Aktif çamur sistemlerinde çözünmüş oksijen parametresinin uygulamaları

Abstract

Bu çalışmada, aktif çamur sistemlerinde çözünmüş oksijen parametresinin uygulamaları araştırılarak kesikli reaktör sistemleri için değişik model yaklaşımlarına göre simülasyonlar yapılmıştır. Tez çalışmasının anlam ve önemi ile çalışmanın amaç ve kapsamının anlatıldığı birinci bölümde oksijen ile ilgili parametreler ve proses bileşenlerinin ilgili model yaklaşımlarının yardımı ile nasıl hesaplanabileceği izah edilmiştir. İkinci bölümde aerobik prosesler tanımlanmış ve bu gruba giren, aynı zamanda tez konusu içinde yer alan aktif çamur sistemleri hakkında bilgiler verilmiştir. Sistemlerin uygulandığı reaktör tipleri hakkında ayrıca açıklamalar yapılmış, havalandırma ile arıtma ortamına çözünmüş oksijen kazandırabileceği düşüncesinden hareketle bu yöntem hakkında ilave bir bilgi verilmiştir. Ayrıca artıma prosesine bağlı çamur oluşumu ve bu olaya etki eden faktörler hakkındaki açıklamalar da bölüm içinde yer almıştır. Üçüncü bölümde aktif çamur sistemleri için geliştirilen model yaklaşımlarının neler olduğu ve nasıl uygulandığı açıklanmış, her bir model için ilgili denklemlerin oluşturulmasında kullanılan matrisler verilmiştir. Model yaklaşımları açıklanırken kesikli reaktör sistemlerinde yapılacak simülasyonlar için bir fikir verebilme amacı esas alınmış, ayrıca modeller birbirleri ile dolaylı olarak kıyaslanmıştır. Model çalışmalarının değerlendirildiği dördüncü bölümde her bir model yaklaşımı için bulunan sonuçlar tablo ve grafikler halinde verilmiş ve sonuçlar kolay ayrışabilen substrat bazında zamana bağlı olarak biokütle değişkenleri, çözünmüş oksijen konsantrasyonu ve oksijen tüketim hızı değerleri için gösterilmiştir. Hesaplamalar esnasında hangi model için hangi kinetik ve stokiometrik katsayı değerlerinin kullanıldığı da ayrıca belirtilmiştir. En önemli değişken olarak değerlendirilen çözünmüş oksijen konsantrasyonunun belirli limitlerin dışına çıkmamasına dikkat edilmiştir. Sonuçlar ve önerilerin yer aldığı en son bölümde tez çalışması boyunca yapılan simülasyonlar ve hesaplamalar doğrultusunda bulunan sonuçlara bağlı olarak çeşitli yorumlar yapılmıştır.

