Biodegradation experiments of the non-ionic surfactants Igepal C-620 (an alkyl-phenol ethoxylate) and Neodol 25-7 (an alcohol ethoxylate) were performed, and biodegradation assessed by concentration, foam potential and surface tension measurements. Igepal C-62 0 biodegraded much slower than Neodol 25-7. The foaming of activated sludge was significantly enhanced by the presence of surfactants, but surfactants alone could not generate a stable foam if the sludge did not contain Nocardia cells. It was also found that products of surfactant biodegradation which were not detected by standard non-ionic surfactant analysis method (CTAS) might still possess some foam-enhancing ability. The results indicate that slowly biodegradable surfactants can enhance the foaming of Nocardia-containing sludge and increase the trapping of Nocardia foam in activated sludge plants.
A study at the Southeast Water Pollution Control Plant, San Francisco, California, was undertaken to evaluate the feasibility of replacing its current secondary effluent chlorination system with a 6.57 m 3 /s ultraviolet (UV) disinfection system. Two vertical lamp units and one horizontal lamp unit were pilot tested in parallel. The effective UV dose for coliform removal and the effects of feedwater characteristics and reactor hydraulics were examined. The horizontal pilot unit gave a more consistent performance than the vertical pilot units. Based on the test results, a UV dose of 65 mW' s/cm 2 would be required for the plant to achieve the target effluent total coliform level (240 CFU/IOO mL) 95% of the time. The large fluctuation of UV disinfection results could be attributed to the wide range of feedwater quality inherent to a combined sewer system. High suspended solids, characteristic of the plant's secondary effluent during storm events, significantly increased the UV dose required to achieve the target coliform level. Existing UV inactivation models were evaluated. The hydraulic behavior of the pilot units was found to significantly affect their test results. Higher virus removal efficiency was observed with the UV systems than the full-scale chlorination system.
Increased anaerobic selector hydraulic retention times (HRTs) in a high-purity oxygen activated sludge process resulted in an increase in soluble orthophosphate release and biodegradable COD removal at aeration solids retention times (SRTs) ranging from 1.1 and 1.7 days. Under operating conditions that included biological foam recycling, a well established phosphorus accumulating organism community was observed at HRTs higher than 55 minutes that resulted in a decrease in filament counts and foam stability, providing effective operating conditions for foam control. While enhanced biological phosphorus removal was observed at HRTs between 20 and 60 minutes, its effectiveness with respect to foam control was compromised due to excessive foam trapping and recycling. Thus, increased anaerobic selector HRTs and low aeration SRTs can provide effective measures for biological foam control under conditions that minimize the impacts of foam trapping or that eliminate biological foam recycling.
Solids retention time (SRT), biological scum trapping and recycle and the dynamic equilibrium between Nocardioform populations in the foam and the ML are the controlling factors in activated sludge foaming events caused by Nocardioform bacteria. The combination of selective wasting and SRT control can ensure long-term foam control to the operation of a pure-oxygen activated sludge system. Polymer addition to the ML, followed by selective wasting of foam can cure a severely foaming ML in a matter of weeks provided the SRT remains below 1.5 days. SRT control and selective wasting will also alleviate a severely foaming ML, but effects will only be observed after three or four months after implementation. A SRT of 0.3 days will result in the complete wash out of Nocardioform bacteria from the activated sludge system, which can then operate at a SRT of 3 days free of Nocardioform.
Increased anaerobic selector hydraulic retention times (HRTs) in a high-purity oxygen activated sludge process resulted in an increase in soluble orthophosphate release and biodegradable chemical oxygen demand removal, confirming that enhanced biological phosphorus removal occurs at aeration solids retention times (SRTs) below 1.7 days. Under operating conditions that included biological foam trapping and recycling, an anaerobic selector with HRTs higher than 55 minutes resulted in a decrease in filament counts and effective foam control. Effective norcardioform control is achieved through the combination of metabolic selective pressure and increased soluble organic substrate removal in the anaerobic selector and low aeration SRT. Water Environ. Res., 79, 472 (2007).
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