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Industrial and Hazardous Waste
Management
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Pre-Treatment
Coagulation
and
Precipitation
Equalization Neutralization
Flotation
Sedimentation
FiltrationSpill Basin
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Introduction
Law of mass action
The mathematical product of the H+
and-
OH ion concentration,expressed as moles/Litre is always equal to 10-14 .
Many wastes contain acidic or alkali materials that require tobe neutralized prior to discharge to receiving water body orprior to chemical or biological treatment.
For biological treatment ,pH should be maintained between
6.5 to 8.5 to ensure optimum biological activity.
The CO2produced during biological oxidation also provides abuffer capacity , which reacts with caustic and acidicmaterials
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Introduction
Organic matter +O2+ bacterial cellsCO2+ H2OHCO3 +
HCO3 CO3
+
+ pH=8.9
CO3
+H2OHCO3
+-OH pH=9
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Neutralization
Process of adjusting the pH of water or wastewater throughaddition of an acid or a base, depending on the target pHand process requirements.
Water or wastewater is generally considered adequatelyneutralized if;
1. Its damage to metals, concrete , or other material is minimal.
2. Its has little effect on fish and aquatic life.
3. It has no effect on biological matter.
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Neutralization
Alkalinity & Acidity
Important source of alkalinity and acidity in natural waters isfrom the carbonate system
Water and wastewater pH controlling ions are HCO3 , [+],
[CO3],[-OH ]
It is determine by titration
pH above 8.3 , two steps take place:
1. pH shifted to 8.3
2. pH shifted to 4.5
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Methods of Neutralization
1. Mixing acidic and alkaline waste streams
2. Passing acid wastes through limestone beds
3. Mixing acid wastes with lime slurry
4. Adding the proper proportion of concentrated solutions of caustic
soda (NaOH) or soda ash (Na2CO3) to acid wastes
5. Adding compressed CO2to alkaline wastes
6. Adding H2SO4to alkaline wastes
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Neutralization of Acidic wastewater
1. Mixing acidic and alkaline waste streams
This can be accomplished within a single plant operation orbetween neighboring industrial plants.
Acid and alkaline wastes may be produced individually within oneplant and proper mixing of these wastes at appropriate times canaccomplish neutralization
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1. Acid wastes neutralization through limestone beds
Passing acidic wastewater through beds of limestone by pumpingthe waste through the bed depending on :
1. Head available
2. Cost involved for neutralization
Two systems involved:
1. Up flow system
2. Down flow system
For down flow system the maximum hydraulic loading rate is 1
gal/min.ft2
Neutralization of Acidic wastewater
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CaCO3+ H2SO4 CaSO4+ H2CO3
The above reaction will continue as long as:
1. excess limestone is available (met by simply providing asufficient quantity of limestone)
2. in an active state( water should not contain lime stonecoating substances such as sulphuric acid or metal salts orhydrochloric acid)
pH is related to the bed depth.
Neutralization of Acidic wastewater
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Limestone Bed
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Problem 1
1. Acid wastes neutralization through limestone beds.
A wastewater flow of 100 gallons/min with 0.1N H2SO4requiredneutralization prior to secondary treatment. This flow is to be neutralized
to a pH of 7 using limestone bed(data given below). Designed aneutralization system specifying
(a) most economical bed depth of limestone
(b) weight of acid per day to be neutralized
(c) limestone requirements on an annual basis
Depth (ft3) 0.5 1 2 3 4
HLR ( gal/ft2.hr) 42 180 850 1440 1600
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Problem 2
1. Acid wastes neutralization through limestone beds.
A wastewater flow of 200 gallons/min with 0.2N HCL required
neutralization prior to secondary treatment. Calculate
(a) weight of acid required on annual basis
(b) limestone requirements on an annual basis
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2. Mixing Acid wastes with Lime Slurries
Depends on type of lime used.
Magnesium fraction of lime is most reactive in strongly acidic
solution and useful below pH 4.2.
Basicity factor
Neutralization can be done by using sodium hydroxide,
sodium carbonate, ammonium hydroxide or magnesium
hydroxide.
Neutralization of Acidic wastewater
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1. CO2Treatment for Alkaline Waste streams
Bottled CO2 is aplied to wastewater as compressed air is applied to
activated sludge process.
It forms a weak acid (carbonic acid) when dissolved in water.
The cost may be prohibitive , when the quantity of alkaline waste is
large
Neutralization of Alkaline wastewater
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2. H2SO4Treatment for Alkaline Waste streams
2NaOH + H2SO4 Na2SO4+ 2H2O
Costly but common method
Storage and feeding equipment requirements are low as a result ofits great acidity
Difficult to handle because of its corrosiveness
Neutralization of Alkaline wastewater
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Selection Criteria(Neutralizing agent)
1. Reaction rate
2. Sludge production and disposal
3. Safety during handling and equipment
4. Side reactionincluding dissolved solids, scale formation and heat
5. Effect of overdosing
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Multistage Neutralization
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Problem
1. 100 gallons/min of wastewater is highly acidic and requiresneutralization prior to secondary treatment. This flow is to be
neutralized to a pH of 7 by lime. Titration are shown in figure fromwhich a 2 stage neutralization system will be used with the total limeconsumption of 2250 mg/l. The first stage requires 2000 mg/l and thesecond stage 250 mg/l.
(a) Determine the amount of lime required in lb/day.
(b) Also determine the size of neutralization tank. Assume detentiontime 5 min