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 suspended particles unpleasant odours and smells reduce BOD (biochemical oxygen demand)

Water pollutants (1)pollutants sources

pesticides Agriculture, insecticides, fungicides, and herbicides. Dichloro-diphenyl-trichloroethane DDT is an example of pesticides.

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. Weed killers (herbicides)

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

Effects of Water pollutants (1)pollutants effects

pesticides • Carcinogenic • Birth Defects• Neurological Disorders• Leads to biological magnification – toxic levels build

up over a period of time in animals at the top of food chains

dioxins • Chloracne, a server form of skin disease• Reproductive and developmental effects• Liver damage and cancer

PCB’s: polychlorinated biphenyls;

• Acne-like breakouts• Hearing and vision problems• Irritation of gastrointestinal treat• Affect reproductive efficiency• Liver damage and cancer

Water pollutants (2)pollutants sources

organic matter 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 detergentsHuman sewageAgricultural run-off from cropsPulp and paper industryChemical and fertilizer manufacturingDetergent

pollutants effectsorganic matter

• Retards the photosynthesis process in plants

nitrates • Babies can develop methemoglobiemia or ‘blue baby syndrome’ caused by a lack of oxygen in the blood. (oxidation of Fe2+ to Fe3+)

• Carcinogenic

phosphates • Eutrophication accelerates plant and algae growth

• Kidney damage• Osteoporosis

Water pollutants (2)

Water pollutants (3) heavy metalsMercury

Source •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)• agriculture

Health hazard

• The most dangerous of the metal pollutants; causes serious damage o the nerves and the brain

• Symptoms of mercury poisoning result from damage to the nervous system: depression, irritability, blindness and insanity

Environ-mental hazard

• Reproductive system failure in fish• Inhibits growth and kills fish• Biological magnification in the food chain

Water pollutants (3) heavy metalsLead

Source •lead paints•lead based solder in water pipes and fittings;•car exhausts (leaded petrol);•car batteries

Health hazard

•Burning pains in the mouth and digestive system followed by constipation or diarrhoea•In severe cases there is a failure of the kidneys, liver and heart which can lead to coma and death•Can cause breain damage particularly in young children

Environme-ntal hazard

• Toxic to plants and domestic animals• Biological magnification in the food chain

Water pollutants (3) heavy metalsCadmium

Source •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•Metal plating

Health hazard

•Replaces zinc in enzymes making them ineffective•Itai-itati disease makes bones brittles and easily broken•Kidney and lung cancer in humans

Environmental hazard

•Toxic to fish•Produces birth defects in mice

Waste-water Treatment If water is discharged untreated in to rivers/

sea, it is eventually decomposed by microorganisms.

Septic tanks or cesspits are used in some areas. The waste is broken down by bacteria.

Waste-water Treatment Classified as primary, secondary or tertiary methods,

including filtration, sedimentation, flocculation and precipitation processes.

Includes both physical and chemical treatment.

Primary treatment: physicalPrimary 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

Primary Treatment First passed through screens and grids which filter out

large insoluble solid objects, and remove floating objects and grease from the surface.

Then is passed into a sedimentation tank (settle). The resulting sludge is removed from the bottom of the tank.

Flocculation: speed up the sedimentation process by adding chemicals which allow suspended particles to join together to form large clumps.

Large flocs are formed by the addition of chemicals into water: Al2(SO4)3(aq) + 3Ca(OH)2(aq) -> 2Al(OH)3(s) + 3CaSO4(s)

(Primary treatment is generally not sufficient to improve the quality of water to safe levels) A typical primary treatment domestic sewage plant can remove about 30-40% of the BOD waste.

Secondary treatment: Secondary treatment: activated activated sludgesludgehttp://en.wikipedia.org/wiki/Activated_sludgehttp://en.wikipedia.org/wiki/Activated_sludge

Secondary treatment:trickler filter Secondary treatment:trickler filter http://en.wikipedia.org/wiki/Trickling_filterhttp://en.wikipedia.org/wiki/Trickling_filter

Secondary treatmentSecondary treatment removes 90% organic waste reducing BOD methods:

activated sludge trickle bed filter

Secondary treatmentSecondary 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 waste and

some nitrates.

Activated Sludge Process: The waste water enters aeration tanks where oxygen is mixed with the water. This creates ideal conditions for aerobic bacteria to grow and flourish. The bacteria then digest the organic matter and any untreated organic matter again settles out as sludge in a second sedimentation or clarifier tank. The water is returned to the aeration tank and the aeration process is repeated until all of the organic matter has been removed.

Removes Dissolved organic matter: Most (about 90%) of organic oxygen-demanding wastes and suspended particles.

Secondary treatmentSecondary treatment

Tertiary treatmentTertiary 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

to precipitate phosphates, aluminium or calcium ions are added which form insoluble phosphates; these phosphates then precipitate out

3Ca2+ (aq) + 2PO43- (aq) Ca3(PO4)2 (s)

2Al3+ (aq) + PO43- (aq) AlPO4 (s)

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-

Distillation Can be used to obtain fresh water from sea water. Sea water is heated and then passed into an evacuated chamber

where it boils, leaving dissolved compounds in solution. The steam is then passed through a condenser which is cooled

by pipes containing more sea water. The warm sea water is then heated

and distilled in turn.

Reverse Osmosis Osmosis: movement of water from a dilute

to a concentrated solution by passing through a semi-permeable membrane.

This process can be reversed if a pressure of 70atm (the osmotic pressure) is applied to the more concentrated salt solution.

The water passes through the semi-permeable membrane and leaves the dissolved salts behind.

(the semi-permeable membrane must be able to withstand high pressures)