Quality of Water

The fecal-oral route is the path by which disease-causing organisms (pathogens) pass from the faeces of an infected person or animal to the mouth of a healthy person. Cholera, typhoid, dysentery, hepatitis A, diarrhoea and worm infections spread in this way.

Flow diagram (the "F-diagram")

 Faeces of infected person/animal
   |      |        |         |
 Fluids  Fields   Flies    Fingers
 (water) (soil)   (vector) (hands)
   |        |       |         |
   +--------+---+---+---------+
                |
              Food
                |
        New host swallows
        (mouth)  --> disease

Explanation

1. An infected person passes pathogens in faeces, often by open defecation.
2. The pathogens reach fluids (drinking water, ponds, wells polluted by faeces), fields (soil and crops fertilised with night soil), flies (which carry germs from faeces to food) and fingers (unwashed hands).
3. These contaminate food or drinking water.
4. A healthy person swallows the pathogens, gets infected and passes them in faeces again, completing the cycle.

Breaking the route

Primary barriers stop faeces reaching the environment (latrines, sanitation); secondary barriers stop the germs reaching the mouth (safe water, handwashing, food hygiene, fly control).

The multiple tube fermentation (MTF) technique estimates the number of coliform bacteria in a water sample as the Most Probable Number (MPN) per 100 ml, using a statistical table. It uses the ability of coliforms to ferment lactose with acid and gas at 35 degrees C within 48 hours. It is done in three stages.

Apparatus and media

Test tubes with inverted Durham tubes, pipettes, incubator at 35 +/- 0.5 degrees C, water bath at 44.5 degrees C, lactose broth (or lauryl tryptose broth), brilliant green lactose bile (BGLB) broth, EC broth, EMB agar, sterile dilution water.

1. Presumptive test

1. Choose sample volumes by the expected pollution. For example, for drinking water: 5 tubes each of 10 ml, 1 ml and 0.1 ml (a 15-tube series).
2. Add the volumes to tubes of lactose broth (double strength for the 10 ml portions) and mix.
3. Incubate at 35 degrees C for 24 hours; if no gas, continue to 48 hours.
4. Gas in the Durham tube plus acid (colour change) is a positive presumptive result. No gas after 48 hours is negative.

2. Confirmed test

1. Transfer a loopful from each positive tube to BGLB broth (for coliform), and to EC broth for faecal coliform / E. coli.
2. Incubate BGLB at 35 degrees C for 48 hours; incubate EC broth at 44.5 degrees C for 24 hours.
3. Gas formation confirms coliform (BGLB) or faecal coliform / E. coli (EC).

3. Completed test

Streak positive confirmed cultures on EMB agar and incubate for 24 hours. Typical E. coli colonies are dark with a green metallic sheen. A Gram stain shows Gram-negative non-spore forming rods, and lactose broth again produces gas.

Calculation of MPN

Count the number of positive tubes in each dilution and read the MPN per 100 ml from the standard MPN table for the combination, for example 5-2-1 (10 ml, 1 ml, 0.1 ml) gives an MPN of about 70 per 100 ml (the 95% confidence limits are also given). For other dilutions, use Thomas' formula:

$$\text{MPN}/100\ \text{ml} = \frac{\text{positive tubes}\times100}{\sqrt{(\text{ml sample in negative tubes})\times(\text{ml sample in all tubes})}}$$

Drinking water should show an MPN of 0 per 100 ml.

What are E. coli and coliform?

Coliform bacteria are Gram-negative, non-spore-forming, rod-shaped bacteria that ferment lactose with gas and acid at 35 to 37 degrees C within 48 hours. They live in soil, vegetation and in the intestines of warm-blooded animals.

Escherichia coli (E. coli) is the main faecal coliform. It lives only in the intestines of humans and warm-blooded animals and is passed in large numbers (about $10^{8}$ to $10^{9}$ per gram) in faeces. It can ferment lactose at 44.5 degrees C and produces indole.

Are they harmful?

• Most E. coli strains are harmless and even help digestion in the gut.
• Some strains (for example E. coli O157:H7, enterotoxigenic and enteropathogenic strains) cause diarrhoea, bloody dysentery, kidney failure (haemolytic uremic syndrome) and can be fatal, especially to children.
• Coliform bacteria in general are usually harmless, but their presence shows that water may have been polluted.
• The main danger is what they stand for: faecal contamination, which means pathogens such as Salmonella, Vibrio cholerae, Shigella, viruses and protozoa may also be in the water.

Why they are tested (indicator organisms)

• Testing for each pathogen is difficult, costly and slow, as many pathogens exist only in small numbers.
• E. coli and coliforms are always present in faeces, easy to detect, survive longer than most pathogens in water and do not multiply in clean water.
• Their absence therefore shows that the water is bacteriologically safe. The standard is zero E. coli per 100 ml (Nepal Drinking Water Quality Standards, WHO).

