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CONTAMINANTCONCENTRATION
REDUCTION
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INTRODUCTION
Contaminant concentration reduction refers to reductionof pollutants in wastewater stream.
Contaminant concentration reduction
reduces requirements for waste treatment system
will be well rewarded by the savings, on account of lowerrequirements of land, equipment and energy.
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INTRODUCTION
The strength of wastes can be reduced by:
Process Changes;
Equipment Modifications;
Segregation of Wastes;
Equalization of Wastes;
By-product Recovery;
Proportioning Wastes; and Monitoring Waste Streams.
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PROCESS CHANGES
Process changes may result in reducedenvironmental pollution load
Textile Industry
Starch had been traditionally used as a sizing agentbefore weaving.
This starch, after removal from the finished cloth, is thesource of 3050% of the mills total oxygen-demandingmatter.
The cellulosic sizing agents, exhibits little or nobiochemical oxygen demand (BOD) or toxic effect instreams.
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PROCESS CHANGES
Coal Mining Waste
Acid mine waste (Mine drainage waste)
Waste from coal washeries
wash raw coal with acid mine waste rather than apublic or private water supply.
The mine drainage waste is neutralized while thecoal is washed clean of impurities.
The initial mine water had a pH of 3, and an iron contentof 551 ppm.
The wastewater finally discharged from the process hada pH of 6.77.1 and an iron content of less than 1 ppm.
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EQUIPMENT MODIFICATIONS
Changes in equipment can reduce the strength ofthe waste, by reducing the amount of pollutantsentering the waste stream.
Pickle factories screens placed over drain lines prevent the escape of
seeds and pieces of vegetables, which adds to thestrength and density of the waste.
Poultry plants traps on the discharge pipeline prevent emission of
feathers and pieces of fat. change the production procedure to dry-collect as
much waste material as possible from manufacturingmachines and operating floors rather than hosingdown the same matter into drains.
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EQUIPMENT MODIFICATIONS
Dairy industry.
Trebler (1944) redesigned the milk cans with smooth necksso that they could be drained faster and completely.
This prevented a large amount of milk waste from entering
streams and sewage plants.
Dairy farmers also installed drip pans to collect milk thatdrains from the cans after they have been emptied
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SEGREGATIONOF WASTES
Segregation of wastes reduces the strength of thefinal waste from an industrial plant. It usually resultsin two wastes: one strong and small in volume, can then be handled
with methods specific to the problem it presents; the other weaker, with large volume.
The two stream can then be treated collectively.
Another type of segregation is the removal of oneparticular process waste from the other processwastes of an industrial plant, which renders themajor part of the waste more amenable totreatment
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SEGREGATIONOF WASTES
Textile Industry
The combined waste of a textile mill would be quite strong,difficult and expensive to treat.
However, when the kiering waste was segregated from the
other wastes, chemically neutralized, precipitated, andsettled.
The supernatant could be treated along with the other threewastes, because the strength of the resulting mixture wasconsiderably less than that of the original combined waste.
GrayWater
WhiteWater
Dye Waste Kier Waste
OxygenConsumedin ppm
1560 31 556 4900
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SEGREGATIONOF WASTES
Metal-finishing plants
The wastes contains both chromium and cyanide, aswell as other metals.
It is necessary to segregate the cyanide-bearing
wastes, make them alkaline, and oxidize them. The chromium wastes, have to be acidified and
reduced.
The two effluents can then be combined andprecipitated in an alkaline solution to remove the metals.
Without segregation, poisonous hydrogen cyanide gaswould develop as a result of acidification.
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SEGREGATIONOF WASTES
Poultry plant
the blood from the killing room floor interfere with thetreatment of the remainder of the chicken waste.
Nemerow and Dasgupta (1991) recommended
the blood be scraped, swept, and disposed of with thescreenings.
In this way, a high BOD waste was segregated from theremaining plant process waste and treated separately.
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EQUALIZATIONOFWASTES
It is dangerous, to arrive at a blanket conclusionthat segregation of strong or dangerous wastes isalways desirable.
Just the reverse technique
Complete Equalization
may be necessary in certain circumstances.
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EQUALIZATIONOFWASTES
Equalization requires holding wastes for a certainperiod, depending on the time taken for the repetitiveprocesses in the plant.
The effluent from an equalization basin is much more
consistent in its characteristics than each separateinfluent to that same basin.
Equalization offers
Stabilization of pH and BOD and settling of solids and heavy
metals
Stable effluents are treated easily and efficiently.
Sometimes equalization may produce an effluent thatwarrants no further treatment.
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EQUALIZATIONOFWASTES
Salt in large quantities is used to cure cucumbers
into pickles.
The large volume of salt brine waste can causedisproportionate objectionable effects whendischarged into a receiving water.
Holding the brine wastes in large vats for slowdischarge with the remaining plant waste over longperiods would minimize the salt effect.
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EQUALIZATIONOFWASTES
A textile-finishing mill that discharges its waste intoa domestic secondary sewage-treatment plantwould upset the efficiency of the plant due tofluctuations, primarily in pH and BOD.
An equalization basin capable of detaining thewaste long enough to reduce the fluctuations in pHand BOD.
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EQUALIZATIONOFWASTES
In addition, the equalized waste can be delivered tothe treatment plant at three different rates of flow:
the highest flow rate corresponded to the time when thegreatest amount of sewage was reaching the plant, and
vice versa.
This will give a more constant dilution of the mills
waste with domestic sewage.
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BY-PRODUCT RECOVERY
By-product recovery is the idealistic aspect of industrial-wastetreatment, that may lead to economic gain.
The low percentage of successful by-products developed fromwaste salvage may dissuade management. However, any use of
waste materials obviously eliminates at least some of the waste some wastes are very difficult to treat at low cost, Under such
circumstances, possibility of building a recovery plant will producea marketable by-product while solving a troublesome wasteproblem.
Both economic considerations and compliance with therequirements of pollution abatement play a major role in anydecisions involving by-product recovery.
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BY-PRODUCT RECOVERY
Metal-plating industries use ion exchangers torecover phosphoric acid, copper, nickel, andchromium from plating solutions. The de-ionizedwater, without any further treatment, is ideal for
boiler-feed requirements.
Paper mills, with the aid of multiple-effectevaporators, recover caustic soda from cookingliquors.
Chemical plants spray dilute waste acids into hot,brick-faced towers to concentrate the acids forreuse.
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BY-PRODUCT RECOVERY
Once a by-product is developed and put intoproduction, it is difficult to identify the new productwith a waste-treatment process.
For example, when sugar is extracted from sugar cane, a
thick syrupy liquid known as blackstrap molasses is left.
This molasses used to be so cheap that it was almostgiven away.
Today, it has many uses, with one of the best-known being
in the production of commercial alcohol.
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PROPORTIONING WASTES
By proportioning its discharge of concentrated wastesinto the main sewer, a plant can often reduce thestrength of its total waste to the point at which it willneed a minimum of final treatment or will cause the
least damage to the stream or treatment plant.
It may prove less costly to proportion one small butconcentrated waste into the main flow, according tothe rate of the main flow, than to equalize the entire
waste of the plant in order to reduce the strength.