Assessing the treatment behavior of municipal wastewater by a modified USBF bioreactor
Mojtaba Mojarrad
1, Amin Noroozi
21
,2
- Department of Chemical Engineering, University of Isfahan, Isfahan, Iran
Corresponding Author’s E-mail ([email protected]) Abstract
In this article, the performance of a modified USBF is investigated in a pilot scale at a capacity of
2250
L for treating municipal wastewater. The raw wastewater is entered in the bioreactor with a chemical oxygen demand (COD) concentration of220020
mg/l. The reactor is studied under different suspended biomass concentrations of about3000
,4000
and5000
mg/l and different total hydraulic retention times (HRT)4
,8
and12
hr. The Lawrence -McCarthy model is used to determine the bio- kinetic coefficients of COD removal behavior. The system has a more favorable efficiency about249
, at the suspended biomass concentration of about5000
mg/l, OLR of1725
kg COD/m3.day and HRT of4
hr. The investigation showed that the yield coefficient (Y), decay coefficient (kd), µmax and saturation constant (Ks) were in the range of073868
-074203
mgVSS/mg COD,070326
-07062 1
/day,1736
-172222 1
/day and225752
-445733
mg sCOD/L.Keywords: bio-kinetic coefficients, domestic wastewater, USBF, wastewater treatment
1
. INTRODUCTIONBecause of strict environmental standards, many researches are conducted in order to optimize wastewater treatment plants using new engineering technologies. The upflow sludge blanket filtration (USBF) process is a novel configuration that incorporates an anoxic selector zone, an aeration unit and an upflow sludge blanket filtration clarifier in an integrated bioreactor [1,2]. In the USBF plant, wastewater enters the anoxic compartment where it mixes with the recycled activated sludge from the bottom of the clarifier. The mixed liquor eventually underflows into the aerobic compartment. After aeration, a stream of the mixed liquor enters the bottom of a prism or cone-shaped clarifier and, as it rises, upward velocity decreases until the flocs of cells become stationary. Then, the sludge flocs are separated from the liquid by upflow sludge blanket filtration and the clear effluent overflows into a collection through and is discharged from the system [2,3].
Mahvi et al (2010) investigated the conventional USBF efficiency in a synthetic municipal wastewater treatment at the aeration times of 2, 4 and 6 hr. The system’s maximum removal efficiencies for BOD5, TKN and TP at the aeration time of 6 hr were 82725, 82.8 and 559, respectively [4]. Mesdaghinia et al (2010) evaluated the system’s performance in terms of COD removal with a synthetic wastewater about 869 at HRT of 6 hr [5].
In this study the performance of a modified USBF bioreactor including the lamella plates in the clarifier section is investigated at different OLR’s and biomass concentration.
1.1 B
IO KINETICC
OEFFICIENTSNowadays, the biological kinetic equations with knowing the regarded biokinetic coefficients are used in the design of the wastewater treatment processes [6]. With use of the biokinetic coefficients, reactor volume, substrate consumption, biomass growth and effluent quality can be predicted [6-8]. The kinetic coefficients used in the activated sludge process are the maximum substrate consumption rate (K), half-saturation
) and specific growth rate (μ
activated sludge wastewater treatment systems. In some of these studies due to the wide range of the biomass concentration, hydraulic retention time and loading rates, some coefficients were not obtained in Metcalf range. Al Malek (2006) calculated the kinetic coefficients for the activated sludge process with a floating membrane in municipal wastewater treatment. The kinetic coefficients were calculated in the biomass concentrations of about 3000, 5000, 10000 and 15000 mg/l. The coefficients Y, Kd and μm were in the standard range, while the coefficient Ks, especially for MLSS concentration of about 15000 mg/l was higher than the standard values [10]. Joseph et al. (1222) determined the operational parameters for municipal wastewater treatment. The kinetic coefficients were calculated in terms of BOD and COD at a HRT of 24 hours. In their research, except of the coefficient Ks, the other coefficients were not in the standard range [11].
Biological models are used for design and the experimental results evaluation. The Lawrence- McCarthy model is applied for the biological reaction kinetics.
