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Treatment of high-strength organic wastewaters using an anaerobic rotating biological contactor
Environment
Pergamon
International, Vol. 21, No. 3, pp. 313-323, 1995 Copyright Q 1995 Elsevier Science Ltd Printed in the USA. All rights reserved 0160-4120/95 $9.50 + .oo
0160-4120(95)00027-5
TREATMENT OF HIGH-STRENGTH ORGANIC WASTEWATERS USING AN ANAEROBIC ROTATING BIOLOGICAL CONTACTOR
Chungsying Department 40227
Lu and An Chin Yeh of Environmental
Engineering,
National Chung-Hsing
University, Taichung, Taiwan
Min-Ray Lin Department of Mathematics Taichung, Taiwan
and Science Education,
National Taichung
Teacher’s
College,
EI 9408-l 97 M (Received 23 August 1994; accepted 25 January 1995)
The removal of high strength organic compounds from the wastewater by an anaerobic rotating biological contactor (AnRBC) was studied experimentally and theoretically. In the steady-state operating condition, the removal efficiencies of soluble COD and BOD could be up to 71% and 76%, respectively, for the organic surface loading rate of 111.4 g COD/m’*d and organic volumetric loading rate of 13.33 kg COD/m3*d. A simple model that accounts for the simultaneous convective transport, dispersion in the axial direction, and substrate utilization by the attached biomass was developed to predict the system performance of the AnRBC reactor. Good agreement between model predictions and experimental data was obtained by comparing dimensionless soluble COD concentration profiles measured at the system effluent of earlier stages. The results presented herein are helpful in providing design and operational criteria for a full-scale AnRBC type system.
INTRODUCTION More stringent requirements for the removal of organic and inorganic substances from the wastewater in recent years necessitate the development of innovative, cost effective wastewater treatment alternatives. The aerobic rotating biological contactor (RBC) is one of the biological processes for the treatment of organic wastewaters. It combines advantages of biological fixed-film (short hydraulic retention time, high biomass concentration, low energy cost, easy operation, and insensitive to toxic substance shock loads) and partial stir. Therefore, the aerobic RBC reactor is widely employed to treat both domestic and industrial wastewaters (Tokus 1989; Gujer and Boller 1990; Ahn and Chang 1991). However, some difficulties limit the practical application of the aerobic
RBC reactor for the treatment of high-strength organic wastewaters including oxygen transfer limitations and excess sludge problems (Ware et al. 1990). The anaerobic rotating biological contactor (AnRBC), a relatively new biological wastewater treatment process, not only has the advantages of the aerobic RBC reactor but also shows potential to solve problems of the aerobic RBC reactor. This process was first introduced by Tait and Friedman (1980) for treating synthetic carbonaceous wastewaters. It was concluded that the AnRBC reactor is both feasible and practical for the removal of high strength organic compounds from the wastewater. Laquidara et al. (1986) used two single-stage AnRBC units for the treatment of synthetic soluble carbohydrate 313
314
C. Lu et al.
NOMENCLATURE
A, C C, ci ci+
D Da KS L ti R S t f U V :a Y, Z
Wetted surface area of disks Substrate concentration Effluent substrate concentration Influent substrate concentration Concentration of substrate at a point infinitesimally beyond the entrance Axial dispersion coefficient Damkohler number for flow convection (=~ma&kL~a&W Substrate concentration at half the maximum specific growth rate Reactor length Area capacity constant (=u,,XJY> Wastewater flow rate Removal coefficient Dimensionless substrate concentration (= C/C3 Time coordinate Mean residence time of RTD data Average flow velocity Volume of the reactor Axial distance Mass of attached biomass per unit wetted surface area of disk Apparent yield coefficient of the attached biomass Dimensionless axial distance (= x/L)
removal from the wastewater did not improve the treatment efficiency of the AnRBC reactor. The COD removal efficiencies of 29-53% were obtained in their study. An evaluation of a simple kinetic model for the treatment of domestic sewage by a single-stage laboratory scale unit of AriRBC has been conducted by Deshpande et al. (1991). The results demonstrated that a simple Monod-type expression can satisfactorily describe the performance of an AnRBC reactor, and the model maintains its validity even with high-strength domestic wastewater. Although the system performance and kinetic model of the AnRBC reactor have been researched for the treatment of organic wastewaters by the above investigators, laboratory studies and field applications of the AnRBC reactor are still relatively limited compared to the aerobic RBC reactor in the literature. Therefore, more information with respect to their predictive model, system performance, kinetics, and microscopic observations would be helpful in providing design and operational criteria for a full-scale AnRBC type system. This paper presents the treatability results of high strength organic wastewater by an AnRBC reactor. A predictive model that takes into account the simultaneous convective transport, as well as dispersion in the axial direction, and substrate utilization by the attached biomass in the AnRBC reactor is also presented. MATERIALS AND METHODS Experimental setup
Greek symbols Maximum specific growth rate for the attached P ma biomass Variance of RTD data lJ2 0 Disk rotational sneed wastewater which is similar to a typical food processing wastewater stream with nutrient and buffer addition. The AnRBC reactor was proved to be an effective process for treating a carbohydrate wastewater at chemical oxygen demand (COD) removal rates between 10 and 140 g/m’.d. Satyanarayan et al. (1987) employed a laboratory-model anaerobic rotating biological drum contactor (AnRBDC) to treat dairy wastewater. It was observed that dairy wastewater can be treated easily with COD removal efficiencies of 60 - 8 1% by an AnRBDC. Lo et al. (1990a; b) investigated the effects of sulfate removal from molasses wastewater on anaerobic digestion using an AnRBC reactor. The experimental results showed that the presence of high sulfate concentration prolonged the start-up process in the AnRBC reactor, however, sulfate
Three bench-scale analogous AnRBC reactors were constructed and identified as AnRBCl, AnRBC2, AnRBC3, respectively. The experimental setup is shown schematically in Fig. 1 and exact dimensions are summarized in Fig. 2 and Table 1. Each AnRBC reactor consisted of four stages, each stage contained fifteen 12 cm diameter acrylic plastic disks with 9.5 mm spacings between disks. These disks were mounted on a 1.6 cm diameter horizontal stainless steel shaft which was supported at both end and rotated by a variable speed drive motor ranging from 0 to 60 r-pm (peripheral disk velocity: 0 to 22.6 m/min). Each stage was separated by fixed baffle plates that contain three holes drilled below the water line for the passage of wastewater to prevent short-circuiting (Fig. 3). Biogas was collected in each stage and vented through a pipe to a gas collection tube filled with saturated sodium chloride liquid, 5% sulfuric acid and methyl red for the evaluation of biogas production rate. The AnRBC reactor was covered and sealed by an external acrylic plastic pipe with a diameter of 17 cm to maintain constant temperature (35 f 0.5”C).
315
Removal of organic compounds f?om wastewater by an AnRBC
variable influent
sp-eed pump
9s collection manifold
--+Pl i-l
1 +
synthetic wastewater
effluent
1
r
+j
l-4
z
,
,
stainless steel shaft ,
gas collection equipmen baffle
disk .i-\
(15 per stage)
sample ports
variable speed drive motor
2nd 1 stage
( stage
3rd
4th
stage
stage
Fig. 1. A schematic diagram of the AnRBC reactor.
