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Biodegradation kinetics of high strength oily pet food wastewater in a membrane-coupled bioreactor (MBR)
Chemosphere 65 (2006) 1204–1211 www.elsevier.com/locate/chemosphere
Biodegradation kinetics of high strength oily pet food wastewater in a membrane-coupled bioreactor (MBR) R. Kurian a, G. Nakhla a
b,*
, A. Bassi
b
Department of Civil & Environmental Engineering, University of Western Ontario, London, Ont., Canada N6A 5B9 b Department of Chemical & Biochemical Engineering, University of Western Ontario, 1151 Richmond Street, Suite 2, London, Ont., Canada N6A 5B9 Received 10 October 2005; received in revised form 20 March 2006; accepted 21 March 2006 Available online 11 May 2006
Abstract Aerobic respirometric experiments were conducted at a temperature of 40 C to determine biokinetic parameters describing growth and decay of aerobic biomass acclimatized in a membrane-coupled bioreactor (MBR). The kinetic parameters were determined with the volatile fatty acids (VFA) acetic acid and propionic acid as substrates as well as the soluble fraction of a high strength rendering wastewater as substrate. The oxygen uptake rate (OUR) and cumulative oxygen uptake (OU) recorded during the experiments were fitted to Monod kinetic model to obtain true yield (Yt), intrinsic maximum substrate removal (k), half-saturation constant (Ks) and endogenous decay coefficient (kd). Ks were determined to be 181, 271 and 806 mg COD l1 and k as 1.89, 1.08 and 0.85 mg COD mg VSS1 h1 for acetate, propionate and rendering wastewater, respectively. The continuous-flow MBR was operated under two HRT conditions, 10 and 5 d, attaining high BOD removal efficiencies of 99% and 87%, respectively. The observed yield in the MBR was 0.03 g VSS g1 COD. 2006 Elsevier Ltd. All rights reserved. Keywords: Acetate; Biokinetics; MBR; Propionate; Rendering wastewater; Respirometry
1. Introduction The growth of industries producing high strength and complex wastewaters makes it imperative to develop suitable and often unique treatment solutions for these wastewaters. Aerobic biological processes operating at higher temperatures are highly advantageous in treating high temperature, high strength industrial wastewaters due to their ability to combine the advantages of conventional aerobic and anaerobic processes that include rapid biodegradation kinetics and reduced biological solids production, respectively (Rozich and Bordacs, 2002; Tchobanoglous et al., 2003). Although membrane-coupled bioreactors (MBR) are not new and have been successfully employed to treat industrial wastewaters over the years (Knoblock et al., 1994; Acharya *
Corresponding author. Tel.: +1 519 661 2111x85470; fax: +1 519 850 2921. E-mail address: [email protected] (G. Nakhla). 0045-6535/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2006.03.050
et al., 2004), few researchers have used MBR technology at high temperatures (Lopetegui and Sancho, 2003; Togna et al., 2003). A literature search shows that though data exist on the operation of submerged MBRs treating wastewater at room temperature (Chang and Judd, 2003; Chang and Kim, 2005) yet information on submerged MBRs, as employed in this study, at elevated temperatures is very limited. Lopetegui and Sancho (2003) successfully operated an external MBR at 55 C to treat paper mill effluent at mixed liquor suspended solids (MLSS) 5050 mg l1. Togna et al., 2003) operated an external MBR to treat beverage wastewater at 50–60 C with MLSS of 15.6 g SS l1, achieving 93% COD removal efficiency. External membranes are known to have the disadvantage of higher energy consumption due to recirculation relative to submerged membranes (Gander et al., 2000). Studies conducted by Kurian et al. (2005) also proved that external MBR operated at 45 C was disadvantageous in treating oily pet food wastewater as used in this study, due to clogging. The authors thus took the novel
R. Kurian et al. / Chemosphere 65 (2006) 1204–1211
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Nomenclature dS dt f k kd Ks MBR P Pi rs S
substrate consumed by biomass in time dt (mg COD l1) time (h) organic fraction of aerobic biomass intrinsic maximum substrate removal rate (mg COD mg VSS1 h1) decay rate (d1) intrinsic half-saturation coefficient (mg COD l1) membrane-coupled bioreactor soluble microbial products in the reactor at any given time (mg COD l1) initial concentrations of soluble microbial products (mg COD l1) rate of substrate removal (mg COD l1 d1) substrate concentration in reactor at given time (mg COD l1)
approach of implementing a submerged MBR operating at 40–43 C, bridging the information gap in the range of transitional temperature between mesophilic and thermophilic. The study presented here highlights the performance of a laboratory scale MBR fed with high strength, oily rendering wastewater over the operational period of 135 d at a temperature of 40–43 C. Respirometry is a well established and reliable technique applicable to biological studies but has been sparsely employed in experiments at high temperatures (du Plessis et al., 2001; Tremier et al., 2005) and less yet, in establishing biokinetics at higher temperatures. Vogelaar et al. (2003) employed respirometric techniques to establish biokinetics with acetate as substrate at 55 C but did not observe significant residual acetate or soluble microbial products (SMP), contrary to previous studies (Lapara et al., 2000a). The study presented here illustrates the use of respirometry as a reliable and reproducible method to establish biokinetic coefficients under aerobic conditions at high temperatures of 40 C, bordering on the lower thermophilic limit, with both simple substrates, acetic and propionic acid, and a complex substrate. Though conversions for acetic acid exist in literature, the development of biokinetic constants for propionic acid and the complex rendering wastewater serves as guideline for future studies.
sCOD soluble COD (mg l1) Seff effluent substrate concentration (mg COD l1) Si initial substrate concentration (mg COD l1) S0 food to microorganism ratio X0 X biomass concentration (mg l1) Xai active VSS (mg l1) Xi inactive VSS (mg VSS l1) Xii initial inactive VSS (mg l1) Xt total biomass (mg VSS l1) Xti initial total VSS (mg l1) Yo observed yield (mg VSS mg1 CODremoved) YP product COD formed per unit substrate COD removed (mg mg1) Yt true yield coefficient (mg VSS mg1 CODremoved) hc sludge retention time or SRT (d)
2.1. Respirometric batch experiments
gen was supplied and provisions were made for a carbon dioxide trap in each reactor. The respirometer was equipped with a water bath for temperature control which was set to 40 C. In conducting the respirometric experiments, precautions were taken to overcome the disadvantage of lower oxygen solubility at the atypically high operational temperature by increasing the headspace and maintaining high speed of mixing. Another precaution taken was to provide a sufficient pre-heating period for the substrate before seeding to avoid expansion of media and headspace air which would result in a reverse pressure through the bubble counters. Adequate carbon dioxide trap of potassium hydroxide was provided and the level of chemical solution in each trap was monitored to ensure no overflow, which could potentially affect the pH in the reactor. The batch reactors were seeded with acclimatized biomass from the continuous aerobic MBR operated at 40–43 C for 135 d. Four sets of respirometric batch experiments were conducted to develop kinetic coefficients for propionic acid and acetic acid biodegradation and two sets of respirometric batch experiments were conducted for the soluble component of rendering wastewater. The blank in each batch was deducted from the other cells of that batch to account for oxygen uptake due to endogenous respiration. The XS 00 was controlled by varying the volume of wastewater (both synthetic and real), diluting with distilled water and seeding it with approximately the same concentration of biomass in each reactor. Samples were taken on the first and final days of the experiment and analyzed for COD, volatile fatty acids (VFA) and volatile suspended solids (VSS).
