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Floc Volume Effects in Suspensions and Its Relevance for Wastewater Engineering Jochen Henkel,* Barbara Siembida-L€osch, and Martin Wagner Technische Universit€at Darmstadt, Institut IWAR, Section for Wastewater Technologies, Petersenstrasse 13, 64287 Darmstadt, Germany ABSTRACT: The aim of this paper is to better understand oxygen transfer reduction caused by floc suspensions. We demonstrate that the overall floc volume significantly influences oxygen transfer depletion. Submerged fine bubble and coarse bubble diffusers are affected in the same way by this phenomenon. The mixed liquor suspended solids concentration (MLSS concentration) is not an appropriate parameter for describing or relating phenomena that are caused by the overall floc volume in activated sludge (e.g., oxygen transfer depression and sludge sedimentation characteristics). A better correlation is achieved by using the mixed liquor volatile suspended solids concentration (MLVSS concentration). To characterize the effects of the overall floc volume in suspensions whose MLVSS concentration cannot be determined (e.g., inorganic iron hydroxide flocs), a new method—the hydrostatic floc volume (HFV)—that approximates the overall floc volume in floc suspensions is introduced. Application of this method demonstrates that oxygen transfer depression caused by iron hydroxide flocs and activated sludge flocs is similar.
’ INTRODUCTION The sedimentation behavior of activated sludge continues to be an active research area.14 The main driver for these investigations is to understand and optimize the settling performance of activated sludge in the clarifier, which is essential for reliable operation of activated sludge plants. The standard procedure applied, in practice, to judge the sedimentation behavior is the integrated sludge volume index (SVI). It relates the sludge volume determined after 30 min to the MLSS concentration. Variations of this method, mainly to overcome the bridging problems with filamentous sludge or at higher sludge concentrations, have been developed, including the dilution (DSVI) or the stirred method (SSVI). Bridging is caused by the interaction of the flocs. Generally, steric interactions, which hinder separation of the free water from the bound water of the flocs, occur more frequently with increasing floc number.5,6 Recently, our group was able to demonstrate that the overall floc volume influences oxygen transfer in floc suspensions.7 This was achieved using iron hydroxide flocs and relating oxygen transfer depression (α-factor) to the overall floc volume. In contrast to the SVI concept, the purpose of the floc volume concept is not to estimate the sedimentation behavior of activated sludge but to use a simple method for estimating the overall volume of all flocs in the system. In an earlier study we hypothesized that the better correlation of oxygen transfer depression and the MLVSS concentration, compared to the MLSS concentration of different activated sludge sources, was obtained because the MLVSS concentration regulates the free water content and better correlates with the overall floc volume in activated sludge.8 One explanation is that r 2011 American Chemical Society
the major part of the activated sludge floc consists of water (>90%), which is bound to organic matter as extracellular polymeric substances (EPS), which again contribute to more than 50% of the organic dried matter of sludge.912 Consequently, it is the amount of bounded water that governs the mass and volume of the entire floc. The MLSS concentration, in contrast to the MLVSS concentration, also includes inorganic materials, such as silt, clay, and sand, which escape removal in the primary settling tank and which, compared to the organic matter, have low water binding capabilities but contribute significantly to the dry solid content (∼1525%). This inorganic content varies, depending on the wastewater influent composition and the primary settling tank performance, which led to the wide range of values for α-factors in the past, if the values were related to the MLSS concentration of different wastewater sources.8 We also hypothesized that oxygen transfer depression caused by iron hydroxide flocs is similar to the mechanism by which activated sludge flocs influence oxygen transfer.7 However, at that time, because a methodology for approximating the overall floc volume of activated sludge was lacking, we were not able to verify this hypothesis. This paper aims to fill these knowledge gaps to better understand oxygen transfer reduction caused by activated sludge flocs and by iron hydroxide flocs. A new method is introduced called Received: May 24, 2011 Accepted: August 18, 2011 Revised: August 14, 2011 Published: September 14, 2011 8788
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the hydrostatic floc volume (HFV) that approximates the overall floc volume in activated sludge and is able to compare results obtained in floc suspensions. Furthermore, we show that the MLSS concentration is not an appropriate parameter to correlate and compare phenomena that are caused by the floc volume.
