Reverse Osmosis (RO)

          Reverse Osmosis (RO) is a water purification technique that uses a semi-permeable membrane to get rid off ions, molecules and larger contaminants (particles) from fresh water.

What is Reverse Osmosis (RO)?

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          To understand the purpose and method of Reverse Osmosis we should understand initially the naturally occurring process of Osmosis.

          Osmosis is a naturally occurring phenomenon and one of the foremost vital processes in nature. This is a method where less concentrated molecules (saline solution) will tend to migrate to more concentrated solution. The objective is an equalized solution.

          The examples of Osmosis are 'Cholera', 'plant roots drawing (absorb) water from the soil' and 'our kidney absorb water from our blood'.

          In the below diagram we showed how osmosis works. A solution that is less concentrated will have a natural tendency to migrate to a solution with a higher concentration. For example, if a container filled with water with a low salt concentration and another container filled with water with high salt concentration and that they were separated by a semi-permeable membrane, then the water with the lower salt concentration would begin to migrate towards the water container with the higher salt concentration, until concentration becomes equal on both sides.

Semi-permeable membrane:
          A semi-permeable membrane is a synthetic or biological membrane that will permit some atoms or molecules to pass but not others. A simple example for semi-permeable membrane is a screen door. It permits air molecules to go through but not pests or something larger than the holes within the screen door. Another example is blood brain barrier, it allows what ever the brain needs and what ever harmful is kept out. The membranes surrounding the organelles.

Reverse Osmosis:
          Reverse osmosis is simply the opposite of osmosis method. Osmosis happens naturally without ant energy required. To reverse the method of osmosis we need to use some energy to the more saline solution. A Reverse Osmosis membrane is a semi-permeable membrane that acts like a filter having extremely tiny pores that enables the passage of water molecules however not the majority of dissolved microscopic contaminants like salts, organic, microorganism and pyrogens. However, we need to "push" the water through the reverse osmosis membrane (or semi-permeable membrane) by applying pressure that is greater than the naturally occurring osmotic pressure in-order to de-salinate (de-mineralize or de-ionize) water in the method, allowing pure water through while holding back majority of contaminants.
          In the below diagram we showed the method of Reverse Osmosis. When pressure is applied to the concentrated solution the water molecules are forced through the semi-permeable membrane and the contaminants are not allowed through.

WATER TREATMENT

          Water occurs in nature "pure" and what ever be the source always contains impurities either in solution or in suspension. The determination of these impurities makes analysis of water necessary and removal and control of these impurities make water treatment essential.

Water sources:

The various sources of water can be broadly classified as:

          a) Rain water

          b) Surface water (River, Streams, Ponds, Lakes and Reservoirs)

          c) Ground water (Springs, Shallow wells and Deep wells)

of the above, logically rain water is the purest but even this collects and dissolves atmospheric gasses and impurities in air. Further, once in contact with the earth's crust, the rain water will gradually dissolve various materials.

Various (Majour) impurities:

The major impurities of water can be classified in three main groups:

          1) Non-ionic and undissolved

          2) Ionic and Dissolved

          3) Gaseous

          In this article we discusses briefly about the water treatment processes indicating the various impurities present in the water and the different types of treatment processes such as 'Filtration' and 'Demineralization', for treating the water for eliminating the suspended and dissolved impurities.

Table of contents

1) Water treatment -- Basic water chemistry

2) Importance of removal of impurities

3) Water softening process
4) Standard Operation Procedure (SOP) of Water treatment plant (for example purpose)

WATER TREATMENT - BASIC WATER CHEMISTRY

          Water occurs in nature "pure" and what ever be the source always contains impurities either in solution or in suspension. The determination of these impurities makes analysis of water necessary and removal and control of these impurities make water treatment essential.

Water sources:

The various sources of water can be broadly classified as:

          a) Rain water

          b) Surface water (River, Streams, Ponds, Lakes and Reservoirs)

          c) Ground water (Springs, Shallow wells and Deep wells)

of the above, logically rain water is the purest but even this collects and dissolves atmospheric gasses and impurities in air. Further, once in contact with the earth's crust, the rain water will gradually dissolve various materials.