The respirometer (Challenge AER-200 Respirometer system, Fayetteville, Arkansas) set-up consisted of six reactors of varying initial substrate to microorganism ratios ðXS 00 Þ and one control, each of 350 ml reactor volume. Pure oxy-
2.1.1. Composition of synthetic wastewater in mineral medium (g l1) Respirometric experiments were conducted with influent of synthetic wastewater prepared with the following
2. Materials and methods
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composition. Synthetic wastewater with single substrate, acetic acid or propionic acid, was prepared to obtain COD of 8.4 g l1. The mineral media consisted of ammonium sulphate (3.7 g l1), KH2PO4 (0.66), MgSO4 Æ 7H2O (1.9), CaCl2 Æ 2H2O (0.63), glycerol (0.3), FeCl3 (0.03), CuSO4 Æ 5H2O (0.002), Na2MoO4 Æ 2H2O (0.004), MnSO4 Æ 2H2O (0.003), ZnCl2 (0.006), CoCl2 Æ 6H2O (0.012) and NaHCO3 for pH control. The synthetic wastewater prepared was then diluted to obtain the required substrate concentration in each reactor. 2.2. Continuous-flow MBR system A laboratory scale continuous aerobic thermophilic MBR, consisting of a 4 l storage tank followed by a 4.5 l clear Plexiglas reactor, was used for the treatment of rendering wastewater, characterized by high COD (18 g l1) of which approximately 63% was contributed by VFAs. Sludge obtained from a laboratory scale thermophilic MBR operating at 45 C for 160 d on the same high strength rendering wastewater was used to seed the reactor. The reactor temperature was maintained at 40–43 C using a Corning hotplate stirrer and the reactor was operated at pH 8–8.3. The temperature selection was made due to limitations related to the maximum operational temperature of the membrane. The membrane module employed was ZW1, Zenon Environmental, Oakville, Ontario, of pore size 0.04 lm and of surface area 0.047 m2 to retain solids in the reactor. The reactor was aerated continuously with compressed air at the rate of 3–5 lpm through the immersed membrane to maintain DO > 1 mg l1. A peristaltic pump (Minipuls3, Gilson Inc., Canada) was used to draw permeate through the immersed membrane unit and pump influent into the reactor. The system operation continued over 87 d at a 5 d hydraulic retention time (HRT) and for 48 d at the 10 d HRT and sludge retention time (SRT) of 45 d for both HRTs. The SRT was calculated based on sludge wastage incurred due to sampling as no other wastage was deemed necessary. 2.3. Model description The oxygen uptake (OU) at any given time in a batch reactor is calculated from Eq. (1) (Grady et al., 1989). The substrate, products and biomass are all expressed in units of COD. The application of Eq. (1) is valid for acclimatized, non-nitrifying growth-associated OU (Grady et al., 1989). The Monod kinetic model (Grady et al., 1989) as shown in Eq. (2) was used to estimate the biokinetic coefficients. OU ¼ ðS i SÞ f ðX t X ti Þ ðP P i Þ K X ai dt S dS ¼ Ks þ S dSMP dS ¼ YP dt dt dX S Ks ¼l X kd X dt Ks þ S Ks þ S
where kd X ai dt X t ¼ X ti þ Y t dS 24 kd X i ¼ X ii þ ð1 f Þ X ai dt 24 X ai ¼ X t X i
ð5Þ ð6Þ ð7Þ
To calculate OU in the batch reactors Eqs. (2)–(4) were solved simultaneously and the resulting values of S, P and X were substituted in Eq. (1) to generate a theoretical curve. For this purpose a set of parameter values must be assumed and curves generated are compared to the actual measured curves. The best-fit curve was determined by establishing the minimum average percentage error (<30%), defined as the absolute difference of theoretical and actual OU divided by the actual OU and minimum relative residual sum of squares (<60) which is the sum of the squares of the difference of theoretical and actual values divided by the actual measured value. 2.4. Analytical methods Parameters analyzed included total suspended solids (TSS), VSS, COD, BOD and VFA. Initial and final day samples from each respirometric reactor were analyzed for TSS and VSS. Particle size distribution, determined using Masterizer 2000 (Malvern Instruments Ltd., UK), revealed that less than 1.5% of the suspended solids were finer than 1.2 lm, and accordingly TSS and VSS were measured using the standard 1.2 lm filter paper (APHA, 2003). BOD, TSS and VSS were determined according to Standard Methods (APHA, 2003). Oil and grease was measured gravimetrically according to Standard Method 5520B (APHA, 2003). Ammonia, nitrate and nitrite were determined using a high performance liquid chromatograph (Dionex Canada Ltd., Oakville, Ontario). Soluble samples were obtained by filtering through 0.45 lm filter paper (Whatman) and used for VFA and sCOD measurements. sCOD determination was done using the HACH Odyssey Analyzer and the heating reactor with standard HACH testing kits. VFA concentrations were determined by a gas chromatograph (Varian CP 3800) with flame ionization detector equipped with a fused silica column (30 m · 0.32 mm · 5 lm). Nitrogen was used as carrier gas at 5 ml min1. Detector and injector temperatures were set at 200 C and the oven was programmed to 110–165 C at a ramp of 20 C min1 for 9.25 min. The minimum detection level for VFA was 1 mg l1.
ð1Þ ð2Þ
3. Results and discussion 3.1. Performance of the continuous-flow MBR
ð3Þ ð4Þ
The laboratory scale continuous-flow MBR system was operated at 40–43 C for 135 d under two HRT conditions of 5 and 10 d. It is noteworthy that the system did attain a
R. Kurian et al. / Chemosphere 65 (2006) 1204–1211
stable performance in spite of the variability in concentrations of the influent contaminants at both HRT conditions (Table 1).
25000
5-day
6000
10-day
5000
20000
4000 15000 3000 10000
2000
5000
1000
Effluent BOD5 (mg/l)
Influent BOD & Reactor VSS (mg/l)
Influent BOD Reactor VSS Effluent BOD
3.2. COD and BOD removal
0
0 0
20
40
60 80 Time (days)
100
120
Influent & Reactor TCOD (mg/l)
Fig. 1. Temporal variation of influent and effluent BOD and reactor VSS.
5-days
60000
10-days
50000 40000
Influent COD
12000
TCOD in reactor
10000
Effluent COD
30000
8000 6000
20000
4000
10000
2000
0
3.3. COD and BOD removal across the membrane
14000 Effluent COD (mg/l)
The influent characteristics varied considerably during both the HRT runs. Yet the system attained pseudosteady-state effluent as evident from the low standard deviations as shown in Table 1, which is testament towards the stability of the system. The variation in influent and effluent COD and BOD is evident from Figs. 1 and 2, respectively. It is evident from these figures that the system attained pseudo-steady-state within a short time during the 10 d HRT. At the 10 d HRT the system attained a remarkable COD and BOD removal efficiency of 97% and 99%, respectively, but the reduction in HRT from 10 to 5 d had a significant impact on the removal efficiencies reducing it to 78% and 87% for COD and BOD, respectively. It is apparent from Fig. 1 that despite the significant influent variability the reactor VSS stabilized at approximately 10,000 mg l1 towards the end of each run, i.e. days 65–85 and days 115–135.
1207
0 0
20
40
60
80
100
120
140
Time (days)
The pore size of the membrane being finer than 0.45 lm resulted in further removal of soluble contaminants. Consistent removal of soluble COD (sCOD) across the membrane was observed throughout the operational period. At the 5 d HRT the membrane affected a 23% sCOD removal while the removal at the 10 d HRT affected by the membrane was 75%. It is interesting to note that based on average conditions the sCOD removed by filtration through the membrane was 1540 mg l1 (75% of 2050 mg sCOD l1 in the reactor) during the 10 d HRT and 1440 mg l1 (23% of 6300 mg sCOD l1 in the reactor) during the 5 d HRT. The sBOD removal across the membrane followed a similar pattern as the sCOD removal with a 24% removal at the 5 d HRT and 78% at 10 d HRT (figure not included).
Fig. 2. Temporal variation of influent, reactor and effluent COD.
3.4. Oil and grease removal The enhanced solubility of oil and grease at higher temperatures is expected to increase the biodegradation and hence contaminant removal as documented by Becker et al. (1997) and Li et al. (2002). In this experiment more than 90% of the oil and grease, which primarily consisted of animal fat, was successfully removed. The doubling of the HRT only increased the removal efficiency of oil and grease marginally from 92% to 95% corresponding to effluent concentrations of 51 mg l1 and 33 mg l1, respectively (Table 1).
Table 1 Performance of the continuous-flow MBR Parameter
Influent 5-day HRT
MLSS at 5-day HRT
Permeate 5-day HRT
Influent 10-day HRT
MLSS at 10-day HRT
Permeate 10-day HRT
TCOD (mg l1) SCOD (mg l1)
18700 ± 3100 (8) 15000 ± 1900 (8)
32100 ± 5800 (8) 6300 ± 1500 (8)
3900 ± 600 (8)
15900 ± 2500 (7) 12600 ± 3800 (7)
12500 ± 6500 (7) 2050 ± 540 (7)
510 ± 100 (7)
TBOD (mg l1) SBOD (mg l1)
9050 ± 1510 (8) 7200 ± 900 (8)
12300 ± 2500 (8) 2400 ± 370 (8)
1500 ± 400 (8)
7700 ± 1250 (7) 6100 ± 1800 (7)
4300 ± 2250 (7) 680 ± 180 (7)
95 ± 13 (7)
Oil and Grease (mg l1) TSS (mg l1) VSS (mg l1)
670 ± 86 (8) 1750 ± 890 (8) 1400 ± 780 (8)
13200 ± 2050 (8) 10600 ± 1400 (8)
51 ± 10 (8) 0±0 0±0
660 ± 95 (7) 1600 ± 1100 (7) 1300 ± 900 (7)
11500 ± 1500 (7) 9900 ± 960 (7)
Numbers in parentheses indicate the number of samples considered.