’ MATERIALS AND METHODS Generally, the experiments in this study are a continuation of the experiments described in previous studies.7,8 In the current work, two pilot-scale membrane bioreactors fed with municipal wastewater were used for activated sludge sampling. Oxygen transfer measurements with iron hydroxide flocs and wastewater sludge were performed in a separate lab-scale column. Hydrostatic Floc Volume (HFV). In a mixture of rigid particles and a liquid, the total volume of the particles, in terms of the total volume of the suspension (solid holdup), can be easily determined by the bulk volume of the particles. However, this relationship cannot be used for elastic flocs that incorporate a great deal of water, as is the case for iron hydroxide flocs and activated sludge flocs. The only method which generates an estimate of floc volume is the sludge volume index (SVI, APHA 2710 D).13 It determines the sludge volume after 30 min of sedimentation and relates it to the suspended solids concentration. It is used to characterize the sedimentation characteristics of activated sludge in the clarifier. However, because sedimentation still continues after 30 min, it is not an appropriate parameter to determine the overall floc volume. Consequently, a method had to be developed that enables approximation of the overall floc volume in these suspensions. The following procedure was applied. A 1 L sample of activated sludge or iron hydroxide suspension was taken and stored in a settling column (7 cm diameter). The sedimentation process of the sample continued until the volume of the settled flocs remained constant. In the case of activated sludge, the flocs started to float because of microbial gas production, and therefore, the sludge respiration had to be suppressed. This was achieved using cyanide, as described by Dobbs et al.,14 at a dose of 0.1 g of cyanide per 1 g of biomass dry content. The final volume of the suspension is called the hydrostatic floc volume (HFV). An example of activated sludge sedimentation and definition of the HFV is summarized in Table 1. It can be seen that after approximately 45 h the suspension reached its terminal volume, which is the HFV. Oxygen Transfer Measurements and α-Factor Calculation. The α-factor is the relationship between the oxygen transfer
coefficient (kLa) in the investigated medium (in our case activated sludge and iron hydroxide suspension) and the oxygen transfer coefficient in clean water (eq 1). α-factor ¼
kL amedium kL acleanw
ð1Þ
The α-factor describes how much better or worse oxygen diffuses into the medium compared to clean water and plays an important role in estimating the required standard oxygen transfer rate (SOTR) in activated sludge, which is the key parameter in submerged diffused aeration systems. To determine oxygen transfer coefficients, the desorption method according to Wagner et al.15 was applied (see also Henkel et al.7,8). Four oxygen sensors (iRAS automation GmbH, Bad Klosterlausnitz, Germany) recorded the change in oxygen concentration in the suspension at constant air flow rate and
Table 1. Example of Floc Sedimentation (MLSS = 7.2 g/L; MLVSS = 6 g/L) elapsed time
volume ratio (mL/L)
0h
1000
2 h 15 min 3 h 15 min
380 340
20 h 20 min
260
44 h 55 min
250 (HFV)
141 h
250
Figure 1. Volumetric mass transfer coefficient during iron hydroxide experiments (fine bubble aeration).
solid concentration. The air flow rate, which was measured with a thermal flow sensor TA10 (Hoentzsch GmbH, Waiblingen, Germany), was then changed and the procedure repeated at the same solid concentration. Three air flow rates were chosen for each test (1, 2, 3 m3/h). Subsequently, the three average kLa values were plotted against the superficial gas velocity (SGV). If applicable, a linear trend line was plotted and calculated. This procedure was repeated for every solid concentration and the clean water test. An example is given in Figure 1. Finally, the α-factor was calculated by dividing the trend line equation at a specific solid concentration and specific air flow rate (2 m3air/(diffuser 3 h)) by the equation obtained during the clean water test. This procedure was applied because it was impossible to repeat exactly the same air flow rate for each test series. Humidity, air pressure, air temperature, and the air blower itself are subject to daily variations that influence the air flow rate, which has to be transformed to standard conditions. Lab-Scale Experiments. The bubble column used in this experiment was 1.30 m high and 0.43 m in diameter. The total water volume in each test was 100 L. In contrast to the iron hydroxide experiments performed by Henkel et al.,7 this time 4.5 kg of ferric chloride was mixed with 250 L of potable water in a separate vessel and bentonite was not added. The pH was then adjusted to 8 with 1 M sodium hydroxide, which substantially increased the salt concentration. As the salt content also influences oxygen transfer, the supernatant of the iron hydroxide flocs was replaced by potable water several times until the salt content reached a concentration equivalent to that of potable water. The final salt content varied between 600 and 850 mg/L. After sedimentation of the iron hydroxide flocs, 150 L of the supernatant was stored separately. 8789
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Table 2. Hydrodynamic Specifications during Lab-Scale Oxygen Transfer Experiments diffuser
number of
riser surface,
superficial gas velocity
specific slit aeration
specific diffuser aeration rate,
type
orifices, n
m2
(SGV), cm3Air/(cm2Surf. 3 s)
rate, cm3Air/(slit 3 s)
m3Air/(diffuser 3 h)
disc
8000
0.15
0.150.55
0.030.10
13
tube
10
0.15
0.150.55
2585
13
Figure 2. Relationship between sludge volumes (dilution method) and MLSS concentration.