Number of stages Disks per stage Disk diameter (mm) Reactor diameter (mm) Disk thickness (mm) Spacing between disks (mm) Wetted surface area of disks (m’) Reactor volume (liters) Surface to volume ratio Reactor material with water bath. The substrate feed rate was controlled by a variable speed peristaltic pump while the water level was controlled by a dynamic head tube resembling a vented inverted siphon on the effluent line. Substrate characteristics during start-up are listed in Table 2. The fourth and twelfth disks in each stage of the AnRBC reactor were designed with four removable sections as shown in Fig. 4 to make detailed microscopic observations of the attached biofilms (two small sections) and measure both total biomass (MISS) and biotilm thickness (two large sections). A special tool was employed to remove these sections so that the anaerobic conditions could be maintained and attached biofilm undisturbed.
4 15 120 130 3 9.5 1.43 11.95 119.7 Polyacrylic Tracer study
The hydraulic characteristics of the AnRBC reactor were studied prior to start-up using an impulse-input response experiment to determine flow patterns in the reactors. A pulse of soluble NaCl was injected at the entrance of the AnRBC reactor to measure the conductivity at the system effluent of each stage. The NaCl concentration was calculated from the calibration curve of conductivity versus soluble NaCl concentration. Effects of varying disk rotational speed (o), hydraulic retention time (HRT), and disk submergence were evaluated and the operating conditions are listed in Table 3.
316
C. Lu et al.
900
I4
1320
mm
)1
350mm bl
Fig. 2. Dimensions of the AnRBC reactor.
Down ‘-
thickness
= 3 mm
I3
A SE -E _I
bl
mm
k
disk
I
Fig. 3. Detailed drawing of the baffle plate.
317
Removal of organic compounds from wastewater by an AnRBC
Table 2. The compositions of synthetic organic wastewater during start-up of the AnRBC reractor.
O-14 days
15-65 days
(g/L)
1
(g/L) (!m (mg/L) (Em @x/L) (g/L) (g/L) (WL) (mg/L) (mg/L) (g/L as CaCO,)
0.18 6 9.65 0.153 7.5 0.89
10 1.8 15 96.5 1.53 75 12 7.27 572 362 50 9.32 7.8-8.0 1.65 21 240
Parameters
C&A
Beef extract NaHC03 FeCl, . 6Hz0 NH&l Na*SO, COD BOD TKN NH3-N Total PO:--P Alkalinity PH COD/BOD COD/N COD/P
7.2-7.4
I -I
Fig. 4. Detailed drawing of removable section disk.
Analytical methods
The following parameters were determined as suggested in Standard Methods (APHA; 1989): soluble COD (508A), soluble BOD (507), NH,-N (417A), total P (424C-II), and alkalinity (403). The volatile acid (VA) was measured by a direct-titration method proposed by Dilallo and Albertson (1961). Dissolved oxygen (DO)
was measured with an electrochemical membrane-type electrode, while pH and oxidation-reduction potential (ORP) were determined by a digital pH meter and an ORP meter, respectively. Conductivity was evaluated by a digital conductivity meter. Biogas production rate was monitored in the gas collection tube.
C. Lu et al.
318
Table 3. The operating conditions during tracer study of the AnRJ3C reactor.
Run no.
1 2 3 4 5 6 7
Flow rate (mL/min) 5.8 5.8 5.8 24.0 3.5 3.0 9.4
HRT
Disk submergence (%)
Cd
(hrs)
6-v)
21.6 21.6 21.6 5.2 35.3 21.6 21.6
6 12 24 12 12 12 12
70 70 70 70 70 40 100
Microscopic observation
Model equations
After start-up of the AnRBC reactor, the small sections were removed and fixed with 25 g L-’ glutaraldehyde at a temperature of 4 “C for 2.5 h and then washed with 0.0 1 M phosphate buffer solution for 10 min three times. Progressively dehydrated from 30% to 100% with ethanol series, dried by critical point dryer, and coated with gold, the samples were scanned at 15 kV with the scanning electron microscopy (BAUSCH & LOMB SEM, Manolab 2 100).
On the basis of above assumptions, the dimensionless mass balance equation in terms of the substrate concentration (COD here) is written as
THEORY
The inside geometry of an AnRBC reactor may be quite complex. However, considerable mathematical simplicity can be attained in analysis if it is characte-rized as an equivalent tube. This is the approach that was employed in this study. Basic
assumptions
The following major assumptions are made in developing a model to predict the system performance of an AnRBC reactor for treating high-strength organic wastewaters: 1) The system is in a steady-state opera-ting condition. 2) The flow is in the axial direction alone and the flow velocity remains constant throughout the reactor. 3) The suspended growth does not usually contribute significant substrate removal in the fixed-film biological reactors and thus is neglected in this study (Clark et al. 1978; Spengel and Dzomback 1992). 4) Due to value of K, in the Monod-type, expression is much higher than substrate concentration, the first-order kinetics is assumed to describe substrate utilization by the attached bamboos in the AnRBC reactor. 5) Since tracer injection was used and was accounted for an instantaneous impulse, the AnRBC reactor is thus treated as a closed-closed reactor (Levenspiel 1972). 6) Complete mixing in the liquid volume is assumed in the determination of biological kinetic parameters.
with boundary conditions (Danckwerts dS dz
+ (+I
-= dS dz
(1 - S)
= 0
0
1953)
at
at
z=o
z=l
(2)
(3)
The quantity, D/UL, called the reactor dispersion number, is a measure of the deviation from the plug flow pattern. If the reactor dispersion number approaches zero, the axial dispersion can be neglected and hence represents a plug flow. If the dispersion number reaches infinity, the axial dispersion is large and hence denotes a completely mixed flow. The value between these two extremes is called an arbitrary flow which represents any degree of partial mixing between plug flow and completely mixed flow. The Damkijhler number (Da) is defined as the ratio of substrate consumption rate by the attached biomass to substrate transport rate by flow convection. Equation 1 is a linear differential equation of the second order with constant coefficients. The analytical solution to Eq. 1, in terms of S as a function of the normalized length of the AnRBC reactor, is written as: S(z)
Z(l+b)
=exp
(zz,
x
exp[ %.&fd-T]
-2.(1-b)exp[k-6?$-dZ!$]
(4)
(l+b)'exp($~)-(1-b)2exp(-~~)
where b=F-zz
(5)
319
Removal of organic compounds from wastewater by an AnRBC
Parameters estimation
Use of Eq. 4 requires estimation of the reactor dispersion number (D/UL) as well as the Damk&ler number (Da) which is determined by the biological kinetic parameters (cl,,,and K J and apparent yield coefficient (YJ of the attached biomass. A summary of estimation of these parameters is provided below. The reactor dispersion number can be experimentally determined by analyzing the response to a tracer impulse. In the closed-closed reactor, the reactor dispersion number can be calculated according to (Levenspiel 1972) Reactor
dispersion
CT2 = 2% 7
number.
