Water quality
Physical Parameters
(i) Temperature
Temperature should less than 40 0C. Hot water accelerates the biological processes. It reduces the dissolved O2 and affects the aquatic organisms.

(ii) pH Value
• A pH range of 6.0 – 9.0 appears to provide protection for the life of fresh water fish and bottom dwelling invertebrates.
• pH of water may also be changed due to dissolved gas from air such as CO2, SO2 and the mixing with industrial effluents.
• Normal pH range of ground water which is useful is 6.0 – 8.5.
If pH < 6.5 it is acidic water which is corrosive. Usually soda ash is used to neutralise the acidity.
• Dolomite can also be used in agriculture. Dolomite slowly neutralises acidity of water.
2H⁺(aq) + CaCO₃.MgCO₃(s) → Ca²⁺(aq) + Mg²⁺(aq) + 2 HCO₃^–(aq)

(iii) Conductivity
• Conductivity is a measure of the ability of an aqueous solution to conduct an electric current.
• Conductivity depends on the following factors.
• Concentration of ions • Mobility of ions
• Oxidation state • Temperature of water

• The unit used to measure the conductance is ohm^-1 = Ω^-1 or Siemens (S). Conductance = 1/Resistance
This measures the ionic strength but does not identify the ions present.

(iv) Turbidity
Sediments in the water prevents the penetration of light to the bottom of water bodies. This will reduce the photosynthesis activity and also creates anaerobic environment introducing a bad smell to the water.

(v) Dissolved amount of oxygen and water quality
• O2 is required for the metabolism of aerobic organisms and also influences some chemical reactions.
• O2 in air dissolves in water. Photosynthesis is the process of producing O2.
• Concentration of dissolved O2 decreases as temperature increases.
• Submerged green plants and algae increase the dissolved O2 content in a water body in the day time.
• Decaying organic matter consumes dissolved O2 in a water body.

Chemical oxygen demand (COD)
(CHO)_n(aq) + O₂(g) → x CO₂(g) + y H₂O(l)
• Amount of oxygen required in mg dm-3 for this oxidation of organic matter wastes to be carried out chemically is called chemical oxygen demand (COD).

• Since the amount of O₂ consumed cannot be easily quantified, dichromate is used for laboratory determination. In this determination the sample is heated with an acid solution of potassium dichromate. All oxidizable organic matter will react with dichromate at this stage.Silver sulphate is generally added as a catalyst.
• Chloride present in water can be oxidized to chlorine under these conditions by dichromate.This is prevented by converting chloride to undissociated mercuric chloride. The excess of unreacted dichromate is determined by titrating with iron(II) ammonium sulphate.
Cr₂O₇^2-(aq) + 14H⁺(aq) + 6e → 2Cr³⁺(aq) + 7H₂O(l)

• Ιn acidic medium;
O₂ (g) + 4H⁺(aq) + 4e → 2H₂O(l)                                                                                                                 (CHO)_n(aq) + O₂(g) → x CO₂(g) + y H₂O(l)

• If the organic matter is oxidized by acid dichromate, then
Cr₂O₇^2-(aq) + 14H⁺(aq) + 6e → 2Cr³⁺(aq) + 7H₂O(l)
1 mol O₂(aq) ≡ 2/3 mol of Cr₂O₇^2-(aq)
Biochemical oxygen demand / Biological oxygen demand (BOD)
(CHO)_n(aq) + O₂(g) → x CO₂(g) + y H₂O(l)
• Amount of O2 required to be carried out for the above oxidation by microorganisms is called biochemical oxygen demand.
• The consumed dissolved oxygen can be used to determine the BOD value.
• Substances which use up dissolved oxygen will contribute to the value of BOD. Pollutants such as human and animal wastes, food canneries, meat etc. controls the BOD.

• When a water sample is saturated with oxygen, the initial concentration of dissolved oxygen can be determined. If it is incubated at 20 0C for a known period, usually 5 days, the microorganisms in the water oxidize the organic matter. The oxygen that remains in the water can be measured. The oxygen used up or the BOD value can be calculated , for the particular water body.

