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MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
Chemical Unit Process
Gyeongsang National UniversityEnviromental Engineering Lab
Ngoc Thuan Le** 김 투안 **
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• The principal chemical unit processes used for wastewater treatment
a. Basic definitions
b. Chemical precipitation
c. Chemical precipitation for phosphorus removal
d. Chemical oxidation
e. Chemical neutralization
Overview
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• For the complete secondary treatment of untreated wastewater, including the removal of either nitrogen or phosphorus or both.
• To remove phosphorus by chemical precipitation
• To be used in conjunction with biological treatment.
• For the removal of heavy metals and specific organic compounds
• For advance treatment of wastewater, the disinfection of wastewater.
Application of chemical unit processes
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• Coagulant is the chemical that is added to destabilize the colloidal particles in wastewater.
• A Flocculent is a chemical, usually positive charge typically organic, added to enhance the flocculation process.
• Microflocculation (perikinetic flocculation), aggregation is brought about by the random thermal motion of fluid molecules.
• Macroflocculation (orthokinetic flocculation), aggregation is brought about by inducing velocity gradients and mixing in the fluid containing the particles to be floccullated, (large particles overtake small particles to form larger particles)
Fundamental of chemical coagulation
Basic Definition
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• The size of colloidal particles: about 0.01-1µm, a net negative surface charge
• The number of colloidal particles in untreated wastewater is from 106 to 1012 /ml.
• Particles solvent interactions:
Hydrophobic or “water-hating”
Hydrophilic or “water-loving”
Association colloids
• Ionization
Nature particles in the wastewater
At high pH At isoelectric pointAt low pH
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• Polyelectrolytes may be devided into two categories:
Natural: include polymers of biological origin such as cellulose derivatives and alginates
Synthesis: simple monomers that are polymerized into high molecular weight substances.
• The action of polyelectrolytes:
Charge neutralization: the cationic polyelectrolytes are used for this purpose
Polymer bridge formation: polymers that are anionic and nonionic, a bridge is formed when two or more particles become absorbed along the length of polymers
Charge neutralization and polymer bridge formation: from using cationic polyelectrolytes of extremely high molecular weight.
Particle destabilization and aggregation with polyelectrolytes
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• Formation of hydrolysis products: in the past, it was thought that free Al+3 and Fe+3 were responsible for particles aggregation, now their hydrolysis products are responsible.
Particle destabilization and removal with hydrolyzed metal ions
Me
H2O
H2O
H2O
OH2
OH2
OH2
3+
Me
H2O
H2O
H2O
OH
OH2
OH2
2+
+ H+
Me: Cr, Al, Fe
In acid: Al(OH)3(s) + 6H3O+(aq) Al3+ (aq) + 6H2O
In base: Al(OH)3(s) + OH-(aq) Al(OH)4
- (aq) + 6H2O
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• In the past, chemical precipitation was used to enhance the degree of TSS and BOD removal
• In current practice, chemical precipitation is used for:
Primary settling facilities
In the independent physical-chemical treatment of wastewater
Removal of phosphorus
Removal of heavy metals
Chemical precipitation for improved plant performance
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
Inorganic chemicals used most commonly for coagulation and precipitation processes in wastewater
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• With chemical precipitation, it is possible to remove
80-90% TSS
50-80% BOD
80-90% bacteria
• Dependent factors
Quantity of chemical used
Mixing time
Loading rates
Operator
Enhance removal of suspended solids in primary sedimentation
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• Phosphorus precipitation with calcium
Using as form of lime, Ca(OH)2
pH of the wastewater increases beyond about 10
Chemical precipitation for phosphorus removal
10Ca2+ + 6PO43- + 2OH- Ca10 (PO4)6(OH)2
Hydroxylapatite
Dosage of lime depend on
Amount of phosphate present
The alkalinity of wastewater
The quantity of lime required is typically about 1.4 to 1.5 times the total alkalinity expressed as CaCO3
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• Phosphate precipitation with aluminum and iron
Al3+ + HnPO43-n AlPO4 + nH
Fe3+ + HnPO43-n FePO4 + nH
• There are many competing reactions because of the effects of alkalinity, pH, trace elements, and ligands in wastewater
• Dosages are established of bench scale test and occasionally by full scale tests.
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
Phosphorus reduction, %
Mole ratio, Al: P
Range Typical
75 1.25:1-1.5:1 1.4:1
85 1.6:1-1.9:1 1.7:1
95 2.1:1-2.6:1 2.3:1
Developed in part from US. EPA (1976)
• Typical alum dosage requirements for various levels of phosphorus removal
• Theoretically, the minimum solubility of AlPO4 occurs at pH 6.3, FePO4 occurs at pH 5.3. In practice, good phosphorus removal anywhere in the range of pH 6.5 to 7.0
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• Factors effecting the choice of chemical for phosphorus removal
a. Influent phosphorus level
b. Wastewater suspended solids
c. Alkalinity
d. Chemical cost (including transportation)
e. Reliability of chemical supply
f. Sludge handling facilities
g. Ultimate disposal methods
h. Compatibility with other treatment processes
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• Phosphorus removal using metal salts and polymers
a. Iron and aluminum salts can be added at a variety of different points in the treatment processes
b. Polyphosphates and organic phosphorus are less easily removed than orthophosphorus
c. Adding aluminum or iron salts after secondary treatment (where organic phosphorus and polyphosphate are transformed into orthophosphorus) results in the best removal.
