Category: water treatment

Coagulation water treatment process

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The purpose of coagulation water treatment process is to removes the colloidal particles from water. The water may contain suspended matter, small or large solid particles. Sedimentation and filtration processes can removes most of the solid particles but the small particles that are remains in colloidal suspension cannot removes. If they clump together and form larger particles, then it would be possible to removes easily. But a negative charge prevents them to coagulate; as like two same magnetic poles repulse each other. They are very stable in colloidal system. If we are able to neutralize this charges, then they would be consolidate into coarse formations. For this purpose we add a chemical that produces positive charges. This chemical is known as coagulant. The positive charges of the coagulant neutralize the negative charges on the colloidal particles. As a result the particles are able to coagulate into coarse formations which are easily removable.

The process of consolidation of colloidal particles by neutralizing the charges with a coagulant, so that they can remove from the treated water by sedimentation or filtration is called coagulation. It is a vital part for drinking water and wastewater treatment.

Coagulants

Coagulants are the chemicals that are used to removes tiny particles in water. We used different types of coagulants in coagulation water treatment process. Generally, we can categories the common type of coagulant into two groups, aluminium base and iron base. The iron base coagulants include ferrous sulfate (FeSO4.7H2O), ferric sulfate, and ferric chloride. On the other hand aluminium base coagulants include aluminium sulfate (Al2(SO4)3.18H2O), aluminum chloride, sodium aluminate and polyaluminum chloride (PACl). Although some other metal salts as like titanium and zirconium are highly effective, but are costly or rare available.

Coagulation Mechanism

The colloidal particles carry electrical charges; normally negative charge. So the opposite charges coagulant is added to the water to overcome the repulsive charge and “destabilize” the suspension. Usually a metallic salt like alum is added as a coagulant to create positively charged ions. Normally 5-10% solution of coagulant is used. The alum and ferrous sulfate are hydrolysis according to the following equation-

Al2(SO4)3 + 6 H2O ↔ 2Al(OH)3 + 3H2SO4
H+ + HCO3– → CO2 + H2O

FeSO4 + 2 H2O → Fe(OH)2 + H2SO4
H+ + HCO3– → CO2 + H2O
But Fe(OH)2 does not act as coagulant, it oxidized into Fe(OH)3 by consuming dissolved oxygen in water and act as coagulant.
H2O + 2 Fe(OH)2 + ½ O2 → Fe(OH)3
This reaction proceeds easily in an alkaline medium.
Coagulation water treatment process

Factors affecting coagulation water treatment

The process of coagulation of water depends on various factors like pH of the medium, temperature of water, coagulant feed concentration, coagulant dosage, type of coagulant, mass and initial turbidity. Moreover it is also depends on pre-treatment and type of pollutants present.

Effect of pH on coagulation

pH affects on the activities of coagulants. The optimum pH for alum coagulation is 6 to 7.5 whereas 5.0 to 8.0 are for iron. If the alkalinity is lower or higher, then the floc does not form properly. As a result, more coagulant is consumed. In this case, it is beneficial to correct the pH by adding acid or base.

Temperature

Temperature is another factor for coagulation water treatment process. It is more significant at lower turbidity. In case of alum, at low temperatures aluminium hydroxide form a strongly hydrated and very stable sol. So in winter season high coagulant are consumed. When the temperature becomes below the 5⁰C, then alum or ferric salts do not work properly. So it should be consider another coagulant like polyaluminum chloride (PACl).

Type of pollutants

The salt composition of soft water and hard water are not same. Hard water contains Ca2+ and Mg2+ ions. They can alter the charge on the colloidal particles.

Optimum dosage

It is very significant to determine the optimum dosage of a coagulant which will give the maximum clarifying effect. Insufficient amount of coagulant cannot able to destabilize properly of the colloidal particles. On the other hand higher dosage can cause excessive sludge production, corrosion and loss of money.

Type of coagulant

All the coagulants are not suitable for all cases. Different temperatures, pH, type of medium may vary the effectiveness of the coagulant. At lower temperature the polyaluminum chloride (PACl) may be more effective than the traditional coagulants like alum or iron salt. Same way, some pH range can be beneficial to use iron salt instead of alum.

