digester. Thermal Hydrolysis Process (THP) mainly impacts COD, N and alkalinity content in anaerobic digestion sidestreams. Surprisingly, the concentration of phosphate is not higher compared to conventional anaerobic digestion, thus offering more attractive recovery possibilities upstream of the digester rather than in sidestreams. All sidestream processes investigated in the present study (struvite, partial nitrification/anammox, ammonia stripping, membranes, bioelectrochemical system, electrodialysis, ion exchange system and algae production) suffer from residual TSS in sidestreams. Above a certain threshold, residual COD and ions can also deteriorate the performance of the process or the purity of the final nutrient-based product. This article also provides a list of characteristics to measure to help in the choice of a specific process.

Valorization of Anaerobic-Fermentation Liquid Digestates-Membrane-Based Process Development

Complete valorization of various wastes and effluents, with significant organic content, remains a great challenge in the pursuit of a circular economy. The approach based on anaerobic fermentation, leading to valuable biogas production, has been broadly accepted and employed as an attractive processing scheme. However, despite notable research efforts, complete valorization of the digestates (involving recovery of nutrients/by-products and full recycling/reuse of treated water) requires additional work for sustainable process development. This study aims to make a contribution in this direction by demonstrating a systematic methodology for valorizing the liquid digestate. The proposed membrane-based processing scheme involves UF-membrane pretreatment of the liquid digestate (for sludge separation) and subsequent NF/RO membrane treatment for reuse/recycling of the permeate; the concentrate, enriched in "nutrients" (phosphate and ammonium compounds), can be utilized for soil fertilization, with further conditioning/processing. By performing targeted laboratory experiments and advanced simulations, the membrane-based process was developed to a relatively high technology-readiness level, including a pilot unit design/construction and preliminary testing with satisfactory results. Through pilot testing in industrial environment, further process development and optimization will be pursued, towards practical applications. The demonstrated methodology is also considered appropriate for systematic development of membrane-based processes to valorize/treat a variety of similar effluents.

Enhancing Urban Wastewater Treatment through Isolated Chlorella Strain-Based Phytoremediation in Centrate Stream: An Analysis of Algae Morpho-Physiology and Nutrients Removal Efficiency

The release of inadequately treated urban wastewater is the main cause of environmental pollution of aquatic ecosystems. Among efficient and environmentally friendly technologies to improve the remediation process, those based on microalgae represent an attractive alternative due to the potential of microalgae to remove nitrogen (N) and phosphorus (P) from wastewaters. In this work, microalgae were isolated from the centrate stream of an urban wastewater treatment plant and a native Chlorella-like species was selected for studies on nutrient removal from centrate streams. Comparative experiments were set up using 100% centrate and BG11 synthetic medium, modified with the same N and P as the effluent. Since microalgal growth in 100% effluent was inhibited, cultivation of microalgae was performed by mixing tap-freshwater with centrate at increasing percentages (50%, 60%, 70%, and 80%). While algal biomass and nutrient removal was little affected by the differently diluted effluent, morpho-physiological parameters (F_V/F_M ratio, carotenoids, chloroplast ultrastructure) showed that cell stress increased with increasing amounts of centrate. However, the production of an algal biomass enriched in carotenoids and P, together with N and P abatement in the effluent, supports promising microalgae applications that combine centrate remediation with the production of compounds of biotechnological interest; for example, for organic agriculture.

New Angled Twin-layer Porous Substrate Photobioreactors for Cultivation of Nannochloropsis oculata

An angled twin-layer porous substrate photobioreactor (TL-PSBR) using LED light was designed to cultivate Nannochloropsis oculata. Flocculation and sedimentation by modification of pH to 11 were determined as the optimal method for harvesting the N. oculata inoculum. The following optimised parameters were found: tilt angle 15°, Kraft 220 g m^-2 paper as substrate material, initial inoculum density of 12.5 g m^-2, 140 µmol photons m^-2 s^-1 light intensity, and a light/dark cycle of 6:6 (h). Test cultivation for 14 days was performed under optimised conditions. The total dried biomass standing crop was 75.5 g m^-2 growth area with an average productivity of 6.3 g m^-2 d^-1, the productivity per volume of used culture medium was 126.2 mg/L d^-1, total lipid content 21.9% (w/w), and the highest productivity of total lipids was 1.33 g m^-2 d^-1. The dry algal biomass contained 3% eicosapentaenoic acid (w/w), 3.7% palmitoleic acid (w/w), and 513 mg kg^-1 vitamin E. The optimisation of N. oculata cultivation on an angled TL-PSBR system yielded promising results, and applications for commercial products need to be further explored.

