Arguably one of the most critical components of public health is the collection and treatment of wastewater and sewage. Wastewater treatment is a necessary evil that is often thought better to be neither seen, heard or, well, smelled. Justifiably, the steps involved in treating wastewater at such facilities are considerable, if not varied. Some schools of thought believe the process to be three or four steps while others suggest there are seven or eight steps involved. Regardless, wastewater treatment is critical to public health, protecting humans and the ecosystem from harmful and toxic elements.

Follow along with Engineering360 as it explores the steps involved in treating wastewater.

Source: eutrophication and hypoxia/ CC BY 2.0Source: eutrophication and hypoxia/ CC BY 2.0

What is wastewater?

Wastewater, also called used water, is water that has been used in the home, in a business, or as part of an industrial process. This water — which comes from a host of sources including toilet water, sink water, shower water, dishwashers, washing machines and even storm runoff — features suspended solids like human waste, food scraps, oils, soaps, and chemicals, all of which need to be removed via wastewater treatment.

What is wastewater treatment?

Wastewater treatment plants remove the solids and particulates — both the seen and the unseen — that end up in the used water. To do this requires a multi-step process that may differ from one facility to another, but the steps likely resemble each other if they are not identical.

Wastewater treatment steps

For the purposes of this article, Engineering360 will look at the widely touted four-step approach to wastewater treatment.

Preliminary treatment

The preliminary treatment step of wastewater treatment involves screening and pumping where incoming wastewater passes through screening equipment to remove solids — ranging from plastics, grease, human waste, wood fragments and rags — from the water. To accomplish this, the wastewater is run through assorted sized screens and sieves that capture larger objects that could potentially endanger wastewater equipment. Smaller grease and sand particles are also removed at this step via desanders and degreasers.

Primary treatment

At the primary treatment step of wastewater treatment, material that settles to the bottom of wastewater tanks in the form of sludge, but at a slower rate than the material in previous steps, is removed. While gravity helps to separate these materials from the wastewater, some municipal wastewater treatment facilities encourage sedimentation using coagulants and flocculants.

Coagulation occurs when chemicals with a positive charge are added to the wastewater. These positively charged chemicals neutralize the negative charge of dissolved particles like dirt. At this step, the particles bind with the chemicals — salt, aluminum and iron for instance — to create larger particles.

Following coagulation, flocculation occurs when wastewater is gently agitated to create larger, heavier particles called flocs. Because flocs are heavier than water, they settle to the bottom of the tank where they are eventually separated out from wastewater

Secondary treatment

At this secondary treatment step of wastewater treatment, organic matter is removed from wastewater via biological means. Using bacteria and microorganisms, wastewater treatment facilities degrade organic matter. Activated sludge treatment occurs here, wherein water under treatment is left in a tank for days under different oxygen conditions. In the tank, bacteria feed on the organic matter. A second settling process occurs here, wherein bacteria grow and settle in the lower part of the tank, thereby creating biological sludge. Once settled, the sludge is activated, and purified water flows out of the top of the tank as effluent.

Tertiary treatment

The tertiary treatment step is the so-called final fourth step where disinfection takes place using either chlorine or ultraviolet light to destroy microscopic organisms. More advanced wastewater treatment facilities will employ an additional layer of protection at this step using filtration with sand beds or other media to further ensure the quality of the effluent that leaves the facility and is discharged to water sources such as lakes and streams.

This final step may also include a so-called sub-step wherein the water is tested and undergoes water analysis to determine that it contains the appropriate levels of pH, ammonia, nitrate, phosphate, residual chlorine and dissolved oxygen.

Though the steps may differ from one facility to the next, the idea is the same among wastewater treatment plants. Check back with Engineering360 for the latest in wastewater technology and more.

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