Physical, chemical and biological characteristics of water

   Quality Of  Water

       

Characteristics Of Water:

              I)   Physical Characteristics:

·         Turbidity:

·         It is caused by suspended materials such as clay, silt or some other finally divided organic and inorganic matter. Turbidity is also due to presence of bacteria, algae, protozoa, fungi. Expressed in ppm (or mg/l)

·         Standard until is that which is produced by 1 mg of finely divided silica (Fullers earth) in 1litre of distilled water.

·         Measured on “Silica Scale”

·         Permissible limit for potable water: 5 to 10 ppm

·         Measured by: Turbidity meters

a)      Jackson Turbid meter: Used to measure high turbidity (> 25 ppm) ----(JTU)

b)      Baylis Turbid meter: Precisely record even low turbidity values (of the order of 0 to 2 ppm) --- BTU

c)       Nephelometer: Commonly used for turbidity of range of 0 to 1 ppm – (NTU & FTU) –Formazan polymer is standard reference suspension.

Turbidity is the measure of resistance to the passage of light through water.

·         Color:

·         It is caused by dissolved organic matters from decaying vegetation or some inorganic-colored soils, growth of algae, metallic ions (Fe and Mn).

·         Measured by comparing the color of water sample with other standard glass tubes (Nessler tubes)

·         Measured by platinum Cobalt Method (Measured on Platinum Cobalt Scale)

·         Permissible limit: 5 to 20 ppm.

·         Measured by Tintometer.

·         Taste & Odor:

·         Due to dissolved organic matter or inorganic salts, dissolved gases etc.

·         Threshold number is the standard unit for measuring.

               Taste is expressed as FIN (Flavor threshold Number)

               Odor is expressed as TON (Threshold Odor Number)

               TON= Dilution ratio

               Dilution Ratio= (A+B)/A

               A= Vol. of raw water sample

              B= Vol. of distilled water used for dilution.

               Permissible Limit: 1 to 3.

               The odor changes with temperature.

               It is tested normally at 20⁰ C to 25⁰ C

              Odor is measured by an apparatus called osmoscope.

Osmoscope:

         The osmoscope is graduated with P⁰ values from 0 to 5.

         P⁰ value of 0 indicates ‘no perceptible odor’

        P⁰ value of 5 indicates ‘extremely strong odor’

·         Temperature: 10⁰C is desirable.

·         Specific Conductivity: To know the dissolved salt content. Determined by ionic water tester.

Impurities in water:

1)      Suspended impurities: They cause turbidity. The concentration of suspended impurities is measured by turbidity.

E.g.: silt, clay, bacteria, fungi, algae.

2)      Colloidal impurities: These are finely divided dispersion of solid particles, not visible to the naked eye. These impurities if associated with organic matter having bacteria becomes the chief source of epidemic.

3)      Dissolved impurities: It includes organic compounds, inorganic salts and gases. The following are various dissolved impurities and their effects.

a)      Salts: CaCl₂ , MgCl₂ , CO₃ , HCO₃ , They cause hardness and alkalinity in water.

b)      Metals: Iron causes red color, Manganese causes brown color.

c)       Lead and arsenic: It cause poisoning.

d)      Gases: Oxygen causes corrosion of metals

·         Chlorine and Ammonia cause bad taste and odor.

·         CO₂ and Hydrogen sulphide cause acidity in water.

II)    Chemical Characteristics:

1)      Total solids, suspended solids and dissolved solids:

·         Total solids can be found by evaporating a sample of water and weighing the dry residue left.

·         Suspended solids can be found by filtering water sample through Whatman filter paper No.44

·         Permissible TDS limit: 500 to 1000 ppm.

Example

A 100 ml water sample is drawn on to a empty dry container whose initial weight is 95.452 gm. After oven drying the sample at 103⁰C for 4 hours its final weight measured to be 95.486 gm. Then total solid’s concentration in mg/l.

Sol: Total Solids = w₂-w₂/v x 10⁶

W₂ = final weight in gm,

W₁ = initial weight in gm,

V = volume of water sample in ml

   = 95.486 – 95.452/100 x 10⁶ =340 mg/l

2)      pH value of water: It indicates hydrogen ion concentration in water.

