Adsorption : filtration hybrid system in wastewater treatment and reuse
- Publication Type:
- Thesis
- Issue Date:
- 2003
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Wastewater contains a matrix of organic and inorganic substances both in dissolved
form and in suspension. Most of the biodegradable substances are removed in primary
and secondary treatment processes. However, the conventional wastewater treatment
processes cannot remove a number of synthetic and refractive organic substances.
These substances can cause tremendous problem in the sewage treatment processes and
in the water body where the effluent from the sewage plant is discharged. These
substances produce odour, colour, and require a large quantity of disinfectant dose
before the wastewater can be discharged into a water body. They can also significantly
deplete the dissolved oxygen level of the water receiving body thus putting all the
aquatic life in danger. The effluent from the sewage treatment plant therefore, needs to
be passed through further treatment process, which is called advanced sewage
treatment process. The advanced treatment processes consist of many treatment
options. Depending upon the characteristics of the sewage and the level of treatment
required, one has to select an appropriate treatment technology. Physico-chemical
processes such as coagulation-flocculation and filtration, adsorption, and membrane
application are some of the most viable treatment processes that can remove the organic
substances to the desirable level. In this study, adsorption, biosorption or biofiltration,
and adsorption-membrane hybrid systems were investigated for the removal of organics
(in terms of total organic carbon (TOC)) from a low strength synthetic wastewater and
a biologically treated secondary effluent from a sewage treatment plant, Sydney.
Adsorption experiments were conducted on low strength synthetic wastewater and the
biologically treated sewage effluent using granular activated carbon (GAC) and powder
activated carbon (PAC). The synthetic wastewater was prepared using three organic
substances (glucose, peptone and yeast extract) and seven inorganic chemicals
(MnS04, CaCI2, NaHC03, NaCl, MgS04·7H20, KH2P04 , and NH2·NH2·H2S04). The
biologically treated sewage effluent was collected from the St. Marys sewage treatment
plant, Sydney. Detailed experimental studies on adsorption equilibrium, batch kinetics
and fixed bed were carried out, and the experimental results were predicted using
suitable mathematical models.
The adsorption equilibrium was analysed with different initial organic concentration of
the synthetic wastewater. The experimental results were then predicted using
association theory (AT), characterization theory (CT), and the Freundlich isotherm. The
experimental results showed unfavourable type of isotherm curve, and hence, the
normal favourable isotherm equations such as Langmuir, Freundlich or Sipps isotherms
were not very successful in describing the adsorption equilibrium results. The AT and
the CT were better in predicting the adsorption equilibrium results than the commonly used
Freundlich isotherm. In this process, the adsorption equilibrium (isotherm)
parameters were determined using a multi variable, non-linear regression, Nelder-Mead
method by optimising an object function defined as the mean percent deviation
between experimental and calculated equilibrium adsorption amounts. The isotherm
parameters were found to be dependent on the initial organic concentration. Hence, it is
important to estimate the isotherm parameters covering a wide range of organIc
concentration. Further, the adsorption equilibrium studies of the individual organic
compounds indicated that the overall effects of the inorganic substances were
unfavourable for the adsorption of organics in the wastewater. The organics of the
synthetic wastewater were found to undergo biodegradation after 8 hours. Thus, the
effect of the background substances in the wastewater, and the biodegradation effect
are another important aspects that need to be considered while evaluating the
effectiveness of the adsorption process for organic removal from the wastewater.
It is equally important to study the adsorption behaviour with time (i.e. adsorption
kinetics). Adsorption kinetics of the organics in the wastewater was determined using
linear driving force approximation (LDFA) model. Basically, the LDFA is a simplified
expression of intraparticle diffusion of adsorbate into adsorbent particles. In this model,
it is assumed that the uptake rate of adsorbate by an adsorbent particle is linearly
proportional to the driving force developed due to the difference between the surface
concentration and the average adsorbed phase concentration of the adsorbate. The main
reason for using the LDFA method was the use of index (or lumped) parameter, total
organic carbon (TOC), to express the total organic contents of the wastewater. The film
mass transfer coefficient (kf) was found to be dependent on the experimental conditions
such as mixing intensity, the adsorbent dose and the initial organic concentrations. The
film mass transfer coefficient (kf) to the adsorbent increased when the mixing intensity
and the adsorbent dose were increased. However, the kf value decreased with the
increase in the initial organic concentration of the solution.
The adsorption isotherm parameters obtained from the association theory (AT) and the
characterization theory (CT), were utilized to fit the experimental results using LDFA
model. The isotherm parameters obtained from both the theories were found equally
effective in predicting the experimental results. The overall effect of the dissolved
inorganic compounds in the synthetic wastewater solution was observed to enhance the
mass transfer rate to the GAC particle. The average value of the overall mass transfer
rate was in the order of 10-6 mls.
