Dialysis bag-microalgae photobioreactor: Novel strategy for enhanced bioresource production and wastewater purification.

Cultivating microalgae in wastewater offers various advantages, but it still faces limitations such as bacteria and other impurities in wastewater affecting the growth and purity of microalgae, difficulty in microalgae harvesting, and extracellular products of microalgae affecting effluent quality. In this study, a novel dialysis bag-microalgae photobioreactor (Db-PBR) was developed to achieve wastewater purification and purer bioresource recovery by culturing microalgae in a dialysis bag. The dialysis bag in the Db-PBR effectively captured the microalgae cells and promoted their lipid accumulation, leading to higher biomass (1.53 times of the control) and lipid production (2.50 times of the control). During the stable operation stage of Db-PBR, the average soluble microbial products (SMP) content outside the dialysis bag was 25.83 mg L-1, which was significantly lower than that inside the dialysis bag (185.63 mg L-1), indicating that the dialysis bag effectively intercepted the SMP secreted by microalgae. As a result, the concentration of dissolved organic carbon (DOC) in Db-PBR effluent was significantly lower than that of traditional photobioreactor. Furthermore, benefiting from the dialysis bag in the reactor effectively intercepted the microorganisms in wastewater, significantly improving the purity of the cultured microalgae biomass, which is beneficial for the development of high-value microalgae products.

A vertical-flow constructed wetland-microalgal membrane photobioreactor integrated system for treating high-pollution-load marine aquaculture wastewater: A lab-scale study.

Individual biological water treatment techniques often prove ineffective in removing accumulated high concentrations of nitrogen and phosphorus in the late stages of biofloc aquaculture. To address this issue, we integrated a previously developed autotrophic denitrification and nitrification integrated constructed wetland (ADNI-CW) with a microalgal membrane photobioreactor (MPBR). Under high nitrogen and phosphorus pollution loads in the influent, the standalone ADNI-CW system achieved removal rates of only 24.17 % ± 2.82 % for total nitrogen (TN) and 25.30 % ± 2.59 % for total phosphorus (TP). The optimal conditions for TN and TP degradation and microalgal biomass production in the Chlorella MPBR, determined using response surface methodology, were an inoculum OD680 of 0.394, light intensity of 161.583 µmol/m2/s, and photoperiod of 16.302 h light:7.698 h dark. Under the optimal operating conditions, the integrated ADNI-CW-MPBR system achieved remarkable TN and TP removal rates of 92.63 % ± 2.8 % and 77.46 % ± 8.41 %, respectively, and a substantial microalgal biomass yield of 54.58 ± 6.8 mg/L/day. This accomplishment signifies the successful achievement of efficient nitrogen and phosphorus removal from high-pollution-load marine aquaculture wastewater along with the acquisition of valuable microalgal biomass. A preliminary investigation of the microbial community composition and algal-bacterial interactions in different operational stages of the MPBR system revealed that unclassified_d__Bacteria, Chlorophyta, and Planctomycetes were predominant phyla. The collaborative relationships between bacteria and Chlorella surpassed competition, ensuring highly efficient nitrogen and phosphorus removal in the MPBR system. This study laid the foundation for the green and sustainable development of the aquaculture industry.

Impact of mixed microalgal and bacterial species on organic micropollutants removal in photobioreactors under natural light.

Single microalgae species are effective at the removal of various organic micropollutants (OMPs), however increased species diversity might enhance this removal. Sixteen OMPs were added to 2 continuous photobioreactors, one inoculated with Chlorella sorokiniana and the other with a microalgal-bacterial community, for 112 d under natural light. Three media were sequentially used in 3 Periods: I) synthetic sewage (d 0-28), II) 10x diluted anaerobically digested black water (AnBW) (d 28-94) and III) 5x diluted AnBW (d 94-112). Twelve OMPs were removed > 30 %, while 4 were < 10 % removed. Removal efficiencies were similar for 9 OMPs, yet the mixed community showed a 2-3 times higher removal capacity (µg OMP/g dry weight) than C. sorokiniana during Period II pseudo steady state. The removal decreased drastically in Period III due to overgrowth of filamentous green algae. This study shows for the first time how microbial community composition and abundance are key for OMPs removal.