Activated sludge systems are evaluated in the group of aerobic processes where all process events occur by oxygen. For an activated sludge reactor, aeration is used for providing dissolved oxygen concentration in the wastewater. Dissolved oxygen is necessary for the activities of microorganisms in degredation of the organic substances and nitrification. The aeration methods are; 1) Diffused aeration 2) Mechanical aeration 3) Turbine aeration 4) Pure oxygen aeration In design conditions the activated sludge systems are divided into three categories; Mixing regime (plug flow, complete mixing), Flow sheet (aeration only, aeration and sludge return, step or series aeration, Fill / Draw), Loading level (high rate, conventional, contact stabilization, two stage aeration, extended aeration). The main activated sludge reactors are considered as complete mix, plug flow and batch. Complete mixed reactors are designed as a continous flow and feed back where biological solids recycled. The last method is also used for the plug flow reactors. The reactor types are Fill / Draw and sequencing batch reactors in batch systems. Components of all reactors are calculated by the substrate and microorganism balances which are applicable to the system concepts. A general system balance is as follows; REACTANT REACTANT REACTANT REACTANT REACTANT ACCUMULATION = FLOW IN - FLOW OUT + PRODUCED - LOST The particles entering the tank are immediately dispersed throughout the tank in a complete mixed reactor system. The particles leave the tank in proportion to their statistical amount. In the continous flow systems, the microorganism concentration in the influent is zero. For the feed back system, influent microorganism concentration exists because of the biological solids recycle. In plug flow reactors, fluid particles pass through the tank and are IX discharged in the same sequence in which they enter. The particles retain their identity and remain in the tank for a time equal to the theoretical detention time. Batch reactors are non-steady processes in their own category, design criteria of the reactors depends on process conditions. There are four different approaches in activated sludge modeling. The first approach is the Classic model and it is the basic model concept for substrate and biomass parameters. Classic model assumes that system has a substrate parameter (S) which contains all degradable organic compounds and biomass parameter (X) which contains the volatile suspended solid fractions, but the active amount of the biomass and the substrate components are not clear. Model equations are written by using Classical model matrix. Endogenous Decay model is conveniently described as an overall decrease in the mixed liquor volatile suspended solids concentration in conventional models, by means of a decay coefficient. When the concept of viability is considered, Xh represents the active or the viable part of biomass and Xp represents the particulate inert organic products. A fraction of the active biomass does not undergo any further reaction during the process and accumulates in the activated sludge. The degradable biomass concentration is 77 percent of the total biomass. Viable biomass in the activated sludge systems depends on the existence of volatile suspended solids in the wastewater and it is oxidized by aeration until the fixed degradation fraction. At the end, the total biomass is equal to the sum of the endogenous decay rate of the active biomass and generation rate of the particulate inert products. This model also assumes two extra biomass fractions; Xs represents the slowly biodegradable particulate fraction of biomass as influent origin. Concentrations of the organic components are taken as COD either in Classic model or in Endogenous Decay model to form their own matrix. Substrate parameters are Ss (readily biodegradable), Si (inert from the influent stream) and Sp (soluble microbial products). By using the ideal kinetic and stokiometric values, the IAWPRC Task Group model assumes that the process is effective to the parameters of the activated sludge system. According to this model, the system has soluble and inert components. The ototrophic and heterotrophic biomass parameters with substrate concentration and dissolved oxygen concentration parameter are main elements of the IAWPRC Task Group model. Organic component values are taken as COD in the model matrix and the components which are proportional to the process time are assumed as positive, the components which are inversely proportional to the process time are assumed as negative. All of the model equations are written as a mass balance. IAWPRC Task Group model describes the oxygen component as an electron acceptor. As a result of this theorem, the main principle is to calculate the activated sludge concentration and the electron acceptor demand. Dissolved oxygen concentration decreases to zero during the treatment process. To avoid this error, a proportion coefficient which is related with dissolved oxygen concentration value is added to the model equations. The proportion coefficient is symbolized as So / (Ko + So), So represents the dissolved oxygen concentration and Ko represents a value which is equal to 0,2. In the Death Regeneration model, death or loss of viability of the microorganisms is first postulated to occur without any utilization of electron acceptor. A major part of the non viable biomass becomes available as Xs (slowly biodegradable substrate) while the rest remains as Xp (inert endogenous mass). Inert fractions have no effect on the process and the active Xh biomass is the main fraction for the readily biodegradable substrate. Dissolved oxygen is only used for growth. Loss of viability or death of heterotrophic biomass is assumed in the treatment while the matrix of the model is formed. The only step using electron acceptor is regeneration and the slowly biodegradable substrate is subsequently converted into a readily biodegradable form by means of hydrolysis and passes through the same growth cycle as its counterpart in the influent. The simulations of the activated sludge systems must contain the basic processes of the overall system and it must be checked if the treatment process is the main parameter of the system components or not. A good simulation could be done after these steps. In this thesis, basic processes were determined first of all and the ideal kinetic and stokiometric values were assumed. All of the values of the organic components were given as COD equivalence to simplify the stokiometric coefficients. The models explained in Chapter 3 were used for calculating the substrate, biomass and oxygen components of the BATCH systems. Computer programs were written by using the RUNGE-KUTTA method and the results were obtained in every 0,5 step of process time. The ideal dissolved oxygen concentration in reactors is between 4 and 5 mg / 1, but the values changing from 3 to 10 mg/ 1 in treatment process were assumed as normal. The oxygen utilization rate values were evaluated after the calculations. The kinetic and stokiometric coefficients given below were used for calculating the Classic model results in substrate, biomass, dissolved oxygen concentration and oxygen utilization rate parameters. Influent substrate concentration was assumed 100, 200, 300, 400 and 500 mgCOD/l respectively. Results were found for one type of substrate and one type of biomass as Classic model assumes. XI Table: 1. Kinetic and Stokiometric Coefficients of the classic Model Endogenous Decay model results were found by using the kinetic and stokiometric values given below. Hydrolysis coefficient was changed between 1 and 10 ( 1/day). Another calculation was also done for a model system where the influent substrate concentration is equal to zero. Process components were assumed as Ss=150 mg COD/ 1, Xs=200 mg COD/ 1, Xp=0 and Xho=500 mg COD/ 1. Observing the relation between the heterotrophic growth and oxygen utilization rate was the main purpose of calculations. It was found that readily biodegradable substrate concentration is increasing by hydrolyzation of Xs and the hydrolysis coefficient is increasing by hydrolyzation of Xs and the hydrolysis coefficient values over 2 (1/day) has no significant effect on the oxygen utilization rate. Table: 2. Kinetic and Stokiometric Coefficients of the Endogenous Decay Model IAWPRC Task Group model results were found by the same method which was used in Endogenous Decay model equations. Proportion coefficient was added to the equations and process components were assumed as XII Ss=150 mg COD/ 1, Xs= 200 mg COD/ 1, Xp=0 and Xho= mg COD/ 1. Kinetic and stokiometric values which were used are given in the table below. Existence of the readily biodegradable substrate concentration was assumed first of all and the results were found. In the second step, the readily biodegradable substrate concentration was assumed as zero. Table: 3. Kinetic and Stokiometric Coefficients of the IAWPRC Model Activated sludge is a complex system and a substrate accumulation will exist in the simulation of all model concepts. Substrate has soluble and particulate fractions, the soluble fraction is applicable to the Monod kinetics. Storage and hydrolys concepts are assumed for the consumption of the particulate substrate. But in the end, oxygen parameters will be more important to the all process components of the activated sludge system.