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Macro- and microscopic in-situ observation of gas bubbles and sludge particles in a biogas tower reactor
Water Research 36 (2002) 2836–2842
Macro- and microscopic in-situ observation of gas bubbles and sludge particles in a biogas tower reactor Torsten Pietscha, Ralf Mehrwaldb, Ralf Grajetzkib, Jan Sensb, Tim Pakendorfc, . Kumpartd, Gerhard Matzd, Herbert M.arklb,* Reinhard Ulrichc, Jorn a
b
Deutsche Hefewerke GmbH and Co. oHG, 22041 Hamburg, Germany Institute of Bioprocess and Biochemical Engineering, Technical University Hamburg-Harburg, 21071 Hamburg, Germany c Department of Optics and Instrumentation, Technical University Hamburg-Harburg, 21071 Hamburg, Germany d Department of Instrumentation, Technical University Hamburg-Harburg, 21071 Hamburg, Germany Received 6 April 2001; received in revised form 25 September 2001; accepted 31 October 2001
Abstract Macroscopic and microscopic in-situ observation of particles and gas bubbles are used to get precise impressions of the hydrodynamical characteristics of a biologically active suspension. Moreover, values of in-situ velocities and particle densities can be gained by using these methods. The suspended anaerobic sludge revealed an extensive fibrous structure (‘fur’) on its surface. The observed microfibers have a profound influence on the settling/flotation behavior of the particles because they increase the effective particle volume, they may trap gas bubbles and they favor agglomeration. The biomass particles do not appear as single spherical objects but due to its fibrous structure on the outside as strongly interacting mass. The compressibility of the bubbles which are entrapped in the sludge agglomerates results in a pressure-dependent density of the sludge particles. r 2002 Published by Elsevier Science Ltd. Keywords: In-situ observation; Velocities; Sludge structure; Gas bubbles; Density
1. Introduction and motivation In the literature, an examination and observation of sludge particles is typically done after they have been taken out of the reactor. To determine physical properties of suspended sludge diverse tests are executed which usually do not reflect the in-situ characteristics. Therefore, this paper aims at two in-situ observation methods in macro- and microscopic scale. From macroscopic view an overall appearance of the sludge and the rising gas bubbles can be obtained. Using a sequence of single pictures which show one item allows to determine in-situ velocities of bubbles and particles. Enlarging the scale (microscopic observation) leads to
*Corresponding author. Tel.: +49-40-42878-3017; fax: +4940-42878-2909. E-mail address: [email protected] (H. M.arkl).
very interesting sights into sludge particles and their insitu hydrodynamic behavior. To understand the mechanisms of gas release from the sludge flocs and to get information about the virtual appearance of anaerobic sludge particles in the threephase system (solid particles–liquid–gas bubbles) an insitu observation is necessary. In other studies, settling velocities are particularly determined in external facilities after the removal of the sludge particles from the reactor (e.g. [1]). The appearance of those sludge particles that are sampled from a reactor and monitored outside is completely different from the in-situ observation because the sludge particles and flocs can alter their shape during the sampling. A mechanical shear is applied to the sampled sludge during the passage through the sampling valve. Moreover, inside the reactor the sludge particles are faced to the hydrostatic pressure which is due to the fluid level above. During the sampling a pressure drop to
0043-1354/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 0 0 4 3 - 1 3 5 4 ( 0 1 ) 0 0 5 1 1 - 5
T. Pietsch et al. / Water Research 36 (2002) 2836–2842
Nomenclature Symbols p Pressure, Pa po Atmospheric pressure, Pa
rsludge rgas rpart rsusp
Greek symbols egas Volumetric gas content in sludge particles eogas Volumetric gas content in sludge particles
ambient pressure is usually applied. Gas bubbles will extend and the structure of the sludge flocs may change. The hydrostatic pressure decreases from the bottom to the top of a reactor. This means a major impact on the size of gas bubbles: On their way upwards they will extend. The produced biogas has to release the sludge agglomerates to be collected at the head space. In the case of dense agglomerates, a considerable increase of gas production may lead to unstable particle structures. Clark [2] reported about a breakage of granules after an increase of the gas production in a 700 l upflow anaerobic sludge blanket (UASB) reactor with about 3 m height. It can be assumed that the particles’ disruption is caused by the enhanced gas production, higher gas content of the particles and subsequent extension of the gas bubbles as the pressure drops. As a result, the generated fine sludge particles are washed out of the reactor. To get information about the ‘real’ appearance and hydrodynamic behavior of active anaerobic biomass under gassing conditions, a macroscopic probe and a process microscope were developed for in-situ observation. Commercially available endoscope technology could not be used because the liquid suspension in the reactor is not a clear liquid. This is a prerequisite for standard endoscopy. Hence, a technology had to be developed to observe particles in a turbid suspension. This paper will describe one macro- and microscopic in-situ observation method and will show some typical results and possibilities of both techniques.
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under atmospheric pressure Density of the wet sludge, kg m3 Density of the gas phase, kg m3 Total density of the three-phase particles, kg m3 Density of the liquid phase, kg m3
Common abbreviation BTR Biogas tower reactor
applied to increase the mixing of the three-phase suspension in the bottom section of the reactor. The reactor is equipped with 72 possible locations to be entered (not shown in Fig. 1) which have different diameters ranging from 8 to 25 mm. At these positions along the reactor ball valves are provided as access ports which permit the insertion. All access ports can be used by the macroscopic probe using different adapters. Only the 14 largest positions with 25 mm diameter can be accessed by the process microscope for observation in microscopic scale. The reactor is equipped with numerous online sensors to monitor process data during the in-situ observation. The online data taken from the process are transferred via ‘fieldbus’ modules to the process computer. The process control system (PCS) using the software
2. Materials and methods 2.1. The laboratory scale biogas tower reactor Fig. 1 shows a schematic drawing of the laboratory plant of the Biogas tower reactor (BTR) [3] which is used for the in-situ observation. The reactor volume is 300 l, height 6 m with a diameter of 0.225 m. The reactor is made of glass and plexiglass. Gas separators along the height of the reactor are used to take out gas and thus to avoid an accumulation of gas bubbles in the upper sections of the reactor. A gas recirculation is
Fig. 1. Schematic drawing of the biogas tower reactor in laboratory scale (volume: 300 l, height: 6 m, diameter: 0.225 m).
