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CARBON OXIDATION - NITRMCATION IN ATTACHED GROWTH ACTIVATED SLUDGE REACTORS
CARBON OXIDATION - MTRMCATION IN ATTACHED GROWTH ACTIVATED SLUDGE REACTORS K. Baskaran and C. Polprasert Environmental Engineering Division, Asian Institute ofTechnology, GP.O. Box 2754, Bangkok 10501, Thailand
ABSTRACT
Experiments on attached-growth activated sludge (AGAS) were conducted to investigate the AGAS efficiency in carbon oxidation-nitrification. Laboratoryscale AGAS reactors were installed with ring lace media to serve as support for the growth of attached-growth microorganisms. It was found that the AGAS reactor with 10% media volume gave the maximum biomass (both dispersed and attached growth) in the reactor. The AGAS reactors were more effective in the carbon oxidation-nitrification than the control reactor without attachedgrowth media. There was less sludge production in the AGAS reactors which should result in lower cost in waste sludge treatment and disposal. A Monoddiffusion model proposed in this study was found to be satisfactory in describing COD removal in the AGAS reactors. KEYWORDS Activated sludge; attached growth; carbon oxidation; nitrification; kinetics. INTRODUCTION In recent years, much efforts have been made to improve the effluent quality by utilizing both dispersed and attached growth microorganisms. The Activated sludge process in which an aeration tank is incorporated with an attached growth media is called attached growth activated sludge (AGAS) process. It was reported in the literature that the AGAS processes were capable of achieving high percentages of carbon oxidation as well as nitrification, while maintaining operational stability. However« information regarding the kinetics and the process efficiency of the combined system on carbon oxidation and nitrification are not presently available. Thus, the process constraints, the mathematical model to predict the effluent quality, and the process design, are not properly investigated. This study was conducted with the following objectives: - to investigate the process efficiency of the AGAS in the removal of carbonaceous and nitrogenous compounds; - to determine the kinetic parameters of the AGAS process; - to investigate the nitrification efficiency of AGAS; and - to develop a mathematical model to predict the effluent chemical oxygen demand (COD) concentrations of the AGAS reactors.
283
284
K. BASKARAN and C. POLPRASERT
MATHEMATICAL MODEL FOR CARBON OXIDATION IN AGAS REACTORS In an AGAS reactor, the combined carbon oxidation-nitrification will be carried out by both suspended and attached growth microorganisms, causing development of mathematical models rather complicated. For simplification, a conceptual model of AGAS is proposed as shown in Fig.l.
Sis
Ses Effluent
Influent Dispersed growth portion Fig.l.
Attached growth portion
Conceptual model of AGAS
where, S o s = influent substrate concentration, mg/L s is s substrate concentration after dispersed growth portion, mg/L S e s = effluent substrate concentration, mg/L According to Fig. 1, the carbon oxidation- nitrification were assumed to occur firstly in the dispersed growth portion and secondly in the attached growth portion.The Monod expression was used to describe the dispersed growth reactions, whereas a diffusion model was used to simulate the attached growth reactions. In a continuous flow reactor, substrate is the rate limiting factor. Using Monod and mass balance equations, the effluent concentration of a dispersed growth reactor is given by: ( 1 + kdec )
(1)
öc ( Um - kd > = * =
where, Ks *d
ec
half velocity constant, mg/L endogenous decay coefficient, day"* solid retention time, days maximum specific growth rate, day"*
Um In the attached growth portion, the biofilm depth was assumed to be limited only by the nutrient availability, whereas substrate uptake rate was limited by carbon concentration. The carbon oxidation within the biofilm was assumed to follow Monod kinetics. The surface flux of carbon into a deep biofilm can be calculated as given below (STRAND, 1986): u
F = [2D ( cf c where, Fcf Dc Uc Xc Yc To kd is of
■ ■ = = *
c xc
)]°-5[S - K Log (1 + is c e
Si s
)] 0 · 5
(2)
surface flux of COD, mg COD/(m2-dav) diffusivity of COD in biofilm, cm*-day"*1 effective maximum specific growth rate of heterotrophs, day -1 heterotrophic biomass concentration in biofilm, mg VSS/mL effective heterotrophic COD yield, mg VSS/mg COD
estimate the carbon removal by the biofilm, the values of D c = 0.29 m2/day; = K C - 0.05 day -1 ; Y c = 0.0269 mg VSS/mg COD [STRAND, 1986] were used.If A the total surface area (cm2) of the attached growth media, then the amount COD penetrated (uptaken) into the biofilm is given by:
Carbon Oxidation - Nitrification s
COD = A ' Fcf where, ScoD = C0D removed by the biofilm, mg COD/day If Q is the flowrate (in L/day), then: S COD therefore, S
-
mg/L
<3>
(4)
Q <5)
■ ^±a " ScoD
Ses
es
A · F cf
285
=
Ks *c
(1 + e c k
A ·
Fcf
-
- 1
(6)
Q
MATERIALS AND METHODS Ring Lace (RL) media , which consists of thin polyyinyl chloride fiber string, loosely wooven into the form of a rope, were used as the attached growth media for the development of attached growth microorganisms in AGAS reactors. The specific surface area and bulk volume of the attached growth media were 628 cm2/m and 0.3 L/m, respectively. The attached growth media elements were installed on a plastic frame and placed in the aeration tank. The amount of the media with the surface area of 628, 1256, 1884 cm2, corresponding to 5%, 10%, 15% of the aeration tank volume, were installed in three laboratory-scale AGAS reactors operating in parallel with a control reactor without attachedgrowth media installment. Each AGAS reactor had 6.6 L of aeration tank and 1.8 L of settling tank separated by an adjustable baffle. Diffused-air aeration was applied to the aeration tanks to maintain the mixed liquor DO concentrations at 3-6 mg/L. A synthetic wastewater having sodium acetate as the main carbon source was used as influent feed to all the reactors. These