8.11 - Sewage Treatment
Learning Objectives
The main objective is to describe best practices in sewage management. The associated skill is describing an environmental concept represented visually.
I. The Four Steps of Sewage Treatment
Sewage treatment has four main steps for cleaning wastewater before it gets discharged.
1. Primary Treatment (Physical Filtration)
This step filters out big solid waste materials like toilet paper, leaves, plastic, or debris. A grit chamber lets inorganics like sand and gravel settle out, and that gets disposed of as sludge. Filtering out large waste and grit is crucial because the microbes in the next step can’t efficiently break down plastic or paper.
Primary treatment starts with bar screens (metal grates) that catch large objects. After the screens and grit chamber, the wastewater sits in a large primary settling tank, where heavier solids sink to the bottom as sludge, and grease and oil float to the top and get skimmed off. Primary treatment removes roughly 60% of the suspended solids and about a third of the organic matter, but nothing that’s dissolved.
2. Secondary Treatment (Biological Decomposition)
This is a biological step using microbes or bacteria to break down organic waste like fecal matter and urine materials. It’s aerobic decomposition, so oxygen has to be added to the aeration tank. The microbes break the waste down into and small nutrient building blocks, primarily nitrogen and phosphorus. Secondary treatment removes only part of the phosphorus and nitrogen.
Aerating the tank speeds this up, because the bacteria need oxygen to break down organic matter quickly. The water then flows into a secondary settling tank, where the bacteria and leftover solids settle out as sludge. Some of that sludge, full of active bacteria, gets pumped back into the aeration tank to keep the process going (this is called activated sludge).
3. Tertiary Treatment (Chemical Filtration)
This is a chemical step using filters and chemical reagents. The goal is to reduce the remaining nitrate, ammonia, and phosphate levels in the waste. It’s considered a best practice for reducing nutrient pollution, eutrophication, and dead zones, but it isn’t used in every facility because of the cost.
Tertiary treatment can also be ecological. Some towns run their treated wastewater through constructed wetlands, where plants take up nitrate and phosphate and microbes break down leftover pollutants before the water reaches a river. Chemical methods include adding chemicals that make phosphate settle out of the water and filtering the water through sand or activated carbon.
4. Disinfectant
The final step uses agents like UV light, chlorine, or ozone. It’s applied to the effluent, the treated wastewater discharged from the plant, to kill leftover bacteria from secondary treatment, like E. coli.
II. Residual Pollutants and Challenges
- Sludge Management: Sludge is the inorganic solid waste and grit that settles out. It’s dried to remove water weight, then taken to a landfill, incinerated, or turned into fertilizer pellets.
- Persistent Organic Pollutants (POPs): Secondary treatment doesn’t remove POPs. Compounds like pesticides or leftover medication compounds (endocrine disruptors) aren’t easily broken down by microbial activity, so they stay in the water after secondary treatment.
- Effluent Quality: Even when treatment systems are working properly, the effluent discharged from wastewater treatment plants usually still contains elevated nitrogen and phosphorus levels (nutrient pollution) along with POPs and endocrine disruptors.
- Combined Sewage and Stormwater Systems: Some cities combine their sanitary sewers, which carry human waste, with stormwater drains. That’s beneficial because the stormwater runoff gets treated. But during heavy rain the plant has limited capacity, so an overflow system releases raw, untreated sewage directly into a river to prevent flooding. That overflow dumps E. coli, ammonia, nitrates, phosphates, and endocrine disruptors into the environment.
Big picture
Sewage treatment relies on a sequence: primary filtration of large debris, then secondary aerobic decomposition using microbes. Tertiary treatment offers crucial reduction of pollutants like nitrate and phosphate, which prevents eutrophication, but it often gets skipped because of cost. No matter the treatment level, the discharged effluent almost always contains trace amounts of POPs and endocrine disruptors, which is a chronic environmental threat.