Improving the Quality of Reclaimed Water via Applying Spirulina platensis to Eliminate Residual Nitrate

Abstract

The application of reclaimed water has been recognized as the key approach for alleviating water scarcity, while its low quality, such as high nitrogen content, still makes people worry about the corresponding ecological risk. Herein, we investigated the feasibility of removing residual nitrate from reclaimed water by applying Spirulina platensis. It is found that 15 mg/L total nitrogen could be decreased to 1.8 mg/L in 5 days, equaling 88.1 % removal efficiency under the optimized conditions. The deficient phosphorus at 0.5-1.0 mg/L was rapidly eliminated but was already sufficient to support nitrate removal by S. platensis. The produced ammonia is generally below 0.2 mg/L, which is much lower than the standard limit of 5 mg/L. In such a nutrient deficiency condition, S. platensis could maintain biomass growth well via photosynthesis. The variation of pigments, including chlorophyll a and carotenoids, suggested a certain degree of influences of illumination intensity and phosphorus starvation on microalgae. The background cations Cu²⁺ and Zn²⁺ exhibited significant inhibition on biomass growth and nitrate removal; thus, more attention needs to be paid to the further application of microalgae in reclaimed water. Our results demonstrated that cultivation of S. platensis should be a very promising solution to improve the quality of reclaimed water by efficiently removing nitrate and producing biomass.

Keywords: Spirulina platensis; microalgae cultivation; nitrate removal; nutrient deficiency; reclaimed water.

Figure 1

The residual ratio (C/C₀) of total nitrogen in the culture solutions of S. platensis under different (a) initial inoculation amounts of 0.13–0.33 g/L (illumination intensity = 6000 lux, initial phosphorus level = 0.5 mg/L), (b) illumination intensities of 2000–6000 lux (initial inoculation amount = 0.33 g/L, initial phosphorus level = 0.5 mg/L), and (c) initial phosphorus levels of 0–1.0 mg/L (initial inoculation amount = 0.33 g/L, illumination intensity = 6000 lux) within 5 days cultivation. The statistically significant differences as compared to the optimized condition (initial inoculation amount = 0.33 g/L, illumination intensity = 6000 lux, and initial phosphorus level = 0.5 mg/L) were represented by * if p < 0.05 and ** if p < 0.01, respectively.

Figure 2

The (a) residual ratio (C/C₀) and (b) decreased concentration of total phosphorus (initial phosphorus levels are 0.5 mg/L and 1.0 mg/L, respectively) in the culture medium of S. platensis within 5 days cultivation under initial inoculation amount of 0.33 g/L and illumination intensity of 6000 lux. The statistically significant differences as compared to the optimized condition (initial inoculation amount = 0.33 g/L, illumination intensity = 6000 lux, and initial phosphorus level = 0.5 mg/L) were represented by * if p < 0.05 and ** if p < 0.01, respectively.

Figure 3

The contents of ammonia in the culture solutions of S. platensis within 5 days cultivation under the different conditions including (a) initial inoculation amount = 0.33 g/L, illumination intensity = 6000 lux, and initial phosphorus level = 0.5 mg/L; (b) initial inoculation amount = 0.13 g/L, illumination intensity = 6000 lux, and initial phosphorus level = 0.5 mg/L; (c) initial inoculation amount = 0.33 g/L, illumination intensity = 2000 lux, and initial phosphorus level = 0.5 mg/L; and (d) initial inoculation amount = 0.33 g/L, illumination intensity = 6000 lux, and initial phosphorus level = 0.0 mg/L. The statistically significant differences as compared to the optimized condition (condition a) were represented by * if p < 0.05 and ** if p < 0.01, respectively.

Figure 4

The biomass productivity of S. platensis under different (a) initial inoculation amounts of 0.13–0.33 g/L (illumination intensity = 6000 lux, initial phosphorus level = 0.5 mg/L), (b) illumination intensities of 2000–6000 lux (initial inoculation amount = 0.33 g/L, initial phosphorus level = 0.5 mg/L), and (c) initial phosphorus levels of 0–1.0 mg/L (initial inoculation amount = 0.33 g/L, illumination intensity = 6000 lux) within 5 days cultivation. The statistically significant differences as compared to the optimized condition (initial inoculation amount = 0.33 g/L, illumination intensity = 6000 lux, and initial phosphorus level = 0.5 mg/L) were represented by * if p < 0.05 and ** if p < 0.01, respectively.

Figure 5

The contents of chlorophyll a and carotenoids in S. platensis after 5 days cultivation under the different conditions including (a) initial inoculation amount = 0.33 g/L, illumination intensity = 6000 lux, and initial phosphorus level = 0.5 mg/L; (b) initial inoculation amount = 0.13 g/L, illumination intensity = 6000 lux, and initial phosphorus level = 0.5 mg/L; (c) initial inoculation amount = 0.33 g/L, illumination intensity = 2000 lux, and initial phosphorus level = 0.5 mg/L; and (d) initial inoculation amount = 0.33 g/L, illumination intensity = 6000 lux, and initial phosphorus level = 0.0 mg/L. The statistically significant differences as compared to the optimized condition (condition a) were represented by * if p < 0.05 and ** if p < 0.01, respectively.

Figure 6

Effects of 0.5 mg/L Cu²⁺, 1.0 mg/L Zn²⁺, 1.1 mg/L Br^-, and 0.2 mg/L I^- on the (a) biomass growth and (b) residual ratio (C/C₀) of total nitrogen for S. platensis within 5 days cultivation under the optimized condition. The statistically significant differences as compared to the control group were represented by * if p < 0.05 and ** if p < 0.01, respectively.