33 ± 4 (7) 0±0 0±0
R. Kurian et al. / Chemosphere 65 (2006) 1204–1211
3.5. Residual substrate The concentrations of various fractions of VFAs are given in Table 2 for the laboratory scale continuous-flow MBR. The continuous flow system exhibited a considerably high concentration of acetate along with other components of VFA in the effluent at the 5 d HRT, which was consumed when the HRT was extended to 10 d. The propionic acid, however, persisted even at 10 d HRT while acetate and other VFA components were minimal. These results are quite contrary to existing literature, where the accumulation and persistence of acetate have been shown to be predominant in aerobic treatment systems operating at temperatures 45–65 C using agricultural wastes and synthetic waste (Chu and Mavinic, 1998; Ugwuanyi et al., 2005). 3.6. Observed yield It is known that at higher temperatures lower observed yields are to be expected owing to the higher decay rates. In this experiment observed yields were calculated based on a plot of cumulative VSS produced versus the cumulative COD removed across the reactor (figure not shown, R2 = 0.84). The cumulative VSS produced accounted for both sludge removed due to sampling and increased VSS measured in the reactor. Graphically the observed yield was found to be 0.03 g VSS g1 COD which is relatively lower than the 0.19 g VSS g1 COD measured in an MBR operated at room temperature, fed with the same wastewater (Acharya et al., 2004) and 0.05 mg SS mg1 COD observed in a thermophilic biofilm reactor (55 C) treating molasses (Suvilampi et al., 2002).
revealed no change in ammonia, nitrates or nitrites. The absence of nitrifying bacteria in the sludge was thus confirmed. 3.8. Respirometric experiments In light of the persistence of acetate and propionate in the effluent of the MBR at 5 d HRT respirometric batch experiments were conducted to further investigate the biodegradation of acetate and propionate. OU and OUR curves obtained from respirometric experiments were fitted with the Monod equation to obtain the kinetic constants discussed below. Fig. 3a is the representative curve obtained from the respirometric experiments for OU for all the three substrates, plotted alongside the theoretical curves predicted by the model. Fig. 3b is the representative curve obtained from the respirometric experiments for OUR with the various substrates. A summary of the biokinetic coefficients determined is presented in Table 3 for acetic acid, propionic acid and rendering wastewater. 3.8.1. Active biomass fraction and s0/x0 ratios The employment of a membrane unit made it possible to maintain high concentration of VSS as evident from
1200 1000 800
OU (mg/l)
1208
Measured
600
Theoretical
400
3.7. Nitrification studies
200
Batch nitrification studies conducted over 4 h periods on the bioreactor sludge at 40 C and room temperature
0 0
50
100
150
Time (hr)
Table 2 VFA data on influent and effluent in continuous-flow MBR Influent
HRT = 5 d
4100 ± 1117 (20)
505 ± 231 (8)
17.6 ± 7.7 (7)
3000 ± 514 (20)
727 ± 375 (8)
165 ± 12 (7)
150 ± 154 (20)
234 ± 52 (8)
0 ± 0 (7)
2000 ± 1155 (20)
176 ± 82 (8)
0 ± 0 (7)
1580 ± 994 (20)
259 ± 41 (8)
0 ± 0 (7)
810 ± 658 (20)
212 ± 65 (8)
0 ± 0 (7)
11640 ± 8569 (20)
2114 ± 638 (8)
182 ± 13 (7)
Fig. 3a. Representative measured and modeled OU curve.
HRT = 10 d
Steady-state data
60
Numbers in parentheses indicate the number of samples considered.
50 OUR (mg/l-hr)
Acetic acid (mg COD l1) Propionic acid (mg COD l1) Isobutyric acid (mg COD l1) Butyric acid (mg COD l1) Valeric acid (mg COD l1) Iso valeric acid (mg COD l1) Reactor VFA (mg COD l1)
Measured Theoretical
40 30 20 10 0 0
50
100
150
Time (hr)
Fig. 3b. Representative measured and modeled OUR curve.
Table 3 Biokinetic coefficients describing substrate degradation Kinetic parameters Yt (mg VSS mg COD1) K (mg COD mg VSS1 h1) Ks (mg COD l1) kd (d1) SMP (mg SMP mg COD1) Xi/Xt lmax (d1)
Acetic acid (9)
Propionic acid (10)
Rendering wastewater (11)
0.29 ± 0.02 1.89 ± 0.10 180.6 ± 14.2 0.23 ± 0.01 0.39 ± 0.07 0.80 ± 0.02 13.1 ± 0.68
0.29 ± 0.03 1.08 ± 0.18 271 ± 32.6 0.23 ± 0.01 0.39 ± 0.05 0.83 ± 0.01 7.53 ± 1.99
0.20 ± 0.01 0.85 ± 0.12 806 ± 192 0.14 ± 0.02 0.56 ± 0.15 0.86 ± 0.04 4.11 ± 0.57
Numbers in parentheses indicate the number of respirometric batch reactor results fitted individually.
Table 1. The high concentration of inactive VSS present in the influent (Table 1) definitely constituted a significant percentage of the reactor VSS. The expected concentration of active biomass (Xai) in the reactor was calculated as per Eq. (8) (Tchobanoglous et al., 2003). X ai ¼
hc Y o DS HRT
Final VFA and COD concentration (mg l-1)
R. Kurian et al. / Chemosphere 65 (2006) 1204–1211
1209 VFA Rendering wastewater
3500
y = 0.88x - 639.38
3000
2
R = 0.99
2500 2000 1500 1000 y = 0.37x - 114.04
500
2
R = 0.76
0 0
1000 2000 3000 4000 5000 Initial VFA and COD concentration (mg l-1)
Fig. 4. Residual VFA and COD as a function of initial concentrations in respirometric batch experiments.
2–4 g COD g VSS1 showed residual VFA of less than 60 mg l1 which was much lower than the lowest residual propionic acid concentrations measured in both batch experiments and continuous-flow system effluent.
ð8Þ
Accordingly Xai was calculated to be 19% of the Xt measured in the reactor (Table 1) based on the total COD removed. Thus the initial fraction of inactive to total biomass used in the respirometric experiments is 0.81, comparable to the theoretical values of 0.80, 0.83 and 0.86 for acetate, propionate and rendering wastewater, respectively. The XS 00 ratio at which the reactors were operated were in the ranges of 2–14 g COD g VSS1, 3–14 g COD g VSS1 and 1–5 g COD g VSS1 for acetate, propionate and rendering wastewater, respectively as calculated based on initial sCOD over the measured VSS. It is known that for respirometric studies the optimum XS 00 is 20 (Grady et al., 1989). In this case the XS 00 values based on active VSS fraction would be much higher than calculated, covering the range above and below 20, as the active biomass fraction was only 19% of the measured VSS. Even though these studies were conducted over a wide range of XS 00 the biokinetic coefficients determined were similar as evident from the low standard deviations as shown in Table 3. The respirometric experiments proved that both acetic acid and propionic acid persisted after the seven day contact time even though, as indicated by the OUR, biological reaction had stopped. The residual VFA reported in the experiments using propionic acid as a carbon source was primarily propionic acid with less than 6% acetic acid (on a COD basis) in all the reactors. The residual VFA concentration was found to be a function of the initial VFA concentration for both acetic and propionic acid as depicted in Fig. 4, and did not exhibit a linear relation to XS 00 . A similar effect was observed in the experiments using complex wastewater as substrate, where the residual COD was found to be proportional to the initial COD concentration as graphically represented in Fig. 4. Experiments conducted with acetic acid at lower XS 00 ratios of
3.8.2. Endogenous decay coefficient (kd) In this experimental study the endogenous decay coefficient was calculated theoretically by curve-fitting and verified by analytical measurements. The value was measured by monitoring the initial and final VSS in four blank reactors without substrate, one in each set, in respirometric tests and calculating kd as first order kinetics as per Eq. (9) (Tchobanoglous et al., 2003). dX ¼ k d X dt
ð9Þ
Accordingly, kd with acetic and propionic acid as substrates was 0.24 d1, comparable to the value determined by curve-fitting as 0.23 d1 (Table 3). In the wastewater experiments kd was calculated based on the change of VSS from 800 mg l1 to 350 mg l1 over 162 h, and found to be 0.12 d1 which was very close to the model predicted value of 0.14 d1. The lower decay rate measured in this experiment may be due to the higher inactive fraction of biomass of 86% compared to the 80–83% in the experiments with VFAs. 3.8.3. True yield (Yt) In this particular study the true yield (Yt) was determined by curve-fitting and verified with experimental values. In the batch experiments conducted on pure substrates (VFA) the change in VSS over the experimental period was plotted against the sCOD removed to obtain the observed yield graphically (R2 = 0.85). The observed yield in the system was accordingly estimated to be 0.09 g VSS g1 COD from the slope of the graph. Applying Eq. (10) (Tchobanoglous et al., 2003) the value of Yt was calculated to be 0.22 g VSS g1 COD, using the contact period of 6.1 d and kd of 0.23 d1. This value is comparable to the curve-fitted value of 0.29 g VSS g1 COD.
