Analysis with dynamic process modeling approach of anaerobic sludge digesters of the urban wastewater treatment plant

The sewage sludges which occurs after treatment process that is called as biomass in wastewater treatment plants, must be finally disposed after satisfying necessary criteria in terms of environment and human health by means of stabilization processes suitable according to characterization of sludge. Sludge disposal is carried out by using options such as regular storage or incineration for these sludge emerging in wastewater treatment plants. However, it is one of the most advantageous options to obtain energy from these sewage sludges when considering the its calorific value and if it's rich carbon content before the disposal process. Application of anaerobic process in treatment systems or anaeorbic way of stabilization processes of the waste sludge is used in the treatment of domestic and industrial wastewater with high pollution load. Anaerobic digestion is an economic bioprocess, not only bring biogas product by removing organic matter in the waste or wastewater as a result of a number of microbial activities, but also plays a key role in the recycling of nutrients and the reduction of greenhouse gas emissions in renewable energy production and pollution control. The biogas produced by anaerobic digestion increases the methane content and the quality of biogas increases proportionally. However, biogas also contains carbon dioxide, hydrogen sulphide, nitrogen, particulate matter and water vapor. The main objective of the anaerobic digestion process is to reduce sludge stabilization and pathogens, but it also reduces the costs by meeting a significant portion of the electricity by heat and power generators that needs of the wastewater treatment plants through biogas. Although these processes may seem simple, bacteria that are involved in the anaerobic digestion process work indirectly or directly with each other, and there are many parameters that affect these processes. In these anaerobic processes, there are four distinct phases in which the organic matter is degraded as far as methane production is concerned. In the first stage called hydrolysis, carbohydrates, lipids and proteins in the complex structure in organic matter are broken down by extracellular enzymes into simple sugars, long chain fatty acids and amino acids. The products of the hydrolysis step are then acidified with acidic bacteria to organic acids and hydrogenated fermenters. In the third step, acetogenic bacteria convert volatile organic acids to acetates and hydrogene. At last, methane is the final product of anaerobic digestion. Methane formation occurs in two ways; methane and carbondioxides with acetate, asidoclastic methanogens, hydrogen and carbon dioxide with hydrogen consuming methanogens. Approximately 70% of the methane gas in the biogas is produced over acetate, and 30% is the result of the synthesis of H2 and CO2. By modeling studies, a number of achievements such as performance of treatment plants, estimation of the efficiencies, development of control systems are obtained. In describing and analyzing the processes, instead of qualitative expressions, technical and clear expressions such as "20% more efficient" can be expressed instead of simple approaches such as "this system is more efficient than the other system" by quantitative or numerical values. Thus, it is possible to reduce risks, reduce costs, and evaluate possible scenarios for development by ensuring that the planned system to be installed by using the model is provided in advance. The pollutant parameters that characterize the input wastewater in wastewater treatment plants have relatively low values and have a direct effect on biogas yield. The actual scale plant discussed in this study performs with a pollutant load of about 30% less than the COD load considered in the design. The planning and design of wastewater treatment systems is based on a fairly comprehensive and complex balance of predicting calculations close to actual conditions. The basic design factors in question need to address many criteria such as population, flow rate, wastewater characteristics, installation site and the discharge standards brought by the legislation. The correct design of the design load in the plants must be in clear terms, such as equipment selection, energy and chemical requirements, process configuration selection, and a proactive approach to the different situations that can be encountered. For this reason, the values considered in the design should be able to tolerate situations that will vary such as flow, concentration, different pollutant loads. Although the main purpose is to use the carbon content in the incoming wastewater to provide discharge limits, when carbon monoxide is integrated into the system, which is an important source of anaerobes for biogas production, carbon must be efficiently used in the system. The simulation of the performance analysis of the plant showed that discharge parameters such as COD, TSS and TN could be easily provided below the discharge limit values. The sludge from the primary sedimentation unit is more suitable than the secondary sludge from the biological treatment process for biogas production by anaerobic digestion. For this reason, the biogas obtained by anaerobic digestion was found to highly weighted over primary sludge. The reason for this is that secondary sludge often contains fewer readily degredable organic substances. 30% of COD removal after primary sedimentation and effluent discharge limits values are well-provided. The efficiency of primary sedimentation can be increased by diverting a part of recycled activated sludge into the influent. However, the physical constraints such as sedimentation efficiency should be evaluated in detail. Moreover, the retention time of primary sedimentation basin is limited. For the efficient use of carbon source for biogas production, it is necessary to determine the COD fractions of wastewater. The results of calculations and simulation studies show that anoxic zones operated in nitrate-limiting conditions. The basically means that more carbon source would be available for additional biogas production. It is suggested to use the denitrification configuration to use less carbon source of anoxic reactors. Anoxic zone is occupied 25% of total volume of BNR tanks. It may be possible with an anoxic volume share as high as 50%. The total SRT of the activated sludge system was calculated to be around 22 days whilst three parallel lanes of activated sludge were in operation. The SRT could be lowered in order to increase the amount of excess sludge, in turn, biogas generation from anaerobic digesters unless disturbing the discharge limits. The required minimum aerobic sludge age should be re-adjusted after actual respirometric measurements of nitrifiers net-growth rate on site. The model simulation study showed that SRT is around 25 days the two digesters running on parallel has 50 % VSS destruction. It has been found that the production of the biogas is 4.979 m3/day. Furthermore, assuming that all the organic matter in the digester could removed by a 33% increase in biogas production up to 6.622 m3/day of biogas will be obtained in the plant. Wastewater characterization is considered an indispensable step in providing all the necessary information in modeling operations. Basically, COD fractions should also be considered on the basis of significant kinetic and stoichiometric values. These fractions selected in mathematical models should be consistent and reliable. Thus, by determining the inert, easy or slow decomposable quantities of organic and inorganic pollutants, detailed information about the pollution character can be provided to clarify the result of purification performance. The use of a general model of activated sludge model systems, including carbon removal, nitrification and denitrification, as well as removal of biological phosphorus, will facilitate the simulation work of systems. An important feature of the general model application is the application of single set kinetic and cytokiometric model parameters for all systems. In this thesis, the anaerobic sludge stabilization unit, which is planned to be established, contributes to the process and working conditions of the integrated wastewater treatment plants, estimates the yields and provides data to better understand the operating conditions and studies the model on the real scale wastewater treatment plant. In this context, the plant's four-month dynamic data is provided to assess the process efficiency and the forecast conditions are compared with the existing conditions based on the model included in the SUMO program. The results of the simulation obtained are found to reflect the actual conditions. Thus, the use of models and simulation tools has been found to be highly functional prior to the establishment of such plants. High molecular weight, colloidal or particulate organic substrates can not be used directly by microorganisms. These substrates can be degraded under hydrolysis process with extracellular enzymes, and this step can be said to be a rate limiting step since it is a slow process in the anaerobic digestion process. Substrates that are formed by the death or lysis of heterotrophic microorganisms can also be used by other anaerobic microorganisms after hydrolysis. Changes in the rate of occurrence of these events affect the rate and degree of anaerobic digestion and increase methane production. As anaerobic death rate is increases, biogas production increases proportionally as cell disruption increases. In addition, an increase in the rate of anaerobic hydrolysis has been shown to affect biogas production positively. No significant increases in biogas production have been observed as the simulated 0.20 value increased by 0.50. However, when anaerobic hydrolysis rate correction factor is lowered to 0.50, which is a reference value of 0.10, a sudden decrease in the amount of biogas is observed between 0.10 and 0.20. This shows that the hydrolysis is a rate limiting step.