Flocculation and Clarification

Flocculation and Clarification:

·         Clariflocculator unit is combination of a flocculation tank and clarification tank. This unit is a circular tank with concentric compartment for flocculation. The effluent in distributed uniformly over the surface of flocculation chamber for effective utilization of the available volume for flocculation. The specially designed flocculation paddles enhance flocculation of the feed solids. The flocculated effluent enters the settling zone from bottom.

·         As water moves upwards, heavy particles settle at bottom, the liquid flows radially outwards and upwards and the clarified liquid is discharged over a peripheral weir into the launder. The deposited sludge is raked to the bottom near the central pocket from which will be discharged into the sludge sump.

·         The clarified water will flow into the storage tank by gravity.

Coagulants and Flocculation

Objective:

Coagulants and flocculants enhance dissolved metal removal and reduce sludge volume during conventional acidic drainage and high-density sludge treatment.

Description:

Coagulants and flocculants are chemicals that can be added during acidic drainage treatment.  Although some chemicals can be considered both coagulants and flocculants (iron and aluminum salts), coagulation and flocculation are two distinct processes. 

Coagulation describes the consolidation of smaller metal precipitate particles into larger metal precipitate particles (flocs).  Coagulants reduce the net electrical repulsive force at the surface of the metal precipitate particles.  The purpose of adding coagulants to acidic drainage waters is to increase the number of flocs present in the treatment water.  As floc density increases, interparticle contact increases due to Brownian motion, promoting agglomeration of colloidal particles into larger flocs for enhanced settling (Qasim et al., 2000).

Coagulants are widely used in water treatment systems and but are not commonly used at conventional acidic drainage treatment operations.  The most common coagulants are aluminum and iron salts.  Aluminum and iron coagulants react with bicarbonate alkalinity (HCO₃^-) in acid drainage creating aluminum, ferric or ferrous hydroxide flocs which attract metals in solution through co-precipitation.  Formation of hydroxide flocs with alum and ferrous sulfate can be represented by the following equations (Faust and Aly, 1999):

       

Flocculation involves the combination of small particles by bridging the space between particles with chemicals (Skousen et al., 1996).  Essentially, coagulants aid in the formation of metal precipitate flocs, and flocculants enhance the floc by making it heavier and more stable.  For this reason, flocculants are sometimes referred to as coagulant aids at water treatment operations (Tillman, 1996; Faust and Aly, 1999).

Two main groups of flocculants exist: mineral which includes activated silica, clays, and metal hydroxides and synthetic which include anionic, cationic, and nonionic compounds.  Activated silica has been used as a flocculant since the 1930’s to strengthen flocs and reduce the potential of deteroriation (Skousen et al., 1996).  It is usually produced on-site by reacting sodium silicate with an acid to form a gel (Tillman, 1996).  When using activated silica, the resultant floc is larger, denser, more chemically stable, and settles faster than iron and aluminum flocs.   

Metal hydroxides, produced from neutralization and precipitation reactions during acidic drainage treatment, can be recycled to serve as flocculants.  This is commonly implemented at high density sludge treatment operations to increase solids content and stability.  

Clay particles have large surface area to mass ratios.  Clays have numerous negative exchange sites on their surfaces for metal adsorption.  The addition of clays into the acid drainage treatment reduces metals and results in more dense flocs which increase settling velocities of the flocs.

Synthetic flocculants consist of polymers which produce negative (anionic), positive (cationic), or both (polyampholytes or nonionic polymers).  Polyampholytes are neutral but release both negative and positive ions when dissolved in water.  The ions released from synthetic polymers (flocculants) adsorb to destabilized particles to form larger flocs.  According to Tillman (1996) cationic polymers are most often used for charge neutralization and are usually used in conjunction with a metallic coagulant to reduce the dose required and amount of sludge produced.  Anionic polymers dissolve in water to provide more reaction sites for positively charged coagulants.  A drawback to using synthetic flocculants is that over-dosage may hinder their efficiency.

