Water Recycling at GIT: A Chemical Engineering Perspective
Smruti Vichare1,Afroz Momin2,Somnath Aghav3
Gharda Institute of Technology,
Lavel.
Abstract:
Gharda Institute of Technology located in rural parts of konkan suffers from serious water shortage in summer months. This problem
is likely to accentuate as a student population on the campus grows. In the forgoing article the water requirement and breakup of waste
generated is critically analyzed. Based on this blueprint for recycling, major part of waste water is framed. This analysis is based on
chemical engineering principles and incorporates latest technologies to address this issue in a cost effective manner. The substitute for
conventional method to treat waste water is the new technology of using potassium ferrate which is economical.
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Pattern of water usage:
As per WHO standards water requirement per person per day is 130lit-150lit. As per estimation out of 150lit/day, 10lit is used for
cooking and drinking which has to be of highest quality, 10lit/day is used for sewage and remaining is used for washing, bathing, and
other miscellaneous needs.
Basis: 150 liter of water per person per day.
Use pattern
•
Cooking
Quality of water
Quantity
of Quantity
of Pollution load
water used
water rejected
Lit/day
Lit/day
Highest
10
Nil
nil
Low
10
10
Highest(10%
and
drinking
•
Sewage
solid)
•
Washing
Moderate
130
130
Low
and bathing
(0.5%
solid)
Table:1
The present practices the sewage is combined with washing and bathing water which after treatment in septic tank is used for
agriculture. This is against established chemical engineering principles, the pollution has to be treated at the source.
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The present practice world over is, first we allow the polluted water to be mixed with large body of good quality water like rivers,
lakes, seas and try to take care of water pollution by dilution at subsequent natural
processes. As against this, the best approach would be to segregate waste water streams on the basis of pollutant load and each stream
be treated separately and then put to best possible use. This approach will reduce the energy and efforts involved in the treatment at
least possible cost. Their are step wise procedure for the treatment of waste water, which is as follows:
Pre-treatment:
Pre-treatment removes materials that can be easily collected from the raw waste water before they damage or clog the pumps and
skimmers of primary treatment clarifiers.
Screening:
The influent waste water is screened to remove all large objects carried in the waste water stream. This is most commonly done with
an automated mechanically raked bar screen in modern plants serving large populations, whilst in smaller or less modern plants a
manually cleaned screen may be used. The raking action of a mechanical bar screen is typically paced according to the accumulation
on the bar screens and/or flow rate. The solids are collected and later disposed in a landfill or incinerated. Bar screens or mesh screens
of varying sizes may be used to optimize solids removal. If gross solids are not removed they become entrained in pipes and moving
parts of the treatment plant and can cause substantial damage and inefficiency in the process.
Grit removal: Pre-treatment may include a sand or grit channel or chamber where the velocity of the incoming wastewater is adjusted
to allow the settlement of sand, grit, stones, and broken glass. These particles are removed because they may damage pumps and other
equipment. For small sanitary sewer systems, the grit chambers may not be necessary, but grit removal is desirable at larger plants.
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Primary treatment:
In the primary sedimentation stage, sewage flows through large tanks, commonly called "primary clarifiers" or "primary sedimentation
tanks." The tanks are used to settle sludge while grease and oils rise to the surface and are skimmed off.
The dimensions of the tank should be designed to effect removal of a high percentage of the floatables and sludge. A typical
sedimentation tank may remove from 60 to 65 percent of suspended solids, and from 30 to 35 percent of biochemical oxygen demand
(BOD) from the sewage.
Secondary treatment:
Filter beds (oxidizing beds): Trickling Filter
In older plants and those receiving variable loadings, Trickling Filter beds are used where the settled sewage liquor is spread onto the
surface of a bed made up of coke (carbonized coal), limestone chips or specially fabricated plastic media. Such media must have large
surface areas to support the biofilms that form. The liquor is typically distributed through perforated spray arms. The distributed liquor
trickles through the bed and is collected in drains at the base. These drains also provide a source of air which percolates up through the
bed, keeping it aerobic. Biological films of bacteria, protozoa and fungi form on the media’s surfaces and eat or otherwise reduce the
organic content. This biofilm is often grazed by insect larvae, snails, and worms which help maintain an optimal thickness.
Overloading of beds increases the thickness of the film leading to clogging of the filter media and ponding on the surface. Recent
advances in media and process micro-biology design overcome many issues with Trickling filter designs.
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Tertiary treatment
The purpose of tertiary treatment is to provide a final treatment stage to raise the effluent quality before it is discharged to the
receiving environment (sea, river, lake, ground, etc.). More than one tertiary treatment process may be used at any treatment plant. If
disinfection is practiced, it is always the final process. It is also called "effluent polishing."
