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UNIT 4

TERTIARY TREATMENT

The purpose of tertiary treatment is to provide a final polishing treatment stage prior to
discharge or reuse of the wastewater.

Chlorination

A water treatment method that destroys harmful bacteria, parasites, and other organisms.
Chlorination also removes soluble iron, manganese, and hydrogen sulfide from the water.

Ozonation

A water treatment process that destroys harmful bacteria and other microorganisms through
an infusion of ozone. Ozone (O3) is a gas created when oxygen molecules are subject to high
electrical voltages.

Ultraviolet radiation

A disinfection process for water and wastewater treatment that involves passing Ultraviolet
(UV) light through water. UV light destroys microorganisms and can reduce dissolved
organic material.

Activated carbon absorption

A physical process that is typically applied as tertiary treatment to remove low concentrations
of contaminants from water that are difficult to remove by other means. Activated carbon has
been processed to make it extremely porous, thereby creating a very large surface area
available for adsorption of contaminants. Activated carbon may have a surface area as great
as 1500 m2/g (7.3 million ft2/lb).

Ion exchange

Ion exchange is a reversible chemical reaction used to remove ions from water and
wastewater. An ion in solution, such as ammonium, copper, calcium, magnesium, and many
others, is exchanged for a similarly charged ion attached to an immobile solid ion exchange
particle. These solid ion exchange particles are either naturally occurring inorganic zeolites or
synthetically produced organic resins
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EVAPORATION (SOLAR AND STEAM)

Evaporation is being considered as an alternative process in an increasing number of

wastewater treatment applications. It can be effective for concentrating or removing salts,
heavy metals and a variety of hazardous materials from solution. Also, it may be used to
recover useful by-products from a solution, or to concentrate liquid wastes prior to additional
treatment and final disposal. Most applications of the technology also produce a high quality,
reusable distillate-a very important feature where water conservation is a priority.

During evaporation, a solution is concentrated when a portion of the solvent, usually water, is
vaporized, leaving behind a saline liquor that contains virtually all of the dissolved solids, or
solute, from the original feed. The process may be carried out naturally in solar evaporation
ponds, or through the use of commercially available evaporation equipment.
Solar evaporation ponds usually are limited by land availability and cost, potential odor
problems, or meteorologic and climatological conditions, whereas mechanical evaporators
are relatively compact, reliable and efficient.

Design and Operation

The evaporation process is driven by heat transferred from condensing steam to a solution at
a lower temperature across a metallic heat transfer surface. The absorbed heat causes
vaporization of the solvent, usually water, and an increase in the solute concentration. The
resulting vapor may be vented to the atmosphere, or condensed for reuse.

Mechanical evaporation is an energy-intensive way to concentrate liquids, and various energy

alternatives should be considered in the selection of the most efficient evaporator. In an ideal
system, one kilogram of condensing steam will evaporate one kilogram of water from the
solution. Such a system has a steam efficiency, or economy, of 1:1 (1 kg of water removed
for every kg. of steam applied). A simple evaporator system (Fig 1) has a single evaporation
chamber, or effect, and is said to have an "economy of one."

Evaporator economy can be increased by increasing the number of effects. A multiple effect
system (Fig 2) uses the vapor from the first effect as the steam source for each subsequent
effect. As the temperature decreases in each succeeding stage, evaporation continues because
the pressure and boiling point also are reduced.

The use of each additional effect increases the system's energy efficiency. For example, a
double-effect evaporator requires approximately 50 percent of the steam consumed by a
single effect unit, and has a theoretical economy of 2. The number of effects can be increased
to the point where the capital cost of the next effect exceeds the savings in energy costs.
The use of vapor compression is another proven technique for reducing energy input. In this
approach (Fig 3), vapor discharged from the evaporator chamber is compressed to the
pressure/temperature values required in the heat exchanger.

Mechanical compressors are used most frequently for accomplishing vapor compression.
Compressors may be of the positive displacement, centrifugal, or axial type. An evaporator
system using mechanical vapor compression often will require only an outside steam source
to initiate operation. This usually can be supplied by a small boiler or resistance heater in the
evaporator feed tank. A steam jet thermal compressor using high pressure steam also may be
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considered. The use of a thermal compressor is approximately equivalent to adding an

additional evaporator effect.