Coagulation jar test

You can determine optimum process condition like dosage, pH by jar test experiments. Generally, it consist several jars filled with equal volume of water. Then test for various dosage of coagulant, pH etc.

Conclusion

Pre-filtration and sedimentation is more effective before the coagulation water treatment process. Moreover, you should follow the following sequence of chemical addition for coagulation of water; firstly add chemicals for pH correction, then add the metal coagulant, after that add the flocculent aid. But remember all the chemicals may not be necessarily for all type of water. In addition, widely many industries practices enhanced coagulation process which is also removes disinfection byproduct (DBP) precursor, color causing compounds.

Internal treatment and control of boiler feed water

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Even if external water treatment is designed and executed, it is inevitable that problems such as corrosion and scale still occur in the long run of the boiler operation. Boiler water treatment chemicals are therefore used for protecting a boiler plant from the problems and are applied for the whole systems including the feed water line and the steam condensate line. In case of internal boiler water treatment, chemicals are added to the boiler to prevent the formation of scale by converting the scale forming compounds to free flowing sludges, which can be removed by-blow down. The chemicals are also added to prevent corrosion, priming and foaming.

Purpose of using boiler water treatment chemicals

  • Controlling pH and alkalinity to prevent scale formation and corrosion
  • Softening to prevent scale formation
  • Sludge dispersing for easy blow down to prevent scaling
  • Oxygen scavenging to prevent corrosion
  • Preventing foaming of boiler water
  • Neutralizing and film forming to prevent corrosion

Boiler water treatment chemicals and their functions

Phosphate base chemicals and alkali agent

Phosphate base boiler compounds for low-pressure boiler contain phosphates and alkali agents. Both phosphates and alkalies take part in preventing scale formation and corrosion on heating surfaces of a boiler by

  • reacting with hardness components
  • keeping silica substances water soluble
  • controlling of alkalinity of boiler water

In a medium pressure boiler caustic or coordinated phosphate treatment is applied. For high pressure boilers, in the light of high heat flux, a coordinated phosphate treatment is employed in order to prevent alkali corrosion caused by the concentrated free alkali. The chemicals generally used are : Na3PO4, NaHPO4, (NaPO3)6, NaOH etc.

Non-phosphate base chemicals

1) Polymer base compounds
Synthetic polyelectrolyte, Natural polyelectrolyte, etc. are used to prevent scale formation. The difference between polymer base and phosphate base boiler compounds is in the function that they prevent the scale formation of hardness components. The preventive mechanism is considered as follows:

  • Inhibit the crystal growth of scale components
  • Inhibit the crystallization of scale components
  • Keep the solids in suspension

2) Chelating agent
In chelate treatment, the hardness components contaminated in feed water are kept in a soluble state by the chelating agent. Typical chelating agents are ethylenediamine tetra acetate (EDTA) and nitrilo triacetate.

Sludge dispersant:
Hardness compounds and silica which enter into the boiler from the feed water become sludges of hydroxyapatite, magnesium hydroxide, magnesium silicate etc. by the effect of boiler compounds. Most of these are easily suspensible in water and are discharged by blow down from the boiler. In order to prevent the accumulation of sludge on the bottom of drum and to prevent scaling on the heating surface by hydroxyapatite, etc., sludge dispersants are frequently employed. Synthetic polymer e.g. acrylic acid polymer is generally used as Sludge dispersant.

Oxygen scavenging chemicals
Although oxygen can be reduced to less than seven parts per billion in modern deaerators, it is still necessary to reduce oxygen even further. This is accomplished by chemical scavenging in which either sodium sulfite (Na2SO3), hydrazine ( N2H4), hydroxylamine derivatives, hydroquinone/progallol-based derivatives is fed into the boiler feed water.

Sodium sulfite
Sodium sulfite is the most common scavenger in industry. The reaction involved is the following,

Na2SO3: +  O2 →  Na2SO4

Such that oxygen is removed to produce soluble sodium sulfite.  Sulfite is fed at about 40 parts to 1 part oxygen.

Hydrazine
The alternative to sulfite treatment is hydrazine, which has traditionally been used on higher pressure boilers. One reaction by which hydrazine removes oxygen is a two step process.