Centrate as a sustainable growth medium: Impact on microalgal inocula and bacterial communities in tubular photobioreactor cultivation systems

Centrate is a low-cost alternative to synthetic fertilizers for microalgal cultivation, reducing environmental burdens and remediation costs. Adapted microalgae need to be selected and characterised to maximise biomass production and depuration efficiency. Here, the performance and composition of six microalgal communities cultivated both on synthetic media and centrate within semi-open tubular photobioreactors were investigated through Illumina sequencing. Biomass grown on centrate, exposed to a high concentration of ammonium, showed a higher quantity of nitrogen (5.6% dry weight) than the biomass grown on the synthetic media nitrate (3.9% dry weight). Eukaryotic inocula were replaced by other microalgae while cyanobacterial inocula were maintained. Communities were generally similar for the same inoculum between media, however, inoculation with cyanobacteria led to variability within the eukaryotic community. Where communities differed, centrate resulted in a higher richness and diversity. The higher nitrogen of centrate possibly led to higher abundance of genes coding for N metabolism enzymes.

Microbial Astaxanthin Production from Agro-Industrial Wastes—Raw Materials, Processes, and Quality

The antioxidant and food pigment astaxanthin (AX) can be produced by several microorganisms, in auto- or heterotrophic conditions. Regardless of the organism, AX concentrations in culture media are low, typically about 10–40 mg/L. Therefore, large amounts of nutrients and water are necessary to prepare culture media. Using low-cost substrates such as agro-industrial solid and liquid wastes is desirable for cost reduction. This opens up the opportunity of coupling AX production to other existing processes, taking advantage of available residues or co-products in a biorefinery approach. Indeed, the scientific literature shows that many attempts are being made to produce AX from residues. However, this brings challenges regarding raw material variability, process conditions, product titers, and downstream processing. This text overviews nutritional requirements and suitable culture media for producing AX-rich biomass: production and productivity ranges, residue pretreatment, and how the selected microorganism and culture media combinations affect further biomass production and quality. State-of-the-art technology indicates that, while H. pluvialis will remain an important source of AX, X. dendrorhous may be used in novel processes using residues.

Biostimulant Capacity of Chlorella and Chlamydopodium Species Produced Using Wastewater and Centrate

Simple Summary

The world population is expected to grow by over 2 billion people in the coming decades, involving an increase in agricultural production. Agriculture demands huge amounts of water and energy, so it is crucial to minimise the use of these resources to ensure a sustainable future. Plant biostimulants can promote germination, plant growth, flowering, and crop productivity, as well as increase nutrient-use efficiencies and resistance to abiotic stress. Microalgae are a novel and interesting source of biostimulants, and they can grow using wastewater. Although there is great interest in developing and applying these natural biostimulants produced from microalgae, there is still only a limited number of well-characterised and stable products available commercially. It is therefore necessary to identify novel strains that have a biostimulant capacity that are robust, that can grow in wastewater, and that are highly productive. This work determines the viability of producing high-quality microalgal biomass using wastewater and assesses the biostimulant capacity of the produced biomass. It is focused on an initial laboratory-scale study to produce these strains in wastewater and a preliminary validation of their biostimulant capacity.

Abstract

The aim of the present study was to assess the potential of producing four microalgal strains using secondary-treated urban wastewater supplemented with centrate, and to evaluate the biostimulant effects of several microalgal extracts obtained using water and sonication. Four strains were studied: Chlorella vulgaris UAL-1, Chlorella sp. UAL-2, Chlorella vulgaris UAL-3, and Chlamydopodium fusiforme UAL-4. The highest biomass productivity was found for C. fusiforme, with a value of 0.38 ± 0.01 g·L⁻¹·day⁻¹. C. vulgaris UAL-1 achieved a biomass productivity of 0.31 ± 0.03 g·L⁻¹·day⁻¹ (the highest for the Chlorella genus), while the N-NH⁴⁺, N-NO₃⁻, and P-PO₄³⁻ removal capacities of this strain were 51.9 ± 2.4, 0.8 ± 0.1, and 5.7 ± 0.3 mg·L⁻¹·day⁻¹, respectively. C. vulgaris UAL-1 showed the greatest potential for use as a biostimulant—when used at a concentration of 0.1 g·L⁻¹, it increased the germination index of watercress seeds by 3.5%. At concentrations of 0.5 and 2.0 g·L⁻¹, the biomass from this microalga promoted adventitious root formation in soybean seeds by 220% and 493%, respectively. The cucumber expansion test suggested a cytokinin-like effect from C. vulgaris UAL-1; it was also the only strain that promoted the formation of chlorophylls in wheat leaves. Overall, the results of the present study suggest the potential of producing C. vulgaris UAL-1 using centrate and wastewater as well as the potential utilisation of its biomass to develop high-value biostimulants.

