What are the steps in waste water treatment?

Waste water treatment involves preliminary treatment, primary treatment, secondary treatment, and tertiary treatment.

The first step in waste water treatment is the preliminary treatment. This involves the removal of large objects and grit (sand and small stones) that may be present in the waste water. Large objects are removed using screens, while grit is removed in grit chambers. This step is crucial as these materials can cause damage and excessive wear to the equipment used in the subsequent stages of the treatment process.

The next step is the primary treatment. This involves the separation of organic and inorganic solids from the waste water. This is achieved through the process of sedimentation, where the waste water is allowed to stand in a large tank, known as a primary sedimentation tank. Here, the heavier solids settle at the bottom of the tank, forming a layer of sludge, while the lighter materials, such as oils and fats, float to the top and form a layer of scum. The sludge and scum are then removed, and the remaining water moves on to the next stage.

The third step is the secondary treatment, which is designed to degrade the organic content of the waste water, which may come from human waste, food waste, soaps and detergent. This is achieved through the use of microorganisms that consume the organic waste as food, converting it into carbon dioxide, water and energy. This process takes place in aeration tanks, where air is pumped in to encourage the growth of the microorganisms.

The final step is the tertiary treatment. This step is not always included in the treatment process, but when it is, it involves the removal of any remaining inorganic compounds and pathogens. This can be achieved through various methods, including filtration, lagooning, nutrient removal, and disinfection. The treated water is then discharged back into the environment, or reused for purposes such as irrigation or industrial cooling.