d. Polymers may be added
(1) to the mixing zone of a highly mixed or internally recirculated clarifier,
(2) preceding a static of dynamic mixer, or
(3) to aerated chanel
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• Phosphorus removal using lime
a. Lime treatment can be used to precipitate a portion of the phosphorus (about 65-80%)
b. Product of precipitation: [Ca5(PO4)3(OH)].
c. pH is about 11 for high lime systems, about 8.5-9.5 for low lime systems
d. After precipitation, the effluent must be recarbonated before biological treatment.
e. In activated sludge systems, the pH of the primary effluent should not exceed 9.5 or 10
5Ca2+ + 3PO43- + OH- Ca5(PO4)3(OH)
Hydroxylapatite
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
Advantages and disadvantages of chemical addition in various section of a treatment plant for phosphorus removal
Level of treatment Advantages Aisadvantages
Primary applicable to most plant, increase BOD and TSS removal, lowest degree of metal leakage, lime recovery demonstrated
Least efficient use of metal, polymer may be required for flocculation, sludge more difficult to dewater than primary sludge
Secondary lower cost, lower chemical dosage than primary, improved stability of activated sludge, polymer not required
overdose of metal cause low pH toxicity, with low alkalinity wastewaters, a pH control system may be necessary, canot use lime because of excess pH, inert solids added to activated sludge mixer liquor, reducing the percentage of volatile solids
Advanced precipitation lowest phosphorus effluent, most efficient metal use, lime recovery demonstrated
Highest capital cost, highest metal leakage
advanced single and two stage filtration
lower cost can be combineed wieth the removal of residual TSS
Length of filter run may be reduced with single-stage filtration additional expense with two stage filtration process
Adapted from US. EPA, 1976
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• Most of metal of interest in wastewater can be precipitated as hydroxides and sulfides
• Dependent factors:
Solubility
pH
Chemical precipitation for removal of heavy metals and dissolved organic substances
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• Oxidizing agents:
ozone (O3),
hydrogen peroxide (H2O2),
permanganate (MnO4),
chloride dioxide (ClO2),
chlorine (Cl2) or (HClO) and
oxygen (O2)
• For reduction of:
BOD,
COD,
ammonia,
nonbiodegradable organic compounds.
Chemical oxidation
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• Oxidation-reduction reaction
Take place between oxidizing agent and reducing agent.
Cu2+ + Zn Cu + Zn2+
Zn - 2e Zn2+ (oxidation)
Cu2+ - 2e Cu (reduction)
Oxidation-reduction reactions often require the presence of one or more catalysts to increase the rate of reaction.
Requirements: transition metal cations, enzymes, pH adjustment…
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
Typical applications of chemical oxidation in wastewater collection, treatment and disposal.
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• Chemical oxidation of BOD and COD
Organic molecule (e.g., BOD)
Intermediate oxygenated molecules
Simple end products (e.g., CO2, H2O…)
Cl, O3,
H2O2
Cl, O3,
H2O2
• Chemical oxidation of nonbiodegradable organic compounds
For treatment of remaining after biological treatment: low molecular weight polar organic compounds and complex organic compounds build around the benzene ring structure
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• Chemical neutralization
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
• Scaling control
Acidifying to reduce pH and alkalinity
Reducing calcium concentration by ion exchange or lime softening
Adding a scale inhibitor chemical (antiscalant) to increase the apparent solubility of CaCO3 in the concentrate stream
Lowering the product recovering rate
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
Chemical storage, feeding, piping, and control systems.
Chemical feeders
Dry feeders Liquid feeders Gas feeders
gravimetric volumetric
BeltLoss in weightSelf-powered
BeltRevolving plateRotaryScrewShakervibratory
Slurry solution AmmoniaChlorineOxygenOzone sulfur dioxide
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
Schematic of typical dry chemical-feed system.
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
Typical volumetric chemical feeders
Conveyor belt Rotary
Screw Vibratory
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
Schematic of typical liquid chemical-feed system.
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
Chemical Application Recommentded mixing times, s
Alum, Al3+ , Ferric chloride, Fe3+ Coagulation of colloidal particles <1
Alum, Al3+ , Ferric chloride, Fe3+ Sweep floc precipitation 1-10
Lime Ca(OH)2 Chemical precipitation 10-30
Chlorine, Cl2 Chemical disinfection <1
Chloramine, NH2Cl Chemical disinfection 5-10
Cationic polymers Destabilization of colloidal particles <1
Anionic polymers particle bridging 1-10
Polymers, anionic Filter aids 1-10
Typical mixing times for various chemicals used in wastewater treatment facilities
MECALF & EDDY|Wastewater Engineering-treatment and reuse Chap.6|Chemical Unit Process
Thank you for your attention!