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

ION EXCHANGE SOFTENING

Ion exchange material:1- Zeolite:2- Synthetic resin

The synthetically resin has a much higher exchange capacity and require less amount of salt for regeneration