Piggery wastewater treatment with algae-bacteria consortia: Pilot-scale validation and techno-economic evaluation at farm level

The efficiency of an outdoor pilot-scale raceway pond treating the wastewaters generated by a large-scale piggery farm in Northern Italy was evaluated. The biomass productivity over 208 days of experimentation was 10.7 ± 6.5 g TSS·m^-2·d^-1, and ammoniacal nitrogen, orthophosphate, and COD average removal efficiencies were 90%, 90%, and 59%, respectively. Results were used to perform a comprehensive techno-economic analysis for integrating algae-based processes in farms of different sizes (100-10000 pigs). The amount of N disposed of on agricultural land could be reduced from 91% to 21%, increasing the fraction returned to the atmosphere from 2.4% to 63%, and the fraction in the biomass from 6.2% to 16%. For intensive farming, the release of 110 t N·ha^-1·y^-1 contained in the digestate could be avoided by including algae-bacteria processes. The biomass production cost was as low as 1.9 €·kg^-1, while the cost for nitrogen removal was 4.3 €·kg N^-1.

Microalgae based wastewater treatment coupled to the production of high value agricultural products: Current needs and challenges

One of the main social and economic challenges of the 21st century will be to overcome the worlds' water deficit expected by the end of this decade. Microalgae based wastewater treatment has been suggested as a strategy to recover nutrients from wastewater while simultaneously producing clean water. Consortia of microalgae and bacteria are responsible for recovering nutrients from wastewater. A better understanding of how environmental and operational conditions affect the composition of the microalgae-bacteria consortia would allow to maximise nutrient recoveries and biomass productivities. Most of the studies reported to date showed promising results, although up-scaling of these processes to reactors larger than 100 m² is needed to better predict their industrial relevance. The main advantage of microalgae based wastewater treatment is that valuable biomass with unlimited applications is produced as a co-product. The aim of the current paper was to review microalgae based wastewater treatment processes focusing on strategies that allow increasing both biomass productivities and nutrient recoveries. Moreover, the benefits of microalgae based agricultural products were also discussed.

A Review about Microalgae Wastewater Treatment for Bioremediation and Biomass Production—A New Challenge for Europe

Microalgae have received much attention in the last few years. Their use is being extended to different fields of application and technologies, such as food, animal feed, and production of valuable polymers. Additionally, there is interest in using microalgae for removal of nutrients from wastewater. Wastewater treatment with microalgae allows for a reduction in the main chemicals responsible for eutrophication (nitrogen and phosphate), the reduction of organic substrates (by decreasing parameters such as BOD and COD) and the removal of other substances such as heavy metals and pharmaceuticals. By selecting and reviewing 202 articles published in Scopus between 1992 and 2020, some aspects such as the feasibility of microalgae cultivation on wastewater and potential bioremediation have been investigated and evaluated. In this review, particular emphasis was placed on the different types of wastewaters on which the growth of microalgae is possible, the achievable bioremediation and the factors that make large-scale microalgae treatment feasible. The results indicated that the microalgae are able to grow on wastewater and carry out effective bioremediation. Furthermore, single-step treatment with mixotrophic microalgae could represent a valid alternative to conventional processes. The main bottlenecks are the large-scale feasibility and costs associated with biomass harvesting.

