Post on 18-Apr-2018
transcript
Bioreactor System and Nanotechnology for Water Treatment
191
5.1 INTRODUCTION
Water is one of the essential enablers of life on earth. Beginning with the
origin of the earliest form of life in seawater, it has been central to the evolution of
human civilizations. Years of intense research have contributed significant
breakthroughs in the treatment of polluted water systems. Bioreactor systems and
application of nanotechnology are recently developed options for the better
treatment of the contaminated water. Bioreactor offers platform for the laboratory
evaluation of large scale systems for industrial applications. Once designed, the
system can be optimised for significant parameters and scale up can be done to
greater level of precision. Noble metals have been similarly associated with the
prosperity of human civilizations through their prominent use in jewellery and
medical applications. The most important reason for the use of noble metals is the
minimal reactivity at the bulk scale, which can be explained by a number of
concepts such as electrochemical potential, relativistic contraction, molecular orbital
theory, etc. Recently, water quality has been associated with the development index
of society. A number of chemical and biological contaminants have endangered the
quality of drinking water. The present work includes novel approaches in the area of
the application of bioreactor systems and noble metal nano particle in the treatment
of contaminated river water system
Reactor systems generally offer effective and promising output for online
applications. Reactor systems can be designed to incorporate multiple stages which
can be sequentially put into operation according to the requirement. This added
advantage is a huge benefit in water treatment processes as it requires multiple
methodologies at various level of treatment.
Chapter 5
192
The polluting factors of contaminated water belong to various categories and
specific treatment strategies are required to tackle each polluting factor. The most
significant polluting aspect of contaminated water system is the presence of high
amount of suspended particles. These particles may be of dissolved or suspended
type, The removal of the total suspended matter is mostly effected by physical or
chemical methods The physical methods are not so cost effective and often result in
the replacement and frequent service requirement of the tangential screens. However
subjecting to natural sedimentation is a cost effective process. But this is a very slow
process and requires more time. Hence additional process aids are required when it
is a matter of large volume of water and particularly when the suspended solids are
relatively high.
The incorporation of additional coagulation and flocculation process to
facilitate better removal of suspended natter contribute to better performance of
subsequent treatment strategies.
Bioreactor may refer to any manufactured or engineered device or system
that supports a biologically active environment. A bioreactor may also refer to a
device or system meant to grow cells or tissues in the context of cell culture. These
devices are being developed for use in tissue engineering or biochemical
engineering or waste management. On the basis of mode of operation, a bioreactor
may be classified as batch, fed batch or continuous. The present bioreactor system
offers multiple treatment strategies contributing to effective removal of suspended
solids, removal of coliforms and hence facilitating chlorination at low dose. This
treatment strategy can be applied online effectively and can be easily scaledup.
Bioreactor System and Nanotechnology for Water Treatment
193
The application of noble metal nanoparticle based chemistry for drinking
water purification is summarized for three major types of contaminants: halogenated
organics including pesticides, heavy metals and microorganisms. Realizing the
molecular nature of contamination in drinking water, significant progress has been
made to utilize the chemistry of nanomaterials for water purification. Scientist
working in the field of environmental nanotechnology view that working at the
nanoscale is not detrimental to the environment. Studies have shown that
nanotechnologies can be used not only to prevent pollution, but also to clean up
pollutants once they have made their way in to the environment. Automatically
precise manufacturing at nanoscale should be able to eliminate chemical pollution
entirely by giving control of processes at the molecular level
Today most of the countries are facing drinking water problems and
conditions are very severe, especially in developing countries. The world is facing
formidable challenges in meeting rising demands of clean water as the available
supplies of freshwater are depleting due to (i) extended droughts, (ii) population
growth, (iii) more stringent health based regulations and (iv) competing demands
from a variety of users USBRSNL (2003), USEPA (1998b), USEPA (1999).Clean
water (i.e., water that is free of toxic chemicals and pathogens) is essential to human
health. In countries such as India, 80% of the diseases are due to bacterial
contamination of drinking water. The World Health Organization (1996)
recommended that any water intended for drinking should contain faecal and total
coliform counts of 0, in any 100 ml sample. When either of these groups of bacteria
is encountered in a sample, immediate investigative action should be taken. The
removal or inactivation of pathogenic microorganisms is the last step in the
Chapter 5
194
treatment of wastewater. USEPA (1998b). The protection of water treatment
systems against potential chemical and biological acts is also becoming a critical
issue in water resources planning (USEPA, 1999; USEPA, 1998).