1210
Yo ¼
R. Kurian et al. / Chemosphere 65 (2006) 1204–1211
Yt 1 þ k d hc
ð10Þ
The observed yield in the lab scale system was calculated to be 0.03 g VSS g1 COD as discussed earlier. The slight variation in values may be due to variability in wastewater characteristics used in the continuous system. The observed yield calculated based on the model predicted Yt of 0.2 g VSS g1 COD and decay coefficient of 0.14 d1 (Table 3) yields a value of 0.03 g VSS g1 COD which is comparable to the measured value. 3.8.4. Half-saturation constant (Ks) The half-saturation constant was obtained by curve-fitting for both acetate and propionate substrate (Table 3). Considering the higher amount of residual propionate in both respirometric batch experiments and continuous-flow experiments the value of Ks defining propionate biodegradation as expected was higher than acetate. The value of Ks obtained by curve-fitting for propionate was 1.5 times that for acetate biodegradation corresponding to values 271 and 180 mg COD l1, respectively. Vogelaar et al. (2003) reported a value of 3 mg COD l1 at 55 C for acetate degradation which is comparatively low since residual acetate and SMP were not observed. Lapara et al. (2000a) reported Ks of 660 mg COD l1 at 55 C for a complex substrate of gelatin and a-lactose. Based on Eq. (11) (Tchobanoglous et al., 2003) Seff of propionic and acetic acid can be calculated using the corresponding Ks values. The maximum substrate removal rates per g of reactor VSS was calculated as per Eq. (12) (Tchobanoglous et al., 2003) for both acetate and propionate at each HRT condition. ð1 þ k d hcÞ K s hc ðlmax k d Þ 1 rs K S ¼ X Ks þ S
S eff ¼
ð11Þ ð12Þ
At the 5 d HRT the ratio of the maximum biomass specific substrate removal rate for acetic to propionic acid is 1.76; very similar to the ratio of k since the ambient concentrations of both acetic and propionic acids in the MBR were significantly above their respective Ks. At the 10 d HRT the same ratio is 0.41, i.e. propionate removal rate was much higher than that of acetate. Although the overall VFA removal rates decreased from 1892 mg d1 at the 5 d HRT to 1139 mg d1 at the 10 d HRT, the absence of VFAs other than acetic and propionic acids at the 10 d HRT clearly indicates that for this complex wastewater, biodegradation of higher VFAs proceed through propionic and acetic acid intermediates. Furthermore, the much higher persistence of propionic acid relative to acetic acid (727 mg l1 versus 505 mg l1) at the 5 d HRT and 165 versus 17.6 mg l1 at the 10 d HRT, coupled with the much higher propionic acid biodegradation rate relative to acetic acid at the 10 d HRT clearly indicates that the rate of generation of propionic acid is much higher than acetic acid
i.e. propionic acid is the primary intermediate for breakdown of VFAs in this complex wastewater. The Ks value determined for the complex wastewater was 806 mg l1 (Table 3), which is extremely high, possibly indicative of inhibition. Yet the Ks value was not high enough to predict the reactor sCOD according to Eq. (11). It can thus be inferred that the presence of inert solids or influent solids did cause inhibitory effects in the continuous-flow system but was not the only inhibitory factor. The wastewater, as mentioned, varied considerably over time not only in COD but also in contaminants such as heavy metals. The influent concentrations were measured in the range of 0.02–1.2 mg l1 of copper, 0.08–6.8 mg l1 of iron, 0.1–2.6 mg l1 of zinc, 0.008–0.03 mg l1 of lead and 1.28–5.2 mg l1 of aluminum and smaller quantities of selenium, arsenic, strontium, etc. Considering the complexity of the wastewater handled it was beyond the scope of this study to identify the exact contaminant or combination of contaminants which may have resulted in inhibition. 3.8.5. Intrinsic maximum substrate removal rate (k) and maximum specific biomass growth rate (lmax) In the respirometric experiments a maximum substrate removal rate of 1.89 g COD g VSS1 h1 and 1.08 g COD g VSS1 h1 was observed for acetate and propionate substrate, respectively and 0.85 g COD g VSS1 h1 for rendering wastewater (Table 3). The maximum specific biomass growth rate calculated as the product of the yield and the maximum intrinsic substrate removal rate was 13.1 d1, 7.5 d1 and 4.1 d1 for acetate, propionate and complex substrate, respectively. These values are within the range of 7.3 d1 aerobically composted sludges at 40 C (Tremier et al., 2005) and 9.6 d1 at 45 C treating complex synthetic wastewater (gelatin and a-lactose) (Lapara et al., 2000b) documented in literature. 3.8.6. Soluble microbial products (SMP) As shown in Table 3, curve-fitting yielded an SMP production of 0.39 g SMP g1 CODC for acetate and propionate. Rearranging Eq. (1) to calculate the only unknown, YP, assuming that initial SMP was nil and replacing the change in biomass with observed yield the following equation is obtained: Y P ¼ ð1 Y o Þ DS OU
ð13Þ
Accordingly calculations were done and the average SMP was found to be 0.3 ± 0.1 g SMP g1 COD for both acetic and propionic acid. The ratio of SMP produced to substrate consumed derived from Eq. (13) for acetate and propionate ranged from 0.1 to 0.4 g SMP g1 COD. Similar calculations for the rendering wastewater yielded an average value of 0.5 ± 0.16 and ranged between 0.2 and 0.7 g SMP g1 COD compared to the curve-fitted value of 0.56 g SMP g1 COD. It must be asserted that scrutiny of the non-VFA sCOD data in the MBR at the 10 d HRT relative to COD removal emphatically corroborates that 73%
R. Kurian et al. / Chemosphere 65 (2006) 1204–1211
of the SMPs formed were biodegraded in the reactor. Using a BOD:COD ratio of 0.65, the amount of total COD removed at the 5 d HRT, calculated as the difference between influent COD and reactor sCOD equivalent of BOD is >8000 mg COD l1. The residual non-VFA sCOD in the MBR at the 5 d HRT is 4172 mg l1, clearly suggesting that 33% of the SMPs produced were biodegraded. 4. Summary and conclusions Respirometric techniques were effective in determining the biokinetic constants describing COD biodegradation and the results are summarized below. • Regardless of the substrate employed, incomplete biodegradation was observed, leaving residual COD at the completion of the experiment. • The residual COD was observed to be proportionate to the initial substrate concentration. • Ks describing propionate biodegradation was determined to be 1.5 times that describing acetate biodegradation. • The high Ks value for the real wastewater indicates the presence of an inhibitory contaminant or contaminants. • The maximum specific biomass growth for acetic acid substrate was 1.75 times that for propionic acid substrate. • Propionate appears to be the predominant intermediate for the biodegradation of butyric, isobutyric, valeric and isovaleric acids. • The low observed yield of 0.03 g VSS g1 COD reveals that the aerobic treatment at higher temperature is a potential solution to difficulties related to high sludge generation observed in conventional systems in treating high strength wastewater.
Acknowledgements The authors express their gratitude to CRESTech and Finnie Distributors, St. Mary’s, Ontario, for financial support. References Acharya, C, Nakhla, G., Bassi, A., Kurian, R., 2004. Treatment of high strength pet food wastewater using two stage membrane bioreactors. In: Proceedings of 77th Annual Conference of Water Environ. Federation, 2004, New Orleans, USA. APHA-AWWA-WPCF, 2003. Standard Method for Examination of Water and Wastewater, 20th ed. American Public Health Association, Washington, DC, USA. Becker, P., Koster, D., Popov, M.N., Markossian, S., Antranikian, G., Markl, H., 1997. The biodegradation of olive oil and the treatment of lipid-rich wool scouting wastewater under aerobic thermophilic conditions. Water Res. 33, 653–660.