Comparative evaluation of low-cost ceramic membrane and polymeric micro membrane in algal membrane photobioreactor for wastewater treatment.

Algal-bacterial membrane photobioreactor (AMPBR) is proven as a highly energy-efficient process for treating domestic wastewater. This study compared the application of polymeric micro-membrane (PMM) and a low-cost ceramic membrane (LCM) to the AMPBR process for treating domestic wastewater with low and high organic pollution levels. Experiments were conducted over 57 days using two PMM-AMPBRs and two LCM-AMPBRs, operating on a 12-h dark/light cycle in a continuous mode. Simulated wastewater containing varying levels of chemical oxygen demand (COD) was fed to reactors for a consistent hydraulic residence time (HRT) of 7 d and a flux rate of 100 L/m2/d. PMM and LCM-AMPBRs demonstrated efficient wastewater treatment capabilities, achieving COD removal rates exceeding 94% and 95% for high and low COD loadings, respectively. PMM-AMPBR achieved 54.1% TN removal at low COD loading, while LCM-AMPBR achieved 57.2%. These removal efficiencies decreased to 45.6% and 47.0% under high COD loading. Total Phosphorus (TP) removal reached 29-33% for PMM-AMPBRs and 21-24% for LCM-AMPBRs, irrespective of COD loading. LCM-AMPBRs showed significantly lower fouling frequency than PMM-AMPBRs. The biomass production rate decreased with increasing COD loading and achieved 40 mg/L/d at low COD loading for both AMPBRs. Net energy return (NER) values for both AMPBRs were close to 0.87, indicating them as energy-efficient processes. Considering the cost-effectiveness and comparable performance, LCM-AMPBR could be a viable alternative to PMM-AMPBR for wastewater treatment, particularly under low COD loading conditions.

Continuous cultivation of mixed-culture microalgae using anaerobic digestion effluent in photobioreactors with different strategies for adjusting nitrogen loading rate.

This study examined continuous mixed-culture microalgae cultivation for nutrient removal from anaerobic digestion (AD) effluents in photobioreactors, while altering the NH4+-N loading rate (NLR) by adjusting either the hydraulic retention time (HRT) (reactor set RH) or the influent NH4+-N concentration (reactor set RS). Both RH and RS demonstrated efficient nutrient removal and microalgae cultivation at NLRs of 4-10 mg NH4+-N/L∙d, reaching peak performance at 10 mg NH4+-N/L∙d. Within this range, RH obtained greater biomass yield and productivity, while RS maintained higher microalgal concentrations. The cultivated biomasses obtained from RH and RS had good settleability and suitable fatty acid compositions as a biodiesel feedstock, although their organic composition varied considerably with NLR and HRT. Parachlorella overwhelmingly dominated the reactors' microalgal communities throughout the experiment, co-existing with various microalgae-associated bacteria. Changes in NLR significantly influenced the bacterial community structures, underscoring its critical role in determining reactor performance and microalgal-bacterial community behavior.

Effect of solar and artificial lighting on microalgae cultivation and treatment of liquid digestate.

A comparative study was carried out to assess the effect of two light sources on microalgae cultivation and the treatment of liquid digestate. The R1 photobioreactor operated with LED lightning allowed to achieve moderate nutrient removal rates whereas soluble COD (Chemical Oxygen Demand) was reduced in 90%. After switching this reactor into sunlight, the removal rate of phosphates increased to 66%. However, the greatest removal rate of both nutrients and sCOD of up to 93% was observed in the R2 photobioreactor operated only under sunlight. Microglena sp. was the dominant algae growing in the R1 reactor, and the main bacteria families detected were Chitinophagaceae, Sphingomonadaceae and Xanthobacteraceae. In contrast, Tetradesmus obliquus dominated in the R2 reactor and Rhodanobacteraceae, Chitinophagaceae and A4b were predominant bacteria in this run. Furthermore, much greater biomass productivity as well as overall biomass density was observed in the R2 photobioreactor cultivated exclusively with solar lightning.

