Aerobik sistemlerde proses stokımetrisi ve kinetiğinin respirometrik olarak değerlendirilmesi

Abstract

ÖZET Biyolojik arıtma sistemlerinin seçimi ve tasarım parametrelerinin belirlenmesi; hem arıtma performansı hem de arıtma tesisinin ilk yatırım ve işletme maliyetleri açısından önem taşımaktadır. Doğru sistem seçimi ve tasarımı ise ancak doğru saptanmış atıksu karakterizasyonu ve arıtılabilirlik verilerine dayandırılarak gerçekleştirilebilir. Aktif çamur sistemlerinde ki son gelişmeler özellikle çok bileşenli modelleme yaklaşımları ve buna bağlı olan atıksu karakterizasyonu ve arıtılabilirlik kavramlarında kendini göstermektedir. Evsel atıksular için tanımlanmış ve denenmiş olan yeni yaklaşımlar bu çalışma kapsamında, evsel ve tarıma dayalı endüstriyel atıksulara uygulanmıştır. Bu çalışmanın amacı; dönüşüm oranının belirlenmesine yönelik bir yöntemin tanımlanması, evsel ve endüstriyel atıksularda çok bileşenli modellemeye imkan verecek karakterizasyon yaklaşımı içinde, yeni verilerin oluşturulması, sistem bileşenleri, stokiyometrik ve kinetik sabitlerin belirlenmesidir. Birinci bölümde, çalışmanın önemi üzerinde durulmuş, çalışmanın amaç ve kapsamı tanımlanmıştır. İkinci bölümde, geniş bir literatür araştırması ile aktif çamur sistemlerinde karbon giderimi açısından önemli olan bütün süreçler incelenmiş ve bu süreçler için verilmiş olan sistem bileşenleri, stokiometrik ve kinetik sabitler gözden geçirilerek, konu hakkında yapılmış çalışmalarla ilgili bilgi verilmiştir. Üçüncü bölümde, dönüşüm oranının belirlenmesine yönelik bir yöntemin tanımlanması ve bu yöntemin evsel ve endüstriyel atıksulara uygulanması verilmiştir. Dördüncü bölümde, yürütülen deneysel çalışmanın düzeni ve analiz yöntemleri açıklanmıştır. Beşinci bölümde, evsel ve endüstriyel atıksularda elde edilen deneysel sonuçlar detaylı olarak verilmiştir. Altıncı bölümde, çok bileşenli içsel solunum yaklaşımı yardımı ile model bileşenlerinin, Oksijen Tüketim Hızı profillerine etkisi araştırılmış, elde edilen deneysel bulgular ve modelleme yardımı ile seçilen evsel ve endüstriyel atıksularda kh, Kx ve Ks kinetik sabitlerin belirlenmiş ve modifiye edilmiş çok bileşenli içsel solunum yaklaşımının gerekliliği üzerinde durularak ve söz konusu model yardımı ile kh, khs, Kx ve Kxx kinetik sabitleri bulunmuştur. Yedinci bölümde, deney verilerinin atıksu bazında değerlendirilmesi yapılarak, bulgular literatür ile karşılaştırılmıştır. Sekizinci bölümde, sonuçlar ve öneriler tartışılmıştır. xxiii

RESPIROMETRIC EVALUATION OF PROCESS KINETIC AND STOICfflOMETRY FOR AEROBIC SYSTEMS SUMMARY Wastewater characterization is now regarded as an indispensable step yielding all the necessary information for a reliable modelling and design of biological treatment processes. It should mainly include fractionation of the chemical oxygen demand, (COD), and assessment of significant kinetic and stoichiometric coefficients. COD fractionation involves identification of inert and biodegradable COD together with readily biodegradable and slowly biodegradable fractions. Experimental methods developed or selected for the assessment of COD fractions should be compatible with the mathematical models defining biological treatment and should yield consistent and reliable values. Modelling is a valuable tool for design of activated sludge systems, because it enables the role and impact of significant parameters to be analysed and so helps to define and optimize the proposed process. In this context, the value of modelling depends upon the accuracy and reliability of available experimental information, the wastewater to be treated and the biochemical reactions involved Experimentally-derived information has to cover various wastewater fractions included as model components, biomass in activated sludge and wastewater and finally, various kinetic and stoichiometric coefficients defining the biochemical processes included in the model. The identification of wastewater characteristics with regard to the organic content is useful both from the standpoint of process kinetics and prediction of effluent quality. Difficulties associated with the correct assessment of organic substrate have caused inconvenience and confusion, to the extent of hindering the progress of activated sludge theory. Collective analytical parameters such as biochemical oxygen demand, (BOD), and chemical oxygen demand, (COD), routinely used to reflect the total organics in wastewaters, do not correspond to the real system variable, i.e. the growth limiting substrate. COD is estimated to be a more useful parameter than BOD, as it enables one to make appropriate correlations among substrate, biomass and dissolved