Altan Ozkan

Reduction of water and energy requirement of algae cultivation using an algae biofilm photobioreactor.

This paper reports the construction and performance of an algae biofilm photobioreactor that offers a significant reduction of the energy and water requirements of cultivation. The green alga Botryococcus braunii was cultivated as a biofilm. The system achieved a direct biomass harvest concentration of 96.4 kg/m(3) with a total lipid content 26.8% by dry weight and a productivity of 0.71 g/m(2) day, representing a light to biomass energy conversion efficiency of 2.02%. Moreover, it reduced the volume of water required to cultivate a kilogram of algal biomass by 45% and reduced the dewatering energy requirement by 99.7% compared to open ponds. Finally, the net energy ratio of the cultivation was 6.00 including dewatering. The current issues of this novel photobioreactor are also identified to further improve the system productivity and scaleup.

Rheological properties of algae slurries for minimizing harvesting energy requirements in biofuel production.

Rheological properties of microalgae slurries were measured as a function of biomass concentration from 0.5 to 80 kg/m(3) for Nannochloris sp., Chlorella vulgaris, and Phaeodactylum tricornutum. At biomass concentrations smaller than 20 kg/m(3), all slurries displayed a Newtonian fluid behavior with less than 30% increase in the effective viscosity from that of the nutrient medium. However, at biomass concentrations larger than 60 kg/m(3), the slurries of the green algae, Nannochloris sp. and C. vulgaris, displayed a shear thinning non-Newtonian behavior with varying degrees of sensitivity to shear rate while that of the diatom, P. tricornutum, was still a Newtonian fluid up to 80 kg/m(3). Moreover, bioenergy pumping effectiveness showed significant deviation among different species in the non-Newtonian regime. Finally, dewatering the slurries to concentration factors larger than 80 did not further increase the total bioenergy harvest effectiveness.

Cell to substratum and cell to cell interactions of microalgae.

This paper reports the cell to substratum and cell to cell interactions of a diverse group of microalgae based on the Extended Derjaguin, Landau, Verwey, Overbeek (XDLVO) approach using the previously reported physico-chemical surface properties. The microalgae included 10 different species of green algae and diatoms from both freshwater and saltwater environments while the substrata included glass, indium-tin oxide (ITO), stainless steel, polycarbonate, polyethylene, and polystryrene. The results indicated that acid-base interactions were the dominating mechanism of interaction for microalgae. For green algae, if at least one of the interacting surfaces was hydrophobic, adhesion at primary minimum was predicted without any energy barrier. However, most diatom systems featured energy barriers for adhesion due to repulsive van der Waals interactions. The results reported in this study are expected to provide useful data and insight into the interaction mechanisms of microalgae cells with each other and with substrata for a number of practical applications including prevention of biofouling of photobioreactors and other man-made surfaces, promotion of biofilm formation in algal biofilm photobioreactors, and developing bioflocculation strategies for energy efficient harvesting of algal biomass. Particularly, Botryococcus braunii and Cerithiopsis fusiformis were identified as promising species for biofloccuation and biofilm formation in freshwater and saltwater aquatic systems, respectively. Finally, based on the observed trends in this study, use of hydrophilic algae and hydrophilic coatings over surfaces are recommended for minimizing biofouling in aquatic systems.

Physico-chemical surface properties of microalgae

This study reports a comprehensive set of experimentally measured physico-chemical surface properties of 12 different microalgae including fresh and seawater species of green algae, diatoms and cyanobacteria. The surface free energy and its components including the acid-base (AB), van der Waals (LW), electron donor/acceptor parameters were quantified based on contact angle measurements along with the Lifshitz-van der Waals acid-base approach using the probe liquid surface tension parameters proposed by van Oss et al. as well as by Della Volpe and Siboni. Moreover, the zeta and surface potentials of all species were determined using electrophoretic mobility measurements along with using Smoluchowskis model. Finally, the free energy of cohesion of the microalgae was also determined based on the calculated surface energy properties. The results showed that the electron donor parameter correlated well with the free energy of cohesion in all groups of microalgae. Moreover, species known to form colonies and exhibit benthic cultures had distinctly hydrophobic surfaces compared to microalgae prefering planktonic growth. These results indicate the importance of surface hydrophobicity for causing biofouiling or flocculation of cultures. Finally, the zeta potentials did not show a distinctive trend with the types of microalgae but the surface potentials were markedly larger for the salt water species. The reported methods and data are expected to provide critical information for researchers and technology developers concerned with cell to cell and cell to substrata interactions of microalgae in algal biomass cultivation and harvesting, biofouling of membranes and surfaces, as well as cell-surface interactions in photosynthetic microbial fuel cell technologies.

Adhesion of algal cells to surfaces

This paper reports the cell–substratum interactions of planktonic (Chlorella vulgaris) and benthic (Botryococcus sudeticus) freshwater green algae with hydrophilic (glass) and hydrophobic (indium tin oxide) substrata to determine the critical parameters controlling the adhesion of algal cells to surfaces. The surface properties of the algae and substrata were quantified by measuring contact angle, electrophoretic mobility, and streaming potential. Using these data, the cell–substratum interactions were modeled using thermodynamic, DLVO, and XDLVO approaches. Finally, the rate of attachment and the strength of adhesion of the algal cells were quantified using a parallel-plate flow chamber. The results indicated that (1) acid–base interactions played a critical role in the adhesion of algae, (2) the hydrophobic alga attached at a higher density and with a higher strength of adhesion on both substrata, and (3) the XDLVO model was the most accurate in predicting the density of cells and their strength of adhesion. These results can be used to select substrata to promote/inhibit the adhesion of algal cells to surfaces.