- exp-UL/D)
- 2(g)2(1
(6)
where u2 is the variance of residence time distribution (RTD) data; f is the mean residence time of RTD data. The variance of a continuous distribution measured at a finite number of equidistant locations is given by: g- tj"cj o2
= j=l
- T2
c
tjCj
(11) Substituting Eqs. 10 and 11 into Eq. 9 and taking the inverse gives: 1 KS 1 -=1 --+R
PC,
P
The values of R and C, are determined directly from experimental data while the values of P and K, can be determined by substituting R and C, into Eqs. 12 for linear regression. Once the values of P and K, have been determined, the Da number can be readily calculated from the experimental conditions. RESULTS AND DISCUSSION
Results are presented in chronological order to allow a clear evaluation of the AnREX reactor: 1) tracer study, 2) start-up, 3) treatability results tier start-up, and 4) predictive model. Tracer study
where tj is time; Cj is the tracer concentration in effluent at time j. The mean residence time, t is approximated by: m j=l
p = km-% ya
(7)
C ‘j j-1
T=
area of disk. According to these definitions, P and R can be expressed as:
(8)
C 'j j - 1
Using the RTD data, the values of u2 and f2 are calculated by Eqs. 7 and 8. Substituting these values into Eq. 6 and employing the trial and error method, the reactor dispersion number can be determined. Damkijhler number. Prior to determination of the Da number, the biological kinetic parameters must be known. Assuming a completely mixed reactor, the mass balance equation inside an AnREX reactor can be written as:
Results obtained with tracer study were analyzed using the reactor dispersion number. Table 4 shows the reactor dispersion number determined from experimental RTD data at system effluent of each stage for varying disk rotational speed (o), hydraulic retention time (HRT), and disk submergence. In general, a reduction of HRT or disk submergence results in an increase of the reactor dispersion number in the AnRlX reactor. On the other hand, the opposite trend is observed for varying o. Effects of varying these parameters on the hydraulic characteristics of the AnREZ reactor were most significant in the first stage. From the results of the reactor dispersion numbers, it can be concluded that the AnRBC reactor was of the plug flow type with large amounts of dispersion (Levenspiel 1972) and the dispersion effects were decreased as an increase of stage number. Start-up
where Q is the wastewater flow rate; and C, is the effluent substrate concentration. Clark et al. (1978) defined the area capacity constant, P, as the maximum amount of substrate removed per unit time per unit area of disk and the removal coefficient, R, as the amount of substrate removed per unit time per unit
The procedure of start-up contained several steps including control of seeding microorganisms, temperature, pH, influent organic loading, and hydraulic retention time. The operating conditions during start-up of the AnRBC reactor are listed in Table 5. The seeding step was to mix the very active anaerobic microorganisms obtained from
320
C. Lu et al.
Table 4. The reactor dispersionnumber determinedfrom experimentalRTD data at the systemeffluent of eachstagefor speed,hydraulicretention time and disk submergence. Run no. 1 2 3 4 5 6 7
stage 1
stage 2
stage 3
stage 4
0.55 0.60 0.95
0.28 0.29 0.49 2.1 0.22 0.28 0.26
0.18 0.17 0.23 1.1 0.16 0.22 0.14
0.14 0.13 0.19 0.5 0.13 0.19 0.10
012 0.64 0.44
varying
disk rational
Table 5. The operatingconditionsduring start-upof the AnRBC reactor. Parameters
O-14 days
15-65 days
Disk rotational speed (rpm) Peripheral velocity (m/min) Disk submergence (W) Digestion temperature (“C) Hydraulic retention time (hours) Flow rate (L/day) Organic surface loading (g-COD/m’ - d) Organic volumetric loading (kg-COD/m3 * d) PH
2 0.75
12 4.5 100 35 +0.5 21.6 13.28 111.4 13.33 6.8-7.2
Taiwan Livestock Research Institute with the synthetic wastewaters as indicated in the second column of Table 2. The synthetic wastewater contained dissolved glucose and beef extract as the primary organic carbon source. Thereafter, put 6 L of the mixtures into each AnRBC reactor for 6 h. The AnRJK reactors were then fed with the synthetic wastewater using operating conditions indicated in the second column of Table 5. After two weeks, the COD removal efficiency achieved 80% and the strength of synthetic wastewater was increased as listed in the last column of Table 2 in order to stimulate the growth of seeding microorganisms. The operating conditions during this period are shown in the last column of Table 5. The AnRBC reactors were run for a period of 6 weeks after attainment of the steady-state condition. Steady-state condition was assumed when the daily changes in the biogas production rate and the effluent COD concentration were within 5% for three successive days. Upon attainment of steady state conditions, the AnRBC reactors were sampled at the system influent and effluent of each stage.
Treatabilityresultsafterstart-up COD and BOD. The treatability results after start-up of the AnRBC reactors are summarized in Table 6. Figures
100 35kO.5 25-30 9.56-l 1.47 5.94-6.93 0.71-0.85 7.2-7.4
5 and 6 show the stead-state concentration profiles of soluble COD and BOD as a function of the stage number. The removal efficiencies of soluble COD and BOD were up to 71% and 76%, respectively, for the influent organic surface loading rate of 111.4 g-COD/m**d and organic volumetric loading rate of 13.33 kg-COD/m3-d. The accumulated removal efficiencies of soluble COD in the first, second, third, and fourth stages were 42%, 58%, 67%, and 71%. Most of the organic compounds were removed in the first two stages. This can be explained by the fact that most easily biodegradable matters (C,H,,O, and beef extract here) were biodegraded in the first two stages. Similar results were observed for the removal of soluble BOD. In view of these results, it was concluded that start-up of the three AnRBC reactors was successful (Tait and Friedman 1980). pH; VA, and alkalinity. Figure 7 shows the results of pH, alkalinity, and volatile acid (VA) measured at the system effluent of each stage. As indicated, pH values were between 6.8 - 7.4, which were in the optimal range of 6.6 - 7.6 for the anaerobic process (MaCarty 1964). The value of VA was highest in the first stage and decreased in the subsequent stages. This may be attributed to the fact that most organic matters were biodegraded and converted to VA in the first stage by the acetogenic microorganisms.
321
Removal of organic compounds from wastewater by an AnRBC
Table 6. Treatability results after start-up of the AnRBC reactor. parameters COD BOD NH,-N Org-N VA Alk TP ORP DO Biogas production
influent (g/L) (g/L) (mg/L) (mg/L) (g as CH,COOH/L) (g as CaCO,/L) (w/L) (mV) bg/L) (L/d) rate
12 7.3 362 210 9.3 50 -210 3.1
8.02
PH
stage 1
stage 2
stage 3
stage 4
7 3.8 421 55 3.9 8
5 2.6 451 58 3.6 8.2
4 2.1 460 54 2.4 8.3
3.5 1.8 466 53 2.3 9
-333 0.0 14.4
-333 0.0 12.1
-340 0.0 5.8
-342 0.0 5.8
7.11
7.21
7.30
7.31
I OIN Stage
number
1
I 2
1 Stage
1 3
-I
4
number
Fig. 5. Steady-state COD concentration profile as a function of tbe stage number after start-up of the AnRBC reactor.
Fig. 6. Steady-state BOD concentration profile as a function of the stage number after start-up of the AnRBC reactor.
The carbonate alkalinity and pH value increased as an increase in the stage number due to a reduction of biodegradation rate of organic matters to VA in the latter stages. Althoughvalues of VA were high in the range of 2.3 to 3.9 g L’, pH values were maintained close to 7.0 by
hibited if the UVA concentrations are in the range of 30 to 60 mg L-l. As a result, the influence of high VA on the biomass activity can be neglected.
the addition
of sodium
bicarbonate
as the buffer
solution.
Theoretically, the concentration of undissociated volatile acid (UVA) only takes l/160 of the total VA concentration in the pH values near 7.0 (Morel 1983). Therefore, the UVA concentrations were estimated between 14.4 and 24.4 mg L-’ in this study. According to Kroeker et al. (1979), the methanogenic microorganisms are in-
Organic nitrogen and ammonium nitrogen. The steady state concentration profiles of organic nitrogen (Org-N) and ammonium nitrogen (NH,-N) as a function of the stage number are shown in Fig. 8. The conversion rate of organic nitrogen to ammonium nitrogen was highest in the first stage and became much slower in the subsequent stages.
322
C. Lu et al.
Alkalinity
’
I
I
1 Stage
2 number
I 3
44
Fig. 7. Alkality, VA, and pH as a function of stage number afterstart-up of the AnRFK reactor.