Pollution prevention and waste phycoremediation by algal-based wastewater treatment technologies: The applications of high-rate algal ponds (HRAPs) and algal turf scrubber (ATS)

Following the escalating human population growth and rapid urbanization, the tremendous amount of urban and industrial waste released leads to a series of critical issues such as health issues, climate change, water crisis, and pollution problems. With the advantages of a favorable carbon life cycle, high photosynthetic efficiencies, and being adaptive to harsh environments, algae have attracted attention as an excellent agent for pollution prevention and waste phycoremediation. Following the concept of circular economy and biorefinery for sustainable production and waste minimization, this review discusses the role of four different algal-based wastewater treatment technologies, including high-rate algal ponds (HRAPs), HRAP-absorption column (HRAP-AC), hybrid algal biofilm-enhanced raceway pond (HABERP) and algal turf scrubber (ATS) in waste management and resource recovery. In addition to the nutrient removal mechanisms and operation parameters, recent advances and developments have been discussed for each technology, including (1) Innovative operation strategies and treatment of emerging contaminants (ECs) employing HRAPs, (2) Biogas upgrading utilizing HRAP-AC system and approaches of O₂ minimization in biomethane, (3) Operation of different HABERP systems, (4) Life-cycle and cost analysis of HRAPs-based wastewater treatment system, and (5) Value-upgrading for harvested algal biomass and life-cycle cost analysis of ATS system.

Supercritical water gasification (SCWG) as a potential tool for the valorization of phycoremediation-derived waste algal biomass for biofuel generation

Phycoremediation is an emerging technology, where algae-based processes were used to effectively remove nutrients, organic wastes, and toxic heavy metals from the polluted environment. The waste algal biomass obtained after phycoremediation, which may contain residual hazardous materials, could still be used as feedstock to produce biofuels/bioenergy preferably through thermochemical conversion technology. This review proposes a synergistic approach by utilizing the phycoremediation-derived algal biomass (PCDA) as feedstock for efficient hazardous waste treatment and clean energy generation via supercritical water gasification (SCWG). The review provides an in-depth study of catalytic, non-catalytic, and continuous SCWG of algal biomass, aiming to lay out the foundations for future study. In addition, the concepts of heat integration as well as water, nutrient, and CO₂ recycling were introduced for a sustainable algae-to-biofuel process, which significantly enhances the overall energy and material efficiency of SCWG. The production of biofuel from algal biomass via other advanced gasification technologies, such as integration with other thermochemical conversion techniques, co-gasification, chemical looping gasification (CLG), and integrated gasification and combined cycle (IGCC) were also discussed. Furthermore, the discussion of kinetics and thermodynamics models, as well as life cycle and techno-economic assessments, appear to provide insights for future commercial applications.

Operation Regimes: A Comparison Based on Nannochloropsis oceanica Biomass and Lipid Productivity

Microalgae are currently considered to be a promising feedstock for biodiesel production. However, significant research efforts are crucial to improve the current biomass and lipid productivities under real outdoor production conditions. In this context, batch, continuous and semi-continuous operation regimes were compared during the Spring/Summer seasons in 2.6 m3 tubular photobioreactors to select the most suitable one for the production of the oleaginous microalga Nannochloropsis oceanica. Results obtained revealed that N. oceanica grown using the semi-continuous and continuous operation regimes enabled a 1.5-fold increase in biomass volumetric productivity compared to that cultivated in batch. The lipid productivity was 1.7-fold higher under semi-continuous cultivation than that under a batch operation regime. On the other hand, the semi-continuous and continuous operation regimes spent nearly the double amount of water compared to that of the batch regime. Interestingly, the biochemical profile of produced biomass using the different operation regimes was not affected regarding the contents of proteins, lipids and fatty acids. Overall, these results show that the semi-continuous operation regime is more suitable for the outdoor production of N. oceanica, significantly improving the biomass and lipid productivities at large-scale, which is a crucial factor for biodiesel production.

Integration of a side-stream microalgae process into a municipal wastewater treatment plant: A life cycle analysis

This study investigates the environmental improvements associated to the integration of a microalgae unit as a side-stream process within an existing municipal wastewater treatment facility in northern Italy. Microalgae are fed on the centrate from sludge dewatering, rich in nutrients, and on the CO₂ in the flue-gas of the combined heat and power unit. The produced biomass is recirculated upflow the water line where it settles and undergoes anaerobic digestion generating extra biogas. A life cycle assessment was performed collecting primary data from an algal pilot-scale plant installed at the facility. Fifteen environmental indicators were evaluated. Compared to the baseline wastewater treatment, the new algal configuration allows an improvement for 7 out of 15 indicators mainly thanks to the electricity savings in the facility. Some recommendations are provided to improve the performance of the algal system in the scaling up.

