10 Coagulation, Flocculation and Precipitation

Babita Khosla

10.1 Coagulation- Flocculation

The treatment of wastewater treatment is a matter of solids separation, since most of the contaminants in wastewater are present in particulate or colloidal form, or are transformed into such forms during the treatment. The separation of particulate matter from the liquid phase is one of the important steps in most wastewater treatment processes. The waters contain both dissolved and suspended particles and the suspended particles vary considerably depending upon the source of water. The larger particulate matter can be easily separated using the conventional filtration, sedimentation techniques while, the colloidal particles are difficult to separate from water because they do not settle by gravity and due to their small size they can easily pass through the pores of filtration media. The colloidal particles are very fine particles having the size over a range of 1nm to 0.1 nm and electrostatic charge on their surface.

For the removal of these particles, these individual colloidal particles must aggregate and grow in size. The aggregation of colloidal particles involves the following separate and distinct steps:

1. Destabilization of the colloidal particles to promote the attachment after they come in contact with each other
2. The transport of colloidal particles to promote their inter-particle collision.

The bringing together of the destabilized particles to form a larger agglomeration is flocculation whereas the overall process involving destabilization, transport and floc formation is called coagulation.

In general, most colloidal material has a negative surface charge and like charges tend to repel each other preventing the phenomenon of coagulation. These characteristics cause the colloidal particles to remain in solution. The destabilisation of colloidal particles to promote coagulation and settlement is achieved by adding chemicals that coat the colloids by opposite charges. The addition of positively charged ions in the solution will destabilise the colloidal matter and allow their settlement. The measurable quantity used to predict the potential of coagulation is Zeta potential and effective coagulation has been found to occur experimentally at zeta potential values ranging from ± 0.5 mV.

In the process of water treatment, clarification of water with coagulating agents has been practiced since ancient times, using a variety of substances. The alum was used as a coagulant in water treatment and more formally for the treatment of public water supplies since seventeenth century (Bratby, 2006). In modern water treatment process, coagulation and flocculation are still essential steps.

Coagulant chemicals are of two main types

1. Primary coagulants
2. Coagulant aids

1. Primary coagulants are the coagulants that neutralize the electrical charge of particles present in the water leading the particles to clump together (Schulz and Okun, 1984). Chemical coagulation has been in practice for several decades to precipitate the soluble heavy metals present in the waste water as hydroxides, thus facilitating their removal by physical separations through the sedimentation process.

Chemically, the coagulants are either metallic salts (such as alum, ferric salts) or polymers. Polymers are man-made organic compounds made up of a long chain of smaller molecules. Polymers can be cationic, anionic, or nonionic. The process of coagulation consists of the following four steps:

The  first  and  the  initial  step  is  simply  addition  of  the  chemical  to  be  used  as coagulant to wastewater. This is followed by the second step, where the solution is mixed rapidly to ensure even and homogeneous distribution of the coagulant throughout the wastewater. In the third step, the solution is mixed again, but this time in a slow speed to encourage the formation of insoluble solid precipitates, the process known as “coagulation.” The final step is the removal of the coagulated particles by way of filtration or decantation.

Mechanism of coagulation

The process of coagulation results from the two basic mechanisms namely Electrokinetic and Orthokinetic. In the electrokinetic coagulation, the zeta potential is reduced by ions or colloids of opposite charge to a level below the vander Waals attractive forces while in orthokinetic coagulation the micelles aggregate and form clumps which agglomerate the colloidal particles (Yılmaz et al., 2007). The addition of high valence cations accompanying the dissolution of the coagulant neutralizes the negative charge present on the colloids before the visible floc formation. The rapid mixing coats the colloidal particles resulting in the formation of microflocs. The microflocs thus formed retain a positive charge in the acidic range because of the adsorption of H+ ions. These microflocs also serves to further neuteralise and coat the colloidal particles while Flocculation agglomerates the colloids with a hydrous oxide floc. In this phase surface adsorption is also active, so the colloids not initially adsorbed are removed by entrapment in the floc.

The outline of operation for effective coagulation as shown in Figure: 2 was given by Riddick in 1964. The alkalinity should be added in the form of Bicarbonate without raising the pH of the water before the addition of Alum or Ferric salts. A rapid mixing of 1-3 minutes is needed to allow the Al3+ and Fe3+ cations to coat the colloids resulting in the formation of microflocs. The coagulant aids such as activated silica or polyelectrolyte are added followed by mixing of 20-30 minutes to build up the floc and control the zeta potential.

Commonly used Coagulants:

The most common type of coagulants used are metal salts like Aluminium coagulants (Aluminium sulphate, Aluminium chloride, Polyaluminium chloride and Sodium aluminate), Iron coagulants (Ferric sulphate, Ferrous sulphate, Ferric chloride) and other chemicals as Hydrated lime Ca(OH)2 and Magnesium carbonate MgCO3.