How they are tested

1. Multiple tube fermentation (MPN) technique: presumptive, confirmed and completed tests in lactose broth, BGLB and EC broth; the result is given as MPN per 100 ml.
2. Membrane filter technique: 100 ml of water is passed through a 0.45 micron filter, which is placed on selective media (m-Endo, m-FC) and incubated; colonies are counted as CFU per 100 ml.
3. Presence-absence test: a rapid test for absence of coliform in 100 ml.
4. Chromogenic substrate test (for example Colilert) and field kits such as H2S strip test: colour change shows E. coli.

Indicator organisms

Indicator organisms are micro-organisms whose presence in water shows faecal pollution and so the possible presence of pathogens. Testing for individual pathogens is difficult, so an easily detected indicator is used. The best-known are coliform group bacteria and E. coli (faecal coliform); others are faecal streptococci and Clostridium perfringens.

An ideal indicator is always present when pathogens are present and absent in safe water, is more abundant and lives longer than pathogens, does not multiply in water, and is easy to detect.

Multiple tube fermentation (MPN) technique

The MTF technique estimates the Most Probable Number of coliform per 100 ml based on gas production from lactose.

Stage 1: Presumptive test

1. Prepare series of tubes of lactose broth with inverted Durham tubes. Inoculate 5 tubes each with 10 ml, 1 ml and 0.1 ml of sample (use higher dilutions for polluted water).
2. Incubate at 35 degrees C for 24 hours; if no gas, for another 24 hours.
3. Gas and acid in the tubes show a positive presumptive result.

Stage 2: Confirmed test

1. Transfer a loopful from each positive tube to brilliant green lactose bile broth, and incubate at 35 degrees C for 48 hours. Gas shows coliform.
2. For faecal coliform / E. coli, transfer to EC broth and incubate at 44.5 degrees C for 24 hours; gas shows E. coli.

Stage 3: Completed test Streak on EMB agar at 35 degrees C for 24 hours. E. coli forms dark colonies with a metallic green sheen. Gram stain should show Gram-negative, non-spore forming rods, and growth in lactose broth should again produce gas.

Calculation of MPN The number of tubes giving a positive result in each dilution is noted (for example 5-1-0), and the MPN per 100 ml is read from the standard MPN table (5-1-0 gives about 33 per 100 ml). For other combinations Thomas' formula is used:

$$\text{MPN}/100\ \text{ml} = \frac{\text{positive tubes}\times100}{\sqrt{(\text{ml in negative tubes})\times(\text{ml in all tubes})}}$$

Interpretation: drinking water must have 0 MPN of E. coli per 100 ml.

Impurities are the substances (physical, chemical or biological) present in water other than H2O molecules. Natural water picks up impurities from the atmosphere, the ground, and human and animal activity.

According to state (size)

According to characteristics

• Physical: turbidity, colour, taste, odour, temperature.
• Chemical: pH, hardness, alkalinity, chloride, sulphate, iron, manganese, nitrate, fluoride, arsenic, dissolved gases, toxic metals.
• Biological: bacteria, viruses, protozoa, algae, helminths and other living organisms, some of which cause disease.

Impurities are also grouped as organic (plant, animal and sewage matter) and inorganic (minerals and salts).

Water-borne diseases are infections caused by pathogens that are present in drinking water contaminated by human or animal faeces and swallowed by the person. They spread by the fecal-oral route.

Types, causes and transmission

Transmission mechanism

An infected person passes the pathogen in stool. Through open defecation, leaking latrines, sewage, or surface run-off, the pathogen reaches rivers, wells and springs. Another person drinks the polluted water or eats food washed in it, gets infected and passes the pathogen again. Poor storage and handling at home also contaminates water. Outbreaks occur mainly in the monsoon in Nepal.

Preventive measures

1. Protect sources (fence springs, locate latrines more than 30 m away and downhill of wells).
2. Treat water: filtration, chlorination, boiling, SODIS, and safe storage in covered containers.
3. Provide sanitation: latrines, safe disposal of excreta and sewage treatment.
4. Hygiene education: handwashing with soap, food hygiene.
5. Regular water quality monitoring and testing for E. coli.
6. Vaccination (cholera, typhoid, hepatitis A) and oral rehydration therapy.
7. Remove arsenic and other chemicals by suitable treatment.

Indicator organisms are micro-organisms, such as coliform bacteria and E. coli, whose presence in water shows faecal pollution and the likelihood that disease-causing organisms are also present. They are used because testing for each pathogen is difficult and costly.

