Production of biodiesel and biolubricant via co-cultivation of Arthrospira platensis and Rhodosporidium babjevae using pulp-paper mill sludge hydrolysate

Abstract The pulp and paper manufacturing industry generates large volumes of sludge, which is characterized by a high COD content known as pulp and paper mill sludge (PPMS). Co-cultivation of Arthrospira platensis and Rhodosporidium babjevae in the present study was evaluated quantitatively by the production of high-value products through lipid biosynthesis by using PPMS hydrolysate. We pretreated PPMS in two steps: dilute-acid hydrolysis followed by enzymatic hydrolysis. Dilute-acid hydrolysis was statistically optimized using a central composite design, yielding optimum conditions of 1.03% (w/w) nitric–sulfuric acid, 134.8 °C, and 23.6 min. Subsequently, the PPMS hydrolysate underwent enzymatic saccharification for 72 h at 50 °C with 2% (w/w) commercial cellulase. The co-cultivation process thereafter was also optimized using Box-Behnken design, where affecting parameters at optimized levels were 3.86 klux light intensity, 1.48 L min-1 airflow rate, 143 h of co-cultivation, and an A. platensis to R. babjevae ratio of 2.2:1. Under these optimized conditions, lipid yield reached 6.56 ± 0.22 g L⁻¹ (57.64 ± 1.0% of DCW), with a carotenoid yield of 41.22 ± 1.57 mg L-1 and COD removal efficiency of 86.36 ± 0.97%. The properties of the synthesized biodiesel follow the EN 14214 standard, and the formulated biolubricant showed tribological performance similar to that of Shell gas compressor oil S3 PY 220.