reactors were operated under ambient conditions at a temperature range of 2529°C. Activated sludge mixed liquor from a domestic wastewater treatment plant in Bangkok was used as inoculum for the laboratory experiments. In the first phase of experiments, the volumetric loading was kept constant at 1.1 kg COD/(m3·day) and all four reactors were operated at different 6 C of 5, 7.5, 10, 15 and 20 days to determine the effect of 6 C on combined carbon oxidation-nitrification and to evaluate the kinetic parameters. In the second phase, 6 C was kept at 10 days and the volumetric loadings were varied at 1.1, 1.9, 2.3 and 3.1 kg COD/(m3-day) . NH3-N concentration of 65 mg/L was maintained in the influent substrate and hydraulic retention time (Θ) was kept at 8 hours throughout the experiments. All four reactors were operated with 100% recycling of settled sludge, and sludge wasting was done from the aeration tank to maintain the desired 6 C value. The reactors were considered to reach steady state conditions when COD or NH3-N concentrations in the effluents remained nearly constant over a 3-4 day period. The data obtained during the steady state conditions were averaged to represent a single data point for each parameter measured. All chemical analysis were carried out according to Standard Methods (APHA-AWWA-WPCF, 1986). RESULTS AND DISCUSSION Effect of θ^ on AGAS Performance (Phase I) The influent and effluent COD concentrations and the total biomass concentrations of all four reactors at steady-state conditions are given in Table 1. All effluent COD concentrations were well below the normal effluent standard. The effect of e c on effluent COD concentrations of the AGAS reactors was not clearly observed, probably because the attached growth microorganisms would stay in the reactors for an extended period of time. A slightly high effluent COD concentration observed in the AGAS reactor with 15% media volume was probably caused by several factors such as improper mixing condition,
286
K. BASKARAN and C. POLPRASERT
limitation of substrate diffusion into the biofilm, inactive biomass within the attached growth media.
and
the presence
of
The removal efficiencies of NH3-N, total-N and the percent nitrification in the four reactors are given in Table 2. NH3-N removal efficiencies in all four reactors were about 99%. Variations in 0 C and percentages of attached-growth media did not have significant effects on NH3-N removal in AGAS reactors, probably because this rather low volumetric loading of 1.1 kg COD/ (m3«day) was favorable for the growth of nitrifying bacteria, which could grow well on the attached-growth media. An increase in percent total-N removal with increasing 6 C values were observed . As shown in Table 1 , the total biomass concentrations in the AGAS reactors were found to increase with increasing e c up to an optimum θ α range of 7.5-15 days. Because the mechanisms of total-N removal in AGAS reactors should be mainly nitrification-denitrification and biomass uptake, the AGAS reactors operating at this 6 C range had higher percent total-N removal than the reactor operating at other 6 C values. Table 1
Effect of θ^ on COD removal & Total Biomass concentration in AGAS Reactors. Volumetric loading = 1.1 kg COD/(m^'day)
VOLUMETRIC INFLUENT EFFLUENT COD (mg/L) PERCENTAGE REMOVAL (%) LOADING COD kg COD/m-d (mg/L) CONTROL 5% MEDIA 10% MEDIA 15% MEDIA CONTROL 5% MEDIA 10% MEDIA 15% MEDIA 1.1 1.9 2.3 3.1
315 623 797 1024
Table 2
8 28 25 24
10 13 9 18
8 14 17 16
13 15 23 25
97.46 95.51 96.86 97.66
96.83 97.91 98.87 98.24
97.46 97.75 97.87 98.44
95.87 97.59 97.11 97.56
Effect of θ^ on Nitrogen Removal in AGAS Reactors. Volumetric Coading = 1.1 kg COD/(ml»day)
PERCENT NITRIFICATION
(%)
PERCENT NH3-N REMOVAL
(%)
PERCENT TOTAL-N REMOVAL
{%)
Θ c lays) CONTROL 5% MEDIA 10% MEDIA 15% MEDIA CONTROL 5% MEDIA 10* MEDIA 15% MEDIA CONTROL 5% MEDIA 10%MEDIA
5 7.5 10 15 20
53.01 51 .54 59.57 55.15 71.43
63.01 47.95 59.57 53.08 57.36
63.01 61.64 59.57 50.00 64.29
65.75 56.16 56.52 60.00 69.64
100.00 99.59 99.28 99.38 99.82
99.36 99.04 99.71 99.08 98.57
99.73 99.73 99.86 99.38 98.57
99.7 99.7 99.5 99.3 99.·8
36.99 36.99 28.99 32.31 26.79
35.62 45.21 28.99 38.46 30.36
35.62 36.99 28.99 38.46 30.36
15%MEDIX 34.2S 39.73 42.03 38.46 30.36
The p e r c e n t n i t r i f i c a t i o n was c a l c u l a t e d from the r a t i o of N 0 3 ~ - N concentration in the effluent to NH3-N concentration in the influent.The percent nitrification was observed to increase slightly with increasing 9 C in accordance with the results previously reported by BALAKRISHNAN and EKENFELDER (1969). The percent nitrification in the AGAS reactor effluents were slightly lower than that in the control reactor which was possibly due to the occurence of denitrification in the anoxic zone developed within the attached growth media. This hypothesis can be supported by the fact that under this volumetric loading, the total-N removal in AGAS reactors were higher than in the control reactor at θ α greater than 5 days. The total biomass (dispersed-growth plus attached-growth) concentrations in the AGAS reactors were 1.3 - 1.5 times larger than that in the control reactor, indicating the benefits of installing attached growth media in the aeration tank. However, the total biomass concentrations in the AGAS reactors were not significantly increased with increasing percentages of the attachedgrowth media, probably because of the limitation on the availability of substrate as well as oxygen transfer into the biofilm in the reactors with high percentages of attached-growth media. Under the above operating conditions, e c did not appear to produce significant effects on carbon oxidation-nitrification in the AGAS reactors. This result might have been due to the presence of attached-growth microorganisms and less population of dispersed-growth microorganisms in the AGAS reactors. However, the development of total biomass concentration in the AGAS reactors was highest at θ € = 10 days. Therefore, the 8 C value of 10 days was selected for use in the second phase of experiments.