Membrane filter technique for E. coli

In this method a known volume of water is filtered through a membrane of 0.45 micron pore size that retains bacteria; the membrane is then incubated on a selective medium and the colonies are counted.

Apparatus: membrane filter unit (funnel, base, vacuum pump), sterile membrane filters (47 mm, 0.45 micron), forceps, petri dishes with absorbent pad, m-FC broth or m-Endo medium, incubator, and sterile dilution water.

Procedure

1. Sterilise the filter unit and place a sterile membrane (grid side up) on the base with sterile forceps.
2. Pour 100 ml of sample (or a suitable dilution for polluted water) in the funnel and filter it with vacuum. Rinse the funnel with sterile water.
3. Remove the membrane and place it, without trapping air bubbles, on the pad saturated with the medium in a petri dish.
4. Incubate: m-Endo at 35 degrees C for 22 to 24 hours (total coliform), or m-FC at 44.5 degrees C for 24 hours (faecal coliform / E. coli).
5. Count the colonies: on m-FC, blue colonies are faecal coliform; on m-Endo, colonies with a golden-green metallic sheen are coliform.

Result

$$\text{Colonies}/100\ \text{ml} = \frac{\text{colonies counted}\times100}{\text{ml of sample filtered}}$$

Select membranes with 20 to 80 coliform colonies for counting. Drinking water must show 0 E. coli per 100 ml.

The membrane filter (MF) technique is a direct way of counting coliform bacteria in a water sample. It gives the count of colonies (CFU) per 100 ml and is faster than the MPN method (results in about 24 hours), uses less media and gives a direct count.

Principle

A measured volume of water passes through a sterile cellulose membrane with 0.45 micron pores. Bacteria are retained on the surface. The membrane is incubated on a selective nutrient medium, where each bacterium grows into a visible colony.

Apparatus and media

Filtration unit (funnel and base), vacuum source, sterile membrane filters, forceps, petri dishes with absorbent pads, m-Endo broth (total coliform) or m-FC broth (faecal coliform), incubator (35 degrees C) or water bath (44.5 degrees C), sterile buffered dilution water, a counting lens.

Procedure

1. Sterilise the unit and place the membrane on the base with sterile forceps.
2. Filter 100 ml of sample (or a dilution) under vacuum, then rinse the funnel.
3. Place the membrane on the medium-soaked pad without air bubbles.
4. Incubate upside down: 35 degrees C for 24 hours for total coliform; 44.5 degrees C for 24 hours for faecal coliform.
5. Count the typical colonies (golden-green sheen with m-Endo; blue colonies with m-FC).

$$\text{Coliform}/100\ \text{ml} = \frac{\text{colonies counted}}{\text{ml filtered}}\times100$$

Merits and limits

It is good for large volumes with low counts (clean water). It is less suitable for turbid water that clogs the filter, and for water with many non-coliform bacteria.

Water-washed diseases are diseases that spread because there is not enough water for personal and domestic hygiene (washing hands, face, body, clothes and utensils). They depend on the quantity of water, not its quality.

Types

1. Diseases of the skin and eyes: scabies, ringworm, skin sores and ulcers, and eye infections such as trachoma and conjunctivitis. Poor washing allows germs and mites to spread by contact.
2. Louse-borne diseases: epidemic typhus and relapsing fever. Lice live and breed in unwashed clothes and hair.
3. Faecal-oral diseases that wash can prevent: diarrhoea, dysentery and typhoid also spread by dirty hands when water for handwashing is short.

Preventive measures

• Provide adequate water in quantity (at least 20 litres per person per day) near the house.
• Promote handwashing with soap after toilet use and before eating, and regular bathing and washing of clothes.
• Provide laundry and bathing places, and tap stands close to the houses.
• Hygiene education and personal cleanliness at schools and health centres.
• Improve sanitation and housing; control lice by regular washing and delousing.
• Treat the infected persons (for example, for trachoma and scabies).

Water-based diseases are diseases caused by pathogens (usually parasitic worms) that spend a part of their life cycle in a water-dwelling animal (an intermediate host such as a snail or crustacean). A person is infected by contact with, or by swallowing, the infected water.

Examples

Control: avoid contact with infested water, filter or treat drinking water, control snails, and provide safe piped or hand-pump water.

Water quality standards for drinking water

Water quality standards are the maximum (or minimum) permissible values of the physical, chemical and biological characteristics of drinking water that make it safe for human consumption. They are set by law so that water supplied to the public does not harm health and is acceptable to the user. In Nepal, the National Drinking Water Quality Standards (NDWQS) 2062 BS apply; many countries follow the WHO guidelines.

