Sewage Treatment

Sewage treatment happens in stages because no single step handles everything in wastewater. Primary treatment is purely physical: wastewater sits in settling tanks where gravity pulls heavy solids (sludge) to the bottom and grease floats to the top, both of which get skimmed or scraped off. This removes maybe 30 to 40 percent of the organic material, but everything still dissolved in the water passes right through. Secondary treatment fixes that with biology. The water moves into an aeration tank, where pumped-in oxygen lets bacteria thrive and literally eat the dissolved organic matter, breaking it down and converting it into more bacterial cells and carbon dioxide. Those bacteria then settle out as more sludge. Tertiary treatment is optional and more targeted, used to strip out specific remaining pollutants like nitrogen, phosphorus, or heavy metals before the water is released.

Even after primary and secondary treatment remove essentially all solids, the water still contains bacteria and viruses too small to settle out or see. That is why chlorination (or another disinfectant like UV light or ozone) is the final step regardless of how clear the water looks: clarity tells you about solids, not about pathogens.

This topic connects directly to the BOD math used throughout the unit. Biochemical oxygen demand measures how much oxygen bacteria need to decompose the organic waste in water, so a high BOD coming into a plant and a low BOD going out shows the secondary stage is doing its job. Skip that stage, relying on primary treatment alone, and a river receiving the discharge gets flooded with organic matter. Bacteria in the river then multiply and consume the dissolved oxygen themselves, starving fish and other aquatic life, which is the same oxygen-depletion problem seen in eutrophication from fertilizer runoff.