494 / 2018-09-09 10:37:41

Nutrient recovery and biomass production during the treatment of anaerobic digestate of dairy slurry by Chlorella vulgaris

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Conventional biological treatments such as activated sludge process cannot remove nutrients from wastewater efficiently, while microalgae have the advantages to quickly uptake nitrogen and phosphorus and recycle them into algal biomass. The present study was to investigate the treatment performance and biomass production when using Chlorella vulgaris to treat unsterilized anaerobic digestate from dairy farm with different dilution ratios (25%, 50%, 75%, 100%). The results showed that the efficiencies of nitrogen and phosphorus decreased with the increase of slurry concentration, which was correlated to microalgal growth. The excellent removals of ammonium, total nitrogen and total phosphorus were achieved in the 25% and 50% treatments, which were 99.9%, 91.0%, 100%, and 93.9%, 84.7%, 100%, respectively. Contrarily, COD removal increased with the increase of slurry concentration, which was attributed to indigenous bacterial metabolism. From the perspective of biomass production, 50% treatment achieved the highest biomass yield and biomass productivities, which were 3.1 g L-1 and 393.6 mg L-1 d-1 respectively. It was also found that the protein content of the harvested biomass increased with the increase of the slurry concentration, while the changes of carbohydrate content presented the opposite trend. The biomass from 25% treatment produced the highest lipid content due to the lack of nitrogen and phosphorus. As the slurry concentration increased, microbial community diversity also increased, corresponding to the elevated COD removal. In the wastewaters with lower dilution ratios, more Bacteroidetes, Candidatus Saccharibacteria, and Chlamydiae were developed with the reduced abundance of Proteobacteria. In summary, lower slurry loads were very beneficial for microalgal growth and nutrients removal, whereas microalgal massive proliferation decreased microbiota diversity and affected community structure.