water pre- treatment process (water packaging)

CHEMISTRY OF WATER PRE-TREATMENT PROCESS:The Raw water from river bears dissolved and suspended impurities as well as organic matters that make it unfit for human consumptions and other industrial processes. The conceptual approach of treatment includes removal of coarse suspended particles, colloids and destruction of harmful microorganisms. Storing of river water in reservoir gives an additional benefit due to natural pre-treatment of water through sedimentation and decantation and thereby reducing organic load and turbidity of DMPT & CWPT Plants to some extent. Turbidity is due to suspended matter in a finely divided state (particle size ranging from 1 to 100 nm). Clay, organic matter and microorganism are contributing causes of turbidity.

1. GENERAL DESCRIPTION OF WATER PRE- TREATMENT SYSTEM:

The water pre-treatment plant and liquid effluent treatment plant package consist of the following systems.1. PT-CW system supplies clarified water to circulating water system (CW system) of the power plantand to any other utilities.2. PT-DM system supplies treated water supply to DM Plant as well as potable water to potable storage tank. 3. The liquid effluent treatment plant is provided for treatment of waste water collected from various area and waste water runoff from Coal Handling Plant.

2. CHARACTERISTIC OF PRE- TREATMENT SYSTEM

Raw water is pumped from Raw water pump house with the help of 3 nos PTCW pumps (2W+1S) for PT-CW system as well as 2nos PTDM pumps (1W+1S) for PT-DM system installed in raw water pump house.

2. A. PLANT CAPACITY:

The plant is designed for treating the raw water, as shown belowPT-CW System - 4000M3/hrPT-DM System - 220M3/hrPotable Filtration System - 25M3/hr for each filter 2. B. RAW WATER QUALITY:

The treatment plant is designed based on the raw water quality given in below.

Sl. No.ParametersUnitsQuality considered for design

1PH--7.2 to 8.2

2TurbidityNTU300

3Total bicarbonateMg/lit as CaCO3120

4Total suspended solidsMg/lit300

5Total iron--1.7

6Odour--Odourless

7Taste--Tasteless

2. C. TREATED WATER QUALITY (PRE-TREATMENT):

1. PT-CW SYSTEM: Following is the water quality for Clarifier:


1TurbidityNTU< 10

2Iron & Manganese content (Total)ppm< 0.3

2. PT-DM SYSTEM: Following is the water quality for Clarifier:


1TurbidityNTU< 10

2Iron & Manganese content (Total)ppm< 0.3

3Organic matterppm< 0.05

3. GSF OUTLET: Following is the water quality at GSF outlet:


1pH

2TurbidityNTU< 2

4. POTABLE FILTERATION SYSTEM: Following is the water quality:


1TurbidityNTU< 5

2Colour--Non-traceable

3Odour--Non-traceable

5. TREATED WATER QUALITY (LIQUIED EFFLUENT TREATMENT) ETP-CLARIFIER (LAMELLA TYPE):

Effluent quality at the outlet of the lamella clarifier is guaranteed for the following:


1Effluent turbidity at designed flowNTU< 10

2Effluent turbidity at 20% overflow condition flowNTU< 15

6. CENTRAL MONITORING BASIN:

The following quality is guaranteed:


1PH--6.5 to 8.2

2TSSMg/l100

3Oil & greaseMg/l10

4BODMg/l30

5CODMg/l250

Moreover, the effluent quality will also meet the norms for effluent discharge as set by WBPCB & CPCB

2. D. PURPOSE OF PRE-TREATMENT:

The purpose of Pre- treatment plant is to remove impurities like organic matter, turbidity and microorganism from raw water. The pre-treatment involves the following steps:1. Aeration in Cascade Aerator2. Prechlorination3. Clarification in Reactor clarifiers4. Filtration in rapid gravity filters.

