Effects of low temperature on Chlorella cultivation in high-concentration human urine wastewater

Abstract: Microalgae (microscopic algae) have been the most promising renewable energy sources in freshwater and marine systems, due to the short growth cycles, high photosynthetic efficiency, environmental adaptability, and cellular lipid content. The cultivation cost of commercial microalgae is ever increasing in recent years. Particularly, chemical reagents are generally added to supply the nutrient sources for microalgae growth. Fortunately, human urine wastewater can be expected for microalgae cultivation, due to the abundant nitrogen, phosphorus, and trace elements for microalgae growth. Microalgae biomass can also be obtained to reduce the microalgae cultivation cost after purification, in order to realize the resource-based and high-value utilization of human urine wastewater. In the case of the microalgae growth temperature above 25.0 ℃, it is very important to explore the microalgae suitable for the low-temperature culture, in order to reduce the heating energy consumption of microalgae culture in winter in the north of China. In addition, human urine is also currently sterilized for microalgae cultivation, due to the high dilution times. More wastewater can be produced from a small amount of original human urine, leading to the high pretreatment cost of microalgae cultivation. In this study, a systematic investigation was carried out to clarify the effects of different temperatures (15.0, 17.5, 20.0, 22.5, and 25.0 ℃) on microalgae cultivation using human urine diluted by low times. The Chlorella pyrenoidosa and Chlorella sp were selected as the microalgae species. An artificial climate incubator was used under the human urine addition ratio of 40% and light intensity of 4000 lx for 14 days. The results showed that the two types of Chlorella grew better in the high concentration of human urine wastewater. The average removal ratios of the total nitrogen, ammonium nitrogen, total phosphorus, and chemical oxygen demand in the culture solution of Chlorella pyrenoidosa reached 78.02%, 79.59%, 79.31%, and 20.11%, respectively, where the maximum biomass dry weight reached 0.502 g/L. Comparatively, the average removal ratios of the total nitrogen, ammonium nitrogen, total phosphorus, and chemical oxygen demand in the culture solution of Chlorella sp. reached 87.90%, 89.55%, 89.29%, and 68.66%, respectively, where the maximum biomass dry weight reached 1.007g/L. There was an increase in biomass production and the total nitrogen, ammonium nitrogen, total phosphorus, and chemical oxygen demand removals of Chlorella pyrenoidosa and Chlorella sp., as the increase of temperature (P<0.05). The increasing rates were smaller under the temperature of higher than 20.0 ℃ than those lower than 20.0 ℃. Meanwhile, there was a strong influence of low temperature on the microalgae biomass, Chlorella protein, and chemical oxygen demand removal ratio than that on the contents of total nitrogen, ammonium nitrogen, and total phosphorus. Better adaptability of Chlorella sp. was also achieved in the human urine at low temperature than that of Chlorella pyrenoidosa, indicating more suitable for cultivation in the north of China. This finding can lay a theoretical foundation for the low-consumption and high-efficiency culture of microalgae in cold regions.