| Low temperatures are known to slow down the rates of chemical and biochemical reactions. However, despite the slowing reaction rates, early studies showed that the efficiency of enhanced biological phosphorus removal (EBPR) systems is unchanged or sometimes better at cold temperatures. In this study, this controversy was investigated using two UCT configuration modified biological nutrient removal (BNR) systems. They were fed with acetate as the sole organic substrate and separately operated at 5°C and 20°C for two years. The results showed that EBPR systems function more efficiently at 5°C than at either 10 or 20°C if the biomass is permitted to acclimate to the temperature. The reason for better system performance was found to be related to reduced competition for substrate in the non-oxic zones as temperature decreased, which resulted in greater populations of phosphate accumulating organisms (PAOs) relative to non-PAOs, and greater EBPR efficiency. Transmission electron microscopy (TEM) pictures of anaerobic and aerobic sludges obtained from 5°C and 20°C systems showed that the colder temperature reduced the variety of microbial organisms present in the EBPR sludge. It has been widely accepted that glycogen metabolism is essential for the storage of polyhydroxyalkanoates (PHA), which is known to be essential for EBPR. However, despite increased intracellular storage of PHAs, mass balances of glycogen showed that its involvement decreased as temperature decreased. Thus, contrary to what has been assumed for the currently accepted EBPR models, there is no strict stoichiometric relationship between PHA storage and glycogen utilization. The results also indicate that temperature not only can exert selective pressure that determines the dominant organisms, but also can force them to use different metabolic pathways. Homeoviscous adaptation ability of EBPR organisms was also investigated for a series of temperatures ranging from 5°C to 20°C. It was observed that membrane fatty acid composition varied for different temperatures. Thus, solute transport and proton motive force were operable even at 5°C. An investigation of the impacts of operational conditions such as low solids retention time (SRT), presence of electron acceptors, and lack of acclimation showed that short- and long-term acclimation of sludge to changing temperatures must be considered during EBPR research. |
