Effect of Mixed Liquor Suspended Solids (MLSS) on Mass Transfer Coefficient in Sparged and Stirred Tank Reactors
For the treatment of industrial wastewaters, the method of biological
oxidation (also known as activated sludge treatment) is used in practice. In
this method, oxygen is transferred from gas to liquid phase. The microbes take
oxygen and chemical organic matters as food source and reduces the COD. Sufficient
availability of oxygen in the liquid phase is prerequisite for the success of
biological oxidation process, however, the mass transfer of oxygen provides
significant resistance to the overall rate of oxidation. For biological oxidation in commercial practice, two types of
systems are commonly employed: diffused aeration and surface aeration. In both
the cases, the MLSS is in the range of 4000 ? 9000 mg/L. It is possible that
the rate of biological oxidation can be increased by an increase in the MLSS
concentration. However, concomitantly, it is important to enhance the mass
transfer rate of oxygen. This requirement demands the knowledge of the mass
transfer characteristics of diffused and surface aeration systems. For diffuser
type systems, Krampe and Krauth [1], cornel et al. [2], Germain and Stephenson [3-5], and Henkel et al. [6] have investigated the effect of MLSS on mass transfer coefficient
(kLa). In all these studies, kLa was found to
decrease by a factor of 5 to 7 when the MLSS concentration was varied from 0
(no MLSS) to 40,000 mg/L. Though good number of studies have been reported for
diffuser type of systems, there is practically no information available in the
published literature regarding this aspect in STR system. Therefore, it was
thought desirable to undertake a systematic study of the effect of MLSS
concentration (0-40,000 ppm) on kLa
in
stirred tank reactors over a wide range of power consumption (0.1 to 1 kW/m3).
The range of parameters was decided on the basis of the practical interest. It
was also thought desirable to investigate simultaneously, the diffused aeration
systems. The experiments were carried out in a 2 L capacity glass stirred
reactor of 130 mm id with 1.7 L working volume. The reactor was fitted with
four standard baffles of 10 mm width. Mass transfer, mixing and cell suspension
were provided by means of impeller agitation. A Remi make stirrer was employed
which was connected to a 50 Volt DC motor equipped with a speed regulator (1.67
to 20 rps). The air compressor was connected to a ceramic sparger through a precalibrated
rotameter. The impeller was a pitched blade turbine (60 mm diameter and 15 mm
blade width). The experiments were also carried out in a diffused aerator (bubble
column reactor) of 3 L working volume. The reactor was acrylic cylindrical
vessel with 100 mm diameter and 600 mm height. Oxygen mass transfer, mixing
and cell suspension were provided by means of flow generated by sparging the
air from the bottom of the reactor. An air compressor was connected to a
ceramic sparger through a control valve and precalibrated rotameter. A DO probe
was immersed in the gas-liquid dispersion. Primary wastewater
from an agrochemical plant was utilized for the experiments. The wastewater was
collected after neutralization, so that it had a uniform COD. The wastewater
contained many complex organic and inorganic compounds with salinity of about
13%. The pH of primarily treated wastewater was found to be in the range of 6.0
to 8.0. The values of COD and BOD were in the range of 1200 to 1350 mgl-1
and 400 to 500 mgl-1, respectively. The activated sludge (AS) was
obtained by filtration of the same agrochemical treatment reactor, which
contained mixed microbial active culture. The recycle sludge and the primary effluent were mixed in such a
way as to get the desired MLSS with values of 6000, 9000, 15000, 20000, 25000,
30000, 35000 and 40000 mg/L. For STR, the impeller speed was in the range of 5
to 11.3 r/s and superficial gas velocity (VG) in the range of 0.19
to 0.5 mm/s. For bubble columns, VG was in the range of 0.5 to 2
mm/s. The MLSS was adjusted to the predetermined value using the filtered
activated sludge. The agitation (rps) and the superficial air velocity (mms-1)
were started at the desired levels. The value of the dissolved oxygen (DO) was
continuously monitored. The agitation and aeration were continued for two hours
for proper adaption as indicated by constant value of dissolved oxygen. At this
time the, air was switched off and whilst the agitation was continued and the
DO was cautiously monitored. At a DO Value of 0.5 mg/L, the air was restarted
and continued until a constant value of DO was obtained. The value of
 was obtained from the nature of DO variation with time.
