Utilization of nano-sized waste lime sludge particles in harvesting marine microalgae for biodiesel feedstock production

In this research, waste lime sludge (LS) nanoparticles procured from integrated pulp and paper mill were used as the flocculent material to harvest the marine microalgae Tetraselmis indica (T. indica). LS contains 80–85% of calcium carbonate in the form of calcite with rhombohedral and scalenohedral structures along with other minerals such as Mg, K, Fe, and Na. XRD results showed that lime sludge particles have an averaged crystallite size of ~ 39 nm. The biomass harvesting efficiency of T. indica was observed at different doses of flocculent (0–160 mg L⁻¹), temperature (30, 35, 40, 45, and 50 °C) and mixing rate (100–300 rpm), respectively. Maximum biomass harvesting of 86% was observed at 50 °C with 140 mg L⁻¹ flocculent dose, pH 7, and 150 rpm in 90 min. It was also observed that LS has no effect on biodiesel profile of T. indica. Scanning electron microscopy (SEM) images revealed agglomeration of microalgal cells and deposition of calcium carbonate on its surface upon treatment with LS at 50 ºC. Experimental data were found to be in good agreement with pseudo-second-order kinetics model. This study indicated that waste lime sludge is a potential material for high harvesting efficiency in low settling time and low-cost harvesting of microalgae and it makes the production of biodiesel cost-effective.