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Chang, I.-S., Judd, S.J., 2003. Domestic wastewater treatment by a submerged MBR (membrane bio-reactor) with enhanced air sparging. Water Sci. Technol. 47, 149–154. Chang, I.-S., Kim, S.-N., 2005. Wastewater treatment using membrane filtration-effect of biosolids concentration on cake resistance. Process Biochem. 40, 1307–1314. Chu, A., Mavinic, D.S., 1998. The effects of macromolecular substrates and a metabolic inhibitor on volatile fatty acid metabolism in thermophilic aerobic digestion. Water Sci. Technol. 38, 55–61. du Plessis, C.A., Barnard, P., Naldrett, K., de Kock, S.H., 2001. Development of respirometry methods to assess the microbial activity of thermophilic bioleaching archaea. J. Microbiol. Methods 47, 189– 198. Gander, M., Jefferson, B., Judd, S., 2000. Aerobic MBRs for domestic wastewater treatment: a review with cost considerations. Sep. Purif. Technol. 18, 119–130. Grady, C.P.L., Dang, J.S., Harvey, D.M., Jobbagy, A., Wang, X.L., 1989. Determination of biodegradation kinetics through use of electrolytic respirometry. Water Sci. Technol. 21, 957–968. Knoblock, M.D., Sutton, P.M., Mishra, P.N., Gupta, K., Janson, A., 1994. Membrane biological reactor system for treatment of oily wastewaters. Water Environ. Res. 66, 133–139. Kurian, R., Acharya, C., Nakhla, G., Bassi, A., 2005. Conventional and thermophilic aerobic treatability of high strength oily pet food wastewater using membrane-coupled bioreactors. Water Res. 39, 4299–4308. Lapara, T.M., Konopka, A., Nakatsu, C.H., Alleman, J.E., 2000a. Thermophilic aerobic wastewater treatment in continuous-flow bioreactors. J. Environ. Eng.-ASCE 126, 739–744. Lapara, T.M., Konopka, A., Nakatsu, C.H., Alleman, J.E., 2000b. Effects of temperature on bacterial community structure and function in bioreactors treating a synthetic wastewater. J. Ind. Microbiol. Biot. 24, 140–145. Li, X.-M., Zeng, G.-M., Liu, J.-J., Chen, J., Lun, S.-Y., 2002. A two-stage anaerobic system for biodegrading wastewater containing terephthalic acid and high strength easily degradable pollutants. J. Environ. Sci. 14, 474–481. Lopetegui, J., Sancho, L., 2003. Aerated thermophilic biological treatment with membrane ultrafilteration: alternative to conventional technologies treating paper mill effluents. Water Sci. Tech. Water Supply 3, 245–252. Rozich, A.F., Bordacs, K., 2002. Use of thermophilic biological aerobic technology for industrial waste treatment PMC Technologies, Exton, PA, USA. Water Sci. Technol. 46, 83–89. Suvilampi, J., Lehtoma¨ki, A., Rintala, J., 2002. Comparison of laboratory-scale thermophilic biofilm and activated sludge processes integrated with a mesophilic activated sludge process. Bioresource Technol. 88, 207–214. Tchobanoglous, G., Burton, F.L., Stensel, D.H., 2003. Wastewater engineering treatment and reuse, 4th ed. The McGraw-Hill Companies, New York, USA. Togna, A.P., Yang, Y., Sutton, P.M., Voigt, H.D., 2003. Testing and process design of a thermophilic membrane biological reactor to treat high-strength beverage wastewater. In: WEFTEC.03, Conference Proceedings, Annual Technology Exhibition & Conference, 76th, Los Angeles, CA, United States. Tremier, A., de Guardia, A., Massiani, C., Paul, E., Martel, J.L., 2005. A respirometric method for characterizing the organic composition and biodegradation kinetics and the temperature influence on the biodegradation kinetics, for a mixture of sludge and bulking agent to be cocomposted. Bioresource Technol. 96, 169–180. Ugwuanyi, J.O., Harvey, L.M., McNeil, B., 2005. Effect of temperature and pH on treatment efficiency and evolution of volatile fatty acids during thermophilic aerobic digestion of model high strength agricultural waste. Bioresource Technol. 96, 707–719. Vogelaar, Jaap C.T., Klapwijk, B., Temmink, H., van Lier, J.B., 2003. Kinetic comparisons of mesophilic and thermophilic aerobic biomass. J. Microbiol. Biotechnol. 30, 81–88.
Biodegradation kinetics of high strength oily pet food wastewater in a membrane-coupled bioreactor (MBR) R. Kurian a, G. Nakhla a
b,*
, A. Bassi
b
Department of Civil & Environmental Engineering, University of Western Ontario, London, Ont., Canada N6A 5B9 b Department of Chemical & Biochemical Engineering, University of Western Ontario, 1151 Richmond Street, Suite 2, London, Ont., Canada N6A 5B9 Received 10 October 2005; received in revised form 20 March 2006; accepted 21 March 2006 Available online 11 May 2006
Abstract Aerobic respirometric experiments were conducted at a temperature of 40 C to determine biokinetic parameters describing growth and decay of aerobic biomass acclimatized in a membrane-coupled bioreactor (MBR). The kinetic parameters were determined with the volatile fatty acids (VFA) acetic acid and propionic acid as substrates as well as the soluble fraction of a high strength rendering wastewater as substrate. The oxygen uptake rate (OUR) and cumulative oxygen uptake (OU) recorded during the experiments were fitted to Monod kinetic model to obtain true yield (Yt), intrinsic maximum substrate removal (k), half-saturation constant (Ks) and endogenous decay coefficient (kd). Ks were determined to be 181, 271 and 806 mg COD l1 and k as 1.89, 1.08 and 0.85 mg COD mg VSS1 h1 for acetate, propionate and rendering wastewater, respectively. The continuous-flow MBR was operated under two HRT conditions, 10 and 5 d, attaining high BOD removal efficiencies of 99% and 87%, respectively. The observed yield in the MBR was 0.03 g VSS g1 COD. 2006 Elsevier Ltd. All rights reserved. Keywords: Acetate; Biokinetics; MBR; Propionate; Rendering wastewater; Respirometry
1. Introduction The growth of industries producing high strength and complex wastewaters makes it imperative to develop suitable and often unique treatment solutions for these wastewaters. Aerobic biological processes operating at higher temperatures are highly advantageous in treating high temperature, high strength industrial wastewaters due to their ability to combine the advantages of conventional aerobic and anaerobic processes that include rapid biodegradation kinetics and reduced biological solids production, respectively (Rozich and Bordacs, 2002; Tchobanoglous et al., 2003). Although membrane-coupled bioreactors (MBR) are not new and have been successfully employed to treat industrial wastewaters over the years (Knoblock et al., 1994; Acharya *
Corresponding author. Tel.: +1 519 661 2111x85470; fax: +1 519 850 2921. E-mail address: [email protected] (G. Nakhla). 0045-6535/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2006.03.050
et al., 2004), few researchers have used MBR technology at high temperatures (Lopetegui and Sancho, 2003; Togna et al., 2003). A literature search shows that though data exist on the operation of submerged MBRs treating wastewater at room temperature (Chang and Judd, 2003; Chang and Kim, 2005) yet information on submerged MBRs, as employed in this study, at elevated temperatures is very limited. Lopetegui and Sancho (2003) successfully operated an external MBR at 55 C to treat paper mill effluent at mixed liquor suspended solids (MLSS) 5050 mg l1. Togna et al., 2003) operated an external MBR to treat beverage wastewater at 50–60 C with MLSS of 15.6 g SS l1, achieving 93% COD removal efficiency. External membranes are known to have the disadvantage of higher energy consumption due to recirculation relative to submerged membranes (Gander et al., 2000). Studies conducted by Kurian et al. (2005) also proved that external MBR operated at 45 C was disadvantageous in treating oily pet food wastewater as used in this study, due to clogging. The authors thus took the novel
R. Kurian et al. / Chemosphere 65 (2006) 1204–1211
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Nomenclature dS dt f k kd Ks MBR P Pi rs S
substrate consumed by biomass in time dt (mg COD l1) time (h) organic fraction of aerobic biomass intrinsic maximum substrate removal rate (mg COD mg VSS1 h1) decay rate (d1) intrinsic half-saturation coefficient (mg COD l1) membrane-coupled bioreactor soluble microbial products in the reactor at any given time (mg COD l1) initial concentrations of soluble microbial products (mg COD l1) rate of substrate removal (mg COD l1 d1) substrate concentration in reactor at given time (mg COD l1)
approach of implementing a submerged MBR operating at 40–43 C, bridging the information gap in the range of transitional temperature between mesophilic and thermophilic. The study presented here highlights the performance of a laboratory scale MBR fed with high strength, oily rendering wastewater over the operational period of 135 d at a temperature of 40–43 C. Respirometry is a well established and reliable technique applicable to biological studies but has been sparsely employed in experiments at high temperatures (du Plessis et al., 2001; Tremier et al., 2005) and less yet, in establishing biokinetics at higher temperatures. Vogelaar et al. (2003) employed respirometric techniques to establish biokinetics with acetate as substrate at 55 C but did not observe significant residual acetate or soluble microbial products (SMP), contrary to previous studies (Lapara et al., 2000a). The study presented here illustrates the use of respirometry as a reliable and reproducible method to establish biokinetic coefficients under aerobic conditions at high temperatures of 40 C, bordering on the lower thermophilic limit, with both simple substrates, acetic and propionic acid, and a complex substrate. Though conversions for acetic acid exist in literature, the development of biokinetic constants for propionic acid and the complex rendering wastewater serves as guideline for future studies.
sCOD soluble COD (mg l1) Seff effluent substrate concentration (mg COD l1) Si initial substrate concentration (mg COD l1) S0 food to microorganism ratio X0 X biomass concentration (mg l1) Xai active VSS (mg l1) Xi inactive VSS (mg VSS l1) Xii initial inactive VSS (mg l1) Xt total biomass (mg VSS l1) Xti initial total VSS (mg l1) Yo observed yield (mg VSS mg1 CODremoved) YP product COD formed per unit substrate COD removed (mg mg1) Yt true yield coefficient (mg VSS mg1 CODremoved) hc sludge retention time or SRT (d)
2.1. Respirometric batch experiments
gen was supplied and provisions were made for a carbon dioxide trap in each reactor. The respirometer was equipped with a water bath for temperature control which was set to 40 C. In conducting the respirometric experiments, precautions were taken to overcome the disadvantage of lower oxygen solubility at the atypically high operational temperature by increasing the headspace and maintaining high speed of mixing. Another precaution taken was to provide a sufficient pre-heating period for the substrate before seeding to avoid expansion of media and headspace air which would result in a reverse pressure through the bubble counters. Adequate carbon dioxide trap of potassium hydroxide was provided and the level of chemical solution in each trap was monitored to ensure no overflow, which could potentially affect the pH in the reactor. The batch reactors were seeded with acclimatized biomass from the continuous aerobic MBR operated at 40–43 C for 135 d. Four sets of respirometric batch experiments were conducted to develop kinetic coefficients for propionic acid and acetic acid biodegradation and two sets of respirometric batch experiments were conducted for the soluble component of rendering wastewater. The blank in each batch was deducted from the other cells of that batch to account for oxygen uptake due to endogenous respiration. The XS 00 was controlled by varying the volume of wastewater (both synthetic and real), diluting with distilled water and seeding it with approximately the same concentration of biomass in each reactor. Samples were taken on the first and final days of the experiment and analyzed for COD, volatile fatty acids (VFA) and volatile suspended solids (VSS).
