E6+Water+Treatment

__**E6**__ __**Water Treatment**__
__E6.1__ __Primary pollutants found in waste water and their sources__ Waste water:- __E6.2__ __Stages of water treatment__ **Primary treatment:-** Removes most of the solid material (around 60%) and some of the BOD waste from the sewage.
 * Pesticides || DDT, herbicides and fungicides ||
 * Nitrates || Found in fertilizers or acid rain. Nitrates are soluble so they accumulate and are toxic at high levels. May cause blue baby syndrome in babies. ||
 * Dioxins || These are very toxic and are formed when organic chlorine compounds are not decomposed at high enough temperatures. ||
 * Heavy metals || These are in the form of ions, for example mercury (from batteries), cadmium (from rechargable batteries) and lead. ||
 * Polychlorobiphenyls (PCBs) || Used in capacitors, transformers and can cause cancer. ||
 * Filtration - Larger objects are filtered from the sewage, using bar screens
 * Settling tanks are then used to settle rocks, stones and small objects from the water, which are sent to landfills
 * Flocculation - Occurs in the sedimentation tank. Aluminium sulfate and calcium hydroxide are added to the water which precipitate and carry solid particles with them to the bottom of the sedimentation tank. The material is removed from the water.

Removes organic materials and biologically oxygen demanding waste (BOD) from the water
 * Secondary treatment:-**
 * The sewage is mixed with activated sludge and oxygen is bubbled in.
 * Bacteria oxidize the organic waste aerobically
 * Water is passed through a sedimentation tank and the activated sludge will be collected, which will be sent to landfills
 * After this treatment the water is disinfected with ozone or chlorine to kill pathogenic bacteria

Further removes toxic metal ions, nitrates and remaining nutrients
 * Tertiary treatment:-**
 * Heavy metal ions and phosphates can be removed by precipitation, e.g. Nickel:
 * Phosphates can be removed by adding aluminium sulfate so precipitation occurs and insoluble phosphates will form.
 * Calcium oxide is added which will remove the phosphates as well as react with the metal ions to form insoluble metal hydroxide which will then be removed


 * Nitrates are soluble so they are more difficult to remove.
 * Anaerobic denitrifying bacteria can reduce nitrates to nitrogen
 * The water can also be passed through algae ponds where nitrates are used as nutrients

Waste water treatment flowchart:- __E6.3__ __Obtaining fresh water from sea water__ This process (desalination) has become increasingly important due to the lack of fresh water.
 * Multi-stage distillation - boiling then condensing the water
 * Reverse osmosis - (carried out under high pressure) the water is forced through a selectively permeable membrane that does not permit large molecules to pass