Eutrophication, due to nutrient discharge from wastewater into the environment, results in damages to the ecological systems. Nutrient recovery is required to reduce eutrophication and to utilize wastewater as resources in sustainability.
Microalgae, microscopic photosynthetic organisms, efficiently take up nutrients and CO2 into their biomass via their metabolism. By cultivating microalgae in wastewater with a supply of flue gas, the culture recovers nutrients from wastewater and uses the CO2 from flue gas. Microalgal bioremediation is more efficient than plant methods, as microalgae grow faster than plants higher up in the food web, and are highly adaptable to various types of wastewater. Microalgae is also preferred to bacterial methods as it produces valuable algal biomass as by-product. Algal biomass, rich in lipids, carbohydrates and proteins, serves as biofuel feedstock or animal feed.
Microalgae are highly versatile in their metabolism. Some are “photoautotrophic,” meaning that, like plants, they use light as energy and CO2 as a carbon source in photosynthesis. “Heterotrophic” microalgae, like many bacteria, use dissolved organic substance as the energy and carbon source for their metabolism. Finally, “mixotrophic” microalgae are capable of both photoautotrophic and heterotrophic types of metabolism.
During autumn and winter in Northern countries, light intensity is low and daytime is short, resulting in lower photoautotrophic production than in spring and summer. Mixotrophic metabolism, which supplies more energy for microalgae by organic carbon addition, can compensate for the photoautotrophic growth reduction in autumns and winters.
In this case study, we investigate the nutrient recovery by mixotrophic algae cultivation in landfill leachate (nitrogen fertilizer) mixed with dairy wastewater (phosphorus and organic carbon sources) in high rate algal ponds in winters and autumns. The algal ponds (see the photo) are 1 cubic meter and located in a green house in the backyard of KalmarEnergi in Moskogen where flue gas and leachate are readily available. This mixotrophic mode showed higher nutrient removal rate than photoautotrophic culture in similar cultivation conditions.
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