Effects of temperature and organic loading on the performance of upflow anaerobic sludge blanket reactors
Shannon Grant and Kwan-Chow Lin
Abstract: A study investigating the effects of temperature and organic loading on the performance of upflow anaerobic sludge blanket reactors was carried out in the laboratory. Laboratory-scale rcactors of 3.2 L volume were semicontinuously fed a synthetic substrate consisting of beef consommC and macro- and micro-nutrient compounds. Temperatures ranged from 10 to 42°C; organic loadings ranged from 2.0 to 30 kg COD/(m3. d). Steady-state process kinetics and efficiencies were evaluated for the various conditions. Based on the assumption that upflow anaerobic sludge blanket reactor kinetics in the temperature range of 10-30°C could be approximated by Monod and modified Arrhenius equation relationships, effluent SCOD concentrations and removals were used to determine the maximum rate of substrate utilization, the half-velocity constant, and the temperature coefficient. A design and operating chart was constructed based on the kinetic coefficients determined from the experimental data.
Key ivorcls: upflow anaerobic sludge blanket reactor, temperature, loading, performance, kinetics.
RCsumC : Une Ctude des effets de la temperature et de charges organiques sur la performance de rCacteurs avec lit de boues expansCes a CtC effectuCe en laboratoire. Des rkacteurs expkrimentaux d'une capacitC de 3,2 L ont CtC alimentis de faqon semi-continue avec un substrat synthCtique composC de consommC de boeuf ainsi que de macro et de microCICments. Les tempkratures variaient entrc 10 et 42°C; les charges organiques variaient entre 2,O et 30 kg de DCO/(m3.d). L'efficacitC et la cinCtique du processus en rCgime permanent ont CtC CvaluCes en fonction des diverses conditions. En prenant comme hypothkse que la cinCtiquc du rCacteur avec lit de boues expansies, h I'intCrieur de la plage de temperatures comprises entre 10 et 30°C, pouvait faire I'objet d'une approximation h I'aide de 1'Cquation de Monod et de l'tquation niodifiCe d'Arrhenius, les concentrations et les quantitCs CliminCes de DCOS ont Cte utilisCes pour determiner le taux maximal d'utilisation du substrat, la constante h demi-vitesse et le coefficient de temperature. Un diagramme de calcul et d'exploitation a CtC dtabli en tenant compte des coefficients cinktiques obtenus partir des donnCes expCrirnenta1e.s.
Mots c l B : rCacteur avec lit de boues expansCes, tenipCrature, charge, performance, cinCtique. [Traduit par la rCdaction]
Introduction
The upflow anaerobic sludge blanket (UASB) reactor has been used to treat a variety of high strength organic waste- waters since Lettinga and co-workers originally developed the process in the 1970s (Lettinga et al. 1985). Typically, the process has been operated in the optimum mesophilic tem- perature range of 30-37°C. However, depending on the temperature of the wastewater and the particular climatic conditions, this optimum temperature range may not always
Received December 14, 1993. Revised manuscript accepted June 28, 1994.
S. Grant AD1 Systems Inc., Fredericton, NB E3B 4V2, Canada. K.-C. Lin Department of Civil Engineering, University of New Brunswick, Fredericton, NB E3B 4H9, Canada.
Note: Written discussion of this paper is welcomed and will be received by the Editor until June 30, 1995 (address inside front cover).
be the most practical one in which to operate. Some wastewaters are warmer than the optimum meso-
philic temperature. For example, many distillery wastewaters in warm climates have temperatures in excess of 40°C. Cool- ing these wastewaters down may not necessarily be the most practical alternative. Likewise, many wastewaters have tem- peratures lower than 30°C and heating the UASB system may not always be desirable.
The purpose of this study was to demonstrate treatment of a high strength synthetic wastewater with laboratory-scale UASB reactors over a wide range of temperatures (10- 42°C). An organic loading (in terms of chemical oxygen demand, COD) of 10 kg COD/(m3 .d ) was treated at tem- peratures of 10, 20, 30, 37, and 42°C. Operation at 55°C was also attempted, but stable operation could not be achieved.
Since overall removal efficiencies are a function of the organic loading rate, as well as temperature and wastewater strength, particularly at suboptimum temperatures, different organic loading rates other than 10 kg COD/(m3 .d) were also used at 10, 20, and 30°C. Steady-state operation was
Can. J. Civ. Eng. 22, 143- 149 (1995) Printed in Canada / Imprimt au Canada
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Fig. 1. Components of the laboratory-scale UASB reactor.
Gas -1
Gas to Atmosphere or to Gas Collection Bag.
