COAGULATION Abdul Haqi Ibrahim Ph D Water Research

COAGULATION Abdul Haqi Ibrahim, Ph. D Water Research Group (WAREG) School of Environmental Engineering Universiti Malaysia Perlis.

INTRODUCTION • Coagulation and flocculation designed to: • Remove infectious agents, • Remove toxic compounds that have adsorbed to the surface of particles, • Remove precursors to the formation of disinfection by products, and • Make the water palatable.

INTRODUCTION • water supplies contain organic and inorganic particles • Both the precipitates and the surface water particles may, for practical purposes, be classified as suspended and colloidal • Colloidal particles are too small to be removed by sedimentation or by sand filtration processes. • The object of coagulation (and subsequently flocculation) is to turn the small particles into larger particles called flocs, either as precipitates or suspended particles. • flocs are readily removed in subsequent processes such as settling, dissolved air flotation (DAF), or filtration

• Coagulation means the addition of one or more chemicals to condition the small particles • Flocculation is the process of aggregation of the destabilized particles and precipitation products.

CHARACTERISTICS OF PARTICLES • Electrical Properties • important electrical property of the colloidal and suspended particles is their surface charge • charge causes the particles to remain in suspension without aggregating for long periods of time. • Surface water particle suspensions are thermodynamically unstable and, given enough time, they will flocculate and settle • aggregation process is very slow • For most particles in water the sign of the charge is negative

Electrical Double Layer • A colloidal dispersion in solution does not have a net charge • negatively charged particles accumulate positive counterions on and near the particle surface. • Thus, as shown in Figure 6 -2 , a double layer forms • The adsorbed layer of cations (known as the Helmholtz layer or the Stern layer) is bound to the particle surface by electrostatic and adsorption forces.

Zeta Potential. • When a charged particle is placed in an electric field, it will migrate to the pole of opposite charge. • This movement is called electrophoresis. • As the particle moves, a portion of the water near the surface moves with it. • This movement displaces the ion cloud and gives it the shape shown • The electric potential between the shear plane and the bulk solution is called the zeta potential.

Particle Stability. • Particles in natural waters remain stable when there is a balance between the electrostatic force of the charged particles and attractive forces known as van der Waals forces. • Because the particles have a net negative charge, the principal mechanism controlling stability is electrostatic repulsion. • Van der Waals forces arise from magnetic and electronic resonance when two particles approach one another • Because the double layer extends further into solution than the van der Waals forces, an energy barrier is formed that prevents particles from aggregating.

Physics of Coagulation • There are four mechanisms employed to destabilize natural water suspensions: • • Compression of the electric double layer, • • Adsorption and charge neutralization, • • Adsorption and interparticle bridging, and • • Enmeshment in a precipitate.

Inorganic coagulants

CHEMISTRY OF COAGULATION • extremely complex. • Basic concepts that will help explain the interaction of coagulants and p. H.

Buffer Solutions. • A solution that resists large changes in p. H when an acid or base is added or when the solution • Atmospheric carbon dioxide (CO 2 ) produces a natural buffer through the following reactions:

Buffer Solutions. • the most important buffer system in water and wastewater treatment. • referred as the carbonate buffer system

Buffer Solutions. The first two cases are common in natural settings when the reactions proceed over a relatively long period of time The second two cases are not common in natural settings. They are used in water treatment plants to adjust the p. H.

Alkalinity. • sum of all titratable bases down to about p. H 4. 5 • It is found by experimentally determining how much acid it takes to lower the p. H of water to 4. 5 • most waters the only significant contributions to alkalinity are the carbonate species and any free H+ or OH • The total H+ that can be taken up by a water containing primarily carbonate species is: • Alkalinity = [HCO 3 -] + 2[CO 32 -] + [OH-] - [H+] where [ ] refers to concentrations in moles/L (Molarity)

Alkalinity. • In most natural water situations (p. H 6 to 8), the OH- and H+ are negligible, such that: • Alkalinity = [HCO 3 -] + 2[CO 32 -] • The pertinent acid/base reactions are • Ka is the equilibrium constant • p. Ka = - log Ka

Alkalinity. • From the p K values, some useful relationships can be found: • Below p. H of 4. 5, essentially all of the carbonate species are present as H 2 CO 3 , and the alkalinity is negative (due to the H+). • At a p. H of 8. 3 most of the carbonate species are present as HCO 3 - and the alkalinity equals HCO 3 -. • Above a p. H of 12. 3, essentially all of the carbonate species are present as [CO 32 -] and the alkalinity equals [CO 32 -] + [OH-]. The [OH-] may not be insignificant at this p. H.

Alkalinity. • p. H starts at above 12. 3 and as acid is added the p. H drops slowly as the first acid (H+ ) addition is consumed by free hydroxide (OH-) • then the acid is consumed by carbonate (CO 32 -) being converted to bicarbonate (HCO 3 - ).