1.1.1. L
AWRENCE-M
CCARTHYM
ODELThe substrate removal rate based on the Lawrence-McCarthy model is described as [6-8]:
i e
es e
dS Q KX S
S S
dt V K S
(1)
The linear form of Equation (1) can be made as Equations (2) and (3):
0
d d
S S
1 Y U K Y K
SRT X
(2)
Where, SRT is the solids retention time (day), Y is the biomass yield (mg VSS/mg COD), U is the substrate utilization rate (mg COD/mg VSS.day), kd is the endogenous decay coefficient (1/day), S0 is the influent substrate concentration (mg COD/L), X is the biomass concentration (mg VSS/L) and Ө is the hydraulic retention time (day).
s 0
K
X 1 1 1
S S K S K U
(3)
Where, Ks is half-velocity constant (mg COD/L) and K is maximum rate of substrate utilization (mg COD/mg VSS.day).
The output substrate concentration is calculated through:
s d
m d
K 1 K
S SRT
1 K
SRT
(4)
where, μm is Specific growth rate (1/day).
2. MATERIALS AND METHODS
2.1. WASTEWATER
Raw wastewater is pumped from south Isfahan wastewater treatment plant settling basin in to the designed bioreactor inlet. Table 1 shows the system input wastewater specifications. The south Isfahan WWTP activated sludge is used for the bioreactor lunching.
Table 1-Specification of the raw wastewater Value Parameter
±072±7 mg/L (
COD)
007207 (
)mg/L BOD5
0.±-0.0 pH
007201 (
)mg/L TSSin
VSS/TSS 7.00
2.2. B
IOREACTORA 2250 L modified bioreactor is used. The general characteristics of the bioreactor are shown in Table 2.
Table 2- The modified bioreactor specifications.
Steel 32 Material
175×175×1 The overall dimensions of pilot (meter)
1200 Useful volume of aerobic section (L)
400 Useful volume of anoxic section (L)
400 Useful volume sedimentation tank (L)
The clarifier position is adjusted at the end of system, after the aeration section and is filled with lamella plates. Aeration section oxygenation is performed by a small bubble diffuser that is located on the bioreactor bottom. An air line is employed at the clarifier bottom to create an airlift system for sludge recycling and excess sludge disposal. A simplified schematic of the pilot scale plant is shown in Figure 1.
Anxic Aeration
Efluent Influent
Excess sludge
sludge recycling Weir
Blower
Sewage pump Influent screen
basket
Diffuser
Lamella plates Raw wastwater
Baffle
Sedimentation tank
Figure1. Schematic view of the modified USBF system applied in this study.
2.3. E
XPERIMENTALP
ROCEDUREThe amount of the total COD removal efficiency is measured in a continuous flow of wastewater stream at suspended biomass concentration of about 3000, 4000 and 5000 mg/l and at three different HRT 4, 8 and 12 hr. The sampling is performed after achieving steady-state condition. The total HRT is regulated based on the system inlet wastewater flow. The aeration and anoxic sections dissolved oxygen are set between (278-472) mg/l and maintained less than 075 mg/l, respectively. The temperature, dissolved oxygen and pH are daily measured in different times. The suspended biomass concentration is measured orderly for controlling microorganism growth. The suspended biomass concentration is regulated by the airlift system. The sampling is carried out at the influent and effluent wastewater from the reactor. All the experiments were conducted according to the standard methods for the examination of water and wastewater [12].
3. RESULTS AND DISCUSSIONS
3.1. REACTOR PERFORMANCE
In this study, in the first step COD removal efficiency was investigated at three HRT levels of 4, 8 and 12 hr
The measurement condition and the COD removal efficiencies results for each run are presented in Table 3. The results indicate that the system has better performance at the suspended biomass concentration of about 3000 and 4000 mg/l, OLR of 1 kg COD/m3.day and HRT of 8 hr.
Table 3-Investigated parameters in steady-state condition COD
(mg/L)
MLSS (mg/L)
HRT (min)
OLR (kg COD/m3.day)
SRT (day)
F/M (kg COD/kg
MLSS.day)
Efficiencies (%)
220020
31040252
Anoxic (min)
Aerobic (min)
Clarifier (min)
Total (min)
0754 0781 1762
2176 1378 574
0718 0722 0754
2171 2278 8478 220
20 45
450 300 150
220 20 45
220 480 240
32200166 220 20 45
450 300 150
220 20 45
220 480 240
0754 0781 1762
2373 1572 572
07135 072025
07405
0271 8276 82
51440226
220 20 45
450 300 150
220 20 45
220 480 240
0754 0781 1762
2272 1271 676
07108 07162 07324
2378 2278 2172
The system efficiency is decreased at lower and higher OLR of 1 kg COD/m3.day. The system has a more favorable efficiency about 249 at the suspended biomass concentration of about 5000 mg/l, OLR of 1725 kg COD/m3.day and HRT of 4 hr. The results show that by decreasing OLR at the suspended biomass concentration of about 5000 mg/l, the system efficiency is decreased. The removal efficiency decreasing can be caused by being the microorganisms in endogenous phase. An increase in the suspended biomass concentration increases the system efficiency and high efficiencies are achieved at shorter HRT’s.