The respirometer (Challenge AER-200 Respirometer system, Fayetteville, Arkansas) set-up consisted of six reactors of varying initial substrate to microorganism ratios ðXS 00 Þ and one control, each of 350 ml reactor volume. Pure oxy-
2.1.1. Composition of synthetic wastewater in mineral medium (g l1) Respirometric experiments were conducted with influent of synthetic wastewater prepared with the following
2. Materials and methods
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composition. Synthetic wastewater with single substrate, acetic acid or propionic acid, was prepared to obtain COD of 8.4 g l1. The mineral media consisted of ammonium sulphate (3.7 g l1), KH2PO4 (0.66), MgSO4 Æ 7H2O (1.9), CaCl2 Æ 2H2O (0.63), glycerol (0.3), FeCl3 (0.03), CuSO4 Æ 5H2O (0.002), Na2MoO4 Æ 2H2O (0.004), MnSO4 Æ 2H2O (0.003), ZnCl2 (0.006), CoCl2 Æ 6H2O (0.012) and NaHCO3 for pH control. The synthetic wastewater prepared was then diluted to obtain the required substrate concentration in each reactor. 2.2. Continuous-flow MBR system A laboratory scale continuous aerobic thermophilic MBR, consisting of a 4 l storage tank followed by a 4.5 l clear Plexiglas reactor, was used for the treatment of rendering wastewater, characterized by high COD (18 g l1) of which approximately 63% was contributed by VFAs. Sludge obtained from a laboratory scale thermophilic MBR operating at 45 C for 160 d on the same high strength rendering wastewater was used to seed the reactor. The reactor temperature was maintained at 40–43 C using a Corning hotplate stirrer and the reactor was operated at pH 8–8.3. The temperature selection was made due to limitations related to the maximum operational temperature of the membrane. The membrane module employed was ZW1, Zenon Environmental, Oakville, Ontario, of pore size 0.04 lm and of surface area 0.047 m2 to retain solids in the reactor. The reactor was aerated continuously with compressed air at the rate of 3–5 lpm through the immersed membrane to maintain DO > 1 mg l1. A peristaltic pump (Minipuls3, Gilson Inc., Canada) was used to draw permeate through the immersed membrane unit and pump influent into the reactor. The system operation continued over 87 d at a 5 d hydraulic retention time (HRT) and for 48 d at the 10 d HRT and sludge retention time (SRT) of 45 d for both HRTs. The SRT was calculated based on sludge wastage incurred due to sampling as no other wastage was deemed necessary. 2.3. Model description The oxygen uptake (OU) at any given time in a batch reactor is calculated from Eq. (1) (Grady et al., 1989). The substrate, products and biomass are all expressed in units of COD. The application of Eq. (1) is valid for acclimatized, non-nitrifying growth-associated OU (Grady et al., 1989). The Monod kinetic model (Grady et al., 1989) as shown in Eq. (2) was used to estimate the biokinetic coefficients. OU ¼ ðS i SÞ f ðX t X ti Þ ðP P i Þ K X ai dt S dS ¼ Ks þ S dSMP dS ¼ YP dt dt dX S Ks ¼l X kd X dt Ks þ S Ks þ S
where kd X ai dt X t ¼ X ti þ Y t dS 24 kd X i ¼ X ii þ ð1 f Þ X ai dt 24 X ai ¼ X t X i
ð5Þ ð6Þ ð7Þ
To calculate OU in the batch reactors Eqs. (2)–(4) were solved simultaneously and the resulting values of S, P and X were substituted in Eq. (1) to generate a theoretical curve. For this purpose a set of parameter values must be assumed and curves generated are compared to the actual measured curves. The best-fit curve was determined by establishing the minimum average percentage error (<30%), defined as the absolute difference of theoretical and actual OU divided by the actual OU and minimum relative residual sum of squares (<60) which is the sum of the squares of the difference of theoretical and actual values divided by the actual measured value. 2.4. Analytical methods Parameters analyzed included total suspended solids (TSS), VSS, COD, BOD and VFA. Initial and final day samples from each respirometric reactor were analyzed for TSS and VSS. Particle size distribution, determined using Masterizer 2000 (Malvern Instruments Ltd., UK), revealed that less than 1.5% of the suspended solids were finer than 1.2 lm, and accordingly TSS and VSS were measured using the standard 1.2 lm filter paper (APHA, 2003). BOD, TSS and VSS were determined according to Standard Methods (APHA, 2003). Oil and grease was measured gravimetrically according to Standard Method 5520B (APHA, 2003). Ammonia, nitrate and nitrite were determined using a high performance liquid chromatograph (Dionex Canada Ltd., Oakville, Ontario). Soluble samples were obtained by filtering through 0.45 lm filter paper (Whatman) and used for VFA and sCOD measurements. sCOD determination was done using the HACH Odyssey Analyzer and the heating reactor with standard HACH testing kits. VFA concentrations were determined by a gas chromatograph (Varian CP 3800) with flame ionization detector equipped with a fused silica column (30 m · 0.32 mm · 5 lm). Nitrogen was used as carrier gas at 5 ml min1. Detector and injector temperatures were set at 200 C and the oven was programmed to 110–165 C at a ramp of 20 C min1 for 9.25 min. The minimum detection level for VFA was 1 mg l1.
ð1Þ ð2Þ
3. Results and discussion 3.1. Performance of the continuous-flow MBR
ð3Þ ð4Þ
The laboratory scale continuous-flow MBR system was operated at 40–43 C for 135 d under two HRT conditions of 5 and 10 d. It is noteworthy that the system did attain a
R. Kurian et al. / Chemosphere 65 (2006) 1204–1211
stable performance in spite of the variability in concentrations of the influent contaminants at both HRT conditions (Table 1).
25000
5-day
6000
10-day
5000
20000
4000 15000 3000 10000
2000
5000
1000
Effluent BOD5 (mg/l)
Influent BOD & Reactor VSS (mg/l)
Influent BOD Reactor VSS Effluent BOD
3.2. COD and BOD removal
0
0 0
20
40
60 80 Time (days)
100
120
Influent & Reactor TCOD (mg/l)
Fig. 1. Temporal variation of influent and effluent BOD and reactor VSS.
5-days
60000
10-days
50000 40000
Influent COD
12000
TCOD in reactor
10000
Effluent COD
30000
8000 6000
20000
4000
10000
2000
0
3.3. COD and BOD removal across the membrane
14000 Effluent COD (mg/l)
The influent characteristics varied considerably during both the HRT runs. Yet the system attained pseudosteady-state effluent as evident from the low standard deviations as shown in Table 1, which is testament towards the stability of the system. The variation in influent and effluent COD and BOD is evident from Figs. 1 and 2, respectively. It is evident from these figures that the system attained pseudo-steady-state within a short time during the 10 d HRT. At the 10 d HRT the system attained a remarkable COD and BOD removal efficiency of 97% and 99%, respectively, but the reduction in HRT from 10 to 5 d had a significant impact on the removal efficiencies reducing it to 78% and 87% for COD and BOD, respectively. It is apparent from Fig. 1 that despite the significant influent variability the reactor VSS stabilized at approximately 10,000 mg l1 towards the end of each run, i.e. days 65–85 and days 115–135.
1207
0 0
20
40
60
80
100
120
140
Time (days)
The pore size of the membrane being finer than 0.45 lm resulted in further removal of soluble contaminants. Consistent removal of soluble COD (sCOD) across the membrane was observed throughout the operational period. At the 5 d HRT the membrane affected a 23% sCOD removal while the removal at the 10 d HRT affected by the membrane was 75%. It is interesting to note that based on average conditions the sCOD removed by filtration through the membrane was 1540 mg l1 (75% of 2050 mg sCOD l1 in the reactor) during the 10 d HRT and 1440 mg l1 (23% of 6300 mg sCOD l1 in the reactor) during the 5 d HRT. The sBOD removal across the membrane followed a similar pattern as the sCOD removal with a 24% removal at the 5 d HRT and 78% at 10 d HRT (figure not included).
