||Welcome to Ask Tom!, a monthly column by our resident water treatment guru, Tom Keenan of
National Environmental Services Agency (NESA). Tom addresses the issues that bug you the most. And Tom knows!! With 35 years experience in providing environmental support services to public and private sector clients on a wide range of environmental issues. Tom has also co-authored and presented training courses on wastewater treatment systems.
articles visit the Ask Tom!
Practical Aspects of an Effluent
Guest article by Shrikant Deshpande, Thermax
Printer friendly PDF
One of the most important topic
being discussed all across the industry today is," The
Effluent Recycle". There are several reasons due to which it
becomes imperative for the industry to consider the recycle of the
wastewater generated by it. The reasons are normally of two types.
The internal reasons like future expansion needing more water for
various usages and external reasons like revision of Pollution
Control Board norms towards immediate or eventual, "Zero
Liquid Discharge". It is very common observation that the
fresh water available all around is being allocated to the
community consumption and industries are asked to manage on its
own. This leaves the choice of tapping a water source like river,
borewell or sea. There are fresh regulations in some areas for
restricting the withdrawal of water from river or deepwell.
It is observed that the lowering of
water table has reduced the availability of ground water and
deteriorated its quality. In some areas it is simply not
available. Because of such and other reasons today industries are
actively considering a recycle of wastewater. There are already
some effluent recycle installations in India and abroad working in
various industries. There is enough data on operation and
maintenance of such system. This paper is intended to share the
practical technical aspect of Effluent Recycle System.
It is very important to carry out
the Feasibility Studies of Effluent Recycle System right upfront.
The Feasibility Studies will normally consider the following
- Capital cost for ETP
modification, Recycle system and reject disposal.
- Evaluation need for recycle.
- Present WWTP Practices.
- Long term planning for water
consumption in future.
- Reject disposal arrangements.
The Effluent Recycle usually makes
an in-house, reliable, good quality additional water source
available for the industry. Hence, even if today it is not
required, the Effluent Recycle plant can be planned for future. It
is obvious that with time all the industries will be asked to go
for, "Zero Liquid Discharge". With present practices of
Effluent Recycle, the cost of Recycle water is higher than the
fresh water available. (It ranges from Rs. 20 - 30 per M3). But
the recycled water quality is usually much better for common
applications like boiler feed, cooling tower make up, some parts
Reject disposal is one of the most
important deterrents in the Feasibility study of recycle system
with Reverse Osmosis. The system generates the reject, which is
high in total dissolved solids, COD & BOD. Some of the ways of
disposal of reject are as follows:
- Mixing with other effluent to
dilute and disposal.
- Evaporation by solar ponds.
- Thermal Evaporation.
- Direct disposal to sea.
Presently, most of the industries
are using option 1 & 2. For reject disposal by solar ponds the
area requirement is one of the constraint (usually 5 mm / M3 /
day.). However, as this is one time investment with negligible
operation and maintenance cost, hence widely in use. The salt
after evaporation is filled in the bags and dumped at a suitable
Thermal evaporation is also
becoming popular. This is usually with evaporator &
crystallizer. The major advantage of Thermal Evaporation is
drastic reduction in solar evaporation pond size, thus needing
less area for overall reject handling system. At present the
capital cost of Thermal Evaporation is prohibitively high and
hence becomes one of the deterrents of feasibility of Effluent
There are some innovative
approaches being actively considered throughout the industries for
tackling the issues of reject disposal. In the industry where salt
is used as filler in process, the reject can be directly used,
hence consumed in house. It may be worthwhile for an industry to
check, if they can use the RO reject somewhere in there process.
The Evaporated salt from solar pond or thermal evaporator can be
sold to the other industries for their usage.
Many times the overall feasibility
of Effluent Recycle System does not fit in, in spite of careful
evaluation of all the options.
Factors for Successful Recycle
After the Feasibility Study is
over, it may be worthwhile to consider some of the factors, which
will affect the successful, recycle system. The major ones are as
- Existing ETP design:
Effluent Recycle system is usually after the tertiary
treatment. The design of existing ETP system is the most
important factor. It is high time that the ETP system
suppliers realize the difference in design of ETP for, "
meeting PCB norms for Disposal" and, " feed to
recycle system". The Recycle System requires the
consistency of parameters at all times. There are several
aspects of ETP design, which can be improved to achieve
consistent and better-treated effluent parameters. It is
recommended to recheck the existing ETP design and the trend
of parameters before going for Recycle.
- Recycle Design Basis: It
is recommended to carry out the analysis of treated effluent
several times in different seasons for varied production
practices and tabulate the results. It may be worthwhile to
separate a highly polluting stream to ETP to achieve
better-treated effluent parameter. Similarly, it may be
worthwhile to consider a not so contaminated stream directly
for recycle without feeding to WWTP. This exercise is
necessary to achieve the consistency of recycle feed
- Good Tertiary Treatment:
Many ETP's do not have adequate tertiary treatment. This needs
to be incorporated.
Use of Recycled Water
The recycling of treated effluent
does not necessarily need treatment by RO. The recycled effluent
without TDS reduction (if not required) can be considered for
usages like gardening, floor washing, toilets etc. It may become
worthwhile in long term to install a separate pumping and piping
system for this usage.
The recycle with Reverse Osmosis is
becoming more and more popular as the treated water quality is far
better for most of the critical application in the industry.
