Aktif çamurda çoklu substrat gideriminin modellenmesi
Modelling of multiple substrate removal in activated suludge process
Yazar: GÜLEN EREMEKTAR
Danışman: PROF. DR. OLCAY TÜNAY
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Çevre Mühendisliği Ana Bilim Dalı
Derece: Doktora, 1997
Konu: Çevre Mühendisliği = Environmental Engineering
Dizin: Aktif çamur = Activated sludge ; Substrat = Substrate

Özet:
ÖZET Bu çalışmada aktif çamur sistemlerinde çoklu substrat gideriminin incelenmesi ve değerlendirilmesi yapılarak bir modelleme yaklaşımı oluşturmak hedeflenmiştir. Birinci bölümde, aktif çamur proseslerinde çoklu substrat giderimi modellemesinin önemi vurgulanmış, çalışmanın amacı açıklanmıştır. İkinci bölümde çoklu substrat giderimi mekanizmasını açıklamak ve konu ile ilgili tanımlan ortaya koymak amaçlanmıştır. Bunun için önce çoğu fiziksel ve matematiksel olarak ifade edilmiş ve deneysel olarak geçerliği büyük ölçüde tanımlanmış kavramlara yer verilmiştir. Sonra çoklu substrat giderim mekanizması ile ilgili kavramlar açıklanarak, literatüre geçmiş çalışmalar ve geliştirilen modeller anlatılmıştır. Üçüncü bölümde, aktif çamurda giderilmesi çoğalma adımı ile kısıtlanan ve çoklu substrat gideriminin mekanizmaları cinsinden birbirleri ile etkileşimi olmayan substratlar esas alınarak oluşturulan model tanımlanmış ve irdelemesi yapılmıştır. Dördüncü bölümde, önerilen modeli değerlendirmek için yapılan deneysel çalışmalar verilmiştir. Deney çalışmaları sonucunda sistemlerin Monod sabitleri belirlenerek modele uygulanmıştır. Ayrıca önerilen modelin değerlendirilmesi önce teorik bazlı yapılmış daha sonra yapılan deney sonuçlan ile uyumluluğu belirlenmiştir. Beşinci bölümde teklif edilen model ile elde edilen sonuçların değerlendirilmesi yapılarak öneriler getirilmiştir. XV

Abstract:
SUMMARY MODELLING OF MULTIPLE SUBSTRATE REMOVAL IN ACTIVATED SLUDGE PROCESS In an era of rapid industrilization today, water contamination (pollution) have become an important environmental problem. Industrial wastewaters which are greatly responsible for the water pollution are purified by widely used biological purification systems. However, about the modelling of these systems, problems become inevitable. These are mainly sourced from the sight that defines the systems as a whole. Although, there are many scientific works about the systems investigated from the specific substrates point of view, limited works are present for certain practices. Hence, this work is planned to make a modelling approach about multi-subtrate selection both for enlightenment of the system and its usage for the practice. The details of the work are focused on the investigation and research of multi-substrate removal from the activated sludge system, and this make a modeling approach. The mechanism of substrate utilization by a bacterial cell can be generally described a sequence of three complex processes: contact of a cell with the molecule of a substrate; transport of the molecule into the cell; and intermediate metabolism of the substrate. Large or sterically incompatible molecules which cannot be readily transported into a cell have to be first broken down or transformed externally to transportable fractions by exoenzymes or wall bound enzymes. On the basis of the described general mechanism it is practical to classify various types of substrates into three groups: single component substrates, which are directly transportable into the cell; multi-component substrates, which are represented by a mixture of several single substrates; and complex substrates which have to be changed externally prior to the transportation into the cell.(Grau.,et.al.1975). The bacteria in activated sludge convert complex particulate and dissolved organic matter into low-molecular weight compounds by extracellular hydrolytic enzymes. The low-molecular compounds are subsequently taken up by the cells and used as energy and carbon sources in cell metabolism Hence, enzyme systems likely to reflect microbial activity would be either enzymes participating in the extracellular hydrolysis or key enzymes in the cellular oxidative metabolism. (Nybroe et.al.1992). When a micro-organism is presented with several carbon sources simultaneously, growth will take place first on the best carbon source, then on the second best, and so on. This growth pattern was termed diauxie by Monod (Monod, 1949). Mechanism responsible for this phenomenon has been shown to be the prevention of the synthesis of inducible enzymes (required for the catabolism of the less) favoured carbon sources. This effect known originally as the glucose effect, has been demonstrated to be caused not by glucose itself but by metabolite arising from the catabolism of the readily available carbon sources, and has been caller more appropriately catabolite xvirepression. Another possible mechanism responsible for diauxic growth is catabolite inhibition in which the function rather than the synthesis of catabolic enzymes, is inhibited by either carbon sources or their intermediary catabolites.