What is Primary Wastewater Treatment?

U.S. wastewater treatment process about 34 billion gallons of wastewater produced by homes and businesses each day. That wastewater must be properly treated in stages before being released into waterways, so that it does not become a source of nitrogen and phosphorous pollution.

Wastewater treatment is a complicated process. To protect human health and environment, hard science must be used to test, treat, and monitor influent and effluent at every stage.


Typically, wastewater treatment happens in three or four stages:

     preliminary treatment, where sticks, plastic bags and other flotsam is strained from the influent;

     primary treatment, where solids are separated from liquid;

     secondary treatment, where biological methods and filters are used to clarify the water; and

     tertiary treatment, where chemicals and filters are used to disinfect and further clarify the water before discharge.

Many factors determine the type of wastewater treatment, including the source of the wastewater, its pollutants, budgetary considerations, and how and where the treated wastewater—and biosolids, or sludge—will be discharged.

In this article, let’s look at what primary wastewater treatment looks like in its various forms, why it’s necessary, and the steps involved.

What are the Steps Involved in Wastewater Primary Treatment?
The steps involved in wastewater primary treatment will vary from plant to plant, but in general, they all have the same goal: begin separating solids from liquids.


Before wastewater enters a treatment plant, it passes through screens that capture sticks, plastic bottles, and other trash. The screens are sometimes made of metal mesh, others, of steel or iron bars. The screens are cleaned of this debris manually or by mechanical scrapers. Some treatment plants use comminutors, devices that screen the water and then grind the captured material. After screening, wastewater typically enters a chamber where grit, sand, gravel, and similar matter sink to the bottom. This step—referred to some as preliminary treatment of wastewater rather than primary treatment—is particularly important in municipalities where stormwater and wastewater flow concurrently.


With the screening completed and the grit removed, wastewater flows into a primary sedimentation tank, where gravity pulls heavier solids to the bottom of the tank for removal. At a later stage, the sedimentation process is repeated, with the help of chemical coagulants.
As heavier material drifts to the bottom of the primary sedimentation, lighter material, such as oil and grease may float to the top, where they it can be skimmed off. However, other suspended organic and inorganic material remains in the primary tank. To remove that material, a process known as dissolved air flotation (DAF) can be used.

Dissolved Air Flotation (DAF)

DAF can be used at any stage of wastewater treatment but is most often used at the primary and tertiary stages. DAF technology is also used to remove low-density solids in the production of drinking water and process water.

Here’s how DAF works: Solids are floated in the clarifier by chemical coagulation and flocculation, and by adding microbubbles. The microbubbles are created in an unpacked saturator which combines 8 percent to 15 percent of recycled water from the clarified water with compressed air. The pressurized air/water mixture is sent through a row of nozzles or special injector depending on the type of DAF technology. For some applications, nitrogen is used as the flotation gas. A pressure drop brings the air out of solution and creates microbubbles, which adhere to the solids and float them to the top of the floatation zone. Sludge is then either hydraulically removed over a stationary weir by raising the water level on a set interval, or mechanically removed with a scraper.

After the wastewater has been through primary treatment processes, it flows into the secondary stage of treatment.
How SUEZ Can Help
SUEZ designs and deploys wastewater technologies and services for municipalities and businesses around the globe. Our expertise covers every aspect of wastewater treatment, including biological treatments, filtration, clarification, biosolid handling, ozonation, UV and other disinfection methods, wastewater reuse, and headworks and equipment servicing.

Contact a representative to learn more about wastewater treatment solutions offered by SUEZ North America.