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Waste water treatment List the primary pollutants found in waste water and identify their sources. Outline the primary, secondary and tertiary stages of waste water treatment, and state the substance that is removed during each stage. Evaluate the process to obtain fresh water from sea water using multi- stage distillation and reverse osmosis. Waste water treatment: why? water is very good solvent so any water sample will always contain solutes e.g. nitrates, phosphates, heavy metals, carcinogenics, ... pathogens and other micro-organisms remove suspended particles remove unpleasant odours and smells reduce BOD Water pollutants (1) pollutants sources Pesticides Agriculture Dioxins Formed as part of large-scale processes like combustion of waste (waste incineration), manufacture of herbicides and paper pulp bleaching with chlorine; extremely toxic. PCB’s: polychlorinated biphenyls; Electrical insulators, molecules added to plastics. Overexposure result in a disease called chloracne which is a skin condition which produces cysts containing a straw coloured liquid; other symptoms are loss of appetite, nausea, vomiting and weakness. Most PCB’s have low toxicity for humans. Water pollutants (2) pollutants Organic matter sources Sewage, agricultural run off e.g. cleaning out of stables, cow sheds, food industry. Nitrates Over-use of fertilizers results in leaching of nitrates into rivers/run-off of fertilizers, animal and human waste. Phosphates Use of phosphate-containing detergents. Water pollutants (3) heavy metals pollutants sources Mercury •mercury cell e.g. used in the electrolysis of brine - greatest dischargers; •batteries; •mercury salts used as fungicides to reduce formation of •moulds on seeds (seed dressing) Lead •lead paints •lead based solder in water pipes and fittings; •car exhausts (leaded petrol); •car batteries Cadmiu m •by-product of the extraction of zinc and lead; cadmium is always found near zinc •used in pigments in ceramics; •zinc-plating always contains some cadmium; •rechargeable batteries Primary treatment: physical Methods: filtration flocculation sedimentation Removes: insoluble solids and liquids most suspended particles some oxygen-demanding wastes (organic matter) (http://en.wikipedia.org/wiki/Sewage_treatment#Filter_beds_.28oxidising_beds.29 Flocculation Involves the addition of alum (Al2(SO4)3 which contains aluminium ions (flocculating agent) which causes coagulation (= coming together) of the nonsettleable (do not settle at the bottom) suspended solids. Al3+ ions (also Ca2+ can be used) react with alkali substances to produce an Al(OH)3 precipitate which traps/attracts the suspended solid particles and causes them to settle at the bottom (=sedimentation). Secondary treatment Activated sludge: oxidation of organic waste by aerobic bacteria encouraged by aeration Trickling filter beds: rotating pipes sprinkle waste water over stones which have bacteria and algae growing on them which consume the organic waste and and some nitrates. Secondary treatment: activated sludge http://en.wikipedia.org/wiki/Activated_sludge Secondary treatment:trickler filter http://en.wikipedia.org/wiki/Trickling_filter Secondary treatment removes 90% organic waste reducing BOD methods: activated sludge trickle bed filter Tertiary treatment: methods Activated carbon-bed: organic waste is oxidized into CO2 and H2O by the carbon activated by heat, also removes dioxins and PCBs. Denitrifying bacteria: nitrates into N2. Chemical precipitation: heavy metal ions are precipitated out by adding anions which form insoluble salts with them (see next slide); also removes phosphates Reverse osmosis and ion exchange: nitrates. Tertiary treatment: precipitation Ions like cadmium, lead, mercury and phosphate ions can be precipitated by adding ions which form insoluble compounds with the heavy metal or phosphate ions. 1. To precipitate phosphates, aluminium or calcium ions are added which form insoluble phosphates: 3Ca2+ (aq) + 2PO43- (aq) Ca3(PO4)2 (s) 2Al3+ (aq) + PO43- (aq) AlPO4 (s) 2. To precipitate heavy metal ions, hydrogen sulphide gas is added; the heavy metal ions form their sulphide salts which have very low solubilities: Pb2+ (aq) + H2S (g) PbS (s) + 2 H+ (aq) Ion exchange X = resin beads Equation of exchange: X – Cl- + NO3- X – NO3 + Cl- Fresh water from seawater Reverse osmosis A high pressure (greater than osmotic pressure i.e. greater than 70 atm) is applied to sea water (low concentration of water) side of a partially permeable membrane (only allows water through and no ions or solute particles). The pure water from the sea water is forced out of the solution through the membrane leaving the salt behind (salt ions cannot pas through) and into pure water (high concentration of water). Fresh water from seawater Sea water is heated in a series of coiled pipes and then introduced into a chamber at lower pressure. Under this lower pressure some of the sea water boils immediately. The water vapour is condensed by contact with cold water pipes which carry the sea water. The condensed water now is free of impurities. In this way the heat released during the condensation is used to preheat some of the water. The remaining sea water that has not boiled yet is directed to another set of chambers each with a lower pressure than the preceding chamber. This method maximises the use of energy.