Fig. 2. Temporal variation of influent, reactor and effluent COD.
3.4. Oil and grease removal The enhanced solubility of oil and grease at higher temperatures is expected to increase the biodegradation and hence contaminant removal as documented by Becker et al. (1997) and Li et al. (2002). In this experiment more than 90% of the oil and grease, which primarily consisted of animal fat, was successfully removed. The doubling of the HRT only increased the removal efficiency of oil and grease marginally from 92% to 95% corresponding to effluent concentrations of 51 mg l1 and 33 mg l1, respectively (Table 1).
Table 1 Performance of the continuous-flow MBR Parameter
Influent 5-day HRT
MLSS at 5-day HRT
Permeate 5-day HRT
Influent 10-day HRT
MLSS at 10-day HRT
Permeate 10-day HRT
TCOD (mg l1) SCOD (mg l1)
18700 ± 3100 (8) 15000 ± 1900 (8)
32100 ± 5800 (8) 6300 ± 1500 (8)
3900 ± 600 (8)
15900 ± 2500 (7) 12600 ± 3800 (7)
12500 ± 6500 (7) 2050 ± 540 (7)
510 ± 100 (7)
TBOD (mg l1) SBOD (mg l1)
9050 ± 1510 (8) 7200 ± 900 (8)
12300 ± 2500 (8) 2400 ± 370 (8)
1500 ± 400 (8)
7700 ± 1250 (7) 6100 ± 1800 (7)
4300 ± 2250 (7) 680 ± 180 (7)
95 ± 13 (7)
Oil and Grease (mg l1) TSS (mg l1) VSS (mg l1)
670 ± 86 (8) 1750 ± 890 (8) 1400 ± 780 (8)
13200 ± 2050 (8) 10600 ± 1400 (8)
51 ± 10 (8) 0±0 0±0
660 ± 95 (7) 1600 ± 1100 (7) 1300 ± 900 (7)
11500 ± 1500 (7) 9900 ± 960 (7)
Numbers in parentheses indicate the number of samples considered.
33 ± 4 (7) 0±0 0±0
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3.5. Residual substrate The concentrations of various fractions of VFAs are given in Table 2 for the laboratory scale continuous-flow MBR. The continuous flow system exhibited a considerably high concentration of acetate along with other components of VFA in the effluent at the 5 d HRT, which was consumed when the HRT was extended to 10 d. The propionic acid, however, persisted even at 10 d HRT while acetate and other VFA components were minimal. These results are quite contrary to existing literature, where the accumulation and persistence of acetate have been shown to be predominant in aerobic treatment systems operating at temperatures 45–65 C using agricultural wastes and synthetic waste (Chu and Mavinic, 1998; Ugwuanyi et al., 2005). 3.6. Observed yield It is known that at higher temperatures lower observed yields are to be expected owing to the higher decay rates. In this experiment observed yields were calculated based on a plot of cumulative VSS produced versus the cumulative COD removed across the reactor (figure not shown, R2 = 0.84). The cumulative VSS produced accounted for both sludge removed due to sampling and increased VSS measured in the reactor. Graphically the observed yield was found to be 0.03 g VSS g1 COD which is relatively lower than the 0.19 g VSS g1 COD measured in an MBR operated at room temperature, fed with the same wastewater (Acharya et al., 2004) and 0.05 mg SS mg1 COD observed in a thermophilic biofilm reactor (55 C) treating molasses (Suvilampi et al., 2002).
revealed no change in ammonia, nitrates or nitrites. The absence of nitrifying bacteria in the sludge was thus confirmed. 3.8. Respirometric experiments In light of the persistence of acetate and propionate in the effluent of the MBR at 5 d HRT respirometric batch experiments were conducted to further investigate the biodegradation of acetate and propionate. OU and OUR curves obtained from respirometric experiments were fitted with the Monod equation to obtain the kinetic constants discussed below. Fig. 3a is the representative curve obtained from the respirometric experiments for OU for all the three substrates, plotted alongside the theoretical curves predicted by the model. Fig. 3b is the representative curve obtained from the respirometric experiments for OUR with the various substrates. A summary of the biokinetic coefficients determined is presented in Table 3 for acetic acid, propionic acid and rendering wastewater. 3.8.1. Active biomass fraction and s0/x0 ratios The employment of a membrane unit made it possible to maintain high concentration of VSS as evident from
1200 1000 800
OU (mg/l)
1208
Measured
600
Theoretical
400
3.7. Nitrification studies
200
Batch nitrification studies conducted over 4 h periods on the bioreactor sludge at 40 C and room temperature
0 0
50
100
150
Time (hr)
Table 2 VFA data on influent and effluent in continuous-flow MBR Influent
HRT = 5 d
4100 ± 1117 (20)
505 ± 231 (8)
17.6 ± 7.7 (7)
3000 ± 514 (20)
727 ± 375 (8)
165 ± 12 (7)
150 ± 154 (20)
234 ± 52 (8)
0 ± 0 (7)
2000 ± 1155 (20)
176 ± 82 (8)
0 ± 0 (7)
1580 ± 994 (20)
259 ± 41 (8)
0 ± 0 (7)
810 ± 658 (20)
212 ± 65 (8)
0 ± 0 (7)
11640 ± 8569 (20)
2114 ± 638 (8)
182 ± 13 (7)
Fig. 3a. Representative measured and modeled OU curve.
HRT = 10 d
Steady-state data
60
Numbers in parentheses indicate the number of samples considered.
50 OUR (mg/l-hr)
Acetic acid (mg COD l1) Propionic acid (mg COD l1) Isobutyric acid (mg COD l1) Butyric acid (mg COD l1) Valeric acid (mg COD l1) Iso valeric acid (mg COD l1) Reactor VFA (mg COD l1)
Measured Theoretical
40 30 20 10 0 0
50
100
150
Time (hr)
Fig. 3b. Representative measured and modeled OUR curve.
Table 3 Biokinetic coefficients describing substrate degradation Kinetic parameters Yt (mg VSS mg COD1) K (mg COD mg VSS1 h1) Ks (mg COD l1) kd (d1) SMP (mg SMP mg COD1) Xi/Xt lmax (d1)
Acetic acid (9)
Propionic acid (10)
Rendering wastewater (11)
0.29 ± 0.02 1.89 ± 0.10 180.6 ± 14.2 0.23 ± 0.01 0.39 ± 0.07 0.80 ± 0.02 13.1 ± 0.68
0.29 ± 0.03 1.08 ± 0.18 271 ± 32.6 0.23 ± 0.01 0.39 ± 0.05 0.83 ± 0.01 7.53 ± 1.99
0.20 ± 0.01 0.85 ± 0.12 806 ± 192 0.14 ± 0.02 0.56 ± 0.15 0.86 ± 0.04 4.11 ± 0.57
Numbers in parentheses indicate the number of respirometric batch reactor results fitted individually.
Table 1. The high concentration of inactive VSS present in the influent (Table 1) definitely constituted a significant percentage of the reactor VSS. The expected concentration of active biomass (Xai) in the reactor was calculated as per Eq. (8) (Tchobanoglous et al., 2003). X ai ¼
hc Y o DS HRT
Final VFA and COD concentration (mg l-1)
R. Kurian et al. / Chemosphere 65 (2006) 1204–1211
1209 VFA Rendering wastewater
3500
y = 0.88x - 639.38
3000
2
R = 0.99
2500 2000 1500 1000 y = 0.37x - 114.04
500
2
R = 0.76
0 0
1000 2000 3000 4000 5000 Initial VFA and COD concentration (mg l-1)
Fig. 4. Residual VFA and COD as a function of initial concentrations in respirometric batch experiments.
2–4 g COD g VSS1 showed residual VFA of less than 60 mg l1 which was much lower than the lowest residual propionic acid concentrations measured in both batch experiments and continuous-flow system effluent.