ORP and DO
The variations of the oxidation-reduction potential (ORP) measured at the system effluent of each stage were in the range of -333 to -342 mV. According to Li (1984), the anaerobic environment is achieved if the ORP is below -330 mV. This implies that the anaerobic condition has been reached during the period of start-up in this study. The ORP decreased (more negative) as an increase of the stage number. It may indicate that the methane fermentation reactions in the latter stages are predominant. The results can also be proved by a reduction of VA as an increase of the stage number in Fig. 7. The dissolved oxygen (DO) concentration was equal to zero at system effluent of each stage. Biogas production. As indicated in Table 6, biogas production rate was decreased as an increase of the stage number. This can be explained by the fact that the removal rates of biodegradable matters are higher in the first stage than those of in the subsequent stages. For the steady-state condition, the total biogas production rate was equal to 38.1 L/d and the gas productivity can be estimated as 0.299 L/g COD removed at 1 atm and 0°C. SEMphotograph. The results of SEM photographs taken from the removable sections of each stage indicate that the primary microorganisms in the outer shell of the biofilm are acetogenic microorganisms for the first and second stages including round-end filament bacteria, cocci, and curved bacteria. The methangenic bacteria are predominant in the outer shell of the biofilm for the third and fourth stages. The morphology of methanogenic mi-
loot\
I OIN
Organic
I 1
ritrogen
I Stage
2 number
~
I 3
Fig. 8. Steady-state organic nitrogen and ammonium nitrogen concentration profiles as a function of stage number after start-up of the AnREE reactor.
croorganisms can be described as rod-shaped with flat ends and very long flexible filaments which tend to aggregate in characteristic bundles. The mean biofilm thickness in each stage was measured from 1.5 mm to a very thin film. A dense microorganism was observed in the disks of the first stage. Model verification
Model parameters were chosen based on the experimental conditions at the end of start- up. The estimated values of D/I-IL were equal to 0.44, 0.26, 0.14, and 0.1 for the first, second, third, and fourth stages of the AnRBC reactor as shown in Run 7 of Table 4, while the values of P and K, were equal to 8.16 mg me2 S’ and 32.9 g L’ (Yeh 1994), respectively. Figure 9 shows the comparison of dimensionless COD concentration profiles predicted by the present model with experimental data measured at the system effluent of each stage. It is seen that the dimensionless COD concentration profiles are predicted well in the earlier stages and are under-predicted in the latter stages. Departures from experimental data in the latter stages can be attributed to the assumption of first-order reaction rate in Eq. 4. CONCLUSIONS
Treatability results of high-strength organic wastewaters by an AnRBC reactor were presented. In the steady-state operating condition (o of 12 rpm, HRT of 2 1.6 h, and 100% disk submergence), the removal effiencies of soluble COD and BOD could be up to 7 1% and 76%, respectively, for organic surface loading rate of
323
Removal of organic compounds from wastewater by an AnRBC
“’ LIN
I
I
2
1 Stage
I
3
I
4
number
Fig. 9. Comparison of steady-state dimensionless COD concentration profiles predicted by the present model with experimental data measured at the system effluent ofeach stage. Line =model prediction; symbol = experimental data.
111.4 g-COD/m’-d and organic volumetric loading rate of 13.33 kg COD/m3-d. In view of the results, the AnREK reactor is both feasible and practical for an effective treatment of high strength organic wastewaters. A simple model that accounts for the axial dispersion and substrate utilization by the attached biomass was also presented to predict the system performance of the AnRBC reactor. Good agreement between model predictions and experimental data was obtained by comparing dimensionless soluble COD concentration profiles measured at the system effluent of earlier stages. This model can be applied to be as an optimization tool to achieve the best design and operation for a full-scale AnRBC type system. Acknowledgment-The authors wish to thank the Taiwan Livestock Research Institute in providing the anaerobic microorganisms. Support from the National Science Council, R0.C. (NSC 82- 0410-E005-080) is gratefully acknowledged.
REFERENCES Ahn, K.H.; Chang, J.S. Performance evaluation of compact RBC-selting tank system. Water Sci. Technol. 23:1467-1476; 1991. Clark, J.H.; Moseng, E.M.; Asano, T. Performance of a rotating biological contactor under varying wastewater flow. J. Water Pollut. Control Fed. 50: 896-911; 1978.
API-IA (Am. Public Health Assoc.) Standard methods for the examination of water and wastewater. 17th edition, New York; 1989. Danckwerts, P.V. Continuous flow systems: distribution of residence times. Chem. Eng. Sci. 2: I-13; 1953. Deshpande, V.P.; Kaul, S.N.; Deshpande, C.V. An evaluation of a simple kinetic model for the treatment of domestic sewage by means of an anaerobic rotating biological contactor. Bioresource Technol. 38: 31-38; 1991. Dilallo,R.; Albertson, O.E. Volatile acid by direct titration. J. Water Pollut. Control Fed. 33: 365; 1961. Gujer, W.; Boller, M. A mathematical model for rotating biological contactors. Water Sci. Technol. 22: 53-73; 1990. IMSL (International Mathematics and Statistics Libraries) Contents Document, Vol. 2, Version 1.0, Houston, Texas; 1987. Koreker, E.J.; Schulte, D.D.; Sparling, A.B.; Lapp, H.M. Anaerobic treatment process stability. J. Water Pollut. Control Fed. 51: 718-721; 1979. Laquidara, M.J.; Blanc, F.C.; O’Shaughnessy, J.C. Development of biotilm, operating characteristics and operational control in the anaerobic rotating biological contactor process. J. Water Pollut. Control Fed. 58: 107-114; 1986. Levenspiel, 0. Chemical reaction engineering. New York, NY: John Wiley & Sons; 1972. Li, A.Y. Anaerobic processes for industrial wastewater treatment. Connecticut, CT: Dorr-Oliver Inc.; 1984. Lo, K.V.; Liao, P.H. Anaerobic treatment of baker’s yeast wastewater: I. start-up and sodium molybdate addition. Biomass 2 1: 207-2 18; 1990. Lo, K.V.; Chen, A.; Liao, P.H. Anaerobic treatment of baker’s yeast wastewater: II. sulfate removal. Biomass 23: 25-37; 1990a. MaCarty, P.L. Anaerobic waste treatment fundamentals-Part two, environment requirements and control. Public Works 95: 123-126; 1964. Morel, F.M.M. Principle of aquatic chemistry. New York, NY: Wiley-Interscience; 1983. Satyanarayan, S.; Thakar, K.; Kaul, S.N.; Badrinath, S.D.; Swarnkar, N.G. Anaerobic rotating biological drum contactor for the treatment of dairy wastes. Indian Chem. Eng. XXIX: 3-6; 1987. Spengel, D.B.; Dzombak, D.A. Biokinetic modelling and scale-up considerations for rotating biological contactor. Water Environ. Res. 64: 223-235; 1992. Tait, S.J.; Friedman, A.A. Anaerobic rotating biological contactor for carbonaceous wastewaters. J. Water Pollut. Control Fed. 52: 2257-2269; 1980. Tokus, R.Y. Biodegradation and removal of phenols in rotating biological contactors. Water Sci. Technol. 21: 1751; 1989. Ware, A.J.; Pescod, M.B.; Starch, B. Evaluation of alternatives to conventional disc support media for rotating biological contactor. Water Sci. Technol. 22: 113-l 17; 1990. Yeh, AC. Treatment of organic wastewaters by the Anaerobic Rotating Biological Contactors. Master Thesis, National ChungHsing University, Taichung, Taiwan; 1994.