Despite the modern success of nanotechnology, the potential health and
environmental risks associated with these applications remain unknown. Prompted
by the discovery that nanoparticles can enter the human body and accumulate in the
environment, government agencies have begun to manage research of
‘nanotoxicology.’ In September 2006, the National Nanotechnology Initiative
reported the intent to research methods to evaluate the toxicity of nanoparticles in
the environment and the human body. This critical information needed is also the
primary focus of the USEPA with regard to nanotechnology. Due to the
overabundance of silver nanoparticles in the consumer market, ‘nanosilver’ has
become a specific area of interest among research scientists. The concerns and
potential consequences related to exposing nanosilver to our water environment
through consumer products are tremendous. If exposed to the environment, silver
nanoparticles may induce the death of bacteria that are surrogate environmental
organisms and vital to all ecosystems. Moreover, in the 1980s, silver ion pollution
from a processing plant endangered the native population of Macoma balthica clams
in the South San Francisco Bay. Since nanosilver is a derivative of silver and silver
ions, the effects of a silver ion pollution in the 1980s may foreshadow a similar and
possibly even worse consequence with nanosilver pollution. Furthermore, the
accumulation of nanosilver in the water environment may have an adverse effect on
the aquatic organisms that inhabit the polluted areas. Current research has shown
that zebrafish embryos exposed to nanosilver result in delayed development,
Bioreactor System and Nanotechnology for Water Treatment
195
defective fetal maturation and death. The most detrimental effect of silver
nanoparticles would be our efforts in recycling water in the wastewater industry.
Several strains of bacteria are implemented in wastewater treatment to digest the
organic substances present in the sludge. Later these organisms can be destroyed
with nanosilver particle. It is clear that the potential consequences of direct or
indirect exposure of nanosilver to the environment are detrimental. As a result, it is
necessary to develop a systematic technique to quantify and analyze the bacterial
toxicity of silver nanoparticles in environmental conditions. In addition, the
abundance of silver nanoparticles in consumer products increases the possibility of
environmental exposure; thus, it is necessary to analyze the toxicity of silver
nanoparticles in such products.
A water filter with 0.3 nm pores (on left) would clean water down to the
atomic level with minimal pressure drop due to drag. Silver coated Nanofilters are
used in many industrial applications for water purification. They are being evaluated
because of their ability to reduce the coliform content and reduce clogging compared
to traditional filtration methods
Nanofilters can help to tackle decontamination of groundwater from
industrial and natural sources. Semi - permeable membrane can act as a molecular
sieve allowing water to pass through while rejecting impurities such as viruses,
spores, bacteria, heavy metals, and other health threats. Nanoscale filters will be able
to actively screen out items matching certain criteria.
Desalination is an area, where nanotechnology could cut costs, save energy,
and improve the lifetime and efficiency of membranes. Today seawater is most
often turned in to drinking water through a 40-year old process called reverse
Chapter 5
196
osmosis, which is slow ,expensive and energy intensive. If nanotechnology can
make the process cheaper and efficient.it could have a large impact and is the need
of the hour in most of the developing countries including India
5.2 REVIEW OF LITERATURE
Bioreactors for treating sewage and wastewater are considered as the most
efficient of these systems. Among the advanced wastewater treatment technologies
the Membrane Bioreactor (MBR) process is an emerging area. It involves a
suspended growth activated sludge system that utilises microporous membranes for
solid/liquid separation in lieu of secondary clarifiers. In addition, it provides a
barrier to certain chlorine resistant pathogens such as Cryptosporidium and Giardia
Membrane Bioreactor systems essentially consists of a combination of
membrane and biological reactor systems. These are used for a wide spectrum of
advanced wastewater treatment processes. In general, MBR applications for
wastewater treatment can be classified into four groups (Stephenson et al., 2000)
namely:
Silver and silver compounds have been used as antimicrobial compounds for
coliform found in waste water (Jain, and Pradeep. 2005). Silver nanoparticles,
nanodots or nanopowder are spherical or flake high surface area metal particles
having high antibacterial activity. Furno (2004) and Moran (2005), have used
Nanoscale silver particles of 1-40 nanometers (nm) with an average particle size of
2-10 micron range, specific surface area of approximately 1m2 g-1 for various water
treatment strategies. Applications for silver nanocrystals include as an anti-
microbial, anti-biotic and anti-fungal agent when incorporated in coatings,
nanofiber, first aid bandages, plastics, soap and textiles, in treatment of certain
Bioreactor System and Nanotechnology for Water Treatment
197
viruses, in self cleaning fabrics, as conductive filler and in nanowire and certain
catalyst applications. It has been reported that Ag nanoparticles were active biocides
against Gram positive Gram-negative bacteria including Escherichia coli,
Staphylococcus aureus, Klebsiella pneumoniae and Pseudomonas aeruginosa. (Jain,
and Pradeep, 2005; Sons, et al., 2004). Sondi (2004) and Ping Li et al., (2005
studied that the Ag nanoparticle of narrow size shows enhanced antibacterial effect
against E. coli.
