Waste+Water+Treatment

**Primary Treatment**

 * Raw sewage is processed in a variety of ways to clean it.
 * The first stage is called primary treatment and involves processes such as filtration, sedimentation and flocculation.
 * Filtration filters out the larger objects from the sewage.
 * Flocculation involves the addition of the chemicals aluminium sulfate and calcium hydroxide. These react giving a precipitate of aluminium hydroxide that is able to attach itself to the suspended solid particles in the sewage. This forces them to sink and a sludge of this material forms at the bottom of what is called the sedimentation tank. This can then be removed from the water.
 * The remaining water is about 60% cleaner and is passed on to the secondary stage of the treatment.

Secondary Treatment

 * This removes the oxygen demanding waste from the water. (BOD)
 * The water is treated with activated sludge.
 * This sludge has been saturated with oxygen and contains bacteria that is able to decompose all the oxygen demanding organic waste aerobically.
 * After treatment the water is disinfected with chlorine or ozone is then fit to drink.

Tertiary Treatment

 * Some countries are increasingly using a third cleaning process to remove dissolved nitrates, phosphates and dangerous metal ions that are not removed from the first 2 cleaning processes.
 * Phosphates and metal ions can be removed by precipitation.
 * In a similar process to flocculation, the water is again treated with aluminium sulphate. This will form insoluble phosphates.
 * If calcium hydroxide is then added it will react with the dissolved metal ions to give the insoluble metal hydroxide that can be easily removed.
 * Nitrates are difficult to remove by this method as all nitrates are soluble.
 * A biological method is used so that the water is passed through a pond of algae that uses up the nitrates as a nutrient.