Algal-bacterial Photobioreactor Research Articles

Technology validation of photosynthetic biogas upgrading in a semi-industrial scale algal-bacterial photobioreactor

The performance of photosynthetic biogas upgrading coupled to wastewater treatment was evaluated in an outdoors high rate algal pond (HRAP) interconnected to an absorption column at semi-industrial scale. The influence of biogas flowrate (274, 370 and 459 L h−1), liquid to biogas ratio (L/G = 1.2, 2.1 and 3.5), type of wastewater (domestic versus centrate) and hydraulic retention time in the HRAP (HRT) on the quality of the biomethane produced was assessed. The highest CO2 and H2S removal efficiencies (REs) were recorded at the largest L/G due to the higher biogas-liquid mass transfer at increasing liquid flowrates. No significant influence of the biogas flowrate on process performance was observed, while the type of wastewater was identified as a key operational parameter. CO2 and H2S-REs of 99% and 100% at a L/Gmax = 3.5 were recorded using centrate. The maximum CH4 content in the biomethane (90%) was limited by N2 and O2 desorption.

Removal of contaminants of emerging concern from urban wastewater in novel algal-bacterial photobioreactors

This work evaluates the removal of five pharmaceuticals and personal care products, i.e., ibuprofen, naproxen, salicylic acid, triclosan and propylparaben, from urban wastewater under two novel algal-bacterial photobioreactor settings. The first configuration (phase A) consisted of an anoxic-aerobic photobioreactor operating at a hydraulic retention time (HRT) of 2d at different concentrations of total organic carbon (TOC) (90mgL−1–200mgL−1). In the second configuration (phase B) an anaerobic step was introduced before the anoxic tank to set a photosynthetic A2O process. In this phase, the HRT varied between 3 and 4d and the TOC was kept constant at 200mgL−1. In addition, the impact of external aeration in the aerobic photobioreactor was assessed. The maximum removals for ibuprofen, naproxen, salicylic acid, triclosan and propylparaben (94±1%, 52±43%, 98±2%, 100±0%, 100±0%, respectively) were recorded during phase B. In phase A, low TOC concentrations triggered higher ibuprofen and naproxen removals likely due to the high contribution of biological oxidation on their removal. In phase B, total or very high removal efficiencies were observed for ibuprofen, propylparaben and triclosan independently on the operating conditions. In contrast, the removal efficiency of naproxen and salicylic acid decreased when the HRT dropped from 4 to 3d in the absence of external aeration, which suggests that biodegradation played a key role in their removal. In addition, sorption might have contributed to the elimination of triclosan and propylparaben from the wastewater.

Performance and kinetics of algal-bacterial photobioreactor (AB-PBR) treating septic tank effluent.

Septic tank effluent contains high organic and nutrient contents. This study aimed to evaluate treatment performance of an algal-bacterial photobioreactor (AB-PBR) treating the septic tank effluent. The experimental unit employed a transparent plastic medium made from recycled drinking water bottles for attached-growth biofilm. Red LED lamp (light intensity ∼100 μmol/m2/s) was applied as an energy source for the growth of algal-bacterial biofilm in the AB-PBR. The experimental results showed that AB-PBR operated at the hydraulic retention time (HRT) of 3 days gave the highest chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP) removal efficiencies of 64, 45 and 35%, respectively, by which the effluent COD concentrations could meet the effluent standards of Thailand, but the effluent TN and TP concentrations needed to be further removed. The Stover-Kincannon model was applied to determine the kinetic values of COD and TN removals with R2 values greater than 0.8. Microbiological examinations indicated Chlorella sp. is the predominant algal species growing in the AB-PBR, while the amplicon sequencing information analytical results revealed the bacterial phylum of Proteobacteria to be the predominant bacterial group.