Carbon Oxidation - Nitrification
287
Bffeet of Volumetric Loading on AGAS Performance (Phase II) The effluent COD concentrations and COD removal efficiencies of the AGAS reactors under steady state conditions at volumetric loadings between 1.1 3.1 kg COD/(m^· day) are given in Table 3. It was found that the effluent COD concentrations of control reactor increased rapidly when the volumetric loadings were increased from 1.1 to 1.9 kg COD/(m·*-day). However, the effluent COD concentrations in the AGAS reactors did not increase much with increasing volumetric loadings which could be due to the presence of high concentrations of biomass in the AGAS reactors (Table 3) . The AGAS reactors with 5% and 10% media volume were able to provide effluents which would satisfy most effluent standards when operating at volumetric loadings up to 3.1 kg COD/(m 3 -day). These reactors were observed to be stable in performance and should be applicable for waste treatment at high organic loadings. Table 3
Effect of Volumetric Loading on COD Removal in AGAS Reactors. 6 C ■ 10 days
INFLUENT COD
Θ
(dajs) 5 7.5 10 15 20
(mg/L) 382 388 315 349 329
EFFLUENT COD CONTROL 5* MEDIA 10% 11 10 8 9 8
7 11 10 8 7
(mg/L) MEDIA 15% 11 7 8 8 6
TOTAL BIOMASS CONCENTRATION (mg/L) MEDIA CONTROL 5% MEDIA 10% 11 17 13 9 15
3420 3480 3770 3565 3270
4460 3780 4625 4820 4545
MEDIA 15% 4240 3520 5140 5020 4530
MEDIA 3900 4150 5350 5055 3985
As the volumetric loadings were increased from 1.1 to 3.1 kg COD/(m 3 «day) , nitrification in the control reactor was reduced from 70% to almost zero, while the AGAS reactors with 5% and 1 0 % media volume had reduction in nitrification from 70% to 50% and from 70% to 43%, respectively (Fig. 2a) . In the AGAS reactor with 15% media, a reduction from 56% to 4% was observed. Limitation of oxygen diffusion and inhibition of nitrifiers due to high COD/TKN ratios in the reactors were hypothesized to be the reasons for the reduction of nitrification. According to USEPA (1975) the COD/TKN ratios should be between 3.0-5.0 for optimum carbon oxidation and nitrification. In this experiment, the COD/TKN ratios were increased from 4.3 to 14.3 which favored the development of the heterotrophs and inhibiting the growth of the nitrifiers.