Microalgae from Biorefinery as Potential Protein Source for Siberian Sturgeon (A. baerii) Aquafeed

The demand for aquafeed is expected to increase in the coming years and new ingredients will be needed to compensate for the low fish meal and oil availability. Microalgae represent a promising matrix for the future aquafeed formulation, however, the high production cost hinders its application. The use of microalgae from biorefinery would reduce the disposal costs for microalgae production. The present study aimed to (i) verify the growth of microalgae on digestate coming from pig farming and (ii) evaluate their potential valorization as dietary ingredient in aquafeed according to a Circular Bioeconomy approach. For these purposes, a microalgae biomass was produced on an outdoor raceway reactor supplied with digestate and used for partial replacement (10% of the diet) in aquafeed for Siberian sturgeon fingerlings (Acipenser baerii). The results obtained confirm the feasibility for growing microalgae on digestate with satisfactory productivity (6.2 gDM m−2 d−1), nutrient removal efficiency and Chemical Oxygen Demand reduction; moreover, the feeding trial carried out showed similar results between experimental and control groups (p > 0.05), in term of growth performance, somatic indices, fillet nutritional composition and intestinal functionality, to indicate that microalgae from biorefinery could be used as protein source in Siberian sturgeon aquafeed.

Enhancement of targeted microalgae species growth using aquaculture sludge extracts

Natural growth-promoting nutrients extracted from aquaculture sludge waste can be used to maximise microalgal growth. This study identified the influence of aquaculture sludge extract (SE) on four microalgae species. Conway or Bold's Basal Media (BBM) was supplemented with SE collected from a Sabak Bernam shrimp pond (SB) and Kota Puteri fish pond (KP), and tested using a novel microplate-incubation technique. Five different autoclave extraction treatment parameters were assessed for both collected SE, i.e., 1-h at 105 °C, 2-h at 105 °C, 1-h at 121 °C, 2-h at 121 °C, and 24-h at room temperature (natural extraction). Microalgae culture in the microplates containing control (media) and enriched (media + SE) samples were incubated for nine days, at 25 °C with the light intensity of 33.75 μmol photons m⁻² s⁻¹ at 12-h light/dark cycle. The total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) in KP SE were 44.0–82.0 mg L⁻¹ and 0.96–8.60 mg L⁻¹. TDN (8.0%–515.0%) and TDP (105%–186 %) were relatively higher in KP SE compared to SB SE. The growth of microalgae species Nannochloropsis ocenica showed significant differences (p < 0.05) between the five extraction treatments from SB and the control. However, Chlorella vulgaris, Neochloris conjuncta, and Nephroclamys subsolitaria showed no significant differences (p > 0.05) in SB SE. N. ocenica, C. vulgaris, and N. conjuncta showed significant differences (p < 0.05) between five extraction treatments from KP and the control while N. subsolitaria showed no significant difference (p > 0.05). The specific growth rate (SGR) in the exponential phase of all microalgae species were relatively higher in SB SE compared to KP SE. While the organic matter content of KP SE was relatively higher, there were no significant differences in microalgae growth compared to SB SE. Nonetheless, modified SE did influence microalgae growth compared to the control. This study shows that modified SE could be used as enrichment media for microalgae cultivation.

Organic wastes/by-products as alternative to CO2 for producing mixotrophic microalgae enhancing lipid production

In this work, white wine lees (WWL), cheese whey (CW), and glycerol (GLY) were used as carbon (C) sources to mixotrophically support the production of the microalga Nannochloropsis salina, replacing CO₂ supply. In doing so, the alga was allowed to grow on C sources dosed at 2 g L^-1, 3 g L^-1, and 4 g L^-1 of C, in the presence and absence of CO₂ supply. WWL and CW were not able to support the algal growth due to a fungal contamination that was genomically identified, while GLY gave interesting results in particular with 3 g L^-1 of C. GLY-C was able to replace CO₂-C completely when the latter was omitted, showing an algal biomass production similar to those obtained in autotrophy. If CO₂-C was provided jointly with GLY-C, biomass production and lipid contents increased more than 30% and 23%, respectively, compared to autotrophy.