Both coagulants and flocculants enhance particle setting rates as particle settling velocity is directly related to particle size and weight.  Naturally, large relatively heavy particles settle more rapidly than smaller, light particles.  In addition to decreasing settling times, heavier flocs can reduce the amount of sludge volume produced in acid drainage treatment facilities.  This is the reason flocculants are used during high-density sludge treatment operations.

Climate:

Conventional lime and limestone and high density sludge treatment of acidic drainage waters can be implemented in all climates. 

Treatment Process:

Coagulants and flocculants are typically released into acidic drainage waters after the addition of the neutralizing reagent, prior to the aeration basin (if present) or settling basin if an aeration basin is not used.  Refer to conventional lime and limestone treatment and high-density sludge.

Requirements and Limitations of Use:

Coagulants and flocculants are used in existing acid drainage treatment facilities when: 1) unique water chemistry or high metal concentrations exist in the acid drainage water or 2) when residence time in the settling basin is not adequate to facilitate efficient metal precipitation and sludge accumulation (Skousen et al., 1996).

Many trace metals such as As (a metalloid), Cd, Cu, Mo, Ni, Pb, and Zn do not precipitate as metal hydroxides, although they can be removed from acidic drainage from co-precipitation on the surfaces of metal hydroxides.  An acidic drainage effluent could contain high amounts of trace metals with inadequate dissolved iron and aluminum concentrations for effective co-precipitation.  Aluminum or iron coagulants can be added during conventional treatment of the above effluent to enhance co-precipitation.  MEND (1994) recommends a 2:1 iron to arsenic ratio for effective arsenic removal.  Additionally, barium sulfate is used to precipitate radionuclides such as radium.

Coagulants and flocculants may also be helpful at acidic drainage treatment operations which either do not use mechanical aerators or have insufficient residence time in the settling basin.  Coagulants and flocculants form larger, heavier, metal precipitate particles which can settle rapidly and scavenge dissolved metals which have not been oxidized.

Alum (aluminum sulfate or Al₂(SO₄)₃ is the most commonly used coagulant in water treatment.  Alum is an effective coagulant for pH values ranging from 5.5 to 7.5 (Table 1).  Alum can be applied as a solid or liquid. 

Iron coagulants are ferric sulfate (Fe₂(SO₄)₃), ferrous sulfate (FeSO₄) and ferric chloride (FeCl₂).  Iron compounds are generally cheaper, produce a heavier floc, and perform over a wider pH range than aluminum coagulants (Tillman, 1996).  However, iron coagulants are not used as much as aluminum due to staining equipment, corrosiveness, and they require more alkalinity than alum.  Ferric sulfate is active over a larger pH range (4.0-6.0, 8.8-9.2) than ferrous sulfate (8.8-9.2) (Table 1) and produces heavier flocs which settle more quickly.  Ferric chloride reacts in a manner similar to ferrous sulfate but is commonly used as an oxidant (Skousen et al., 1996).  Ferric chloride is effective over a much greater pH range than aluminum sulfate, ferric sulfate, and ferrous sulfate (Table 1).

Table 1. Coagulants and their effective pH ranges.  From Tillman (1996).

Effective pH Range	Coagulant
5.5 to 8.0	Aluminum Sulfate
4.0 to 6.0, 8.8-9.2	Ferric Sulfate
8.8 to 9.2	Ferrous Sulfate
4.0 to 11.0.	Ferric Chloride

The above coagulants are also considered flocculants because they form metal hydroxides.  Reactions involving iron and aluminum coagulants require alkalinity and can decrease pH.  Therefore, additional alkalinity in the form of a neutralizing reagent must be added to account for the extra alkalinity use (Refer to calculations).

Alkalinity requirements for coagulants (Faust and Aly, 1999):

Assumptions

1)      all alkalinity in system is bicarbonate (HCO₃^-)

2)      calculated alkalinity expressed as CaCO₃

3)      100% coagulant content-additional calculations may be required based on state of coagulant (solid or liquid) and coagulant content.