4Fe3O4 + O2 → 6Fe2O3

6Fe2O3 + N2H4 → 4Fe3O4 + 2H2O + N2

One interesting feature of these reactions is that in the first reaction the oxygen breaks down the protective oxide coating to form rust. In the second reaction, the hydrazine reduces the rust to the desired passive form. Only three parts 35 percent hydrazine solution is required per part of oxygen.
In using either hydrazine or sulfite, it is important to apply the following rules,

  • Feed the scavenger at earliest practical point
  • Feed continuously
  • Maintain residual sulfite or hydrazine according to levels recommended below

After the scavenging process of oxygen removal, the dissolved oxygen should be less than ten parts per billion.

Corrosion inhibitors
Volatile amines and filming amines are typically used as corrosion inhibitors to prevent the steam and condensate lines from corrosion. Volatile amines inhibit the corrosion by controlling the condensate pH. Filming amines are applied to form a water repellent film on metal surfaces which prevents direct contact with the corrosive substances such as oxygen and carbon di-oxide. Volatile amines eg., ammonia (NH3), cyclohexyl amine (C6H11NH2) etc., and filming amines eg., alkyl amines (RNH2) ( R=C10-C22) are used as corrosion inhibitors.

Antifoaming agents
Antifoaming agents are chemicals used to control foaming on the boiler water surface, caused by an increase of the dissolved or suspended solids or by contamination of oil and fat or organic matter in the boiler water. Chemicals of amide based, alcohol based or fatty acid ester based chemicals are used as antifoaming agents. These chemicals change the surface tension of the water.

Water quality control method for boiler feed water system

1) Water quality control target value
The boiler water quality control target value is set on the basis of standard criteria and considerations of the structure and operation conditions of a boiler. The following standard values for different parameters of boiler water should be maintained for smooth operation of boiler.

a) Control items for feed water

  • pH
  • Total hardness
  • Dissolved oxygen
  • Total iron and total copper
  • Conductivity
  • Silica
  • Hydrazine

b) Control items for boiler water

  • pH
  • Alkalinity
  • Total evaporation residue
  • Electrical conductivity
  • Chloride ion
  • Phosphate ion
  • Sulfite ion or hydrazine
  • Silica

2) Boiler water treatment methods

  • Caustic treatment
  • Coordinated phosphate treatment
  • volatile treatment

3) Concentration control of boiler water
Blow down is done to control the concentration of boiler water and to discharge sludge, impurities contained in the feed water and solids in the chemicals added.

4) Control of chemical injection
The purpose of controlling the chemical injection includes the adjustment of the water quality of the boiler system to the specified values by sufficient effect of the functions of chemicals chosen for an individual boiler. Some chemicals do not exert their total effect and in addition, may cause secondary problems depending on the excess or shortage of dosage and the point or method of injection. The control of chemical injection involves the following

  • Dissolution and dilution
  • Determination of dosage-Initial dosage, maintenance dosage
  • Injection point

External Boiler feed water treatment

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Water is treated properly before feeding boiler. External Boiler Feed water treatment is the decrease or elimination of contaminants from water outside the boiler. It involves removal or reduction of some undesirable characteristics and addition of desirable characteristics to make water suitable and less troublesome for a proposed application before it is delivered to the point of use.

With object to prepare boiler feed water, the water after conventional treatment is further treated before using as boiler feed water. The following steps are associated in external boiler Feed water treatment.

  • Demineralization to remove dissolved minerals, salts or acids.
  • Dearation to remove dissolved oxygen

Demineralization of boiler feed water
The following steps consist in demineralization section:
– Cation exchanger
– Degasifier
– Anion exchanger
– mixed Bed polisher

Cation exchanger
In demineralization process, hydrogen cation exchanger is generally employed to remove all cations from water. Filtered water is passed though the cation tower, which is filled with hydrogen cation exchanger. In cation exchanger the following reactions are taken place. Therefore the scale forming cations such as Ca, Mg ions are replaced by the labile hydrogen, and water is set free from those ions.

Ca(HCO3)2 + 2HR  =  CaR2 + 2H2O + 2CO2

Mg(HCO3)2 + 2HR  =  MgR2 + 2H2O + 2CO2

NaHCO3 + HR  =  NaR + H2O + CO2

Here the symbol R stand for the organic radical.
The following reactions are taken place between the resins and sulfates or chlorides ions.

