Björn Podola

Application of a prototype-scale Twin-Layer photobioreactor for effective N and P removal from different process stages of municipal wastewater by immobilized microalgae

In the view of limited phosphorous resources and tightened discharge regulations, the recovery of phosphate and nitrate from wastewater is of great interest. Here, the integration of microalgae into wastewater treatment processes is a promising approach. A prototype-scale Twin-Layer photobioreactor immobilizing the green alga Halochlorella rubescens on vertical sheet-like surfaces was constructed and operated using primary and secondary municipal wastewater. The process was not impaired by suspended solids, bacteria or loss of algal biomass by leaching. The average areal microalgal growth was 6.3 gm(-2) d(-1). After treatment, P and N concentrations in the effluents could efficiently be reduced by 70-99%, depending on element and type of wastewater. Mean effluent values of ⩽ 1.0mg L(-1)P and 1.3 mg L(-1)N met the legal discharge limits of the European Water Framework Directive and show a potential to comply with upcoming, more stringent legislation.

Selective real-time herbicide monitoring by an array chip biosensor employing diverse microalgae

A multiple-strain algal biosensor was constructed for the detection of herbicides inhibiting photosynthesis. Nine different microalgal strains were immobilised on an array biochip using permeable membranes. The biosensor allowed on-line measurements of aqueous solutions passing through a flow cell using chlorophyll fluorescence as the biosensor response signal. The herbicides atrazine, simazine, diuron, isoproturon and paraquat were detectable within minutes at minimal LOEC (Lowest Observed Effect Concentration) ranging from 0.5 to 100μgL−1, depending on the herbicide and algal strain. The most sensitive strains in terms of EC50 values were Tetraselmis cordiformis and Scherffelia dubia. Less sensitive species were Chlorella vulgaris, Chlamydomonas sp. and Pseudokirchneriella subcapitata, but for most of the strains no general sensitivity or resistance was found. The different responses of algal strains to the five herbicides constituted a complex response pattern (RP), which was analysed for herbicide specificity within the linear dose-response relationship. Comparisons of herbicide-specific RP to reference RPs of the five herbicides always showed the lowest deviation of the herbicide-specific RP tested with the reference RP of the same herbicide for the triazine and phenylurea herbicides. We therefore conclude that, in principle, identification of a specific herbicide is possible employing the algal sensor chip.

Porous Substrate Bioreactors: A Paradigm Shift in Microalgal Biotechnology?

Many of the demands in the production of microalgae at a technical scale cannot presently be met by state-of-the-art cultivation technologies based on suspensions. Immobilized cultivation using porous substrate bioreactors (PSBRs) is characterized by a reduction of liquid reaction volumes by several orders of magnitude and has solved several volume-related problems. Recently, PSBRs demonstrated potential for both established and novel applications in microalgal biotechnology, and first insights into biophysical processes have provided an understanding of the benefits of PSBR biofilm cultivation. Further efforts should primarily focus on scale-up and engineering challenges in this emerging field and, additionally, provide experience in the long-term operation of bioreactors. The results may contribute to assessing the technical and economic potential of PSBR cultivation.

A long-term operating algal biosensor for the rapid detection of volatile toxic compounds

A method for the stable long-termimmobilization of microalgal cultures wasdeveloped. Immobilized Klebsormidiumcultures were used in a biosensor systemfor air monitoring. The measurement ofbiosensor response was performed usingseveral parameters obtained from the PAMchlorophyll fluorescence technique. To testbiosensor response on toxic compoundsmethanol and formaldehyde, classified asvolatile organic compounds (VOC), were usedin concentrations relevant to human health.Our results showed that quantitativedetection of methanol vapour by thebiosensor is possible within minutes atconcentrations from 75 to 350 ppm.Additionally, due to reversibility of thebiosensor response signal and long-termstability, the biosensor was operationalfor 30 days with repeated exposure periodsto methanol vapour. We conclude that thealgal biosensor, in principle, is suitableto detect volatile toxic compounds such asmethanol and formaldehyde.

Microscale profiling of photosynthesis‐related variables in a highly productive biofilm photobioreactor

In the present study depth profiles of light, oxygen, pH and photosynthetic performance in an artificial biofilm of the green alga Halochlorella rubescens in a porous substrate photobioreactor (PSBR) were recorded with microsensors. Biofilms were exposed to different light intensities (50–1,000 μmol photons m−2 s−1) and CO2 levels (0.04–5% v/v in air). The distribution of photosynthetically active radiation showed almost identical trends for different surface irradiances, namely: a relatively fast drop to a depth of about 250 µm, (to 5% of the incident), followed by a slower decrease. Light penetrated into the biofilm deeper than the Lambert‐Beer Law predicted, which may be attributed to forward scattering of light, thus improving the overall light availability. Oxygen concentration profiles showed maxima at a depth between 50 and 150 μm, depending on the incident light intensity. A very fast gas exchange was observed at the biofilm surface. The highest oxygen concentration of 3.2 mM was measured with 1,000 μmol photons m−2 s−1 and 5% supplementary CO2. Photosynthetic productivity increased with light intensity and/or CO2 concentration and was always highest at the biofilm surface; the stimulating effect of elevated CO2 concentration in the gas phase on photosynthesis was enhanced by higher light intensities. The dissolved inorganic carbon concentration profiles suggest that the availability of the dissolved free CO2 has the strongest impact on photosynthetic productivity. The results suggest that dark respiration could explain previously observed decrease in growth rate over cultivation time in this type of PSBR. Our results represent a basis for understanding the complex dynamics of environmental variables and metabolic processes in artificial phototrophic biofilms exposed to a gas phase and can be used to improve the design and operational parameters of PSBRs. Biotechnol. Bioeng. 2016;113: 1046–1055.

Continuous removal of zinc from wastewater and mine dump leachate by a microalgal biofilm PSBR

Bio-removal of heavy metals from wastewater by microalgae has been investigated for decades. However, technical and economical limitations of cultivation systems for microalgae still impair progress toward application. Recently, a novel type of bioreactor for (immobilized) biofilm cultivation, the Porous Substrate Bioreactor (PSBR), has been shown to optimize biomass feedstock production and harvest, offering novel possibilities for application in the treatment of wastewater. We used two types of laboratory-scale Twin-Layer PSBRs to remove zinc (2-3 mg Zn L(-1)) from synthetic wastewater and real mine dump leachate in a continuous and batch process. The selection and use of a biofilm of a Zn-resistant strain of the green alga Stichococcus bacillaris (EC50 of 28.9 mg Zn L(-1) based on Pulse-amplitude modulated (PAM) chlorophyll fluorescence analysis) led to a high zinc absorption capacity of 15-19 mg Z ng(-1) algal dry matter. The removal capacity for zinc correlated positively with biomass production and was thus, light dependent. Bio-removal properties observed here combined with biomass productivities of PSBR systems compare favorably with other algal-based bio-sorption technologies.

Erratum to: Growing microalgae as aquaculture feeds on twin-layers: a novel solid-state photobioreactor

In the sentence “With a surface productivity of 2 g m day and a flow rate of 6 L per meter TL module, approximately 1 kWh is required for pumping to produce 1 kg of algal dry mass” the flow rate should include the time (in hours). The corrected sentence is shown below. “With a surface productivity of 2 g m day and a flow rate of 6 L h per meter TL module, approximately 1 kWh is required for pumping to produce 1 kg of algal dry mass”.