Separator
reached at organic loading rates of 2, 3.5, 5.0, and 10 kg COD/(m3. d) at 10°C, 10, 15, and 20 kg COD/(m3 . d) at 20°C, and 10, 20, and 30 kg COD/(m3.d) at 30°C. This allowed comparison of the dual effect of temperature and organic loading, and provided enough data to approximate UASB process kinetics in the 10-30°C temperature range.
Materials and methods
Five identical 3.2 L laboratory-scale, plexiglas UASB reac- tors were used in this study. The reactors were approxi- mately 0.90 m high and 0.09 m in diameter. Intermittent, slow, gentle agitation of the granular sludge bed portion of the reactor was accomplished by a stirring rod controlled by a mechanical stir motor on top of the reactor. The reactors had a gas-solids separator and settling compartment in the upper portion of the reactor to help minimize effluent sus- pended solids concentrations and maximize sludge retention. No flow recirculation was used. Figure 1 illustrates the com- ponents of the laboratory-scale UASB reactors.
The reactors were intermittently fed for 17 s every 90 s by means of a peristaltic pump. In this manner, a continuous flow rate of 4.6 L/d was simulated. This allowed for a hydraulic retention time of 16 h to be maintained.
The sludge height for each unit was kept at approximately 50% of the reaction chamber height by periodic sludge wast- ing from the top half of the reactor.
The feed used was a synthetic waste of which commercial grade beef consommC was the organic component. Beef con- sommC was used because it was readily available and inex- pensive and produced a relatively soluble organic wastewater that could be consistently reproduced on a day-to-day basis. The organic component of the beef consommC consisted of 74.2 % carbohydrate and 25.8 % protein.
The exact influent COD concentration, and therefore the exact concentration of beef consommC used, was dependent on the fixed flow rate of 4.6 L/d and target organic loading rate. Sodium bicarbonate was added for alkalinity (the exact
amount was dependent on temperature and organic loading). A small amount of potassium sulfate and sodium sulfate and a trace metal solution were added to provide sufficient macro- and micro-nutrients.
The reactor pH in the sludge bed was monitored at least once per week (more often for the first few days after loading conditions were changed). This was to ensure that pH was maintained at 6.8 or slightly higher by alkalinity addition (in the form of sodium bicarbonate) with the feed. The steady- state pH in the sludge bed for all reactors was between 6.8 and 7.0 during the study.
The soluble and total COD measurements (SCOD and COD, respectively) were done by the Hach method (Hach Company 1989). Suspended solids (SS), volatile suspended solids (VSS), and 5-day biochemical oxygen demand (BOD,) measurements were done in accordance with standard methods (APHA, AWWA, and WPCF 1985).
The UASB reactors were operated at each condition until a period of steady state was reached. Steady state was a period of at least 2 weeks in which SS and SCOD concentra- tions remained constant. Typically, the reactors were run for a period of 1.5-4 months to ensure that a period of steady state had been reached under each new temperature or load- ing condition.
Periodic visual and scanning electron microscope exami- nation of the sludge during the study period revealed that the sludge maintained good granulation under all loading condi- tions between 10 and 37°C. The sludge formed after more than 2 months at 42°C appeared to be much less granular, having much smaller and more flocculent sludge particles. Sludge density in the sludge bed at 42°C was also lower than that at other temperatures (43 g VSS versus 51 -74 g VSS).
Results and discussion
Steady-state results The steady-state results for various temperatures and organic loadings are summarized in Table 1. The change in influent
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Table 1. Steady-state effluent concentrations and efficiencies.
Effluent concentration Effluent removal Influent Avg. VSS in Specific organic
Temperature COD reactor Organic load load SS SCOD COD BOD, SCOD COD BOD, ("C) (mg/L) (g) (kg COD/(& . dl) (kgl(kg. dl) (mglL) (mglL) (mglL) (mgIL) (%) (%) (%)
Fig. 3. Percent SCOD removal versus organic loading.
100
Fig. 2. SS concentration In effluent versus organic loading.
4500
a 3
C & 1500. - - -
(I)
4000
55-1 0 5 10 15 20 25 30
Organic Loading (kgIrn3. d)
- . .. -
Organic Loading (kg COD/^^. d)
COD concentration from 1400 to 21 000 mg/L at the fixed flow rate allowed the organic loading to vary from 2 to 30 kg COD/(mL d). The specific organic loading in kg COD/ (kg VSS . d) was calculated from the organic loading and the average amount of VSS in the reactor. Organic concentra- tions in the effluent (in terms of SCOD. COD. and BOD,)
30°C, and to a lesser extent the higher sludge production, caused a significant increase in effluent SS concentrations.