More recently, it has been demonstrated that the bactericidal effect of silver
was caused by silver chelation preventing DNA from unwinding. The anti-microbial
effects of silver, in zerovalent and ionic form, have been widely studied in great
detail. (Jain and Pradeep, 2005., Sondi, and Sondi 2004., Aymonier, 2002). It has
also been used widely as a common disinfectant for surgical masks (Lia et al.,
2006), textile fibers (Dubas et al., 2006), wound dressing (Maneerung et al., 2008),
etc. Significant efforts have been devoted to study the toxic effects of silver
nanoparticles on a broad spectrum of micro-organisms including E. coli (Jain and
Pradeep, 2005, Sondi, and Sondi 2004, Morones, et al., 2005, Pal et al., 2007 and
Lok, et al., 2007). Pseudomonas aeruginosa (Pal, et al., (2007). Vibrio cholera (Pal,
et al., 2007). Bacillus subtilis and HIV-1 (Elechiguerra, 2005). Prior to discussing
the chemistry behind the biocidal activity of silver nanoparticles, it is useful to
understand how silver ions act against micro-organisms. While the precise details
are not yet elucidated, protein inactivation and loss of replication ability of DNA are
suggested. A few important observations are highlighted. It was pointed out that
cells protect the DNA by forming a defense around the nucleus, when the cells are
subjected to external stimuli such as heat. Under severe external stimulus, the
Chapter 5
198
defense mechanism fails leading to the denaturation of the DNA (loss of replication
ability). A similar observation was found in the case of silver ions with E. coli and
Staphylococcus aureus. (Feng, 2000). The formation of protective layers around
DNA and DNA's condensation was clearly evident in the study (Feng. 2000). The
large-scale movement in the cellular components in the presence of silver ions is
indeed surprising and reflects the ability of the cell to protect itself against external
stimuli. It was also found that the interaction of silver ions with sulfur present in
many proteins, leads to protein inactivation (Liau, 1997). While the external addition
of sulfur-containing compounds led to the neutralization of anti-bacterial activity of
silver ions, the presence of sulfur in silver-rich regions confirmed the interaction
between sulfur and silver.
The nature of the charge on the cell surface (due to presence of different
functional groups) and the anti-bacterial composition plays a key role in determining
the effectiveness. It was found that the same nature of charge on the antimicrobial
composition and cell surface (negative charge) leads to repulsion and decreased
contact (Hamouda 2000). The ability of silver to absorb oxygen in atomic form has
been widely utilized immensely for many organic reactions such as conversion of
methanol to formaldehyde. It is revealed that bulk silver in an oxygen charged
aqueous medium catalyzes the complete destructive oxidation of microorganisms
(Davies 1997).
