Nitrate and phosphate removal by Spirulina platensis

Abstract

The cyanobacterium Spirulina platensis was used to verify the possibility of employing microalgal biomass to reduce the contents of nitrate and phosphate in wastewaters. Batch tests were carried out in 0.5 dm³ Erlenmeyer flasks under conditions of light limitation (40 μmol quanta m⁻² s⁻¹) at a starting biomass level of 0.50 g/dm³ and varying temperature in the range 23–40°C. In this way, the best temperature for the growth of this microalga (30°C) was determined and the related thermodynamic parameters were estimated. All removed nitrate was used for biomass growth (biotic removal), whereas phosphate appeared to be removed mainly by chemical precipitation (abiotic removal). The best results in terms of specific and volumetric growth rates (μ =0.044 day⁻¹, Q _x =33.2 mg dm⁻³ day⁻¹) as well as volumetric rate and final yield of nitrogen removal ($Q_{{{\text{N}} - {\text{NO}}^{ - }_{3} }}$ =3.26 mg dm⁻³ day⁻¹, $Y_{{{\text{N}} - {\text{NO}}^{ - }_{3} }}$=0.739) were obtained at 30°C, whereas phosphorus was more effectively removed at a lower temperature. In order to simulate full-scale studies, batch tests of nitrate and phosphate removal were also performed in 5.0 dm³ vessels (mini-ponds) at the optimum temperature (30°C) but increasing the photon fluence rate to 80 μmol quanta m⁻² s⁻¹ and varying the initial biomass concentration from 0.25 to 0.86 g/dm³. These additional tests demonstrated that an increase in the inoculum level up to 0.75 g/dm³ enhanced both NO₃ ⁻ and PO₄ ³⁻ removal, confirming a strict dependence of these processes on biomass activity. In addition, the larger surface area of the ponds and the higher light intensity improved removal yields and kinetics compared to the flasks, particularly concerning phosphorus removal (μ =0.032–0.050 day⁻¹, Q _x =34.7–42.4 mg dm⁻³ day⁻¹,$Q_{{{\text{N}} - {\text{NO}}^{ - }_{{\text{3}}} }}$=3.24–4.06 mg dm⁻³ day⁻¹, $Y_{{{\text{N}} - {\text{NO}}^{ - }_{3} }}$=0.750–0.879, $Q_{{{\text{P}} - {\text{PO}}^{{3 - }}_{4} }}$=0.312–0.623 mg dm⁻³ day⁻¹, and $Y_{{{\text{P}} - {\text{PO}}^{{3 - }}_{4} }}$=0.224–0.440).