Figure 3. Relationship between sludge volumes and MLVSS concentration.
The oxygen transfer experiments started at the highest iron hydroxide concentration (26.1 g/L). After each experiment, a fixed amount of iron hydroxide flocs was removed from the system and replaced with the stored supernatant. kLa and the α-factor were determined as described in section. A fine bubble and a coarse bubble aeration device were tested to investigate whether iron hydroxide flocs show the same behavior as observed previously with activated sludge.8 The specifications of the setup are listed in Table 2. Finally, the same experiments were performed with activated sludge and the fine bubble aeration device. The activated sludge was taken from a membrane bioreactor fed with real wastewater from a municipal wastewater plant (DarmstadtEberstadt, Germany). To achieve a range of sludge concentrations, the sludge was diluted with permeate from the membrane bioreactor. The highest sludge concentration tested was 17.5 g/L. Sedimentation Experiments. Once the hydrostatic floc volume procedure was established, it was applied during operation of two pilot-scale membrane bioreactors. The membrane bioreactors had a water volume of 1 m3 (1.7 m water depth), and the wastewater flow was 100 L/h. The organic load was 0.9 kg of COD/d. The F/M ratio depended on the actual MLVSS concentration in the tank but ranged from 0.45 to 0.02 kg BOD5/ (kg MLVSS 3 d). Additionally, the sludge volume was determined after 30 min without dilution to compare the results.13
different kinds of sludges showing different settling behavior at the same MLSS concentration could show similar settling behavior at the same MLVSS concentration if the overall floc volume is the main reason for bridging. Figure 2 depicts the sludge volume, determined with the dilution method, as a function of the MLSS concentration of three sludges. The data for the greywater sludge was generated during the 2 year operation of two membrane bioreactors fed with synthetic greywater.7,8 The data for the wastewater sludges were generated during the operation of two separate membrane bioreactors fed with real wastewater. A linear relationship can be observed in all three cases, although the variation of the sludge volume at a certain MLSS concentration was quite large. Whereas the wastewater sludges showed a similar development with increasing MLSS concentration, the greywater sludge volume increased more slowly with increasing MLSS concentration. Calculation of the SVI indicates that the sedimentation characteristic of membrane bioreactor wastewater sludge at a specific MLSS concentration was worse (for example, SVI 83 mL/g at MLSS of 12 g/L) than that of the greywater sludge (SVI ≈ 33 mL/g at MLSS of 12 g/L). Microscopic investigation of the sludges revealed that neither filamentous bacteria nor bulking sludge were present in the MBR greywater sludge and the MBR wastewater sludge 2010. During colder periods, especially in January and February, the MBR wastewater sludge 2009 did contain filamentous bacteria. The major difference between sludges was the organic content, which was around 38% and 82% for the greywater sludge and the wastewater sludge, respectively. The nontypical low ignition loss of greywater sludge was caused by the presence of a large amount of abrasive materials, used in toothpaste, and zeolites, used in washing agents, in the greywater influent. If the same data are plotted against MLVSS concentration (Figure 3), each type of sludge showed a much closer relationship than before. We concluded that the overall floc volumes of greywater and wastewater sludge at a certain MLVSS concentration were
’ RESULTS AND DISCUSSION Sludge Volume Index. The sludge volume index provides a rough estimate of how well sludge settles in the clarifier. It is calculated from the ratio of sludge volume after 30 min to MLSS concentration. As noted, the dilution method and/or stirring method have been applied to determine sludge volume after 30 min, when sludge settling properties are hindered by bridging between flocs.5,16 We hypothesized that the MLVSS concentration better correlates to the overall floc volume in activated sludge. One conjecture generated by this hypothesis was that
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Figure 4. Sludge volume after 30 min of sedimentation (blue triangles) and hydrostatic floc volume (yellow rectangles).