CaSO4 + 2HR  =  CaR2 + H2SO4

MgSO4 + 2HR  =  MgR2 + H2SO4

NaCl + HR  =  NaR + HCl

Regeneration with sulfuric acid is the most widely used and most economical method of regeneration. The reactions, in condensed form, may be indicated as follows:

CaR2 + H2SO4 =  CaSO4 + 2HR

MgR2 + H2SO4 =  MgSO4 + 2HR

2NaR + H2SO4 =  Na2SO4 + 2HR

Carbonic acid is produced by the reactions with bi-carbonate salts. In sequence which is removed as CO2 gas. Sulphate and chloride ions in water give rise to mineral acids. The resins are turn into corresponding salts and the feed water becomes acidic soft water.

Degasifier
Most of the CO2 is removed from acidic soft water in degasifier. This is obtained by spreading water over raching rings and air that flows there in counter current direction by the blower.

Anion Exchanger
Then water is passed though the anion exchanger. Which is filled with two types of organic synthetic resin, i.e highly basic or weakly basic. Both types can remove strongly ionized acids for example sulfuric acid, hydrochloric acid or nitric acid. But only highly basic anion exchange resin can remove weakly ionized acids such as silicic and carbonic acids. For the anion exchange of a strongly ionized acid the following reaction may take place, where R4N represents the complex anion-exchanger radical.

H2SO4 + 2R4NOH → (R4N)2SO4 + 2H2O

Regeneration
Highly basic anion exchangers are regenerated with caustic soda, and weakly basic anion exchanger may be regenerated with caustic soda, soda ash or sometimes ammonium hydroxide.

(R4N)2SO4 + 2NaOH  →   2R4NOH + Na2SO4

Mixed Bed Polisher
Lastly the water is passed though the mixed bed polisher containing cation exchange resins and anion exchange resins to remove the ions left in water after passing though anion exchange resins demineralized water thus produced is used as make-up for boiler feed water.

Deaeration
Mechanical Deaerator
The simplest process for removing dissolved gases like oxygen is by the use of an open tank containing the water and dissolved oxygen, where most of the dissolved oxygen is stripped of by means of low pressure steam.

The Deaerator consists of a horizontal tank provided with a packed tower. During normal operation, the demineralized water is introduced above the packed bed & the stripping steam is introduced under the bed, controlled by the pressure controller. During start up steam can be introduced at the bottom of the tank, to heat up the stored quality of water.

The deaerator is operated at approx. 132⁰C in order to ensure proper shipping of the water, it is significant that an additional of steam be available in the bed, as a result the inlet temperature of the water to the Deaerator is kept (5-20)⁰C bellow the temperature level in the deaerator. It is checked that a reasonable flow of steam leaves the deaerator through the flow orifice. The deaerated water is called as the boiler feed water. The quality of stripped demineralized water is determined by analyzing samples taken from sample line. The boiler feed water from the deaerator pumped by the BFW pumps to the auxiliary boiler.

Boiler feed water treatment

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The function of the boiler is producing steam. The effectiveness of the boiler is depends on the water. Hence the boiler feed water quality is an important factor for boiler system. Lots of problems in the boiler system caused by water can be prevented in many cases if the water quality is controlled properly. Therefore, it is required boiler feed water treatment perfectly. In boiler, water quality control target values are set on the basis of standard criteria and operation condition of a boiler and these are maintained for smooth running of the boiler.

Necessity of boiler feed water treatment:

  • To obtain maximum boiler efficiency
  • To enhance the life time of boiler tubes
  • To prevent corrosion due to condensate quality
  • To prevent down time caused by troubles arisen from water quality

Boiler Problems caused by water

  • Scale Problem: Scale development is highly unwanted. If the boiler water contaminant with some dissolved solids, it will reduce the boiler performance and boiler life. When the steam is generated, the solid content become concentrated and form deposit in the bottom of the boiler. As a result poor heat transfer and it need to be overheating the boiler tube. Therefore decreases the effectiveness of the boiler. More over increase the fuel and maintenance cost.
  • Corrosion Problem: If the boiler feed water contains some gases as like oxygen, carbondioxide, and ammonia it will harmful for boiler. These gases will react with the boiler plate and others metals and lead to boiler corrosion.
  • Priming and foaming problem