The application of adsorption system in practice is usually carried out in the fixed bed
adsorption mode. The adsorbent (usually GAC) is packed in a column and the target
pollutants are passed through either end to be adsorbed by the adsorbent. In this study,
the fixed bed adsorption study was carried out in acrylic columns in the laboratory. The
GAC bed depth, organic concentration of the feed solution, and the filtration velocity
through the GAC bed were varied to evaluate the effectiveness of the fixed bed
adsorption system. The experiments were carried out with both the biologically treated
sewage effluent and the synthetic wastewater. The experimental results were predicted
using the dynamic adsorption model. The film mass transfer coefficient (kf) was
obtained by fitting the fixed bed experimental data. The kf increased when filtration
rate was increased, but it decreased with the increase in the organic concentration of the
feed solution. As expected, the value of kf remained constant with the increase in GAC
bed depth. The effect of axial dispersion coefficient was negligible, as the GAC bed
depth and the size of the GAC particles used in this study were shallow and small
respectively. The average value of the overall mass transfer rate in the fixed bed study
was also in the order of 10-6 mls but slightly less than that obtained in batch kinetics
study.
The fixed bed system with attached microorganisms on the surface of the adsorbent
(fixed bed medium) is referred to as a biofilter, where the organics are adsorbed
(biosorption) and biodegraded by the microorganisms. The fixed bed adsorption
experimentations were conducted for a longer duration to investigate the biological
activity on the granular activated carbon (GAC). The experimental results showed the
growth of microorganisms on the surface of GAC particles. In other words, the
adsorption system turned into biosorption or biofiltration system after few weeks of
operation. The adsorption capacity of the GAC particles slowly exhausted with the
growth of microorganisms with time. The overall organic removal efficiency of the
system was however, not impaired by the growth of microorganisms. The organics
were removed by the processes of biosorption and subsequent biodegradation. The
biomass growth rate was found to fluctuate with time in pattern. Despite the
fluctuation in the biomass, the TOC removal efficiency of the biofiltration system was
consistent at 55 % for 77 days of continuous operation. Moreover, the daily
backwashing provided at 30 % bed expansion to avoid filter clogging did not have
adverse effect on the TOC removal efficiency of the biofilter. The organic removal
efficiency of the biofilter changed when the filtration rate was altered from that in
which the biofilter was acclimatized~ however the organic removal pattern remained
consistent with time. This result suggests that the biofilter should be operated in the
same filtration velocity at which it is acclimatized to attain maximum efficiency of the
filter.
A practical mathematical model was developed incorporating both adsorption and
biodegradation of organics. The organic removal efficiency of the biofilter was
successfully predicted using kinetics data obtained from the previous studies. The
model was sensitive to the biofilm thickness and decay constant.
The adsorption-membrane hybrid system is emerging as a cost-effective membrane
process for the organic removal. In this system, the organics are adsorbed on the
adsorbent and the organic laden adsorbents are removed by the membrane separation
process. In this study, the adsorption-membrane hybrid system was evaluated using
submerged hollow fibre (pore size 0.1 ~m), and the external loop crossflow
rnicrofiItration. Powdered activated carbon (PAC) was used to reduce the direct organic
loading onto the membrane surface. The main function of membrane in these studies
was to remove the organic laden PAC particles. The submerged PAC-Membrane
hybrid system was found effective in removing dissolved organic substances both from
the synthetic wastewater and the biologically treated effluent of a sewage treatment
plant. The system has potential for its long-term application in the treatment of
wastewater without the need of frequent membrane cleaning. This preliminary study
showed that the PAC-membrane hybrid system could be used for a long time
effectively (over 47 days). At the initial stage of operation, the organic removal was
mainly due to adsorption by PAC, but during the long-term application of the system,
the adsorption capacity of the PAC was exhausted gradually, and the microbial
communities developed on the PAC, in the suspension of the reactor, and on the
membrane surface actively participated in the biodegradation of the organics.
An empirical mathematical model was developed for the submerged hollow fibre
membrane hybrid system. The model predicted the organic removal efficiency of the
system satisfactorily. A new term, membrane correlation coefficient (MCC) was
introduced in the model to account for the adsorption of organics onto membrane
surface. The MCC and the filtration rate (flux) were found to be the main model
parameters that controlled the quality of the effluent from the system. Greater the value
of MCC, better was the organic removal efficiency of the system. The MCC value was
found to increase with the increase in the PAC dose to the system. Since only the short-term
experiments were conducted in this study, the biological degradation of the
organics was not included in the model. It is necessary to incorporate the biological
degradation part in the model to predict the long-term efficiency of the system.
The external loop cross-flow microfiltration system with prior PAC addition was also
tested using the synthetic wastewater. This study showed that the use of PAC helped
not only in the organic removal but also in the enhancement of the filtration flux. The
use of PAC was instrumental in increasing the operational life the membrane hybrid
system by reducing the organic fouling on the membrane. The conventional pressure
filtration models, cake filtration model (CFM) and standard blocking model (SBM)
were used to successfully predict the experimental results. Since CFM was more
effective in predicting the volume of the permeate flux from the hybrid system, one
could infer that the fouling mechanism of the membrane was mainly due to the
formation of cake layer on the membrane surface. However, the experimental
conditions used in the hybrid system were not so favourable for removing the organics
from the synthetic wastewater. The organic removal efficiency of the PAC-membrane
hybrid system was only 250/0 for the PAC dose of 150 mg/L. The organic removal
efficiency of the system depends mainly on the characteristics of the adsorbent and the
influent wastewater solution, and the adsorbent dose.
This study shows that activated carbon can effectively be used in different operational
modes and in different treatment processes to remove organics from the wastewater,
and to produce effluent of high quality that can be reused for many purposes.
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