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T. Pietsch et al. / Water Research 36 (2002) 2836–2842
Table 1 Typical composition of the artificial pre-acidified sucrose feed solution (macroelements) for degradation in the biogas tower reactor in laboratory scale Fructose Glucose Acetic acid Propionic acid Butyric acid KOH NaOH NH4Cl (NH4)3PO4 CaCl2 6H2O MgCl2 6H2O KH2PO4 FeCl2 Na2S Yeast extract Trace elements
10.000 2.000 90 2 60 3 3 400 700 74 305 136 5 60 250 according to Dubach
mg l1 mg l1 mmol l1 mmol l1 mmol l1 mg l1 mg l1 mg l1 mg l1 mg l1 mg l1 mg l1 mg l1 mg l1 mg l1 [4]
industrial computer control concept (iCCC) with the real time operating system Lynx-OS is applied for controlling the reactor. The reactor is fed with artificial preacidified sucrose solution the composition of which is shown in Table 1. The sucrose has almost totally been converted to fructose, glucose, acetic acid, propionic acid and butyric acid. The pH of the solution ranges between pH 4 and 5. The reactor was inoculated with a mixture of nongranulated sludge from a full-scale municipal wastewater treatment plant and granulated sludge from a UASB plant treating pulp and paper effluent. The reactor was operated for a period of more than 2 years. The start-up is not subject of this paper. The observations of the sludge covered a period of approximately 1 year in which the sludge appearance remained stable. The organic loading rate was typically between 5 and 7 kg DOC m3 reactor per day. The DOC removal efficiency was >85% with hydraulic retention times within 2–4 days. 2.2. Macroscopic in-situ observation The macroscopic probe is shown in operation in Fig. 2. Silica fibers transport the illumination light to the head of the probe where it is emitted. The reflected light is returned to the CCD camera without any enlarging lenses. For illumination the permanent emitter KL1500Z (Schott) was used which filtered wavelength above 800 nm. Taking the observation window with its dimensions of 2 6 mm2 the suspension passing in front of this window can be observed. Due to the chosen dimension the macroscopic appearance of particles and bubbles as well as their movements can be observed. Typically, video movies are recorded.
Fig. 2. Macroscopic probe in measuring position for observation and detection of velocities (particles or gas bubbles). Possible observation window 2 6 mm2, no enlarging lenses are integrated.
Using framegrabber technology sequences of single pictures can be generated from the video films. Based on the time interval between two single pictures the velocities of the observed particles and bubbles can be determined. It must be stated that only a two-dimensional analysis (horizontal and vertical axes) is possible. The third dimension which is axially into the picture can not be assessed by this method. This probe is mainly designed to determine the overall appearance of a three-phase suspension and to gain the velocities of the observed items. 2.3. Microscopic in-situ observation This instrument is called process microscope and it has been designed for operation in ‘optically dark’ suspensions. Using pulsed dark-field illumination, it permits direct observation of the particles which constitute the biomass in the reactor. In contrast to the macroscopic probe this instrument is aimed to observe the structure of sludge particles. The microscope is built into a stainless steel tube of 24 mm outer diameter. Its length of 350 mm permits insertion to any depth up to the full diameter of the reactor. At the input end of the microscope, an observation window (4 mm diameter, 0.5 mm thick) and a concentric illuminating ‘axicon’ lens seal the airfilled optical system from the aqueous medium of the reactor, see Fig. 3a. The ‘probe volume’ of the microscope, right outside the window, is observed through a commercial microscope objective [5]. A system of three high-quality lenses relays the primary image to the output end. There it is projected onto a color video camera and routed to a closed-circuit TV system for convenient observation, recording, image processing, etc. The choice of this ‘endoscope’ geometry, with illumination and observation of the probe volume from the same side, minimizes the perturbation of the flow
T. Pietsch et al. / Water Research 36 (2002) 2836–2842
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ULTROPAK microscope objective
observation window
z
to relay lenses and camera from Xe - lamp silica fiber, guiding the illumination light
probe volume colllimating lens
intensity of illumination
axicon lens (silica)
∆ z = 0.3 mm
zo 0
(a)
diameter 24 mm
0.2
0.4
0.6
0.8 z
(b) distance from window [mm]
Fig. 3. (a) Cross-sectional view of the microscope head, showing the microscope objective and 2 of the 18 illuminating beams. The region of their intersection in front of the window is the probe volume. (b) Calculated [9] intensity distribution of the illuminating light along the axis of the microscope.
pattern in the reactor by the body of the microscope. A transmission geometry, by comparison, would require the insertion of two bodies, one for illumination and one for observation. Nevertheless, with the present geometry, flow is restricted in the probe volume right in front of the window to a motion parallel to the window. Another characteristic of the ‘endoscope’ geometry is that the image is produced by light scattered through E901. For the biological phase objects of interest here, it has the consequence that objects with sharply defined boundaries are predominantly seen. Imaging of soft, diffuse, weakly scattered boundaries requires particularly bright illumination, preferably with short-wavelength (blue) light and a particularly low level of stray light. The microscope was designed accordingly. The microscope shows the probe volume which can be ‘seen’ by the optical observation system and which simultaneously is illuminated by the Xenon flash lamp. This definition of the probe volume has to be understood in order to appreciate observations with the instrument. Along the two lateral dimensions the microscope can see a field of view measuring 2.3 2.0 mm2 at the window. Along the third, axial dimension the microscope receives light from all points in a cone extending from that field of view. Its semiaperture is the numerical aperture (NA=0.18) of the objective lens. From that cone, however, only points inside a thin slice yield a sharp image, other points appear blurred. The thickness (E30 mm) of that slice is the ‘depth of field’ of the objective. As a consequence, the illuminating light should be directed only into this ‘depth of field’, so as to minimize the amount of light returning into the microscope from points outside the region of sharp imaging. Such light appears as ‘stray light’, reducing the image contrast.