w
l
1.5 2 25 3 Volumetric loading, kg COD/im^doy)
(a) Nitrification
I
1.5 2 2.5 3 Volumetric boding , kg COD/lm-?doy)
(b) NH3-N removal
Ί
1.5 2 2.5 3 Volumetric loading , kg COO/tnrAlay)
(c) Total-N removal
Fig. 2. Effect of Volumetric Loading on the Perfomance of AGAS Reactors; O c = 10 days. The NH3-N and total-N removal in all four reactors under different volumetric loadings are shown in Fig. 2(b) & (c), respectively. NH3-N removal efficiencies in the control reactor reduced from 99% to 60% when the volumetric loadings were increased from 1.1 to 3.1 kg COD/ (m3·day) or an increase of COD/TKN ratios from 4.3 to 14.3. The AGAS reactors with 5% and 10%
288
K. BASKARAN and C. POLPRASERT
media showed rather complete removal of NH3-N at the above loading rates. This result suggests that the development of the attached growth microorganisms in an activated sludge reactor will improve the process stability with respect to nitrogen removal. In the attached growth media, the nitrifiers would be predominant at deeper layers of the biofilm where the substrate had been reduced by the dispersed and attached growth organisms to a certain level suitable for their growth. Therefore, the effect of inhibition due to high volumetric loading on the nitrifiers in the AGAS reactors was much less than that in the control reactor where the nitrifiers were present in the mixed liquor. Development of anaerobic condition and insufficient diffusion of oxygen to the deeper layers of the biofilm in the AGAS reactor with 15% media might have caused the reduction in NH3-N removal efficiencies. In the case of total-N removal, it was observed that the percent removal in the control reactor and the AGAS reactor with 15% media were considerably higher than those in the other two AGAS reactors. It was found that with the increase of volumetric loadings, the dispersed-growth microorganisms in the control reactor also increased up to about 4000 mg/L, which resulted in higher nitrogen uptake. In case of the AGAS reactor with 15% media there was probably a loss of nitrogen through denitrification due to limited DO within the biofilm. Although the AGAS reactor with 1 0 % media had a lower percent nitrification than that in the 5% media reactor (Fig. 2a) , the former had higher total-N removal (Fig. 2c) which implied that part of the N03~-N had been denitrified. The highest percentage of total-N removal was attained in the control reactor (Fig. 2c) which was due mainly to biomass uptake; accordingly a large quantity of dispersed biomass needs to be treated and disposed of. Thus, the cost incurred for the disposal of the waste sludge produced in the control reactor is high. But in case of the AGAS reactors where the dispersed growth biomass is less, the cost incurred for treatment and disposal of the waste sludge is less, eventhough there is slightly lower total-N removal in the AGAS reactors. The discharge of nitrified effluents from the AGAS reactors would not cause as much oxygen demand as that of the effluent of the control or conventional activated sludge units where the percentages of nitrification were lower (Fig. 2a) . The result of this study suggest that attached-growth media should be installed in aeration tanks to enhance biomass uptake, nitirfication and denitrification for the purpose of nitrogen r e m o v a l . However, under high volumetric loadings, the AGAS reactors should not have too much attachedgrowth media installed to avoid the occurrence of anaerobic conditions within the biofilm which will affect the nitrification process. Kinetics of AGAS The kinetic parameters of the AGAS reactors obtained from the first-phase experiments are given in Table 4. In general, the K s , k and \im values of the AGAS reactors were lower than the control unit becuase of the presence of more dispersed growth biomass in the control reactor. In the determination of the kinetic parameters, it was assumed that the total biomass was active in the substrate utilization. Since the AGAS reactors contained attached growth biomass that were obviously less active in substrate utilization, thus the lower values of K s , k and yim were obtained. Table 4: Kinetic Parameters for AGAS Reactors Operating at Volumetric Loading s 1.1 ka COD/(mA· day) PARA)1ETER k d (day _ 1 ) K s (mg/L COD) k (mg MLSS/mg COD.day) Y (mg COD/mg MLSS) U m (day _ 1 )
REACTOR CONTROL 0.057 128 4.5 0.52 2.36
5% MEDIA 0.026 67 2.08 0.5 1.04
10% MEDIA 15% 1 MEDIA 0.026 72 2.08 0.45 0.94
0.094 43 1.25 0.89 1.11
289
Carbon Oxidation - Nitrification
The mathematical model given in Eq. (6) was used to describe the performance of the AGAS reactors using the kinetic parameters shown in Table 4. The experimental results obtained from the Phase I were used to test the model applicability. The effluent COD concetrations calculated from the model with the 95% confidence limits and the experimental data obtained are present in Fig. 3. The experimental results for the AGAS reactors with 5% and 10% media were within the 95% confidence limits, but in case of the AGAS reactor with 15% media, most of the predicted values were lower than the experimental results. This difference might have been due to the presence of high density and clamping of the attached growth microorganism causing the actual surface area of the attached growth media to be less than the value used in the model calculation.
35l· f25 a 20 O υ Ι! Z ,
Reactor - 5 % media O Model O Experimental data
Reoctor - 10 % media
Reoctor - 1 5 % media
9 5 % Confidence Limits
% o 10 15 Solid retention time (days)
20
Fig. 3. Prediction of AGAS Performance by the Monod-Diffision Model CONCLUSIONS From the experimental results obtained, the following conclusions are made: - SRT of 10 days gave the maximum biomass (both dispersed and attached growth) concentration in the AGAS reactors operating at volumetric loading of 1.1 kg COD/(m^»day) and Θ = 8 hours. - AGAS reactors with 5% and 10% media volume were more effective in carbon oxidation-nitrification than the control reactor when operating at volumetric loadings between 1.1 and 3.1 kg COD/(m3·day). - At high volumetric loadings, the AGAS reactor with 15% attached-growth media achieved about the same degree of total-N removal as that of the control unit, but the cost incurred for further treatment and disposal of the waste sludge would be less for the AGAS reactor. - The overall benefits of AGAS reactors appeared to be less sludge production and better nitrification efficiency, even when operating at high volumetric loadings. - The Monod-diffusion model (Eq. 6) was found to be satisfactory in describing the COD removal in the AGAS reactors operating under the conditions experimented in this study. REFERENCES APHA-AWWA-WPCF (1985), Standard Methods for Examination of Water and Wastewater, 16th. Edition, APHA, Washington D . C U S A . BALAKRISHNAN, S. and ECKENFELDER, W.W. (1969), Nitrogen Relationship in Biological Treatment Process - I, Nitrification in the Activated Sludge Process, Water Research, Vol. 3, No. 1, pp. 73-81. METCALF & EDDY (1979), Wastewater Engineering; Treatment, Disposal, Reuse., McGraw Hill Series in Water Resources and Environmental Engineering, N.Y., USA. STRAND, S.E. (1986), Model of Ammonia and Carbon Oxidation in Biofilms, J. Env., Eng., Div.. ASCE, Vol. 112, No. 4, pp. 785-803. USEPA, (1975), Process Design Manual for Nitrogen Control, USEPA, Technology Transfer. Cincinnati, Ohio, USA.