When available, waste heat from other process streams also may be captured to lower
evaporation costs. For example, hot process fluids may be pumped through the heating tubes
instead of steam, recovering heat and transferring it to the fluid to be evaporated, or energy
from hot flue gases can be converted to steam in a reboiler and subsequently used in an
evaporator.

Types of Evaporators

Evaporators can be categorized according to the arrangement of their heat transfer surface
and the method used to impart energy (heat) to the solution. Some common types of
evaporators include

Vertical tube falling film: Recirculating liquid is introduced at the top of a vertical tube
bundle and falls in a thin film down the inside of the tubes. The liquid absorbs heat from
steam condensing on the outside of the tubes and the water in the liquid is vaporized. This
type of evaporator usually is selected for higher viscosity liquids and for concentrating heat-
sensitive solutions that require low residence times.

Horizontal tube spray film: Recirculating liquor is heated and sprayed over the outside of a
horizontal tube bundle carrying low pressure steam, condensing water vapor inside the tube.
Vapor from the evaporator chamber can be used as steam in a subsequent effect, or
mechanically compressed and reused as the heating medium in the stage where it was
generated.
Scale forming on the outside of the tubes can be removed periodically through chemical
cleaning. Horizontal tube designs can be applied in locations with low headroom
requirements, and are especially beneficial in indoor installations.

Forced circulation: Recirculating liquor is pumped through a heat exchanger under pressure
to prevent boiling and subsequent scale formation in the tubes. The liquor then enters a
separator chamber operating at a slightly lower pressure or partial vacuum, causing flash
evaporation of water, and formation of insoluble crystals in the liquor (Fig 6).
Forced circulation evaporators, or crystallizers, are often used for applications requiring high
solids concentration or crystallizing, or in applications involving large amounts of suspended
solids. Energy costs for forced circulation units can be more than for other evaporation
systems because of their high recirculation rates.

Combined and hybrid systems: Combining different types of evaporators, or combining them
with other processes to reduce capital and operating costs, or meet specific treatment
objectives, often is possible. One fairly common arrangement uses a falling film evaporator
followed by a forced circulation crystallizer. In this scheme, an evaporator concentrates the
wastewater stream to 20 to 30 percent solids, and a crystallizer further concentrates it to a
solid. Energy costs may be reduced by using steam vented from the evaporator to operate the
crystallizer.

Hybrid designs are becoming more common in zero liquid discharge applications. A hybrid
system may consist of an evaporator or evaporator/crystallizer preceded by a reverse osmosis
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or electrodialysis preconcentration step. The concentrate, or reject, from the preconcentrator

becomes the feed for the evaporator.

Although a hybrid system adds complexity, it can reduce the size of the evaporator unit, as
well as the system's energy needs. But note that not all wastewaters, especially those with
high scaling tendencies, are candidates for hybrid systems. Table 1 shows the energy savings
obtained by selecting an 85 gpm hybrid system.

Evaporator Applications
Because evaporation is an energy- and capital-intensive process, the selection and design of
an evaporator system must be carefully considered for each application.

Evaporators have been used successfully in many industrial wastewater treatment

applications, e.g., power and chemical plant wastewaters, metal finishing wastes, spent pulp
liquors, emulsified oil streams, high soluble BOD (sugar) streams, and nonvolatile aqueous
organic or inorganic streams containing dyes, acids and bases.

Zero liquid discharge: Federal, state and local regulations governing industrial wastewater
discharges continue to become more stringent. All direct dischargers must obtain a National
Pollutant Discharge Elimination System (NPDES) permit that sets the maximum permissible
limit for regulated pollutants. NPDES permits are subject to renewal, and permitted discharge
levels may be lowered to reflect changes in the receiving water body. Dischargers therefore
must take into account their wastewater production as well as possible future variations.

In many industrial plants, evaporators can be installed to achieve zero liquid discharge of
wastewater. These systems often consist of a falling film evaporator followed by a
crystallizer and filter press (Fig 7). A rotary spray dryer may take the place of a crystallizer.
However, precautions must be taken to control fugitive dust emissions. Figure 7 illustrates
the water balance for a hybrid zero liquid discharge system used to treat a power plant's
cooling tower blowdown.

Water reuse: In this area evaporation has several advantages over conventional physical-
chemical processes, one of the most significant being the high quality of the distillate. Most
installations can produce a distillate TDS of less than 10 mg/l, and in some cases, less than 2
mg/l.

