A Simplified Mathematical Model to Determine the Degradation Coefficient and Order of Decay for Activated Sludge Process (original) (raw)
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International Journal of Environment and Pollution, 2005
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ANZIAM Journal, 2006
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Environmental Modeling & Assessment, 2008
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Activated sludge process (ASP), which is a suspended growth system, is used on a large scale for biological treatment of wastewater. ASP requires air supply. The aeration facilities are designed to meet the oxygen demand of the process while maintaining in the aeration tank minimum desirable dissolved oxygen (DO) concentration. The presence of DO in wastewater is desirable because it prevents the formation of bad odors and toxic gases. The last two form under anaerobic conditions in which anaerobic microorganisms produce hydrogen sulfide and methane. The operation of aeration facilities is continuous which increases the operational cost of the treatment plants. However, the intermittent aeration ASP is now widely attractive both for reducing power requirements of aeration facilities and for upgrading existing biological treatment plants for nitrogen removal. In Iraq, nearly, all sewage treatment plants imply ASP with continuous aeration scheme. But, due to the present state of electrical power supply in Iraq, the stop of aeration process due to power shut down cannot be avoided, i.e., mostly all sewage treatment plants in Iraq are under the condition of forced intermittent aeration system. In designing an ASP, the volume of aeration tank is calculated using a set of design criteria. The designer based on his engineering judgment usually, specifies the tank dimensions. So there is a need of knowing how tank dimensions are specified. Also, in deciding the power required for a mechanical aeration system, a value is randomly chosen based on accepted range for power per unit volume values. The requirements highlighted in the above paragraph can be accomplished using a mathematical model. The Activated Sludge Model No. 1 (ASM1) is usually considered as the reference model, which was primarily developed for municipal ASPs to describe the removal of organic carbon substances and nitrogen. However, with respect to DO dynamics, the model does not include all factors affecting the rate of DO supply. In this study, A model was developed for simulating the DO supply in ASP. The model is a modified version of ASM1 that incorporates all the factors affecting the rate of oxygen supply, such as type of aeration system, tank geometry, and the physical characteristics of treated sewage including the total dissolved solids (TDS) and temperature. The developed model, which is labeled as ASP-DO include nine state variables and eight reaction processes. It solves nine simultaneous nonlinear ordinary differential equation using the 4th order Rung Kutta method. ASP-DO program was written in Maple13 language. It was used as a tool for; studying how aeration tank dimensions affect the concentrations of dissolved oxygen and other interesting components of sewage, determining the power input per aeration basin volume for an ASP equipped with mechanical aeration system, investigating the effects of aeration cut in Hamdan Sewage Treatment Plant (HSTP) on dissolved oxygen level in ASP and the performance of ASP, and determining the best intermittent aeration condition, that will maintain the desirable minimum level of dissolved oxygen in the aeration tanks. The most important conclusions of this study are; as water depth in the aeration tank of ASP increases, the concentration of DO increases, as L/W ratio of the aeration tank increases, the DO concentration increases, ASP-DO model can be used for determining the power input per unit volume of aeration tanks equipped with surface aeration system and it is not recommended to chose a random value based on known values range given in scientific resources, ASPDO model can be, successfully, used to track the variations in DO concentration under different design and operation conditions, the current state of electrical power supply in Basrah may lower the percent of soluble substrate removal to about 47% when air on/off is 3hr/3hr, and the intermittent aeration system can be applied in HSTP with 1.5hr/0.5 hr air on/off.
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Water Science and Technology, 1995
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