ABSTRACT
Experiments on attached-growth activated sludge (AGAS) were conducted to investigate the AGAS efficiency in carbon oxidation-nitrification. Laboratoryscale AGAS reactors were installed with ring lace media to serve as support for the growth of attached-growth microorganisms. It was found that the AGAS reactor with 10% media volume gave the maximum biomass (both dispersed and attached growth) in the reactor. The AGAS reactors were more effective in the carbon oxidation-nitrification than the control reactor without attachedgrowth media. There was less sludge production in the AGAS reactors which should result in lower cost in waste sludge treatment and disposal. A Monoddiffusion model proposed in this study was found to be satisfactory in describing COD removal in the AGAS reactors. KEYWORDS Activated sludge; attached growth; carbon oxidation; nitrification; kinetics. INTRODUCTION In recent years, much efforts have been made to improve the effluent quality by utilizing both dispersed and attached growth microorganisms. The Activated sludge process in which an aeration tank is incorporated with an attached growth media is called attached growth activated sludge (AGAS) process. It was reported in the literature that the AGAS processes were capable of achieving high percentages of carbon oxidation as well as nitrification, while maintaining operational stability. However« information regarding the kinetics and the process efficiency of the combined system on carbon oxidation and nitrification are not presently available. Thus, the process constraints, the mathematical model to predict the effluent quality, and the process design, are not properly investigated. This study was conducted with the following objectives: - to investigate the process efficiency of the AGAS in the removal of carbonaceous and nitrogenous compounds; - to determine the kinetic parameters of the AGAS process; - to investigate the nitrification efficiency of AGAS; and - to develop a mathematical model to predict the effluent chemical oxygen demand (COD) concentrations of the AGAS reactors.
283
284
K. BASKARAN and C. POLPRASERT
MATHEMATICAL MODEL FOR CARBON OXIDATION IN AGAS REACTORS In an AGAS reactor, the combined carbon oxidation-nitrification will be carried out by both suspended and attached growth microorganisms, causing development of mathematical models rather complicated. For simplification, a conceptual model of AGAS is proposed as shown in Fig.l.
Sis
Ses Effluent
Influent Dispersed growth portion Fig.l.
Attached growth portion
Conceptual model of AGAS
where, S o s = influent substrate concentration, mg/L s is s substrate concentration after dispersed growth portion, mg/L S e s = effluent substrate concentration, mg/L According to Fig. 1, the carbon oxidation- nitrification were assumed to occur firstly in the dispersed growth portion and secondly in the attached growth portion.The Monod expression was used to describe the dispersed growth reactions, whereas a diffusion model was used to simulate the attached growth reactions. In a continuous flow reactor, substrate is the rate limiting factor. Using Monod and mass balance equations, the effluent concentration of a dispersed growth reactor is given by: ( 1 + kdec )
(1)
öc ( Um - kd > = * =
where, Ks *d
ec
half velocity constant, mg/L endogenous decay coefficient, day"* solid retention time, days maximum specific growth rate, day"*
Um In the attached growth portion, the biofilm depth was assumed to be limited only by the nutrient availability, whereas substrate uptake rate was limited by carbon concentration. The carbon oxidation within the biofilm was assumed to follow Monod kinetics. The surface flux of carbon into a deep biofilm can be calculated as given below (STRAND, 1986): u
F = [2D ( cf c where, Fcf Dc Uc Xc Yc To kd is of
■ ■ = = *
c xc
)]°-5[S - K Log (1 + is c e
Si s
)] 0 · 5
(2)
surface flux of COD, mg COD/(m2-dav) diffusivity of COD in biofilm, cm*-day"*1 effective maximum specific growth rate of heterotrophs, day -1 heterotrophic biomass concentration in biofilm, mg VSS/mL effective heterotrophic COD yield, mg VSS/mg COD
estimate the carbon removal by the biofilm, the values of D c = 0.29 m2/day; = K C - 0.05 day -1 ; Y c = 0.0269 mg VSS/mg COD [STRAND, 1986] were used.If A the total surface area (cm2) of the attached growth media, then the amount COD penetrated (uptaken) into the biofilm is given by:
Carbon Oxidation - Nitrification s
COD = A ' Fcf where, ScoD = C0D removed by the biofilm, mg COD/day If Q is the flowrate (in L/day), then: S COD therefore, S
-
mg/L
<3>
(4)
Q <5)
■ ^±a " ScoD
Ses
es
A · F cf
285
=
Ks *c
(1 + e c k
A ·
Fcf
-
- 1
(6)
Q
MATERIALS AND METHODS Ring Lace (RL) media , which consists of thin polyyinyl chloride fiber string, loosely wooven into the form of a rope, were used as the attached growth media for the development of attached growth microorganisms in AGAS reactors. The specific surface area and bulk volume of the attached growth media were 628 cm2/m and 0.3 L/m, respectively. The attached growth media elements were installed on a plastic frame and placed in the aeration tank. The amount of the media with the surface area of 628, 1256, 1884 cm2, corresponding to 5%, 10%, 15% of the aeration tank volume, were installed in three laboratory-scale AGAS reactors operating in parallel with a control reactor without attachedgrowth media installment. Each AGAS reactor had 6.6 L of aeration tank and 1.8 L of settling tank separated by an adjustable baffle. Diffused-air aeration was applied to the aeration tanks to maintain the mixed liquor DO concentrations at 3-6 mg/L. A synthetic wastewater having sodium acetate as the main carbon source was used as influent feed to all the reactors. These reactors were operated under ambient conditions at a temperature range of 2529°C. Activated sludge mixed liquor from a domestic wastewater treatment plant in Bangkok was used