Alkalinity. • At about p. H 8. 3 the carbonate is essentially all converted to bicarbonate, at which point there is another somewhat flat area where the acid is consumed by converting bicarbonate to carbonic acid.

Alkalinity. • alkalinity serves as a measure of buffering capacity. • Alkalinity = [HCO 3 -] + 2[CO 32 -] + [OH-] - [H+] • The greater the alkalinity, the greater the buffering capacity. Alkaline vs Alkalinity • Alkaline water has a p. H greater than 7, while a water with high alkalinity has a high buffering capacity

Alkalinity. • Alkalinity is not expressed in but rather in mg/L as Ca. CO 3. • In order to convert species to mg/L as Ca. CO 3 , multiply mg/L as the species by the ratio of the equivalent weight of Ca. CO 3 to the species equivalent weight:

Example • A water contains 100. 0 mg/L CO 32 - and 75. 0 mg/L HCO 3 at a p. H of 10. Calculate the alkalinity exactly at 25 C. Approximate the alkalinity by ignoring [OH- ] and [H+]. • First, convert CO 32 -, HCO 3 - , OH- , and H+ to mg/L as Ca. CO 3.

Example p. H = - log [H+] i. e. let say the concentration of H+ is 0. 0000001 M the p. H is p. H = -log [0. 0000001] = - log [1 x 10 -7] = -7 Hence – (-7) = 7

• The OH- concentration is determined from the ionization of water, that is:

Aluminium • Aluminum can be purchased as either dry or liquid alum • More concentrated solution, there can be problems with crystallization of the alum during shipment and storage. • The alternative is to purchase dry alum • When alum is added to a water containing alkalinity, the following reaction occurs:

Aluminium • Each mole of alum added uses six moles of alkalinity and produces six moles of carbon dioxide. • reaction shifts the carbonate equilibrium and decreases the p. H. • long as sufficient alkalinity is present and CO 2 (g) is allowed to evolve, the p. H is not drastically reduced and is generally not an operational problem.

Aluminium • When sufficient alkalinity is not present to neutralize the sulfuric acid production, the p. H may be greatly reduced • If above reaction occurred, lime or sodium carbonate may be added to neutralize the acid formed because the precipitate will dissolve.

Example • Estimate the amount of alkalinity (in mg/L) consumed from the addition of 100 mg/L of alum.

Example

Important remarks (aluminium) • Aluminum ion does not really exist as Al 3+ , and that the final product is more complex than Al(OH)3. • When the alum is added to the water, it immediately dissociates, resulting in the release of an aluminum ion surrounded by six water molecules. • The aluminum ion starts reacting with the water, forming large Al ・ OH ・ H 2 O complexes. • The complex is a very large precipitate that removes many of the colloids by enmeshment as it falls through the water.

Iron • An example of the reaction of Fe. Cl 3 in the presence of alkalinity is: • without alkalinity • Forming hydrochloric acid, which in turn lowers the p. H.

p. H and Dose • Two important factors in coagulant addition are p. H and dose • determined from laboratory tests called a “jar test”

Example of Jar Test • Six beakers are filled with the raw water, and then each is mixed and flocculated uniformly by identical paddle stirrers driven by a single motor • typical test is conducted by first dosing each jar with the same alum dose and varying the p. H in each jar. • The test is then repeated in a second set of jars by holding the p. H constant at the optimum p. H and varying the coagulant dose.

• on a raw water containing 15 NTU and a HCO 3 - alkalinity concentration of 50 mg/L expressed as Ca. CO 3. • The turbidity was measured after the mixture was allowed to settle for 30 minutes • The objective is to find the optimal p. H, coagulant dose, and theoretical amount of alkalinity that would be consumed at the optimal dose.

• Result Jar Test 1

• Result Jar Test 1

• Plot the result • In actual practice, the laboratory technician would probably try to repeat the test using a p. H of 6. 25 and varying the alum dose between 10 and 15 to pinpoint the optimal conditions.

• lack of sufficient alkalinity will require the addition of a base to adjust the p. H into the acceptable range. • Lime (Ca. O), calcium hydroxide Ca(OH) 2 , sodium hydroxide (Na. OH), and sodium carbonate (Na 2 CO 3 ), also known as soda ash, are the most common chemicals used to adjust the p. H.

impact of the lack of alkalinity • Estimate the p. H that results from the addition of 100 mg/L of alum to a water with no alkalinity, and estimate the amount of sodium hydroxide (Na. OH) in mg/L required to bring the p. H to 7. 0.

The optimum p. H range for alum is approximately 5. 5 to 7. 7 with adequate coagulation possible between p. H 5 and 9 under some conditions

• To determine if base needs to be added when alkalinity is present, estimate the amount of: 1. alkalinity present and 2. calculate the amount of alkalinity “destroyed, ” as in Example 6 -2. 3. If the amount destroyed exceeds the amount present, estimate the excess alum 4. Use this amount to estimate the amount of base to add.