Alkalinity required for alum

Reaction between alum and natural alkalinity:

From the equation, it is apparent that 1 mole alum reacts with 3 moles Ca(HCO₃)₂

On a weight basis, 1 mg/L alum reacts with:

                                            

Converting Ca(HCO₃)₂ to CaCO₃:

                            

The above calculation indicates that a 1 mg/L addition of hydrated alum at an acidic drainage treatment plant requires an additional 0.45 mg/L CaCO₃ alkalinity.

Alkalinity required for ferrous sulfate

Reaction between ferrous sulfate and natural alkalinity:

From the equation, it is apparent that 1 mol Fe₂(SO₄)₃ reacts with 3 moles Ca(HCO₃)₂

On a weight basis, 1 mg/L  Fe₂(SO₄)₃ reacts with:

           

Converting Ca(HCO₃)₂ to CaCO₃:

           

The above calculation indicates that a 1 mg/L addition of hydrated alum at an acidic drainage treatment plant requires an additional 1.10 mg/L CaCO₃ alkalinity.

Predicted Performance:

Coagulants are not commonly used in acidic drainage treatment operations.  However, iron and aluminum salts can be used for trace metal (and metalloid) removal under special circumstances.  Water treatment operations have experienced good results using coagulants to remove dissolved metals (Tillman, 1996; Faust and Aly, 1999).

Metal hydroxide flocculants, formed during acidic drainage treatment, is recycled to create high density sludge.  High density sludge treatment processes can increase solids content from 1-5% to as high as 40% (MEND, 1994). 

Synergistic Technologies:

Conventional Lime and Limestone Treatment

High-Density Sludge (HDS)

Oxidants

Costs:

List costs of aluminum and iron salts

Flocculants used in high density sludge operations are recycled metal hydroxides.  Refer to high density sludge for additional costs associated with this technology.

Case Studies:

No available case studies on the use of coagulants or flocculants were found in the acidic drainage treatment literature.  Refer to high density sludge for metal hydroxide flocculants.

References:

Faust, S.D., and Aly, O.M., 1999, Chemistry of Water Treatment: New York, Lewis

Publishers, 581 p.

MEND, 1994. MEND Report 3.32.1: Acid mine drainage-Status of chemical treatment

and sludge management practices, Mine Environmental Neutral Drainage (MEND), Canada.

Qasim, S.R., Motley, E.M., and Zhu, G., 2000, Water Works Engineering: Planning,

Design, and Operation: Upper Saddle River, New Jersey, Prentice Hall, 844 p.

Skousen, J., Lilly, R., and Hilton, T., 1996, Special chemicals for treating acid mine

drainage, in Skousen, J., and Ziemkiewicz, P.F., eds., Acid Mine Drainage: Control and Treatment: Morgantown, West Virginia, West Virginia University and National Mine Land Reclamation Center, p. 173-180.

Tillman, G.M., 1996, Water Treatment: Troubleshooting and Problem Solving: Chelsea,

Michigan, Ann Arbor Press, 156 p.

CLARIFIER PRINCIPLE OF OPERATION

PRINCIPLE OF OPERATION

The Solids Contact Clarifier works on two basic principles of Coagulation / Flocculation and Hydraulic Separation.

Coagulation and flocculation occur in the flocculation zone when the feed stream comes in intimate contact with chemical mixtures and suspended sludge particles from previously treated water.  This contact also promotes floc growth as smaller particles agglomerate into larger heavier particles.

The hydraulic separation principle uses an upflow design to move water into the settling zone for clarification.

WORKING

Raw water enters the central draft tube above the re-circulator impeller where it is mixed with treatment chemicals and re-circulated precipitates.  Precipitate re-circulation is accomplished by a variable speed impeller which acts as an air lift pump.  The mixture of raw water and sludge rises through the central draft tube and is discharged into the flocculation compartment.

This mixture undergoes gentle hydraulic turbulence as it passes through this zone.

Part of the water (equivalent to the instantaneous influent flow rate) enters the clarified water zone and rises toward the effluent collector.

