Waste Treatment- Primary vs. Secondary vs. Tertiary

Difference Between Waste Treatment- Primary And Secondary And Tertiary


Sewage-treatment plants use a series of steps to remove any biological and chemical contaminants that are a risk to human health or the environment. Such plants eliminate final traces of suspended solids; halt the undesirable growth of algae; reduce nutrient content; and remove undesirable taste, color, and odor. The treated sewage is then released into rivers, lakes, or the ocean. The quality of the water released depends on the condition of the incoming water and the treatment and purification processes used.

The three standard sewage-treatment stages include primary, secondary, and tertiary steps. Primary treatment is almost always applied. Although secondary treatment is recommended for most sewage, many plants are not equipped to perform it. Tertiary treatment, a relatively expensive cleansing step, is used even less frequently, usually only when water of drinking quality is desired.

Primary treatment.

This treatment removes large floating or suspended particles, heavier particles called grit (such as sand or gravel), and any excessive grease or oil from the sewage. A series of screens, grit chambers, and sedimentation tanks are used at this stage.

If no further treatment is performed, the wastewater is disinfected by the addition of chlorine and discharged into a stream or a body of water. If further treatment is needed, the wastewater goes through the secondary-treatment step.

Secondary treatment.

The second step in the process uses aerobic microorganisms (bacteria that thrive in the air) to break down organic matter left in the sewage. The process—called biological oxidation—involves trickling filters, activated sludge, and stabilization ponds. Unless tertiary treatment will be used, the wastewater is disinfected with chlorine and then discharged.

Sludge remaining from the primary- and secondary-treatment processes is sent to a sludge digester for further processing. The digester relies on aerobic bacteria to break down volatile matter in the sludge over the course of two or three weeks. Methane, a by-product of this step, can be captured and used as a fuel source. The remaining sludge is incinerated, deposited in a landfill, or recycled as fertilizer or for use as a soil conditioner.

Tertiary treatment.

Also called advanced wastewater treatment, tertiary treatment transforms liquid sewage into water of drinking quality. Chemical treatments remove undesirable constituents that remain after the secondary treatment. These unwanted materials include nitrates, which can cause public-health problems, and nitrogen and phosphorus, which encourage the growth of algae. The specific methods applied in tertiary treatment depend on the source of wastewater being treated. For example, carbon-absorption, reverse-osmosis, or distillation processes remove organic materials. In contrast, coagulation and sedimentation treatments eliminate heavy metals.

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