Minimization of this stray light is achieved by ‘dark field illumination’. The illuminating light is well collimated and directed by an ‘axicon’ lens from all sides into the probe volume, as shown in Fig. 3a. The illuminating rays nearly graze the axicon surface, so as to keep the ‘depth of illumination’ as close to the window and as short as possible. The resulting ‘depth of illumination’ has a width of DzE300 mm, its center is situated at a position zo E250 mm outside the window, see Fig. 3b. The 30 mm ‘depth of field’ is adjusted accordingly to zo : No further ‘focusing’ of the microscope is necessary. The microscope produces a sharp image of the region Dz and a slightly blurred image of the neighboring probe volume. Snapshots are taken, using the flash illumination until, by chance, a particle of interest happens to be ‘in focus’, as judged from the images observed. The Xenon flash lamp (Hamamatsu LL6604) was selected for its short pulse duration (3 ms) and broad spectrum, with a strong emphasis at the blue end. The short pulse illumination serves to ‘freeze’ the motion of the freely floating particles. Thus, at the given resolution of the microscope objective of E2 mm, a maximum particle velocity of E1 m s1 is acceptable before motion blurr deteriorates the image quality. From the Xe lamp, the light is guided to the microscope by a bundle of 18 fibers (200/220 mm all silica fibers). Correspondingly, there are 18 collimated beams illuminating the probe volume. The arrangement of the 18 beams is vaguely discernible in Fig. 5d, where the gas bubble act as a convex mirror showing the far-field distribution of the illuminating light. The color video camera (Hitachi HVC-20) uses three CCD-arrays of 795 596 pixels each, with ‘progressive scan’ reconstruction of the color images.
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2.4. Determination of the gas content in microbial pellets The average gas content of the biomass particles has been experimentally determined. Therefore, a fermenter (Bioengineering, volume 16 l) which could be pressurized between 0.2 and 3 bar total pressure is used. The total volume of the liquid is measured varying the pressure in steps of approximately 0.3 bar. The gas content is gained by averaging these data set and assuming the biosuspension to be incompressible.
3. Results and discussion 3.1. Macroscopic in-situ observation: determination of velocities A video movie of the gassing suspension which is taken by the macroscopic probe shows interesting sights of tiny and large movements of gas bubbles and sludge flocs. Especially, the effect of fast rising bubbles on the movement of sludge flocs was observed. The gas bubbles kick the sludge particles from underneath and push them in an upward direction. High biological activity, which means lots of rising bubbles, decreases the settling velocities of sludge particles and flocs considerably. The macroscopic probe videos show settling characteristics of sludge particles in an impressive way. It helped a lot to get a broader understanding of sedimentation. Using the framegrabber technology sequences of single pictures illustrated in Fig. 4 can be gained from the video films. Now, the distinct velocities of gas bubbles as well as sludge particles can be determined
Fig. 4. In-situ sequence of gas bubbles (see white arrow) and biomass flocs for the detection of their velocities: Time lag between each single picture: 100 ms, window dimensions: 2 6 mm2, velocity of the gas bubbles: 5 mm s1. Pictures taken with macroscopic probe.
dividing the moved distance by the time lag between the individual pictures. In this case, the determined velocity of gas bubbles is about 5 mm s1. Apart from the result shown here, complete in-situ settling velocity profiles of gassing sludge particles in the BTR have been determined using this method [6]. 3.2. Microscopic in-situ observation: structure of the suspended sludge Using the in-situ microscope an observation of the biologically active particles is possible. The aim is to get an impression of how the release of gas bubbles from clusters of microorganisms looks like and how the active biomass flocs appear in the biosuspension under gassing conditions. Fig. 5 shows the in-situ pictures of biologically active sludge in the BTR. The morphology of the particles and flocs in the reactor is very diverse. A large variety of shapes and sizes exist. Many particles form larger agglomerates and many of these agglomerates contain gas bubbles. The most striking feature, however, is the presence of a ‘fur’ of long, thin fibers at the surface of most particles. The fibers have lengths of up to 0.5 mm. It should be noted here that the fibers were not observed in samples of sludge taken from the reactor and placed under a standard laboratory microscope. During such a transfer apparently the delicate fibers attach to the granules and are no longer discernible. The existence of fibrous structures on the outer surface of pellets (‘fur’) was unexpected and for the first time observed so clearly in the BTR. It showed the tremendous interaction of the different particles. These fibrous structures on the outer surface of the biomass pellets influence their fluid dynamics substantially because a pellet with an even surface moves totally different than a pellet with the mentioned fibers. It should be mentioned that extensive literature reports about fibrous material on methanogenic sludge exist but typically no movies from in-situ movements of active sludge particles are gained. From optical observation (e.g. Fig. 5b) high portions of sludge flocs could be identified which are filled with biogas bubbles (white spots). These bubbles are entrapped in the flocs compared to also existing ‘free’ bubbles (see Fig. 5d). Most of the observed ‘free’ gas bubbles which are not incorporated into flocs or pellets are bigger than the incorporated ones and show a diameter typically ranging from 100 to 300 mm. In case of an overflow the gas separator, e.g. when there is more gas rising than taken out, the overflowing gas bubbles are much larger in diameter than the above mentioned ‘free’ bubbles and they can be as large as 5 mm. The biomass shows various kinds of agglomerated structures ranging from compact pellets like Fig. 5a up to loose flocs (see Fig. 5c).
