Aktif çamur sistemlerinde biyolojik azot giderimi ve tasarım seçenekleri

Abstract

Organik bileşiklerden sonra atıksudaki en önemli parametrelerden olan azotun çeşitli formlarda bulunduğu yüzeysel ve yeraltı sularından giderilmesi insan sağlığı ve doğal denge üzerindeki olumsuz etkileri yüzünden önem kazanmaktadır. Bu çalışmada, çeşitli ülkelerin nitrifikasyon ve denitrifikasyon İçin geliştirdikleri modeller incelenmiş, birbiri ile kıyaslanmış ve farklı getirdikleri yaklaşımlar değerlendirilmiştir. Birinci bölümde çalışmanın, anlamı, amacı ve kapsamı kısaca özetlenmiştir. İkinci bölümde azotun kaynaklan, formları, sulardan giderilme amaçlan belirtilmiştir, ve nitrifikasyon ve denitrifikasyon proseslerinin stokiometrisi ve kinetiği açıklanmıştır. Üçüncü bölümde nitrifikasyon ve denitrifikasyon sistemleri için hazırlanan farklı modellerin akım şemalan ve bu akım şemalarının açıklamalan verilmiştir. Bölüm sonundaki sayısal tasarım örneği ile, çalışmada verilen teorik esasların uygulaması gerçekleştirilmiş ve son olarak tüm modeller için aynı giriş koşullan söz konusu olması durumunda elde edilen tasanın büyüklükleri tablo halinde özetlenmiştir. Dördüncü bölümde, üçüncü bölümde verilen farklı yaklaşımlar irdelenmiş, farklı tasanm büyüklüklerin hesaplanabilmesi için seçilen uygulama ve kabuller karşılaştırılmış ve değerlendirilmiştir. Beşinci bölümde sonuç ve öneriler verilmiş, nitrifikasyon denitrifikasyon proseslerinin farklı yaklaşımlar sonucu tasanm esaslarının değiştiği ancak uygulamada elde edilen sayısal değerlerin modelden modele büyük farklılık taşımadığı belirtilmiştir.

Man's influence on the environment is receiving increasing public and scientific attention. The quality of some of the nation's water bodies has been subjected to continuing degradation as a result of man's activities. While there has been considerable success in reversing this trend, one roadblock to greater progress often has been the lack' of the necessary technology to reliably and economically remove the pollutants which are the cause of degradation of receiving waters. While conventional technology is well developed for removing organics from wastewater, the processes for the control of nitrogen in wastewater effluents have been developed only recently. Nitrogen control techniques are divided into two broad categories. The first group of nitrogen control processes is involved with the conversion of organic and ammonia nitrogen to nitrate nitrogen, a less objectionable form. These processes are termed nitrification procewsses. The second category involves processes which result in the removal of nitrogen from the wastewater, not just merely the conversion from one form to another form in the wastewater. This letter group includes biological mtrification-demtrification,ion exchange, ammonia stripping and breakpoint chlorination. Various compounds containing the element nitrogen are becoming increasingly important in wastewater management programs because of the many effects that nitrogeneous materials in wastewater effluent can have on the environment. Nitrogen, in its various forms, can deplete dissolved oxygen levels in receiving waters, stimulate aquatic growth, exhibit toxicity toward aquatic life, affect chlorine disinfection efficiency, present a public health hazard, and affect the suitability of wastewater for reuse. Biological and chemical processes which occur in wastewater treatment plants and in the natural environment can change the chemical form in which nitrogen exists. Such change may eliminate one deleterious effect of nitrogen while producing, or leaving unchanged, another effect. For example, by converting ammonia in raw wastewater to nitrate, the oxygen-depleting and toxic effects of ammonia are eliminated, but the biostimulatory effects may not be changed significantly. Nitrogenous materials may enter the aquatic environment from either natural or man- caused sources. Further, the quantities from natural sources are often increased by man's activity. That is why it seems to be useful to have an understanding of the various sources of nitrogenous materials and to have an appreciation of the quantities of nitrogen which may be expected from each. Although the source of nitrogen causing a specific pollution is often obvious, diffuculty may be encountered in determining which of several possible sources is most important. As an example, if a stream with excessive aquatic growths due to nitrogen receives effluent from a sewage treatment plant, drainage from fertilized cropland, and runoff from pastures or feedlots, the contributions of nitrogen from the treatment plant may be a small fraction of that from the other two sources. Thus, in analysing a nitrogen pollution problem, care must be taken to ensure all possible sources are investigated