Filtration:
Sand Filtration removes much of the residual suspended matter. Filtration over activated carbon, also called carbon adsorption,
removes residual toxins.
Lagooning:
Lagooning provides settlement and further biological improvement through storage in large man-made ponds or lagoons. These
lagoons are highly aerobic and colonization by native macrophytes, especially reeds, is often encouraged. Small filter feeding
invertebrates such as Daphnia and species of Rotifera greatly assist in treatment by removing fine particulates.
Disinfection:
The purpose of disinfection in the treatment of waste water is to substantially reduce the number of microorganisms in the water to be
discharged back into the environment. The effectiveness of disinfection depends on the quality of the water being treated (e.g.,
cloudiness, pH, etc.), the type of disinfection being used, the disinfectant dosage (concentration and time), and other environmental
variables. Cloudy water will be treated less successfully, since solid matter can shield organisms, especially from ultraviolet light or if
contact times are low.
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Generally, short contact times, low doses and high flows all militate against effective disinfection. Common methods of disinfection
include ozone, chlorine, ultraviolet light, or sodium hypochlorite. Chloramine, which is used for drinking water, is not used in waste
water treatment because of its persistence.
Chlorination remains the most common form of waste water disinfection in North America due to its low cost and long-term history of
effectiveness. One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may
be carcinogenic or harmful to the environment. Residual chlorine or chloramines may also be capable of chlorinating organic material
in the natural aquatic environment. Further, because residual chlorine is toxic to aquatic species, the treated effluent must also be
chemically dechlorinated, adding to the complexity and cost of treatment.
Ultraviolet (UV) light can be used instead of chlorine, iodine, or other chemicals. Because no chemicals are used, the treated water has
no adverse effect on organisms that later consume it, as may be the case with other methods. UV radiation causes damage to the
genetic structure of bacteria, viruses, and other pathogens, making them incapable of reproduction. The key disadvantages of UV
disinfection are the need for frequent lamp maintenance and replacement and the need for a highly treated effluent to ensure that the
target microorganisms are not shielded from the UV radiation (i.e., any solids present in the treated effluent may protect
microorganisms from the UV light). In the United Kingdom, UV light is becoming the most common means of disinfection because of
the concerns about the impacts of chlorine in chlorinating residual organics in the wastewater and in chlorinating organics in the
receiving water. Some sewage treatment systems in Canada and the US also use UV light for their effluent water disinfection.
Ozone (O3) is generated by passing oxygen (O2) through a high voltage potential resulting in a third oxygen atom becoming attached
and forming O3. Ozone is very unstable and reactive and oxidizes most organic material it comes in contact with, thereby destroying
many pathogenic microorganisms. Ozone is considered to be safer than chlorine because, unlike chlorine which has to be stored on
site (highly poisonous in the event of an accidental release), ozone is generated onsite as needed. Ozonation also produces fewer
disinfection by-products than chlorination. A disadvantage of ozone disinfection is the high cost of the ozone generation equipment
and the requirements for special operators.
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Odour Control:
Odours emitted by sewage treatment are typically an indication of an anaerobic or "septic" condition. Early stages of processing will
tend to produce smelly gases, with hydrogen sulfide being most common in generating complaints. Large process plants in urban areas
will often treat the odours with carbon reactors, a contact media with bio-slimes, small doses of chlorine, or circulating fluids to
biologically capture and metabolize the obnoxious gases. Other methods of odour control exist, including addition of iron salts,
hydrogen peroxide, calcium nitrate, etc. to manage hydrogen sulfide.
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Conclusion:
As the chemical engineering principle is to treat the pollution at the cause but the conventional method used is diluting the waste water
by collecting it the from sewage , washing , bathing, etc. into fresh water bodies and then treating it to remove the impurities. In GIT
also the municipal method of collecting the waste water from sewage, washing, bathing is observed. But instead, if we collect the
waste water from washing and bathing in different tank and further treated would reduce the problems faced by GIT in summers. The
water after treatment can even be used for drinking if reverse osmosis is carried out on the waste water. Or otherwise, the water can be
used for car washing, bathing, etc.
The conventional method is so time consuming and many processes has to be carried out to purify the water. Instead a new technique
is developed using Potassium Ferrate to treat the water which is obtained after pre-treatment. This is a compound which removes all
the impurities from water and forms precipitate of iron. This is a new method which should be known to every one of us, as a
Chemical Engineer!
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References:
1. Specialty chemicals in the environment , A.T.Stone, march 2000.
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