T. Pietsch et al. / Water Research 36 (2002) 2836–2842
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Fig. 5. In-situ pictures of the biosuspension in the biogas tower reactor (laboratory scale plant) made with the in-situ-microscope. Image size represents an area of 2.26 1.69 mm2, resolution of the mounted CCD-Camera: 768 574 pixels, 1 pixel corresponds to about 3 3 mm, xenon flash lightning with a flash duration of 3–5 ms. (a) Sludge pellet with fibrous structures on the outer surface. (b) Sludge floc with incorporated gas bubbles (white spots). (c) Fibrous sludge floc with a sludge pellet in its center, single gas bubble with sludge floc (in the back) and a sludge pellet (in the right corner, front). Pictures taken with process microscope.
The microscopic in-situ observation revealed the broad diversity of the sludge and the fact that sludge particles may entrap high portions of gas bubbles. 3.3. Pressure-dependent density of the suspended sludge particles The gas bubbles expand in the case of reduced hydrostatic pressure. As a consequence, the total density of those flocs holding bubbles declines and may reach a lower value than that of the surrounding liquid suspension. The result is the flotation of such biomass flocs. Using the density of the gas phase rgas ; the volumetric gas content in the particles egas ; the density of the sludge rsludge and the total density of the biomass particles rpart can be calculated according to Eq. (1). rpart ¼ egas rgas þ ð1 egas Þrsludge :
ð1Þ
Assuming ideal behavior of the gas phase, the volumetric gas content in the particles egas under other pressure p than the atmospheric pressure po is given by Eq (2). egas ¼ eogas
po : p
ð2Þ
The following values have been applied in this study (density of the biosuspension is denoted as rsusp and is assumed to be water): rsludge ¼ 1050 kg m3 (own measured data [7]), rgas ¼ 1:71 kg m3 (CO2 at 371C,
pressure: 1 bar, VDI Heat Atlas [8], eogas ¼ 0:055; rsusp ¼ 993 kg m3 at 371C [8]. Fig 6 shows the calculated influence of hydrostatic pressure on the total density of biomass particles. Depending on the gas content entrapped in the biomass, the total density increases from the top to the bottom of the BTR. Applying different pressures to the biomass suspension (see Section 2.4) leads to the average gas content of the particles. These measurements determined the mean gas content to about 5.5 vol% of the total particle volume under atmospheric pressure. This corresponds with a gas content of 3.4 vol% in the bottom module of the BTR under operating conditions of 1.6 bar using Eq. (2). If the total density of the particles is lower than the density of the surrounding biosuspension, which is assumed to be water, the particles are not settling but floating (marked as cross-shaded area in Fig. 6). The identification of the total density of active sludge particles under operating conditions is important to understand floating and settling. Apart from biological aspects with regard to the above-mentioned knowledge, sludge flotations might be seen as a result of physicochemical effects. The literature reports extensively how the feed composition determines the sludge structure and density. Moreover, the hydrodynamic regime in a reactor strongly influences the appearance of the sludge structure. The presented methods offer a brilliant tool for further precise research work concerning factors affecting the sludge structure (e.g. shock loadings,
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T. Pietsch et al. / Water Research 36 (2002) 2836–2842
movement from lower to upper sections of the reactor. The calculation of pressure-dependent total density of biomass pellets is shown.
Acknowledgements
Fig. 6. Total density of sludge particles as a function of the depth below the fluid level. Density of the solid sludge: 1050 kg m3, density of the gas at 371C, 1 bar: 1.71 kg m3, density of biosuspension at 371C: 993 kg m3. The cross-shaded area shows the range of density in which flotation of sludge will occur. The volumetric gas content in the particles in the bottom of the reactor under atmospheric pressure is experimentally determined as 5.5 vol% of the volume of a particle.
variations of sludge during long-term observation, change of feed composition).
4. Conclusion The in-situ observation is a brilliant tool to get ‘real’ information about the behavior of biologically active sludge flocs under operating conditions. The macro- and microscopic observations aim at different scopes which can be the determination of in-situ velocities or insights into the sludge structure. The observed fibers on the outside of the particles could be seen so clearly in the BTR for the first time. The in-situ video movies revealed that the movement of the biomass flocs is hydrodynamically more like a mass rather than individuals. The in-situ observation showed the entrapment of gas bubbles in sludge flocs and thus revealed pressure-dependent hydrodynamics of the sludge particles. A flotation of sludge agglomerates, that contain gas bubbles, may occur due to the
The authors gratefully acknowledge the financial support from the Deutsche Forschungsgemeinschaft DFG in the framework of the Sonderforschungsbereich 238 ‘In-situ Measuring Techniques and Dynamic Modelling of Multi-Phase Systems’.
References [1] Fukuzaki S, Nishio N, Sakurai H, Nagai S. Characteristics of methanogenic granules grown on propionate in an upflow anaerobic sludge blanket (UASB) reactor. J Ferment Bioeng 1991;71(1):50–7. [2] Clark JN. Utilization of acid and sweet wheys in a pilotscale UASB digester. NZ J Dairy Sci Technol 1988;23(4):305–27. [3] M.arkl H, Reinhold G. The biogas tower reactor, a new concept for anaerobic sewage treatment (Der Biogas. die anaerobe Turmreaktor, ein neues Reaktorkonzept fur Abwasserreinigung). Chem-Ing Tech. 1994;66(4):534–6. [4] Dubach A. Anaerobic digestion of fructose using methanogenic mixcultures (Anaerobe Verg.arung von Fructose mit methanogenen Mischkulturen). Dissertation, Universit.at Zurich, . 1987. [5] ULTROPAK, UO 6.5x, NA=0.18; Ernst Leitz GmbH, Wetzlar, Germany, 1972. . [6] Pietsch T. Ruckhalt . suspendierter Biomasse und ortliche . Verteilung geloster Gase in einem Biogas-Turmreaktor (Retention of suspended biomass, local distribution of dissolved gases in a biogas tower reactor). Dissertation, Tu Hamburg-Harburg, Logos-Verlag, 2000. [7] Alphenaar PA, Per!ez MC, Lettinga G. The influence of substrate transport limitation on porosity and methanogenic activity of anaerobic sludge granules. Appl Microbiol Biotechnol 1993;39(2):276–80. [8] VDI-Heat Atlas, Verein Deutscher Ingenieure, editor. Dusseldorf: . VDI-Verlag, 1993. [9] Zemax-SE, Optical Design Program, Version 6.1g, Focus Software Inc, 1997.
