Investigating why recycling gravity harvested algae increases harvestability and productivity in high rate algal ponds |
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Authors: | JBK Park RJ Craggs AN Shilton |
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Affiliation: | 1. National Institute of Water and Atmospheric Research Ltd. (NIWA), Aquatic Pollution Group, P.O. Box 11-115, Hamilton, New Zealand;2. School of Engineering and Advanced Technology, Massey University, Private Bag 11 222, Palmerston North, New Zealand |
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Abstract: | It has previously been shown that recycling gravity harvested algae promotes Pediastrum boryanum dominance and improves harvestability and biomass production in pilot-scale High Rate Algal Ponds (HRAPs) treating domestic wastewater. In order to confirm the reproducibility of these findings and investigate the mechanisms responsible, this study utilized twelve 20 L outdoor HRAP mesocosms operated with and without algal recycling. It then compared the recycling of separated solid and liquid components of the harvested biomass against un-separated biomass. The work confirmed that algal recycling promoted P. boryanum dominance, improved 1 h-settleability by >20% and increased biomass productivity by >25% compared with controls that had no recycling. With regard to the improved harvestability, of particular interest was that recycling the liquid fraction alone caused a similar improvement in settleability as recycling the solid fraction. This may be due to the presence of extracellular polymeric substances in the liquid fraction. While there are many possible mechanisms that could account for the increased productivity with algal recycling, all but two were systematically eliminated: (i) the mean cell residence time was extended thereby increasing the algal concentration and more fully utilizing the incident sunlight and, (ii) the relative proportions of algal growth stages (which have different specific growth rates) was changed, resulting in a net increase in the overall growth rate of the culture. |
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Keywords: | Algae Biofuels High Rate Algal Ponds Wastewater treatment Biomass production Algal species control Algal recycling |
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