Effects Of Low Dissolved Oxygen Research Articles

Oxygen transfer dynamics and activated sludge floc structure under different sludge retention times at low dissolved oxygen concentrations

In activated sludge systems, the aeration process consumes the most energy. The energy cost can be dramatically reduced by decreasing the operating dissolved oxygen (DO) concentration. However, low DO may lead to incomplete nitrification and poor settling performance of activated sludge flocs (ASFs). This study investigates oxygen transfer dynamics and settling performances of activated sludge under different sludge retention times (SRTs) and DO conditions using microelectrodes and microscopic techniques. Our experimental results showed that with longer SRTs, treatment capacity and settling performances of activated sludge improved due to smaller floc size and less extracellular polymeric substances (EPS). Long-term low DO conditions produced larger flocs and more EPS per unit sludge, which produced a more extensive anoxic area and led to low oxygen diffusion performance in flocs. Long SRTs mitigated the adverse effects of low DO. According to the microelectrode analysis and fractal dimension determination, smaller floc size and less EPS in the long SRT system led to high oxygen diffusion property and more compact floc structure that caused a drop in the sludge volume index (SVI). In summary, our results suggested that long SRTs of activated sludge can improve the operating performance under low DO conditions.

Oxygen Control Challenge for Advanced Wet Processing

A control of Dissolved Oxygen (DO) in liquids and oxygen concentration in ambient atmosphere for semiconductor processing is described. The effects of low DO in wet processing for galvanic corrosion and water mark formation were evaluated using various oxygen concentration conditions with HF or HCl mixtures. It was found that the low DO concentration condition (25ppb) prevented the occurrence of defects (voiding, trenching or water marks). The results from processing a device sample showed that low DO concentration condition is required for advanced CMOS processing.

Quantifying the chronic effect of low DO on the nitrification process

Our previous study indicated that a low dissolved oxygen (DO) could enrich and shift nitrifier community, making complete nitrification feasible under long-term low DO conditions. This research determined nitrifier kinetic constants, and quantified the chronic effect of low DO on the overall nitrification process. For ammonia oxidizing bacteria (AOB), the half-velocity constants of DO on the growth (KDO-g) and decay (KDO-d) were 0.29 and 0.48mgL−1, respectively. For nitrite oxidizing bacteria (NOB), those values were 0.08 and 0.69mgL−1, respectively. The low KDO-g values for both AOB and NOB suggest that a DO of greater than 1mgL−1 does not provide further benefit to nitrification, and the lower KDO-g value for NOB suggests that nitrite oxidation is less impacted by a low DO. The KDO-d values of 0.48 and 0.69mgL−1 for AOB and NOB, respectively, suggest that a low DO of less than 1mgL−1 significantly inhibits the decay of both AOB and NOB, resulting in their enrichment. The relationship between the operational DO and required SRT for complete nitrification was developed to provide a theoretical foundation for operating an advanced wastewater treatment plant under low DO, to significantly improve aeration energy efficiency.

Long-Term Low DO Enriches and Shifts Nitrifier Community in Activated Sludge

In the activated sludge process, reducing the operational dissolved oxygen (DO) concentration can improve oxygen transfer efficiency, thereby reducing energy use. The low DO, however, may result in incomplete nitrification. This research investigated the long-term effect of low DO on the nitrification performance of activated sludge. Results indicated that, for reactors with 10 and 40 day solids retention times (SRTs), complete nitrification was accomplished after a long-term operation with a DO of 0.37 and 0.16 mg/L, respectively. Under long-term low DO conditions, nitrite oxidizing bacteria (NOB) became a better oxygen competitor than ammonia oxidizing bacteria (AOB) and, as a result, no nitrite accumulated. Real-time PCR assays indicated that the long-term low DO enriched both AOB and NOB in activated sludge, increasing the sludge nitrification capacity and diminishing the adverse effect of low DO on the overall nitrification performance. The increase in the population size of nitrifiers was likely resulted from the reduced nitrifier endogenous decay rate by a low DO. Under long-term low DO conditions, Nitrosomonas europaea/eutropha remained as the dominant AOB, whereas the number of Nitrospira-like NOB became much greater than Nitrobacter-like NOB, especially for the 40 day SRT sludge. The enrichment and shift of the nitrifier community reduced the adverse effect of low DO on nitrification; therefore, low DO operation of a complete nitrification process is feasible.