as inoculum for the laboratory experiments. In the first phase of experiments, the volumetric loading was kept constant at 1.1 kg COD/(m3·day) and all four reactors were operated at different 6 C of 5, 7.5, 10, 15 and 20 days to determine the effect of 6 C on combined carbon oxidation-nitrification and to evaluate the kinetic parameters. In the second phase, 6 C was kept at 10 days and the volumetric loadings were varied at 1.1, 1.9, 2.3 and 3.1 kg COD/(m3-day) . NH3-N concentration of 65 mg/L was maintained in the influent substrate and hydraulic retention time (Θ) was kept at 8 hours throughout the experiments. All four reactors were operated with 100% recycling of settled sludge, and sludge wasting was done from the aeration tank to maintain the desired 6 C value. The reactors were considered to reach steady state conditions when COD or NH3-N concentrations in the effluents remained nearly constant over a 3-4 day period. The data obtained during the steady state conditions were averaged to represent a single data point for each parameter measured. All chemical analysis were carried out according to Standard Methods (APHA-AWWA-WPCF, 1986). RESULTS AND DISCUSSION Effect of θ^ on AGAS Performance (Phase I) The influent and effluent COD concentrations and the total biomass concentrations of all four reactors at steady-state conditions are given in Table 1. All effluent COD concentrations were well below the normal effluent standard. The effect of e c on effluent COD concentrations of the AGAS reactors was not clearly observed, probably because the attached growth microorganisms would stay in the reactors for an extended period of time. A slightly high effluent COD concentration observed in the AGAS reactor with 15% media volume was probably caused by several factors such as improper mixing condition,
286
K. BASKARAN and C. POLPRASERT
limitation of substrate diffusion into the biofilm, inactive biomass within the attached growth media.
and
the presence
of
The removal efficiencies of NH3-N, total-N and the percent nitrification in the four reactors are given in Table 2. NH3-N removal efficiencies in all four reactors were about 99%. Variations in 0 C and percentages of attached-growth media did not have significant effects on NH3-N removal in AGAS reactors, probably because this rather low volumetric loading of 1.1 kg COD/ (m3«day) was favorable for the growth of nitrifying bacteria, which could grow well on the attached-growth media. An increase in percent total-N removal with increasing 6 C values were observed . As shown in Table 1 , the total biomass concentrations in the AGAS reactors were found to increase with increasing e c up to an optimum θ α range of 7.5-15 days. Because the mechanisms of total-N removal in AGAS reactors should be mainly nitrification-denitrification and biomass uptake, the AGAS reactors operating at this 6 C range had higher percent total-N removal than the reactor operating at other 6 C values. Table 1
Effect of θ^ on COD removal & Total Biomass concentration in AGAS Reactors. Volumetric loading = 1.1 kg COD/(m^'day)
VOLUMETRIC INFLUENT EFFLUENT COD (mg/L) PERCENTAGE REMOVAL (%) LOADING COD kg COD/m-d (mg/L) CONTROL 5% MEDIA 10% MEDIA 15% MEDIA CONTROL 5% MEDIA 10% MEDIA 15% MEDIA 1.1 1.9 2.3 3.1
315 623 797 1024
Table 2
8 28 25 24
10 13 9 18
8 14 17 16
13 15 23 25
97.46 95.51 96.86 97.66
96.83 97.91 98.87 98.24
97.46 97.75 97.87 98.44
95.87 97.59 97.11 97.56
Effect of θ^ on Nitrogen Removal in AGAS Reactors. Volumetric Coading = 1.1 kg COD/(ml»day)
PERCENT NITRIFICATION
(%)
PERCENT NH3-N REMOVAL
(%)
PERCENT TOTAL-N REMOVAL
{%)
Θ c lays) CONTROL 5% MEDIA 10% MEDIA 15% MEDIA CONTROL 5% MEDIA 10* MEDIA 15% MEDIA CONTROL 5% MEDIA 10%MEDIA
5 7.5 10 15 20
53.01 51 .54 59.57 55.15 71.43
63.01 47.95 59.57 53.08 57.36
63.01 61.64 59.57 50.00 64.29
65.75 56.16 56.52 60.00 69.64
100.00 99.59 99.28 99.38 99.82
99.36 99.04 99.71 99.08 98.57
99.73 99.73 99.86 99.38 98.57
99.7 99.7 99.5 99.3 99.·8
36.99 36.99 28.99 32.31 26.79
35.62 45.21 28.99 38.46 30.36
35.62 36.99 28.99 38.46 30.36
15%MEDIX 34.2S 39.73 42.03 38.46 30.36
The p e r c e n t n i t r i f i c a t i o n was c a l c u l a t e d from the r a t i o of N 0 3 ~ - N concentration in the effluent to NH3-N concentration in the influent.The percent nitrification was observed to increase slightly with increasing 9 C in accordance with the results previously reported by BALAKRISHNAN and EKENFELDER (1969). The percent nitrification in the AGAS reactor effluents were slightly lower than that in the control reactor which was possibly due to the occurence of denitrification in the anoxic zone developed within the attached growth media. This hypothesis can be supported by the fact that under this volumetric loading, the total-N removal in AGAS reactors were higher than in the control reactor at θ α greater than 5 days. The total biomass (dispersed-growth plus attached-growth) concentrations in the AGAS reactors were 1.3 - 1.5 times larger than that in the control reactor, indicating the benefits of installing attached growth media in the aeration tank. However, the total biomass concentrations in the AGAS reactors were not significantly increased with increasing percentages of the attachedgrowth media, probably because of the limitation on the availability of substrate as well as oxygen transfer into the biofilm in the reactors with high percentages of attached-growth media. Under the above operating conditions, e c did not appear to produce significant effects on carbon oxidation-nitrification in the AGAS reactors. This result might have been due to the presence of attached-growth microorganisms and less population of dispersed-growth microorganisms in the AGAS reactors. However, the development of total biomass concentration in the AGAS reactors was highest at θ € = 10 days. Therefore, the 8 C value of 10 days was selected for use in the second phase of experiments.