·         pH = log₁₀ (1/H⁺),

if pH > 7 it is alkaline: if pH<7 it is acidic.

·         Alkalinity is caused by bicarbonates (HCO₃) of Ca and Mg and carbonates (CO₃), hydroxides (OH) of Ca, Mg, k, Na.

·         Acidity is caused by Mineral Acids, free CO₂, Sulphide of Fe & Aluminum.

·         Alkalinity Effects: Incrustation and sediment deposit in pipelines and difficult in chlorination.

·         Acidity Effects: Tuberculation & Corrosion of pipe lines.

·         Permissible limit: 6.6 to 8.5

·         pH is measured by potentiometer and Calorimetric method.

3)      Hardness:

A characteristic which prevents formation of lather or foam with soap.

Effects of hardness:

 Scaling of boilers, greater soap consumption, corrosion and incrustation of pipe lines, food becomes tasteless etc.,

Types of Hardness:

1.       Carbonate Hardness (CH)

2.       Non-Carbonate Hardness (NCH)

·         Temporary or carbonate hardness: Caused by HCO₃ and CO₃ of Ca & Mg.

·         Can be removed to some extent by simple boiling or removed fully by addition of lime.

·         Permanent or non-Carbonate hardness: Caused by SO₄, Cl, NO₃ of Ca & Mg.

·         Can be removed by water softening methods such as Lime soda process, Demineralization process and Zeolite process.

Permissible limits: For boiler feed waters<75 ppm.

For drinking purpose, between 75 & 115 ppm.

If Hardness is <75 ppm is called ‘soft’

If Hardness is >200 ppm is called ‘Hard’

Measurement: Measured in terms of p p m or mg/lit of CaCO₃

·         Measured by EDTA test (Ethylene Diamine Tetrameric Acid test). In the titration process, Eri chrome black T is used to show the end of titration.

·         After determining number of Ca & Mg ions present in water by titration process with versa nates solution hardness is estimated as follows;

Total Hardness: in mg/l as CaCO₃

= [Ca in mg/l x Equivalent wt. of CaCO₃/ Equivalent wt. of Ca]  

+ [Mg in mg/ l x Equivalent wt. of CaCO₃/ Equivalent wt. of Mg]

Combining weight or equivalent weight = Molecular weight / valency

(Ca⁺⁺=40; Mg⁺⁺ = 24; Sr⁺⁺ = 87.6; CaCO₃-- = 100; CO₃ -- = 60; HCO₃- =61; OH- = 17)

So, Total Hardness = Ca⁺⁺mg/l x (50/20) +Mg⁺⁺mg/l x (50/12)

TH = CH +NCH

If TH > alkalinity, then CH = Alkalinity.

So, NCH = TH – CH.

If TH < Alkalinity, then CH = TH, NNCH = 0.

·         Hardness is also expressed in degree of hardness.

·         One British degree of hardness (Clark’s Scale) = 14.25 ppm of CaCO₃

·         One American degree of hardness = 17.15 ppm of CaCO₃

= [HCO₃ in mg/l x Equivalent wt. of CaCO₃/Equivalent wt. of HCO₃]

 + [CO₃ in mg/l x Equivalent wt. of CO₃]

So, Total alkalinity = HCO₃- mg/l x (50/61) + CO₃- mg/l x (50/30)

4)      Chloride content:

Present in the form of Sodium Chloride. Imparts salty taste.

·         Permissible limit: 250 mg/l. Presence of high quantity indicates pollution from sewage and other human & industrial waste.

5)      Nitrogen Content: Indicates presence of organic matter and the extent to which it has undergone decomposition.

Different forms:

i)        Free Ammonia: indicates presence of undecomposed organic matter and it represents first stage decomposition permissible limit: less than 0.15 ppm

ii)      Albuminoid Nitrogen: Indicates decomposition of organic matter has started, limit shall not exceed 0.3 mg/lit

iii)     Nitrites: Indicates presence of partly decomposed organic matter. Permissible limit: nil

iv)     Nitrates: Indicates fully oxidized organic matter. Permissible limit:< 45 ppm.