Dissolved ionic compounds

Hardness
• Hardness isdue to dissolved metal ions, namely Ca2+ and Mg2+. Here the contribution of Ca and Mg are the most considered and other metals are negligible because they are less soluble in water. Therefore, we discuss Ca and Mg only here. Hard water has no adverse health effects. Hard water is less desirable because it requires more soap for cleaning. It forms scum and curd and it toughens vegetables during cooking and forms scales in boilers, hot water heaters and pipes. The composition of ground water naturally reflects the underlying geology, the residence time in the rock, the previous composition of the ground water and in some instances, the flow path. Due to the slower movement of ground water in the aquifers as compared to that of surface water, the composition of the former shows a negligible variation with time for a given aquifer

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Ca²⁺(aq) / Mg²⁺(aq) + 2HCO₃^–(aq) → CaCO₃(s) + MgCO₃(s) + CO₂(g) + H₂O(l)

• Aquifer – A porous permeable rock layer containing water (Groundwater).

Iron (Fe)
• The primary source of iron in water is rock layers containing iron ore. Iron is typically dissolved in water and when brought to the surface, can form ‘rust’ which may settle out. Another source of iron is iron-reducing bacteria, which depends upon iron to live. The commonest iron containing water is red , laundry spotting, metallic in taste and staining of plumbing fixtures. These are usually due to the presence of iron above 0.3mg dm^-3. Iron affects the taste of drinking water.

Fluoride (F^–)
• Varying amounts of fluoride are found in groundwater of different areas of Sri Lanka. For example, due to the apatite ore in Eppawala fluoride, concentration of ground water in the surrounding areas is high. Fluoride can affect teeth during the period when permanent teeth are being formed. For tropical countries the fluoride content should not exeed 0.6 mg dm^-3
Nitrates
• Nitrate is a common contaminant found mainly in ground water. High nitrate concentrations can be particularly dangerous to babies under six months, since nitrates interfere with the ability of blood to carry oxygen. Nitrate also causes cancer. Fertilizers, human and animal sewage are usually enriched with of nitrogenous compounds which may enter into a ground water body as a result of leaching. The conversion of ammonia to nitrate is brought about by highly specialized soil bacteria.

Phosphates
• Phosphate ions are added to water by chemical fertilizers and artificial detergents. Due to nitrates ions and phosphate ions, an eutrophication condition arises in water and enhances the growth of algae. As a result, amount of dissolved oxygen in water decreases.

Methods of water purifications

Sedimentation
• The separation of a suspension of solid particles into a concentrated slurry. The supernatant liquid, after sedimentation is clear.
• If the effluent is suitable it will be discharged into waterways. Otherwise it is passed to a secondary treatment.

Coagulation
• Muddy river water in large water supply schemes can be coagulated using aluminium salt(alum).
• Water is stored in large tanks and can be coagulated with A1(III) or Fe(III).
• A gelatinous precipitate of aluminum hydroxide or iron(III) hydroxide is formed. As it settles and sinks to the bottom, the precipitate carries suspended material with it as sludge.
Flocculation and Filtration
• During the flocculation smaller particles are agglomerated to form bigger particles and these particles are filtered.
• After that water is passed slowly through sand filters. Different types of sand filters are used for water flow.
• Fine sand
• Course sand
• Gravel
• Stones

• Filtration removes microbes and other suspended particles from water. Many filters will also remove some harmful chemicals found in water.

Disinfection process
• Use of chlorine
Cl₂, ClO₂ ,chloroamines are used as disinfectants. They kill bacteria by oxidising. The residual Cl₂ prevents the formation of further bacteria. But excess Cl₂ reacts with organic substances and forms harmful substances including trihalomethanes and chlorinated phenols.
• Use of ozone
Ozone also destroys bacteria by oxidation. But it dissociates quickly. Ozone does not give further protection from bacteria. Therefore the water disinfected by ozone has to be used quickly. Since ozone doesn’t give any side effects it is preferably used by human. Unlike Cl₂, ozone needs no storage and it can be generated easily.

• Use of UV radiation
It kills both the bacteria and viruses. As ozone it cannot have a further protection from bacteria.