Havasız arıtmanın fizikokimyasal dengelerle modellenmesi

Abstract

Bu çalışmada, havasız reaktörlerin fîziko-kimyasal dengelerle modellemesi yapılarak elde edilen sonuçlar yardımı ile havasız arıtma uygulama çalışmalarına kolaylıklar sağlanması hedeflenmiştir. Birinci bölümde tezin konusu ve kapsamı açıklanarak, çalışmanın önemine değinilmiştir. İkinci bölümde, havasız arıtmada ve çeşitli substratlann fermantasyon özellikleri anlatılarak, avantaj ve dezavantajları ile çevre şartlarının anaerobik süreçlere etkileri açıklanmıştır. Üçüncü bölümde, anaerobik reaktörlerin modellemesi ile ilgili daha önce yapılan çalışmalar anlatılmıştır. Dördüncü bölümde, havasız antma sistemlerinin modellemesi fîziko-kimyasal dengelerle kurulmuştur. Mikroorganizma ve substrat konsantrasyonu arasındaki ilişkiyi belirlemek için Monod kinetiği anlatılmıştır. Ayrıca laboratuvar çalışması ve oradan çıkan sonuçlarla hesaplanan katsayıların bulunması anlatılmıştır. Anaerobik antma modellemesinin bilgisayar programlan da bu bölümde anlatılmıştır. Beşinci bölümde ise sonuçlar özetlenmiştir.