It is largely understood that cellular membranes play a critical role in
maintaining the viability of cells. The cellular permeability in the case of gram-
negative bacteria such as E. coli is largely controlled by the presence of a
lipopolysaccharide (LPS) layer on the outer surface of the cellular membrane. The
Bioreactor System and Nanotechnology for Water Treatment
199
heavily saturated fatty acids on LPS links it to the membrane backbone, which itself
contains many negative ions. Thus, LPS binds cations, which is also confirmed by
the presence of Mg2+/Ca2+ as an electrostatic linker to bind adjacent LPS chains. The
affinity of LPS towards cations has been utilized for permeation of polycationic
antibiotics in the cytoplasm. It is also suggested that the binding of even simple
cations to LPS weakens the membrane backbone which may lead to the
disintegration of the membrane. On the contrary, negatively charged ions have been
reported to bind with Mg2+/Ca2+, which also leads to loss of cellular viability. In the
context of observations for silver ions, it is appropriate to understand the
observations for silver nanoparticles. Silver nanoparticles cause irreparable damage
to the cellular membrane (Sondi, 2004, Pal et al., 2007, Gorgoi, 2006) which enables
the accumulation of nanoparticles in the cytoplasm. It is suggested that action of
silver nanoparticle arises due to this damage and not its toxicity (Pal et al., 2007).
The pits in the cell wall, post-treatment, are quite significant. An important aspect of
the biocidal action of silver nanoparticles is the requirement of supported
nanoparticles for anti-bacterial effects. As explained in an earlier section on
biosynthesis of metal nanoparticles, cells protect themselves from metal toxicity
through the action of cellular proteins which bind to the nanoparticle surface leading
to nanoparticle aggregation and thus rendering nanoparticles immobile. This was
suggested by the studies of nanoparticle solutions with bacteria. It is therefore
expected that small size nanoparticles are able to easily penetrate across membranes
(Morones, et al., 2005., Pal et al., 2007). Similarly, antibacterial activity of
nanocrystals is found to have a dependence on crystal shape. The activity is found to
be higher for truncated triangular nanoplates when compared with nanorods and
Chapter 5
200
spherical particles. The interaction of bacteria with high atom density crystal plane
has been proposed (Morones, et al., 2005) exhibiting higher anti-bacterial activity
was reported (Pal et al., 2007). Recently, it has been proposed that the activity of
silver nanoparticle arises due to the formation of superoxide, which has been
detected by the dismutation activity of superoxide dismutase. The addition of
dismutase leads to reduction in anti-bacterial activity (Chang 2007). It is also
suggested that the chemistry of silver ions is important in the anti-bacterial effect of
silver nanoparticles. That chemisorbed Ag+ ions is important in determining the
silver nanoparticle toxicity was confirmed by the non-toxicity of oxidized
nanoparticles to silver-resistant E. coli strains (Lok, 2007).
5.3 MATERIALS AND METHODS
5.3.1 Designing of the Reactor
This bioreactor consists of a reservoir and three specially designed treatment
units. The reservoir is a rectangular tank made up of polyacrylic material with a
capacity of 10 liters. The tank measures about 30cm in length, 35cm in width and 50
cm in height (Fig.5.1).
Bioreactor System and Nanotechnology for Water Treatment
201
Fig. 5.1
The reservoir unit of 35cm X 30cm X50cm with 5l capacity carrying the polluted water for treatment
The reservoir unit is connected to two pipe lines. Of the two pipelines, one
line offers flow of untreated water as the control from reservoir and the other line
carries the three in line water treatment units (Fig.5.2).
Fig. 5.2
The three single treatment units connected in series contributing to multistage treatment system for the treatment of polluted water along with
the control flow unit
Chapter 5
202
The treatment units comprises of three Units. The units are same in
dimension and each extends upto 100 cm. Each unit is designed with provisions for
adding the coagulant/ flocculant and also for filtering out the coagulated fraction
(Fig.5.3).
Fig. 5.3
A single treatment unit of 100 cm length in the multistage treatment system for polluted water.
Each of these units is also connected to a sedimentation/ filtration device.
The container for adding flocculent is attached to a funnel that drops down to the
water (Fig.5.4). The sedimentation unit, which is attached to the treatment units, is a
removable device. After each treatment, the sedimentation unit is detached, cleaned
and refitted for future use.
Bioreactor System and Nanotechnology for Water Treatment
203
Fig. No.5.4
A single unit showing the provision for adding coagulants or flocculants for inline treatment
The diameter of pipe fitted between the flocculation unit and sedimentation unit
is comparatively reduced. This reduction in size helps in the effective mixing of
flocculants and water. The funnel of the flocculation unit is 5.5 cm in diameter and 8cm
in height. The sedimentation device is 6cm in diameter and 18 cm in height (Fig.5.5).