The effects of hypoxia on fishes: from ecological relevance to physiological effects

Hypoxia is an ever increasing threat to aquatic systems. While fluctuating levels of dissolved oxygen (DO) can be a natural phenomenon, hypoxia caused by eutrophication and organic pollution is now considered to be amongst the most pressing and critical water pollution problems in the world, particularly in densely populated regions. The effects of low DO on fishes are an area of great concern and thriving study. Researchers have examined the effects of low DO on fishes from the cellular to community level. The purpose of the current paper is to review the effects of low DO on complex fish behaviour, community and fish physiology. Our review will also highlight studies in which DO is known to interact with a known contaminant. Throughout the paper we will highlight areas in need of future research such as chronic exposure, interactive effects of DO and contaminants, an increased understanding of how hypoxia affects communities of organisms, and finally a need for an increase in freshwater studies.

Combined effects of pulp and paper effluent, dehydroabietic acid, and hypoxia on swimming performance, metabolism, and hematology of rainbow trout

Experiments were conducted to examine the effects of a thermomechanical (TMP)/bleached kraft pulp and paper mill effluent (BKME), dehydroabietic acid (DHAA), hypoxia, and combinations of hypoxia and effluent on juvenile rainbow trout. In the first two experiments, trout were exposed for 4 weeks to 0%, 10%, 30%, and 70% TMP/BKME or 0, 35, 110, and 250 μg L −1 DHAA, respectively. Endpoints of those dose–response studies included critical swimming speed, oxygen consumption, and hematology. Reduced swimming performance was found for fish exposed to 70% TMP/BKME. Moderate increases in mean cell hemoglobin concentration at 70% TMP/BKME and blood glucose at 30% and 70% TMP/BKME were also seen. The opposite trend for glucose was found for DHAA-exposed fish, where a slight decrease in glucose was seen at 110 and 250 μg L −1 DHAA. The third experiment examined the effects of 15% v/v TMP/BKME exposure at 2.5 and 5.0 mg L −1 dissolved oxygen (DO) for 4 weeks. This experiment found no effect of low DO on swimming ability. An interactive effect between DO and effluent exposure was seen only on hematocrit, where effluent caused an increase in hematocrit at 5 mg L −1 and a decrease at 2.5 mg L −1 DO. Effluent exposure in this experiment resulted in a greater number of smaller red blood cells. The current study demonstrated physiological effects in rainbow trout exposed to varying concentrations (15–70% v/v) of a TMP/BKME and no substantial effects of DHAA exposure. With the exception of the reduced swimming performance in fish exposed to TMP/BKME, the observed effects are considered relatively small in magnitude but are occurring at concentrations of effluent that occur in the receiving environment.

Nitrification at low levels of dissolved oxygen with and without organic loading in a suspended-growth reactor

Laboratory-scale mixed flow reactors with DO (dissolved oxygen) control were operated at 25°C using substrate containing 80 mg l −1 of NH 3-N. DO of 0.5 mg l −1 produced no effect on ammonia oxidation of the reactor. Determination of the biomass index based on the substrate utilization rate revealed that low levels of DO elevated the growth yield of ammonia oxidizers by double. This high growth yield compensated the reduced specific substrate utilization rate at low levels of DO. Kinetic analysis showed that the μ m value was not significantly changed by low DO because of the elevated growth yield. Growth yield of nitrite oxidizers was not increased by low DO. Nitrite oxidation was strongly inhibited by 0.5 mg l −1 of DO, and accumulated nitrite reached almost 60 mg l −1. The saturation constant in terms of DO ( K o ), estimated based on specific substrate utilization rate, was 0.32 mg l −1 for ammonia oxidation, while this value for nitrite oxidation could not be obtained. The interacting effect of low DO and organic loading was examined by adding glucose to the substrate. The inhibitory effect on ammonia oxidation by organic loading through heterotrophic activity was enhanced by low DO. The results suggest that the K o value in a system with organic loading is larger than the above value which was estimated in a system without organic loading. Simultaneous nitrification and denitrification took place to some extent when glucose was fed at low DO.