Determination the effect of sludge age on system kinetics and biological treatability in activated sludge processes

Activated sludge systems have been the foundation of the biological wastewater treatment since more than 100 years all over the world, and these systems are still among most commonly used processes for wastewater treatment. In the design step of wastewater treatment plants or the optimization of existing wastewater treatment plants; biochemical processes of microbial population such as hydrolysis, growth and endogenous decay must be well-defined, and system kinetics and design parameters must be verisimilar to the real conditions, in order to maximize the treatment performance and minimize the operation and maintenance cost. Currently, kinetics of activated sludge systems are determined by laboratory scale experimental studies supported by model-based simulations, yielding kinetic and stoichiometric coefficients, which are then used as a basis for wastewater treatment plant design. Moreover, existing wastewater treatment plants are also revised by using the results of these studies. Not only the system kinetics, but also variable wastewater characteristics and the mass balances changing with design parameters must be integrated to the modeling studies as a holistic approach to obtain the most accurate results for the design or revisions of wastewater treatment plants. In recent studies, oxygen uptake rate (OUR) profiles are widely used to determine the kinetics of growth and hydrolysis mechanisms. However, slowly biodegradable organic matter accumulating in activated sludge reactors leads to increase in OUR levels and it becomes difficult to estimate the biomass activity and kinetic coefficients. Therefore, while conducting respirometry based modeling studies slowly biodegradable organic matter accumulation should be taken into consideration and real system simulations should also be conducted parallel with obtained kinetic coefficients. This approach also enables determination of active biomass concentration, which has a significant effect on parameter estimation. Hence, more realistic biomass composition can only be determined with this holistic evaluation. With this approach, operational and control parameters could be monitored due to the dynamic determination of influent and effluent characteristics of the plant and operational cost could be minimized by system optimization. COD fractionation of wastewater being treated are significant components of the activated sludge models. This fractionation provides the identification of organic matter with different biodegradation properties and necessary experimental support to the model for the accurate estimation. In terms of biodegradation characteristics, total influent COD has two major components: the total nonbiodegradable COD or inert COD, CI, and the total biodegradable COD, CS. The inert COD fraction may be further subdivided into two components, soluble inert COD, SI, and particulate inert COD, XI. Since the soluble inert fraction of influent wastewater bypasses the system, effluent quality is significantly affected by this fraction. In this respect, assessment of the inert fraction of the organic matter is very important because it indicates indirectly the other main fraction – biodegradable organic matter, in other words substrate available for growth mechanism of the microorganisms. The biodegradable fraction of COD, CS, may also be subdivided into two major fractions which have been identified as readily biodegradable COD, SS, and slowly biodegradable COD, XS. Experimental observations show that there is a significant difference between the biodegradation rates of these two components. In the context of this study; laboratory experiments were conducted with the samples taken from a large-scale municipal wastewater treatment plant, which is operated with the same influent wastewater and two different sludge ages. Firstly, inert COD fractions of the influent wastewater COD were determined by experimental setup which involves running two aerated batch reactors, one fed with the total COD (CT) and the other with the soluble COD (ST). By measuring the final threshold soluble and total COD values of the reactors, inert COD fractions of influent wastewater were calculated. As a result of this experimental study, the soluble inert COD fraction (SI) of influent wastewater was calculated as 8-9%, while the particulate inert COD fraction (XI) was calculated as 5-6%. After these experiments, biodegradable COD fraction (CS) of influent wastewater was determined as 86%. In the second experimental part of the thesis, the biodegradable fractions of the influent wastewater and the system kinetics were defined by respirometric analysis for both systems. In this study, maximum heterotrophic growth rate, 𝜇̂􀯁, was determined as 6 day-1 and 3.7 day-1 for the systems with the sludge age of 3 days and 9 days, respectively. This result was compatible with literature and shows the decreasing trend of the heterotrophic growth rate with increasing sludge age in activated sludge systems. In the modeling step, integration of both systems were provided using the kinetics and the performance data obtained from the existing plant, and the fractions of effluent and sludge streams were determined at steady-state conditions. Therefore, a municipal wastewater treatment plant operated with two different sludge ages was studied holistically by the modeling studies using both conventional measurements and respirometric analyzes. As a result of the holistic evaluation of wastewater treatment plant, hydrolysis rates of the slowly biodegradable organic matter in the system working for carbon removal with the sludge age of 3 days and 9 days are found as 1.5 day-1 and 2.5 day-1, respectively. According to the results of the modeling studies conducted using SUMO Wastewater Process Simulator software, the accumulation of slowly biodegredable organic matter in the sludge of biological treatment step of the plant (θX = 3 days) was determined as 16.6%. The similar accumulation rate in the advanced biological treatment step of the plant (θX= 9 days) was found as 4.1%. These results were also verified by using the mass balance equations For carbon removal system with the sludge age of 3 days, equations gave that 84% of the influent slowly biodegradable organic matter was removed, while 16% of it had accumulated in the activated sludge. The same calculations showed that 96% of the influent slowly biodegradable organic matter was removed in the system with the sludge age of 9 days, while 4% of it had accumulated in the sludge. It was concluded that an increase in the sludge age led to an increase in hyrolysis rate and the removal rate of slowly biodegredable organic matter was higher in the system with higher sludge age. Therefore, in the activated sludge systems operated at low sludge ages, accumulated slowly biodegredable organic matter is being removed from the system with excess biological sludge. The accumulation of slowly biodegredable organic matter in the sludge causes problems in determination of active hetetrophic biomass of total sludge. In the scope of this thesis, the content of the study is presented by giving the significance and aim of the study in the first and second chapters. The third chapter includes the kinetic models and relevant mathematical equations which are the basis of this study, including the state of the art. The material and methods that are used for the experimental and modeling studies are given in detail in the fourth chapter. Finally, the results and the discussions of the study are given in the fifth and the sixth chapters, respectively.