1. AERATION IN CASCADE AERATOR:

In Water Treatment, aeration is practiced for the following purposes1. To increase the dissolved oxygen content of the water;2. To expel the undesirable dissolved gases, such as carbon dioxide and other volatile substances those cause taste and odour.3. To oxidise Fe+2 & Mn+2 so that those can easily be precipitated from their soluble state in the Clarifier.The following chemical reactions represent the oxidation of iron and manganese: 4 Fe+2 + O2 + 10 H2O 4 Fe (OH)3 + 8 H+ 2 Mn+2 + O2 + 2 H2O 2 MnO2 + 4 H+The above process of aeration is achieved in the Cascade Aerator where water is allowed to fall in thin layers; thereby exchange of gases takes place between water and atmosphere. Removal of iron and manganese is effected by oxidation, followed by the separation of Fe+3 and Mn+4 as ferric hydroxide [Fe(OH)3] and manganese dioxide[MnO2] precipitates. Manganese is usually found in low concentrations compared to iron and following oxidation, the water can be subjected to direct filtration. In the case of iron, oxidation is followed by settling and filtration, when Fe (II) is greater than about 5 mg/l. direct filtration is used when the iron concentration is less than about 5 mg/l, in the absence of turbidity. Although the aeration requirements are small, the rate of reaction is slow and pH dependent, e.g.: At pH 6.9, 90% oxidation of iron requires about 40 minutes reaction time. At pH 7.2, 90% oxidation of iron requires only 10 minutes reaction time.In some waters, aeration alone is adequate for complete oxidation of iron because the removal of carbon dioxide by aeration raises the pH above 7.5. Manganese removal by aeration at high pH values is a slow and inefficient process and strong oxidants such as chlorine is therefore usually necessary. Chlorination will also be effective in oxidizing iron. 0.14 mg of Oxygen is required per mg of Fe oxidized with the reduction of alkalinity by 1.80 mg expressed as CaCO3, per mg of Fe oxidized. Each mg of Fe results in 2 mg of suspended solids [Fe (OH)3], following oxidation. 0.29 mg of Oxygen is required per mg of Mn oxidized with the reduction of alkalinity by 1.80 mg expressed as CaCO3, per mg of Mn oxidized. Each mg of Mn results in 1.6 mg of suspended solids (MnO2).

2. CHLORINATION:Chlorination is an important unit operation in a water treatment practice. This process is brought about by addition of chlorine solution to the water body. In water treatment plant chlorination is effected in two stages

(i) Pre-Chlorination(ii) Post-Chlorination.

Pre-chlorination is done to reduce algae and other micro-organisms population, which may otherwise hinder the process of coagulation and filtration.Post chlorination is recommended on the filtered water to make it suitable for human consumption.Chlorine acts as a disinfectant by the,a. Destruction of micro-organism b. Oxidation of Fec. Removal of taste and odour producing compoundsd. Oxidation of organic compounds

For effective chlorination in water treatment practice it is necessary to ascertain (a) dose of chlorine required (b) optimum contact period and (c) the presence of residual chlorine (0.3 to 0.5 mg/l). From experience, it has been found that a contact period of 30minutes is adequate to disinfect water.When Cl dissolves in water the following reaction occurs:Cl + HO HOCl + HClHOCl ClO +H

The direction the above reaction is dependent on pH of the medium:

Sl. No.SpeciespH interval

1Cl22

2HOCl2.5-7.0

3OCl-8.5

The germicidal efficiency of HOCl is due to the relative ease with which it penetrates cell walls of microorganisms* OCl ion is more effective germicide than HOCl.*The optimum pH range for effective chlorination is 6.0- 7.0

3. CLARIFICATION:Clarification is the term used to encompass coagulation, flocculation and sedimentation reactions which are involved in the removal of suspended and colloidal matter from raw water. The term coagulation refers to the process of destabilization by adding a coagulant and the term flocculation to the agglomeration of the neutralized colloidal particles.