ð8Þ
Accordingly Xai was calculated to be 19% of the Xt measured in the reactor (Table 1) based on the total COD removed. Thus the initial fraction of inactive to total biomass used in the respirometric experiments is 0.81, comparable to the theoretical values of 0.80, 0.83 and 0.86 for acetate, propionate and rendering wastewater, respectively. The XS 00 ratio at which the reactors were operated were in the ranges of 2–14 g COD g VSS1, 3–14 g COD g VSS1 and 1–5 g COD g VSS1 for acetate, propionate and rendering wastewater, respectively as calculated based on initial sCOD over the measured VSS. It is known that for respirometric studies the optimum XS 00 is 20 (Grady et al., 1989). In this case the XS 00 values based on active VSS fraction would be much higher than calculated, covering the range above and below 20, as the active biomass fraction was only 19% of the measured VSS. Even though these studies were conducted over a wide range of XS 00 the biokinetic coefficients determined were similar as evident from the low standard deviations as shown in Table 3. The respirometric experiments proved that both acetic acid and propionic acid persisted after the seven day contact time even though, as indicated by the OUR, biological reaction had stopped. The residual VFA reported in the experiments using propionic acid as a carbon source was primarily propionic acid with less than 6% acetic acid (on a COD basis) in all the reactors. The residual VFA concentration was found to be a function of the initial VFA concentration for both acetic and propionic acid as depicted in Fig. 4, and did not exhibit a linear relation to XS 00 . A similar effect was observed in the experiments using complex wastewater as substrate, where the residual COD was found to be proportional to the initial COD concentration as graphically represented in Fig. 4. Experiments conducted with acetic acid at lower XS 00 ratios of
3.8.2. Endogenous decay coefficient (kd) In this experimental study the endogenous decay coefficient was calculated theoretically by curve-fitting and verified by analytical measurements. The value was measured by monitoring the initial and final VSS in four blank reactors without substrate, one in each set, in respirometric tests and calculating kd as first order kinetics as per Eq. (9) (Tchobanoglous et al., 2003). dX ¼ k d X dt
ð9Þ
Accordingly, kd with acetic and propionic acid as substrates was 0.24 d1, comparable to the value determined by curve-fitting as 0.23 d1 (Table 3). In the wastewater experiments kd was calculated based on the change of VSS from 800 mg l1 to 350 mg l1 over 162 h, and found to be 0.12 d1 which was very close to the model predicted value of 0.14 d1. The lower decay rate measured in this experiment may be due to the higher inactive fraction of biomass of 86% compared to the 80–83% in the experiments with VFAs. 3.8.3. True yield (Yt) In this particular study the true yield (Yt) was determined by curve-fitting and verified with experimental values. In the batch experiments conducted on pure substrates (VFA) the change in VSS over the experimental period was plotted against the sCOD removed to obtain the observed yield graphically (R2 = 0.85). The observed yield in the system was accordingly estimated to be 0.09 g VSS g1 COD from the slope of the graph. Applying Eq. (10) (Tchobanoglous et al., 2003) the value of Yt was calculated to be 0.22 g VSS g1 COD, using the contact period of 6.1 d and kd of 0.23 d1. This value is comparable to the curve-fitted value of 0.29 g VSS g1 COD.
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Yo ¼
R. Kurian et al. / Chemosphere 65 (2006) 1204–1211
Yt 1 þ k d hc
ð10Þ
The observed yield in the lab scale system was calculated to be 0.03 g VSS g1 COD as discussed earlier. The slight variation in values may be due to variability in wastewater characteristics used in the continuous system. The observed yield calculated based on the model predicted Yt of 0.2 g VSS g1 COD and decay coefficient of 0.14 d1 (Table 3) yields a value of 0.03 g VSS g1 COD which is comparable to the measured value. 3.8.4. Half-saturation constant (Ks) The half-saturation constant was obtained by curve-fitting for both acetate and propionate substrate (Table 3). Considering the higher amount of residual propionate in both respirometric batch experiments and continuous-flow experiments the value of Ks defining propionate biodegradation as expected was higher than acetate. The value of Ks obtained by curve-fitting for propionate was 1.5 times that for acetate biodegradation corresponding to values 271 and 180 mg COD l1, respectively. Vogelaar et al. (2003) reported a value of 3 mg COD l1 at 55 C for acetate degradation which is comparatively low since residual acetate and SMP were not observed. Lapara et al. (2000a) reported Ks of 660 mg COD l1 at 55 C for a complex substrate of gelatin and a-lactose. Based on Eq. (11) (Tchobanoglous et al., 2003) Seff of propionic and acetic acid can be calculated using the corresponding Ks values. The maximum substrate removal rates per g of reactor VSS was calculated as per Eq. (12) (Tchobanoglous et al., 2003) for both acetate and propionate at each HRT condition. ð1 þ k d hcÞ K s hc ðlmax k d Þ 1 rs K S ¼ X Ks þ S
S eff ¼
ð11Þ ð12Þ
At the 5 d HRT the ratio of the maximum biomass specific substrate removal rate for acetic to propionic acid is 1.76; very similar to the ratio of k since the ambient concentrations of both acetic and propionic acids in the MBR were significantly above their respective Ks. At the 10 d HRT the same ratio is 0.41, i.e. propionate removal rate was much higher than that of acetate. Although the overall VFA removal rates decreased from 1892 mg d1 at the 5 d HRT to 1139 mg d1 at the 10 d HRT, the absence of VFAs other than acetic and propionic acids at the 10 d HRT clearly indicates that for this complex wastewater, biodegradation of higher VFAs proceed through propionic and acetic acid intermediates. Furthermore, the much higher persistence of propionic acid relative to acetic acid (727 mg l1 versus 505 mg l1) at the 5 d HRT and 165 versus 17.6 mg l1 at the 10 d HRT, coupled with the much higher propionic acid biodegradation rate relative to acetic acid at the 10 d HRT clearly indicates that the rate of generation of propionic acid is much higher than acetic acid
i.e. propionic acid is the primary intermediate for breakdown of VFAs in this complex wastewater. The Ks value determined for the complex wastewater was 806 mg l1 (Table 3), which is extremely high, possibly indicative of inhibition. Yet the Ks value was not high enough to predict the reactor sCOD according to Eq. (11). It can thus be inferred that the presence of inert solids or influent solids did cause inhibitory effects in the continuous-flow system but was not the only inhibitory factor. The wastewater, as mentioned, varied considerably over time not only in COD but also in contaminants such as heavy metals. The influent concentrations were measured in the range of 0.02–1.2 mg l1 of copper, 0.08–6.8 mg l1 of iron, 0.1–2.6 mg l1 of zinc, 0.008–0.03 mg l1 of lead and 1.28–5.2 mg l1 of aluminum and smaller quantities of selenium, arsenic, strontium, etc. Considering the complexity of the wastewater handled it was beyond the scope of this study to identify the exact contaminant or combination of contaminants which may have resulted in inhibition. 3.8.5. Intrinsic maximum substrate removal rate (k) and maximum specific biomass growth rate (lmax) In the respirometric experiments a maximum substrate removal rate of 1.89 g COD g VSS1 h1 and 1.08 g COD g VSS1 h1 was observed for acetate and propionate substrate, respectively and 0.85 g COD g VSS1 h1 for rendering wastewater (Table 3). The maximum specific biomass growth rate calculated as the product of the yield and the maximum intrinsic substrate removal rate was 13.1 d1, 7.5 d1 and 4.1 d1 for acetate, propionate and complex substrate, respectively. These values are within the range of 7.3 d1 aerobically composted sludges at 40 C (Tremier et al., 2005) and 9.6 d1 at 45 C treating complex synthetic wastewater (gelatin and a-lactose) (Lapara et al., 2000b) documented in literature. 3.8.6. Soluble microbial products (SMP) As shown in Table 3, curve-fitting yielded an SMP production of 0.39 g SMP g1 CODC for acetate and propionate. Rearranging Eq. (1) to calculate the only unknown, YP, assuming that initial SMP was nil and replacing the change in biomass with observed yield the following equation is obtained: Y P ¼ ð1 Y o Þ DS OU
ð13Þ
Accordingly calculations were done and the average SMP was found to be 0.3 ± 0.1 g SMP g1 COD for both acetic and propionic acid. The ratio of SMP produced to substrate consumed derived from Eq. (13) for acetate and propionate ranged from 0.1 to 0.4 g SMP g1 COD. Similar calculations for the rendering wastewater yielded an average value of 0.5 ± 0.16 and ranged between 0.2 and 0.7 g SMP g1 COD compared to the curve-fitted value of 0.56 g SMP g1 COD. It must be asserted that scrutiny of the non-VFA sCOD data in the MBR at the 10 d HRT relative to COD removal emphatically corroborates that 73%
R. Kurian et al. / Chemosphere 65 (2006) 1204–1211
of the SMPs formed were biodegraded in the reactor. Using a BOD:COD ratio of 0.65, the amount of total COD removed at the 5 d HRT, calculated as the difference between influent COD and reactor sCOD equivalent of BOD is >8000 mg COD l1. The residual non-VFA sCOD in the MBR at the 5 d HRT is 4172 mg l1, clearly suggesting that 33% of the SMPs produced were biodegraded. 4. Summary and conclusions Respirometric techniques were effective in determining the biokinetic constants describing COD biodegradation and the results are summarized below. • Regardless of the substrate employed, incomplete biodegradation was observed, leaving residual COD at the completion of the experiment. • The residual COD was observed to be proportionate to the initial substrate concentration. • Ks describing propionate biodegradation was determined to be 1.5 times that describing acetate biodegradation. • The high Ks value for the real wastewater indicates the presence of an inhibitory contaminant or contaminants. • The maximum specific biomass growth for acetic acid substrate was 1.75 times that for propionic acid substrate. • Propionate appears to be the predominant intermediate for the biodegradation of butyric, isobutyric, valeric and isovaleric acids. • The low observed yield of 0.03 g VSS g1 COD reveals that the aerobic treatment at higher temperature is a potential solution to difficulties related to high sludge generation observed in conventional systems in treating high strength wastewater.
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