Figure 3 shows the percent SCOD removal versus organic loading. This graph illustrates that SCOD removal efficiency decreased as temperature decreased (with the exception of 42°C) and (or) as organic loading increased.
and their percent removals are shown for the various condi- Kinetic rate data (&, k, and 8) tions. Effluent SS concentrations are also listed for com- Steady-state SCOD effluent concentrations and removals at parison. 10, 20, and 30°C were used to estimate the half-velocity
Figure shows effluent SS concentrations Versus Organic constant, K,, and the maximum rate of substrate utilization, loading. From the figure it can be seen that effluent SS k, by assuming that UASB reactor kinetics could be approxi- increased with decreasing temperature (with the exception of mated by the Monad relationship given by the following 42°C) and (or) increasing organic loading. The increase in equation: effluent SS at a lower temperature for a given organic load- ing is presumably due to ;he increase in-viscosi~y at lower
[ l l XV - K, 1 1 ---
temperatures. The increase in effluent SS for higher organic + - loading is mainly due to an increase in gas and sludge pro-
(SO-se)Q k S e k
duction. The intense gas activity in the laboratory-scale where k is the maximum specific substrate utilization rate UASB reactors at 20 and 30 kg CODl(m3.d) for 20 and (Ittime); Se is the substrate concentration in the effluent
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Fig. 4. Determination of K, and k values at 10°C.
12
11 - .I
R2 = 0.92
4 I I I
0 0.001 0.002 0.003 0.004 0.005 0.006
1 IS, (Llmg)
Fig. 5. Determination of K, and k values at 20°C.
1.2 1 I
0 0.0005 0.001 0.001 5 0.002 0.0025
1 IS, (Llmg)
(masslvolume); K, is the half-velocity coefficient (mass1 volume); and X is the biomass concentration (masslvolume) (Monod 1949).
This equation was used to determine the maximum sub- strate utilization rate, k, and the half-velocity coefficient, K,, from a linear plot of (XV)/[(S, - S,)Q] versus l/Se for 10, 20 and 30°C. The slope of such a plot is equal to KJk, and the Y intercept is equal to l lk .
Figures 4, 5 , and 6 show the graphs of (XV)/[(S, - Se)Q] versus l/Se for 10, 20, and 30°C, respectively. From regression results, the determined maximum specific rates of
substrate utilization, k, were 0.22, 0.80, and 1.38 d- and the half-velocity coefficients, Ks, were 236, 393, and 675 mg1L for temperatures of 10, 20, and 30°C, respec- tively. Results from t-tests confirm that all the slopes and intercepts in the regression equations are significantly differ- ent from zero at a significance level of cu = 0.10 for tempera- tures of 10, 20, and 30°C, except the slope for the plot at 20°C. In view of the few data points, the values of K, and k derived from the slope and intercept estimates are approx- imate.
The determination of the maximum specific rate of sub-
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Fig. 6. Determination of K, and k values at 30°C.
0 0.0005 0.001 0.001 5 0.002 0.0025 0.003 1/S, (Llmg)
Fig. 7. Determination of 0 for 10-30°C.
strate utilization allows for direct comparison in the drop of kinetic rates from 30°C to 10°C. The maximum specific rate of substrate utilization at 30°C was 1.7 times that at 20°C and the maximum rate at 20°C was 3.6 times that at 10°C. The factor of 1.7 between k values at 30°C and 20°C is roughly in line with the rule of thumb known as the Qlo or van't Hoff's rule, which states that the reaction rate doubles for a 10" temperature rise. However, the factor of 3.6 between 20°C and 10°C indicates a significant deviation from this rule. Therefore, the UASB process appears to be much more sensitive to a drop in temperature between 20°C
and 10°C than between 30°C and 20°C, at least in respect to the conditions used in this study.
Figure 7 shows a plot of the natural logarithm of the reac- tion rates versus the inverse of the respective temperatures in kelvin (K). The slope of the line given by this graph is equal to the negative of the activation energy, E, divided by the ideal gas constant, R (= 8.32 J/(mol. K)). The activation energy, E, was calculated to be 65.1 Idlmol. The change in temperature, AT, required for a doubling of the reaction rate was calculated to be 7.6"C for the temperature range of 10 to 30°C.
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Fig. 8. Application of kinetic rate data - SCOD removal efficiency versus organic loading rate and temperature.
404 1 0 5 10 15 20 25 30 35 40
Organic Loading (kg C O D I I ~ ~ . d)
The following variation of the Arrhenius equation was used to calculate the temperature coefficient, 8, between temperatures of 10, 20, and 30°C:
where 8 is the temperature coefficient, TI and T2 are tem- peratures, and K, and K2 are the reaction rates at tempera- tures TI and T2, respectively.