Influence of alkalinity and temperature on photosynthetic biogas upgrading efficiency in high rate algal ponds

Algal-bacterial photobioreactors have emerged as a cost-effective platform for biogas upgrading. The influence on biomethane quality of the inorganic carbon concentration (1500, 500 and 100 mg L−1) and temperature (12 and 35 °C) of the cultivation broth was evaluated in a 180 L high rate algal pond (HRAP) interconnected to a 2.5 L absorption column via settled broth recirculation. The highest CO2 and H2S removal efficiencies (REs) from biogas were recorded at the highest alkalinity (CO2-REs of 99.3 ± 0.1 and 97.8 ± 0.8% and H2S-REs of 96.4 ± 2.9 and 100 ± 0% at 12 and 35 °C, respectively), which resulted in CH4 concentrations of 98.9 ± 0.2 and 98.2 ± 1.0% at 12 and 35 °C, respectively, in the upgraded biogas. At the lowest alkalinity, the best upgrading performance was observed at 12 °C (CO2 and H2S-REs of 41.5 ± 2.0 and 80.3 ± 3.9%, respectively). The low recycling liquid to biogas ratio applied (0.5) resulted in a negligible O2 stripping regardless of the alkalinity and temperature, which entailed a biomethane O2 content ranging from 0 to 0.2 ± 0.3%.

Seasonal variation of biogas upgrading coupled with digestate treatment in an outdoors pilot scale algal-bacterial photobioreactor

The yearly variations of the quality of the upgraded biogas and the efficiency of digestate treatment were evaluated in an outdoors pilot scale high rate algal pond (HRAP) interconnected to an external absorption column (AC) via a conical settler. CO2 concentrations in the upgraded biogas ranged from 0.7% in August to 11.9% in December, while a complete H2S removal was achieved regardless of the operational month. CH4 concentrations ranged from 85.2% in December to 97.9% in June, with a limited O2 and N2 stripping in the upgraded biogas mediated by the low recycling liquid/biogas ratio in the AC. Biomass productivity ranged from 0.0 g m−2 d−1 in winter to 22.5 g m−2 d−1 in summer. Finally, microalgae diversity was severely reduced throughout the year likely due to the increasing salinity in the cultivation broth of the HRAP induced by process operation in the absence of effluent.

Continuous photosynthetic abatement of CO2 and volatile organic compounds from exhaust gas coupled to wastewater treatment: Evaluation of tubular algal-bacterial photobioreactor

The continuous abatement of CO2 and toluene from the exhaust gas by an indigenous microalgal-bacterial consortium was investigated in a pilot tubular photobioreactor interconnected to an absorption column using diluted centrate in seawater as a free nutrient source. The removal efficiency of CO2 and toluene was maximised in the vertical absorption column by identifying an optimum liquid to gas (L/G) ratio of 15. The photobioreactor supported steady-state nitrogen and phosphorus removals of 91±2% and 95±4% using 15% diluted centrate at 14 and 7 d of hydraulic retention time (HRT), respectively. A decrease in the removal efficiencies of nitrogen (36±5%) and phosphorus (58±10%) was recorded when using 30% diluted centrate at 7 d of HRT. The volumetric biomass productivities obtained at an HRT of 7 d accounted for 42±11 and 80±3mg TSS L−1 d−1 using 15 and 30% centrate, respectively. Stable CO2 (76±7%) and toluene removals (89±5%) were achieved at an L/G ratio of 15 regardless of the HRT or centrate dilution. Hence, this study demonstrated the potential of algal-bacterial systems for the continuous removal of CO2 and volatile organic compounds from exhaust gas coupled with the simultaneous treatment of centrate.

Comparative evaluation of piggery wastewater treatment in algal-bacterial photobioreactors under indoor and outdoor conditions

This work evaluated the performance of four open algal-bacterial photobioreactors operated at ≈26days of hydraulic retention time during the treatment of 10 (×10) and 20 (×20) times diluted piggery wastewater (PWW) under indoor (I) and outdoor (O) conditions for four months. The removal efficiencies (REs) of organic matter, nutrients and zinc from PWW, along with the dynamics of biomass concentration and structure of algal-bacterial population were assessed. The highest TOC-RE, TP-RE and Zn-RE (94±1%, 100% and 83±2%, respectively) were achieved indoors in ×10 PWW, while the highest TN-RE (72±8%) was recorded outdoors in ×10 PWW. Chlorella vulgaris was the dominant species regardless of the ambient conditions and PWW dilution. Finally, DGGE-sequencing of the bacterial community revealed the occurrence of four phyla, Proteobacteria being the dominant phylum with 15 out of the 23 most intense bands.

