Akışkan yataklı reaktörde biyolojik denitrifikasyon sürecinin incelenmesi

Abstract

-IV- Ö Z E T Evsel ve endüstriyel kökenli atıksuların, alıcı yüzeysel sulara sürekli ve kontrolsuz boşaltımı, çökelebilir katı maddelerinbiyokimyasal oksijen ihtiyacının ve mikrobiyolojik kökenli kirleticilerin yanında azot ve fosfor gibi mineral besi maddelerinin yarattığı kirliliği gündeme getirmiştir. Bunlardan azot çeşitli formlarıyla, oksijen ihtiyacı, ötröfikasyon sürecini hızlandırma, zehirlilik gibi sorunların kaynağıdır. Azotun arıtılmasında konvangiyonel yöntemlerin yüksek oranlarda verimlilik sağlayamaması, biyo lojik denitrifikasyon sürecini, en önemli seçeneklerden biri olarak güncelleştirmiştir. Bu çalışma, biyolojik denitrifikasyon reaksiyonunun kinetik parametrelerinin hesaplanmasını ve pH, sıcaklık nitrit gibi çevre faktörleri etkilerinin incelenmesini, biyolojik denitrifikasyon sürecinin, yeni bir reaktör tipi seçeneği olan akışkan yataklı reaktörlerde uygulanmasının, kinetik özellikleri ile mühendislik uygulamasında kullanılabilecek tasarım kriterlerinin açı ğa çıkarılmasını amaçlamaktadır. Birinci bölümde, çalışmanın önemi, amacı ve kapsamı açıklanmıştır. İkinci bölümde, biyolojik denitrifikasyon reaksiyonu üzerin de literatürde daha önce yapılmış çalışmalar, mikrobiyoloji, kinetik ve stokiometri, pH, sıcaklık gibi çevre faktörlerinin etkileri kriterlerinden ayrıntılı olarak verilmiştir. Üçüncü bölümde, biyolojik denitrifikasyon sürecinin, askıda çoğalma sistemlerinde, biyofüm çoğalma sistemlerinde çeşitli uygu lamalarına yönelik, deneysel sonuçlar ve tasarım kriterleri, literatür verilerinden derlenmiştir. Dördüncü bölümde, çalışmada kullanılan deney düzenekleri olarak tam karışmalı ve kesikli reaktörler, akışkan yataklı reaktör ve-V- deneysel yöntemler açıklanmıştır. Bu bölümde ayrıca, deneysel verilerin eldesinde kullanılmak üzere, akışkan yataklı reaktöre özgün olarak geliştirilen, biyofilm kalınlığı, biyofilm kuru madde yoğunluğu ve biyokütle konsantrasyonu gibi parametrelerin ölçüm yöntemleri açıklanmıştır. Beşinci bölümde, deneysel bulgular verilmekte ve açıklanmakta dır, tam karışmalı ve kesikli reaktörlerde, askıda çoğalma sistemle rinde, biyolojik denitrifikasyon reaksiyonunun kinetik parametreleri ve optimum pH ortamı bulunmuştur. Reaksiyon üzerinde sıcaklık etkisinin incelenmesi sonucu, akışkan yataklı reaktörün, askıda çoğalma sistemlerine nazaran sıcaklık değişikliklerine daha dayanıklı olduğu ortaya konmuş, denitrifikasyon hızının pH ve sıcaklığa bağlı değişimini tanımlayan bir model oluşturulmuştur. Biyolojik denit rifikasyon reaksiyonunun nitratın nitrite indirgenmesi aşamasınca sınırlandığı ve nitritin tanımlanan bir aralık dışında inhibisyon etkisinin olmadığı saptanmıştır. Bu bölümde, akışkan yataklı reaktörde biyolojik denitrifikas yon sürecinin tasarımına yönelik, biyofilm kalınlığı, biyofilm ka lınlığı artış hızı ve biyofilm kuru madde yoğunluğu parametreleri geliştirilmiş ve matematik olarak tanımlanmıştır. Reaksiyon kineti ğinin sıfırıncı mertebeden olduğu ancak biyofilm kalınlığı cinsin den tanımlanan bir sınır ötesinde 1/2. mertebeye uygunluk gösterdiği ve bu olgunun nedeninin, nitratın biyofilm içine diffüzyonunun sınırlanması olduğu açıklanmıştır. Biyolojik denitrifikasyon sürecinde besi maddesi olarak kullanılan fosforun, MONOD kinetiği uyarınca giderildiği gösterilmiştir. Tüm bu bulguların birlikte ele alınması ile, pratik kullanım özelliklerine sahip, biyolojik denitrifikasyon sürecini akışkan yataklı reaktörde tanımlayan bir model oluşturulmuştur. Altıncı bölümde sonuçlar sunulmaktadır.