(Chian and Mateles 1968). The pattern of diauxic growth in batch culture represents a sequence in which growth takes place successively on one or more substrates, all of which are present at the beginning of the growth process but are consumed successively during the growth period (Gaudy et al., 1963). On the other hand, if two or more substrates are contained in the medium feed in continuous culture, the situation differs from the batch case in that the microorganisms are always exposed to all substrates and might attack only one or another, depending on the growth or dilution rates.,. Either enzymes necessary for the utilization of various substrates are not synthesised by the microbial cell in the presence of a repressing substrate (enzyme repression) or these catabolic enzymes are formed but remain inactive termed enzyme inhibition (Stumm- Zollinger, 1966) The effect of the presence of conventional organic substrates on the biodegradation of toxic waste components is of great practical importance. One of the major problems currently confronting environmental engineering and science professionals is the biological destruction of toxic organic chemicals in waste treatment facilities, polluted groundwater, hazardous waste sites, and the like. In these situations, it is inevitable that the toxic, or inhibitory, components will be found in mixtures with non-toxic, or conventional wastes. When alternative carbon sources are presented to microbial populations, a plethora of possible substrate interactions can occur. Interactions such as diauxie have been reported by a number of investigators for cells utilising glucose as an alternative carbon source. (Rozich and Colvin 1986). In the metabolism of multiple substrates, frequently reported degradation patterns include diauxie, simultaneous utilization and competitive inhibition. Diauxie or sequential utilization results from catabolite repression or catabolite inhibition. Simultaneous utilization of substrates may or may not result in growth. Simultaneous utilization of substrates that do not support growth İs termed cometabolism. As defined by Dalton, cometabolism is the transformation of a nongrowth substrate in the obligate presence of a growth substrate or another transformable compound (Dalton and Stirling, 1982). More generally, cometabolism also includes transformations by resting cells if no growth results (Chang et.al.1993). According to diddle, cometabolism is defined as transformation of a nongrowth substrate by growing cells in the presence of a growth substrate by resting cells in the absence of a growth substrate, or by resting cells in the presence of an energy substrate. A growth substrate is defined as an electron donor that provides reducing power and energy for cell growth and maintenance. Many cometabolic enzymes and cofactors are induced by growth substrate, although cometabolic agents may also be induced by other factors, or they may be produced constitutively. An energy substrate is defined as an electron donor that provides reducing power and energy, but does not by itself support growth. Although the term cometabolism has traditionally been applied to oxidation, many reductive transformations also satisfy the definition of cometabolism given previously, i.e., they depend upon the concurrent or previous utilization of a growth or energy substrate, and their transformation does not appear to facilitate growth. Many one and two carbon halogenated compounds are coreduced by xviisulphate-reducers, methanogens, facultative anaerobes, aceto-bacteria and Clostridia (Criddle, 1993). Microbial growth is an efficiently regulated system of thousands of chemical reactions. Some of these regulatory processes become readily apparent when the growth takes place in a multisubstrate environment. Microbial growth on multiple substrates exhibits a variety of behavior ranging from simultaneous utilization of all substrates to sequential utilisation of substrates, resulting in multiexponential growth phases. Concept of a cybernetic framework for modeling the growth of microorganisms on multiple substrates have been introduced by Kompala and others (Kompala et al., 1986). This approach differs from kinetic modeling efforts by incorporation of the optimal nature of microbial regulatory processes. It was established therein that cybernetic models can describe most of the available experimental observations of microbial growth on mixed substrates with quantitative accuracy. Thus, both batch and continuous culture data could be predicted by suitable choice of model parameters. However, the basic premise, the most important attribute of Kompala' s modelling effort, that information obtained from growth on single substrate experiments on each of the substrates will yield all of the information required for predicting growth in mixed substrates had remained to be verified. Basic postulates have been presented by Kompala as follows: given multiple substrates, on which growth rates are distinctly different the microbes prefer to grow on the fastest substrate with the growth curve showing multiauxial behavior; more generally, the growth behavior ranges from simultaneous utilization of multiple substrates (which occurs when the growth rates are very nearly the same) to sequential utilization with intermediate lag periods; the growth rate on a mixture of substrates is never greater than the maximum of the growth rates on individual substrates; while growing on a slower substrate, if a faster substrate is added, the microbes inhibit the activity of the already enzymes for the slower substrates. In natural ecosystems one would not expect complete dominance of one organism by competitive elimination to occur, as the organisms may not be solely growth limited by one substrate alone. The substrate level may be more complex than this simple case, and include mixed substrates and/or multiple growth-limiting substrates. Relatively few experimental studies have been concerned with competition for mixed substrates by mixed microbial populations. In this case the kinetics of substrate utilization is governed by not only the number and type of substrates, but also by the interactions among the various microbial species. Owing to competition for nutrients among organisms, shifts in the species structure of the mixed population may occur. The behavior of mixed organisms