Fig. 5.5
The sedimentation and filtration unit in a single unit of multistage treatment system for polluted water
(diameter 6 cm, height 18 cm, No of pores 10/cm2)
Chapter 5
204
The first unit assigned as the coagulation unit for alum, the second unit is
assigned for flocculation with Moringaseed powder and the third unit for mild
chlorination.
The whole device, except the reservoir unit, is supported by adjustable
metallic stands (Fig.5.6). The reservoir unit is kept high above all the treatment units
(fig.5.1).
Fig. 5.6
The metallic supporting stand of adjustable height to support the inline treatment units and supporting line
5.3.2 Working of the Bioreactor
The contaminated water is first collected in the reservoir unit. Water from
this reservoir unit is allowed to flow through the coagulation unit I, Flocculation
UnitII, Unit, Mild Chlorination unit III and the nanofilter unit towards the end. The
Bioreactor System and Nanotechnology for Water Treatment
205
specially designed control units and outflow units aids in collecting water from
every stages of treatment (Fig.5.7).
Finally provisions are given for attaching nanofilters also. The nano particles
used are silver nano particles (Fig.5.8). The water is allowed to passes through the
coagulation unit at first and then into the flocculation unit, chlorination unit and
finally into the nanofilter (Fig.5.9).
Fig 5.7
The complete multistage inline water treatment system supported by the metallic stands
Chapter 5
206
Fig. 5.8
The silver particle coated nano filter connected to the inline multistage water treatment system.
Fig. 5.9
The complete multistage inline water treatment stage with nano filter connected at the end.
After the completion of the three stages of treatment, water flows out from
the pipe line. Samples of treated water can be collected after each stage and can be
verified for any qualitative change that can occur in the water sample after the
treatments. The untreated water makes a continuous flow through the control tube
Bioreactor System and Nanotechnology for Water Treatment
207
and samples can be withdrawn at uniform intervals for comparison of the efficiency
of the treatment strategies.
5.3.3 Alum and Moringa seed powder treatment
Alum and moringa seed powder treatment was done as mentioned in the
section 4.3. The alum at 30mg/l dose was packed in the first stage of the reactor
followed by morinda seed powder at (30 mg/l) in the second stage. The water was
circulated through the packed reactor system under constant reservoir head.
Chlorination was done the last stage at the minimum concentration to get complete
(.1-.75 mg/l) elimination of total coliform.
5.3.4 Silver Nano particle treatment for waste water treatment.
The nano particle was synthesized and it was adhered to the nano filter of
pore size 45 to 47 nm. The water was allowed to pass through the filter with
pressure. The coated filter was incorporated at the end of the reactor system.
5.4 RESULTS
The bioreactor systems offer rapid and efficient treatment system for the
complete purification of the contaminated water. Bioreactor system contributes
multiple strategies for the integrated treatment of the polluted water.
In the present study complete treatment of the polluted water was attempted
with a three stage reactor system. The reactor system could offer facility for alum
coagulation Moringa seed flocculation, mild chlorination and finally nanofilter
treatment.
The treatment was carried out with and without nanofilter treatment. On
treating the polluted water with alum, Moringa seed powder followed by mild
Chapter 5
208
chlorination the COD, BOD and the MPN were considerably reduced. The BOD
was reduced from 80± mg/l to 24± mg/l. Similarly the COD was reduced from 142±
mg/l to 132± mg/l. The MPN was also brought to minimum from 1200 It is most
striking that after the treatment with the alum and moringa seed powder much of the
coliforms were eliminated. Finally mild chlorination (0.75 mg/L) could bring the
MPN to minimum value. On treating the water with alum, moringa seed powder,
mild chlorination followed by nano treatment the coliform content was completely
eliminated with much reduced BOD and COD at the same low dose of chlorination.