Examples (Nepal standard): turbidity 5 NTU (maximum 10), pH 6.5 to 8.5, TDS 1000 mg/l, hardness 500 mg/l, chloride 250 mg/l, iron 0.3 mg/l, arsenic 0.05 mg/l, nitrate 50 mg/l, residual chlorine 0.1 to 0.2 mg/l, and E. coli 0 per 100 ml.

Significance of WHO guidelines

1. Scientific basis: The WHO "Guidelines for Drinking-water Quality" give health-based guideline values drawn from toxicology and epidemiology reviews and are updated periodically.
2. Basis for national standards: Countries, including Nepal, adapt them to local conditions, cost and risks.
3. Public health protection: The guideline for E. coli (zero per 100 ml), arsenic (0.01 mg/l), fluoride (1.5 mg/l), nitrate (50 mg/l), lead (0.01 mg/l) and others prevent acute and chronic disease.
4. Risk-based approach: They promote Water Safety Plans, covering catchment to tap, instead of only end-product testing.
5. Guides for monitoring and treatment: They help in selection of treatment, monitoring frequency and emergency response.
6. Basis for comparison and planning: They permit comparison of supplies and set targets for developing countries.

Hardness of a ion $= \text{concentration (mg/l)}\times\dfrac{50}{\text{equivalent weight}}$ as CaCO$_3$. Equivalent weights: Ca = 20.04, Mg = 12.15, Sr = 43.81, HCO$_3$ = 61, CO$_3$ = 30.

(i) Concentration of Ca and Mg

Let Ca = Mg = $x$ mg/l.

$$\begin{aligned} \text{TH} &= x\left(\frac{50}{20.04}+\frac{50}{12.15}\right) = x(2.495+4.115) = 6.610x = 300\\ x &= 45.38\ \text{mg/l} \end{aligned}$$

Ca$^{2+}$ = Mg$^{2+}$ = 45.38 mg/l.

(ii) Alkalinity

Only HCO$_3^-$ is present (no CO$_3^{2-}$ or OH$^-$), so

$$\text{Alkalinity} = 150\times\frac{50}{61} = 122.91\ \text{mg/l as CaCO}_3$$

(iii) Carbonate and non-carbonate hardness

The alkalinity (122.91) is less than the total hardness (300), so carbonate hardness equals alkalinity.

$$\begin{aligned} \text{CH} &= 122.91\ \text{mg/l}\\ \text{NCH} &= \text{TH}-\text{CH} = 300-122.91 = 177.09\ \text{mg/l} \end{aligned}$$

Answer: Ca = Mg = 45.38 mg/l; alkalinity = 122.91 mg/l as CaCO$_3$; CH = 122.91 mg/l; NCH = 177.09 mg/l.

Total hardness (TH) $= 150$ mg/l, carbonate hardness (CH) $= 60$ mg/l. The three bivalent metallic ions are taken as Ca$^{2+}$, Mg$^{2+}$ and Sr$^{2+}$ present in equal concentration $x$ mg/l (equivalent weights 20.04, 12.15 and 43.81).

Non-carbonate hardness

$$\text{NCH} = \text{TH}-\text{CH} = 150-60 = 90\ \text{mg/l as CaCO}_3$$

Alkalinity

Total hardness is greater than carbonate hardness, so all alkalinity is used in carbonate hardness: alkalinity = CH $= 60$ mg/l as CaCO$_3$.

Concentration of the bivalent ions

$$\begin{aligned} \text{TH} &= x\left(\frac{50}{20.04}+\frac{50}{12.15}+\frac{50}{43.81}\right) = x(2.495+4.115+1.141) = 7.752x = 150\\ x &= 19.35\ \text{mg/l} \end{aligned}$$

Answer: NCH = 90 mg/l as CaCO$_3$; alkalinity = 60 mg/l as CaCO$_3$; each bivalent ion = 19.35 mg/l.

Total hardness (TH) $= 160$ mg/l, carbonate hardness (CH) $= 70$ mg/l. The three bivalent metallic ions are taken as Ca$^{2+}$, Mg$^{2+}$ and Sr$^{2+}$ present in equal concentration $x$ mg/l (equivalent weights 20.04, 12.15 and 43.81).

Non-carbonate hardness

$$\text{NCH} = \text{TH}-\text{CH} = 160-70 = 90\ \text{mg/l as CaCO}_3$$

Alkalinity

Total hardness is greater than carbonate hardness, so all alkalinity is used in carbonate hardness: alkalinity = CH $= 70$ mg/l as CaCO$_3$.

Concentration of the bivalent ions

$$\begin{aligned} \text{TH} &= x\left(\frac{50}{20.04}+\frac{50}{12.15}+\frac{50}{43.81}\right) = x(2.495+4.115+1.141) = 7.752x = 160\\ x &= 20.64\ \text{mg/l} \end{aligned}$$

Answer: NCH = 90 mg/l as CaCO$_3$; alkalinity = 70 mg/l as CaCO$_3$; each bivalent ion = 20.64 mg/l.