Not only does the recovered water from an evaporator meet most discharge specifications, it
can almost always be recycled for reuse in manufacturing or cooling applications. In one
metal finishing installation, distillate was recycled as process rinse water at a volume ten
times less than the quantity of city water required to do the job.

Evaporators can minimize the production of regulated waste residues, and increase the
potential for recovering valuable metals from those wastes. Unlike ion exchange, evaporation
is not as sensitive to traces of oil, and does not produce regeneration wastes that require
additional treatment.
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ADVANCED OXIDATION SYSTEM

In many water and wastewater treatment applications, there are a number of pollutants that
are difficult to reduce by physical, chemical, or biological means alone. In more recent years,
there has been a growing concern regarding pharmaceutical drugs in drinking water and
aquatic environments. Pesticides get caught in runoff from farms into freshwater supplies.
Personal care products are typically washed down the drain into whatever system they are
linked to. Landfill leachate is a toxic cocktail of compounds that can leak into groundwater
sources. Such contaminants fall into the category of micropollutants, because they are so
small. Their size alone is part of the reason, they are so difficult to remove from water and
wastewater by certain means. More efficient removal requires a more powerful oxidation
process, this process is called an advanced oxidation process (AOP).

This process creates powerful oxidizing agents in the form of hydroxide (OH–), but more
specifically, its neutral variant the hydroxyl radical (⦁OH). Its oxidation potential is twice that
of chlorine, a commonly used disinfectant. Hydroxyl radicals are the driving forces behind
many advanced oxidation processes. Ozone (O3), hydrogen peroxide (H2O2), and ultraviolet
light (UV) are often used in various combinations to produce ⦁OH in sufficient quantities to
degrade organic (and some inorganic) pollutants. This process can reduce these pollutant
concentrations, potentially from hundreds of parts per million (ppm) to just a few parts per
billion (ppb).

These radicals are non-selective, therefore, they attack almost all organic materials. After
these contaminants are broken down once by the ⦁OH radical they form intermediates. Those
intermediates themselves react with the oxidants and mineralize into stable compounds.
Advanced oxidation has been around for several years. Therefore, this process has more than
proved its usefulness, however, it is still being researched and optimized accordingly.

A powerful treatment process like the advanced oxidation process has many benefits, but it
also has its share of disadvantages.

Here are just a few of the pros and cons of this particular process:

Pros
 Rapid reaction rates
The OH molecule has some of the fastest reaction rates of all of the oxidants used to treat
water and wastewater due to its high oxidation potentials and their non-selective nature.
These quick reactions result in much lower retention times than other conventional treatment
processes.

 Small footprint
Because of the oxidation power of the ⦁OH radical, Advanced oxidation process units do not
require much land area to process the needed flow rate for the system.

 Theoretically, do not introduce new hazardous substances into water

One of the issues with chlorine disinfection is the highly toxic byproducts (DPB’s) that can
result after treatment. To prevent these byproducts, an extra de-chlorination step is often
required before anything else can be accomplished with the treated water. The ⦁OH molecule
can combine to create water. The biggest issues would be with bromate formation and excess
peroxide, but these can be dealt with in a well-designed advanced oxidation process system.
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 Mineralization of organics
AOP can convert the organic materials within the water into stable inorganic compounds like
water, carbon dioxide, and salts.

 Can treat nearly all organic materials and can remove some heavy metals
The highly reactive nature of ⦁OH means these molecules will attack almost any organic
materials without discriminating, and therefore, can remove many different contaminants in
one reactor vessel, including reducing a few heavy metals.

 Can work for disinfection

Especially when used with UV disinfection, the oxidation power of AOP systems make them
capable of acting as a disinfection step for any pathogens that may be present in the water.

 No sludge production as with chemical or biological processes

An advanced oxidation process does not treat water and wastewater by transferring pollutants
into another phase. Other treatment processes create solids like sludge that need to be filtered
out and dealt with separately.

 Does not concentrate waste for further treatment

Treatment solutions such as membranes result in increased concentrations of the waste
contaminants, since they merely separate clean water from the pollutant compounds. AOP
meanwhile directly reacts with the pollutants and reduces them to harmless compounds. This
process therefore, decreases their concentrations in the effluent.