oxygen in terms of electron equivalence. It cannot however, identify the growth limiting components directly related to the corresponding process rate expressions, especially in complex media such as domestic and industrial wastewaters. From a modelling standpoint, COD cannot differentiate between biodegradable and inert organic matter or between readily and slowly biodegradable fractions. The inert COD may be present in the influent or it may be generated during biological treatment as residual microbial products. This residual COD fraction, not so critical for conventional wastes, becomes significant for industrial effluents and especially in strong wastes as it XXIVleads to misinterpretation of the biological treatability and kinetic analysis; the soluble inert COD fraction also becomes a challenging factor in meeting the effluent limitations expressed in terms of COD for a number of industrial categories. Current understanding of activated sludge modelling for carbon and nitrogen removal involves a number of kinetic and stoichiometric coefficients, excluding parameters related to biological phosphorus removal. A significant proportion of these coefficients are defined for specific functions in the models and therefore, are not sensitive to wastewater characteristics. A smaller group however, including maximum heterotrophic and autotrophic growth rates jIHandjIA, heterotrophic yield, YH, hydrolysis rate constant, Kh and correction factors for anoxic conditions, Tig and % are very much site specific and need to be experimentally determined for each case. This study was devoted to the evaluation of biological treatability of the wastewaters, not in a conventional way as it is often tested in different size batch or continuous test reactors, but with a treatability oriented characterization approach using the recent possibilities associated with respirometric techniques enabling the experimental assessment of the determination of Yh and major kinetic constants such as jIHand bn and the determination of the readily biodegradable COD, Ss, a very significant organic fraction for these type of wastewaters. Respirometric analysis of different domestic sewage together with textile, dairy, meat processing, tannery and confectionery wastewaters were carried out for the experimental assessment of the readily biodegradable COD. The accuracy and the reliability of the experimental procedure was tested using synthetic sewage and different wastewater mixtures. The respirometric procedure and substrate to biomass F/M (CTı/XTı) ratio, all all stoichiometric and kinetic constants were investigated using model simulation of the respirometry curve together with the collected experimental data. The Readily Biodesradable CODJSs The correct assessment of the readily biodegradable COD concentration, Ss is important because this fraction is conceived as the rate limiting substrate component for heterotrophic growth. Methods used for its determination rely on respirometric measurements conducted under aerobic or anoxic conditions, in continuous or batch reactors. In the aerobic batch test adopted in this investigation, the selection of an appropriate initial F/M ratio provides a clear differentiation between oxygen utilization rates, (OUR), induced by Ss and Xs. The initial OUR stays constant over a period where the maximum growth rate is sustained; after the depletion of Ss, it drops to a lower level only correlated with the hydrolysis of Xs and the endogenous respiration. The readily biodegradable substrate, Ss, may be calculated from the following relationship: 1 4 Ss = - AÖ2 1-Yh xxvwhere A02 is the difference between total respiration and respiration due to hydrolyzed substrate and endogenous metabolism and YH is the heterotrophic yield. The Heterotrophic Yield Coefficient, YH Soluble COD analyses were also made on samples taken for OUR measurements, yielding soluble COD profiles together with OUR profiles during the experiment. Respiration due to endogenous respiration was assumed to be negligible compared to growth, as commonly accepted in the literature. It was also assumed that the level of soluble residual microbial products within the limited duration of the experiment would not be significant enough to affect the change of COD concentration due to growth. This way, the proposed method was followed to determine YH, using the simultaneously obtained COD and OUR data on the basis of the following expression. Y..1- A°> H ACOD(soL) The Maximum Heterotrophic Growth Rate, u ` The maximum heterotrophic growth rate, is again determined by means of respirometric tests, on the basis of the initial OUR value at the F/M ratio recommended for the test. In this framework, the method developed by KAPPELER and GUJER (1992) involving a batch test with centrifuged wastewater and a very small amount of biomass corresponding to an initial COD/VSS ratio of 4 was used. For the observed OUR profile, the following linear expression may be derived, with a slope of £H - brf. OUR.