Why Phosphate Accumulating Organisms (PAOs) Win the Competition in EBPR Systems

EBPR mechanism and biochemistry have been investigated since last two decades. However, an apparent controversy exists when temperature impacts on EBPR performance were studied. While early researchers (Barnard et al., 1986, Daigger et al. 1987) concluded that EBPR performance was unchanged or even better at cold temperatures several other researchers (Brdjanovic et al. 1987, Beatons et al. 1999) showed that it was not true. In this paper, the results of a recent EBPR research (Erdal U.G. 2002) are presented to explain the reasons for this apparent controversy. In addition, two unknown questions, physical state of cell walls and substrate transport, in EBPR research are answered. The results of the study showed that, the main reasons for the deteriorated EBPR performance were the unaclimitized sludge to temperature and inadequate SRT. The reduced competition for substrate in the non-oxic zones at low temperatures results in an increased population of PAOs relative to non-PAOs and greater EBPR efficiency at steady state if the SRT of the system is above the critical washout SRT for the prevailing temperature. The study showed that EBPR bacterial community have ability to alter their membrane fatty acid composition to keep their membrane fluid as temperature changed. Saturation curves with distinct Km>Ks values and lock of acetate transport in energy limited conditions strongly suggested that acetate transport is an energy requiring process (active transport). Even though many universally accepted EBPR models (Mino et al. 1987) suggest that the primary purpose of glycogen is to provide reducing power for PHA formation, EBPR have also ability to alter their matabolic pathways (i.e., glycolysis) to gain energy for acetate transport, growth and other cell functions. However, the energy gain rather than poly-P cleavage is kinetically limited. Due to maintenance of readily available energy source (poly-P), PAOs can quickly uptake acetate in anaerobic zone while competitors utilize slow energy generation pathway (i.e., glycolysis). This unique ability gives them very selective advantage to compete with other organisms for substrate uptake in anaerobic zone of EBPR system and makes them winner.