Recirculating packed-bed biofilm photobioreactor combined with membrane ultrafiltration as advanced wastewater treatment.

Packed-bed biofilm photobioreactor combined with ultrafiltration membrane was investigated for intensifying the process for secondary wastewater effluent treatment. Cylindrical glass carriers were used as supporting material for the microalgal-bacterial biofilm, which developed from indigenous microbial consortium. Glass carriers allowed adequate growth of the biofilm with limited suspended biomass. Stable operation was achieved after a start-up period of 1000 h, where supernatant biopolymer clusters were minimized and complete nitrification was observed. After that time, biomass productivity was 54 ± 18 mg·L-1·day-1. Green microalgae Tetradesmus obliquus and several strains of heterotrophic nitrification-aerobic denitrification bacteria and fungi were identified. Combined process exhibited COD, nitrogen and phosphorus removal rates of 56 ± 5%, 12 ± 2% and 20 ± 6%, respectively. Membrane fouling was mainly caused by biofilm formation, which was not effectively mitigated by air-scouring aided backwashing.

Algae communication, conspecific and interspecific: the concepts of phycosphere and algal-bacteria consortia in a photobioreactor (PBR).

Microalgae in the wild often form consortia with other species promoting their own health and resource foraging opportunities. The recent application of microalgae cultivation and deployment in commercial photobioreactors (PBR) so far has focussed on single species of algae, resulting in multi-species consortia being largely unexplored. Reviewing the current status of PBR ecological habitat, this article argues in favor of further investigation into algal communication with conspecifics and interspecifics, including other strains of microalgae and bacteria. These mutualistic species form the 'phycosphere': the microenvironment surrounding microalgal cells, potentiating the production of certain metabolites through biochemical interaction with cohabitating microorganisms. A better understanding of the phycosphere could lead to novel PBR configurations, capable of incorporating algal-microbial consortia, potentially proving more effective than single-species algal systems.

Wastewater from textile digital printing as a substrate for microalgal growth and valorization.

This study aims at evaluating an innovative biotechnological process for the concomitant bioremediation and valorization of wastewater from textile digital printing technology based on a microalgae/bacteria consortium. Nutrient and colour removal were assessed in lab-scale batch and continuous experiments and the produced algae/bacteria biomass was characterized for pigment content and biomethane potential. Microbial community analysis provided insight of the complex community structure responsible for the bioremediation action. Specifically, a community dominated by Scenedesmus spp. and xenobiotic and dye degrading bacteria was naturally selected in continuous photobioreactors. Data confirm the ability of the microalgae/bacteria consortium to grow in textile wastewater while reducing the nutrient content and colour. Improvement strategies were eventually identified to foster biomass growth and process performances. The experimental findings pose the basis of the integration of a microalgal-based process into the textile sector in a circular economy perspective.

Development of a bench-scale photobioreactor with a novel recirculation system for continuous cultivation of microalgae.