`, w In = (UH -bH)t OUR, H H The Activity Coefficient The method defined by EKAMA and MARAIS (1986) for the determination of Ss, may also be used for the assessment of j!H with the understanding that the initially observed OUR reflects maximum growth conditions, neglecting the interferences induced by hydrolysis and endogenous respiration. In fact, OURi is proportional to the product £H XH as given by the following expression: MHXH=T^r-OUR1 The total biomass, XT measured during the experiment may be related to the active biomass, XH by means of an activity coefficient, fa : XH = faXT xxviSince jîH is determined, as previously described, by a similar respirometric method (KAPPELER and GUJER, 1992) a parallel test may be run to yield fa: f YH OUR, The activity coefficient, fa is essential to calculate the correct value for XH which will be used in the curve-fitting evaluations for the evaluation of other kinetic constants. The Endogenous Respiration Rate, bH The method to calculate bH involves plotting the change of OUR with time in a batch reactor devoid of substrate. The slope of the OUR profile yields the value of bH on the basis of the following equation: In OUR = ln[l.42 (1 - fe )bHXH1 ] - b ` t The Inert COD Fraction The procedure proposed by (ORHON et al.) basically uses the principle that both soluble and particulate inert microbial products, SP and XP can be expressed as a constant fraction of the influent biodegradable COD, CSi. The experiment requires two aerated batch reactors, one started up with the unfiltered wastewater (CTi) and the other with the filtered wastewater (Sn). In each reactor, total and soluble COD are monitored for a period long enough to ensure the depletion of all biodegradable substrate and the mineralization of all biomass so that the measured Cn and Sn values reach their constant threshold levels containing only initial inert COD and residual products. Both reactors are initially seeded by a minimal amount of biomass previously acclimated to the wastewater to avoid the interference of residual COD released through the decay of the initial inoculation. It is suggested that the acclimation be carried out in fill and draw systems operated at F/M ratios over 1.0 g COD/g VSS.day to ensure a highly viable biomass, and the amount of seed is adjusted between 10-50 mg/1, depending on the nature of the wastewater to be tested. The previous method for the assessment of Sn is applicable to wastewater samples with a reasonable large particulate COD fraction. This component may be too small or practically negligible in some industrial wastewaters or in the effluent from the chemical treatment step of multi-stage treatment schemes commonly used for complex strong wastes. For such wastes, the procedure proposed by GERMİRLİ et al. may be followed; Two aerobic batch reactors of filtered wastewater and glucose, seeded with a very small amount of biomass previously acclimated to the glucose-wastewater mixture are run in paralel. Glucose is selected because it is the central compound of vital importance which appears in the metabolic pathways of the biodegradation of every organic matter. At the end of the experiment where all biodegradable substrates in the two reactors are depleted, the difference between the residual COD levels may be manipulated to yield XXV1 1the initial inert COD in the wastewater tested and the ratio between the soluble residual products and the initial COD, Ysp. The Slowly BhdeeradaMe COD Fractions Slowly biodegradable organic fractions, Xsi, are generally determined from mass balance. In models where slowly biodegradable COD is not ftuther differentiated as rapidly and slowly hydrolyzable components, and initial biomass is not separately identified, the following mass balance yields Xsi, provided that Ssi is experimentally estimated. Csi = Ssi + Xsi Rapidly hyrolyzable COD, if considered as a model component, is generally assumed to be predominantly soluble, so that it may be defined with a reasonable approximation for domestic wastewaters by means of the mass balance expression below: Sm= Sti - Su - Ssi A similar mass balance on the particulate COD fractions may be used to calculate the slowly hydrolyzable COD. Results of extensive experimental work, carried out on COD fiactionation, kinetic and stoichiometric constants of a