Evaluation of the dynamics of microalgae population structure and process performance during piggery wastewater treatment in algal-bacterial photobioreactors

The dynamics of microalgae population during piggery wastewater (PWW) treatment in four open photobioreactors operated at 27days of hydraulic retention time, and inoculated with Chlorella sp. (R1), Acutodesmus obliquus (R2), Oscillatoria sp. (R3) and in the absence of inoculum (R4), were evaluated for 6months. In addition, the algal-bacterial biomass concentration, removal of organic matter, nutrients and heavy metals were also assessed. The results revealed a high diversity and rapid variations in the structure of microalgae populations, Chlorella sp. being dominant in R4 throughout most of the operational period. Steady state average biomass concentration ranged from 2445–2610mg/L in R1–R3 to 3265mg/L in R4. No significant differences were recorded in the removal efficiencies (REs) of total organic carbon (86–87%), inorganic carbon (62–71%), total nitrogen (82–85%) and total phosphorous (90–92%). Finally, Zn-REs accounted for 26% in R3, 37% in R2, and 49% in R1 and R4.

A comparative analysis of biogas upgrading technologies: Photosynthetic vs physical/chemical processes

Two biogas upgrading technologies, viz. an innovative algal-bacterial photobioreactor and a conventional activated carbon filter coupled with a water scrubber, were comparatively evaluated in terms of environmental, economic and social performance by using the IChemE Sustainability Metrics. The upgrading of 300Nm3/h of biogas generated from the anaerobic digestion of mixed sludge in a wastewater treatment plant was used as a model scenario for the comparative analysis. Despite the algal-bacterial photobioreactor entailed 1860 times higher land requirements, the two-stage physical/chemical technology exhibited ×3.8 higher energy consumptions and larger environmental impacts in terms of material and water consumption and greenhouse gas emissions (the latter by a factor of ~45). The investment cost for the algal-bacterial photobioreactor was 1.6 times higher than that of its physical/chemical counterpart due to the biomass drying unit required to produce an algae-based fertilizer. However, the operating cost of the physical/chemical technology was ~7 times higher due to the frequent replacement of the activated carbon. A further analysis of the net present value (NPV 20) revealed that photosynthetic upgrading would yield revenues from year 5 of operation mainly due to the sale of the algal bio-fertilizer produced, even without tax incentives for bio-methane.

Enhanced carbon, nitrogen and phosphorus removal from domestic wastewater in a novel anoxic-aerobic photobioreactor coupled with biogas upgrading

This work evaluated the performance of an innovative anoxic-aerobic algal-bacterial photobioreactor coupled with biogas upgrading for the treatment of domestic wastewater via nitrification-denitrification. The process, which incorporated a biomass settling step followed by recycling to the anoxic tank, was operated at a hydraulic retention time of 2days, a sludge retention time of ≈11days under a 12h/12h light/dark irradiation cycle at 392μEm−2*s−1. An increase in the removal efficiency of TN from 38% to 81%, NH4+ from 39% to 97%, and P-PO43− from 59% to 64% were recorded when additional CO2 was supplied to the photobioreactor via biogas scrubbing, which supported an almost complete nitrification of the NH4+ to NO3− and promoted microalgae growth (with the subsequent enhancement in N and P assimilation). TOC removal remained constant at 90±2% regardless of the addition of CO2, while the effluent biomass concentration averaged 26±12mgTSS/L. A DGGE-sequencing analysis of the bacterial community revealed the occurrence of 10phyla, Proteobacteria being the dominant phylum. Finally, the morphological characterization of the microalgae population dynamics revealed a gradual dominance of the genus Scenedesmus, which accounted for 94–100% at the end of the experiment.

