Water

Chapter 19

Hard Water

Hard water is water that does not form a lather easily with soap.

There are 2 types of hardness.

1. Temporary hardness is referred to as hardness in water which can be removed by boiling.

2. Permanent hardness cannot be removed by boiling.

Water hardness in Irelands

Temporary Hardness• Temporary hardness is caused by the presence in the water of calcium

hydrogencarbonate Ca(HCO3)2 and magnesium hydrogencarbonate Mg(HCO3)2.

• This form of hardness arises when rain water, containing the weak acid carbonic acid (H2CO3), runs over limestone rocks(CaCO3 ) dissolving out Ca2+ according to

CaCO3 + HCO3– + H+ Ca(HCO3)2

The dissolved Ca(HCO3)2 is now responsible for the temporary hardness.

ScaleWhen this hard water is boiled a reaction occurs which results in the

calcium ions being precipitated out of solution in the form of CaCO3

which is almost insoluble. This precipitate forms the deposit called

‘scale’ or ‘fur’ which is often clearly visible on the inside of kettles and

boilers in hard-water regions. heat

Ca(HCO3)2(aq)CaCO3(s) + H2O(l) + CO2(g)

heat

Ca(HCO3)2(aq)CaCO3(s) + H2O(l) + CO2(g)

Water softening.

Any process which removes or reduces the degree of hardness of a water sample is termed water softening. Boiling a sample of water which has temporary hardness is an example of a water softening method.

Permanent Hardness

• Permanent hardness cannot be removed by boiling .

• Permanent hardness is caused by the presence of dissolved calcium and magnesium sulphates and calcium and magnesium chlorides. CaSO4 CaCl2

• An example of how this type of hardness can be acquired by

water is where water is flowing through, or over, gypsum rocks (calcium sulphate). The calcium sulphate is slightly soluble in water and so the water contains dissolved calcium sulphate after passing through the rocks, giving a hardness of up to about 500 p.p.m., again recorded as 500 p.p.m. CaCO3.

• The sum of the temporary hardness and the permanent hardness is referred to as the total hardness.

Methods of Removing Hardness from Water - Water Softening

A number of methods are used for removing hardness.

1. Distillation

2. Addition of washing soda

3. Ion Exchange resins and de-ionisers.

4. Calgon

Estimation of the Total Hardness in Water

This titration involves titrating a sample of water (usually but not

always 100 cm3) with a solution of disodium

ethylenediaminetetraacetic acid (Na2edta) in the presence of a

buffer which holds the pH at about 10.

Na2edta + Ca2+ Caedta + 2Na+

The indicator is Eriochrome Black T and it changes colour at the

end-point from wine-red to blue.

An unusual feature is that the indicator is added as a tiny pinch of

solid.

Total Hardness

Wine Red Blue

Calculations• The calculation involves application of the

formula

where V = volume, M = molarity, n = number of moles in the balanced equation.

Having got the molarity of the Ca2+ ions, express this as moles CaCO3 per litre and then as mg per litre, i.e. parts per million (p.p.m.)

Water Treatment

Water Treatment

The various stages of water treatment are.

1. Screening- removal of large particles

2. Settling

The precipitation of the particles causing turbidity is achieved using

flocullation agents. The most common flocullation agent is are

aluminium sulphate, Al2(SO4)3.

Polyelectrolytes are often added to speed up flocculation or

coagulation. Aluminium contamination is probably the most

controversial of these agents; there has been

ongoing speculation linking it with neurodegenerative diseases

such as Alzheimer’s disease.

Flocullation

floc floating at the surface of a basin

Mechanical system to push floc out of the water basin

Filtration + Chlorination

3. Filtration

The purpose of filtration is to remove particles from the water, whetherthese exist in the raw water naturally or whether they have been producedby the coagulation process. Filtration is usually achieved by the downwardpassage of water through about a metre of finely divided inert material(sand or anthracite) which is on a support bed of coarser material(usually gravel). Drains at the bottom of the filter collect the water as itfilters through.

4.Chlorination

Elemental chlorine and compounds of chlorine are regularly added towater during the treatment of the water for public supplies. Chlorination isone of a number of possible treatments whose purpose is to disinfect thewater to keep the pathogen content down to a safe level.

Filtration

5. Fluoridation

Fluoridation

Over the past forty years a number of studies have shown

some correlation between fluoride concentration in water

and the incidence of tooth decay. It would appear that at

fluoride concentrations of around 1.0 p.p.m. Maximum

benefit is obtained. The chemicals added to water to supply

fluoride ion include simple salts such as sodium fluoride,

NaF, and calcium fluoride, CaF2.

Sewage treatment(Waste water treatment)

Two measuring parameters are regularly used in analysis of wastewater effluent. These are Biochemical oxygen demand (BOD) and chemical oxygen demand (COD)

Biochemical oxygen demand (BOD) is taken as a measure of the degree of pollution of a water sample based on the quantity of oxygen consumed by the micro organisms present in a one litre sample stored in the dark at 20 °C for five days. It can be expressed in milligrams of oxygen per litre of sample, i.e. mg l – 1 or p.p.m.