Carbon Oxidation - Nitrification
287
Bffeet of Volumetric Loading on AGAS Performance (Phase II) The effluent COD concentrations and COD removal efficiencies of the AGAS reactors under steady state conditions at volumetric loadings between 1.1 3.1 kg COD/(m^· day) are given in Table 3. It was found that the effluent COD concentrations of control reactor increased rapidly when the volumetric loadings were increased from 1.1 to 1.9 kg COD/(m·*-day). However, the effluent COD concentrations in the AGAS reactors did not increase much with increasing volumetric loadings which could be due to the presence of high concentrations of biomass in the AGAS reactors (Table 3) . The AGAS reactors with 5% and 10% media volume were able to provide effluents which would satisfy most effluent standards when operating at volumetric loadings up to 3.1 kg COD/(m 3 -day). These reactors were observed to be stable in performance and should be applicable for waste treatment at high organic loadings. Table 3
Effect of Volumetric Loading on COD Removal in AGAS Reactors. 6 C ■ 10 days
INFLUENT COD
Θ
(dajs) 5 7.5 10 15 20
(mg/L) 382 388 315 349 329
EFFLUENT COD CONTROL 5* MEDIA 10% 11 10 8 9 8
7 11 10 8 7
(mg/L) MEDIA 15% 11 7 8 8 6
TOTAL BIOMASS CONCENTRATION (mg/L) MEDIA CONTROL 5% MEDIA 10% 11 17 13 9 15
3420 3480 3770 3565 3270
4460 3780 4625 4820 4545
MEDIA 15% 4240 3520 5140 5020 4530
MEDIA 3900 4150 5350 5055 3985
As the volumetric loadings were increased from 1.1 to 3.1 kg COD/(m 3 «day) , nitrification in the control reactor was reduced from 70% to almost zero, while the AGAS reactors with 5% and 1 0 % media volume had reduction in nitrification from 70% to 50% and from 70% to 43%, respectively (Fig. 2a) . In the AGAS reactor with 15% media, a reduction from 56% to 4% was observed. Limitation of oxygen diffusion and inhibition of nitrifiers due to high COD/TKN ratios in the reactors were hypothesized to be the reasons for the reduction of nitrification. According to USEPA (1975) the COD/TKN ratios should be between 3.0-5.0 for optimum carbon oxidation and nitrification. In this experiment, the COD/TKN ratios were increased from 4.3 to 14.3 which favored the development of the heterotrophs and inhibiting the growth of the nitrifiers.
w
l
1.5 2 25 3 Volumetric loading, kg COD/im^doy)
(a) Nitrification
I
1.5 2 2.5 3 Volumetric boding , kg COD/lm-?doy)
(b) NH3-N removal
Ί
1.5 2 2.5 3 Volumetric loading , kg COO/tnrAlay)
(c) Total-N removal
Fig. 2. Effect of Volumetric Loading on the Perfomance of AGAS Reactors; O c = 10 days. The NH3-N and total-N removal in all four reactors under different volumetric loadings are shown in Fig. 2(b) & (c), respectively. NH3-N removal efficiencies in the control reactor reduced from 99% to 60% when the volumetric loadings were increased from 1.1 to 3.1 kg COD/ (m3·day) or an increase of COD/TKN ratios from 4.3 to 14.3. The AGAS reactors with 5% and 10%
288
K. BASKARAN and C. POLPRASERT
media showed rather complete removal of NH3-N at the above loading rates. This result suggests that the development of the attached growth microorganisms in an activated sludge reactor will improve the process stability with respect to nitrogen removal. In the attached growth media, the nitrifiers would be predominant at deeper layers of the biofilm where the substrate had been reduced by the dispersed and attached growth organisms to a certain level suitable for their growth. Therefore, the effect of inhibition due to high volumetric loading on the nitrifiers in the AGAS reactors was much less than that in the control reactor where the nitrifiers were present in the mixed liquor. Development of anaerobic condition and insufficient diffusion of oxygen to the deeper layers of the biofilm in the AGAS reactor with 15% media might have caused the reduction in NH3-N removal efficiencies. In the case of total-N removal, it was observed that the percent removal in the control reactor and the AGAS reactor with 15% media were considerably higher than those in the other two AGAS reactors. It was found that with the increase of volumetric loadings, the dispersed-growth microorganisms in the control reactor also increased up to about 4000 mg/L, which resulted in higher nitrogen uptake. In case of the AGAS reactor with 15% media there was probably a loss of nitrogen through denitrification due to limited DO within the biofilm. Although the AGAS reactor with 1 0 % media had a lower percent nitrification than that in the 5% media reactor (Fig. 2a) , the former had higher total-N removal (Fig. 2c) which implied that part of the N03~-N had been denitrified. The highest percentage of total-N removal was attained in the control reactor (Fig. 2c) which was due mainly to biomass uptake; accordingly a large quantity of dispersed biomass needs to be treated and disposed of. Thus, the cost incurred