Cons

 Relatively high capital and operating/maintenance costs

Perhaps the biggest drawback of the AOP process is its costs. The most significant are the
operating and maintenance costs from the required energy and chemical reagents to operate
the system.

 Complex chemistry tailored to specific contaminants

Advanced oxidation processes have several different variants. These variants need to be
carefully selected to efficiently treat the water/wastewater in question. This process is also a
dosage dependent process, so the appropriate amounts of ⦁OH molecules are formed to
achieve the desired level of treatment. Such complex chemistry, will likely need highly
skilled engineers to design the system correctly.

 Removal of residual peroxide may need to be considered

Advanced oxidation process systems utilizing hydrogen peroxide should be carefully
controlled for residual H2O2 as it can have potential negative effects on later treatment steps.
This residual hydrogen peroxide may be harmful to humans. However, careful design of the
system can prevent excess residual H2O2 and any associated consequences.
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Oxidation is the driving force for many water and wastewater treatment solutions. One
solution, in particular, is designed around maximizing the oxidation potential of the system.
This particular process is the AOP process.

The advanced oxidation (AOP) process, dubbed so by William Glaze and company in 1987,
typically refer to processes that produce hydroxyl radicals (⦁OH). These radicals are the
primary oxidants that carry out this process, breaking down compounds into intermediates
and then mineralizing those intermediates into simple compounds like water, carbon dioxide,
and salts. There are several different ways these radicals can be produced. In general, these
AOP molecules are formed as particular compounds degrade, compounds such as ozone (O3)
and hydrogen peroxide (H2O2) in particular. Other components, like ultraviolet light (UV),
are used as catalysts in the reaction to encourage the compounds to break down accordingly.
However, as with many water/wastewater treatment solutions, there are upsides and
downsides to the different options. Therefore, it is important to carefully choose an AOP
process that will work best with your particular application. How do you make such a choice?
By knowing some of the basic information of each AOP process. First, it would help to know
a bit about some of the options available in each AOP process and how these options work.

Types of AOP:

Ozone
Ozone interacts with hydrogen-containing compounds and decomposes in a series of steps to
reduce to ⦁OH radicals in an alkaline solution. It can be used on its own as an AOP, typically
at higher pH levels due to the abundance of hydroxide ions present. O3 itself is also a
powerful oxidant and acts as a secondary oxidizer in the overall process, though the reactions
are much slower. If used, O3 must be generated on site and used quickly, as it has a very short
half-life. In addition, if bromide is one of the contaminants in the wastewater, there is
potential for the formation of bromates molecules, which are highly toxic.

Hydrogen Peroxide
Hydrogen peroxide cannot be used as a standalone oxidation treatment like ozone. It is not as
strong of a secondary oxidizer as O3 but it can react with hydrogen and oxygen-containing
compounds in a less complex process than ozone. It does not need to be produced on site, but
it does need to be kept in careful storage as it is unstable. H2O2 also needs to be monitored for
residuals leftover after treatment. The compound can be toxic to humans, so it may need to be
treated for.

Ultraviolet Light
Ultraviolet light is used quite commonly as a disinfectant, for its ability to kill or prohibit the
reproduction of a number of pathogens. As it is merely a wavelength of light, UV is not itself
an oxidant, but it transfers massless photons to chemical compounds; breaking their bonds
quickly and easily. However, being light driven, some contaminants including suspended
solids can reduce the efficiency of UV interaction by blocking it from the target compounds.

Combinations
Most often, the treatments above are used in some combination with one another: O3/UV,
O3/H2O2, H2O2/UV, O3/H2O2/UV. These combinations use the strengths of these individual
processes to improve the efficiency of the overall AOP process. However, each combination
does have some of drawbacks, therefore, the optimized process is chosen based on
application.
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Next, it is pertinent to mention what considerations must be taken into account when
choosing the correct AOP process for a particular application.

Things to consider:

Water composition
Perhaps most obviously, the composition of the influent water should be carefully considered.
AOP is a heavily involved chemical process, and therefore, decisions on what process to use
are dependent on what particular pollutants are within the water to be treated.

Treatment goals
Environmental regulations or reuse considerations determine how much the water/wastewater
needs to be treated. More lax standards may only need a simple process, while stricter
standards require something a little stronger.

UV dose
As with the typical UV disinfection process, an appropriate amount of UV exposure is
required to achieve the desired results without drawing unnecessary levels of energy making
this process uneconomical.