Influence of the gas-liquid flow configuration in the absorption column on photosynthetic biogas upgrading in algal-bacterial photobioreactors

The potential of an algal-bacterial system consisting of a high rate algal pond (HRAP) interconnected to an absorption column (AC) via recirculation of the cultivation broth for the upgrading of biogas and digestate was investigated. The influence of the gas-liquid flow configuration in the AC on the photosynthetic biogas upgrading process was assessed. AC operation in a co-current configuration enabled to maintain a biomass productivity of 15gm−2d−1, while during counter-current operation biomass productivity decreased to 8.7±0.5gm−2d−1 as a result of trace metal limitation. A bio-methane composition complying with most international regulatory limits for injection into natural gas grids was obtained regardless of the gas-liquid flow configuration. Furthermore, the influence of the recycling liquid to biogas flowrate (L/G) ratio on bio-methane quality was assessed under both operational configurations obtaining the best composition at an L/G ratio of 0.5 and co-current flow operation.

Influence of light intensity on bacterial nitrifying activity in algal-bacterial photobioreactors and its implications for microalgae-based wastewater treatment

The influence of irradiance on the nitrifying activity in photobioreactors of a bacterial consortium enriched from a wastewater treatment bioreactor was assessed using independent ammonium oxidation kinetic batch tests and respirometric assays. Culture irradiance below 250 μmol m−2 s−1 did not show a significant effect on nitrification activity, while irradiance at 500 and 1250 μmol m−2 s−1 caused a decrease of 20 and 60% in the specific total ammonium nitrogen removal rates and a reduction of 26 and 71% in the specific NO3− production rates, respectively. However, no significant influence of irradiance on the affinity constant of NH4+ oxidation was observed. The increasing nitrite accumulation at higher light intensities suggested a higher light sensitivity of nitrite oxidizers. Additionally, NH4+ oxidation respirometric assays showed a decrease in the oxygen uptake of 14 and 50% at 500 and 1250 μmol m−2 s−1, respectively. The experimental determination of the light extinction coefficient (λ) of the nitrifying bacterial consortium (λ = 0.0003 m2 g−1) and of Chlorella sorokiniana (λ = 0.1045 m2 g−1) allowed the estimation of light penetration in algal-bacterial high rate algal ponds, which showed that photoinhibition of nitrifying bacteria can be significantly mitigated in the presence of high density microalgal cultures.

Biogas upgrading from vinasse digesters: a comparison between an anoxic biotrickling filter and an algal‐bacterial photobioreactor

AbstractBACKGROUNDBiological technologies for biogas upgrading such as anoxic biotrickling filters (BTF) and algal–bacterial photobioreactors (PBR) constitute a cost‐effective, environmentally friendly alternative to conventional physical–chemical methods. Nonetheless, there is a lack of studies comparing their performance when treating real biogas.RESULTSA H2S removal efficiency >98% was consistently recorded in both systems, with elimination capacities up to 26 and 12.4 g S‐H2S m−3 h−1 in the BTF and PBR, respectively, at empty bed residence times (EBRTs) of ∼ 30 min. Both bioreactors demonstrated a high robustness towards fluctuations in biogas composition and flowrate, maintaining nearly complete desulfurization despite the variations in sulfur load and EBRT. The BTF also showed an immediate recovery from a 15 days operational shut‐down, and the ability to utilize the nutrients from nitrate‐supplemented digestate during biogas desulfurization. In addition, the PBR supported an average CO2 removal of 23.0 ± 11.8%, increasing to 62% at higher pH of 8.1, with a carbon fixation rate of 285 mg CO2 L−1 d−1.CONCLUSIONSThe BTF was confirmed as a robust technology for the desulfurization of real biogas, even when supplemented with effluent from the digester. High H2S removal was also achieved in the PBR, the CO2 fixation capacity of microalgae enhancing biogas purification. © 2015 Society of Chemical Industry

Minimization of biomethane oxygen concentration during biogas upgrading in algal–bacterial photobioreactors