3.1 DETERMINATION OF BIOKINETIC COEFFICIENTS
The biokinetic coefficients of the model are determined by plotting the U versus 1/SRT and 1/S versus 1/U diagrams in Equations 2 and 3. The regressed results of Equations 2 and 3 are shown in Figure 2a and 2b at steady-state condition and suspended biomass concentration of about 3000 mg/l. The F/M ratio is in 0718 to 0754 ranges at this condition.
Figure 2. Determination of kinetic coefficients at MLSS of about 3333 mg/L (a) Ks and K (b) Y and kd The biokinetic coefficients of the model are determined at suspended biomass concentration of about 4000 mg/l according to the steady-state results presented in Figure 3a and 3b. The F/M ratio is in 07135 to 07405 ranges at this condition.
(a) (b)
Figure 3. Determination of kinetic coefficients at MLSS of about 4333 mg/L (a) Ks and K (b) Y and kd The F/M ratio is in a range of 07108 to 07324 at suspended biomass concentration of about 5000 mg/l.
Figure 6 and 2 are plotted according to steady-state results and the kinetic coefficients are obtained from these diagrams.
Figure 4. Determination of kinetic coefficients at MLSS of about 5333 mg/L (a) Ks and K (b) Y and kd
The obtained kinetic coefficients are shown in Table 4 at the investigated biomass concentrations. All coefficients magnitudes are in the activated-sludge processes ranges except Ks coefficient. The values of Ks coefficient are much higher than that of the reported range in Metcalf & Eddy, while ranges were as same as the reported in the literature. This indicates that the coefficient Ks value is affected by the kd value estimation;
therefore, any uncertainty in the kd estimation will affect the Ks value [14]. This issue is more sensible with an increase in the suspended biomass concentration.
Table 4- The biokinetic coefficients for the modified USBF system at different MLSS concentrations µmax=Y*K Ks (mg/L)
kd(1/day) Y (mg/mg)
MLSS (mg/L)
1736 175256 172222 225752
325731 445733 07062
070514 070326 073868
074053 074203 3000
4000 5000
The kinetic coefficients reported from different studies and the obtained results from this study are shown in Table 5. Based on the Y definition that is the ratio of produced substrate to consumed substrate, it is revealed that an increase in the suspended biomass concentration increases the Y value, while decreases the kd value and the amount of produced sludge. The results of Table 3 indicate that an increase in the SRT increases the suspended biomass concentration. An increase in the suspended biomass concentration leads to an increase in the μm, which could be also attributed to the same reason given before.
(a) (b)
(a) (b)
Table 5- The Lawrence-McCarthy model kinetic coefficients obtained from different study at COD Substrate Y (mg/mg) kd(1/day) Ks (mg /L) µmax=Y*K Source
Domestic 0731-0735 07016-07068 43-223 172 Pala, 2005
Domestic 0762 0702 22 3725 Lawrence, 1220
San, 1222
Domestic 074-0762 0702-0702 22-60 372-3725
MWW 1728 0712 3676 0728 Joseph, 1222
Synthetic 0742-0758 0703207151 282-2233 1728-6746 Al-Malack,2006
MWW 0762-1725 0702-07031 31172-508 1726-3712 Mardani,2011
MWW 074-078 07025-07025 15-20 2-10 Metcalf&Eddy,2005
Domestic 0738-.042 070326-07062 225-445 1736-1722 This study
4. Conclusions
The modified USBF bioreactor has a superior performance than the conventional activated sludge process in the viewpoints of organic loading rates, biomass concentration, organic and hydraulic shock, and efficiency.
It is shown that the bioreactor efficiencies are 23 and 249 at the biomass concentrations of about 3000 and 4000 mg/L and organic loading rate of 1 kg COD/m3.day, respectively, while it has a better performance about 249 at the biomass concentration of about 5000 mg/L and OLR of 172 kg COD/m3.day. All determined biokinetic coefficients except Ks are in the reported activated-sludge biokinetic coefficients range.
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