This paper proposes a new metabolic model for acetate uptake by a mixed culture of phosphate- and glycogen-accumulating organisms (PAOs and GAOs) under anaerobic conditions. The model uses variable overall stoichiometry based on the assumption that PAOs may have the ability of using the glyoxylate pathway to produce the required reducing power for polyhydroxyalkonate (PHA) synthesis. The proposed model was tested and verified by experimental results. A sequencing batch reactor system was operated for enhanced biological phosphorus removal (EBPR) with acetate as the sole carbon source at different influent acetate/phosphate ratios. The resulting experimental data supported the validity of the proposed model, indicating the presence of GAOs for all tested HAc/P ratios, especially under P-limiting conditions. Strong agreement is observed between experimental values and model predictions for all model components, namely, PHB production, PHA composition, glycogen utilization, and P release.

One of the two trains of the 37,000 m3/d Annapolis, Maryland step aeration activated sludge treatment plant was modified for single-sludge anoxic-aerobic operation, and then fixed-film media were integrated into the aerobic zone to enhance nitrification. Rope-like Ringlace media was selected for integration, and 30,000 meters were installed in a volume of 475 m3 for a pilot demonstration. The purpose of the integrated fixed-film media was to upgrade the short hydraulic retention time (HRT) basin (6 hrs nominal) for efficient, year-round nitrogen removal without construction to increase basin volume. An engineering study had concluded that upgrading the facility for year round complete nitrification, without nitrogen removal, would cost US$24 million. The modified train was operated for 12 months, six in the plug-flow MLE configuration, and six in a step-feed configuration. The integrated Ringlace media increased the nitrification rate per unit volume to 225% of that observed in the control section, attaining a value of 1.75 kg/d NH3-N per linear meter at 15°C. The media also increased denitrification in the aerobic media section to the extent that between 30 and 88% of the nitrates formed in the section were denitrified within it, permitting a potential 25% or more reduction in the volume of the anoxic zone. An IFAS retrofit design was developed which incorporated step-feed operation, and reduced the projected construction cost to US$9.2 million.

The effect of influent organic compounds on the performance of a biological nutrient removal system was investigated using a pilot plant system operated as a UCT (University of Cape Town) process. The system was fed domestic sewage and operated at a sludge age of 13 days. The effects of separate addition of formic, acetic, propionic, butyric, isobutyric, valeric, and isovaleric acid on phosphorus release under anaerobic conditions, and phosphorus uptake under aerobic conditions, were studied. The effects of the organic acid additions on the removal of nitrogen and COD, and changes in SOUR and MLSS, were also studied. All added substrates, except formic acid, caused significant increases in phosphorus release in the anaerobic stage, and subsequent phosphorus uptake in the aerobic stage with an increase in phosphorus removal efficiency. It was also found that the branched organic acids, isobutyric and isovaleric, caused more phosphorus release in the anaerobic stage and better phosphorus removal efficiencies in the system, compared with the nonbranching forms of the same organic acids. The most recent biochemical model, proposed by Comeau et al. (1986) and Wentzel et al. (1986) was also tested using the data collected in this investigation. Both models, in most cases, overestimated the ratios of phosphorus release to volatile fatty acid utilized. All added substrates caused no change in either COD or TKN removals. For engineering applications, it is suggested by this research, that at least 20 mg COD equivalent of acetic acid is needed for the removal of 1 mg phosphorus.

It is well known and firmly established that the rate of chemical and biochemical reactions slow down as temperature decreases. Nevertheless, several studies have reported that the efficiency of enhanced biological phosphorus removal (EBPR) improves as temperature decreases. However, several recent studies have reported that EBPR reaction rates decrease with temperature decrease in accordance with the Arrhenius relationship. This study was designed to more thoroughly investigate this controversy using two UCT plants fed with a synthetic wastewater consisting primarily of acetate as the COD form, and a small amount of supplemental yeast extract. Experiments were performed over temperatures ranging from 5 to 20 degrees C. The results showed that, even though the kinetic rates decrease as temperature decreases, EBPR systems perform better at colder temperatures. The reason for better system performance is apparently related to reduced competition for substrate in the non-oxic zones, which results in an increased population of PAOs and, thus, greater EBPR efficiency. The proliferation of PAOs apparently occurs because they are psychrophilic whereas their competitors are not. The experiments showed that the EBPR sludges accumulated high concentrations of both PHA and glycogen at 20 degrees C, but accumulated more PHA and much less glycogen at 5 degrees C. Although the results could be interpreted as the result of changes in the PAO-GAO competition, Mann-Whitney non-parametric comparisons of transmission electron microscopy examinations revealed no indication of the presence of GAOs population under any temperature conditions. Regardless, mass balances of the glycogen data showed that the involvement of glycogen is less at cold temperature, even though EBPR was greater. Unlike current EBPR models (e.g. Mino model), the results suggest that glycogen metabolism is not a precursor for EBPR biochemistry. The results also indicate that temperature not only may cause selective pressure on the dominant organisms, but also may force them to use a different metabolic pathway as temperature decreases.

The organic substances most frequently mentioned as precursors of trihalomethanes in public water supplies are the naturally occurring humates found in all surface waters and in many groundwaters. However, laboratory data have shown that algae—both their biomass and their extracellular products—also react readily with chlorine to produce THMs.