Pergamon
International, Vol. 21, No. 3, pp. 313-323, 1995 Copyright Q 1995 Elsevier Science Ltd Printed in the USA. All rights reserved 0160-4120/95 $9.50 + .oo
0160-4120(95)00027-5
TREATMENT OF HIGH-STRENGTH ORGANIC WASTEWATERS USING AN ANAEROBIC ROTATING BIOLOGICAL CONTACTOR
Chungsying Department 40227
Lu and An Chin Yeh of Environmental
Engineering,
National Chung-Hsing
University, Taichung, Taiwan
Min-Ray Lin Department of Mathematics Taichung, Taiwan
and Science Education,
National Taichung
Teacher’s
College,
EI 9408-l 97 M (Received 23 August 1994; accepted 25 January 1995)
The removal of high strength organic compounds from the wastewater by an anaerobic rotating biological contactor (AnRBC) was studied experimentally and theoretically. In the steady-state operating condition, the removal efficiencies of soluble COD and BOD could be up to 71% and 76%, respectively, for the organic surface loading rate of 111.4 g COD/m’*d and organic volumetric loading rate of 13.33 kg COD/m3*d. A simple model that accounts for the simultaneous convective transport, dispersion in the axial direction, and substrate utilization by the attached biomass was developed to predict the system performance of the AnRBC reactor. Good agreement between model predictions and experimental data was obtained by comparing dimensionless soluble COD concentration profiles measured at the system effluent of earlier stages. The results presented herein are helpful in providing design and operational criteria for a full-scale AnRBC type system.
INTRODUCTION More stringent requirements for the removal of organic and inorganic substances from the wastewater in recent years necessitate the development of innovative, cost effective wastewater treatment alternatives. The aerobic rotating biological contactor (RBC) is one of the biological processes for the treatment of organic wastewaters. It combines advantages of biological fixed-film (short hydraulic retention time, high biomass concentration, low energy cost, easy operation, and insensitive to toxic substance shock loads) and partial stir. Therefore, the aerobic RBC reactor is widely employed to treat both domestic and industrial wastewaters (Tokus 1989; Gujer and Boller 1990; Ahn and Chang 1991). However, some difficulties limit the practical application of the aerobic
RBC reactor for the treatment of high-strength organic wastewaters including oxygen transfer limitations and excess sludge problems (Ware et al. 1990). The anaerobic rotating biological contactor (AnRBC), a relatively new biological wastewater treatment process, not only has the advantages of the aerobic RBC reactor but also shows potential to solve problems of the aerobic RBC reactor. This process was first introduced by Tait and Friedman (1980) for treating synthetic carbonaceous wastewaters. It was concluded that the AnRBC reactor is both feasible and practical for the removal of high strength organic compounds from the wastewater. Laquidara et al. (1986) used two single-stage AnRBC units for the treatment of synthetic soluble carbohydrate 313
314
C. Lu et al.
NOMENCLATURE
A, C C, ci ci+
D Da KS L ti R S t f U V :a Y, Z
Wetted surface area of disks Substrate concentration Effluent substrate concentration Influent substrate concentration Concentration of substrate at a point infinitesimally beyond the entrance Axial dispersion coefficient Damkohler number for flow convection (=~ma&kL~a&W Substrate concentration at half the maximum specific growth rate Reactor length Area capacity constant (=u,,XJY> Wastewater flow rate Removal coefficient Dimensionless substrate concentration (= C/C3 Time coordinate Mean residence time of RTD data Average flow velocity Volume of the reactor Axial distance Mass of attached biomass per unit wetted surface area of disk Apparent yield coefficient of the attached biomass Dimensionless axial distance (= x/L)
removal from the wastewater did not improve the treatment efficiency of the AnRBC reactor. The COD removal efficiencies of 29-53% were obtained in their study. An evaluation of a simple kinetic model for the treatment of domestic sewage by a single-stage laboratory scale unit of AriRBC has been conducted by Deshpande et al. (1991). The results demonstrated that a simple Monod-type expression can satisfactorily describe the performance of an AnRBC reactor, and the model maintains its validity even with high-strength domestic wastewater. Although the system performance and kinetic model of the AnRBC reactor have been researched for the treatment of organic wastewaters by the above investigators, laboratory studies and field applications of the AnRBC reactor are still relatively limited compared to the aerobic RBC reactor in the literature. Therefore, more information with respect to their predictive model, system performance, kinetics, and microscopic observations would be helpful in providing design and operational criteria for a full-scale AnRBC type system. This paper presents the treatability results of high strength organic wastewater by an AnRBC reactor. A predictive model that takes into account the simultaneous convective transport, as well as dispersion in the axial direction, and substrate utilization by the attached biomass in the AnRBC reactor is also presented. MATERIALS AND METHODS Experimental setup
Greek symbols Maximum specific growth rate for the attached P ma biomass Variance of RTD data lJ2 0 Disk rotational sneed wastewater which is similar to a typical food processing wastewater stream with nutrient and buffer addition. The AnRBC reactor was proved to be an effective process for treating a carbohydrate wastewater at chemical oxygen demand (COD) removal rates between 10 and 140 g/m’.d. Satyanarayan et al. (1987) employed a laboratory-model anaerobic rotating biological drum contactor (AnRBDC) to treat dairy wastewater. It was observed that dairy wastewater can be treated easily with COD removal efficiencies of 60 - 8 1% by an AnRBDC. Lo et al. (1990a; b) investigated the effects of sulfate removal from molasses wastewater on anaerobic digestion using an AnRBC reactor. The experimental results showed that the presence of high sulfate concentration prolonged the start-up process in the AnRBC reactor, however, sulfate
Three bench-scale analogous AnRBC reactors were constructed and identified as AnRBCl, AnRBC2, AnRBC3, respectively. The experimental setup is shown schematically in Fig. 1 and exact dimensions are summarized in Fig. 2 and Table 1. Each AnRBC reactor consisted of four stages, each stage contained fifteen 12 cm diameter acrylic plastic disks with 9.5 mm spacings between disks. These disks were mounted on a 1.6 cm diameter horizontal stainless steel shaft which was supported at both end and rotated by a variable speed drive motor ranging from 0 to 60 r-pm (peripheral disk velocity: 0 to 22.6 m/min). Each stage was separated by fixed baffle plates that contain three holes drilled below the water line for the passage of wastewater to prevent short-circuiting (Fig. 3). Biogas was collected in each stage and vented through a pipe to a gas collection tube filled with saturated sodium chloride liquid, 5% sulfuric acid and methyl red for the evaluation of biogas production rate. The AnRBC reactor was covered and sealed by an external acrylic plastic pipe with a diameter of 17 cm to maintain constant temperature (35 f 0.5”C).
315
Removal of organic compounds f?om wastewater by an AnRBC
variable influent
sp-eed pump
9s collection manifold
--+Pl i-l
1 +
synthetic wastewater
effluent
1
r
+j
l-4
z
,
,
stainless steel shaft ,
gas collection equipmen baffle
disk .i-\
(15 per stage)
sample ports
variable speed drive motor
2nd 1 stage
( stage
3rd
4th
stage
stage
Fig. 1. A schematic diagram of the AnRBC reactor.
