Treatment and analysis Water Treatment and analysis

Water Treatment Average household uses 300 litres per day Criteria Colourless Odourless Safe to drink (no active bacteria) Fluorinated To ensure water is of an adequate standard for drinking, it must be treated

7 Stages Water from Lakes and Rivers Screening Flocculation Settlement Filtration Chlorination Fluoridation pH Adjustment Water to homes and industry

Flocculation * flocculating agent ( or Screening * Wire mesh 1st stage * Removes large solids and floating debris like twigs, plastics etc Flocculation * flocculating agent ( or 2nd stage flocculant) usually aluminium sulphate (alum) is added. * Makes smaller suspended solids coagulate or stick together in large clumps, so they are easier to remove at the next stage

Settlement * Large tanks 3rd stage * Water goes in at the bottom and rises slowly to the top, at < 2m/hr * Particles settle to the bottom * Over 90% of suspended solids removed at this stage Filtration * Large beds of sand 4th stage * Removes remaining susp solids * Sand supported on a layer of gravel * Sand cleaned regularly * Water now clear but may contain harmful bacteria

Chlorination * Cl2 gas is added 5th stage * Sterilises water * Very small quantities 0.2 – 0.5 ppm * Controlled carefully * Monitored by bacteriological exam of the water Fluoridation * NaF or H2SiF6 added 6th stage (hexafluorosilicic acid) * Added by law as helps to reduce dental decay by strengthening the enamel * Small quantities ~ 1ppm

Water is now ready for consumption and use pH Adjustment 7th stage Optimum level is between 7 - 9 Too Acidic * may cause damage to pipes * may be corrected by addition of Ca(OH)2 (lime)to raise the pH * If very hard water, might be softened by addition of NaCO3 which is a base Too Basic * may be corrected by addition of dilute H2SO4 to lower the pH Water is now ready for consumption and use

Oxygen Dissolves from air into water Low solubility in water Temperature inversely affects solubility Temp Solubility of O2 E.g. 11.3 mg/L @ 10oC 9.2 mg/L @ 20oC

Water pollution The release of substances into the environment that damage the environment is called pollution 3 main types Eutrophication Organic Waste Heavy metals

Eutrophication Enrichment with nutrients (fertilisers / nitrates / phosphates) Due to run off from land (erosion from land) / due to pollution / dispersion in water / absorption by plants Rapid growth of plants (algae) on surface / algal blooms formed Light blocked from (photosynthesis reduced in) plants below surface Decay by bacteria of algae, algal blooms / increase in bacteria / Dissolved oxygen depleted (lowered, used up) / oxygen level falls Fish levels reduced or killed off

Discharge of Organic Waste Domestic sewage, slurry, silage effluent, effluent from food processing factories, milk, industrial waste, etc Bacteria and other micro-organisms feed Waste is broken down, O2 used up Organic matter + O2 CO2 + H2O O2 used up, reducing levels of fish and possibly killing off fish life. O2 gone, anaerobic bacteria take over, river will smell due to presence of by product, Hydrogen Sulphide, H2S

Heavy Metals Lead Cadmium Mercury Pb2+ Cd2+ Hg2+ Cumulative poisons Industrial effluents Battery dumping Regulations have brought about improvements Removed by precipitation

BOD Test Biochemical Oxygen B.O.D. test Demand BOD is defined as the amount of dissolved O2 consumed by biological action when a sample of water is kept in the dark at 200C for 5 days

The Winkler method is used to determine the amount of dissolved Oxygen in a water sample 2 bottles sampled 1st one tested straight away >> ppm of O2 2nd one tested after 5 days >> ppm of O2 Difference between the two readings is the BOD level, as this is the amount of O2 used up.

Winkler Method 1 cm3 Manganese (II) Sulphate 1cm3 alkaline potassium iodide solution (KI) 1cm3 H2SO4

Titrated against standard sodium thiosulphate Mn2+ + OH- ions react together Mn(OH)2 reacts with O2 Mn(OH)3 acid added I- + Mn(OH)3 iodine liberated I2 Titrated against standard sodium thiosulphate 1 O2 : 4 S2O32- White precipitate Brown precipitate

Sewage Treatment 3 main stages Primary treatment physical Secondary Treatment biological Tertiary Treatment chemical

Primary Treatment The physical stage The incoming sewage is screened to remove debris and non-biodegradable material. It passes into large deep sedimentation (settlement) tanks where about 50% of the suspended solids and about 33% of BOD are removed Liquid on top is then passed onto to secondary treatment

Secondary Treatment The biological stage The incoming sewage from first stage of treatment is passed into aeration tanks where mechanical stirrers aerate it. This allows aerobic bacteria and other micro-organisms to decompose the solid matter into a harmless sludge called Activated Sludge. Sewage then flows into settling tanks, sludge can be reused, or or recyled to make methane About 95% of BOD removed

Tertiary Treatment The chemical stage When the liquid passes out of the second round of settling, it is clean and inoffensive, but it may contain compounds of nitrates and phosphates Phosphates from washing powders and washing up liquids, and nitrates from organic material in sewage Must be removed before water discharged into river as could cause eutrophication.

Phosphate removal aluminium sulphate added, aluminium phosphate precipitates out Iron (III) chloride added, iron (III) phosphate precipitates out Lime may also be used Insoluble phosphate compounds allowed to settle out before discharge into waterway Nitrate removal difficult and expensive as nitrogen may be present in many forms, NH3, NO2- , NO3-, organic compounds containing N

Instrumental Methods of Analysis pH meter Atomic Absorption Spectrometry Colorimetry – determining conc. of free chlorine in swimming pool water DPD tablets added, reacts with chlorine in water Compare colour with ones of known concentration Comparator - disc of precalibrated solutions Or Colorimeter – measure absorbance of known solutions and plot graph