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

Laboratory experiments were conducted using pure cultures of Nocardia amara e and Acinetobacter calcoaceticus under aerobic and anaerobic/aerobic cycli c conditions to determine the control of Nocardia in mixed cultures by biol ogical selection. The objective was to investigate the ability of anaerobic /aerobic sequencing conditions such as in enhanced biological phosphorus re moval (EBPR) process to control Nocardia through competitive selection of A cinetobacter. Aerobic batch experiments showed that Nocardia due to its low er maximum specific growth rate (mu(max) = 2.3/d), and observed yield (Y-ob s = 0.23 g VSS produced/g COD removed) than those of Acinetobacter (mu(max) = 6.2/d, and Y-obs = 0.35 g VSS produced/g COD removed) will not be able t o compete in the dual culture utilizing the same carbon source. Under anaer obic/aerobic conditions in sequencing batch reactors (SBR), Nocardia was wa shed out in about 17 days, whereas, Acinetobacter grew successfully to a st eady 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 de tectable levels after 56 h under aerobic conditions and was completely wash ed out under alternating anaerobic/aerobic conditions in about 24 h. Acinet obacter grew successfully in the dual culture in both completely aerobic an d alternating anaerobic/aerobic SBR to 125-140% of the initial concentratio n. 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 Nocard ia cannot compete well with Acinetobacter under anaerobic/aerobic condition s such as in EBPR, and hence will be washed out of such systems. (C) 2000 E lsevier Science Ltd. All rights reserved.