Number of stages Disks per stage Disk diameter (mm) Reactor diameter (mm) Disk thickness (mm) Spacing between disks (mm) Wetted surface area of disks (m’) Reactor volume (liters) Surface to volume ratio Reactor material with water bath. The substrate feed rate was controlled by a variable speed peristaltic pump while the water level was controlled by a dynamic head tube resembling a vented inverted siphon on the effluent line. Substrate characteristics during start-up are listed in Table 2. The fourth and twelfth disks in each stage of the AnRBC reactor were designed with four removable sections as shown in Fig. 4 to make detailed microscopic observations of the attached biofilms (two small sections) and measure both total biomass (MISS) and biotilm thickness (two large sections). A special tool was employed to remove these sections so that the anaerobic conditions could be maintained and attached biofilm undisturbed.
4 15 120 130 3 9.5 1.43 11.95 119.7 Polyacrylic Tracer study
The hydraulic characteristics of the AnRBC reactor were studied prior to start-up using an impulse-input response experiment to determine flow patterns in the reactors. A pulse of soluble NaCl was injected at the entrance of the AnRBC reactor to measure the conductivity at the system effluent of each stage. The NaCl concentration was calculated from the calibration curve of conductivity versus soluble NaCl concentration. Effects of varying disk rotational speed (o), hydraulic retention time (HRT), and disk submergence were evaluated and the operating conditions are listed in Table 3.
316
C. Lu et al.
900
I4
1320
mm
)1
350mm bl
Fig. 2. Dimensions of the AnRBC reactor.
Down ‘-
thickness
= 3 mm
I3
A SE -E _I
bl
mm
k
disk
I
Fig. 3. Detailed drawing of the baffle plate.
317
Removal of organic compounds from wastewater by an AnRBC
Table 2. The compositions of synthetic organic wastewater during start-up of the AnRBC reractor.
O-14 days
15-65 days
(g/L)
1
(g/L) (!m (mg/L) (Em @x/L) (g/L) (g/L) (WL) (mg/L) (mg/L) (g/L as CaCO,)
0.18 6 9.65 0.153 7.5 0.89
10 1.8 15 96.5 1.53 75 12 7.27 572 362 50 9.32 7.8-8.0 1.65 21 240
Parameters
C&A
Beef extract NaHC03 FeCl, . 6Hz0 NH&l Na*SO, COD BOD TKN NH3-N Total PO:--P Alkalinity PH COD/BOD COD/N COD/P
7.2-7.4
I -I
Fig. 4. Detailed drawing of removable section disk.
Analytical methods
The following parameters were determined as suggested in Standard Methods (APHA; 1989): soluble COD (508A), soluble BOD (507), NH,-N (417A), total P (424C-II), and alkalinity (403). The volatile acid (VA) was measured by a direct-titration method proposed by Dilallo and Albertson (1961). Dissolved oxygen (DO)
was measured with an electrochemical membrane-type electrode, while pH and oxidation-reduction potential (ORP) were determined by a digital pH meter and an ORP meter, respectively. Conductivity was evaluated by a digital conductivity meter. Biogas production rate was monitored in the gas collection tube.
C. Lu et al.
318
Table 3. The operating conditions during tracer study of the AnRJ3C reactor.
Run no.
1 2 3 4 5 6 7
Flow rate (mL/min) 5.8 5.8 5.8 24.0 3.5 3.0 9.4
HRT
Disk submergence (%)
Cd
(hrs)
6-v)
21.6 21.6 21.6 5.2 35.3 21.6 21.6
6 12 24 12 12 12 12
70 70 70 70 70 40 100
Microscopic observation
Model equations
After start-up of the AnRBC reactor, the small sections were removed and fixed with 25 g L-’ glutaraldehyde at a temperature of 4 “C for 2.5 h and then washed with 0.0 1 M phosphate buffer solution for 10 min three times. Progressively dehydrated from 30% to 100% with ethanol series, dried by critical point dryer, and coated with gold, the samples were scanned at 15 kV with the scanning electron microscopy (BAUSCH & LOMB SEM, Manolab 2 100).
On the basis of above assumptions, the dimensionless mass balance equation in terms of the substrate concentration (COD here) is written as
THEORY
The inside geometry of an AnRBC reactor may be quite complex. However, considerable mathematical simplicity can be attained in analysis if it is characte-rized as an equivalent tube. This is the approach that was employed in this study. Basic
assumptions
The following major assumptions are made in developing a model to predict the system performance of an AnRBC reactor for treating high-strength organic wastewaters: 1) The system is in a steady-state opera-ting condition. 2) The flow is in the axial direction alone and the flow velocity remains constant throughout the reactor. 3) The suspended growth does not usually contribute significant substrate removal in the fixed-film biological reactors and thus is neglected in this study (Clark et al. 1978; Spengel and Dzomback 1992). 4) Due to value of K, in the Monod-type, expression is much higher than substrate concentration, the first-order kinetics is assumed to describe substrate utilization by the attached bamboos in the AnRBC reactor. 5) Since tracer injection was used and was accounted for an instantaneous impulse, the AnRBC reactor is thus treated as a closed-closed reactor (Levenspiel 1972). 6) Complete mixing in the liquid volume is assumed in the determination of biological kinetic parameters.
with boundary conditions (Danckwerts dS dz
+ (+I
-= dS dz
(1 - S)
= 0
0
1953)
at
at
z=o
z=l
(2)
(3)
The quantity, D/UL, called the reactor dispersion number, is a measure of the deviation from the plug flow pattern. If the reactor dispersion number approaches zero, the axial dispersion can be neglected and hence represents a plug flow. If the dispersion number reaches infinity, the axial dispersion is large and hence denotes a completely mixed flow. The value between these two extremes is called an arbitrary flow which represents any degree of partial mixing between plug flow and completely mixed flow. The Damkijhler number (Da) is defined as the ratio of substrate consumption rate by the attached biomass to substrate transport rate by flow convection. Equation 1 is a linear differential equation of the second order with constant coefficients. The analytical solution to Eq. 1, in terms of S as a function of the normalized length of the AnRBC reactor, is written as: S(z)
Z(l+b)
=exp
(zz,
x
exp[ %.&fd-T]
-2.(1-b)exp[k-6?$-dZ!$]
(4)
(l+b)'exp($~)-(1-b)2exp(-~~)
where b=F-zz
(5)
319
Removal of organic compounds from wastewater by an AnRBC
Parameters estimation
Use of Eq. 4 requires estimation of the reactor dispersion number (D/UL) as well as the Damk&ler number (Da) which is determined by the biological kinetic parameters (cl,,,and K J and apparent yield coefficient (YJ of the attached biomass. A summary of estimation of these parameters is provided below. The reactor dispersion number can be experimentally determined by analyzing the response to a tracer impulse. In the closed-closed reactor, the reactor dispersion number can be calculated according to (Levenspiel 1972) Reactor
dispersion
CT2 = 2% 7
number.