Digestate treatment with algae-bacteria consortia: A field pilot-scale experimentation in a sub-optimal climate area

This paper addresses the efficiency of a microalgae-based agricultural digestate treatment at pilot-scale in an outdoor raceway pond (880 L, pH-dependent CO₂ dosage) and in a bubble column (74.5 L, air-bubbling). Specifically, nitrogen removal, evolution of the algae-bacteria consortium, and the actual process applicability in the Po Valley climate are discussed. The performance of the two reactors varied seasonally. The average algal productivity in the raceway was 32.4 ± 33.1 mg TSS·L^-1·d^-1 (8.2 ± 8.5 g TSS·m^-2·d^-1) while in the PBR it was 25.6 ± 26.8 mg TSS·L^-1·d^-1; the average nitrogen removal was 20 ± 29% (maximum 78%) and 22 ± 29% (maximum 71%) in the raceway and in the column, respectively. Nevertheless, nitrification had a key role as 61 ± 24% and 52 ± 32% of the nitrogen load was oxidized in the raceway and in the column, respectively.

Utilization of centrate for the outdoor production of marine microalgae at the pilot-scale in raceway photobioreactors

In this study, the outdoor production of marine microalgae in raceway photobioreactors was investigated, modifying the centrate percentage in the culture medium (20, 30, 40 and 50%) and using two different dilution rates (0.2 day^-1 and 0.3 day^-1). The data obtained showed that microalgae are capable of producing biomass in addition to recovering the nutrients contained in the centrate. The best results for biomass productivity and light efficiency were obtained when the centrate was set at 20% with a dilution rate of 0.3 day^-1. The biomass productivity was 32.42 g m^-2·day^-1 while the photosynthetic efficiency was 0.74 g_biomass·E^-1 (3.66%). Regarding the nutrients, nitrogen (the majority being in the form of ammonium [NH₄⁺]) and phosphorus were only fixed into biomass when optimal conditions were set; if this was not the case, they were lost to stripping or precipitation. The maximal nutrient removal capacities under the optimal conditions were 28.72 mg_N·l^-1·day^-1 and 3.99 mg_P·l^-1·day^-1. Population changes were determined by the dilution rate set whilst the centrate percentage had little effect. Four strains were present in the culture, Nannochloropsis g. being the main one. Biochemical changes did not vary greatly between the conditions set for the culture, with a composition rich in proteins and carbohydrates being observed. One can conclude that to produce marine microalgal biomass for a range of potential commodities such as feed, biofertilizers and biofuels, it is possible to use centrate from anaerobic digestion as the sole nutrient source, as a way of reducing costs.

Long-term photosynthetic CO2 removal from biogas and flue-gas: Exploring the potential of closed photobioreactors for high-value biomass production

The long-term performance of a tubular photobioreactor interconnected to a gas absorption column for the abatement of CO₂ from biogas and flue-gas was investigated. Additionally, a novel nitrogen feast-famine regime was implemented during the flue-gas feeding stage in order to promote the continuous storage of highly-energetic compounds. Results showed effective CO₂ (~98%) and H₂S (~99%) removals from synthetic biogas, supported by the high photosynthetic activity of microalgae which resulted in an alkaline pH (~10). In addition, CO₂ removals of 99 and 91% were observed during the flue-gas operation depending on the nutrients source: mineral salt medium and digestate, respectively. A biomass productivity of ~8 g m^-2 d^-1 was obtained during both stages, with a complete nitrogen and carbon recovery from the cultivation broth. Moreover, the strategy of feeding nutrients during the dark period promoted the continuous accumulation of carbohydrates, their concentration increasing from 22% under normal nutrition up to 37% during the feast-famine cycle. This represents a productivity of ~3 g-_{carbohydrates} m^-2 d^-1, which can be further valorized to contribute to the economic sustainability of the photosynthetic CO₂ removal process.

Utilization of centrate from urban wastewater plants for the production of Scenedesmus sp. in a raceway-simulating reactor

This work investigates the production of the native microalgae strain Scenedesmus sp. in semi-continuous mode at lab scale in open raceway-simulating reactors and using centrate as the culture medium. The biomass productivity and nutrient removal capacity of Scenedesmus sp. at different dilution rates were investigated indoors as well as its tolerance to centrate as the culture medium at different concentrations. A biomass productivity of 7.80 g/m² day was obtained at 200 μE/m² s, 5 cm culture depth, 0.30 1/day of dilution rate and 60% centrate while nitrogen and phosphorus removal rates were 1.50 g/m² day and 0.15 g/m² day, respectively. The produced biomass characterization under these conditions showed a lipid content of 12.60% d wt. along with a favorable fatty acids profile with 57.70% of total fatty acids composed of saturated and monounsaturated fatty acids. Subsequently, the effect of light intensity and culture depth on biomass productivity and nutrient uptake as well as the biochemical composition and fatty acids profile was studied using two irradiance levels (200 and 1000 μE/m² s) and four culture depths (5 cm, 10 cm, 15 cm and 20 cm). Under optimal conditions of 1000 μE/m² s, 60% centrate, 0.30 1/day dilution rate and 15 cm culture depth, a maximum biomass productivity of 22.20 g/m² day was obtained. Nitrogen and phosphorus removal rates of 2.00 gN/m² day and 0.40 gP/m² day, respectively, were recorded. An amount of 11.70% d wt. of lipids was determined along with a suitable fatty acids profile for biofuel production.