Settled precipitates (sludge) are moved continuously along the floor toward the centre of the unit by means of a slowly rotating scraper, which covers the entire floor area.  The accumulated sludge is transferred to the sludge pit at the centre of the unit, where sludge-thickening pickets concentrate the sludge, reducing the total amount of blowdown.

Excess sludge is removed by a blowdown system.  Backflushing with water under pressure clears the blow-off line.  Opening of the blow-off valve permits the sludge to flow to waste.  These operations are either manual or automatic.  Most of the water and suspended precipitates enter the lower end of the draft tube to be re-circulated, providing positive solid contact regardless of sludge inventory level.  Clear water rises and is uniformly collected.

PLATE TYPE CLARIFIER

The  Plate Type Clarifier is a new development in filtration technology for effective removal of High Suspended Solids present in the water.

Clarity of liquid overflow and desnity of underflow discharge are the two fundamental process requirements of all gravity settling equipment. 

In many applications the area needed to provide the desired overflow clarity exceeds that required for thickening of the settled solids.  This means that, in a cylindrical settling tank, the lower section including the rakes and drive mechanism is overdimensioned.

Pre-treatment is given to water to make it suitable for subsequent treatment, which makes it suitable for use in a particular process.  Actually what we term as pre-treatment plant is a misnomer.  The pre-treatment plant generally consists of Coagulations, Clarification and Filtration.  If one examines the worldwide statistics of water treatment plants - 85% of the plants will fall in this category.

COAGULATION :

The suspended solids in water acquire a negative charge which prohibits these particles incoming together to form a large mass for easier settleability.  Coagulants such as Alum, Ferrous sulphate, Ferric chloride - which have positive charged ions like aluminum, Iron are then dosed into the water to hasten the settling of the suspended particles.  Suitable dosing system comprising tanks, pumps etc. are employed for this purpose.  Coagulated particles are known as “Flocs”.

CLARIFICATIONS :

The purpose of clarification is to bring the flocs together to form a larger mass.  The larger the mass of flocs, the heavier the particles and easier settleability.

Inclined plate clarifier with solid contract re-circulation achieves solid - liquid separation by directing the liquid between a series of inclined plates.  The settling surface of each plate is equivalent to its horizontal projection.  These plates are normally spaced approx 50 mm apart, with the result that large settling surfaces are concentrated within a relatively small floor area.

EQUIPMENTS :

FLOCCULATION CUM FLASH MIXING TANK :

This tank is constructed with MS plates and having a circular cross section with a diameter of 2200 mm and a conical bottom.  There are TWO compartments in this tank.  In flash mixing chamber a motorized agitator with 0.5 H.P. motor is mounted.  In flocculator chamber a geared motor       mixer is mounted.  This tank is internally painted with coal tar epoxy after sand blasting.

CLARIFIER :

Constructed from MS plates, rectangular in cross section with a conical     hopper bottom.  FRP plated are fitted inside the clarifier with an equidistant of 50 mm apart.

OPERATION :

Raw water is pumped to a chamber outside the flash-mixer tank, Coagulant (Alum) is dosed into this chamber through a dosing pump.  Water then enters the flash-mixing chamber.  Mixing of coagulant with water takes place with the help of motorized mixer and then it enters the flocculation chamber.  Slow speed agitator mounted on this chamber rotates at gentle speed, which coalesces the floc.  The overflow of flocculator tank passes into the clarifier chamber.  The clarifier has a launder at its inlet.  Water flows through this launder to the bottom portion and slowly rises up in between the plates.   The particles settle down on the plates and slide down to the sludge-collecting zone.  Clear water moves up and is collected in the outlet launder through pipes having small orifices.  These small orifices regulate the laminar flow of water to the outlet launder.   A sample to be drawn from the bottom portion of clarifier where ½” tapping is provided to check for sludge concentration.  Whenever sludge concentration is found more, open the valves provided at the drain nozzles and drain the sludge till clear water comes out.

Cross sectional drawing showing the operation is enclosed for reference.

MAINTENANCE :

The advantage of this type of clarifier is fact that not only it saves area but very easy to operate and maintain.  the only rotary equipments are two small motors, a small gearbox, which can be repaired / replaced quickly if needed.