- exp-UL/D)
- 2(g)2(1
(6)
where u2 is the variance of residence time distribution (RTD) data; f is the mean residence time of RTD data. The variance of a continuous distribution measured at a finite number of equidistant locations is given by: g- tj"cj o2
= j=l
- T2
c
tjCj
(11) Substituting Eqs. 10 and 11 into Eq. 9 and taking the inverse gives: 1 KS 1 -=1 --+R
PC,
P
The values of R and C, are determined directly from experimental data while the values of P and K, can be determined by substituting R and C, into Eqs. 12 for linear regression. Once the values of P and K, have been determined, the Da number can be readily calculated from the experimental conditions. RESULTS AND DISCUSSION
Results are presented in chronological order to allow a clear evaluation of the AnREX reactor: 1) tracer study, 2) start-up, 3) treatability results tier start-up, and 4) predictive model. Tracer study
where tj is time; Cj is the tracer concentration in effluent at time j. The mean residence time, t is approximated by: m j=l
p = km-% ya
(7)
C ‘j j-1
T=
area of disk. According to these definitions, P and R can be expressed as:
(8)
C 'j j - 1
Using the RTD data, the values of u2 and f2 are calculated by Eqs. 7 and 8. Substituting these values into Eq. 6 and employing the trial and error method, the reactor dispersion number can be determined. Damkijhler number. Prior to determination of the Da number, the biological kinetic parameters must be known. Assuming a completely mixed reactor, the mass balance equation inside an AnREX reactor can be written as:
Results obtained with tracer study were analyzed using the reactor dispersion number. Table 4 shows the reactor dispersion number determined from experimental RTD data at system effluent of each stage for varying disk rotational speed (o), hydraulic retention time (HRT), and disk submergence. In general, a reduction of HRT or disk submergence results in an increase of the reactor dispersion number in the AnRlX reactor. On the other hand, the opposite trend is observed for varying o. Effects of varying these parameters on the hydraulic characteristics of the AnREZ reactor were most significant in the first stage. From the results of the reactor dispersion numbers, it can be concluded that the AnRBC reactor was of the plug flow type with large amounts of dispersion (Levenspiel 1972) and the dispersion effects were decreased as an increase of stage number. Start-up
where Q is the wastewater flow rate; and C, is the effluent substrate concentration. Clark et al. (1978) defined the area capacity constant, P, as the maximum amount of substrate removed per unit time per unit area of disk and the removal coefficient, R, as the amount of substrate removed per unit time per unit
The procedure of start-up contained several steps including control of seeding microorganisms, temperature, pH, influent organic loading, and hydraulic retention time. The operating conditions during start-up of the AnRBC reactor are listed in Table 5. The seeding step was to mix the very active anaerobic microorganisms obtained from
320
C. Lu et al.
Table 4. The reactor dispersionnumber determinedfrom experimentalRTD data at the systemeffluent of eachstagefor speed,hydraulicretention time and disk submergence. Run no. 1 2 3 4 5 6 7
stage 1
stage 2
stage 3
stage 4
0.55 0.60 0.95
0.28 0.29 0.49 2.1 0.22 0.28 0.26
0.18 0.17 0.23 1.1 0.16 0.22 0.14
0.14 0.13 0.19 0.5 0.13 0.19 0.10
012 0.64 0.44
varying
disk rational
Table 5. The operatingconditionsduring start-upof the AnRBC reactor. Parameters
O-14 days
15-65 days
Disk rotational speed (rpm) Peripheral velocity (m/min) Disk submergence (W) Digestion temperature (“C) Hydraulic retention time (hours) Flow rate (L/day) Organic surface loading (g-COD/m’ - d) Organic volumetric loading (kg-COD/m3 * d) PH
2 0.75
12 4.5 100 35 +0.5 21.6 13.28 111.4 13.33 6.8-7.2
Taiwan Livestock Research Institute with the synthetic wastewaters as indicated in the second column of Table 2. The synthetic wastewater contained dissolved glucose and beef extract as the primary organic carbon source. Thereafter, put 6 L of the mixtures into each AnRBC reactor for 6 h. The AnRJK reactors were then fed with the synthetic wastewater using operating conditions indicated in the second column of Table 5. After two weeks, the COD removal efficiency achieved 80% and the strength of synthetic wastewater was increased as listed in the last column of Table 2 in order to stimulate the growth of seeding microorganisms. The operating conditions during this period are shown in the last column of Table 5. The AnRBC reactors were run for a period of 6 weeks after attainment of the steady-state condition. Steady-state condition was assumed when the daily changes in the biogas production rate and the effluent COD concentration were within 5% for three successive days. Upon attainment of steady state conditions, the AnRBC reactors were sampled at the system influent and effluent of each stage.
Treatabilityresultsafterstart-up COD and BOD. The treatability results after start-up of the AnRBC reactors are summarized in Table 6. Figures
100 35kO.5 25-30 9.56-l 1.47 5.94-6.93 0.71-0.85 7.2-7.4
5 and 6 show the stead-state concentration profiles of soluble COD and BOD as a function of the stage number. The removal efficiencies of soluble COD and BOD were up to 71% and 76%, respectively, for the influent organic surface loading rate of 111.4 g-COD/m**d and organic volumetric loading rate of 13.33 kg-COD/m3-d. The accumulated removal efficiencies of soluble COD in the first, second, third, and fourth stages were 42%, 58%, 67%, and 71%. Most of the organic compounds were removed in the first two stages. This can be explained by the fact that most easily biodegradable matters (C,H,,O, and beef extract here) were biodegraded in the first two stages. Similar results were observed for the removal of soluble BOD. In view of these results, it was concluded that start-up of the three AnRBC reactors was successful (Tait and Friedman 1980). pH; VA, and alkalinity. Figure 7 shows the results of pH, alkalinity, and volatile acid (VA) measured at the system effluent of each stage. As indicated, pH values were between 6.8 - 7.4, which were in the optimal range of 6.6 - 7.6 for the anaerobic process (MaCarty 1964). The value of VA was highest in the first stage and decreased in the subsequent stages. This may be attributed to the fact that most organic matters were biodegraded and converted to VA in the first stage by the acetogenic microorganisms.
321
Removal of organic compounds from wastewater by an AnRBC
Table 6. Treatability results after start-up of the AnRBC reactor. parameters COD BOD NH,-N Org-N VA Alk TP ORP DO Biogas production
influent (g/L) (g/L) (mg/L) (mg/L) (g as CH,COOH/L) (g as CaCO,/L) (w/L) (mV) bg/L) (L/d) rate
12 7.3 362 210 9.3 50 -210 3.1
8.02
PH
stage 1
stage 2
stage 3
stage 4
7 3.8 421 55 3.9 8
5 2.6 451 58 3.6 8.2
4 2.1 460 54 2.4 8.3
3.5 1.8 466 53 2.3 9
-333 0.0 14.4
-333 0.0 12.1
-340 0.0 5.8
-342 0.0 5.8
7.11
7.21
7.30
7.31
I OIN Stage
number
1
I 2
1 Stage
1 3
-I
4
number
Fig. 5. Steady-state COD concentration profile as a function of tbe stage number after start-up of the AnRBC reactor.
Fig. 6. Steady-state BOD concentration profile as a function of the stage number after start-up of the AnRBC reactor.
The carbonate alkalinity and pH value increased as an increase in the stage number due to a reduction of biodegradation rate of organic matters to VA in the latter stages. Althoughvalues of VA were high in the range of 2.3 to 3.9 g L’, pH values were maintained close to 7.0 by
hibited if the UVA concentrations are in the range of 30 to 60 mg L-l. As a result, the influence of high VA on the biomass activity can be neglected.
the addition
of sodium
bicarbonate
as the buffer
solution.
Theoretically, the concentration of undissociated volatile acid (UVA) only takes l/160 of the total VA concentration in the pH values near 7.0 (Morel 1983). Therefore, the UVA concentrations were estimated between 14.4 and 24.4 mg L-’ in this study. According to Kroeker et al. (1979), the methanogenic microorganisms are in-
Organic nitrogen and ammonium nitrogen. The steady state concentration profiles of organic nitrogen (Org-N) and ammonium nitrogen (NH,-N) as a function of the stage number are shown in Fig. 8. The conversion rate of organic nitrogen to ammonium nitrogen was highest in the first stage and became much slower in the subsequent stages.
322
C. Lu et al.
Alkalinity
’
I
I
1 Stage
2 number
I 3
44
Fig. 7. Alkality, VA, and pH as a function of stage number afterstart-up of the AnRFK reactor.