and that the ammount to be expected from each is accurately estimated. Once an estimate is made, nitrogen control measures can be oriented toward the more significant sources. Natural sources of nitrogenous substances include precipitation, dustfall, nonurban runoff, and biological fixation. Amounts from all may be increased in some way by man. It may be quite diffcult to determine quantities which might be expected under completely natural conditions. In order to find levels of nitrogenous substances in precipitation which are as close to natural as possible, it is necessary to take samples far from urban or agricultural areas. Even these values may be suspect,however. The quantities of nitrogen in nonurban runoff from non-fertilized land may be expected vary greatly, depending on the erosive characteristics of the soil. Biological fixation may add nitrogen to both soil and surface water environments. Of particular interest interest is the role of fixation in eutrophication of lakes. The activities of man may increase quantities of nitrogen added to the aquatic environment from three of the sources discussed above: precipitation, dustfall, and nonurban runoff. These sources are increased principally by fertilization of agricultural land and the combustion of fuels. Other man-related sources include runoff from urban areas and livestock feedlots, municipal wastewater effluents, subsurface drainage from agricultural lands and from septic tank leach fields, and industrial wastewaters. It was previously noted that nitrogenous compounds discharged from wastewater treatment facilities can have several deleterious effects. Although biostimulation of receiving waters has generated the most concern in recent years, other less well XI publicized impacts can be of major importance in particular situations. These impacts include toxicity to fish life, an increase in the dissolved oxygen depletion in receiving waters, adverse public health effects - principally in groundwater, and a reductionin the suitability for reuse. A major problem in the field of water pollution is eutrophication, evcessive plant growth and/or algae blooms resulting from over fertilization of rivers, lakes, and estuaries. Results of eutrophication include deterioration in the appearance of previously clear waters, odor problems from decomposing algea and a lower dissolved oxygen level which can adversely affect fish life. Four basic factors are required for algal growth: nitrogen, phosphorus, carbon dioxide and light energy. The absence of any one will limit growth. In special cases, trace micronutrients such as cobalt, iron, manganese may be limiting factors under natural conditions. Good generalization concerning which factor is growth limiting are difficult to make. Light and carbon dioxide are essentially impossible to control. Both nitrogen and phodphorus are present in waste discharges and hence subject to control. The questions which must usually be answered when faced with a eutrophication problem are: is nitrogen or phosphorus the limiting nutrient and can the amount entering the receiving water be significantly reduced by removing that nutrient from the waste stream? In some cases algal assay procedures may allow a conclusion as to which nutrient is limiting. Under some circumstances, however, removal of both nitrogen and phosphorus may be undertaken to limit algal growth. The principal toxicity problem is from amonia in the molecular form (NH3) which can adversely affect fish life in receiving waters. Factors which may increase ammonia toxicity at a given pH are; greater concentrations of dissolved oxygen and carbon dioxide, elevated temperatures, and biocarbonate alkalinity. Ammonium can be biologically oxidized to nitrite and then to nitrate in receiving waters and thereby add to the oxygen demand imparted by carbonaceous materials. The public health hazard from nitrogen is associated with the nitrate form and is limited principally to groundwater where high concentrations can occur. Nitrate drinking water is associated methemoglobinemia, a sometimes fatal blood disorder which affects infants less then tthan three months old. When water high in nitrate is used for preparing infant formulas, nitrate is reduced to nitrite in the stomach after ingestion. The nitrites react with hemoglobin in the blood to form methemoglabin, XII which is incapable of carrying oxygen. The result is suffocation accompanied by a bluish tinge to the skin, which accounts for the use of the "blue babies" in conjunction with methemoglobinia. In suspect areas water should be analyzed for both nitrite and nitrate since either form will cause methemoglobinemia. While direct wastewater reuse for domestic water supply is not yet a reality because of public health considerations, plans for