Indicator organisms are micro-organisms (usually bacteria) that are always found in faeces of humans and warm-blooded animals, easily detected in water, and used to show that the water has been contaminated with faecal matter and may contain pathogens.

Testing for each pathogen (cholera, typhoid, viruses, protozoa) is difficult, slow and costly, so an indicator is tested instead.

Qualities of a good indicator

• Present in large numbers in faeces, and present whenever pathogens are present.
• Absent in unpolluted water.
• Survives in water at least as long as pathogens and is not killed more quickly by disinfection.
• Does not multiply in water.
• Easy and cheap to detect and count.

Examples

• Total coliform group bacteria.
• Faecal coliform and E. coli (best indicator of recent faecal pollution).
• Faecal streptococci (enterococci) and Clostridium perfringens (for older pollution).

The drinking water standard is zero E. coli / faecal coliform per 100 ml.

Use Thomas' formula for the Most Probable Number:

$$\text{MPN}/100\ \text{ml} = \frac{\text{No. of positive tubes}\times100}{\sqrt{(\text{ml of sample in negative tubes})\times(\text{ml of sample in all tubes})}}$$ $$\text{MPN}/100\ \text{ml} = \frac{9\times100}{\sqrt{3.246\times6.666}} = \frac{900}{4.652} = 193.5$$

Answer: MPN $\approx 193$ per 100 ml.

The bacteriological examination of water tests for the presence of faecal bacteria to find if the water is safe to drink. As testing for each pathogen is not practical, indicator organisms (coliform and E. coli) are used.

1. Sampling

Collect the sample in a sterile, clean glass bottle (with sodium thiosulphate if the water is chlorinated to remove residual chlorine). Flame the tap, let water run for 2 to 3 minutes, fill the bottle leaving an air space, and keep it cool (below 10 degrees C). Test within 6 hours (maximum 24 hours).

2. Tests

• Standard plate count (total bacterial count): 1 ml of sample is placed on nutrient agar and incubated at 37 degrees C for 24 hours (and 22 degrees C for 72 hours); colonies are counted. Treated water should have a low count (less than 100 per ml).
• Multiple tube fermentation test (MPN): presumptive test in lactose broth at 35 degrees C, confirmed test in BGLB or EC broth, and completed test on EMB agar. The result is given as MPN of coliform per 100 ml.
• Membrane filter technique: 100 ml is filtered through a 0.45 micron membrane and incubated on m-Endo or m-FC medium; coliform colonies are counted.
• Presence-absence test and field kits (H2S strip test, Colilert): quick tests for screening.

3. Interpretation

Zero E. coli per 100 ml must be found in drinking water (Nepal Drinking Water Quality Standards and WHO). A positive result shows faecal contamination, and the water should be treated (chlorination) and the source protected.

Natural water contains many living organisms; some are harmless, some are harmful and some give problems of taste and odour.

Control

Protect sources from sewage and runoff, control algae by copper sulphate or chlorine, provide filtration and disinfection (chlorination, UV, ozone), and cover storage tanks to prevent mosquito breeding.

The fecal-oral route is the path by which pathogens in the faeces of an infected person reach the mouth of another person.

Sketch (F-diagram)

        Faeces
      /  |   |  \
 Fluids Fields Flies Fingers
      \  |   |  /
         Food
          |
      Mouth of new host

Faeces carry the germs to fluids (water), fields (soil), flies and fingers; from there they contaminate food and drinking water, which is swallowed by a new host.

Preventive measures (barriers that break the route)

Primary barriers (stop faeces from entering the environment):

• Build and use toilets; stop open defecation.
• Safe disposal and treatment of excreta and sewage.

Secondary barriers (stop germs reaching the mouth):

• Protect and treat drinking water (chlorination, filtration, boiling); store water safely in covered vessels.
• Wash hands with soap after defecation, after cleaning a baby, before cooking and eating.
• Keep food covered and clean; wash vegetables and fruits; avoid raw or stale food.
• Control flies by covering latrine pits and waste.
• Hygiene education and community-led total sanitation.

Water-related insect vector diseases (water vector diseases) are diseases transmitted by insects that breed in water or bite near water. The pathogen is not in drinking water; it is carried by the insect (vector) from an infected to a healthy person.

Two examples

1. Malaria

• Cause: Plasmodium parasite.
• Vector: female Anopheles mosquito, which breeds in stagnant, clear water (ponds, paddy fields, puddles).
• Transmission: the mosquito bites an infected person, picks up the parasite, and transmits it when it bites another person.
• Symptoms: fever with chills and sweating.

