Elsevier

Bioresource Technology

Volume 111, May 2012, Pages 343-352
Bioresource Technology

Harvesting microalgal biomass using submerged microfiltration membranes

https://doi.org/10.1016/j.biortech.2012.02.009Get rights and content

Abstract

This study was performed to investigate the applicability of submerged microfiltration as a first step of up-concentration for harvesting both a freshwater green algae species Chlorella vulgaris and a marine diatom Phaeodactylum tricornutum using three lab-made membranes with different porosity. The filtration performance was assessed by conducting the improved flux step method (IFM) and batch up-concentration filtrations. The fouling autopsy of the membranes was performed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and Fourier transform infrared spectroscopy (FTIR). The cost analysis was estimated based on the data of a related full-scale submerged membrane bioreactor (MBR). Overall results suggest that submerged microfiltration for algal harvesting is economically feasible. The IFM results indicate a low degree of fouling, comparable to the one obtained for a submerged MBR. By combining the submerged microfiltration with centrifugation to reach a final concentration of 22% w/v, the energy consumption to dewater C. vulgaris and P. tricornutum is 0.84 kW h/m3 and 0.91 kW h/m3, respectively.

Highlights

► A submerged microfiltration membrane was applied for harvesting Chlorella vulgaris and Phaeodactylum tricornutum broth. ► The effect of algal species, up-concentration level and membrane fouling were studied and a cost estimation was performed. ► Both algal broths have a comparable filterability. ► The filterability of algal broths was function of up-concentration and membrane properties. ► The cost estimation shows the promising prospect of proposed filtration–centrifugation process.

Introduction

Recently, microalgal biomass has been recognized as a promising alternative source of raw material for biofuel production, but a lack of an economical and efficient method to harvest algal biomass is a major drawback to boost their full-scale application (Greenwell et al., 2010). Before entering downstream processing, algal broth normally requires harvesting, up-concentration and drying. At present, microalgae are only produced on a limited scale for high value products, such as food supplements, natural pigments and polyunsaturated fatty acids (Raja et al., 2008). Because of their low concentration in the culture medium (0.5–2 g/l) and small size, typically a few micrometers, harvesting microalgal biomass is a major challenge. Most existing microalgal production systems use energy intensive centrifugation, which represent a major fraction of the total energy demand of the production process (Grima et al., 2003). Hence, the net energy output in the case of biofuel production is seriously decreased.

Membrane technology is generally cheaper than applying centrifuges and is known to be not energy intensive. It thus forms a very promising technology for algal harvesting and additionally offers the advantages of almost complete retention of biomass (Mouchet and Bonnelye, 1998) as well as potential disinfection via removal of protozoa and viruses (Judd, 2006). Furthermore, no chemicals such as coagulants or flocculants are required, thus preventing their accumulation in the biomass or the recycled streams that exist in a coagulation–flocculation process (Vandamme et al., 2011).

Most literature on microalgal harvesting confirms the effectiveness of micro- and ultra-filtration in cross-flow configuration (Rossignol et al., 1991, Zhang et al., 2010). This configuration offers a high productivity due to the high cross-flow velocity and shear rates exposed onto the membrane surface. However, it consumes considerable energy due to high applied pressures and liquid velocities (Le-Clech et al., 2006). Furthermore, over-exposure of microalgal biomass to shear, especially in intake and pumping systems may break microalgal cells to form smaller particles, colloids and dissolved organic matters or promote release of exopolymeric substances (EPS). These small particles are known to cause severe membrane fouling by enhancing pore blocking and producing a less porous cake layer on the membrane surface (Babel and Takizawa, 2010; Ladner et al., 2010). The cell breakage may also lead to the loss of targeted products from the cell interior. Therefore, application of submerged microfiltration that applies lower pressures in absence of any cross flow velocity is expected to be more efficient (Babel and Takizawa, 2010). This system is commonly applied in submerged membrane bioreactors (MBRs) for wastewater treatment, as it is cheaper due to the absence of pressure resistant membrane housings, and proven to offer lower energy consumption (Judd, 2006, Le-Clech et al., 2006). In such immersed system, the shear is generally provided by coarse air bubbles, thus the limited exposure of microalgal cell to enhanced shear rates is also expected to reduce EPS release and thus to better sustain the filtration operation.

In this study, a submerged microfiltration was applied to harvest both a freshwater microalgal species Chlorella vulgaris and a marine diatom Phaeodactylum tricornutum. Both Chlorella and Phaeodactylum are promising species for the production of microalgal biomass for food, feed, or fuel, and are currently intensively studied (Greenwell et al., 2010, Raja et al., 2008). The effect of membrane properties was observed by testing three lab-made polyvinylidene fluoride (PVDF) membranes. The filtration performances were evaluated using the improved flux-step method (IFM) (van der Marel et al., 2009) and batch up-concentration filtrations. The IFM results were used to compare the fouling propensity of the tested membranes for microalgal species, and to provide information on ranges of applicable fluxes in a full-scale system. The membrane fouling was evaluated by observing scanning electron microscopy (SEM) images of fresh, fouled and cleaned membranes and by applying both Fourier transform infrared spectroscopy (FTIR) and energy-dispersive X-ray spectroscopy (EDX) to identify the organic and inorganic fouling, respectively. The energy consumption for a full-scale application of submerged microfiltration process in algae harvesting was also tentatively estimated by adapting the data taken from a related full-scale submerged MBR for wastewater treatment. These values were used as a basis of comparison with other microalgae up-concentration processes.

Section snippets

Cultivation and characterization of microalgae

C. vulgaris (SAG, Germany, 211-11B) was cultured in Wright’s cryptophytes (WC) medium prepared from pure chemicals dissolved in disinfected tap water (Guillard and Lorenzen, 1972). P. tricornutum (UGent, Belgium, Pt 86) was cultured in WC medium prepared in deionized water to which 30 g/l synthetic sea salt (Homarsel, Zoutman, Belgium) was added. Both species were grown in two separated plexiglas bubble column photobioreactors, with a working volume of 30 l and diameter of 20 cm. Degassing was

Membrane characterization

The SEM images of the fresh/new membrane surface and the properties of the membranes used in this study are shown in Fig. S1 of supplementary materials and Table 1 respectively. As expected, all membranes have an asymmetric structure as can be seen from the SEM images. The membrane pore size and/or surface porosity, as analyzed by imageJ, decreases with increasing polymer concentration in the casting solutions (van der Marel et al., 2010). A higher polymer concentration leads to an increased

Conclusions

This study reveals the potential of submerged microfiltration as a low-cost microalgae harvesting process. The IFM results suggest lower degrees of fouling compared to conventional submerged MBRs within the range of operational parameters. The energy estimation gave a promising prospect for this technology to be applied at larger scales as a low-cost and energy efficient technology. Furthermore, it widens the possibility to apply microalgae technology both for high and low value products.

Acknowledgements

K.U.Leuven for support in the frame of the CECAT excellence, GOA, FWO (G.0808.10 N) and IDO financing, and the Flemish Government for the Methusalem funding and the Federal Government for an IAP grant.

Institute for the promotion of Innovation by Science and Technology-Strategic Basic Research (IWT-SBO) project Sunlight and the K.U.Leuven Research Coordination Office-Industrial Research Fund (DOC-IOF) project Algae-Tech

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