Alum: The most popular and economical coagulant in water treatment application is alum   (Al2(SO4)3.18H2O). When alum is added to water, the reaction is

Al2(SO4)3).18H2O + 3Ca(OH)2 → 2Al(OH )3 + 3CaSO4 + 18H2O

The formation of an aluminium hydroxide floc is the outcome of the reaction between the coagulant and the alkalinity of the water present as hydroxides of calcium and magnesium. If the water to be treated has insufficient alkalinity or `buffering’ capacity, additional alkali such as hydrated lime, sodium hydroxide or sodium carbonate must be provided for the reaction.

With the addition of sodium carbonate commonly known as soda ash, the reaction is as following:

Al2 (SO4)3 + 3NaCO3 + 3H2 O → 2Al(OH)3 + 3Na2SO4 + 3CO2

The aluminium hydroxide actually exists in the chemical form Al2O3.xH2O and is amphoteric in nature means that it can act as either an acid or a base. The alum floc is least soluble at pH 7.0. The pH control is important in coagulation process, not only in the removal of turbidity and colour but also to maintain satisfactory minimum levels of dissolved residual aluminium in the  clarified water. The optimum coagulation pH value should be attained by adding sulphuric or similar strong acid rather than excess coagulant.

The coagulation process with alum as the coagulant is capable of achieving significant removal of organic contaminants, but the pH of the water during coagulation process has profound influences on effectiveness of coagulation and the best results are achieved in slightly acidic condition. Simultaneously, the optimum pH for alum coagulation is influenced by the concentration of organic matter in the water. For water of higher orga nic mattercontent, the optimum pH is displaced to be slightly more acidic values (AWWA, 1979). Thus conventional coagulation practices may provide excellent organic removal if the coagulant dose and pH conditions are adjusted into the optimum range.

Iron salts: The iron salts most commonly used as coagulants include ferric sulfate, ferric chloride and ferrous sulfate. These compounds often produce good coagulation when conditions are too acidic for use of alum but have the disadvantage of being more difficult to handle.

Ferric Sulphate available in the form of red-brown powder or as granules. The formation of a ferric hydroxide floc is the result of the reaction between the acidic coagulant and the alkalinity of the water, which usually consists of calcium bicarbonate.

Fe2(SO4)3 + 3Ca(HCO3 )2  → 2Fe(OH )3 + 3CaSO4 + 6CO2

Ferric Chloride is available in anhydrous form as a green-black powder, and also as a dark-brown syrupy liquid or as crystal ferric chloride.

2FeCl 3 + 3Ca(HCO3 )2  → 2Fe(OH )3 + 3 CaCl2 + 6 CO2

An insoluble hydrous ferric oxide is produced over a pH range of 3.0 to13.0.The floc charge is positive in the acidic range and negative in the basic range with mixed charges over the pH range 6.5 to 8.0.

2. Coagulant Aids

Coagulant aid is an inorganic material and is used along with main coagulant. Coagulant aids when added increase the density and provide toughness to the flocs so that they will not break up during the mixing and settling processes.The common coagulant aids used are Bentonite, Calcium carbonate, Sodium silicate and polyelectrolytes. Polyelectrolytes are high molecular weight polymers have adsorbable groups and form bridges between particles or charged flocs. There are three types of polyelectrolytes: a cationic (adsorbed on negative colloids), anionic

(adsorbed on positive colloids) and nonionic. These polyelectrolytes replaces ionic group on the colloid and permits hydrogen bonding between the colloid and the polymer.

Coagulation Control Test

The quality of water changes with season and weather so, the coagulant dosages have to be adjusted accordingly for variations in water turbidity and organic material. An underdose of coagulant may cause the sample to appear cloudy with no floc and settling and an overdose of coagulant may form dense floc that may be fragile and fluffy which will not settle well when the mixing is turned off. However, coagulant dosages cannot be calculated, they have to be determined experimentally by jar test. The jar test is a simple effective method that simulates the coagulation/flocculation process of the existing or proposed water treatment plant. It is a widely used laboratory test for coagulation and flocculation control in water treatment plant operations and design. The purpose of a jar test is to select the type of coagulant used and dosage selection, coagulant aid and dosage selection, determination of optimal pH, determination of the point of chemical addition, optimization of mixing time and intensity for rapid and slow mixings (Amirtharajah and O’Melia, 1990).

Jar Test Procedure

The jar test procedure involves the following steps:

1. Fill the jar testing apparatus containers with sample water and keep it on the magnetic stirrer. Add the coagulant to each container in small increments at a pH of 6.0. After each addition stir rapidly for 1 minute followed by a 3-5 minute slow mix. The rapid mix stage helps to disperse the coagulant throughout each container. Continue the addition until a visible floc is formed.
2. Using this dosage, place 1000 ml of sample in each of six beakers and adjust the pH of each beaker in the range of 4.0-9.0 with standard alkali.
3. Rapid mix each sample for 3 minutes followed by gentle mixing for 15 to 20 minutes to allow flocculation.
4. Measure the effluent concentration of each settled sample.
5. Plot the percentage removal versus pH and select the optimum pH. Using this pH optimize the coagulant dosage and plot percentage removal versus coagulant dosage and select the optimum dosage.

test 10.2 Precipitation

Precipitation is a method for the removal of metalllic suspended solids, fats, oils, greases, and other organic substances from wastewater that are either dissolved or suspended in solution. These dissolved or suspended particles can be made to settle out of solution as a solid precipitate, which can then be easily separated using sedimentation, filtration or centrifugation. The voluminous precipitate formed can capture ions and particles causing sweeping of ions and particles from the wastewater. (Tchobanoglous and Burton, 1991). The process of precipitation can also be assisted through the use of coagulants.