Various (Majour) impurities:

The major impurities of water can be classified in three main groups:

          1) Non-ionic and undissolved

          2) Ionic and Dissolved

          3) Gaseous

1) Non-ionic Impurities:  These are mainly, salt, mud, dirt and other suspended matter, micro-organisms, bactiria and other organic matter, oil and corrosion products. It goes without saying that drinking water and most industrial water supplied should be clear and organic-free.

2) Ionic and dissolved Impurities:  Any salt which dissolves in water disassociates into positively charged ions called "Cations" and negetively charged ions called "Anions". Since these permit the water to conduct electricity, these salts are called electrolytes.
          Some of the most common Cations in water are:
      Calcium, Magnesium, Sodium and Iron, and rarely Ammonium, Potassium and Manganese. These cations are associated with Anions like Bicarbonates, Carbonates, Hydroxides (the sum of which is termed as Alkalinity), Sulfates and Chlorides. Presence of Nitrites and Phosphates is normally not very common. In the water treatment field, the preferred method of expression of these dissolved impurities is in terms of Equivalent Calcium Carbonate, abbrevated to as "CaCO3". This is because Calcium Carbonate is a good common denominator as it has a molecular weight of 100, which facilitates calculations.
          Moreover, in this form of analysis, the sum of Cations or total Cation always equals the total Anions. Quantitatively, these are expressed in parts per million or milligram/ltr. One part per million equals one ten thousandth of one percent (0.0001%). One part per million means one part in a million parts, for example, one liter in a million liters of water or one Kg in a million Kgs of water.
          Of all the dissolved impurities, hardness is perhaps the most troublesome. Hardness is due to compounds of Calcium and Magnesium. On heating water caontaining these salts, Carbon Dioxide is released from solution and the Bicarbonates are converted into Carbonates which are insoluble and form scales and deposites. Other salts of Calcium and Magnesium like Sulfates and Chlorides have lower solubility than Sodium salts and participated out at high temperatures. Bicarbonates of Calcium and Magnesium are known as the "Alkaline hardness" or "Temporary hardness" and chlorides, sulfates, nitrates etc., of Calcium and Magnesium are known as "Neutral" or "Permanent hardness". Sodium salts are highly soluble but can be corrosive if present in large quantities such as Sodium Chloride or Sodium Bicarbonate.
          Dissolved Silica is another troublesome impurity, especially in water fed to Boilers of very high temperatures and pressures. Even in lower pressure boilers, it could form a very hard type of scale by acting as a binding agent.
          The natural water contains solid, liquid and gaseous impurities and therefore, this water cannot be used for the generation od steam in the boilers. The impurities present in the water should be removed before it'suse in steam generation. The necessity for reducing the corrosive nature & quantity of dissolved and suspended solids in feed water has become increasingly important with the advent of high pressure, critical & supercritical boilers.

Impurities in water:
The impurities present in the feed water are classified as given below -

-- Undissolved and suspended solid materials.
-- Dissolved salts and minerals.
-- Dissolved gases.
-- Other materials (as Oil, Acid) either in mixed or unmixed forms.

A. Undissolved and suspended materials: Turbidity and sediments:

          Turbidity in the water is suspended insoluble matter including coarse particles (mud, sediment, sand etc.,) that settle rapidly on standing. Amounts range from almost zero in most ground waters and 60,000 ppm in muddy and turbulent river water. The turbidity of feed water should not exceed 5 ppm. These materials can be removed by settling, coagulation and filtration. Their presence is undesirable because heating or evaporation produces hard stony scale deposits on the heating surface and clog the fluid system. Both are objectionable as they cause damage to the boiler system.

B. Dissolved salts and minerals:

a) Calcium and Magnesium salts: The calcium and Magnesium salts present in the water in the form of carbonates, bicarbonates, sulfates and chlorides. The presence of these salts is recognized by the hardness of water (Hardness of water is tested by soap test). The hardness of water is classified as temporary and permanent hardness. The temporary hardness is caused by the bicarbonates of calcium and magnesium and can be removed by boiling. The boiling converts the soluble bicarbonates into less soluble carbonates which can be removed by simple blow down method. The permanent hardness of the water is caused by the presence of chlorides, sulfates and nitraes of calcium and magnesium and they cannot be removed just by boiling because they form a hard scale on heating surfaces.
          A standard amount of measurement of hardness is taken as being the amount of Calcium Carbonate (CaCO3) in th water and is referred to in part per million (ppm) or grains per gallon (grains/gallon*17.1=ppm).