Water treatment for boiler
Water is treated suitably before feeding boiler. It also requires some physical and chemical treatment inside boiler to prevent any detrimental effects due to water. The main function of boiler water treatment is to protect the boiler from corrosion and scale formation. The water conditioning for boiler is broadly classified into two types of boiler feed water treatment:

  1. External boiler water treatment
  2. Internal boiler water treatment

Necessity of cooling tower water treatment

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Cooling water is used in any kind of industry and trouble in a cooling water system affects the operation of the whole production plant as well as the system itself. Therefore, the suitable quality of the cooling water must be controlled so that trouble free operation may be ensured.

Problems in cooling water system and their prevention
Problems commonly encountered with cooling water are

  • Corrosion
  • Scaling
  • Fouling

Cooling water corrosion
Corrosion is the destructive attack of a metal by chemical or electrochemical reaction with its environment. Corrosion can be viewed as the process of returning metals to their natural state – the ores from which they were originally obtained. Corrosion entails the conversion of a metal from the atomic to the ionic state, with the loss of one or more electrons. Corrosion of metals in water involves electrochemical reaction of the metal in presence of dissolved oxygen. The basic reactions of the electrochemical corrosion of carbon steel (the most widely used material for heat exchanger) are given below:
Anodic reaction Fe → Fe2+ + 2e–

The electrons that are released flow through the steel, as they do through the wire of a galvanic cell, to a cathodic region where they react with dissolved oxygen.
Cathodic reaction O2 + 2H2O + 4e– → 4OH–

The Fe ions formed in the anodic region travel to the cathodic regions through the water adjacent to the steel, just as ions travel through a salt bridge in a galvanic cell. In the cathodic regions Fe2+ ions react with oxygen to form Ferrous hydroxide.
2Fe + 2H2O + O2 → 2Fe2+ + 4OH– → 2Fe(OH)2

Ferrous hydroxide precipitates from solution. However, this compound is unstable in oxygenated solutions and oxidized to the ferric salt (rust)
2Fe(OH)2 + H2O + O2 → 2Fe(OH)3
or Fe2O3 + 3H2O (corrosion products)

The anodic reaction cannot proceed without a corresponding cathodic reaction, which is a reduction process.

Effects of cooling water corrosion

  • Reduction of the efficiency and life time of heat exchanger
  • Interruption of production and contamination of products
  • Accident due to sudden failure of cooling system
  • Reduction of the strength of material
  • Reduction of the flow rate of water
  • Increased cost of maintenance

Factors influencing corrosion in cooling water system

  • Dissolved salts
  • Dissolved gases
  • Water temperature
  • pH of the cooling Water
  • Flow rate of water
  • Deposits/Fouling

Control of cooling water corrosion

  • Changing corrosion environment
  • Removing aggressive components from cooling water
  • Using corrosion inhibitors in cooling water

Cooling water corrosion inhibitors: A corrosion inhibitor is a substance which when added in small concentrations to a corrosive environment decreases the corrosion rate.

Principles of cooling water corrosion inhibition action
Corrosion inhibitors for cooling water system are water soluble; however they form insoluble films on metal surfaces. This film is called protective film and inhibits corrosion reaction by preventing the hydration of metal ions or reduction of dissolved oxygen on the metal surface. An inhibitor becomes effective only after its concentration in water reaches a certain level. The minimum concentration required for an inhibitor to become effective is known as its critical concentration. Thus, a critical concentration must be maintained constantly if an inhibitor is to exhibit the desired effect.

Classification of cooling water corrosion inhibitors

  • Oxidized film type: Chromates, Molybdates, Nitrites
  • Precipitated film type: 1. Submerged ion type: Poly phosphate. Zn salt, phosphonets
    2) Metal salt type: Triazol, Mercapto benzotriazole
  • Adsorption film type: Amine, surfactant- organic products
  • Mixed inhibitors: Zn/chromate, Zn /polyphosphates, Zn /phosphonates

Cooling water Scaling
Scaling is the formation of thick layer on metal surface of suspended or dissolved substances in cooling stream by means of different physical and chemical action.

