Cultivation of lipid-producing microalgae in struvite-precipitated liquid digestate for biodiesel production

Using liquid digestate from the biogas industry as a medium to culture lipid-producing microalgae is considered mutually beneficial for digestate valorization and for reducing the cost of microalgal cultivation. However, the low transmittance and high ammonium (NH₄⁺-N) levels in liquid digestate negatively influence microalgae growth.

Struvite precipitation was used to pretreat liquid digestate. To obtain struvite-precipitated supernatant with an ideal transmittance, NH₄⁺-N concentration, salinity, and N:P ratio for microalgal growth, there should be a 1:1.2:1.2 NH₄⁺:Mg²⁺:PO₄^3- molar ratio in the liquid digestate, with KH₂PO₄ and MgCl₂ added through continuous stirring. The addition and stirring was subsequently stopped when the pH reached 8.5. Of the nine tested microalgae species, Dictyosphaerium ehrenbergianum exhibited the best growth in the supernatant. The biomass productivity and lipid content of D. ehrenbergianum cultured in the struvite-precipitated supernatant were 161.06 mg/l/days and 34.33%, respectively, which was higher than when cultured in the standard BG-11 medium. Moreover, the struvite-precipitated supernatant improved the accumulation of monounsaturated fatty acids and saturated fatty acids.

This study described a new way to combine liquid digestate treatment and microalgal biodiesel production. The struvite-pretreated liquid digestate can be used to culture D. ehrenbergianum for biodiesel production.