3. A. MECHANISM OF COAGULATION:

In coagulation chemical such as alum is added to water. In natural water colloidal silica particles are negatively charged due to preferential adsorption of negative ions from the vicinity. Al3 ions coming from alum neutralize the colloidal particles and thereby permitting them to coalesce. In addition to this alum reacts with bicarbonate alkalinity of water to form of Al(OH) called floc which enmeshes and entraps the finer particles forming masses that are sufficiently heavy to settle rapidly. Al (SO) + 3 Ca (HCO) = Al (OH) + 3CaSO + 6COCoagulation phenomenon can better be explained as follows:A colloidal silica particle preferentially adsorbs negative ions on its surface which drags some positive ions in its vicinity. Therefore, in such cases at equilibrium there exists an electrical double layer at the particle-liquid interface. One part of the double layer consists of negative ions fixed on the surface of silica particle and the second layer in the aqueous phase consists of ions with overall positive charge predominance. The solution side of this double layer containing counter ions can be divided into two parts - a fixed part of the counter ions at a distance of a few Angstrom from the solid surface (stern layer) and another diffuse portion of that counter ions with concentration gradually falling off or increasing to that uniform value as prevalent in the bulk. The zeta potential is the difference in potential between the fixed part of the double layer on the solution side and bulk solution. The colloidal particle having large positive or negative zeta potential repels each other and therefore has high dispersion stability. Addition of alum reduces zeta potential of the colloid and allowing the particle to coalesce. There are a number of factors which can affect the coagulation & flocculation processes e.g. pH, temperature, nature of particles present in raw water and the chemical composition of raw water but the most important factor is pH. So, for a coagulant to act efficiently, it is necessary that there is sufficient alkalinity in the natural water. When, the alkalinity is found to be less than required for complete neutralization of coagulant, an additional, source for providing alkalinity is employed. The most common source used is lime which furnishes residual alkalinity, and this promotes the coagulation efficiency. But almost care has to be taken during the usage of lime for coagulation process. Excessive lime increase pH by providing OH- ions, which will dissolve Al(OH)3 On the other hand Al(OH)3 dissolves in H+ ions also. Hence, a neutral pH is to be maintained during coagulation process. Lime is added in such a fashion that a pH is 7 7.5 is maintained during coagulation. It has been established that in pH 7 7.5 range the solubility if Al(OH)3 is less significant.

Important points:*The pH range for satisfactory coagulation with alum is 6.0-7.0.*Low temperature inhibits coagulation. Temperature variation above 2F might cause thermal upset in the clarifier bed.*Polyelectrolytes are more effective coagulating & flocculating agents than alum but they are more expensive.*The main aim of coagulation is to remove colloidal silica from raw water as far as possibleCauses of Clarifier Bed disturbance:

i) Sudden upsurge of heavy turbid water.ii) Fall in pH due to excess alum dosing.iii) Sudden fall in temperature.

4. A. FILTRATIONThe suspended and colloidal matter in the river water is reduced to consistently normal proportions by clarification. The final purification is effected by passing the treated water through rapid gravity sand filters. Rapid sand filters as the name implies have high filtration rates and are designed to receive coagulated and settled water. When the water flows down through the medium, suspended and most of the colloidal impurities in the water are left behind in the pores or upon the medium itself. This process of separating impurities from the carrying water is called filtration. Filtration is an important and active process in the natural purification of water. A number of different processes combine to produce the overall removal of impurities associated with the water by way of filtration through granular substances. The most important of these processes involve several Physical actions such as straining, sedimentation, etc. Straining is the simplest of filtration phenomena. It takes place, almost entirely, at the surface of the filter where the water enters the pores of the filter bed. Initially, straining removes only those substances which are larger than the pore openings. As the filtration is continued, the substances those are strained out accumulate on the surface of the filter bed and lead to the formation of a mat thereon. Removal of this mat becomes necessary when the resistance to filtration is mounted to excessive values. This causes head loss in the filter, which prevent filters to have desired filtered water output. The dirt collected over the filter bed is removed by back washing. The process of washing consists in loosening the impurities adhering to the sand particles by agitation of the bed with air and floats them to drain by means of a reverse of current of filtered water.

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water pre- treatment process (water packaging)

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