ORP and DO
The variations of the oxidation-reduction potential (ORP) measured at the system effluent of each stage were in the range of -333 to -342 mV. According to Li (1984), the anaerobic environment is achieved if the ORP is below -330 mV. This implies that the anaerobic condition has been reached during the period of start-up in this study. The ORP decreased (more negative) as an increase of the stage number. It may indicate that the methane fermentation reactions in the latter stages are predominant. The results can also be proved by a reduction of VA as an increase of the stage number in Fig. 7. The dissolved oxygen (DO) concentration was equal to zero at system effluent of each stage. Biogas production. As indicated in Table 6, biogas production rate was decreased as an increase of the stage number. This can be explained by the fact that the removal rates of biodegradable matters are higher in the first stage than those of in the subsequent stages. For the steady-state condition, the total biogas production rate was equal to 38.1 L/d and the gas productivity can be estimated as 0.299 L/g COD removed at 1 atm and 0°C. SEMphotograph. The results of SEM photographs taken from the removable sections of each stage indicate that the primary microorganisms in the outer shell of the biofilm are acetogenic microorganisms for the first and second stages including round-end filament bacteria, cocci, and curved bacteria. The methangenic bacteria are predominant in the outer shell of the biofilm for the third and fourth stages. The morphology of methanogenic mi-
loot\
I OIN
Organic
I 1
ritrogen
I Stage
2 number
~
I 3
Fig. 8. Steady-state organic nitrogen and ammonium nitrogen concentration profiles as a function of stage number after start-up of the AnREE reactor.
croorganisms can be described as rod-shaped with flat ends and very long flexible filaments which tend to aggregate in characteristic bundles. The mean biofilm thickness in each stage was measured from 1.5 mm to a very thin film. A dense microorganism was observed in the disks of the first stage. Model verification
Model parameters were chosen based on the experimental conditions at the end of start- up. The estimated values of D/I-IL were equal to 0.44, 0.26, 0.14, and 0.1 for the first, second, third, and fourth stages of the AnRBC reactor as shown in Run 7 of Table 4, while the values of P and K, were equal to 8.16 mg me2 S’ and 32.9 g L’ (Yeh 1994), respectively. Figure 9 shows the comparison of dimensionless COD concentration profiles predicted by the present model with experimental data measured at the system effluent of each stage. It is seen that the dimensionless COD concentration profiles are predicted well in the earlier stages and are under-predicted in the latter stages. Departures from experimental data in the latter stages can be attributed to the assumption of first-order reaction rate in Eq. 4. CONCLUSIONS
Treatability results of high-strength organic wastewaters by an AnRBC reactor were presented. In the steady-state operating condition (o of 12 rpm, HRT of 2 1.6 h, and 100% disk submergence), the removal effiencies of soluble COD and BOD could be up to 7 1% and 76%, respectively, for organic surface loading rate of
323
Removal of organic compounds from wastewater by an AnRBC
“’ LIN
I
I
2
1 Stage
I
3
I
4
number
Fig. 9. Comparison of steady-state dimensionless COD concentration profiles predicted by the present model with experimental data measured at the system effluent ofeach stage. Line =model prediction; symbol = experimental data.
111.4 g-COD/m’-d and organic volumetric loading rate of 13.33 kg COD/m3-d. In view of the results, the AnREK reactor is both feasible and practical for an effective treatment of high strength organic wastewaters. A simple model that accounts for the axial dispersion and substrate utilization by the attached biomass was also presented to predict the system performance of the AnRBC reactor. Good agreement between model predictions and experimental data was obtained by comparing dimensionless soluble COD concentration profiles measured at the system effluent of earlier stages. This model can be applied to be as an optimization tool to achieve the best design and operation for a full-scale AnRBC type system. Acknowledgment-The authors wish to thank the Taiwan Livestock Research Institute in providing the anaerobic microorganisms. Support from the National Science Council, R0.C. (NSC 82- 0410-E005-080) is gratefully acknowledged.
REFERENCES Ahn, K.H.; Chang, J.S. Performance evaluation of compact RBC-selting tank system. Water Sci. Technol. 23:1467-1476; 1991. Clark, J.H.; Moseng, E.M.; Asano, T. Performance of a rotating biological contactor under varying wastewater flow. J. Water Pollut. Control Fed. 50: 896-911; 1978.
API-IA (Am. Public Health Assoc.) Standard methods for the examination of water and wastewater. 17th edition, New York; 1989. Danckwerts, P.V. Continuous flow systems: distribution of residence times. Chem. Eng. Sci. 2: I-13; 1953. Deshpande, V.P.; Kaul, S.N.; Deshpande, C.V. An evaluation of a simple kinetic model for the treatment of domestic sewage by means of an anaerobic rotating biological contactor. Bioresource Technol. 38: 31-38; 1991. Dilallo,R.; Albertson, O.E. Volatile acid by direct titration. J. Water Pollut. Control Fed. 33: 365; 1961. Gujer, W.; Boller, M. A mathematical model for rotating biological contactors. Water Sci. Technol. 22: 53-73; 1990. IMSL (International Mathematics and Statistics Libraries) Contents Document, Vol. 2, Version 1.0, Houston, Texas; 1987. Koreker, E.J.; Schulte, D.D.; Sparling, A.B.; Lapp, H.M. Anaerobic treatment process stability. J. Water Pollut. Control Fed. 51: 718-721; 1979. Laquidara, M.J.; Blanc, F.C.; O’Shaughnessy, J.C. Development of biotilm, operating characteristics and operational control in the anaerobic rotating biological contactor process. J. Water Pollut. Control Fed. 58: 107-114; 1986. Levenspiel, 0. Chemical reaction engineering. New York, NY: John Wiley & Sons; 1972. Li, A.Y. Anaerobic processes for industrial wastewater treatment. Connecticut, CT: Dorr-Oliver Inc.; 1984. Lo, K.V.; Liao, P.H. Anaerobic treatment of baker’s yeast wastewater: I. start-up and sodium molybdate addition. Biomass 2 1: 207-2 18; 1990. Lo, K.V.; Chen, A.; Liao, P.H. Anaerobic treatment of baker’s yeast wastewater: II. sulfate removal. Biomass 23: 25-37; 1990a. MaCarty, P.L. Anaerobic waste treatment fundamentals-Part two, environment requirements and control. Public Works 95: 123-126; 1964. Morel, F.M.M. Principle of aquatic chemistry. New York, NY: Wiley-Interscience; 1983. Satyanarayan, S.; Thakar, K.; Kaul, S.N.; Badrinath, S.D.; Swarnkar, N.G. Anaerobic rotating biological drum contactor for the treatment of dairy wastes. Indian Chem. Eng. XXIX: 3-6; 1987. Spengel, D.B.; Dzombak, D.A. Biokinetic modelling and scale-up considerations for rotating biological contactor. Water Environ. Res. 64: 223-235; 1992. Tait, S.J.; Friedman, A.A. Anaerobic rotating biological contactor for carbonaceous wastewaters. J. Water Pollut. Control Fed. 52: 2257-2269; 1980. Tokus, R.Y. Biodegradation and removal of phenols in rotating biological contactors. Water Sci. Technol. 21: 1751; 1989. Ware, A.J.; Pescod, M.B.; Starch, B. Evaluation of alternatives to conventional disc support media for rotating biological contactor. Water Sci. Technol. 22: 113-l 17; 1990. Yeh, AC. Treatment of organic wastewaters by the Anaerobic Rotating Biological Contactors. Master Thesis, National ChungHsing University, Taichung, Taiwan; 1994.