By Scott Johnson
Updated Aug 30, 2022
Sewage treatment’s core mission is to transform human and industrial waste into a product that is safe for humans and the environment. Modern treatment plants deploy a combination of physical, chemical, and biological processes to systematically remove solids and solute contaminants. The workflow is typically segmented into Preliminary, Primary, Secondary, Advanced, and Disinfection stages, each progressively targeting finer pollutants.
During the initial phases, solids are tackled through screening, sedimentation, and skimming. The Preliminary stage focuses on removing coarse matter—often via mechanical screens—while also aiming to eliminate grit. High flow rates and velocities discourage premature settling. Once this stage is complete, the influent moves to the Primary stage where the flow is slowed to promote sedimentation. Settled solids are scraped from the bottom, floating material is skimmed, and a substantial portion of oil, grease, and soap is removed.
Secondary treatment harnesses aerobic biological processes to break down residual organic matter—including human waste, food residues, oils, and soaps. Microorganisms metabolize these organics, converting some solids into coagulated masses that are easier to separate. Subsequent sedimentation or filtration removes the microbial biomass from the effluent. While most solids are eliminated, nutrients such as nitrogen and phosphorus often persist and may require further treatment.
Advanced processes target the remaining solids and nutrients that survive secondary treatment. These can be added as discrete stages or integrated into earlier steps. Chemical methods effectively remove phosphorus and heavy metals, while biological pathways are employed for nitrogen removal. Additional procedures may be necessary to eliminate specific industrial or agricultural contaminants. A final filtration stage can further polish the effluent by capturing any residual solids.
Disinfection safeguards the environment by neutralizing pathogenic microorganisms before effluent release. Common methods include chlorination, ozone, ultraviolet light, or other chemical disinfectants. Chlorination remains the most widespread technique, but it leaves residual chlorine that can harm ecosystems. Ozone and UV treatments produce cleaner effluent and are increasingly adopted for their ecological advantages.
