One litre of sterile medium containing a COD of 8500 mg/L is seeded with 1 mg of microorganisms as COD from the same species. The medium is highly buffered and at the ambient temperature this species can double every 2.1 hours.

(a) If 1 mg of substrate COD produces 0.66 mg (as COD) of biomass (the other 0.34 mg of COD is oxidized to provide energy for growth), assuming log growth, how much biomass as COD and substrate exist after 24 hours?

(b) If the microorganisms grow in the log phase until the supply of original substrate is exhausted, how much time will elapse? Why should the medium be buffered?