2. Dengue (also yellow fever, Japanese encephalitis, filariasis)

• Cause: Dengue virus.
• Vector: Aedes aegypti mosquito, which breeds in clean water held in containers, tyres and water tanks.
• Transmission: by mosquito bite in daytime.
• Symptoms: high fever, severe headache, joint pains and rash.

Control

Drain stagnant water, cover storage tanks, use larvicides and fish that eat larvae, use bed nets and repellents, spray insecticides, and give community awareness.

Dissolved oxygen (DO) is determined by the Winkler (iodometric) titration method, usually with the azide modification, in which DO is fixed with manganese hydroxide and measured as liberated iodine.

Reagents

Manganous sulphate solution, alkali-iodide-azide reagent, concentrated sulphuric acid, starch indicator, standard sodium thiosulphate (0.025 N).

Procedure

1. Fill a 300 ml BOD bottle with the sample carefully without air bubbles.
2. Add 2 ml MnSO4 and 2 ml alkali-iodide-azide reagent below the surface; stopper and mix by inverting. A brown-white precipitate forms; let it settle.
3. Add 2 ml of concentrated H2SO4, restopper and mix until the precipitate dissolves. Iodine is liberated in an amount equal to the DO.
4. Take 200 ml (or 203 ml) of the solution in a flask and titrate with 0.025 N thiosulphate until the yellow colour becomes pale.
5. Add 1 ml of starch (blue colour appears) and continue the titration until the blue colour just disappears. Note the volume used.

Reactions

Mn(2+) + 2OH(-) gives Mn(OH)2; with oxygen it gives MnO(OH)2 (brown). In acid, MnO(OH)2 + 2I(-) + 4H(+) gives Mn(2+) + I2 + 3H2O. Then I2 + 2S2O3(2-) gives 2I(-) + S4O6(2-).

Calculation

$$\text{DO (mg/l)} = \frac{V\times N\times8\times1000}{V_s}$$

where $V$ is the ml of thiosulphate, $N$ its normality and $V_s$ the sample volume (200 ml). With 0.025 N thiosulphate and 200 ml sample, 1 ml of titrant equals 1 mg/l of DO.

Taste and odour in water arise from the following causes:

1. Algae and other micro-organisms: blue-green algae, diatoms and actinomycetes give fishy, grassy, earthy or musty smells, especially in lakes and reservoirs in warm weather.
2. Decaying organic matter: decomposing leaves, vegetation and animal matter give musty or swampy taste and odour.
3. Dissolved gases: hydrogen sulphide (rotten egg smell) in ground water, and methane.
4. Dissolved minerals and salts: iron and manganese (metallic taste), sodium chloride (salty), magnesium and sulphate (bitter taste), high TDS.
5. Sewage and industrial wastes: phenols, oils, chemicals, pesticides, and detergents.
6. Chlorine and its compounds: excess chlorine or chlorophenols from disinfection of water that has phenols.
7. Pipes and storage: corrosion of iron or copper pipes, bacterial growth in dead ends, and stagnant water.

Removal methods include aeration, activated carbon, chlorination or ozonation, and protecting the source.

Hardness is the property of water that prevents soap from lathering and causes scale in pipes and boilers. It is caused by dissolved salts of calcium and magnesium (and sometimes iron, strontium and manganese).

Causes

• Bicarbonates of Ca and Mg cause temporary (carbonate) hardness, which is removed by boiling.
• Sulphates, chlorides and nitrates of Ca and Mg cause permanent (non-carbonate) hardness.
• Water dissolves these salts when it passes through limestone, dolomite, chalk and gypsum rocks, so ground water is generally harder than surface water.

Effects

• Wastes soap, since hard water forms scum with soap before a lather forms; more soap and detergent is needed.
• Forms scale in boilers, hot water pipes, kettles and heat exchangers, reducing efficiency and sometimes causing explosion.
• Causes clogging of pipes and reduction of carrying capacity.
• Gives bad taste, and makes cooking slow (pulses and vegetables do not cook well).
• Harms textile, paper and dyeing industries; spoils the texture of clothes and skin irritation.
• Very high hardness is not seriously harmful to health; some studies suggest soft water is related to heart disease.

Hardness above 500 mg/l as CaCO3 is not acceptable; it is reduced by lime-soda softening or ion exchange.

Hardness of a ion $= \text{concentration (mg/l)}\times\dfrac{50}{\text{equivalent weight}}$ as CaCO$_3$. Equivalent weights: Ca = 20.04, Mg = 12.15, Sr = 43.81, HCO$_3$ = 61, CO$_3$ = 30. Strontium contributes 2.0 mg/l as given. Na$^+$ and K$^+$ do not cause hardness.