Prospects, recent advancements and challenges of different wastewater streams for microalgal cultivation

Microalgae are recognized as one of the most powerful biotechnology platforms for many value added products including biofuels, bioactive compounds, animal and aquaculture feed etc. However, large scale production of microalgal biomass poses challenges due to the requirements of large amounts of water and nutrients for cultivation. Using wastewater for microalgal cultivation has emerged as a potential cost effective strategy for large scale microalgal biomass production. This approach also offers an efficient means to remove nutrients and metals from wastewater making wastewater treatment sustainable and energy efficient. Therefore, much research has been conducted in the recent years on utilizing various wastewater streams for microalgae cultivation. This review identifies and discusses the opportunities and challenges of different wastewater streams for microalgal cultivation. Many alternative routes for microalgal cultivation have been proposed to tackle some of the challenges that occur during microalgal cultivation in wastewater such as nutrient deficiency, substrate inhibition, toxicity etc. Scope and challenges of microalgal biomass grown on wastewater for various applications are also discussed along with the biorefinery approach.

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.

Microalgae Cultivation on Anaerobic Digestate of Municipal Wastewater, Sewage Sludge and Agro-Waste

Microalgae are fast-growing photosynthetic organisms which have the potential to be exploited as an alternative source of liquid fuels to meet growing global energy demand. The cultivation of microalgae, however, still needs to be improved in order to reduce the cost of the biomass produced. Among the major costs encountered for algal cultivation are the costs for nutrients such as CO₂, nitrogen and phosphorous. In this work, therefore, different microalgal strains were cultivated using as nutrient sources three different anaerobic digestates deriving from municipal wastewater, sewage sludge or agro-waste treatment plants. In particular, anaerobic digestates deriving from agro-waste or sewage sludge treatment induced a more than 300% increase in lipid production per volume in Chlorella vulgaris cultures grown in a closed photobioreactor, and a strong increase in carotenoid accumulation in different microalgae species. Conversely, a digestate originating from a pilot scale anaerobic upflow sludge blanket (UASB) was used to increase biomass production when added to an artificial nutrient-supplemented medium. The results herein demonstrate the possibility of improving biomass accumulation or lipid production using different anaerobic digestates.

Influence of hydraulic retention time on indigenous microalgae and activated sludge process

Integration of the microalgae and activated sludge (MAAS) process in municipal wastewater treatment and biogas production from recovered MAAS was investigated by studying the hydraulic retention time (HRT) of semi-continuous photo-bioreactors. An average total nitrogen (TN) removal efficiency (RE) of maximum 81.5 ± 5.1 and 64.6 ± 16.2% was achieved at 6 and 4 days HRT. RE of total phosphorous (TP) increased slightly at 6 days (80 ± 12%) HRT and stabilized at 4 days (56 ± 5%) and 2 days (55.5 ± 5.5%) HRT due to the fluctuations in COD and N/P mass ratio of the periodic wastewater. COD and organic carbon were removed efficiently and a rapidly settleable MAAS with a sludge volume index (SVI_10) of less than 117 mL g(-1) was observed at all HRTs. The anaerobic digestion of the untreated MAAS showed a higher biogas yield of 349 ± 10 mL g VS(-1) with 2 days HRT due to a low solids retention time (SRT). Thermal pretreatment of the MAAS (120 °C, 120 min) did not show any improvement with biogas production at 6 days (269 ± 3 (untreated) and 266 ± 16 (treated) mL gVS(-1)), 4 days (258 ± 11(untreated) and 263 ± 10 (treated) mL gVS(-1)) and 2 days (308 ± 19 mL (treated) gVS(-1)) HRT. Hence, the biogas potential tests showed that the untreated MAAS was a feasible substrate for biogas production. Results from this proof of concept support the application of MAAS in wastewater treatment for Swedish conditions to reduce aeration, precipitation chemicals and CO2 emissions.