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ENVIRONMENTAL ENGINEERING
441
Lecture 5:
Water Treatment (2)
Coagulation and flocculation
Philadelphia UniversityFaculty of Engineering
Department of Civil EngineeringFirst Semester, 2013/2014
OBJECTIVES OF WATER TREATMENT
“ To remove harmful microorganisms or chemicals, thereby preventing the spread of disease in order to supply clean and safe water for public demand”
WATER SOURCE
Potable water/Palatable water
Water source could be Ground water (well) most likely better quality than
surface water) Deep Shallow
Surface Water Lakes River Reservoir
Raw water quality varies with the source, and if the source is surface water, the quality vary seasonally, particularly with flooding, upland and lowland, etc
WATER TREATMENT SYSTEM
Treatment system can be classified as Simple disinfection ; i.e ground water (simply
chlorination if only has bacteria) Filter plant
Rapid mixing, flocculation, sedimentation, filtration and disinfestation to remove color, turbidity, taste & odor, organic matter and bacteria)
Softening plant; use the same unit operation as filter system but use different chemical. Softening plant in primarily used to remove hardness
SELECTION OF TREATMENT PROCESSES
Selection of the set of Treatment Processes will be based on : Sources of water intake Characteristic of waterDetailed raw water quality analysis for a minimum
of one year, or longer, at periods of high, low and medium flows. The parameters to be looked at all those listed in the legislative standards
Cost Suitability of the processes for removing
particulate impurities. Example: Turbidity; suitable process might be
coagulation , sedimentation or filtrationFor pathogen, pre or post chlorination, UV,
ozone, or chloramines
COAGULATION Colloids are suspended particle very small to
settle in a reasonable time (sedimentation) and too small to trap to be trapped in the pores of a filter (Filtration);
They possess a negative charges that repels other colloidal particles before they colloid with one another .
In order to destabilize the colloids, we must neutralize the charge by addition of an ions of the opposite charge for the colloids
COAGULATION Positive ions is added to water to reduce the surface
charge to the point where the colloids are not repelled from each other.
Coagulants tend to be positively charged. Due to their positive charge, they are attracted to the negative particles in the water
The combination of positive and negative charge results in a neutral , or lack, of charge
WATER TREATMENT COAGULANTS
Particles in water are negative; coagulants usually positively charged. The two most common coagulants are:
1. Alum- aluminum sulfate
2. Ferric chloride or ferrous sulfate
Two important factors in coagulation addition are pH and dose; which must be determined from lab test
COAGULANT ALUM
Alum- (aluminum sulfate): Alum is attracted to the positive charges
created by aluminum hydroxides. 1. Trivalent Al+3 charge attracts neg – particles2. Forms flocs of aluminum hydroxide (AlOH3).
3. Impacted by mixing, alkalinity, turbidity and temp.
4. Ideal pH range 5-8
ALUM- (ALUMINUM SULFATE)-
Al2(SO4)3·14H2O + 6 HCO3- 2(Al(OH3). 3H2O(S) + 6CO2 +8H2O + 3SO4
-
2
2Al+3 + negatively charged colloids makes neutral surface charge
WHY IS ALKALINITY SO IMPORTANT??
No bicarbonate (low alkalinity, low pH sulfuric acid!):Al2(SO4)3·14H2O 2Al(OH3).3H2O(S) +3H2SO4+2H2O
Optimum pH: 5.5 to 6.5 Operating pH: 5 to 8
JAR TEST
One of the most common method to evaluate coagulation efficiency
To determine the optimal pH and the coagulant dose
Measure : pH Turbidity- SS removal Sludge volume
WATER TREATMENT -IRON
Iron salt- (Iron chloride or sulfate)-FeCl3+ 3 HCO3
- +3H2O Fe(OH3). 3H2O(S) + 3CO2 +3Cl-
No bicarbonate (low alkalinity, low pH hydrochloric acid!):
FeCl3+ 6 H2O Fe(OH3). 3H2O(S) + 3HCl
2. Work over a larger pH range (4-9) than alum
WATER TREATMENT- COAGULANT AID
Activated silica (sodium silicate)- helps improve coagulation, decreases volume of coagulant necessary. Typically is sodium silicate.
Produce stable solution, that has negative surface charges
Resulting larger, denser flocs and settle faster
For treating highly colored, low turbidity water
Require proper equipment and close operational control
WATER TREATMENT- COAGULANT AID
Bentonite (clay)- helps improve coagulation, decreases volume of coagulant necessary. 1. high in color, low turbidity, low mineral content water
2. 10-50 mg/L dosage 3. Heavier denser floc that settles faster
WATER TREATMENT- COAGULANT AID
Polymers:Water-soluble organic polymersLong chained carbon, high MWt, many active sitesused as both primary coagulants and coagulant
aids. Act as "bridges" between the already formed
particles : • Anionic—ionize in solution to form negative sites
along the polymer molecule. • Cationic—ionize to form positive sites. Positive and negative charges-
polyamorphotype• Non-ionic—very slight ionization. effectiveness: particles type, turbidity present,
and the turbulence (mixing) available during coagulation.
WATER TREATMENT- COAGULANT AID
pH adjuster Sulfuric Acid for lowering the pH Lime Ca(OH)2 or Soda Ash Na2CO3 for raising the
pH
SOFTENING
Objective: to remove hardnessHardness: Sum of All polyvalent cations
express as CaCO3
Carbonate hardness (soft) (CH) Non-Carbonate hardness (hard)
If pH< 8.3; that means all HCO3 is the dominant form of alkalinity
NCH= Total Hardness (TH) – Alkalinity (HCO3-)
Softening can be accomplished by either lime soda or Ion exchange
SOFTENING- LIME SODA
The objective is to precipitate the Ca as CaCO3 at pH 10.3 and Mg as Mg(OH)2 at pH 11
Ca2+ + CO32- CaCO3 (s)
Mg2+ + 2OH- Mg(OH)2 (s)
Mg is more expensive to remove than Ca More expensive to remove NCH than CH,
because we need to add another chemical to provide CO3
2-
Can not produce a water completely free of hardness because of the solubility of CaCO3 and Ma(OH)2 in water; physical limitation in mixing and contact, and the lack of sufficient time for the reaction
ION EXCHANGE SOFTENINGReversible interchange of ion between a soild
and liquid phase in which no permanent change in the structure of the solids
The water containing the hardness (such as Ca or Mg) is passed through the ion exchange material. The hardness in the water exchange with an ion (Na) from the ion- exchange materials.
Ca(HCO3)2 + 2NaR CaR2 + 2NaHCO3
Alkalinity does not changing Removal of 100% hardness from water until
the exchange capacity is reached “breakthrough”
Regeneration of the material by using NaCl; CaR2 + 2NaCl 2NaR + CaCl2
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