-VI- SUMMARY Until recently the historical approach to waste treatment was to ensure the removal of settleable solids, reduce the bio chemical oxygen demand and eliminate bacterial contaminants. In- recent years the advent of severe eutrophication problems has brought attention to nitrogen control. Furthermore, additional oxygen demand to biochemical degradation, toxicity, severe det rimental.' effects of drinking water containing nitrates to infants are among. the environmental pollution problems caused by domestic and industrial nitrogen discharges to surface waters. Control of aitrogen in aquatic environment is required for a variety of reasons in addition to its role as an algal nutrient. In ground water recharge situations, the build-up of oxidized nitrogen can likewise pose a serious problem. The increasing importance of controlling the discbarge of nitrogenous material from wastewaters has prompted extensive research aimed at the development of nitrogen removal processes. Selective ion exchange for both ammonia and nitrate nitrogen removal, ammonia stripping by air at elevated p£, algae harvesting reverse osmosis, breakpoint chlorination and the biological process of nitrification and denitrification, are examples of such processes which have received much attention» The principles of biological nitrif icationrdenitrif icat ion have been recognized for nearlya century, nitrifying organisms having been isolated as early as.1890 by Winogradsky. The biological nitrif ication- denitrification process is perhaps the most common scheme for nitrogen removal from municipal wastewaters in use today. In the treatment plant, following the release of organically- bouad nitrogen as ammonia by hAerotropbic bacteria during the. organic removal stage, ammonia is oxidized primarily to nitrate by autotrophic bacteria. Nitrate conversion may then take' place through two mechanisms, assimilatory or dissinilatory denitrification. In assimilatory denitrification, nitrate is used in cell synthesis.-VII- In dissimilatory denitrification, nitrate seryes as the hydrogen acceptor in the oxidation-reduction reactions of the carbon substrate to provide energy for a- 11 growth and is converted to gaseous and products principally Nj by heterotrophic bacteria. almost all of the early studies on this subject only considered experimental assesment of a specific problem without. further elaboration on the integration of the several aspects of biological denitrification. The. results cited in these studies could only be considered within the framework of the experimental study and could not be extrapolated for the general treatment of the fluidized bed process. This study was basically motivated from the above considerations and was mainly geared to provide a rational approach to biological denitrif ication process in fluidized-bed reactors from the engineering point of view. In this study, a general survey of kinetic parameters, environmental factor effects such as pH, temperature and nitrite, on biological denitrification process is evaluated on kinetic aspects and engineering design criteria. In the first chapler, the basic principles leading to the description of the subject of concern are out lined and the scope' and objectives of this study are defined. In the second chapter, emphasis is placed upon studies in the literature dealing with biological denitrification. Special attention was directed towards, the kinetics and stbichiometry a£ the reaction and pH, temperature effects on the removal rate. During the past a large part on the technological development in this world have been accomplished by `trial and error` methods, which fortunately has lead to a lot of suitable methods and processes. However, if an optimal strategy for the treatment is aimed at, this approach most often has to be accompanied by or followed by a theoretical development and reconsideration of all the basic elements of the process. Therefore, the work on the description of biological denitrification kinetics by biofilm theories is rewieved in detail.