A sewage treatment plant with a capacity of 10 million litres per day (MLD) is treating the sewage through an aerobic biological process. The inlet and outlet BOD are measured as 100 and $30 \mathrm{mg} / \mathrm{l}$, respectively. If the mixed microbial culture has an observed biomass yield of $0.5 \mathrm{~g}$ Volatile Suspended Solids (VSS)/g BOD, the sludge production rate (in kg per day, rounded off to one decimal place) when the plant is operating at $80 \%$ capacity will be ____________.