for the disposal of the waste sludge produced in the control reactor is high. But in case of the AGAS reactors where the dispersed growth biomass is less, the cost incurred for treatment and disposal of the waste sludge is less, eventhough there is slightly lower total-N removal in the AGAS reactors. The discharge of nitrified effluents from the AGAS reactors would not cause as much oxygen demand as that of the effluent of the control or conventional activated sludge units where the percentages of nitrification were lower (Fig. 2a) . The result of this study suggest that attached-growth media should be installed in aeration tanks to enhance biomass uptake, nitirfication and denitrification for the purpose of nitrogen r e m o v a l . However, under high volumetric loadings, the AGAS reactors should not have too much attachedgrowth media installed to avoid the occurrence of anaerobic conditions within the biofilm which will affect the nitrification process. Kinetics of AGAS The kinetic parameters of the AGAS reactors obtained from the first-phase experiments are given in Table 4. In general, the K s , k and \im values of the AGAS reactors were lower than the control unit becuase of the presence of more dispersed growth biomass in the control reactor. In the determination of the kinetic parameters, it was assumed that the total biomass was active in the substrate utilization. Since the AGAS reactors contained attached growth biomass that were obviously less active in substrate utilization, thus the lower values of K s , k and yim were obtained. Table 4: Kinetic Parameters for AGAS Reactors Operating at Volumetric Loading s 1.1 ka COD/(mA· day) PARA)1ETER k d (day _ 1 ) K s (mg/L COD) k (mg MLSS/mg COD.day) Y (mg COD/mg MLSS) U m (day _ 1 )
REACTOR CONTROL 0.057 128 4.5 0.52 2.36
5% MEDIA 0.026 67 2.08 0.5 1.04
10% MEDIA 15% 1 MEDIA 0.026 72 2.08 0.45 0.94
0.094 43 1.25 0.89 1.11
289
Carbon Oxidation - Nitrification
The mathematical model given in Eq. (6) was used to describe the performance of the AGAS reactors using the kinetic parameters shown in Table 4. The experimental results obtained from the Phase I were used to test the model applicability. The effluent COD concetrations calculated from the model with the 95% confidence limits and the experimental data obtained are present in Fig. 3. The experimental results for the AGAS reactors with 5% and 10% media were within the 95% confidence limits, but in case of the AGAS reactor with 15% media, most of the predicted values were lower than the experimental results. This difference might have been due to the presence of high density and clamping of the attached growth microorganism causing the actual surface area of the attached growth media to be less than the value used in the model calculation.
35l· f25 a 20 O υ Ι! Z ,
Reactor - 5 % media O Model O Experimental data
Reoctor - 10 % media
Reoctor - 1 5 % media
9 5 % Confidence Limits
% o 10 15 Solid retention time (days)
20
Fig. 3. Prediction of AGAS Performance by the Monod-Diffision Model CONCLUSIONS From the experimental results obtained, the following conclusions are made: - SRT of 10 days gave the maximum biomass (both dispersed and attached growth) concentration in the AGAS reactors operating at volumetric loading of 1.1 kg COD/(m^»day) and Θ = 8 hours. - AGAS reactors with 5% and 10% media volume were more effective in carbon oxidation-nitrification than the control reactor when operating at volumetric loadings between 1.1 and 3.1 kg COD/(m3·day). - At high volumetric loadings, the AGAS reactor with 15% attached-growth media achieved about the same degree of total-N removal as that of the control unit, but the cost incurred for further treatment and disposal of the waste sludge would be less for the AGAS reactor. - The overall benefits of AGAS reactors appeared to be less sludge production and better nitrification efficiency, even when operating at high volumetric loadings. - The Monod-diffusion model (Eq. 6) was found to be satisfactory in describing the COD removal in the AGAS reactors operating under the conditions experimented in this study. REFERENCES APHA-AWWA-WPCF (1985), Standard Methods for Examination of Water and Wastewater, 16th. Edition, APHA, Washington D . C U S A . BALAKRISHNAN, S. and ECKENFELDER, W.W. (1969), Nitrogen Relationship in Biological Treatment Process - I, Nitrification in the Activated Sludge Process, Water Research, Vol. 3, No. 1, pp. 73-81. METCALF & EDDY (1979), Wastewater Engineering; Treatment, Disposal, Reuse., McGraw Hill Series in Water Resources and Environmental Engineering, N.Y., USA. STRAND, S.E. (1986), Model of Ammonia and Carbon Oxidation in Biofilms, J. Env., Eng., Div.. ASCE, Vol. 112, No. 4, pp. 785-803. USEPA, (1975), Process Design Manual for Nitrogen Control, USEPA, Technology Transfer. Cincinnati, Ohio, USA.