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

CaCO3 + HCO3– + H+  Ca(HCO3)2 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.

Ca(HCO3)2(aq).  CaCO3(s) + H2O(l) + CO2(g) Scale When 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.

A number of methods are used for removing hardness. 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 Blue Wine Red

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, whether these exist in the raw water naturally or whether they have been produced by the coagulation process. Filtration is usually achieved by the downward passage 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 it filters through. 4.Chlorination Elemental chlorine and compounds of chlorine are regularly added to water during the treatment of the water for public supplies. Chlorination is one of a number of possible treatments whose purpose is to disinfect the water 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 Method The 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 Algal bloom in Orielton Lagoon, Australia, Fish Kill in the Salton Sea as a result of eutrophication

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 solids The 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) 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.