Anaerobic digestion is a process which has been included in both municipal and industrial waste water treatment process trains since the nineteen thirties. It is an effective method for stabilizing concentrated biologically decomposable organic wastes. Moreover, digestion does not require aeration equipment such as blowers or mechanical aerators. Anaerobic decomposition of organic wastes is a stepwise process. Extracellular enzymes serve on important role in the initial solubilization of complex organic materials. However, in the fermentation of soluble industrial organic wastes this first step is bypassed The solubilized organics are subsequently decomposed by "acid forming bacteria" into short chain fatty acids, referred to as volatile acids, as well as carbon dioxide, hydrogen, and other minor products. Ultimately, the volatile acids are converted to methane, carbondioxide, and additional organisms by a generalized class of microbes referred to as the "methane forming bacteria". It is this conversion of volatile acids to methane and carbon dioxide that is considered to be the rate limiting reaction in the series. Although the formation of methane and carbon dioxide from complex organics is a series reaction, it is important to recognise that all three steps occur simultaneously in a plant-sized digester. An additional adventage derived from the digestion process is the production of a useful by-product, methane. Approximately sixty-five percent by volume of the digester gas is methane while the balance consists mostly of carbon dioxide. The anaerobic metabolism of a complex subsrate, including suspended organic matter, can be regarded as a three step process: l.step: Hydrolysis of suspended organics and soluble organics of high molecular weight. 2.step: Degradation of small organic molecules to various volatile fatty acids, ultimately acetic acid. 3.step: Production of methane, primarily from acetic acid but also from hydrogen and carbondioxide. Xlll Of the three steps, the second one is rather quick, while the two others are slow. This accounts for many instability problem encountered in anaerobic processes. Basicly, however, the anaerobic processes are not more unstable than aerobic, ones. One of the reason why this is a rather rare view, is that engineering design practise for anaerobic processes through the years have been operating with rather small safety factors and very poor process control. In anaerobic sludge treatment the solids retention time, 0C, and the hydraulic retention time, 0, are almost identical. The ratio 0C 1 0 might be increased to 1.5-2, due to withdrawal of digester supernatent. In anaerobic wastewater treatment, however, the ratio between solids retention time and hydraulic retention time can be increased to 10-100. This reduces the reactor volume considerably and makes anaerobic wastewater treatment economically interesting as compared to aerobic processes. For anaerobic processes the cost of aeration increases with increased concentration of organic matter, and above some 5-10 kgCOD/m3 the system becomes oxygen transfer limited, which results in increased hydraulic retention time in order to ensure sufficient oxygen supply to the process. For such industrial wastes, anaerobic treatment has long been economically attractive. Anaerobic digesters are complicated systems which may be approximated to physico-chemical systems (liquid + gaseous environment) that interact with biological systems (biomass and related metabolism). Defining a reliable model for a physico - chemical system is relatively easy because of the low number of variables, knowledge of parameter relationships and because interfering factors such as ionic strength can be taken into account. On the other hand, biological systems found in anaerobic digesters are difficult both to define and study because they comprise a great number of interacting microbial species and because metabolism and interactions between a species are far from being completely understood. Yet most anaerobic studies deal specifically with biological aspects while the limitations set by the physico - chemical system are hardly considered. This fact is partly understandable if one considers that most anaerobic studies carried out in the last two decades have been related to the mesophilc degradation of sludges with high natural buffer potential (organic nitrogen -» NH4HC03) which is very important in keeping environmental conditions relatively constant. Present emphasis on the anaerobic treatment of high and low buffer potential at temperatures ranging from 5 to 65 °C makes the study of physico -chemical system important both for the definition of the digester environmental conditions and for proces control. The complex biological system has been simulated by an ideal catalyst which is part of the physico - chemical system. The anaerobic digester is then modelled by a simple catalyst reactor where the substrate is converted to CH4,C02 insoluble compounds (to simulate biomass synthesis) and acetic acid (to simulate overload conditions). XIV The following assumptions are considered to hold valid for the digester model: 1. The digester is a completely stirred tank reactor. 2. An ideal catalyst convert substrate at rate independent of pH and temperature variations. 3. In-reactor catalyst consentration is constant. 4. Substrate is made of corbohydrates. Substrate convertion to CH4 and C02 is defined by the ratio a = moles CH4 1 {moles CH4 + moles C02 ) = 0.5 in general, a depends on substrate composition (C/O/H ratio) or on the oxidation state of carbon. 5. Fraction Y of the substrate is converted to insoluble compounds to simulate biomass synthesis. 6. To simulate imballance between acidfying and methanogenic populations the substrate which is not converted to CH4,C02 and biomass is discharged as acetic acid. 7. Gas and liquid phases are in equilibrium. 8. Steady state conditions prevail. 9. Influent inorganic carbon concentration and CH4 solubility in water are neglected. (CH4 solubility is low i.e. 14ppm by volume at 35 °C) 10. Cations M* are added to the influent as hydroxides. 1 1. The influence of ionic strength on activity coefficients are neglected. 12. Water vapour pressure is considered. Substrate (as C6HX206) and cation (as Na + ) concentrations in the influent, in- reactor acetic + acetate concentration and temperature were assumed as independent variables. pH and PCÛ2, were assumed as dependent variables. pH is important as methanogens have a narrow optimum pH range. Pco, is a relevent parameter for the determination of biogas quality and it is commonly thought that Pco variations are caused by the biological system. This possibility is not denied but this investigation is only concerned with the substential variations in Pco due to the physico - chemical system. It is considered that some or all of the following process variables should be monitored to insure an early warning of process upset. (1) rate of total gas production, (2) Gas composition, (3) volatile acid concentration, (4) pH, and (5) Alkalinity. XV Field testing the response of each of the variable to digester upset and failure would be difficult, if not impossible. However, a detailed study of the behaviour of all the common process variables utilized in the digester operation was performed using computer simulations. Organic, hydraulic, and toxic overloading were simulated. Moreover all of the process variables which have been used by plant operators as well as research workers to monitor the condition of a digester will ultimately register a sharp increase or decrease as a digester fails. It was possible to determine which process variables responded to the imposed digester overloading in sufficient time to permit corrective action to be taken. Digester process variables which were evaluated as potential process stability indicators. Rate of methane production, total substrate concentration, total volatile acids, %C02 in Dry Gas, pH, Bicarbonate Alkalinity, Organism concentration, Specific Growth Rate, Dry Gas Flow Rate, Total volatile acids: Total Alkalinity Ratio, Total Alkalinity, Dissolved Carbon Dioxide, Unionized Acetic Acid. Although all of the these variables yielded save process information, the first four offered the greatest potential from the standpoint of indicating stability. They demonstrate a definite response to the simulated organic, hydraulic and toxic digester overloading in sufficient time to allow an operator to sends the digester instability and implement corrective action. The simulated responses of (a) rate of methan production, (b) total substrate concentration, (c) percent Carbon Dioxide in the dry gas, and (d) pH to the three common types of overloading are presented in the figures below. Toxic overloading can occour when the contituents of certain industrial waste spills are pomped to a digester without being sufficiently diluted. The primary cause of hydraulic overloading in municipal digesters is an effective digester residence time which is too brief for a net organism growth. In effect the organisms are diluted from the reactor faster than they can reproduce. On the other hand, the rate of methane increases in the early stages of organic overloading and does not decline until process approaches failure. In effect, it provided an early warning of impending instability due to toxic overloading. The initial objective of this work was to construct a mathematical model for the anaerobic digested This model was programmed on a computer to evaluate process stability indicators and factors which increase digested stability. Mathematical modeling is an evolutionary process. Because of the inherent limitations in quantitavely describing the physical behaviour of process and operations, there is a continuous need to refine and improve a mathematical model. This model depicts the anaerobic digester as a continuous flow, completely mixed three-phase reactor in which organic compounds are biologically converted to methane and carbon dioxide. The model was formulated by first describing the reaction mechanisms and the coupling these expressions with the pertinent reactor equations for a conventional digestion tank. XV! An improved dynamic model of the anaerobic digester was presented which can be used for simulating digester design and operating conditions the simulations enable digester performance to be evaluated semiquantitatively under a variety of conditions. This form of analysis can frequantly reduce the amount of laboratory and pilot experimental studies by rapidly screening process alternatives. Alternatives which the simulations indicate as having the greatest potential can be further studied by pilot testing. The digester was modeled as a completely mixed continious flow reactor. Methane formation was considered as the rate limiting step. Therefore, the reaction kinetics were written solely for the volatile acid conversion step. Spesific growth rate was described by and inhibition function in which the unionized volatile acid acted as both the rate limiting and rate inhibiting substrate. The model included expressions for the interactions among the gas, liquid, and solid phases as well as a first order function for toxic dead. The model was programmed on a hybrid computer to fasilitate simulating digester overloading and control strategies. Control strategies which were evaluated from the myriad of simulations included a) gas scrubbing and recycle, b) base addition, c) sludge and organizms recycle, and d) flow reduction gas scrubbing and recycle a control action which has not as yet been field tested, appeared to overcome cation toxicity and carbonate precipiteation difficulties which can accompany digester pH adjustment.