Novel operational strategies to reduce the O2 concentration in the upgraded biogas were evaluated in a 180L algal–bacterial photobioreactor interconnected to a 2.5L external absorption column during the simultaneous treatment of diluted anaerobically digested or raw vinasse and biogas upgrading. The lowest biomethane O2 levels (0.7±0.2%) were recorded when raw vinasse was fed directly into the absorption column, which resulted in CO2 and H2S removals from biogas of 72±1% and 100±0%, respectively. Process operation at a Hydraulic Retention Time (HRT) of 7d under the above configuration also supported the maximum total carbon, nitrogen and phosphorus removals of 72±4%, 74±3% and 78±5%, respectively. Biomass productivity ranged from 11.4±1.8 to 13.5±2.2gm−2d−1 during microalgae cultivation in diluted anaerobically digested vinasse, while this productivity increased to 16.9±0.7gm−2d−1 when feeding diluted raw vinasse. The good settling characteristics of the algal–bacterial flocs resulted in an average harvesting efficiency of 98.6±0.5% at a HRT in the settler of 23min, regardless of the treated vinasse. The morphological and molecular characterization of the microbial communities showed a high microalgae diversity and bacterial species richness, regardless of the operational conditions (Shannon–Wiener indices ranging from 2.8 to 3.3).

A review on the state-of-the-art of physical/chemical and biological technologies for biogas upgrading

The lack of tax incentives for biomethane use requires the optimization of both biogas production and upgrading in order to allow the full exploitation of this renewable energy source. The large number of biomethane contaminants present in biogas (CO2, H2S, H2O, N2, O2, methyl siloxanes, halocarbons) has resulted in complex sequences of upgrading processes based on conventional physical/chemical technologies capable of providing CH4 purities of 88–98 % and H2S, halocarbons and methyl siloxane removals >99 %. Unfortunately, the high consumption of energy and chemicals limits nowadays the environmental and economic sustainability of conventional biogas upgrading technologies. In this context, biotechnologies can offer a low cost and environmentally friendly alternative to physical/chemical biogas upgrading. Thus, biotechnologies such as H2-based chemoautrophic CO2 bioconversion to CH4, microalgae-based CO2 fixation, enzymatic CO2 dissolution, fermentative CO2 reduction and digestion with in situ CO2 desorption have consistently shown CO2 removals of 80–100 % and CH4 purities of 88–100 %, while allowing the conversion of CO2 into valuable bio-products and even a simultaneous H2S removal. Likewise, H2S removals >99 % are typically reported in aerobic and anoxic biotrickling filters, algal-bacterial photobioreactors and digesters under microaerophilic conditions. Even, methyl siloxanes and halocarbons are potentially subject to aerobic and anaerobic biodegradation. However, despite these promising results, most biotechnologies still require further optimization and scale-up in order to compete with their physical/chemical counterparts. This review critically presents and discusses the state of the art of biogas upgrading technologies with special emphasis on biotechnologies for CO2, H2S, siloxane and halocarbon removal.

Evaluation of wastewater treatment in a novel anoxic–aerobic algal–bacterial photobioreactor with biomass recycling through carbon and nitrogen mass balances

Algal–bacterial symbiosis, implemented in an innovative anoxic–aerobic photobioreactor configuration with biomass recycling, supported an efficient removal of total organic carbon (86–90%), inorganic carbon (57–98%) and total nitrogen (68–79%) during synthetic wastewater treatment at a hydraulic and sludge retention times of 2days and 20days, respectively. The availability of inorganic carbon in the photobioreactor, determined by its supply in the wastewater and microalgae activity, governed the extent of nitrogen removal by assimilation or nitrification–denitrification. Unexpectedly, nitrate production was negligible despite the high dissolved oxygen concentrations, denitrification being only based on nitrite reduction. Biomass recycling resulted in the enrichment of rapidly settling algal flocs, which supported effluent total suspended solid concentrations below the European Union maximum discharge limits. Finally, the maximum nitrous oxide emissions recorded were far below the emission factors reported for wastewater treatment plants, confirming the environmental sustainability of this innovative photobioreactor in terms of global warming impact.