(Table 5.1 and 5.2)
Table 5.1
Effect of various combinations of water treatment without nano filter
Parameters Control Combinations of Alum(1mg/L)Muringa( 2mg/l),Chlorination(0.75 mg/L)
BOD (mg/L) 80±0.04 24±0.81
COD ( mg/L) 142±1.4 132±1.21
MPN 1200±1.06 0
Table 5.2
Effect of various combinations of water treatment with nanofilter
Parameters Control Combinations of Alum(1mg/L)Muringa ( 2mg/l),Chlorination(0.75 mg/L) Nano filter
BOD(mg/L) 80±0.04 12±0.31
COD(mg/L) 132±1.27 38±0.12
MPN 1200±1.06 0
Bioreactor System and Nanotechnology for Water Treatment
209
The FT/IR analysis done for the treated water with alum, Moringa seed
powder and low dose chlorination there was no indication of chlorination derived
byproducts. (Fig.5.10) Even the GC/MS analysis could not bring any evidences of
chlorination derived byproducts. (Fig.5.11)
cm-1
Fig. 5.10
FT/IR analysis of the polluted water sample after integrated treatment of the water sample with the combinations of Alum, Muringa seed powder followed
by low dose chlorination in the multistage reactor system.
% T
Chapter 5
210
10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0 14.50.0
0.5
1.0
1.5
2.0
2.5(x10,000,000)
TIC
50.0 75.0 100.0 125.0 150.0 175.0 200.0 225.0 250.0 275.0 300.0 325.0 350.0 375.0 400.0 425.0 450.0 475.0 500.00.0
25.0
50.0
75.0
100.0
%
7343
12983
115 213157 256185
228 356 480284 342 429403 504325
25.0 50.0 75.0 100.0 125.0 150.0 175.0 200.0 225.0 250.0 275.0 300.0 325.0 350.0 375.0 400.0 425.0 450.0 475.0 500.0 525.0 550.00.0
25.0
50.0
75.0
100.0
%
43
73
41
129
213157115 256185
281 429 503355327 462401314 550534
Fig. 5.11
MS of the peak obtained at 12.5 min in the analysis of the polluted river sample with the combinations of Alum, Muringa seed powder followed by chlorination
at low dose in the multistage reactor system
Bioreactor System and Nanotechnology for Water Treatment
211
The situation was even better in the case of nanotreated water where there
was no evidence of chlorination derived by products through FT/IR and GC/MS
analysis. But there was complete elimination of coliforms at a low dosage of
chlorination. (Fig.5.12 and Fig. 5.13)
Fig. 5.12
FT/IR analysis of the polluted water sample after integrated treatment of water sample with the combinations of Alum, Muringa seed powder, mild chlorination and followed by nanofilter treatment in the multistage
reactor system.
Chapter 5
212
10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 16.00.0
1.0
2.0
3.0
4.0(x1,000,000)
TIC
50.0 75.0 100.0 125.0 150.0 175.0 200.0 225.0 250.0 275.0 300.0 325.0 350.0 375.0 400.0 425.0 450.0 475.0 500.00.0
25.0
50.0
75.0
100.0
%
55
41
83
111
264125 180 222166303282 380341 490443402
Fig. 5.13
GC/MS analysis of the of the peak obtained at 11, 25 min of water sample after integrated treatment of water sample with the combinations of Alum, Muringa
seed powder followed by chlorination at low dose in the multistage reactor system.
5.5 DISCUSSION
The presence of chemical contaminants in the aquatic environment is of
significant concern to the society due to the possible health risks to humans, wild
life and domestic animals, The source of chemical contamination of surface waters
includes waste from industrial and domestic waste water plants, runoff from
Bioreactor System and Nanotechnology for Water Treatment
213
agricultural land and leachates from landfills and storage lagoons. The source of
water pollution of Pamba river water is also from similar sources. During the pilgrim
season there is huge inflow of human waste from partially treated sources besides
direct disposals. The river is also acting as a sink for a variety of organic pollutants
during the pilgrim season.
Water is used for several purposes by humans but the level of purity of the
water being consumed is very critical since it has a direct effect on human health.
Excessive turbidity in drinking water may represent a health concern as it can
provide food and shelter for pathogens and possibly promote excess growth of
pathogens in the distribution system. Although turbidity is not a direct indicator of
health risk, there is strong relationship between turbidity and coliform content.
Turbidity could be effectively removed by coagulation and flocculation followed by
sedimentation. This could also eliminate much of the suspended coliform from the
contaminated water and hence could also reduce the requirement of high dose of
chlorination.
Concern over the negative aspects of chlorination especially super
chlorination had emerged in the early 1970s. This came mostly as result of the
toxicity of residual chlorine to fish and other sensitive aquatic organisms and also
due to the accumulation of cancer causing trihalomethanes and other chlorinated
organics. Despite these environmental concerns, superchlorination of contaminated
drinking water system is still practised in many communities with and without
dechlorination. Chlorine is a strong oxidising agent and its application in water
treatment is likely to modify the chemical and biological nature of treated water.