wide range of domestic and industrial wastewaters are given in Table 1, Table 2, Table 3, Table 4, Table 5. The experimental accuracy was investigated by testing the ability of the procedure to recover and reflect known amounts of readily biodegradable COD, using synthetic substrate, synthetic substrate-sewage mixtures and industrial-domestic wastewater mixtures. Experiments were conducted on raw samples, since the resulting data were expected to reflect the character of the wastewaters investigated. The COD fractionation data on domestic sewage presented above, may be compared with similar results outlined in literature, reported for the same type of wastewater in different parts of the world. An evaluation for the COD fractionation of industrial wastewaters is not possible as there are no similar data available in the literature. The results show clearly that the total organic content characterized by the total COD concentration, On gives inadequate, if not misleading information about the biological treatability of different wastewaters. Another common and significant feature of experimental observations is the fact that the slowly biodegradable COD should be considered as the major rate limiting process component for heterotrophic growth in biological treatment systems. In fact, in the conventional kinetic approach, the entire BOD or the COD concentration in the wastewater is conceived as the rate limiting substrate in heterotrophic growth. With the introduction of COD fractionation, this approach is totally abandoned in the new models which now state that heterotrophic growth is controlled alone by the readily biodegradable substrate which is either present in the wastewater, or generated through the hydrolysis of slowly biodegradable organic matter. As clearly shown in this study, xxvmI «J o © «I -! es <s o ?o `^es* <.< © *- es L2 ^J «o i i *-* eş i °£ £ § ^` o 2 £ H o o ^ T f` O « N ?* O o oo - es _ -H ^ « oo « (<. Ö Ö Ö Ö O O VD vo es o 1 I en o O ^ es 2 en i -!. ı -i en §°Sen a s ` *. a <n ^ es <n I 1 I t I 1 I I I I I o` *``' o en v£>,_ en I.. _. l.i I I I. v^o.i.'V- <>o(Jtenesenr--'-< en «- ' ~.n £j <J/ OÖ O O -< `M O in NO es oo. o en ı o o ?`* 0° «- -, O 00 `O 2 W) -< oo ~ oo o/ es oo en oo O o ` >o S r~.^ -3- `, en ?* s `> s *°. ? ***. a ?*. `>. Ö © ©` => O °' `* °' en o ^ lîflîlîîîo^^psf c!>«SI hwtflWK^i 8 I Q O Îf D O ! 8 c ~. XXIXTable 2. The general evaluation of dairy and meat industry wastewaters Parameter Dairy Industry Mean Range Meat Industry Mean Range Table 3. The general evaluation of confectionery and textile industry wastewaters Parameter Confectionery Industry Mean Textile Industry Mean Range XXXTable 6. The general evaluation of tannery industry wastewaters Parameter Mean Range Table 7. The accuracy of the OUR experiments tested with duplicate runs under different conditions XXXIthe initial readily biodegradable COD is a relatively small fraction of the total COD content of wastewaters. Therefore, biological treatment systems are not controlled by the depletion of the readily biodegradable organics, but by hydrolysis of slowly biodegradable organics, a much slower process compared to heterotrophic growth. Consequently appropriate design of biological treatment, especially for industrial wastewaters and the assurance of the desired effluent quality should solely account for the hydrolysis of organics, a factor entirely overlooked in conventional practice. Another issue of practical significance is the problem created by more and more stringent COD criteria, imposed as effluent limitations for domestic and industrial wastewaters, with the belief that they can be met with biological treatment at longer hydraulic detention times and sludge ages. The COD fractionation dala presented in the paper provide clear evidence that such limitations cannot be achieved, due to inert COD fractions of wastewaters. They further indicate that the effluent quality with respect to soluble organics, is likely to deteriorate at higher sludge ages with the accumulation of residual metabolic products. The average readily biodegradable COD fraction was determined as 9% of the total COD of Istanbul domestic sewage, corresponding to an SSi concentration of around 50 mgl`1. These ratios compared with similar data reported for different countries show that sewage character with respect to COD fractionation is quite site-specific. The readily biodegradable COD fraction of the industrial wastewaters tested were found to be higher than that of domestic sewage, varying from 15 % for meat processing to 23% for tannery effluents. The experimental results indicate that the readily biodegradable, the slowly