Macro- and microscopic in-situ observation of gas bubbles and sludge particles in a biogas tower reactor Torsten Pietscha, Ralf Mehrwaldb, Ralf Grajetzkib, Jan Sensb, Tim Pakendorfc, . Kumpartd, Gerhard Matzd, Herbert M.arklb,* Reinhard Ulrichc, Jorn a
b
Deutsche Hefewerke GmbH and Co. oHG, 22041 Hamburg, Germany Institute of Bioprocess and Biochemical Engineering, Technical University Hamburg-Harburg, 21071 Hamburg, Germany c Department of Optics and Instrumentation, Technical University Hamburg-Harburg, 21071 Hamburg, Germany d Department of Instrumentation, Technical University Hamburg-Harburg, 21071 Hamburg, Germany Received 6 April 2001; received in revised form 25 September 2001; accepted 31 October 2001
Abstract Macroscopic and microscopic in-situ observation of particles and gas bubbles are used to get precise impressions of the hydrodynamical characteristics of a biologically active suspension. Moreover, values of in-situ velocities and particle densities can be gained by using these methods. The suspended anaerobic sludge revealed an extensive fibrous structure (‘fur’) on its surface. The observed microfibers have a profound influence on the settling/flotation behavior of the particles because they increase the effective particle volume, they may trap gas bubbles and they favor agglomeration. The biomass particles do not appear as single spherical objects but due to its fibrous structure on the outside as strongly interacting mass. The compressibility of the bubbles which are entrapped in the sludge agglomerates results in a pressure-dependent density of the sludge particles. r 2002 Published by Elsevier Science Ltd. Keywords: In-situ observation; Velocities; Sludge structure; Gas bubbles; Density
1. Introduction and motivation In the literature, an examination and observation of sludge particles is typically done after they have been taken out of the reactor. To determine physical properties of suspended sludge diverse tests are executed which usually do not reflect the in-situ characteristics. Therefore, this paper aims at two in-situ observation methods in macro- and microscopic scale. From macroscopic view an overall appearance of the sludge and the rising gas bubbles can be obtained. Using a sequence of single pictures which show one item allows to determine in-situ velocities of bubbles and particles. Enlarging the scale (microscopic observation) leads to
*Corresponding author. Tel.: +49-40-42878-3017; fax: +4940-42878-2909. E-mail address: [email protected] (H. M.arkl).
very interesting sights into sludge particles and their insitu hydrodynamic behavior. To understand the mechanisms of gas release from the sludge flocs and to get information about the virtual appearance of anaerobic sludge particles in the threephase system (solid particles–liquid–gas bubbles) an insitu observation is necessary. In other studies, settling velocities are particularly determined in external facilities after the removal of the sludge particles from the reactor (e.g. [1]). The appearance of those sludge particles that are sampled from a reactor and monitored outside is completely different from the in-situ observation because the sludge particles and flocs can alter their shape during the sampling. A mechanical shear is applied to the sampled sludge during the passage through the sampling valve. Moreover, inside the reactor the sludge particles are faced to the hydrostatic pressure which is due to the fluid level above. During the sampling a pressure drop to
0043-1354/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII: S 0 0 4 3 - 1 3 5 4 ( 0 1 ) 0 0 5 1 1 - 5
T. Pietsch et al. / Water Research 36 (2002) 2836–2842
Nomenclature Symbols p Pressure, Pa po Atmospheric pressure, Pa
rsludge rgas rpart rsusp
Greek symbols egas Volumetric gas content in sludge particles eogas Volumetric gas content in sludge particles
ambient pressure is usually applied. Gas bubbles will extend and the structure of the sludge flocs may change. The hydrostatic pressure decreases from the bottom to the top of a reactor. This means a major impact on the size of gas bubbles: On their way upwards they will extend. The produced biogas has to release the sludge agglomerates to be collected at the head space. In the case of dense agglomerates, a considerable increase of gas production may lead to unstable particle structures. Clark [2] reported about a breakage of granules after an increase of the gas production in a 700 l upflow anaerobic sludge blanket (UASB) reactor with about 3 m height. It can be assumed that the particles’ disruption is caused by the enhanced gas production, higher gas content of the particles and subsequent extension of the gas bubbles as the pressure drops. As a result, the generated fine sludge particles are washed out of the reactor. To get information about the ‘real’ appearance and hydrodynamic behavior of active anaerobic biomass under gassing conditions, a macroscopic probe and a process microscope were developed for in-situ observation. Commercially available endoscope technology could not be used because the liquid suspension in the reactor is not a clear liquid. This is a prerequisite for standard endoscopy. Hence, a technology had to be developed to observe particles in a turbid suspension. This paper will describe one macro- and microscopic in-situ observation method and will show some typical results and possibilities of both techniques.
2837
under atmospheric pressure Density of the wet sludge, kg m3 Density of the gas phase, kg m3 Total density of the three-phase particles, kg m3 Density of the liquid phase, kg m3
Common abbreviation BTR Biogas tower reactor
applied to increase the mixing of the three-phase suspension in the bottom section of the reactor. The reactor is equipped with 72 possible locations to be entered (not shown in Fig. 1) which have different diameters ranging from 8 to 25 mm. At these positions along the reactor ball valves are provided as access ports which permit the insertion. All access ports can be used by the macroscopic probe using different adapters. Only the 14 largest positions with 25 mm diameter can be accessed by the process microscope for observation in microscopic scale. The reactor is equipped with numerous online sensors to monitor process data during the in-situ observation. The online data taken from the process are transferred via ‘fieldbus’ modules to the process computer. The process control system (PCS) using the software
2. Materials and methods 2.1. The laboratory scale biogas tower reactor Fig. 1 shows a schematic drawing of the laboratory plant of the Biogas tower reactor (BTR) [3] which is used for the in-situ observation. The reactor volume is 300 l, height 6 m with a diameter of 0.225 m. The reactor is made of glass and plexiglass. Gas separators along the height of the reactor are used to take out gas and thus to avoid an accumulation of gas bubbles in the upper sections of the reactor. A gas recirculation is
Fig. 1. Schematic drawing of the biogas tower reactor in laboratory scale (volume: 300 l, height: 6 m, diameter: 0.225 m).