Chemical dose
In order to achieve an acceptable concentration of ⦁OH radicals, sufficient doses of O3 and/or
H2O2 need to be added in order to do that. Again, this is a chemistry intensive process and
chemistry demands proper dosing or else you will not get the desired results.

Energy consumption
The AOP process can be a fairly energy intensive system in some cases, certain system
configurations or applications requiring more than others.

Cost
AOP process systems tend towards higher costs in general, but certain systems do cost more
than others. The most significant costs deal with operational aspects to keep up with the need
for input chemicals and energy based on contaminant levels.

Lastly, you may require the assistance of a process engineering firm that specializes in water
treatment. In many cases, choosing the proper system can fall to past experience. It can be
difficult to tell how a system will work just based on theory. It can take hours of research and
testing, and an engineer may have dealt with all of that in a previous, yet similar application.

So, carefully consider your options and how they may apply to you. After doing this, find a
firm who has done work on a similar project before and they can potentially advise you on
how it might work out.
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MEMBRANE TECHNOLOGY
Membrane separation processes cover an astonishingly wide range of separation, using
different types of membranes. There are five types of membrane separation processes, for
water purification:
1. Microfiltration
2. Ultra‐filtration
3. Nano‐filtration
4. Reverse osmosis
5. Electrochemical separation
All the separation processes separate solute from solution based on their molecular size. All
the membrane technologies are essentially separation technologies depending upon size of
the constituent to be separated or on the ionic charge or absence of charge, solution diffusion
in to the matrix or some such transport phenomenon. None of the filtration technologies
destroy the pollutants.
1. Microfiltration This technology is useful in separating suspended particle up to 0.1
micron or above.

2. Ultra‐filtration this is the size exclusion based pressure driven membrane separation
process range of 10‐100 Å.
Typical rejected species or constituents includes sugar, bio‐molecules, polymers, colloidal
particles and high molecular weight organic substances depending upon their molecular
weight, molecular size and also shape.
These membrane are classified according to their molecular weight cut off (MWCO),which is
usually defined as smallest molecular weight species for which membranes have more than
90% rejection.
Different available configuration of UF membrane are:
1. Flat membrane in plate and frame structure
2. Tubular
3. Spirally wounded module
4. Hollow fibretype
Spirally wound module are the most commonly used.
3. Nanofiltration The pore size of this membrane is maller than Ultra
Filtration membrane.
The organic compounds bearing very low molecular weight linear chain structure are rejected
while monovalent cations combined with monovalent anions to form compound or salts pass
through the permeate. The divalent cations or anions do not pass though the permeate. This
property of rejection due to ionic charges is made use in the softening of water for the various
applications. The molecular weight cut for such types of membranes is lower than that of UF
membrane.
Applications of Nanofiltration
1. Decoloringof effluents and removal of spent mineral acids used to scavenge organics and
heavy metal impurities.
2. Heavy metals are rejected preferentially by Nanofiltration membrane, purification of acid
and base has become economically possible due to Nanofiltration.
3. Dye factories effluent contains highly concentrated dye, salts and some acids.
Nanofiltration can very effectively separate the dye and concentrate it to. This way of
concentration and purification reduces the loss for dye thereby effecting a reduction in
ETP load.
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REVERSE OSOMOSIS

Reverse osmosis reverse osmosis is a membrane technology used for separation.

osmosis: when more concentrated solution is separated by semi permeable membrane flow of
less concentrated solution towards the more concentrated solution takes place due to
difference in osmotic pressure of two solution.
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 In a typical RO system the solution is first filtered through a rough filter like sand or
active carbon, or dual filter etc.
 If solution contains Ca++, Mg salts, iron, carbonates, then acid dosing system is
introduced.
 The pH is adjusted and the solution is then filter through micro cartridge filter (5-10
micron).
 The pretreated water is then pumped in to the RO tank with a high pressure pump.
 The membrane separates the pollutants in concentrated form in the reject stream and the
pure water is collected as a permeate.