The parameters like MLSS, MLVSS were measured using the standard
methods. pH was measured using pH meter [TOSHCON industries private ltd India.,
Model CL 46). DO was measured using DO meter [HACH company, HQ40d multi
parameter model; ± 0.10]. RPM was noted from a vicinity probe (Remi make 50 V
DC motor; ± 10). The experimental data was analyzed using the following
equations and the values of mass transfer coefficient were estimated.
dissolved oxygen. The mass balance for oxygen is given by:
particles and size/shape of particles. The activated sludge in reactor behaves like non-Newtonian fluids.
The rheology of sludge was studied using Anton Paar Modular compact Rheometer (MCR
102) and for the MLSS concentrations of 3000, 6000,
9000, 15000, 20000, 25000, 30000, 35000 and 40000 mg/L. Â The different MLSS concentrations solutions were prepared with both
water and primary effluent. The rheological properties were measured in the
shear rate range of 10-100 (1/s). With three different impeller speeds and at three different superficial
gas velocities, the mass transfer study is shown in Fig 1. It can be seen from
these figures that the value of kLa exponentially decreases with an
increase in the MLSS concentration. However, beyond about 30,000 MLSS, it
remains practically constant. Figures 1A to 1D show a larger error bar at low
MLSS. This is because of the presence of highly active replicating and
substrate degrading microbes. On contrast, at high MLSS, the microbe
replication is very low and has more inactive microbes. It was observed that,
in STR value of kLa decreases by a factor of 1.6 to 1.8 when MLSS
was increased from 6000 to 40000. However, in bubble column, in the same range
of MLSS, the reduction in kLa was bound to be by a factor of 2.5 to 5. This is because the level of shear rate is much higher in STR as
compared to sparged reactors. Thus, in sparged reactors, the apparent viscosity
mainly depends on MLSS concentration. On contrast, in stirred reactors, the
shear rate is high and the variation in apparent viscosity with MLSS
concentration was found to be nominal as compared with that in sparged
reactors. As a consequence, the dependence of kLa with MLSS is much
stronger is sparged reactors as compared to that was stirred reactors.
oxidation (also known as activated sludge treatment) is used in practice. In
this method, oxygen is transferred from gas to liquid phase. The microbes take
oxygen and chemical organic matters as food source and reduces the COD. Sufficient
availability of oxygen in the liquid phase is prerequisite for the success of
biological oxidation process, however, the mass transfer of oxygen provides
significant resistance to the overall rate of oxidation. For biological oxidation in commercial practice, two types of
systems are commonly employed: diffused aeration and surface aeration. In both
the cases, the MLSS is in the range of 4000 ? 9000 mg/L. It is possible that
the rate of biological oxidation can be increased by an increase in the MLSS
concentration. However, concomitantly, it is important to enhance the mass
transfer rate of oxygen. This requirement demands the knowledge of the mass
transfer characteristics of diffused and surface aeration systems. For diffuser
type systems, Krampe and Krauth [1], cornel et al. [2], Germain and Stephenson [3-5], and Henkel et al. [6] have investigated the effect of MLSS on mass transfer coefficient
(kLa). In all these studies, kLa was found to
decrease by a factor of 5 to 7 when the MLSS concentration was varied from 0
(no MLSS) to 40,000 mg/L. Though good number of studies have been reported for
diffuser type of systems, there is practically no information available in the
published literature regarding this aspect in STR system. Therefore, it was
thought desirable to undertake a systematic study of the effect of MLSS
concentration (0-40,000 ppm) on kLa
in
stirred tank reactors over a wide range of power consumption (0.1 to 1 kW/m3).
