The mixing of microalgae bioreactors is important as it circulates cells in and out of the light zone and reduces boundary layers, benefiting photosynthesis and nutrient uptake. However, mixing is also a major economical factor, impacting on power consumption and operating costs, and it remains a barrier for those locations where no electrical supply is available. Combining solar panels with algae ponds can help to alleviate the power cost burden, but if solar panels are the sole source of generated electricity for a regional raceway pond system, the paddle wheels will only operate during day time. This study investigated the growth of halophilic green alga Tetraselmis suecica, in outdoor paddle wheel-driven open ponds under three different mixing regimes. The first experimental condition was a continuously mixed regime, while in the second and third conditions, the paddle wheels were stopped overnight and restarted 1 h after sunrise or 1 h before sunrise, respectively. Growth rate, biomass productivity and cell weight showed no statistically significant difference during the period when the culture was growing under optimal environmental conditions. Chlorophyll fluorescence measurements of photosystem II maximum quantum yield in the light (F v ′/F m ′) over a 24-h period found no significant differences between cultures, further suggesting that under favourable conditions ceasing mixing overnight may have no undesirable impact on Tetraselmis health or productivity. These findings imply a potential 37 % power saving and a 33 % reduction in energy-associated costs. During a heat stress event, Tetraselmis was negatively impacted in all ponds to the same degree; however, the continuously mixed pond recovered faster than ponds that were not mixed overnight.
Industrial-scale microalgae production will likely require large energyintensive technologies for both culture and biomass recovery; energy-efficient and cost-effective microalgae dewatering and water management are major challenges. Primary dewatering is typically achieved through flocculation followed by separation via settling or flotation. Flocculants are relatively expensive, and their presence can limit the reuse of de-oiled flocculated microalgae. Natural flocculation of microalgae-autoflocculation-occurs in response to changes in pH and water hardness and, if controlled, might lead to less-expensive "flocculant-free" dewatering. A better understanding of autoflocculation should also prompt higher yields by preventing unwanted autoflocculation. Autoflocculation is driven by doublelayer coordination between microalgae, Ca +2 and Mg +2 , and/or mineral surface precipitates of calcite, Mg(OH) 2 , and hydroxyapatite that form primarily at pH > 8. Combining surface complexation models that describe the interface of microalgae: water, calcite:water, Mg(OH) 2 :water, and hydroxyapatite:water allows optimal autoflocculation conditions-for example pH, Mg, Ca, and P levels-to be identified for a given culture medium.
El cultivo de microalgas en sinergia con el tratamiento de efluentes tiene mucho potencial para ser un enfoque óptimo en la gestión del agua dulce. La gestión de efluentes de una manera en la que se maximice la recuperación de nutrientes, como nitrógeno y fósforo, al mismo tiempo que se produzca biomasa microalgal, puede incluso ser una alternativa de negocio agro-industrial, especialmente cuando no hay prácticas de gestión de efluentes implementadas (Fenton y Ó hUallacháin, 2012). Los consorcios diseñados de microalga-bacteria tienen un alto potencial de complementariedad para la remoción tanto de nitrógeno y fósforo como de compuestos orgánicos de carbono. En este estudio se cultivaron exitosamente las microalgas Scenedesmus dimorphus y Chlorella pyrenoidosa en consorcio con la bacteria promotora del crecimiento de plantas Azospirillum brasilense en mezclas de dos efluentes de la industria láctea, el purín vacuno y el suero ácido de queso, sin esterilizar ni diluir en diferentes condiciones de laboratorio hasta la escala de estanques abiertos al exterior.
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