b) Sodium and Potassium salts:  These are extremly soluble in water and do not deposit unless highly concentrated. Their presence is troublesome as they are alkaline in nature and accelerated the corrosion.

c) Chlorides:  Majority of the chlorides cause increased corrosive action of water.

d) Iron:  Most common soluble iron in water is ferrous bicarbonate. The water cantaining ferrous bicarbonate deposits become yellowish and reddish sediments of ferric hydroxide if exposed to air. Majority of ground surface water contains less than 5 ppm but even 0.3 ppm can create trouble in feed water system by soft scale formation and accelerating the corrosion.

e) Manganese:  It also occurs in similar form and it is also equally troublesome.

f) Silica:  Most natural water contains silica from 1 to 100 ppm. Its presence is highly objectionable as it forms very hard scale in Boilers and forms insoluble deposits in turbine blades. In modern high pressure boilers its presence is reduced as low as 10-50 ppb.

g) Microbiological Growth:  Various growth occur in surface water (lake & river). The micro-organisms include diatoms, molds, bacterial slimes, algae, manganese & sulphate reducing bacteria and many others. These can cause coating on Heat Exchanger and clog the flow passages and reduce the heat transfer rates.

h) Colour:  Surface waters from swampy areas become highly colored due to decaying vegetation. Colour of feed water is objectionable as it causes foaming in Boilers and may interfere with treatment processes. It is generally removed by chlorination and absorption by activated carbon.

C. Dissolved Gases:

a) Oxygen:  It presents in surface water in dissolved form with variable percentage depending upon the water temperature and other solid contents in water. Its presence is highly objectionable as it corrosive to iron, zinc, brass and other minerals. It causes corrosion and pitting of water lines, boiler tubes. Its effect is furthur accelerated at high temperatures.

b) Carbon Dioxide:  The river water contains 50 ppm and well water contains 2 to 50 ppm of C02. It also causes the corrosion of stream, water and condensate lines, It also helps to accelerate the corrosive action of oxygen.
          wThe other gases are H2S, CH4, N2, and many others but their percentages are negligible, therefore, their effects are not discussed here.

D. Other minerals:
a) Free Mineral Acid:   Usually present as Sulphuric or hydrochloric acid and causes corrosion. The presence is reduced by neutralization with alkalis.

b) Oil:  Generally, the lubricating oil is carried with steam into the condenser and through the feed system to the Boiler. It causes sludge, scale and foaming in Boilers. It is generally removed by strainers and baffle seperators.
          The effects of all the impurities present in the water are the scale formation on the different parts of the boiler system and corrosion. The scale formation reduces the heat transfer rates and clog the flow passage and endager the life of the equipments by increasing the temperature above safe limit. The corrosion phenomenon reduces the life of the plant rapidity. Therefore, it is absolutely necessary to reduce the impurities below a safe limit for the proper working of the power plant.

C. Dissolved Gases:
The atmosphereic gases found in naturally occuring waters, only two Carbon Dioxide and Oxygen, are the main causes of many corrosion related problems.

WATER TREATMENT - IMPORTANCE OF REMOVAL OF IMPURITIES

          The major concern in industrial water treatment, where thw water used directly or indirectly in an industrial process, is to treat the water to be suitable for that particular application/process. The use of water in boiler for steam generation is an obvious industrial use. Depending on the process, varying degrees of purity of treated water are required. For example, a textile processing unit will require soft and clea water for process use; a chemical plant will require pure water for process not exceeding 1.0 mg/ltr of dissolved impurities or electronic components manufacturing unit require ultra pure water containing total dissolved impurities not exceeding 0.5 mg/ltr or less. So depending upon the requirement, various water treatment processes are adopted to ttreat the water, to make the water suitable for that particular application. The details of the process are as follows:

FILTRATION

          Filtration is the process of passing a liquid containing suspended matter through a suitable porous material (filtering medium) to efficiently remove the suspended matter in the liquid. This is basically a physical treatment of water.

          Filtration is employed in the treatment of industrial water in order to remove or reduce suspended solids and turbidity. This is of special importance in boiler feed water as otherwise there will be formation of sludge and slit deposits. These will restrict flow, causes overheating and consequent failure of water wall tubes. Furthur, in combination with hardness, these sludge and slit deposits will add to the volume and has the insulating effect of scale deposits.