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T. Pietsch et al. / Water Research 36 (2002) 2836–2842
Table 1 Typical composition of the artificial pre-acidified sucrose feed solution (macroelements) for degradation in the biogas tower reactor in laboratory scale Fructose Glucose Acetic acid Propionic acid Butyric acid KOH NaOH NH4Cl (NH4)3PO4 CaCl2 6H2O MgCl2 6H2O KH2PO4 FeCl2 Na2S Yeast extract Trace elements
10.000 2.000 90 2 60 3 3 400 700 74 305 136 5 60 250 according to Dubach
mg l1 mg l1 mmol l1 mmol l1 mmol l1 mg l1 mg l1 mg l1 mg l1 mg l1 mg l1 mg l1 mg l1 mg l1 mg l1 [4]
industrial computer control concept (iCCC) with the real time operating system Lynx-OS is applied for controlling the reactor. The reactor is fed with artificial preacidified sucrose solution the composition of which is shown in Table 1. The sucrose has almost totally been converted to fructose, glucose, acetic acid, propionic acid and butyric acid. The pH of the solution ranges between pH 4 and 5. The reactor was inoculated with a mixture of nongranulated sludge from a full-scale municipal wastewater treatment plant and granulated sludge from a UASB plant treating pulp and paper effluent. The reactor was operated for a period of more than 2 years. The start-up is not subject of this paper. The observations of the sludge covered a period of approximately 1 year in which the sludge appearance remained stable. The organic loading rate was typically between 5 and 7 kg DOC m3 reactor per day. The DOC removal efficiency was >85% with hydraulic retention times within 2–4 days. 2.2. Macroscopic in-situ observation The macroscopic probe is shown in operation in Fig. 2. Silica fibers transport the illumination light to the head of the probe where it is emitted. The reflected light is returned to the CCD camera without any enlarging lenses. For illumination the permanent emitter KL1500Z (Schott) was used which filtered wavelength above 800 nm. Taking the observation window with its dimensions of 2 6 mm2 the suspension passing in front of this window can be observed. Due to the chosen dimension the macroscopic appearance of particles and bubbles as well as their movements can be observed. Typically, video movies are recorded.
Fig. 2. Macroscopic probe in measuring position for observation and detection of velocities (particles or gas bubbles). Possible observation window 2 6 mm2, no enlarging lenses are integrated.
Using framegrabber technology sequences of single pictures can be generated from the video films. Based on the time interval between two single pictures the velocities of the observed particles and bubbles can be determined. It must be stated that only a two-dimensional analysis (horizontal and vertical axes) is possible. The third dimension which is axially into the picture can not be assessed by this method. This probe is mainly designed to determine the overall appearance of a three-phase suspension and to gain the velocities of the observed items. 2.3. Microscopic in-situ observation This instrument is called process microscope and it has been designed for operation in ‘optically dark’ suspensions. Using pulsed dark-field illumination, it permits direct observation of the particles which constitute the biomass in the reactor. In contrast to the macroscopic probe this instrument is aimed to observe the structure of sludge particles. The microscope is built into a stainless steel tube of 24 mm outer diameter. Its length of 350 mm permits insertion to any depth up to the full diameter of the reactor. At the input end of the microscope, an observation window (4 mm diameter, 0.5 mm thick) and a concentric illuminating ‘axicon’ lens seal the airfilled optical system from the aqueous medium of the reactor, see Fig. 3a. The ‘probe volume’ of the microscope, right outside the window, is observed through a commercial microscope objective [5]. A system of three high-quality lenses relays the primary image to the output end. There it is projected onto a color video camera and routed to a closed-circuit TV system for convenient observation, recording, image processing, etc. The choice of this ‘endoscope’ geometry, with illumination and observation of the probe volume from the same side, minimizes the perturbation of the flow
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ULTROPAK microscope objective
observation window
z
to relay lenses and camera from Xe - lamp silica fiber, guiding the illumination light
probe volume colllimating lens
intensity of illumination
axicon lens (silica)
∆ z = 0.3 mm
zo 0
(a)
diameter 24 mm
0.2
0.4
0.6
0.8 z
(b) distance from window [mm]
Fig. 3. (a) Cross-sectional view of the microscope head, showing the microscope objective and 2 of the 18 illuminating beams. The region of their intersection in front of the window is the probe volume. (b) Calculated [9] intensity distribution of the illuminating light along the axis of the microscope.
pattern in the reactor by the body of the microscope. A transmission geometry, by comparison, would require the insertion of two bodies, one for illumination and one for observation. Nevertheless, with the present geometry, flow is restricted in the probe volume right in front of the window to a motion parallel to the window. Another characteristic of the ‘endoscope’ geometry is that the image is produced by light scattered through E901. For the biological phase objects of interest here, it has the consequence that objects with sharply defined boundaries are predominantly seen. Imaging of soft, diffuse, weakly scattered boundaries requires particularly bright illumination, preferably with short-wavelength (blue) light and a particularly low level of stray light. The microscope was designed accordingly. The microscope shows the probe volume which can be ‘seen’ by the optical observation system and which simultaneously is illuminated by the Xenon flash lamp. This definition of the probe volume has to be understood in order to appreciate observations with the instrument. Along the two lateral dimensions the microscope can see a field of view measuring 2.3 2.0 mm2 at the window. Along the third, axial dimension the microscope receives light from all points in a cone extending from that field of view. Its semiaperture is the numerical aperture (NA=0.18) of the objective lens. From that cone, however, only points inside a thin slice yield a sharp image, other points appear blurred. The thickness (E30 mm) of that slice is the ‘depth of field’ of the objective. As a consequence, the illuminating light should be directed only into this ‘depth of field’, so as to minimize the amount of light returning into the microscope from points outside the region of sharp imaging. Such light appears as ‘stray light’, reducing the image contrast.