Sources of water scale
Scale usually consists of calcium carbonate, iron oxides and sulfides,  silica, calcium phosphates and sulfates etc. The sources of water scale are as follows:

  • From soluble constituents in water

    • Ca2+ + 2HCO3– → CaCO3 + H2O + CO2
    Mg2+ + SiO2 → MgSiO3
    Ca2+ + SO42- → CaSO4

  • From treatment products

    • Ca + 2PO43- → Ca3(PO4)2
    Zn2+ + 2PO43- → Zn3(PO4)2

  • From soluble contaminants in the air

    • H2S + CrO42- → Cr2O3
    H2S + Fe2+ + FeS

Mechanism of scale formation
Initially a very thin layer of the scale components is formed on the metal surface. Gradually crystal nuclei are formed from the numerous microcrystals to glow the crystal grains. The coarse grained crystals are then coagulated to promote the formation of scale. The solubility product of the dissolved ions, temperature, pH of the cooling stream and corrosion product are countable factors for scale formation in the heat exchanger.

Effect of scale formation
Since the thermal conductivity of the scale is extremely low in comparison with that of the tube material, the scale adhesion remarkably lowers the thermal efficiency of the heat exchanger. Excessive scale growth in the tube may clog them. Cooling water scale also accelerates the corrosion on the metal surface. Effects of scaling are as follows:

  • Reduced heat transfer
  • Loss of cooling capacity
  • Restriction of water flow
  • Obstruct spray nozzles
  • Acceleration of corrosion

Results in:
– Reduce System efficiency
– Production loss
– Unscheduled emergency equipment shutdown
– Increased-pumping energy cost
– Increased maintenance cost
– Increased wastewater treatment costs

Factors effecting scale formation
– Calcium and carbonate ion concentration
– pH of cooling water
– Water temperature
– Flow rate
– Dissolved salts

Control of water scale formation
Scale formation may be inhibited or controlled by taking the following measures
– Removing the hardness from the water using a water softener
– By adding scale inhibitors to the cooling stream which enlarge the solubility of the hardness salts
– By avoiding excessive temperature difference
– By adjusting pH of the cooling stream
– By keeping low concentration of suspended materials/ions in the cooling stream which the hardness salts can remain soluble.

Scale inhibitors
These are substances which when added to a cooling water system prevents the formation of scale on metal surfaces.

Mechanism of scale inhibition
Scale inhibitors present the scale formation by the following way
– Prevents the formation coarse grains of deposits
– Prevents coagulation of crystal grains
– prevents the formation of coarse grained crystals by distorting the crystal surface of the scale forming crystal nuclei.

Chemicals used for scale inhibition
– Poly phosphates, phosphonates, sulfonates, phosphate esters
-Synthetic polymer, polyacrylates, polymetltacrylates, maleic anhydride copolymers.

Fouling
Fouling is the formation of deposits of foreign materials on heat transfer surface of heat exchanger and other cooling equipment.

Types of fouling
– General fouling: porous deposits, dirt, silt, Sand, corrosion products
– Microbiological fouling: algae, fungi, Bacteria, Slime

Effects of fouling
– Loss of heat transfer
– Plugging pipe lines, heat exchanger
– Acceleration of corrosion
– Loss of cooling capacity
– Restriction of water flow
– Obstructs spry nozzle
– Increased-pumping energy cost
– increased maintenance cost

Control of fouling
– By adding inhibitors to cooling water
– Maintaining optimum velocity of cooling water (3-10 ft / sec )
– Minimizing suspended materials by filtration
– Using chlorine, slimicides or biocides

Factors influencing the slime formation by microorganism
– Water temperature
– pH of cooling water
– Nutrients in cooling water
– Dissolved oxygen

Monitoring of cooling water treatment
– Corrosion rate measurement with test coupons
– Corrosion rate measurement with corrosion meter
– Corrosion monitoring by using heat exchanger
– Estimation of scale inhibitor effect by water quality analysis
– Measurement of scaling rate by using monitor heat exchanger
– Monitoring of the prevention effects of slime adhesion and sludge accumulation.