Hence extreme care should be taken in chlorinating drinking water and every
Chapter 5
214
possibility should be evaluated for low dose chlorination minimising the formation
of chlorination derived byproducts. Hence in the present study an attempt was made
to evaluate the performance of an integrated treatment system using an online multi
stage reactor system.
The designing of the bioreactor itself was truly novel as it offered provision
for coagulation followed by filtration for the online treatment of the contaminated
water. The reservoir by its high water head offered good flow rate. Both the
control system and the treatment system were connected serially and offered
regulation for the flow of the water. At any time comparison could be made for the
effectiveness offered by the individual or the integrated system in treating the
water (Fig 5.1 to 5.9).
The length of the individual treatment unit and the diameter of the pipe
connecting the mixing and sedimentation unit were taken with a view to offer
maximum mixing inbetween. The designed system carried three such units offering
coagulation and sedimentation separately. Any type of treatment orienting
coagulation followed by sedimentation could be carried out with this system. In the
present attempt, the treatment strategies selected were low dose alum treatment, low
dose moringa seed treatment, low dose chlorination followed by nanofilter
treatment.
The effectiveness of aluminum and iron coagulants arises principally from
their ability to form multi-charged polynuclear complexes with enhanced adsorption
characteristics. The seed kernels of M. oleifera contain significant quantities of low
molecular-weight water soluble proteins that carry a positive charge. When the
crushed seeds are added to raw water, the proteins produce positive charges acting
Bioreactor System and Nanotechnology for Water Treatment
215
like magnets and attracting the predominantly negatively charged bacteria and
thereby sediments them. When the contaminated water was put into treatment with
alum, moringa seed powder followed by chlorination at low dose of 0.75 mg/l there
was reduction in both BOD and COD. The coliform count was also nil .But when
the strategies were repeated along with the introduction of nanofilter treatment at the
end, the reduction of both BOD and COD were enhanced much at the same dosage
of chlorination. The coliform count was also nil. Therefore the introduction of a
nanofilter treatment offered better treatment facility at the same dose of low
chlorination (Table 5.1 and 5.2).
The mechanism of the antimicrobial action of silver ions is not completely
known. However, the effect of silver ions on bacteria is linked with its interaction
with thiol group compounds found in the respiratory enzymes of the bacterial cells.
Silver binds to the bacterial cell wall and cell membrane and inhibits the respiration
process. In case of E-coli, silver acts by inhibiting the uptake of phosphate and
releasing phosphate, mannitol, succinate, proline and glutamine from the E-coli cell .
In addition, it was shown that Ag+ ions prevent DNA replication by binding to the
polynucleotide molecules, hence resulting in bacterial death. When all these positive
strategies were taken together for water treatment the results obtained were truly
encouraging. The complete removal of coliforms could be effected at a very low
dosage of 0.75 mg/l chlorination whereas the chlorination requirement was 30 mg/l
when taken as a single treatment strategy. 2 mg/l was strong enough to produce the
toxic chlorination derived products. Chlorination at a dosage of 0.75 was incapable
of producing any toxic halogenated compound which was evidenced by the
spectroscopic analysis (Fig. 5.10 to Fig. 5.13).
Chapter 5
216
In the FT/IR spectra of Fig. 5.10 and Fig. 5.12 there were no peaks at 2880
cm-1, 1258 cm-1 indicating the presence of aliphatic CH3, there was no peak at 1100
cm-1 representing CCl and there was no peak at 700 cm-1 representing residual
chlorine. All these representations were there when the contaminated water was
treated with chlorination alone at high rate of 30 mg/l (Fig.3.7 to 3.18). In the
GC/MS analysis also (Fig.5.11 and Fig.513) there was no representations
corresponding to halogenated alkanes.
The spectroscopic analysis and the values obtained in the analysis of BOD,
COD and MPN strongly suggested that the present treatment strategy adopted was
effective and was bringing coliform count to nil without affecting the organic load.
This strategy was highly useful as it could be applied online during the flow of
contaminated water; it could be scaled up easily and could be implemented
successfully in field trials.