The range of parameters was decided on the basis of the practical interest. It
was also thought desirable to investigate simultaneously, the diffused aeration
systems. The experiments were carried out in a 2 L capacity glass stirred
reactor of 130 mm id with 1.7 L working volume. The reactor was fitted with
four standard baffles of 10 mm width. Mass transfer, mixing and cell suspension
were provided by means of impeller agitation. A Remi make stirrer was employed
which was connected to a 50 Volt DC motor equipped with a speed regulator (1.67
to 20 rps). The air compressor was connected to a ceramic sparger through a precalibrated
rotameter. The impeller was a pitched blade turbine (60 mm diameter and 15 mm
blade width). The experiments were also carried out in a diffused aerator (bubble
column reactor) of 3 L working volume. The reactor was acrylic cylindrical
vessel with 100 mm diameter and 600 mm height. Oxygen mass transfer, mixing
and cell suspension were provided by means of flow generated by sparging the
air from the bottom of the reactor. An air compressor was connected to a
ceramic sparger through a control valve and precalibrated rotameter. A DO probe
was immersed in the gas-liquid dispersion. Primary wastewater
from an agrochemical plant was utilized for the experiments. The wastewater was
collected after neutralization, so that it had a uniform COD. The wastewater
contained many complex organic and inorganic compounds with salinity of about
13%. The pH of primarily treated wastewater was found to be in the range of 6.0
to 8.0. The values of COD and BOD were in the range of 1200 to 1350 mgl-1
and 400 to 500 mgl-1, respectively. The activated sludge (AS) was
obtained by filtration of the same agrochemical treatment reactor, which
contained mixed microbial active culture. The recycle sludge and the primary effluent were mixed in such a
way as to get the desired MLSS with values of 6000, 9000, 15000, 20000, 25000,
30000, 35000 and 40000 mg/L. For STR, the impeller speed was in the range of 5
to 11.3 r/s and superficial gas velocity (VG) in the range of 0.19
to 0.5 mm/s. For bubble columns, VG was in the range of 0.5 to 2
mm/s. The MLSS was adjusted to the predetermined value using the filtered
activated sludge. The agitation (rps) and the superficial air velocity (mms-1)
were started at the desired levels. The value of the dissolved oxygen (DO) was
continuously monitored. The agitation and aeration were continued for two hours
for proper adaption as indicated by constant value of dissolved oxygen. At this
time the, air was switched off and whilst the agitation was continued and the
DO was cautiously monitored. At a DO Value of 0.5 mg/L, the air was restarted
and continued until a constant value of DO was obtained. The value of
 was obtained from the nature of DO variation with time.
The parameters like MLSS, MLVSS were measured using the standardmethods. pH was measured using pH meter [TOSHCON industries private ltd India.,
Model CL 46). DO was measured using DO meter [HACH company, HQ40d multi
parameter model; ± 0.10]. RPM was noted from a vicinity probe (Remi make 50 V
DC motor; ± 10). The experimental data was analyzed using the following
equations and the values of mass transfer coefficient were estimated.
                                                                                              Â
Where C is DO and C* is saturation value of DO.
In the aerobic biological system, the living cells consume thedissolved oxygen. The mass balance for oxygen is given by:
                                                                                                          Â
Where OUR is oxygen uptake rate
Sludge rheological properties depend on the affinity amongparticles and size/shape of particles. The activated sludge in reactor behaves like non-Newtonian fluids.
The rheology of sludge was studied using Anton Paar Modular compact Rheometer (MCR
102) and for the MLSS concentrations of 3000, 6000,
9000, 15000, 20000, 25000, 30000, 35000 and 40000 mg/L. Â The different MLSS concentrations solutions were prepared with both
water and primary effluent. The rheological properties were measured in the
shear rate range of 10-100 (1/s). With three different impeller speeds and at three different superficial
gas velocities, the mass transfer study is shown in Fig 1. It can be seen from
these figures that the value of kLa exponentially decreases with an
increase in the MLSS concentration. However, beyond about 30,000 MLSS, it
remains practically constant. Figures 1A to 1D show a larger error bar at low
MLSS. This is because of the presence of highly active replicating and
substrate degrading microbes. On contrast, at high MLSS, the microbe
replication is very low and has more inactive microbes. It was observed that,
in STR value of kLa decreases by a factor of 1.6 to 1.8 when MLSS
was increased from 6000 to 40000. However, in bubble column, in the same range
of MLSS, the reduction in kLa was bound to be by a factor of 2.5 to 5. This is because the level of shear rate is much higher in STR as
compared to sparged reactors. Thus, in sparged reactors, the apparent viscosity
mainly depends on MLSS concentration. On contrast, in stirred reactors, the
shear rate is high and the variation in apparent viscosity with MLSS
concentration was found to be nominal as compared with that in sparged
reactors. As a consequence, the dependence of kLa with MLSS is much
stronger is sparged reactors as compared to that was stirred reactors.
Reference
Fig. 1. Effect of MLSS concentration, impeller speed and
superficial gas velocity on Mass transfer coefficient.