Minimization of this stray light is achieved by ‘dark field illumination’. The illuminating light is well collimated and directed by an ‘axicon’ lens from all sides into the probe volume, as shown in Fig. 3a. The illuminating rays nearly graze the axicon surface, so as to keep the ‘depth of illumination’ as close to the window and as short as possible. The resulting ‘depth of illumination’ has a width of DzE300 mm, its center is situated at a position zo E250 mm outside the window, see Fig. 3b. The 30 mm ‘depth of field’ is adjusted accordingly to zo : No further ‘focusing’ of the microscope is necessary. The microscope produces a sharp image of the region Dz and a slightly blurred image of the neighboring probe volume. Snapshots are taken, using the flash illumination until, by chance, a particle of interest happens to be ‘in focus’, as judged from the images observed. The Xenon flash lamp (Hamamatsu LL6604) was selected for its short pulse duration (3 ms) and broad spectrum, with a strong emphasis at the blue end. The short pulse illumination serves to ‘freeze’ the motion of the freely floating particles. Thus, at the given resolution of the microscope objective of E2 mm, a maximum particle velocity of E1 m s1 is acceptable before motion blurr deteriorates the image quality. From the Xe lamp, the light is guided to the microscope by a bundle of 18 fibers (200/220 mm all silica fibers). Correspondingly, there are 18 collimated beams illuminating the probe volume. The arrangement of the 18 beams is vaguely discernible in Fig. 5d, where the gas bubble act as a convex mirror showing the far-field distribution of the illuminating light. The color video camera (Hitachi HVC-20) uses three CCD-arrays of 795 596 pixels each, with ‘progressive scan’ reconstruction of the color images.
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2.4. Determination of the gas content in microbial pellets The average gas content of the biomass particles has been experimentally determined. Therefore, a fermenter (Bioengineering, volume 16 l) which could be pressurized between 0.2 and 3 bar total pressure is used. The total volume of the liquid is measured varying the pressure in steps of approximately 0.3 bar. The gas content is gained by averaging these data set and assuming the biosuspension to be incompressible.
3. Results and discussion 3.1. Macroscopic in-situ observation: determination of velocities A video movie of the gassing suspension which is taken by the macroscopic probe shows interesting sights of tiny and large movements of gas bubbles and sludge flocs. Especially, the effect of fast rising bubbles on the movement of sludge flocs was observed. The gas bubbles kick the sludge particles from underneath and push them in an upward direction. High biological activity, which means lots of rising bubbles, decreases the settling velocities of sludge particles and flocs considerably. The macroscopic probe videos show settling characteristics of sludge particles in an impressive way. It helped a lot to get a broader understanding of sedimentation. Using the framegrabber technology sequences of single pictures illustrated in Fig. 4 can be gained from the video films. Now, the distinct velocities of gas bubbles as well as sludge particles can be determined
Fig. 4. In-situ sequence of gas bubbles (see white arrow) and biomass flocs for the detection of their velocities: Time lag between each single picture: 100 ms, window dimensions: 2 6 mm2, velocity of the gas bubbles: 5 mm s1. Pictures taken with macroscopic probe.
dividing the moved distance by the time lag between the individual pictures. In this case, the determined velocity of gas bubbles is about 5 mm s1. Apart from the result shown here, complete in-situ settling velocity profiles of gassing sludge particles in the BTR have been determined using this method [6]. 3.2. Microscopic in-situ observation: structure of the suspended sludge Using the in-situ microscope an observation of the biologically active particles is possible. The aim is to get an impression of how the release of gas bubbles from clusters of microorganisms looks like and how the active biomass flocs appear in the biosuspension under gassing conditions. Fig. 5 shows the in-situ pictures of biologically active sludge in the BTR. The morphology of the particles and flocs in the reactor is very diverse. A large variety of shapes and sizes exist. Many particles form larger agglomerates and many of these agglomerates contain gas bubbles. The most striking feature, however, is the presence of a ‘fur’ of long, thin fibers at the surface of most particles. The fibers have lengths of up to 0.5 mm. It should be noted here that the fibers were not observed in samples of sludge taken from the reactor and placed under a standard laboratory microscope. During such a transfer apparently the delicate fibers attach to the granules and are no longer discernible. The existence of fibrous structures on the outer surface of pellets (‘fur’) was unexpected and for the first time observed so clearly in the BTR. It showed the tremendous interaction of the different particles. These fibrous structures on the outer surface of the biomass pellets influence their fluid dynamics substantially because a pellet with an even surface moves totally different than a pellet with the mentioned fibers. It should be mentioned that extensive literature reports about fibrous material on methanogenic sludge exist but typically no movies from in-situ movements of active sludge particles are gained. From optical observation (e.g. Fig. 5b) high portions of sludge flocs could be identified which are filled with biogas bubbles (white spots). These bubbles are entrapped in the flocs compared to also existing ‘free’ bubbles (see Fig. 5d). Most of the observed ‘free’ gas bubbles which are not incorporated into flocs or pellets are bigger than the incorporated ones and show a diameter typically ranging from 100 to 300 mm. In case of an overflow the gas separator, e.g. when there is more gas rising than taken out, the overflowing gas bubbles are much larger in diameter than the above mentioned ‘free’ bubbles and they can be as large as 5 mm. The biomass shows various kinds of agglomerated structures ranging from compact pellets like Fig. 5a up to loose flocs (see Fig. 5c).
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Fig. 5. In-situ pictures of the biosuspension in the biogas tower reactor (laboratory scale plant) made with the in-situ-microscope. Image size represents an area of 2.26 1.69 mm2, resolution of the mounted CCD-Camera: 768 574 pixels, 1 pixel corresponds to about 3 3 mm, xenon flash lightning with a flash duration of 3–5 ms. (a) Sludge pellet with fibrous structures on the outer surface. (b) Sludge floc with incorporated gas bubbles (white spots). (c) Fibrous sludge floc with a sludge pellet in its center, single gas bubble with sludge floc (in the back) and a sludge pellet (in the right corner, front). Pictures taken with process microscope.