industrial reuse are being carried out in several areas. When reclaiming wastewater for industrial purposes, ammonia may need to be removed in order to prevent corrosion. Further, nitrogen compounds can cause biostimulation in cooling towers and distribution structures. In the past several years the number of processes utilized in wastewater treatment has increased rapidly. Many of these processes have been developed with the spesific purpose of transforming nitrogen compounds or removing nitrogen from the wastewater stream. Others can remove several compounds, including significant amounts of nitrogen. Still others may remove only a small amount of nitrogen which is a small part of the total. Biological processes for control of nitrogenous residuals in effluents can be classified in two broad areas. First, a process designed to produce an effluent where influent nitrogen (ammonia and organic nitrogen) is substantially converted to nitrate nitrogen can be considered. This process, nitrification, is carried out by bacterial populations that sequentially oxidize ammonia to nitrate with intermediate formation of nitrite. Nitrification will satisfy effluent or receiving water standards where reduction of residual nitrogenous oxygen demand due to ammonia is mandated or where ammonia reduction for other reasons is required for the treatment system. The second type of process, denitrification, reduces nitrate to nitrogen gas and can be used following nitrification when the total nitrogenous content of the effluent must be reduced. v - The two principal genera of importance in biological nitrification processes are Nitrosomonas and Nitrobacter. Both of these groups are classed as autotrophic organisms. These organisms are distinguished from heterotrotrophic bacteria in that they derive energy for growth from the oxidation of inorganic nitrogen compounds, rather than from the oxidation of organic matter. Another feature of these organisms is that inorganic carbon (carbon dioxide) is used for synthesis rather than organic carbon. Each group is limited to the oxidation of specific species of nitrogen compounds. Nitrosomonas can oxidize ammonia to nitrite, but cannot complete the oxidation to nitrate. On the other hand, Nitrobacter is limited to the oxidation of nitrite to nitrate. Since complete nitrification is a sequential reaction, treatment processes must be designed to provide an environment suitable to the growth of both groups of nitrifying bacteria. XIII The stoichiometric reaction for oxidation of ammonium to nitrite by Nitrosomonas is: Mf4+ + 1.5 02 -* 2 H* + H20 + NO; The loss of free energy by this reaction at physiological concentrations of the reactants has been estimated by various investigators to be between 58 and 84 kcal per mole of ammonia. The reaction for oxidation of nitrite to nitrate by Nitrobacter is: no; + o.5 o2 -* no; This rection has been estimated to release between 15.4 to 20.9 kcal per mole of nitrite. Thus, Nitrosomonas obtains more energy per mole of nitrogen oxidized than Nitrobacter. If it is assumed that the cell synthesis per unit of energy produced is equal, there should be greater mass of Nitrosomonas formed than Nitrobacter per mole of nitrogen oxidized. This is in fact the case. The overall oxidation of ammonium by both groups is : nh; +2o2-> no; + 2 H + + H20 The biological process of denitrification involves the conversion of nitrate nitrogen to a gaseous nitrogen species. The gaseous product is primarily nitrogen gas but also may be nitrous oxide or nitric oxide. Gaseous nitrogen is relatively unavailable for biological growth, thus denitrification converts nitrogen which may be in an objectionable form to one which has no significant effect on environmentally quality. As oppesed to nitrification, a relatively broad range of bacteria can accomplish denitrification, including Pseudomonas, Micrococcus, Archromabacter and Bacillus. These groups accomplish nitrate reduction by what is known as a process of nitrate dissimilation whereby nitrate or nitrite replaces oxygen in the respiratory processes of the organism under anoxic conditions. Because of the ability of these organisms to use either nitrate or oxygen as the terminal electron acceptors while oxidizing organic matter, these organisms are termed facultative heterotrophic bacteria. Combined dissimilation-synthesis equation for denitrification is: NO; + 1.08 CH3OH + 0.24 H2C03 -* 0.056 C^J7N02 + 0.47 N2 + 1.68 H20 + HC03 In this framework, kinetic and stoichiometry of nitrification-denitrification processes are given for a better understanding of biological nitrogen removal. And since there are many design approaches for nitrogen removal the main aim of the workshop is to compare some of the design models and to evoluate the different approaches.