Total hardness

Alkalinity and carbonate hardness

Carbonate alkalinity is zero, so the alkalinity is due to HCO$_3^-$:

$$\text{Alkalinity} = 72\times\frac{50}{61} = 59.00\ \text{mg/l as CaCO}_3$$

Alkalinity is less than the total hardness, so CH = alkalinity. NCH $= 101.02-59.00 = 42.03$ mg/l.

Answer: Total hardness = 101.02 mg/l; carbonate hardness = 59.00 mg/l; non-carbonate hardness = 42.03 mg/l (all as CaCO$_3$).

Hardness of a ion $= \text{concentration (mg/l)}\times\dfrac{50}{\text{equivalent weight}}$ as CaCO$_3$. Equivalent weights: Ca = 20.04, Mg = 12.15, Sr = 43.81, HCO$_3$ = 61, CO$_3$ = 30. Strontium is given as 25.26 mg/l hardness. Na$^+$, K$^+$, Cl$^-$, SO$_4^{2-}$ and NO$_3^-$ do not cause hardness or alkalinity.

Total hardness

Alkalinity

$$\begin{aligned} \text{HCO}_3^-:&\ 92\times\frac{50}{61} = 75.39\\ \text{CO}_3^{2-}:&\ 112\times\frac{50}{30} = 186.67\\ \text{Total alkalinity} &= 262.05\ \text{mg/l as CaCO}_3 \end{aligned}$$

Carbonate and non-carbonate hardness

Alkalinity (262.05) is less than total hardness (397.57), so CH = alkalinity and

$$\text{NCH} = 397.57-262.05 = 135.52\ \text{mg/l}$$

Answer: Carbonate hardness = 262.05 mg/l; non-carbonate hardness = 135.52 mg/l; total hardness = 397.57 mg/l (as CaCO$_3$).

Total hardness TH $= 280$ mg/l, carbonate hardness CH $= 75$ mg/l, Ca$^{2+}$ = Mg$^{2+}$ $= x$ mg/l.

Total alkalinity

TH is greater than CH, so the alkalinity is fully used in carbonate hardness: total alkalinity = CH $= 75$ mg/l as CaCO$_3$. (Non-carbonate hardness $= 280-75 = 205$ mg/l.)

Concentration of Ca and Mg

$$\begin{aligned} \text{TH} &= x\left(\frac{50}{20.04}+\frac{50}{12.15}\right) = 6.610x = 280\\ x &= 42.36\ \text{mg/l} \end{aligned}$$

Answer: Total alkalinity = 75 mg/l as CaCO$_3$; Ca$^{2+}$ = Mg$^{2+}$ = 42.36 mg/l.

Hardness of a ion $= \text{concentration (mg/l)}\times\dfrac{50}{\text{equivalent weight}}$ as CaCO$_3$. Equivalent weights: Ca = 20.04, Mg = 12.15, Sr = 43.81, HCO$_3$ = 61, CO$_3$ = 30. Na$^+$, K$^+$ and SO$_4^{2-}$ do not contribute to hardness or alkalinity.

Total hardness

$$\begin{aligned} \text{Ca hardness} &= 65\times\frac{50}{20.04} = 162.18\\ \text{Mg hardness} &= 35\times\frac{50}{12.15} = 144.03\\ \text{TH} &= 306.21\ \text{mg/l as CaCO}_3 \end{aligned}$$

Carbonate hardness

Alkalinity due to HCO$_3^-$ $= 248\times\dfrac{50}{61} = 203.21$ mg/l as CaCO$_3$. It is less than TH, so

$$\text{CH} = 203.21\ \text{mg/l},\qquad \text{NCH} = 306.21-203.21 = 103.00\ \text{mg/l}$$

Answer: Total hardness = 306.21 mg/l; carbonate hardness = 203.21 mg/l; non-carbonate hardness = 103.00 mg/l (as CaCO$_3$).

Hardness is expressed as CaCO$_3$ equivalent. Each cation of concentration $C$ (mg/l) contributes $C \times \dfrac{50}{\text{eq. wt}}$ to hardness, which gives the standard factors: Ca 2.5, Mg 4.1, Sr 1.14.

$$TH = 2.5\,Ca + 4.1\,Mg + 1.14\,Sr$$

(i) Non-carbonate hardness

$$NCH = TH - CH = 150 - 77 = 73\ \text{mg/l as CaCO}_3$$

(ii) Concentration of principal cations

Let $Ca = Mg = Sr = x$.

$$\begin{aligned} 150 &= (2.5 + 4.1 + 1.14)\,x = 7.74\,x \\ x &= \frac{150}{7.74} = 19.38\ \text{mg/l} \end{aligned}$$

So Ca = Mg = Sr = 19.38 mg/l each.