Microalgae cultivation can be used to increase the sustainability of carbon emitting processes, converting the CO2 from exhaust gases into fuels, food and chemicals. Many of the carbon emitting industries operate in a continuous manner, for periods that can span days or months, resulting in a continuous stream of gas emissions. Biogenic CO2 from industrial microbiological processes is one example, since in many cases it becomes unsustainable to stop these processes on a daily or weekly basis. To correctly sequester these emissions, microalgae systems must be operated under continuous constant conditions, requiring photobioreactors (PBRs) that can act as chemostats for long periods of time. However, in order to optimize culture parameters or study metabolic responses, bench-scale setups are necessary. Currently there is a lack of studies and design alternatives using chemostat, since most works focus on batch assays or semi-continuous cultures. Therefore, this work focused on the development of a continuous bench-scale PBR, which combines a retention vessel, a photocollector and a degasser, with an innovative recirculation system, that allows it to operate as an autotrophic chemostat, to study carbon sequestration from a biogenic CO2-rich constant air stream. To assess its applicability, the PBR was used to cultivate the green microalga Haematococcus pluvialis using as sole carbon source the CO2 produced by a coupled heterotrophic bacterial chemostat. An air stream containing ≈0.35 vol% of CO2, was fed to the system, and it was evaluated in terms of stability, carbon fixation and biomass productivity, for dilution rates ranging from 0.1 to 0.5 d-1. The PBR was able to operate under chemostat conditions for more than 100 days, producing a stable culture that generated proportional responses to the stimuli it was subjected to, attaining a maximum biomass productivity of 183 mg/L/d with a carbon fixation efficiency of ≈39% at 0.3 d-1. These results reinforce the effectiveness of the developed PBR system, making it suitable for laboratory-scale studies of continuous photoautotrophic microalgae cultivation.

Photosynthesis driven continuous hydrogen production by diazotrophic cyanobacteria in high cell density capillary photobiofilm reactors.

Hydrogen (H2) is a promising fuel in the context of climate neutral energy carriers and photosynthesis-driven H2-production is an interesting option relying mainly on sunlight and water as resources. However, this approach depends on suitable biocatalysts and innovative photobioreactor designs to maximize cell performance and H2 titers. Cyanobacteria were used as biocatalysts in capillary biofilm photobioreactors (CBRs). We show that biofilm formation/stability depend on light and CO2 availabilityH2 production rates correlate with these parameters but differ between Anabaena and Nostoc. We demonstrate that high light and corresponding O2 levels influence biofilm stability in CBR. By adjusting these parameters, biofilm formation/stability could be enhanced, and H2 formation was stable for weeks. Final biocatalyst titers reached up to 100 g l-1 for N. punctiforme atcc 29133 NHM5 and Anabaena sp. pcc 7120 AMC 414. H2 production rates were up to 300 µmol H2 l-1h-1 and 3 µmol H2 gcdw-1h-1 in biofilms.

Design and experimental validation of an optimized microalgae-bacteria consortium for the bioremediation of glyphosate in continuous photobioreactors.

Glyphosate will be banned from Europe by the end of 2022, but its widespread use in the last decades and its persistence in the environment require the development of novel remediation processes. In this work, a bacterial consortium was designed de novo with the aim to remove glyphosate from polluted water, supported by the oxygen produced by a microalgal species. To this goal, bioinformatics tools were employed to identify the bacterial strains from contaminated sources (Pseudomonas stutzeri; Comamonas odontotermitis; Sinomonas atrocyanea) able to express enzymes for glyphosate degradation, while the microalga Chlorella protothecoides was chosen for its known performances in wastewater treatment. To follow a bioaugmentation approach, the designed consortium was cultivated in continuous photobioreactors at increasing glyphosate concentrations, from 5 to 50 mg L-1, to boost its acclimation to the presence of the herbicide and its capacity to remove it from water. C. protothecoides tolerance to glyphosate was verified through batch experiments. Remarkably, steady state conditions were reached and the consortium was able to live as a community in the reactor. The consortium activity was validated in both synthetic and real wastewater, where glyphosate concentration was reduced by about 53% and 79%, respectively, without the detection of aminomethylphosphonic acid formation.

Process Technologies of Cyanobacteria.

Although the handling and exploitation of cyanobacteria is associated with some challenges, these phototrophic bacteria offer great opportunities for innovative biotechnological processes. This chapter covers versatile aspects of working with cyanobacteria, starting with up-to-date in silico and in vitro screening methods for bioactive substances. Subsequently, common conservation techniques and vitality/viability estimation methods are compared and supplemented by own data regarding the non-invasive vitality evaluation via pulse amplitude modulated fluorometry. Moreover, novel findings about the influence the state of the pre-cultures have on main cultures are presented. The following sub-chapters deal with different photobioreactor-designs, with special regard to biofilm photobioreactors, as well as with heterotrophic and mixotrophic cultivation modes. The latter topic provides information from literature on successfully enhanced cyanobacterial production processes, augmented by own data.