Complete biological N removal from anaerobic digester sidestreams in algal-bacterial photo-bioreactors

Complete biological N removal from anaerobic digester sidestreams in algal-bacterial photo-bioreactors

Microalgal-Biotechnology As a Platform for an Integral Biogas Upgrading and Nutrient Removal from Anaerobic Effluents

The potential of a pilot high rate algal pond (HRAP) interconnected via liquid recirculation with an external absorption column for the simultaneous removal of H2S and CO2 from biogas using an alkaliphilic microalgal-bacterial consortium was evaluated. A bubble column was preferred as external absorption unit to a packed bed column based on its ease of operation, despite showing a comparable CO2 mass transfer capacity. When the combined HRAP-bubble column system was operated under continuous mode with mineral salt medium at a biogas residence time of 30 min in the absorption column, the system removed 100% of the H2S (up to 5000 ppmv) and 90% of the CO2 supplied, with O2 concentrations in the upgraded biogas below 0.2%. The use of diluted centrates as a free nutrient source resulted in a gradual decrease in CO2 removal to steady values of 40%, while H2S removal remained at 100%. The anaerobic digestion of the algal-bacterial biomass produced during biogas upgrading resulted in a CH4 yield of 0.21-0.27 L/gVS, which could satisfy up to 60% of the overall energy demand for biogas upgrading. This proof of concept study confirmed that algal-bacterial photobioreactors can support an integral upgrading without biogas contamination, with a net negative CO2 footprint, energy production, and a reduction of the eutrophication potential of the residual anaerobic effluents.

Carbon and nutrient removal from centrates and domestic wastewater using algal–bacterial biofilm bioreactors

The mechanisms of carbon and nutrient removal in an open algal-bacterial biofilm reactor and an open bacterial biofilm reactor were comparatively evaluated during the treatment of centrates and domestic wastewater. Comparable carbon removals (>80%) were recorded in both bioreactors, despite the algal-bacterial biofilm supported twice higher nutrient removals than the bacterial biofilm. The main carbon and nitrogen removal mechanisms in the algal-bacterial photobioreactor were assimilation into algal biomass and stripping, while stripping accounted for most carbon and nitrogen removal in the bacterial biofilm. Phosphorus was removed by assimilation into algal-bacterial biomass while no effective phosphorous removal was observed in the bacterial biofilm. Carbon, nitrogen and phosphorus removals of 91 ± 3%, 70 ± 8% and 85 ± 9%, respectively, were recorded in the algal-bacterial bioreactor at 10d of hydraulic retention time when treating domestic wastewater. However, the high water footprint recorded (0.5-6.7 Lm(-2)d(-1)) could eventually compromise the environmental sustainability of this microalgae-based technology.

Molecular characterization of bacterial communities in algal–bacterial photobioreactors treating piggery wastewaters

The influence of the loading rate and the seasonal environmental conditions on the bacterial communities established in High Rate Algal Ponds (HRAPs) treating diluted swine manure was investigated using 16S rRNA Denaturing Gradient Gel Electrophoresis (DGGE) and further phylogenetic analysis. Two HRAPs were successfully operated at different loading rates (10 and 20 times diluted swine manure, resulting in average Surface Organic Loading Rates (SOLR) ranging from 13 to 99gm−2d−1 and from 21 to 180gm−2d−1, respectively) for 9 months. The DGGE analysis revealed a high diversity of bacteria symbiotically living with microalgae. The environmental conditions rather than the organic loading rate were the most significant factors determining the bacterial community structure. Hence, despite a higher biodiversity was recorded in summer, both HRAPs presented a similar community regardless of their different organic loading. Most of the species (≈54%) belonged to Verrucomicrobium, an ubiquitous phylum of aerobic bacteria commonly found in eutrophic environments. Firmicutes and Proteobacteria (Gammaproteobacteria) were the other dominant phyla. Some of the bacteria present in the HRAPs were highly resistant to UV radiation and floc-forming species, which might explain the reasonable good settling characteristics of the algal–bacterial biomass in the ponds. The present work provides new insights on the complex interactions between microalgae and bacteria in algal-based wastewater treatment systems.