This study was aimed at harvesting of microalgae, Chlorella pyrenoidosa, by using Bottom ash (BA). Utilization of an inexpensive and an easily available BA as a flocculant can be the alternative solution for reducing the harvesting cost of the microalgae and on the other side best way to utilize the unused waste from coal based thermal power plants. Herein, aimed flocculation process was evaluated and optimized through the statistical method, response surface methodology (RSM) with Box Behnken Design (BBD). Optimized conditions for harvesting were found to be best with BA concentration of 100 mgL(-1), temperature (50 degrees C), pH (6.5), and incubation time (60 min) for achieving a maximum yield of 97.9%. The results obtained indicated that BA is a promising flocculant that requires low dose, temperature, and time for a high coagulation and agglomeration of the microalgae. BA is a cost effective and an environmentally friendly flocculant that could neutralize the Zeta potential (zeta) of surface charge of the microalgal cell surface. The morphological changes of microalgae were also analyzed on various temperatures with scanning electron microscopy (SEM), and elemental composition on surface by energy dispersive X-ray spectroscopy (EDS) images with and without flocculant. (C) 2021 Elsevier B.V. All rights reserved.