Essentials of a Good Recycle
In case Reverse Osmosis is part of
effluent recycle system the pretreatment of tertiary treated
effluent becomes very essential. Most of the Reverse Osmosis
membranes are delicate and prone to fouling by COD / BOD, oil and
grease, colloids, heavy metals etc, hence after careful evaluation
of the recycle feed water, the appropriate units must be
incorporated as a pretreatment to Reverse Osmosis. Pretreatment is
usually for the following parameters:
- COD & BOD: Most of
the ETP's are designed to reduce the BOD and COD up to the PCB
disposal norms (250 / 30). However, for Reverse Osmosis these
values have to be much lesser.
- Oil & Grease: The RO
membranes are prone to fouling / choking with oil and grease.
The desired value for RO feed is nil.
- Heavy Metals: They tend
to foul the membranes and affect irreversibly. This needs to
be reduced to nil.
- Dissolved/Colloidal Silica:
The dissolved silica is one of the major parameter in deciding
the recovery of Reverse Osmosis system. Hence it is preferable
to reduce it as low as possible. Colloidal Silica usually
slips from most of media filtration system hence needs to be
- Excessive Hardness: The
excessive hardness in the feed water limits the Reverse
Osmosis recovery severely; hence this needs to be removed
The success of a good recycle
system is in efficient pretreatment of various foulants. Hence it
is advised not to compromise on pretreatment system to reduce the
initial capital cost. An inadequate pretreatment will result in
enormous problems in future thus proving costlier.
Pretreatment Units for Recycle
With the advancement in technology
in water treatment over a period of time there are several options
available for pretreatment to Reverse Osmosis. A combination of
various units can be considered on case-to-case basis. Some of the
unit operations are as below:
- Media Filtration: A good
media filtration is very essential to reduce the suspended
parameters and colloids. There are options like sand
filtration, anthracite filtration or garnet filtration. The
design of the media filter for recycle pretreatment is usually
different from regular water treatment filters.
- Carbon Filtration: A good
activated carbon filter with high quality carbon goes a long
way in polishing the effluent and making it more suitable for
RO feed. Here the quality and size of carbon has to be
different from regular filters.
- Disinfectant by Oxidants:
It becomes inevitable to dose chlorine (liquid or gas), ozone,
H2O2 for reduction of organic matter. The dosages of these
chemicals need to be correctly selected. The dosing equipment
should be adequately sized and provided with necessary
interlocks and alarms. The retention time for adequate
reaction of oxidant is very essential for good results.
- Advanced Oxidation Technology
(Photo Chemical Oxidation): There are some evidences of
successful usage of advance oxidation technology for reduction
of COD. However, in absence of adequate data these usually
needs pilot testing.
- Organic Scavenger (Resin
Based): It is observed that the incorporation of resin
based organic scavenger after the filtration is effective in
further polishing of organic matter and reduction of SDI.
There is enough past data for use of this technology for
- Reactivator-Type Clarifier:
A sludge recirculation type clarifier is being increasingly
used as the first unit in a recycle system. This helps in
reduction of dissolved silica and hardness, precipitation of
heavy metals and other ions and control of colloids in
addition to effective removal of suspended matter. This also
helps in reduction, of organic impurities associated with
- Ultrafiltration (Dead End
Tubular): Popularly known as "UF". It is
becoming the best choice as a pretreatment for recycle RO
system. This effectively reduces the colloids and organic
matter and helps in maintaining the constant SDI to RO feed.
This enhances the life and operating cost of RO substantially.
The tubular UF systems are usually back-washable hence does
not get fouled easily. If UF is incorporated in pretreatment,
the carbon filtration, micron filtration etc can be
eliminated. There are certain evidences that due to the
constant good quality of water after UF the RO block size can
The UF is becoming popular in India
recently and is being actively offered by various OEM's in the
Water Treatment Industry. The capital cost of the UF system is
presently high due to heavy import components but is likely to
reduce over a period of time.
The RO block for an effluent
recycle system needs to be designed differently as compared to
normal brackish water. It is understood that in spite of all the
precautions in pre treatment some of the impurities will slip into
the RO block thus causing fouling to the membranes. To make the RO
block safer in design following factors needs to be considered.
- Surface Area: Adequate
membrane surface area to be provided (GFD of 8- 10).
- Choice of Correct Membrane:
The RO membrane should be fouling resistance, smooth surface
with enough feed spacer thickness. The past data on a
particular membrane module for similar application must be
collected. It is also important that the membrane selected is
"Tough". It should be able to regain its performance
ever after repeated cleaning.
- Membrane Cleaning/Flushing
System: It is very essential to have adequate membrane
flushing and cleaning system. This helps in keeping the
membrane surface clean and extend the life. Additional
disinfectant dosing system can also be incorporated.
- Control & Interlocks:
IT is essential to provide the system with adequate
instrumentation, controls, interlocks to facilitate the smooth
operation and timely corrective actions.
In addition to all the above good
operation and maintaining practices shall go a long way in success
of an Effluent Recycle System.
About the Author
Mr. Deshpande is a Chemical
Engineer with 15 years experience in water treatment, RO &
effluent recycle. Presently he is working with Thermax. He
can be contacted e-mail address: firstname.lastname@example.org
Help others by posting
your comments, suggestions and experiences with water or
wastewater treatment or any other concerns you may have on
our On-Line Help Forum. For
past Ask Tom! Articles, visit the Ask Tom!
Guest articles for the
Ask Tom! Column are always welcome, for more information please
contact Tom Keenan directly at his email address: email@example.com