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.

The effects of light regime on carbon cycling, nutrient removal, biomass yield, and polyhydroxybutyrate (PHB) production by a constructed photosynthetic consortium.

Microalgae can add value to biological wastewater treatment processes by capturing carbon and nutrients and producing valuable biomass. Harvesting small cells from liquid media is a challenge easily addressed with biofilm cultivation. Three experimental photobioreactors were constructed from inexpensive materials (e.g. plexiglass, silicone) for hybrid liquid/biofilm cultivation of a microalgal-bacterial consortia in aquaculture effluent. Three light regimes (full-spectrum, blue-white, and red) were implemented to test light spectra as a process control. High-intensity full-spectrum light caused photoinhibition and low biomass yield, but produced the most polyhydroxybutyrate (PHB) (0.14 mg g-1); a renewable bioplastic polymer. Medium-intensity blue-white light was less effective for carbon capture, but removed up to 82 % of phosphorus. Low-intensity red light was the only net carbon-negative regime, but increased phosphorus (+4.98 mg/L) in the culture medium. Light spectra and intensity have potential as easily-implemented process controls for targeted wastewater treatment, biomass production, and PHB synthesis using photosynthetic consortia.

Creating value from purple phototrophic bacteria via single-cell protein production.

Mixed culture purple phototrophic bacteria (PPB) is a rapidly emerging technology for resource recovery from wastewaters. PPB biomass can be used as single-cell protein, with a high protein content complemented by value-added components (e.g. pigments and polyhydroxyalkanoates), merging functionalities within a single product. This has the potential to increase the value and impact the economic feasibility, justifying higher capital costs for PPB photobioreactors for real-life applications. Artificial illumination is prohibitively expensive, and naturally illuminated, outdoor units are a critical next step. However, information required for informed technoeconomic assessment of single-cell protein from PPB is still missing and can only be determined in dedicated larger-scale, outdoor systems. Larger scale units are also required to supply feed for larger cohort trials. Although data from microalgae research can be used as starting point to estimate costs, they cannot be translated directly for PPB, as the organisms and metabolic growth are fundamentally different.

Lab-scale photobioreactor systems: principles, applications, and scalability.

Phototrophic microorganisms that convert carbon dioxide are being explored for their capacity to solve different environmental issues and produce bioactive compounds for human therapeutics and as food additives. Full-scale phototrophic cultivation of microalgae and cyanobacteria can be done in open ponds or closed photobioreactor systems, which have a broad range of volumes. This review focuses on laboratory-scale photobioreactors and their different designs. Illuminated microtiter plates and microfluidic devices offer an option for automated high-throughput studies with microalgae. Illuminated shake flasks are used for simple uncontrolled batch studies. The application of illuminated bubble column reactors strongly emphasizes homogenous gas distribution, while illuminated flat plate bioreactors offer high and uniform light input. Illuminated stirred-tank bioreactors facilitate the application of very well-defined reaction conditions. Closed tubular photobioreactors as well as open photobioreactors like small-scale raceway ponds and thin-layer cascades are applied as scale-down models of the respective large-scale bioreactors. A few other less common designs such as illuminated plastic bags or aquarium tanks are also used mainly because of their relatively low cost, but up-scaling of these designs is challenging with additional light-driven issues. Finally, this review covers recommendations on the criteria for photobioreactor selection and operation while up-scaling of phototrophic bioprocesses with microalgae or cyanobacteria.

Influence of wettability and surface design on the adhesion of terrestrial cyanobacteria to additive manufactured biocarriers.