biodegradable and the inert COD fractions in relative to total COD are a useful characterization of a strong industrial waste. Parallel tests run at different F/M ratios and under different biomass acclimation conditions, together with experiments conducted using synthetic waste and domestic- synthetic waste mixtures provided acceptable proof that the respirometric approach adopted in the study was quite reliable. Experiments under aerobic conditions are best interpreted by model simulation of the OUR curve using applicable kinetic and stoichiometric data; as such, the experimental assessment of the readily biodegradable COD should only be conceived as part of a comprehensive investigation of wastewater characterization and treatability. The results showed that the observation of a distinct lower plateau was essential for an accurate evaluation and this was only possible with the choice of an appropriate F/M ratio and test duration. The maximum specific growth rate of heterotrophs for İstanbul Kadıköy domestic sewage was evaluated to vary in the range of 2.7-6.5 d`1, with a mean value of 4.7 d`1. A much wider jIH range of 1.5 - 7.0 d`1 is depicted in the literature, either measured by similar respirometric techniques or adopted for model simulation, giving a clear indication that £H is quite site and model-specific and it may be affected by compounds in sewage with a inhibitory effect. The results provided strong proof to the general understanding that the growth of heterotrophs is very much wastewater-specific and xxxushould be observed for a period involving a statistically meaningful number of experiments for the characterization of a given domestic sewage. Parallel studies with synthetic substrate simulating the readily biodegradable substrate content of domestic sewage and with synthetic waste-domestic sewage mixtures did not reflect any indication of substrate limitation on £H; the assessment of £H was not affected by the low Ss levels, supporting the validity of very low Ks values attributed to domestic sewage. The results, as listed in Table 2,3,4,5 and 6 show that the magnitude of maximum heterotrophic growth rate was practically the same for most of the samples tested. This is quite expectable, since the respirometric approach only accounts for the readily biodegradable substrate and not the entire organic load as practiced previously, for the evaluation of fiHi and no drastic change in the composition of Ss is likely to take place from one industry to another. The mean value of Mh f°r meat processing effluent was 3.8 d`1 and varied within a narrow range of 3.6-4.2 d`1 for the three samples studied. For the textile effluent, the result was practically the same (£H=3.9 d'1) and was not appreciably affected by chemical treatment. The dairy and confectionery effluents were respectively characterized by a slightly lower £H, with a mean value of 3.1 d`1 and a range of 2.9-3.3 d`1. The tannery effluent after chemical treatment, a wastewater with a complex and variable structure, exhibited a wider fiH range of 3.0-5.7 d`1, with a still compatible mean value of 4.8 d`1. The same tests performed on raw tannery samples, only yielded a mean fiH value of 2.1 d`1, less than half of what has been measured after chemical treatment, providing a clear evidence of the significant inhibitory effects of chromium, sulfur and other toxic compounds on heterotrophic growth. Comparable results were also obtained for the assessment of fiH for different industrial effluents, explainable by the fact that the respirometric procedure only accounts for Ss, and the nature of Ss should not be expected to show significant variation from one industrial effluent to the other, because by definition, Ss, is the most easily degradable fraction, which is the same in the most wastewaters (fatty acids etc). The mean value of YH for domestic sewage was between 0.66-0.67 mgCOD/mgCOD for the six samples studied. The mean value of YH for raw textile effluent was 0.62 mgCOD/mgCOD for the two samples studied and was not appreciably affected by chemical treatment The tannery effluent, raw and after chemical treatment, exhibited a wider YH range of 0.64-0.71 mgCOD/mgCOD, with a still compatible mean value of 0.67 mgCOD/mgCOD. The level of the yield coefficient, YH determined as 0.62-0.71 is quite compatible both with theoretical and experimental values associated with domestic sewage, lending support to the respirometric method proposed for its assessment. The value of bn was evaluated to change from 0.11-0.24 day`1 for all domestic and industrial wastewaters, depending on the temperature of the wastewater samples used in the experiments. xxxiu