A BIOCHEMICAL MECHANISM FOR COD STABILIZATION IN EBPR ANAEROBIC ZONES

An extensive investigation of enhanced biological phosphorus removal (EBPR) metabolism based on the performance of laboratory-scale EBPR systems operated at two different temperatures, 20 and 5°C, was performed in an effort to elucidate biochemical mechanisms that contribute to the stabilization of COD in the anaerobic zones, i.e. anaerobic stabilization (AnS), of EBPR systems. The systems were fed acetate, a non-fermentable substrate, as the only organic carbon substrate to minimize the diversity of anaerobic biochemical reactions that might occur. Substantial AnS, an average of 9% of total COD removal for 9 observations, with a range from 3.0 to nearly 15%, was measured in the two EBPR systems operated at 20°C, but none was observed in the system maintained at 5°C, even though its configuration was identical to one of the 20°C systems. Off-gas measurements from the anaerobic and aerobic stages of a batch test utilizing the 20°C biomass did not reveal the presence of any gaseous products such as H2 or CH4 that might have explained the loss of electrons from the EBPR systems, in conformance with results reported by Wable and Randall (1994). It was concluded that only CO2 was released in significant quantities from the anaerobic zones of the experimental systems.The EBPR biomasses maintained in two UCT configuration systems at 20 and 5°C exhibited different patterns of storage and consumption, and showed major metabolic differences whereby they maintained their energy and reducing equivalents balances. This was determined by the detection and quantification of enzymatic activities, and further confirmed and elucidated using solid-state NMR analysis. The results strongly imply that the bacterial populations of the biomasses were different, and, indeed, the population was shown to be much less diverse at 5°C (U. Erdal, 2002). The primary anaerobic metabolic difference between the 20 and 5°C systems was that in the 20°C system the carbons followed through the branched TCA cycle, whereas the 5°C system preferred the glyoxylate cycle. Mass balancing the reducing equivalents indicated that the 20°C system generated 15.7% more NADH than needed, thereby requiring the operation of the branched TCA cycle, likely resulting in AnS. By contrast, the 5°C system produced less reducing equivalents than needed utilizing the glyoxylate cycle, indicating that another, as yet unidentified, source of reducing equivalents was needed. It also was determined that the 20°C system consumed 116 mmols of NADH to accomplish gluconeogenesis whereas the 5°C system required only 48, which would account for a large difference in aerobic oxygen consumption. The results clearly showed that steady-state EBPR is much greater at lower temperatures, and phosphorus removal by the 5°C UCT system was 2.5X that of the 20°C system. This occurs because glycogen degradation is very temperature sensitive, which strongly favors the PAOs over the GAOs. The enzymatic activity data clearly showed that EBPR metabolism used the EMP pathway at both experimental temperatures, and did not use the ED or PP pathways.