E6 Water treatment

75% of the Earth's surface is covered by water. 97.3% of this is ocean, 2.1% glaciers and ice caps, 0.6% ground water, 0.015% lakes and rivers, and 0.001% in the atmosphere.

E.6.1 List the primary pollutants found in waste water and identify their sources.

Water is capable of hydrogen bonding and is highly polar- this allows is to dissolve many chemicals and as a result some toxic substances, bacteria and viruses can be carried by water. Pollutants in water arise from human usage of water for personal use and industry, and the subsequent release of water into the environment. Water unsuitable for drinking, irrigation, industrial use or washing is considered polluted water.

Pollutants include:

- Heavy metals. i.e. Cadmium from rechargeable batteries, metal plating and pigments. Copper from household plumbing, copper mining and smelting. Mercury from batteries, mercury salts (fungicides), mercury cells in chlor-alkali industry, discharge from pulp and paper mills. Lead, Zinc.

- Pesticides from agricultural practices, like DDT, fungicides and herbicides.

-Nitrates: from acid rain and artificial fertilizers. Causes infantile methaemoglobinaemia (oxygen starvation) because of less acid in babies' stomachs that are converted to nitrite by bacteria- this oxidises the iron in haemoglobin irreversibly preventing it from binding with oxygen. In adults, nitrites are converted into nitrosamines (carcinogenic).

-Dioxin: Waste materials containing organochlorine compounds form dioxins when they are not incinerated at high enough temperatures. It accumulates in fat and liver cells and causes malformation of fetuses.

-Polychlorinated biphenyls (PBCs): One to ten chlorine molecules attached to a biphenyl molecule, chemically stable and high electrical resistance. Accumulate in fatty tissues and remain in the environment; affect reproductive efficiency, impair learning in children, carcinogenic.

E.6.2 Outline the primary, secondary and tertiary stages of waste water treatment, and state the substance that is removed during each stage.

Raw sewage can either be discharged untreated into water bodies, or, in rural areas, deposited into cesspits/septic tanks where they are broken down before they leach into the ground. However, the treatment of waste enables fresh water to be recycled from it.

Primary treatment: Removes 60% of solid material and 1/3 of oxygen demanding wastes. Coarse mechanical filters are used to remove large objects and then a sedimentation tank allows for suspended solids to settle out as sludge. Calcium hydroxide and aluminium sulphate are added to form aluminium hydroxide, which precipitates together with suspended dirt particles in flocculation. Grease is removed by skimming, and the effluent is discharged into a waterway or for secondary treatment.

Secondary treatment: removes 90% of oxygen-demanding wastes, where waste are aerobically degraded using oxygen and bacteria. Two methods- a) waste water is left to trickle through a bed of stones with bacteria; b) sewage is aerated with pure oxygen in a sedimentation tank. Sludge contains active microorganisms that digest organic waste (activated sludge) and some of it is recycled. Water is discharged into a water way where chlorine is added for disinfection or ozone.

Tertiary treatment: expensive; removes heavy metal ions, nitrates, phosphates and residual organic compounds.

- Precipitation- Aluminium sulphate and calcium oxide can be used to precipitate phosphates. Heavy metal ions can be precipitated as insoluble hydroxides/basic salts by the addition of calcium hydroxide or sodium carbonate, or insoluble sulphides by the bubbling of hydrogen sulphide. However some metal hydroxides redissolve as the complexes are soluble (Zn, Hg, Cd).

- Activated carbon bed- carbon is activated by heating periodically to high temperatures and this removes dissolved organic material by oxidising the adsorbed organic compounds to carbon dioxide and water and regenerates the carbon surface.

- Removal of nitrates- ion exchange zeolites can be used to exchange hydroxide ions for nitrates but this is very expensive. The other method would be to use denitrifying bacteria to reduce them to nitrogen or to pass the water through algae pools as algae utilize nitrates as nutrients.

E.6.3 Evaluate the process to obtain fresh water from sea water using multi-stage distillation and reverse osmosis.

Desalination removes salts from sea water (3.5%) that make the water unfit for consumption and for industrial or agricultural purposes. Seawater is heated in a series of coiled pipes before being introduced to a partially evacuated chamber where the water boils instantaneously under the reduced pressure and is condensed and piped off as freshwater. A multi-stage distillation system is often used so that heat released from the condensation of steam can be reused to heat more seawater. This distillation system separates more volatile water from the less volatile salts, and is quite a costly system to maintain.

Reverse osmosis is when pressure is applied to a solution across a semi-permeable membrane that is greater than the osmotic pressure (of water diffusing across the membrane down the concentration gradient) which in turn causes osmosis to occur in the reverse direction, leaving the salts behind as water is forced out of the salt solution. In this case, the membrane used is cellulose ethanoate. It does not require a phase change and hence needs less energy, but still requires energy to produce the necessary pressure (70atm) for reverse osmosis.