on mixed substrates depends on the similarly in the organism's affinities for the mixed substrates. If two organisms have closely overlapping preferences for a growth-limiting substrate, they should compete severely for this common nutrient source, and this may result in the exclusion of the inferior organism from the system (Rozich and Colvin, 1986). When there is more than one substrate present as a nutrient source for growth of a micro-organism, the nutrients will, in general, be utilised sequentially. The effects of mixed substrates on enzyme synthesis and activity in microorganisms have been studied in pure culture of various species of microorganisms, where it has been found xviiithat physiologically two different responses can be distinguished. One is catabolite repression, the other one is catabolite inhibition. In batch culture, the growth of microorganisms may take place on one or more substrates. Initially, all of the substrates are present and are utilised concurrently or successively during the growth period. In continious culture, the growth pattern is different from that of the batch case because of the existence of much lower steady-state concentrations of substrates, and catabolite inhibition and repression effects may be absent or reduced. In this study, a model in the presence of more than one substrate and a group of micro-organism is suggested. The model is related to the substrates which affect from each other and their desire to use up the enzyme present in the media. If the two substrates presence in the media equation is given as follows: H=U,nl Si 1 + - |+Si V Ks2. Kb + 1-W S2 Ks2 V Ksi. General equation: n-Z- yUini.Si t=l Jxsi + Si + JVsi u = specific growth rate; um = maximum specific growth rate; S = substrate concentration; Ks = half saturation coefficient. An experimental work was conducted to support suggested multiple substrate model given above. The aim of the experimental study is to verify the validity of the proposed multi-substrate model by the aid of the experimental data. In the experimental study, glucose, benzoic acid and their mixture were used to investigate the effect of multi-substrates on the substrate removal mechanism. The reason for the selection of the glucose and benzoic acid was that their non-interacting properties. The experiments were conducted in batch activated sludge systems. Specific parameters such as glucose, benzoic acid and OUR (oxygen utilization rate) were measured to monitor the system besides conventional parameters such as COD and VSS. All measurements were in accordance with Standard Methods (APHA, 1989). Three reactors fed with glucose, benzoic acid and glucose - benzoic acid mixture were operated under different F/M ratios for the asssesment of kinetic coefficients. xixAssesment of the kinetic coefficients were made by using two differet methods: Levenspiel method (Levenspiel 1980) and respirometric methods (Ekama et a!., 1986; Kappeler and Gujer, 1992). The results obtained in both methods; Levenspiel and respirometric, show a similar trend. The kinetic coefficients determined in batch tests on total COD basis are as follows: For glucose, u^ = 3/day"1, Ks = 10 mg/1; for benzoic acid jim = 3.5/day"1, Ks = 7 mg/1 and for glucose - benzoic acid mixture H(G+B)m = 3.8/day"1, Ks = 8 mg/1. Theoretical evaluation and verification of the model is conducted on the data obtained by Levenspiel Method. In this context, the removal curve of the model is evaluated to be between the glucose and benzoic acid curve can be explained as one substrate will accelerate the other substrates' removal or they all accelerate each others removal. The following results can be drawn from the evaluation of the results obtained with the suggested model. The removal rate of substrates which are present in a mixture is always smaller than that of present singly. According to this, multi-substrate removal will be encountered as a lower rate process compared to single substrate removal. If one of the substrates is removed much faster than the other, then the removal rate of this substrate will be dominant in the system, as also shown in the theoretical elaboration. In contrast, when the removal rates of substrates are closer to each other, then, depending on the kinetic coefficients, important reductions can be observable in their removal rates. In this work, according to pre-determined aim a systematic examination and evaluation of multi-substrate removal in the activated sludge system is made. Under certain conditions a model which has a high potential for applicability is developed. Its use is elaborated and a number of proposals about the topic is made.. The mechanism of substrate removal in activated sludge is taking place during chemical and physical changes and conversions. Processes like adsorption, hydrolysis, storage and assimilation are evaluated because of their roles in the substrate removal. According to this evaluation, except hydrolysis process, it was shown that the use of substrate for growth is a basic and limiting mechanism.. In the case of basic mechanisms of growth, removal rate can be completely defined as the interactions among the substrates and their removal rates with respect to each other.. The interactions among the substrates are collected in four sub-titles; catebolite repression, catebolite inhibition, cometabolism and inhibition. Out of these, substrate inhibition is another way for the removal of a substrate.. According to explanations given above, the developed model is based on the substrates whose removal, in the activated sludge, is limited with a growth step. Also, these substrates do not interact with each other according to mechanisms mentioned above. xx