Activated sludge

Primary Treatment(Physical)

Pre-treatment

The incoming sewage is pushed through mechanically raked screens to macerate the sewage and remove large debris.

The pre-treated sewage flows into primary settling tanks. The sewage enters at the centre of the tank, (c. 12 m in diameter and c. 2 m deep), and rises allowing sludge to settle.

The decanting liquid is transferred to the secondary treatment system. The settling tanks have a skimmer mechanism at the top to remove floating particles and a scraper on the settling tank floor (the base of the tank is hopper-shaped, i.e. sloping to the centre) to gather the settling sludge.

Primary

Solids and large floating debris are screened from the waste water

 Remaining solids are removed by allowing the waste to settle in sedimentation tanks

Secondary Treatment(Biological)

• This involves the biological degradation of the nutrient content of the effluent. This is usually done aerobically using percolating filters, activated sludge digestion units, aeration basins or biotowers.

• Percolating filters and activated sludge digestion units are commonly used in sewage works but all four methods can be found in use separately or in pairs industrially.

Secondary Treatment

Activated Sludge MethodThe activated sludge method operates aerobically. The sewage is fed continuously into aerated tanks that is kept oxygenated by mechanical agitators.The micro-organisms break down the organic material into Carbon Dioxide and Water.

Waste water treatment plant

Tertiary Treatment(Nitrates and Phosphates)

While primary and secondary treatment of effluent largely

concentrate on the reduction of COD/BOD levels they have a

lesser impact on phosphate and nitrate concentrations under the

conditions normally applied. High levels of nitrogen and

phosphorus are environmentally harmful as they act as nutrients

which give rise to algal bloom, leading to eutrophication.

Ammonium salts and nitrates are the common sources of nitrogen

and usually originate from animal waste or fertilisers.

Phosphates are precipitated by treatment with lime, Ca(OH)2,

aluminium sulphate, Al2(SO4)3, or iron(III)sulphate, Fe2(SO4)3. In

each case an insoluble salt is produced which can be filtered off.

Tertiary Treatment(Nitrates and Phosphates)

Removal of nitrates and phosphates Phosphates are removed by precipitation with Aluminium Phosphate Nitrates are removed by biological nitrification.

Tertiary treatment is a costly process

Water pollution

• Eutrophication.• The over-enrichment of waters by nutrients, such as nitrate and

phosphate, gives rise to a problem known as eutrophication.

• Added nutrients act as fertilisers and result in increased growth of algae and other plant matter in waterways. This increased growth is often very apparent from algal blooms and scums on stretches of waterways.

When this type of algal bloom is followed by death and decay of

animal and plant life in a competition for depleting oxygen

supplies,the term eutrophication is used.

Eutrophication

Fish Kill in the Salton Sea as a result of eutrophication

Algal bloom in Orielton Lagoon, Australia,

Suspended and Dissolved Solids

Suspended solids can be particles of plant and animal remains or silt.

These neither sink nor float; they are held in suspension in the liquid

but are not dissolved.

The amount of suspended solids in a sample of water can be

determined by weighing a dried sheet of fine grade filter paper and

filtering through it a known volume of water (a reasonably large volume of

water will usually be needed, e.g. one litre). The filter paper is then

washed with distilled water, dried carefully and reweighed. The increase

in mass is the mass of solids suspended in the sample.

Suspended solids are usually expressed in p.p.m.

Total Dissolved solidsThe dissolved solids can be determined by taking a known volume

of filtered water (to ensure that all suspended solids have been

removed) in a previously weighed dry beaker, and then boiling the

contents gently to dryness.

The dissolved solids will remain in the beaker and their mass can

be accurately found by reweighing the beaker when it has cooled.

The concentration of dissolved solids should also be expressed in p.p.m.

Estimation of Dissolved Oxygen in Water

Estimation of the concentration of dissolved oxygen in water is

used in the determination of the quality of surface waters and also

in waste waters, particularly from biological treatment plants.

The most common titrimetric procedure to measure dissolved

oxygen is called the Winkler method. It relies on reactions involving

manganese ions, iodide ions and oxygen. Under alkaline conditions

the oxygen dissolved in the water oxidises the Mn2+ ions to Mn3+

ions. When the mixture is acidified, the Mn3+ ions are reduced

back to Mn2+ ions by the iodide ions.

This reaction liberates iodine whose concentration can be estimated

by titration against standard sodium thiosulphate solution. The

concentration of the iodine in the final solution is twice the oxygen

concentration of the original solution.

Winkler method

Biochemical Oxygen Demand (BOD)

• Biochemical Oxygen Demand (BOD)

The BOD test was first devised by the Royal Commission on Sewage in the early 1900s.

The biochemical oxygen demand is the amount of dissolved oxygen consumed by biochemical action when a sample of water is kept in the dark at 20 °C for five days.