Applications

RO is more useful to separate salts and organic compounds from textile effluents. Some
of the wastewater varieties from textile industry that can be treated by RO for recovery of
reusable water such as:

1. Rayon industry process wastewater.

2. Textile dyes house effluent. Up to 80% of warm dye house wastewater can be recovered
for recycle by RO membrane.
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ION EXCHANGE

Ion Exchange can be used in wastewater treatment plants to swap one ion for another for the
purpose of demineralization. There are basically two types of ion exchange systems, one
which is using the anion resins and another is the cation exchange resins. The materials can
be further broken down to individual grouping depending on whether it is a strong base
cation, weak acid cation, strong base anion, or weak base anion. Regardless of which type of
exchangers, the resins can very sensitive to fouling caused by presence of organic matter in
the untreated water. Thus it is imperative that before the influent undergoes the treatment
process, another separate section is needed to remove most of the suspended solids and if
possible the soluble organics as well so that it will put lesser loads towards the ion exchange
unit.

A typical scenario on how the whole process works can be simplified in the example given
here. For instance, in a common demineralizer, influent water which passes through a cation
exchange resin will be stripped off of it metallic cations salt to become acids whereby loss of
the ions will be replaced with a similar corresponding amount of hydrogen ions. The resultant
acids will then be removed through another alkaline regenerated anion exchange resins in
which this time round, the anions present in the wastewater will be substituted with
equivalent amount of hydroxides.

As the capacity of the bed always has a certain fixed limit, eventually the resin will become
exhausted and thus has to undergo regeneration process. The cation exchange resin is
regenerated using either hydrochloric or sulfuric acid, producing waste brine in the process
while the anion resin will be regenerated with sodium hydroxide.

Individual ion exchange bed, other than grouped together according to its functional groups
can also be divided into different types based on its solubility. Naturally for it to be effective,
the materials must be insoluble under normal conditions and for that to happen, usually high
molecular weight compound has to be selected. Bead size is also another criteria that
determines the type of resin suitable for general application. Basically a certain granular size
is needed so that the molecules that form the whole structure do not hold on together too
compact and there must be a certain void volume to prevent massive liquid head loss. The
characteristics on how the beads are arranged will also affect the ion exchange bed physical
durability whether it is good enough to withstand expansion and contraction due to change of
temperature without risk of bursting or collapsing taking place.
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One more thing to take note is that depending on the procedures used for backwashing and
especially in cases whereby there is an uncontrolled high flowrate caused by mishandling,
this can lead to blowoff of resins coming out from the vessel and resulting in capacity loss of
the beds. Therefore, typical attrition losses can be expected to be within 5 to 20% depending
on which type of ion exchange resin used in the systems.

Ion exchange softening, also known as Sodium Zeolite Softening is a typical example of how
the ion exchange process works to purify the water by removing the hardness level caused by
presence of calcium and magnesium. This is normally used in steam boilers and industrial
water treatment applications. When the bed gets completely saturated with Ca and Mg ions, it
has to be regenerated mainly by increasing the amount of sodium ions. This is done by first
performing backwashing to minimize compactness of the bed and to release trapped air
pockets. After that, it will be followed by the actual regeneration process using brine solution.
Once this is completed, rinsing will be carried out to remove whatever excess brine left in the
exchange bed. In modern designs, all these operations are performed using automated
programs which can detect the level of saturation and prompt the operators to act
accordingly. Overall, depending on the influent characteristics, this will determine the
frequency needed to carry out the whole cycle of regeneration.

A typical scenario on how the whole process works can be simplified in the example given
here. For instance, in a common demineralizer, influent water which passes through a cation
exchange resin will be stripped off of it metallic cations salt to become acids whereby loss of
the ions will be replaced with a similar corresponding amount of hydrogen ions. The resultant
acids will then be removed through another alkaline regenerated anion exchange resins in
which this time round, the anions present in the wastewater will be substituted with
equivalent amount of hydroxides. As the capacity of the bed always has a certain fixed limit,
eventually the resin will become exhausted and thus has to undergo regeneration process. The
cation exchange resin is regenerated using either hydrochloric or sulfuric acid, producing
waste brine in the process while the anion resin will be regenerated with sodium hydroxide.
 Page 14 of 16

ACTIVATED CARBON TREATMENT

Introduction
Activated carbon is a material used to filter harmful chemicals from contaminated water and
air. It is composed of black granules of coal, wood, nutshells or other carbon-rich materials.
As contaminated water or air flows through activated carbon, the contaminants sorb (stick) to
the surface of the granules and are removed from the water or air. Granular activated carbon
or “GAC” can treat a wide range of contaminant vapors including radon and contaminants
dissolved in groundwater, such as fuel oil, solvents, polychlorinated biphenyls (PCBs),
dioxins, and other industrial chemicals, as well as radon and other radioactive materials. It
even removes low levels of some types of metals from groundwater.