similar, which resulted in similar sedimentation characteristic, as observed in this study, and similar oxygen transfer behaviors of different kinds of sludges observed in our previous study.8 This is another indicator for the close relationship of MLVSS concentration to the overall floc volume in activated sludge. Hydrostatic Floc Volume. As noted in the former section, the sludge volume method, especially at high suspended solids concentrations, delivers a relatively wide range of sludge volumes at a certain MLSS or MLVSS concentration. Additionally, sludge volumes larger than 1000 mL/L are an artifact of the dilution method, caused by multiplication of the dilution applied to the sludge, rather than an approximation of the overall floc volume present in the sludge. Because the MLVSS concentration may only be used to approximate the overall floc volume in biological floc aggregates and we wanted to compare activated sludge flocs with iron hydroxide flocs (inorganic flocs), we developed a new method (see Materials and Methods) to approximate the overall floc volume in floc suspensions that does not destroy the floc structure and is easy to apply. We emphasize that it is not an alternative to the SVI method, which aims at characterizing activated sludge sedimentation behavior in the clarifier. Figure 4 shows the results of sludge volume determination according to the APHA method,13 without stirring or dilution, and the HFV method, related to the MLVSS concentration. The volume ratio increased with increasing MLVSS concentration in both cases. A linear correlation was observed for the HFV method up to a floc volume of about 500 mL/L at an MLVSS concentration of 11 g/L. The trend line follows the equation HFV ¼ 43:85 3 MLVSSð( 22Þ
ð2Þ
Compared to the sludge volume method with dilution presented in the section Sludge Volume Index, the HFV method still delivers reasonable values at elevated MLVSS concentrations and, additionally, shows smaller deviations. The values achieved with the sludge volume method without dilution demonstrate the effect of bridging that occurs at sludge volumes higher than 300 mL/L. The steric interferences of the flocs hinder sedimentation and result in a sharp increase in sludge volumes. It seems as if the HFV method resolves this bridging effect and correlates well with the MLVSS concentration. It indicates that this method may be used to correlate the overall floc volume of activated sludge. Oxygen Transfer at Various Floc Concentrations. In a previous study, we demonstrated that the MLVSS concentration affects oxygen transfer depression (α-factor) of submerged fine
Figure 5. α-Factor of iron hydroxide flocs and activated sludge flocs related to the concentration of suspended solids.
Figure 6. α-Factor of iron hydroxide flocs and activated sludge flocs related to the hydrostatic floc volume.
bubble and coarse bubble aeration systems in the same way.8 To test the hypothesis that the mechanism of oxygen transfer depression caused by activated sludge flocs and iron hydroxide flocs is similar, we exposed iron hydroxide flocs to fine and coarse bubble aeration systems. Figure 5 illustrates the results of this experiment. Additionally, one set of tests was performed with activated sludge in the same column to compare the results. Figure 5 shows that the α-factor in the experiments with iron hydroxide decreased linearly with increasing MLSS concentration, irrespective of the aeration system (coarse bubble aeration or fine bubble aeration). This result is identical to the data derived from activated sludge in the earlier studies. However, even though the decrease is also linear for activated sludge flocs, the slope, compared to the iron hydroxide flocs, is different. Hence, it could be concluded that activated sludge flocs reduce oxygen transfer more strongly than iron hydroxide flocs. As mentioned above, one reason why we developed the HFV method was the theory that the MLSS concentration did not correlate with the overall floc volume of various suspensions. In Figure 6 we compare the α-factors from Figure 5 to the HFV. In contrast to the results shown in Figure 5, all suspensions now show similar trends. This result is remarkable because hydroxide flocs and activated sludge flocs are very different in 8791
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Environmental Science & Technology their water binding capacities and densities.17,18 The results support the hypothesis that the overall floc volume is the actual driver for oxygen transfer depression in suspensions that contain flocs and that the MLSS concentration insufficiently correlates with the overall floc volume. These results also suggest that the HFV is a useful parameter for comparing phenomena that can be attributed to the floc volume, such as gas transfer. In oxygen transfer studies activated sludge has been considered to be a pseudohomogeneous medium with non-Newtonian pseudoplastic fluid properties.19 One way of characterizing these fluids is to measure their apparent viscosity. Although the apparent viscosity of activated sludge has often been assumed to be responsible for the decrease in the α-factor, supporting systematic studies are scarce. Some authors correlated the apparent viscosity to the α-factor of different sludges but only with limited success, especially if the results of these studies are compared with each other.20,21 It is debatable whether apparent viscosity is a meaningful parameter in terms of oxygen transfer in floc suspensions. A promising solution to this dilemma would be to replace the idea of a pseudohomogeneous medium with the new concept of a suspension consisting of flocs that interact with the air bubble. Interpretations of how the floc influences oxygen transfer into the free water fraction could then incorporate results that come from investigations of solid suspensions in the field of chemical engineering. A parameter used to compare oxygen transfer in solid suspensions is the solid holdup, which relates the overall volume occupied by the solids to the overall volume of the suspension. With increasing solid holdup (εS), a decrease in oxygen transfer is commonly observed.2224 Mena et al.25 summarize eight ways in which the gasliquid system can be affected by solids. Transferring these observations to activated sludge leads to the proposition that the following phenomena may influence oxygen transfer. 