Adhesion of Chlorella vulgaris on Hydrophilic and Hydrophobic Surfaces

This experimental study reports the adhesion rate and adhesion density of Chlorella vulgaris on hydrophilic glass, and hydrophobic indium tin oxide (ITO) surfaces at constant shear rate. Cultivation of algae as biofilms offers an energy and water efficient method for algal biofuel production. In order to design algal biofilm cultivation systems, algal adhesion and biofilm formation on substrates with different surface properties must be known. To assess this, a parallel plate flow chamber was used to quantify the adhesion rate of the commonly used algae Chlorella vulgaris to the surfaces under controlled shear rates. The contact angle and zeta potential measurements were made both for the algal cells and the adhesion surfaces to model adhesion. The experimental results were compared with the predictions of the Derjaguin, Landau, Verwey, Overbeek (DLVO), extended DLVO (XDLVO) theories, and the thermodynamic model. The experiments showed that the rate of adhesion over the hydrophobic surface was 81 cells mm−2 min−1 which was 3 times larger than that of the hydrophilic surface for the first forty minutes of the adhesion experiments. Moreover, the final adhesion density over the hydrophobic surface was 6182 mm−2 after an experimental duration of 320 minutes which was 2.7 times that of the hydrophilic surface. Detachment studies done with increased shear rates showed that the adhesion strength of algae was also higher over the hydrophobic surface. The experimental results fit best with the results from the XDLVO theory. However, the model was inaccurate in predicting high detachment rate from the hydrophilic surface with increased shear rates. Results show the importance of surface material selection for the initial adhesion of cells. These results can be used for selection and design of surface materials for optimizing initial adhesion of algae cells in algal biofilm photobioreactors. Furthermore, the results can also be used for the design of planktonic photobioreactors to avoid biofouling.Copyright

Rheological Study of Algae Slurries for Minimizing Pumping Power

This paper reports the rheological properties of algae slurries as a function of cell concentration. From both energy and economic perspectives, the algae slurry for producing biofuels should have rheological attributes that minimizes the pumping power requirements while delivering the maximum amount of biomass from the cultivation fields to the biorefinery. To achieve this, an accurate knowledge of the rheological properties of algae slurries as a function of cell concentration is necessary. This study measures the rheological properties of eight different concentrations of Nannochloris sp. in ASP-m nutrient media ranging from 0.5 to 80 kg dry biomass/m3 . Strain controlled dynamic frequency sweep tests, transient step rate tests, and steady rate sweep tests were performed with an ARES-TA Rheometer using a double wall couette cup and bob attachment. Shear rates ranged from 5–270 s−1 . The results show that the concentrations of 10 kg/m3 and below behaved as Newtonian fluids with a dynamic viscosity of 1.1×10−3 Pa-s while the concentrations of 20 kg/m3 and above behaved as shear thinning non-Newtonian fluids. Finally, an energy analysis was performed where a non-dimensional bioenergy transport efficiency was defined as the ratio of the energy content of transported algae biomass to the required pumping power. The results show that an optimal biomass concentration minimizing pumping requirements occurs at the highest dry biomass concentration.Copyright

Novel Algae Biofilm Photobioreactor for Reduced Energy and Water Usage

This paper reports the design and performance of a novel photobioreactor that decreases the water requirements of algae cultivation and energy requirements of harvesting and downstream processing for biofuel production compared to conventional technologies. The photobioreactor cultivates algae as a biofilm, immobilized on carbonated concrete surface. In this study the well known lipid producer Botryococcus braunii was used. The nutrient solution was flown over the surface to enhance the mass transfer of nutrients in and metabolites out of the algae biofilm. The prototype featured a footprint area of 0.275 m2 and has been operated for 35 days. The algae concentration in the photobioreactor reached 30.73 kg/m3 with a maximum total lipid content of 12.3% by dry weight. The water requirement for cultivation was reduced up to by about 41.58 times and energy required for nutrient delivery was estimated to be reduced by about 230 times with respect to raceway ponds.© 2010 ASME

Steady and Dynamic Rheological Properties of Dense Slurries of Chlorella vulgaris

This experimental study reports the steady and dynamic rheological properties of dense slurries of the green algae Chlorella vulgaris. Biofuel production from algae growth is a promising technology that has the potential to serve as a significant component of the world’s revised energy mix. Along with providing a renewable fuel source, algae production acts as a CO2 sink, potentially reducing net CO2 emissions. Design and operation of algae biofuel production facilities require accurate knowledge of the flow characteristics of algae slurries and estimation of the pumping and harvesting energy requirements. Reliable rheological data is needed to optimize production processes to lower costs and increase yields. This study reports steady state viscosity measurements conducted using the ARES TA rotational rheometer using the common algae strain Chlorella vulgaris over the packing factor range from 0.1 to 0.8. Viscoelastic data was gathered using oscillatory tests conducted on the rotational rheometer with a double wall coquette fixture geometry. Dynamic frequency sweep tests were used to recover the storage shear modulus (G′ ), and the loss shear modulus (G″ ), which correspond to the elastic and viscous properties of the fluid, respectively. Apparent viscosity of the cell suspensions increased with increasing packing factors. Packing factors lower than 0.3 exhibited Newtonian characteristics, whereas at larger packing factors the behavior was shear-thinning. The algae suspensions exhibited both viscous and elastic behavior when subjected to oscillatory flow, behaving as a dilute solution. Finally, the frequency of the gel point increased with increasing packing factor.Copyright