          1. Filtration is employed as a pre-treatment to softening or demineralising plants to protect the resins in them. It is also employed for treating potable water.

          2. The weakly basic anion resins exchanges only the strong acids such as Hydrochloric acid, Sulphuric acid and Nitric acid.

          There are basically two types of Filters - Gravity type, and Pressure type. For industrial applications the latter are preferred; they can be linked into the mains as they work under pressure and used in conjunction with other water treating equipment like softners, demineralisers without recourse to re-pumping. Also, Presure Filters are easier to install and operate, require less space and minimal civil work.

          Filter media commonly employed are graded and washed sand of effective size 0.35 mm to 0.5 mm resting on supporting underbed of crushed gravel and pebble of four varying sizes, with the coarsest size at the bottom of the bed. The sand depth is 500 mm and the underbed depth also 500 mm.

          The major components of a pressure filter are a steel pressure vessel with dished ends (normally vertical and cylindrical, though larger flows horizontal and cylindrical vessels are employed); internals comprising raw water distributor and filtered water collector-cum backwash water distributor; external pipe work and valves; filter media and instruments like pressure gauges, flow indicator etc.,

          Backwashing of the filter bed has to be carried out periodically (normally once in 24 hours, more frequently if the pressure drop across the bed exceeds 0.7 kg/cm and which indicates accumulation of dirt in the bed) with filtered water at a minimum head of 10 MWC. If air agitation facilitates are provided then the backwash rate can be reduced. The normal backwash time is 5-6 minutes and the scour time 2-3 minutes.

          NormallyFilters should not be fed with water carrying suspended matter and turbidity content of more than 30 to 50 NTU. Above these limits the water should be settled and clarified before Filtration. Also to increase the efficiency of Filtration and to ensure that even fine Suspensions are removed, the ususal practice is to dose Coagulant Chemicals like alum, ferrous sulfates or sodium aluminate at the inlet to the filter by means of a effluent. Activated carbon is used as a filtering Medium when oil/chlorine removal, etc, are required. A layer of processes Manganese Dioxide is incorporated in sand filters for iron removal. A layer of anthracite once the sand bed enhances the filtering capasity by providing in-depth filtration. When anthracite is used for filter can handle turbidity of upto 100 NTU.   

WATER SOFTENING PROCESS

          The objective of treatment in a Base Exchange Softner is to convert hardness forming salts of Calcium and Magnesium to soluble Sodium salts. Due to the low solubility of Calcium and Magnesium Salts, they tend to precipitate and form scales when the temperature of water us increased.

          Sodium salts are highly soluble and hence do not form hard scales. The hard water, to be treated, flows through bed of bead type Polystyrene Cation Exchange resins in sodium form, which exchanges sodium ions with Calcium and Magnesium ions present in hard water.

There are two types of hardness present in water.

1. Temporary Hardness: This occurs due to presence of Carbonates and Bicarbonates of Calcium and Magnesium.

2. Permanent Hardness: This occurs due to presence of Sulfates, chlorides and nitrates of Calcium and Magnesium.

          The output capasity of the water softner is inversely proportional to the hardness present in the raw water. The resign has to be regenerated periodically with Sodium Chloride solution of 10-13% concentration.

The ion exchange reaction for the service runcan be presented as follows:

Ca++      +     2 NaR = CaR2       +        2 Na+

Mg++      +    2 NaR = MgR2      +           Na+

(hard water)   (resin in Na form)    (exhausted resin)    (soft water)

where R is the resin.

          From the above reaction, it will be seen that resign retains the Calcium and Magnesium and releases equivalent of sodium to the water leaving the Softner. After the Softner has produced specific quantity of soft water, the softner should be regenerated with Sodium Chloride. The reaction for the regeneration is given below:

CaR2 + 2NaCl       =     2 NaR     +          CaCl2

MgR2 + 2NaCl      =     2 NaR     +          MgCl2

(exhausted resin)  (common salt)   (regenerated Resin)  (waste to drain)

 Where R is the resin.

          It should be noted that the same Softner could produce less amount of Soft Water between two regenrations, with increase in raw water hardness. The quality of soft water may also get affected due to this increase/change in composition of raw water.