The efficiency of oxygen transfer depends on many factors including the type, size and shape of diffusers and the tank geometry. In this paper, the effect of the depth of water in the tank and, the extension of coverage area of diffusers on each of oxygen transfer capacity (OC), efficiency (E) and, on a percentage of oxygen absorption (δ) is tested. Experimental procedure is adopted to evaluate the effect of these parameters. The results of the study showed that, both the depth of water and the extent of coverage area of diffuser had a significant effect on the tested parameters. The values of oxygen transfer capacity (OC) and efficiency (E) ranged from 18 to 170 grO2/m3. hr and from 2 to 17 grO2/m3air, respectively; depending on the depth of water in tank and the ratio of diffusers coverage area. The percentage of oxygen absorption ranged from 0.45–5.4% depending on the testing conditions. Specific mathematical models to describe the effect of each parameter were also derived. The exponential form of equation proved to be efficient in describing the effect of a depth water on oxygen transfer capacity (OC) whereas; the linear form of equation was good enough in representing the effect of the other parameters.

The key element in developing an analytical or mathematical procedure is to determine the main factors that possess potential importance in oxygen transfer process. In this paper, the factors that affect the oxygen mass transfer coefficient (K L a) in activated sludge units are determined. A dimensional analysis procedure is adopted to develop a mathematical model, which can be used for calculating these factors. A very high accurate model with a correlation factor of (98.889 %) is obtained from this analysis. The solution of the model shows that: The main dimensionless groups which controlling the oxygen mass transfer in this units are: Reynolds number, Froude number, the ratio of bubbles diameter to length of its bath in water, the ratio of water depth in the tank to tank length, and the ratio of diffusers area to tank area. Each of the Reynolds number and the ratio of area of diffusers to the area of tank have positive effect on the oxygen mass transfer coefficient. Increasing any of airflow rates, the area of diffusers coverage in the tank and the length of the path of bubbles in water significantly increase the oxygen mass flow rate while this ability can also be increased by decreasing the diameter of bubbles in the system.

Oxygen transfer capacity has greatest influence on activated sludge performance. In this study, experimental procedure is carried out to determine the effect of airflow rate and the level of diffusers submergence, on the oxygen transfer rate of diffused air systems. In addition, a number of readings were also collected from published papers. The data were selected to cover various scales of plants and operating conditions. After sorting out, all data were treated and then combined to the experimental readings in order to expand the applicability of the results. Individual mathematical models to describe the effect of each parameter were also derived. The results of the study showed that, increasing the airflow rate at fixed water depth and diffusers submergence enlarge the oxygenation capacity of the system. The equation, which controls this relationship, is linear. At diffusers submergence of 4.6 m, the slope of the equation was 11.8. With reduce the depth of diffusers to about 0.4 m; the slope of equation was decrease to 2.3. At constant airflow rate, the depth of diffusers has a significant effect on both of the oxygenation capacity and oxygen transfer efficiency of the system. Exponentional form of equation is shown to be efficient in expressing the relationship between the submergence and the oxygenation capacity. At 0.4 m, diffusers submergence, the oxygen transfer efficiency was 1.8 whereas; this value is rising to about 11.5 at 4.6m submergence.

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We investigated the effect of land use based on the dominant phytoplankton and physical-chemical variables in the different areas of the Tigris River, within the city of Mosul. Agricultural and urban activities have a significant impact on the water quality of the river. Regardless of physical and chemical variables, phytoplankton act as a bio-indicator of water quality due to their fast and sensitive response to changes in the environment. Our research was conducted in the Tigris River within the city of Mosul by examining the phytoplankton species and the physical-chemical variables at 16 sites during each vegetation period. Point and non-point source pollutants have affected the Tigris River within the city of Mosul, getting into the river from upstream through agricultural activities and by urban activities in the middle section of the city, respectively from both banks. Based on our results, we observed the highest phytoplankton abundance during the October sampling periods, while the lowest occurred during the July sampling period, which was associated with maximum water temperature and absence of rain. According to our study, land use (e.g., agricultural, and urban activities) greatly affected the dominant phytoplankton species and physical-chemical variables of the Tigris River. Oscillatoria sp. dominated all seasons in the agricultural region, while at the same time, we observed an increase in the number of phytoplankton species caused by the nutrient availability upstream on the river. The effects of climate have very significant and characteristic effects in this area, which basically determine the community of organisms and the water quality; the effects resulting from anthropogenic activity significantly modify this. Based on our investigation, in the part of the Tigris River connected to Mosul, we found a clear connection between the pollution caused and the effects of different land uses, through the examination of the algal community and physical-chemical variables in different periods of the year.