Search

Glycolysis has been generally accepted to be the source of reducing power used for the synthesis of polyhydroxyalkanoates (PHAs) in the anaerobic metabolism of polyphosphate accumulating organisms (PAOs). However, the tricarboxylic acid cycle (TCA) has also been suggested to contribute to the generation of reducing equivalents, creating some controversy in this research field over the last two decades. Various research approaches have been applied in order to clarify this issue, including the analysis of intracellular compounds, labelled carbon substrates and gene expression. However, the conclusions of these studies seem not to be consolidated. The extent of TCA cycle involvement could be related to the experimental methods employed, the community structure of the cultures used, and also the operational conditions employed. This mini-review analyses the historical findings related to the source of reducing power in PAOs, and highlights the different approaches used in the previous studies. Key factors influencing the generation of reducing power through different metabolic pathways are discussed, and further research directions are also proposed.

Disinfection efficacy was evaluated in four pilot-scale systems that were operated at four water utilities across North America. Through this approach, factors which are difficult to simulate in a laboratory environment (such as water properties, process conditions, climatic conditions) were assessed, in order to determine their influence on disinfectant effectiveness. The results from this study show that a critical level of disinfectant residual ( C crit) was necessary for inhibiting heterotrophic bacterial growth to less than 500 CFUml−1 in the effluent of these pilot-scale systems. When free chlorine was the disinfectant, this critical residual concentration was found to be approximately 0.6 mg L−1. A total chlorine concentration of 1 mg L−1 resulted in low bacterial counts with chloramine disinfectants, though approximately 10–20% of this residual was found to be comprised of slowly degradable organic chloramines which are less effective as disinfectants. A higher ductile iron pipe corrosion rate was observed with a free chlorine disinfectant as compared with chloramines. The results from this study are likely to be of interest to water utilities responsible for the operation and optimization of drinking water distribution systems.

The effectiveness of enhanced biological phosphorus removal (EBPR) systems is directly affected by the competition of polyphosphate accumulating organisms (PAOs) and glycogen accumulating organisms (GAOs). This study investigated the short-term effects of carbon source on PAO and GAO performance. The tests were designed to clearly determine the impact of volatile fatty acid (VFA) composition on the performance of two types of biomass, one enriched for PAOs and the other for GAOs. The two populations were enriched in separate reactors using identical operating conditions and very similar influent compositions with acetate as the sole carbon source. The only difference was that a very low level of phosphorus was present in the influent to the GAO reactor. The abundance of PAOs and GAOs was quantified using fluorescence in-situ hybridisation. The results clearly show that there are some very distinctive differences between PAOs and GAOs in their ability to utilise different carbon substrates. While both are able to take up acetate rapidly and completely, the GAOs are far slower at consuming propionate than the PAOs during short-term substrate changes. This provides a potentially highly valuable avenue to influence the competition between PAOs and GAOs. Other VFAs studied seem to be less usable in the short term by both PAOs and GAOs, as indicated by their much lower uptake rates.

The control of the nitrate recirculation flow in a predenitrification system is addressed. An elementary mass balance analysis on the utilisation efficiency of the influent biodegradable COD (bCOD) for nitrate removal indicates that the control problem can be broken down into two parts: maintaining the “anoxic” zone anoxic (i.e. nitrate is present throughout the anoxic zone) and maximising the usage of influent soluble bCOD for denitrification. Simulation studies using the Simulation Benchmark developed in the European COST program show that both objectives can be achieved by maintaining the nitrate concentration at the outlet of the anoxic zone at around 2 mgN/L. This setpoint appears to be robust towards variations in the influent characteristics and sludge kinetics.