Sewage treatment is a multi-stage process designed to remove contaminants from wastewater, transforming it into an effluent suitable for discharge into the environment or for various reuse applications. This process safeguards public health by eliminating harmful pathogens and protects aquatic ecosystems from pollution caused by untreated discharges. It addresses the volume of wastewater generated daily, preventing environmental overload and preserving water resources.
Preliminary and Primary Treatment
Preliminary treatment begins by physically removing large objects to prevent damage to downstream equipment. Wastewater passes through screens, capturing debris like rags, plastics, and other large solids. Next, the flow enters grit chambers, where water velocity is slowed. This allows heavier inorganic particles, like sand and gravel, to settle by gravity, protecting mechanical components from abrasion.
After these removals, the wastewater proceeds to primary sedimentation tanks. Here, the flow is further slowed, enabling suspended solids and organic matter to settle to the bottom as sludge. Lighter materials, such as oils and greases, float to the surface and are skimmed off. This stage separates 50-70% of suspended solids and some organic matter, preparing the water for biological treatment.
Secondary Treatment
Secondary treatment follows physical separation, focusing on the biological breakdown of dissolved and suspended organic matter. This stage utilizes microorganisms to consume and metabolize organic pollutants, transforming them into less harmful forms. The goal is to reduce the biochemical oxygen demand (BOD) and suspended solids.
The activated sludge process is a common method where wastewater is mixed with a biological floc of bacteria and protozoa in aeration tanks. Air or pure oxygen is introduced into these tanks, creating an aerobic environment that supports the growth of these microorganisms. These microbes break down organic compounds, synthesizing new cells and releasing carbon dioxide and water. The mixture then flows into secondary clarifiers, where the biological floc, heavier with consumed organic matter, settles to the bottom, separating from the treated water. A portion of this settled “activated sludge” is recycled back to the aeration tanks to maintain the process.
Another method is the trickling filter, which passes wastewater over a fixed bed of media like rocks, gravel, or plastic. Microorganisms form a biofilm on the surface of this media, and as wastewater trickles downward, these microbes absorb and metabolize the organic pollutants. Air circulates through the media, providing oxygen for aerobic biological activity. As the biofilm thickens, older layers may slough off. This material is then removed in a settling tank.
Advanced or Tertiary Treatment
Advanced or tertiary treatment represents an additional stage designed to remove specific contaminants not addressed in earlier processes. This step is implemented when higher water quality is required for discharge into sensitive receiving waters or for water reuse applications. Objectives include the removal of nutrients like nitrogen and phosphorus, as well as pathogens. Excess nitrogen and phosphorus can lead to harmful algal blooms and ecosystem imbalances if discharged untreated.
Techniques for nutrient removal often involve biological processes tailored to convert ammonia to nitrate and then to nitrogen gas, or to enable specific bacteria to store phosphorus. Chemical precipitation can also be used, where chemicals are added to bind with phosphorus, forming solids that can be filtered out. For pathogen removal, disinfection is a final step. Common disinfection methods include chlorination, which uses chlorine to kill or inactivate harmful bacteria and viruses, and ultraviolet (UV) light irradiation, which damages the DNA of pathogens, preventing their reproduction without adding chemicals. Filtration, such as sand filtration or more advanced membrane filtration, can also remove fine particles and some microorganisms.
Managing Treated Water and Solids
The sewage treatment process yields two primary outputs: treated water, known as effluent, and separated solids, referred to as sludge. The treated effluent, having met stringent quality standards, is discharged safely into natural water bodies such as rivers, lakes, or oceans. Increasingly, this treated water is also being beneficially reused for irrigation, industrial cooling, or aquifer recharge, conserving freshwater resources.
The accumulated solids, or sludge, require further processing to reduce their volume and stabilize them before final disposal or beneficial use. Sludge undergoes various treatments, including thickening and dewatering, which significantly reduce its water content and volume. Digestion processes, such as anaerobic digestion, stabilize the organic matter, often producing biogas (rich in methane) that can be captured and used as an energy source. Once treated, stabilized sludge, sometimes termed biosolids, can be applied to agricultural land as a soil amendment or fertilizer, incinerated, or disposed of in landfills.