-VII in the third chapter, the conventional biological denitrifi- cation processes are relieved. The main approaches in the literature were to use suspended or attached culture systems. The significance of the need to adopt fluidized bed technology for this process and the need to establish an experimental set-up for this purpose is clearly pointed out. In the fourth chapter, the completely mixed batch reactors and fluidized bed reactor are described and experimental procedures are outlined. Meanwhile, the procedures for the prediction of bio film thickness, biofilm build-up rate, and biofilm dry density are presented. In the fifth chapter, the experimental data are presented and discussed. The general approach was to make a bridge between two former fields of research, the microbial level often treated from a scientist point of view and the technological level at which an empirically descriptive approach has been taken. The kinetic parameters of biological denitrification reaction with an external carbon source (methanol) are evaluated and optimum pH -was found to be 8. Jtmax ~°'037 hour:1 KCH 0H (methanol removal rate) «= 0,150 mgCH3OH/mgVSS.h 'SkT-N (Dcnitri*iÇation rate) = 0,0585 mgNCÇ-N/mgVSS.h Yck0H :?** °»25 m8 VSS/mgCHaOH /<h-U ` °'63 mg vss/m8 N0İ-N Stochiometric removal ratio - 2,56 mgCHsOH/ mg N0£-N Ks,CH30H ^h.ml-n <<l'0m&/1 The removal of methanol and nitrate in suspended culture, systems were found to be in zero-order intrinsic removal rate. ' The temperature coefficient, kfc, was predicted as 0,054 °C~ x inIX - suspended-culture systems, 0,023 `c`1 in fluidized-bed systems. Meanwhile, Qu yalues were 3,5 and 2,9 respectively. These results indicated, the relative stability of fluidized-bed reactor, in comparision with suspended-culture system, towards temperature fluctuations. A. basic model predicting the pH and temperature effects in suspended culture systems is set forth. 0,054 (t-25) 'SjO^-N, t f 'SkÇ-N, 25 °C 10 [I + 0,03 (10* (8_pH) - 1)1 The mechanism of nitrite removal and nitrite inhibition ön nitrate reduction is discussed. One of the 2 ATP molecules, produced for the energy demand in biological denier if ication reaction is used during the reduction of nitrate to nitrite, while the other during the reduction of nitrite to nitrous oxide. The reaction rate is controlled by nitrate reduction step. Except a specific region no inhibitory effect of nitrite was found around reasonable concentrations. The region mentioned above can be descrihed as; NOT >_ 50 mg/1 ; N§ S 5 mg/1 Therefore, dhe definition `competetive-inhihition` can't be used in this circumstances. Two-phase, complementary reaction must be preferred. In the fluidized-bed reactor, the mathematical relation between biofilm thickness and superficial upward flow velocity and definition of biofilm thickness build-up are presented as engineering design and operational control parameters. -^- = 102. 1,8 Ztaa*~Z. r_ p Ks v +V. At *N0j-N I =0,156 - S'S s r V max SA zero order intrinsic removal rate when biofilm thickness is smaller Chan 500 microns, 1/2 order bulk reaction when biofilm thickness is greater than 500 microns is found to he the general characteristics of the biological denitrif ication kinetics in biofilm systems. The significance of the diffusion effect in dominating the removal kinetics is shown and the diffusion rate limiting property of nitrate is proposed. The biofilm dry density and biomass concentration versus biofilm thickness graph is performed and a maximum was found at 300 micron biofilm thickness. Phosphorus, which was used as a nutrient, was shown to be reaction rate limiting component, and removal rate in Monod kinetics was experimentally predicted.-The ideal, methanol: nitrate: phosphate- phosphorus feed rate; Hethanol/nitrate/phosphorus «= 100 / 40 / 1 With the integration of engineering and scientific work, ajnathematic model predicting, denitrif ication rate for biological denitrif ication process in fluidized-bed reactor is presented. Along this frame-work, the scientific level and technological level were conducted in correlation with each other, in.order to. describe biological denitrif ication process in fluidized-bed reactor as a new alternative. 7İN0İ =1/3 ^°l-max eV3 H - 0,06 V8 +0,0075x.... MCÇ - 0,5 `t,-..? X ?[ -S- + 0,02 V*0»025?.. J ' Vs The results are summarized in the sixth chapter.