To facilitate industrial wastewater treatment, a bioreactor containing 500 m^3 of wastewater was inoculated by 10 m^3 of bacterial suspension with a cell concentration of 5x10^9 cell/mL and an average cell mass of 3x10^-12 g/cell. Calculate the mean specific growth rate of the microbial community treating the industrial wastewater if the biomass concentration increased to 5.3 g/L after 18 hours of batch cultivation.