The value of 6 for the range of 10-30°C was calculated to be 1.10 based on this equation. A t-test confirmed that the slope was different from zero at a significance level of a = 0.15. The values of 8 were determined to be 1.14 and 1.06 for the individual temperature ranges of 10-20°C and 20-30°C, respectively. This again illustrates that the effect of temperature on process kinetics is not linear in the range of 10-30°C, and that the process is more temperature sensi- tive between 10 and 20°C than between 20 and 30°C.
Henze and Harremoes (1985) compiled data from a num- ber of sludge activity tests reported by different investigators in the temperature range of 10-30°C. The natural log- arithms of reported methane production rates from these activity tests were divided by a methane production rate for 35°C and then plotted on the Y axis versus their respective temperatures on the X axis. The slope of the straight line of these points gave a temperature coefficient, 8, of 1.10. Thus, the determination of kinetic rates and the temperature coeffi- cient in Henze and Harremoes' study was carried out by a different method from that used here but the 6 values obtained were identical.
Application of kinetic rate data The following equation shows that the effluent SCOD con- centration, S,, is a function of the SCOD removal effici- ency, eff, and the influent SCOD concentration, So, in the following manner:
Substituting [3] for S, into [ l ] and rearranging gives the fol- lowing relationship:
This equation and eq. [2] allow for the SCOD removal efficiency, eff, to be calculated (by trial and error) for any UASB loading condition and temperature (in the range of 10-30°C) by using the experimentally determined kinetic rate parameter data (Ks, k, and 8).
Figure 8 was constructed using [2] and [4] in conjunction with the calculated kinetic parameters (K,, k, and 8) and a UASB loading condition based on the following assumptions:
Effective UASB volume = 3.2 L, Flow = 4.6 L/d, MLVSS in UASB (XV) = 70000 mg, kzonc = 0.8 g COD/& MLVSS. d), Ks(200C) = 393 mg/L COD, 6k(IO-20"C) = 1.14, 8k(20-30"C) = 1.06.
These assumptions were typical of the loading conditions used in this study.
Trial-and-error determinations of SCOD removal effici- ency, eff, for various influent SCOD concentrations, So, at different temperatures (10 - 30°C) were used to construct Fig. 8. Experimental data from the research are plotted on the figure to illustrate the reasonably close degree of fit between the experimental data and the simulated values from this method.
Figure 8 clearly illustrates the relationship between SCOD removal efficiency, organic loading rate, and temperature. The figure could be used as a rough design and operating chart for UASB reactors treating similar type of soluble wastewaters, assuming that the operating pH, upflow veloc- ity, reactor design and configuration, and the degree of waste acidification were similar. For example, the figure shows that a designer (or operator) would not want to have an organic loading rate exceeding approximately 10 kg COD/ (m3.d) if he (she) wanted to maintain at least an 85% SCOD removal efficiency at 15°C.
The figure illustrates three key points: ( i ) SCOD removal efficiency decreases with decreasing
temperature and increasing organic loading. (ii) There can be relatively small differences between
SCOD removal efficiencies for different temperatures if the organic loading rate is sufficiently low enough.
(iii) The higher the temperature is, the higher the maxi- mum organic loading rate can be in order to maintain a desired SCOD removal efficiency within the temperature range of 10-30°C.
Conclusions
1. The effluent SCOD and SS concentrations from laboratory-scale UASB reactors increased with an increase in organic loading and (or) a decrease in temperature.
2. At temperatures of 10, 20, and 30°C, the maximum specific rates of substrate utilization, k, were determined to be 0.22, 0.80, and 1.38 d-I, and the half-velocity constants,
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Ks, were 236, 393, and 675 mg/L, respectively, in this study. 3. Under the conditions studied, the temperature coeffi-
cient, 19, was determined to be 1.14 between 10°C and 20°C and 1.06 between 20°C and 30°C; the overall temperature coefficient was 1.10 between 10°C and 30°C. The overall value confirmed that obtained by Henze and Harremoes (1985) for anaerobic treatment processes.
4. From the experimentally determined kinetic param- eters, a design and operating chart was constructed for the selection of organic loading rates that could be used to meet a given SCOD removal efficiency for temperatures in the range of 10-30°C for the treatment of similar wastewaters by the UASB process under similar operating conditions
Acknowledgements
This paper is based on the M.Sc.E. thesis submitted by the first author to the University of New Brunswick. The finan- cial support of the Natural Sciences and Engineering Research Council of Canada is acknowledged.
References
Jointly published by America Public Health Association, America Water Works Association, and Water Pollution Control Federation, Washington, D.C.
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Henze, M., and Harremoes, P. 1985. Literature review, anaerobic treatment of wastewater in fixed film reactors. Workshop on Anaerobic Treatment Processes, Association of Environmental Engineering Professors, Purdue University, Ind.
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Monod, J . 1949. The growth of bacterial cultures. Annual Review of Microbiology, 3: 37 1 - 394.
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