The FRP plates can be lifted one by one by hand (Weight approx 6 kg) from the platform.  They can be cleaned and put back again with out affecting treated water quality.   This exercise is to be monitored bimonthly or monthly.

ADVANTAGES  :

1.                  Heavy Duty rigid construction of tank, sludge hopper and plate packs.

2.                  White spacing of plates to handle high density feed pulps and coarse solid particles.

3.                  Lower capital costs

4.                  Lower installation costs.

-          One piece or pre-fabricated delivery

-          Smaller foundations

-          Less building space

-          Rectangular plan suits conventional building structures

5.                  Prevents short circuiting and reduces turbulences from surface wind or feed temperature variations.

6.                  Simple flat sheet and standard section construction

-          Special materials and protective coatings easily incorporated.

-          Repair and replacement of parts is simplified.

water treatment

India has 16 percent of the world's population, but comparatively only 2.5 percent of the earth's land mass and 4 percent of its water resources. These already limited water resources are depleting rapidly while at the same time the demands on them are increasing. India has intermittent drinking water supplies and poor transmission and distribution networks for water. Currently, 75 percent of the rural population and 85 percent of the urban population have access to public water supplies. However, municipal agencies in many Indian towns and cities are unable to increase their water supply capacities to match population growth, especially in the urban areas. According to a recent government assessment, the water requirement for industrial use will quadruple from the current 30 billion cubic meters to 120 billion cubic meters by 2025.

The Indian water sector consists of both a drinking water/bottled water segment and a wastewater treatment equipment segment. Both are growing at a relatively high pace due to increasing health concerns and a scarcity of clean water. The total Indian water market exceeds $ 8 billion. The government sector contributes a little more than 50 percent, with the rest of the business coming from the private industrial sector. The overall water market is growing at 10 -12 percent annually, with even higher growth rates in the industrial and drinking water segments. The wastewater treatment market segment is highly fragmented and unorganized. Imports constitute approximately $110 million of the $690 million market for municipal and industrial water treatment equipment. The U.S. is India's principal source of imports of water treatment equipment, with an estimated share of 40 percent.

A growing population has increased the demand for drinking water and rapid urbanization has required increased sewage treatment. Many industries had to adopt water-recycling systems due to the scarcity of water. Growing public concern, media pressure, and renewed legislation has left industries with no option but to install water treatment equipment. At the state and local level, there is a compulsory requirement of environment clearances from pollution control boards. The recent Supreme Court directive to move polluting units out of Delhi is also likely to act as an impetus to future sets of water treatment equipment. In addition, many existing treatment plants will need to be replaced or upgraded to meet with more stringent standards.

The water treatment market is moving gradually from chemical treatment and demineralisation plants to membrane technology. However, several large user segments such as refineries and power plants continue to use demineralisation technology. Zero discharge systems and wastewater recycling are becoming increasingly popular in India.

Role of Automation in Water & Waste Water Treatment Industry

Water being a critical utility in many industries (pharma, electronics, power, etc), it is important to evaluate the benefits of automation on water and wastewater treatment plants. Let us consider certain examples to understand the importance of automation in water & waste water treatment.

• Bottled water Industry & beverage Industry: If the conductivity and pH of the treated water are not monitored and controlled, they will not meet the norms specified for packaged / bottled water specified by BIS.
• Pharma industry: This industry requires high purity water for formulations (purified water) and Injectables. The quality parameters to be maintained are Conductivity (<1mS/cm) and TOC (<500 ppb). Apart from this, most high purity water systems today prefer non-chemical treatment processes favoring RO-EDI-UF system. One of the feed limitations to RO is that there should be no free chlorine content. This necessitates the need of an ORP (Oxidation-Reduction Potential) meter.
• Waste water treatment: The key design parameters for efficient waste water treatment includes monitoring dissolved oxygen (DO) levels during various cycles of operation. In the absence of instrumentation to monitor it, the treated effluent will not be able to consistently meet the discharge norms with respect to BOD/COD/ammonical nitrogen/phosphorous levels.

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