The method used for precipitation will depend upon the contaminants to be removed from the waste water. Different methods were adopted for precipitation and they are described as below.

Metals Removal

Heavy metals are generally precipitated as hydroxide through the addition of lime or caustic soda. The addition of these chemicals change the pH of the water to a pH of minimum solubility of metallic ions present in it. The pH of minimum solubility varies with the metal and shown in the graph below (Figure 6).

When   treating   industrial   waste   water  containing   metallic   ions   as   the  contaminants,   the pretreatment of the waste water is necessary to remove substances that may interfere with the precipitation of these metals specifically ammonia and cyanide. These interfering contaminants form complexes  and  limit  the  removal  of  metals  by  precipitation.  Further,  cyanide  can be removed by alkaline chlorination, catalytic oxidation and ammonia can be removed by stripping  or other suitable methods prior to the removal of metals. Once rendered insoluble, these compounds will tend to precipitate and settle.

Since the optimal pH for precipitation depends both on the metal to be removed and on the counter ion used (hydroxide, carbonate, or sulfide), the best treatment procedure must be determined on a case by- case basis. After the optimum pH for precipitation of various metallic contaminants is established, the settling process can be accelerated by addition of a coagulant, which gathers the insoluble metal compound particles into a coarse floc that can settle rapidly.

Removal of Fats, Oils and Greases

Fats, oils, and greases are typically organic substances having density less than water. So, float on the surface forming the surface slicks rather than settling to the bottom of the water to be treated. They behave in this way because they are hydrophobic, nonpolar substances and are insoluble in water. These free floating slicks can be removed by skimming the surface of the solution. However, oils, fats, and greases can also be emulsified in aqueous solution by the addition of the surfactants. The addition of surfactants will disperse the oils and fats into small globules, which remain suspended in the aqueous solution. The substances like soaps and detergents act as surfactants and help in making hydrophobic substances soluble in water. These dispersed globules can be removed by destabilizing the electrical charge attractions that keep the localized globules of oil in solution. This can be done with the addition of a polymer which act as coagulant and leads to charge neutralization. In this way, the charge attraction of the oily particles is disrupted, allowing them to separate from the aqueous solution.

Phosphorus Removal

The treatment for phosphorus removal involves the addition of lime or metal salts (most commonly ferric chloride or aluminum sulfate, also called alum) to react with soluble phosphate to form solid precipitates that are removed by solids separation processes including clarification and filtration. Chemical treatment is the most common method used for phosphorus removal to meet effluent concentrations below 1.0 mg/L. When lime is used, a sufficient amount of lime must be added to increase the pH of the solution to at least 10, creating an environment in which excess calcium ions can react with the phosphate to produce an insoluble precipitate (hydroxylapatite). Lime is an effective phosphate removal agent, but results in a large sludge volume. When ferric chloride or alum is used, the iron or aluminum ions in solution

will react with phosphate to produce insoluble metal phosphates. The degree of insolubility for these compounds is pH dependent (Details discussed in Module 22). Moreover, many competing chemical reactions can take place alongside these, meaning that the amount of metal salt to add to the solution cannot simply be calculated on the basis of the phosphate concentration, but must be determined in the laboratory for each case (Tchobanoglous and Burton, 1991).

Suspended Solids

The fine particles suspended in solution can escape the filtration, sedimentation and other similar removal processes. Their small size allows them to remain suspended over extended periods of time. These suspended particles in the wastewater are negatively charged and so the cationic polymers are generally used to reduce the surface charge of the suspended particles thus causing particle to get coagulated and settled (Tchobanoglous and Burton, 1991). Alternatively, lime can be used as a clarifying agent for removal of particulate matter. The calcium hydroxide reacts in the wastewater solution to form calcium carbonate, which itself acts as a coagulant, sweeping particles out of solution.

The amount of chemicals required for treatment depends on the physic-chemical parameters as pH, alkalinity, phosphates present of the wastewater. The interference of the co-contaminants actually makes it difficult to calculate the quantities of chemical needed for the precipitation. The accurate doses should be determined by jar tests and should be confirmed by field evaluations.

References:

• Amirtharajah,  Appiah  and  O’Mella,  Charles  R.,  1990.  “Coagulation  Processes:
• Destabilization, Mixing, and Flocculation. Water Quality and Treatment, A Handbook of Community Water Supplies. “Ed, Pontius, Frederick W., AWWA 4th Ed. McGraw- Hill, Inc. NY.
• AWWA Committee Report, 1979. “Organic Removal by Coagulation: A review and Research Needs”. Journal AWWA 71(10): 585 – 603
• Bratby, J., 2006. “Coagulants, in Coagulation and Flocculation in Water and Wastewater Treatment.” 2nd ed., IWA Publishing, London, 50- 68.