The present work considers the application of an observer based control system to the observation and control of biomass concentration in a hybrid bioreactor system in the presence of measurement delays. A Proportional Integral (PI) controller was used and the performance of the system was assessed in terms of the maximum sampling intervals for fixed values of delays and maximum allowable delays for particular sampling partition diameters. The results obtained were analysed relative to a previous work which considered a feedback linearising controller to perform the same funtions. A comparative study was carried out to determine the superiority of the new controller for this application. The PI controller provided better responses and allowed the observer to converge at relatively higher values of sampling time and delays.