Competitive growth of Nocardia and Acinetobacter under anaerobic/aerobic batch operation

Laboratory experiments were conducted using pure cultures of Nocardia amarae and Acinetobacter calcoaceticus under aerobic and anaerobic/aerobic cyclic conditions to determine the control of Nocardia in mixed cultures by biological selection. The objective was to investigate the ability of anaerobic/aerobic sequencing conditions such as in enhanced biological phosphorus removal (EBPR) process to control Nocardia through competitive selection of Acinetobacter. Aerobic batch experiments showed that Nocardia due to its lower maximum specific growth rate (μmax=2.3/d), and observed yield (Yobs=0.23 g VSS produced/g COD removed) than those of Acinetobacter (μmax=6.2/d, and Yobs=0.35 g VSS produced/g COD removed) will not be able to compete in the dual culture utilizing the same carbon source. Under anaerobic/aerobic conditions in sequencing batch reactors (SBR), Nocardia was washed out in about 17 days, whereas, Acinetobacter grew successfully to a steady state concentration approximately equal to its initial concentration. When equal mass of Nocardia and Acinetobacter were grown as a dual culture in SBR, Nocardia concentration decreased from about 200 mg VSS/L to near detectable levels after 56 h under aerobic conditions and was completely washed out under alternating anaerobic/aerobic conditions in about 24 h. Acinetobacter grew successfully in the dual culture in both completely aerobic and alternating anaerobic/aerobic SBR to 125–140% of the initial concentration. Polyphosphate accumulation was not seen either in single cultures or in dual cultures of Nocardia and Acinetobacter under anaerobic/aerobic cyclic conditions in a sequencing batch reactor. These results suggest that Nocardia cannot compete well with Acinetobacter under anaerobic/aerobic conditions such as in EBPR, and hence will be washed out of such systems.