·         Excess causes the disease called “Methemoglobinemia” (Blue baby disease)

·         Total Kjeld ahi Nitrogen (TKN) = Free ammonia + organic nitrogen

6)      Metals & Chemical Substances:

a)       Iron & Manganese:

Permissible limit: iron < 0.3 ppm, Mn < 0.05 ppm.

·         Excess iron & Mn cause discoloration of clothes washed in such water and incrustation of water mains due to deposits.

b)      Copper: Permissible limit – 1 to 3 mg/l.

·         Affects human lungs and other respiratory organs.

c)       Sulphide: Permissible < 250 mg/l

·         Causes laxative effect on human body and diarrhea.

d)      Fluoride:

·         Fluoride < 1 ppm, cause formation of cavities in the teeth.

·         Fluoride > 1.5 ppm causes Fluorosis (mottling and discoloration of teeth) and deformation of bones.

·         Permissible limit: between 1 ppm and 1.5 ppm.

7)      Dissolved gases:

a)       H₂S: It gives bad taste and odor

b)      CO₂: Indicates biological activity and causes corrosion in pipe lines.

c)       Dissolved oxygen (D.O): To know the extent of pollution of water.

·         Determined by Winkler’s method.

·         Since D.O is consumed by unstable organic matter, D.O less than its saturated level indicates presence of organic matter, therefore pollution.

·         For fresh river water saturation D.O at 20⁰ C =9.2 mg/l.

·         Permissible limit: For potable water should be between 5 to 10 ppm.

d)      Nitrogen gas: Indicates presence of organic matter.

8)      Biochemical oxygen Demand (B.O.D): Permissible Limits: B.O.D. of safe drinking water = Nil.

iii)     Microscopic Characteristics:

·         To study about the presence of Bacteria, virus, Algae, Protozoa.

·         Bacteria is single celled organisms

Bacteria (1 to 4 microns in size): Classification:

1)      Non pathogenic bacteria: They are Harmless; e. g Escherichia coliform or E-coli.

2)      Pathogenic bacteria: They are harmful & causes water borne disease.

·         Detection of “E-Coli” indicates the probable presence of ‘pathogens’

·         E-Coli bacteria is used as indicator organism.

Classification based on shape:

1)      Cocci – Sphere shaped bacteria

2)      Bacilli – Rod shaped bacteria

3)      Spirally – Twisted rod-shaped bacteria

Classification based on living conditions:

1)      Aerobic bacteria: It survives in the presence of Oxygen

2)      Anaerobic bacteria: It survives in the absence of Oxygen

3)      Facultative bacteria: Survives with or without Oxygen.

To detect and measure coliform bacteria, the following tests are available.

a)      Total count test: In this test bacteria are cultivated on specially prepared medium of agar for different dilutions of sample of water with distilled water. The diluted sample is placed in an incubator for specified time at specified temperature. The bacteria colonies so formed are counted and results are computed per 100 ml

·         For drinking water, the total count shall not be more than 1 per 100 ml

b)      E- Coli test: Divided into

i)        Presumptive test

ii)       Confirmed test

iii)     Completed test

In these tests, if gas is produced after incubation, it is reported as +ve and further tests are to be followed to confirm E- Coil. If no gas is produced it is presumed that there is no bacteria and the test is treated as negative.

 3)      Membrane filter technique:

Recent method. Sample is filtered through sterile membrane with pore size 5 to 10 mm. The bacteria is retained on the membrane. The membrane is put in contact with suitable nutrient. [ M- Endo’s medium] which inhibits growth of bacteria other than coli from group. Then placed in incubator at prescribed conditions and the visible colonies are counted with microscope.

               MPN (Most Probable Number):

It is the bacterial density which is most likely to be present in water. It is used to report the amount of bacteria present.

·                              To determine MPN, confirmed or completed tests results are required.

·                              Permissible limits: MPN should be Nil.

Micro Organic plants: These are tiny plants.

E.g.: Algae, plankton etc.

·                               They cause bad taste & odor and interface with smooth working of filter.

·                              To control algae, copper sulphide chemical is mixed in water.

Protozoa: These are unicellular animals.