The microscopic in-situ observation revealed the broad diversity of the sludge and the fact that sludge particles may entrap high portions of gas bubbles. 3.3. Pressure-dependent density of the suspended sludge particles The gas bubbles expand in the case of reduced hydrostatic pressure. As a consequence, the total density of those flocs holding bubbles declines and may reach a lower value than that of the surrounding liquid suspension. The result is the flotation of such biomass flocs. Using the density of the gas phase rgas ; the volumetric gas content in the particles egas ; the density of the sludge rsludge and the total density of the biomass particles rpart can be calculated according to Eq. (1). rpart ¼ egas rgas þ ð1 egas Þrsludge :
ð1Þ
Assuming ideal behavior of the gas phase, the volumetric gas content in the particles egas under other pressure p than the atmospheric pressure po is given by Eq (2). egas ¼ eogas
po : p
ð2Þ
The following values have been applied in this study (density of the biosuspension is denoted as rsusp and is assumed to be water): rsludge ¼ 1050 kg m3 (own measured data [7]), rgas ¼ 1:71 kg m3 (CO2 at 371C,
pressure: 1 bar, VDI Heat Atlas [8], eogas ¼ 0:055; rsusp ¼ 993 kg m3 at 371C [8]. Fig 6 shows the calculated influence of hydrostatic pressure on the total density of biomass particles. Depending on the gas content entrapped in the biomass, the total density increases from the top to the bottom of the BTR. Applying different pressures to the biomass suspension (see Section 2.4) leads to the average gas content of the particles. These measurements determined the mean gas content to about 5.5 vol% of the total particle volume under atmospheric pressure. This corresponds with a gas content of 3.4 vol% in the bottom module of the BTR under operating conditions of 1.6 bar using Eq. (2). If the total density of the particles is lower than the density of the surrounding biosuspension, which is assumed to be water, the particles are not settling but floating (marked as cross-shaded area in Fig. 6). The identification of the total density of active sludge particles under operating conditions is important to understand floating and settling. Apart from biological aspects with regard to the above-mentioned knowledge, sludge flotations might be seen as a result of physicochemical effects. The literature reports extensively how the feed composition determines the sludge structure and density. Moreover, the hydrodynamic regime in a reactor strongly influences the appearance of the sludge structure. The presented methods offer a brilliant tool for further precise research work concerning factors affecting the sludge structure (e.g. shock loadings,
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movement from lower to upper sections of the reactor. The calculation of pressure-dependent total density of biomass pellets is shown.
Acknowledgements
Fig. 6. Total density of sludge particles as a function of the depth below the fluid level. Density of the solid sludge: 1050 kg m3, density of the gas at 371C, 1 bar: 1.71 kg m3, density of biosuspension at 371C: 993 kg m3. The cross-shaded area shows the range of density in which flotation of sludge will occur. The volumetric gas content in the particles in the bottom of the reactor under atmospheric pressure is experimentally determined as 5.5 vol% of the volume of a particle.
variations of sludge during long-term observation, change of feed composition).
4. Conclusion The in-situ observation is a brilliant tool to get ‘real’ information about the behavior of biologically active sludge flocs under operating conditions. The macro- and microscopic observations aim at different scopes which can be the determination of in-situ velocities or insights into the sludge structure. The observed fibers on the outside of the particles could be seen so clearly in the BTR for the first time. The in-situ video movies revealed that the movement of the biomass flocs is hydrodynamically more like a mass rather than individuals. The in-situ observation showed the entrapment of gas bubbles in sludge flocs and thus revealed pressure-dependent hydrodynamics of the sludge particles. A flotation of sludge agglomerates, that contain gas bubbles, may occur due to the
The authors gratefully acknowledge the financial support from the Deutsche Forschungsgemeinschaft DFG in the framework of the Sonderforschungsbereich 238 ‘In-situ Measuring Techniques and Dynamic Modelling of Multi-Phase Systems’.
References [1] Fukuzaki S, Nishio N, Sakurai H, Nagai S. Characteristics of methanogenic granules grown on propionate in an upflow anaerobic sludge blanket (UASB) reactor. J Ferment Bioeng 1991;71(1):50–7. [2] Clark JN. Utilization of acid and sweet wheys in a pilotscale UASB digester. NZ J Dairy Sci Technol 1988;23(4):305–27. [3] M.arkl H, Reinhold G. The biogas tower reactor, a new concept for anaerobic sewage treatment (Der Biogas. die anaerobe Turmreaktor, ein neues Reaktorkonzept fur Abwasserreinigung). Chem-Ing Tech. 1994;66(4):534–6. [4] Dubach A. Anaerobic digestion of fructose using methanogenic mixcultures (Anaerobe Verg.arung von Fructose mit methanogenen Mischkulturen). Dissertation, Universit.at Zurich, . 1987. [5] ULTROPAK, UO 6.5x, NA=0.18; Ernst Leitz GmbH, Wetzlar, Germany, 1972. . [6] Pietsch T. Ruckhalt . suspendierter Biomasse und ortliche . Verteilung geloster Gase in einem Biogas-Turmreaktor (Retention of suspended biomass, local distribution of dissolved gases in a biogas tower reactor). Dissertation, Tu Hamburg-Harburg, Logos-Verlag, 2000. [7] Alphenaar PA, Per!ez MC, Lettinga G. The influence of substrate transport limitation on porosity and methanogenic activity of anaerobic sludge granules. Appl Microbiol Biotechnol 1993;39(2):276–80. [8] VDI-Heat Atlas, Verein Deutscher Ingenieure, editor. Dusseldorf: . VDI-Verlag, 1993. [9] Zemax-SE, Optical Design Program, Version 6.1g, Focus Software Inc, 1997.