1 The increased floc volume decreases the interfacial area between the bubble and the liquid phase, since it makes contact with the bubble surface and hinders transfer to the liquid phase. 2 The tendency of bubbles to coalesce is favored by attachment of small, hydrophobic flocs, which leads to larger bubbles with a lower specific surface area. 3 Turbulence in the bubble wake is diminished due to accumulation of flocs in the wake area. 4 With increasing floc number, the possibility of collision during bubble formation at the orifices increases, which reduces the bubble formation frequency and leads to larger bubbles at the orifice. 5 The decrease in the free water content leads to an increase in gas holdup, related to the liquid phase at the same air flow rate. This shifts the critical superficial gas velocity, which is responsible for the change from the homogeneous flow regime to the heterogeneous flow regime, to lower values. 6 The increased floc volume also increases the probability of collision between the flocs themselves and increases the apparent viscosity. This again leads to a decrease in the liquid velocity at the same air flow rate, which results in formation of larger bubbles at the orifice and enhances the probability of bubble coalescence. All these effects would lead to a decrease of the volumetric oxygen transfer coefficient kLa in activated sludge. Since suppression of the α-factor for fine bubble and coarse bubble aeration systems was similar and bubble formation and bubble rise
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characteristics of coarse bubbles are not affected by the liquid properties,26 only two phenomena remain that could explain the similar behavior. The first is the reduction of turbulence in the bubble wake area; the second describes suppression of the oxygen transfer from the bubble to the liquid phase. However, further investigations are required to determine which mechanism is responsible for this phenomenon.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +1 303 273 3871, fax: +1 303 273 3413; e-mail:
[email protected].
’ ACKNOWLEDGMENT The authors wish to thank Prof. Cornel, our department head, for his time and inspiring discussions, the German Federal Ministry of Education & Research, and the Faudi Foundation for their financial support. Ms. Anita Curt, our laboratory assistant at the wastewater treatment plant in Eberstadt, deserves special thanks for the enormous number of samples she processed during this work. ’ REFERENCES (1) Chen, Y.; Yang, H.; Gu, G. Effect of acid and surfactant treatment on activated sludge dewatering and settling. Water Res. 2001, 35 (11), 2615–2620. (2) Li, X. Y.; Yang, S. F. Influence of loosely bound extracellular polymeric substances (EPS) on the flocculation, sedimentation and dewaterability of activated sludge. Water Res. 2007, 41 (5), 1022–1030. (3) Schuler, A. J.; Jang, H. Causes of variable biomass density and its effects on settleability in full-scale biological wastewater treatment systems. Environ. Sci. Technol. 2007, 41 (5), 1675–1681. (4) Wilen, B.-M.; Lumley, D.; Mattsson, A.; Mino, T. Relationship between floc composition and flocculation and settling properties studied at a full scale activated sludge plant. Water Res. 2008, 42 (16), 4404–4418. (5) Dick, R. I.; Vesilind, P. A. The Sludge Volume Index: What Is It? Water Pollut. Control Fed. 1969, 41 (7), 1285–1291. (6) Bye, C. M.; Dold, P. L. Sludge volume index settleability measures: Effect of solids characteristics and test parameters. Water Environ. Res. 1998, 70 (1), 87–93. (7) Henkel, J.; Cornel, P.; Wagner, M. Free Water Content and Sludge Retention Time: Impact on Oxygen Transfer in Activated Sludge. Environ. Sci. Technol. 2009, 43 (22), 8561–8565. (8) Henkel, J.; Lemac, M.; Wagner, M.; Cornel, P. Oxygen transfer in membrane bioreactors treating synthetic greywater. Water Res. 2009, 43 (6), 1711–1719. (9) Vaxelaire, J.; Cezac, P. Moisture distribution in activated sludges: a review. Water Res. 2004, 38 (9), 2215–2230. (10) Smollen, M. Moisture retention characteristics and volume reduction of municipal sludges. Water SA 1988, 14 (1), 25–28. (11) Frolund, B.; Palmgren, R.; Keiding, K.; Nielsen, P. H. Extraction of extracellular polymers from activated sludge using a cation exchange resin. Water Res. 1996, 30 (8), 1749–1758. (12) Wilen, B. M.; Jin, B.; Lant, P. The influence of key chemical constituents in activated sludge on surface and flocculating properties. Water Res. 2003, 37 (9), 2127–2139. (13) APHA, Standard methods for the examination of water and wastewater; American Public Health Association, American Water Works Association, Water Pollution Control Federation: Washington DC, 2005. 8792
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