Low DO Levels Research Articles

Development of small outdoor microcosms for studying contaminant transformation rates and mechanisms under various water column conditions

AbstractThree outdoor microcosm treatments were developed to simulate aerobic (aerobic), thermally stratified (stratified), and anaerobic (anaerobic) water columns for assessing contaminant transformation rates under controlled, quasi‐natural conditions. Twelve 3.2‐m‐diameter by 1.4‐m‐deep fiberglass tanks (11.3 m3 water volume) were used in the study, four for each treatment. The tanks were variably manipulated both physically (with internal frames and plastic layers) and chemically (with sediment trays and grass supplements) to facilitate the three environments. The manipulations successfully created three different and reproducible (both between tanks with common treatments and over time) aquatic systems for further study. The aerobic treatment produced a good simulation of a P‐limited, mesotrophic water column. Alternately, both the stratified and anaerobic treatments developed eutrophic to hypereutrophic water conditions and had nutritionally balanced total nitrogen (TN) and total phosphorus (TP) levels (approximately 14:1 mg N/mg P). Mild thermal stratification was achieved in the stratified units and low‐oxygen conditions were established in the anaerobic units. A trial study, using the herbicide alachlor [2‐chloro‐2′,6′‐diethyl‐N‐(methoxymethyl)acetanilide], was performed to test the utility of the three microcosm designs for assessing comparative in situ contaminant transformation rates. This experiment was initiated by adding alachlor to all tanks to a final concentration of 50 μg/L. Alachlor levels and other chemical parameters were then monitored for 36 d. The estimated first‐order alachlor decay coefficients and half‐lives for the aerobic, stratified, and anaerobic treatments were 0.011/d (63 d), 0.024/d (29 d), and 0.020/d (35 d), respectively. Subsequent analyses showed that the highest rates of alachlor decay occurred in units with grass supplements, low DO and pH levels, and comparatively high TP levels. These results suggest that lower nutrient (i.e., low TP levels), photosynthesis‐dominated aquatic systems produce lower alachlor decay rates, whereas higher nutrient, decomposition‐dominated systems produce higher decay rates. This project shows that diverse physical and chemical environments can be created in small outdoor microcosms and that these systems can be used successfully to assess contaminant decay rates in natural systems.

Study of the Factors Affecting Activated Sludge Settling in Domestic Wastewater Treatment Plants

A study has been made of the influence of the process parameters on the settling characteristics of sludge in domestic wastewater treatment plants. The experiments were conducted at a pilot plant situated at the Wastewater Treatment Plant of Valencia, Spain (1,000,000 inhabitants). The influent to the pilot plant was the same as that of the plant. It was found that F/M ratios in the range of 0.25-0.60 g DBO5/g MLVSS day produce values of SVI representative of good settleability of the sludge. Values of sulphide concentration in the influent above 1.0 mg/l negatively affect the process, causing bulking and floating sludge. High values of fats were found to have no influence on the SVI, but did affect the performance of depuration. At low DO levels, the appearance of bulking with high SVI values was observed. Throughout the experiments, the permanence of nutrients (basically N and P) above that required for the growth of the microorganisms was proved.

Influence of dissolved oxygen on growth of young eel.

Experiments were carried out to examine the influence of nearly constant and of diurnal fluctuated dissolved oxygen level on growth, feeding and food conversion rates of Japanese eel Anguilla japonica. Eel at average weight of 2-5g were reared in 16l rearing tanks with continously supplied freshwater (23.4-25.0°C) of different DO levels. DO level was controlled by introducing N2 and O2 gas into the inflowing water. Growth, feeding and food conversion rates of eel decreased when they were exposed to con-start DO levels below 40% but were maintained almost at the same degree at the oxygen level from 60% to above air-saturation. Growth rate also decreased when DO level diurnally fluctuated (12:12h) from 90% or 60% to levels below 40%. Effects of diurnal fluctuations of DO on growth of eel were similar whether the DO level at recovery period was kept at 90% or 60%. But in the case of 60%, a longer exposure to alternate low DO levels brought more severe decrease in growth rate than 12h. Consequently, the desirable DO level for normal growth in both constant and fluctuated condition is expected to be more than 60%. No effects of DO level on protein digestibility and food requirement for maintenance of eel were observed. The reduction in food conversion rate at low DO level is presumed to be caused mainly by the decrease in food consumption of eel.

DISEASONAL THERMAL AQUACULTURE— 1. EFFECT OF TEMPERATURE AND DISSOLVED OXYGEN ON SURVIVAL AND GROWTH OF Macrobrachium rosenbergii

AbstractSurvival and growth of the giant shrimp (Macrobrachium rosenbergii) is profoundly influenced by changes in temperature and dissolved oxygen (Do) of thermal effluents from power plants which discharge into aquaculture systems. These changes are due to seasons as well as plant shutdown in areas such as the northeast United States. In laboratory studies simulating aquaculture ponds the Critical and Lethal Thermal Maximum (CTMax, LTMax) and Minimum (CTMin, LTMin) at high and low DO levels have been measured for shrimp pre‐acclimated to 20–36°C. CTMax and CTMin rise with temperature of acclimation. The same is true for LTMax and LTMin. Low DO reduces the upper temperature tolerance as shown by a drop in CTMax and LTMax. It can become a critical factor in ponds during the peak of the summer, especially at nights. By contrast, Do has little or no effect on CTMin and LTMin. Dissolved oxygen is not a limiting factor at low temperatures. Growth and Conversion Ratio were evaluated in chronic studies at various temperatures, DO being maintained at air saturation. Growth nearly doubled for every 5°C increase in temperature over the 20130°C range while there was no appreciable change in conversion ratio.