Preparation of activated carbon

The medium for an activated carbon filter is typically petroleum coke, bituminous coal,
lignite, wood products, coconut shell or peanut shell. The carbon medium is “activated” by
subjecting it to stream (a gas like water, argon or nitrogen) and high temperature (800-
1000°C) usually without oxygen. In some cases, the carbon may also undergo an acidic wash
or be coated with a compound to enhance the removal of specific contaminants. The
activation produces carbon with many pores and a high specific surface area. It is then
crushed to produce a granular or pulverised carbon product.

Working
Activated carbon treatment generally consists of one or more columns or tanks filled with
GAC. Contaminated water or vapors are usually pumped through a column from the top
down, but upward flow is possible. As the contaminated water or air flows through the GAC,
the contaminants sorb to the outer and inner surfaces of the granules. The water and air
exiting the container will be cleaner. Regular testing of exiting water or air is conducted to
check contaminant levels. If testing shows that some contaminants remain, the water or air
may need to be treated again to meet the treatment levels.

The GAC will need to be replaced when the available surfaces on the granules are taken up
by contaminants and additional contaminants can no longer sorb to them. The “spent” GAC
may be replaced with fresh GAC or “regenerated” to remove the sorbed contaminants. To
regenerate spent GAC, it is usually sent to an offsite facility where it is heated to very high
temperatures to destroy the contaminants. If a lot of GAC needs to be regenerated, equipment
to heat the GAC and remove the sorbed contaminants can be brought to the site.
 Page 15 of 16

Depending on the site, treated groundwater may be pumped into a nearby stream or river or
back underground through injection wells or trenches. At some sites, a sprinkler system can
distribute the water over the ground surface so that it seeps into soil. The water also may be
discharged to the public sewer system for further treatment at a sewage treatment plant.

Time
It only takes a few minutes for water or vapors to pass through an activated carbon filter.
However, the time it takes to clean up a site with activated carbon treatment will depend on
how long it takes to bring all the contaminated groundwater or contaminant vapors to the
ground surface for treatment. This can take several months to many years. Treatment may
take longer where:
• Contaminant concentrations are high or the source of dissolved contaminants has not been
completely removed.
• The volume of contaminated groundwater or vapors is large.
• Treatment of groundwater or vapors involves several other cleanup methods. These factors
vary from site to site.

Safety
Activated carbon treatment is safe to use. Treated water is sampled and analyzed regularly to
ensure that the carbon continues to adequately sorb contaminants. If concentrations start to
increase in the treated water, the carbon is reactivated or replaced. The tanks are cleaned or
replaced with care to avoid releasing contaminants. Larger filters are often preferred because
they do not have to be replaced as often as small ones. When treatment is complete, the used
carbon may contain hazardous contaminants that require special handling and disposal at a
hazardous waste facility.

Activated carbon treatment generally will not disrupt the surrounding community. Initial
construction of systems to extract groundwater or contaminant vapors from the ground may
involve the use of heavy equipment. This may cause a temporary increase truck traffic in the
neighborhood as equipment is brought to the site or when carbon tanks are exchanged.
However, the treatment system itself is not particularly noisy while running. Depending on
the amount of groundwater or vapors that need to be treated, tanks of activated carbon can
range in size from a 55-gallon drum to a tank that is 20 feet tall and 10 feet or more in
diameter.
 Page 16 of 16

QUALITY PARAMETERS AT ENTRY AND EXIT OF RO

Table summarizes the limits of quality parameters at entry and exit of reverse osmosis. It is
recommended to respect these limits to ensure successful operation of the membrane system.
Otherwise, more frequent cleaning and/or sanitization may become necessary. The
concentrations correspond to the entry to the membrane for a continuous feed stream,
including any influences to the feed water from dosing chemicals or piping materials in the
pretreatment line.

feed water at entry (RO feed)

product water (RO permeate)
small quantity of wastewater (RO reject)
publicly owned treatment works (POTW)

feed water at entry (RO feed)

treated water at exit (After RO)
purified water (PW)
WFI [water for injection]