(iii) Total alkalinity

Carbonate hardness is the part of hardness balanced by alkalinity. When alkalinity is less than total hardness, carbonate hardness equals alkalinity (the alkalinity is due to bicarbonate). Since CH = 77 < TH = 150:

$$\text{Total alkalinity} = CH = 77\ \text{mg/l as CaCO}_3$$

Answer: NCH = 73 mg/l; Ca = Mg = Sr = 19.38 mg/l; total alkalinity = 77 mg/l (as CaCO$_3$).

Hardness is expressed in CaCO$_3$ equivalent. Using the standard factors, $TH = 2.5\,Ca + 4.1\,Mg$ (Ca and Mg in ppm). Here TH = 400 ppm, CH = 150 ppm.

(i) Concentration of calcium and magnesium

Let $Ca = Mg = x$.

$$\begin{aligned} 400 &= 2.5x + 4.1x = 6.6x \\ x &= \frac{400}{6.6} = 60.61\ \text{ppm} \end{aligned}$$

Ca = Mg = 60.61 ppm (as the ions). In CaCO$_3$ terms: Ca hardness = $2.5 \times 60.61 = 151.5$ ppm and Mg hardness = $4.1 \times 60.61 = 248.5$ ppm.

(ii) Total alkalinity and NCH

Carbonate hardness (150) is less than total hardness (400), so the alkalinity is fully used up in forming carbonate hardness:

$$\text{Total alkalinity} = CH = 150\ \text{ppm as CaCO}_3$$ $$NCH = TH - CH = 400 - 150 = 250\ \text{ppm}$$

Answer: Ca = Mg = 60.61 ppm; total alkalinity = 150 ppm; NCH = 250 ppm.

pH is $-\log_{10}[H^+]$. When two samples are mixed, the hydrogen-ion amounts (moles) add, so the final $[H^+]$ is the volume-weighted average. (Buffering by alkalinity is ignored, as is usual in this type of problem.)

Sample 1: 700 ml, pH 6, so $[H^+] = 10^{-6}$ mol/l. Sample 2: 500 ml, pH 8, so $[H^+] = 10^{-8}$ mol/l.

$$\begin{aligned} [H^+]_{mix} &= \frac{700\times10^{-6} + 500\times10^{-8}}{700+500} \\ &= \frac{7.0\times10^{-4} + 5.0\times10^{-6}}{1200} = 5.8750e-07\ \text{mol/l} \end{aligned}$$ $$pH = -\log_{10}(5.8750e-07) = 6.23$$

Answer: pH of the mixture = 6.23 (acidic, because the acidic sample dominates).

pH $=-\log_{10}[H^+]$. On mixing, the moles of H$^+$ add, so the mixture's $[H^+]$ is the volume-weighted average (buffering is ignored).

$$\begin{aligned} [H^+]_{mix} &= \frac{400\times10^{-6} + 700\times10^{-8}}{400+700} \\ &= \frac{4.0\times10^{-4} + 7.0\times10^{-6}}{1100} = 3.7000e-07\ \text{mol/l} \end{aligned}$$ $$pH = -\log_{10}(3.7000e-07) = 6.43$$

Answer: pH of the mixture = 6.43.

The moles of H$^+$ in each sample add on mixing, so the final hydrogen-ion concentration is the volume-weighted average. Buffering is ignored.

$$\begin{aligned} [H^+]_{mix} &= \frac{7.0\times10^{-4} + 3.0\times10^{-5} + 5.0\times10^{-6}}{700+300+500} \\ &= \frac{7.35\times10^{-4}}{1500} = 4.9000e-07\ \text{mol/l} \end{aligned}$$ $$pH = -\log_{10}(4.9000e-07) = 6.31$$

Answer: pH of the mixture = 6.31.

Acidity is measured by the hydrogen-ion concentration, $[H^+] = 10^{-pH}$. The pH scale is logarithmic, so each unit of pH changes $[H^+]$ ten times. (The question compares the two samples, P with pH 6.5 and Q with pH 7.8; "sample B" is read as sample Q.)

$$\begin{aligned} [H^+]_P &= 10^{-6.5} = 3.162\times10^{-7}\ \text{mol/l} \\ [H^+]_Q &= 10^{-7.8} = 1.585\times10^{-8}\ \text{mol/l} \end{aligned}$$ $$\frac{[H^+]_P}{[H^+]_Q} = 10^{7.8-6.5} = 10^{1.3} = 19.95$$

Answer: Sample P (pH 6.5) is about 20.0, i.e. nearly 20 times, more acidic than sample Q (pH 7.8).