Productive biofilms are gaining growing interest in research due to their potential of producing valuable compounds and bioactive substances such as antibiotics. This is supported by recent developments in biofilm photobioreactors that established the controlled phototrophic cultivation of algae and cyanobacteria. Cultivation of biofilms can be challenging due to the need of surfaces for biofilm adhesion. The total production of biomass, and thus production of e.g. bioactive substances, within the bioreactor volume highly depends on the available cultivation surface. To achieve an enlargement of surface area for biofilm photobioreactors, biocarriers can be implemented in the cultivation. Thereby, material properties and design of the biocarriers are important for initial biofilm formation and growth of cyanobacteria. In this study, special biocarriers were designed and additively manufactured to investigate different polymeric materials and surface designs regarding biofilm adhesion of the terrestrial cyanobacterium Nostoc flagelliforme (CCAP 1453/33). Properties of 3D-printed materials were characterized by determination of wettability, surface roughness, and density. To evaluate the influence of wettability on biofilm formation, material properties were specifically modified by gas-phase fluorination and biofilm formation was analyzed on biocarriers with basic and optimized geometry in shaking flask cultivation. We found that different polymeric materials revealed no significant differences in wettability and with identical surface design no significant effect on biomass adhesion was observed. However, materials treated with fluorination as well as optimized biocarrier design showed improved wettability and an increase in biomass adhesion per biocarrier surface.

Aggregation of purple bacteria in an upflow photobioreactor to facilitate solid/liquid separation: Impact of organic loading rate, hydraulic retention time and water composition.

Purple non-sulfur bacteria (PNSB) form an interesting group of microbes for resource recovery from wastewater. Solid/liquid separation is key for biomass and value-added products recovery, yet insights into PNSB aggregation are thus far limited. This study explored the effects of organic loading rate (OLR), hydraulic retention time (HRT) and water composition on the aggregation of Rhodobacter capsulatus in an anaerobic upflow photobioreactor. Between 2.0 and 14.6 gCOD/(L.d), the optimal OLR for aggregation was 6.1 gCOD/(L.d), resulting in a sedimentation flux of 5.9 kgTSS/(m2.h). With HRT tested between 0.04 and 1.00 d, disaggregation occurred at the relatively long HRT (1 d), possibly due to accumulation of thus far unidentified heat-labile metabolites. Chemical oxygen demand (COD) to nitrogen ratios (6-35 gCOD/gN) and the nitrogen source (ammonium vs. glutamate) also impacted aggregation, highlighting the importance of the type of wastewater and its pre-treatment. These novel insights to improve purple biomass separation pave the way for cost-efficient PNSB applications.

Passively immobilized cyanobacteria Nostoc species BB 92.2 in a moving bed photobioreactor (MBPBR): Design, cultivation, and characterization.

The cyanobacterium Nostoc sp. BB 92.3. had shown antibacterial activity. A cultivation as biofilm, a self-forming matrix of cells and extracellular polymeric substances, increased the antibacterial effect. A new photobioreactor system was developed that allows a surface-associated cultivation of Nostoc sp. as biofilm. High-density polyethylene carriers operated as a moving bed were selected as surface for biomass immobilization. This system, well established in heterotrophic wastewater treatment, was for the first time used for phototrophic biofilms. The aim was a cultivation on a large scale without inhibiting growth while maximizing immobilization. Cultivation in a small photobioreactor (1.5 L) with different volumetric filling degrees of carriers (13.4%-53.8%) in a batch process achieved immobilization rates of 70%-85% and growth was similar to a no-carrier-control. In a larger photobioreactor (65 L) essentially all of the biomass was immobilized on the carriers and the space-time yield of biomass (0.018 gcell dry weight L-1 day- ​​​​​​​1 ) was competitive compared to phototrophic biofilm cultivations from literature. The use of carriers increased the gas exchange in the reactor by a factor of 2.5-3 but doubled the mixing time. Enriched gassing with carbon dioxide resulted in a short-term increase in growth rate, but unexpectedly it also adversely changed the growth morphology.