Wastewater, commonly known as sewer water, encompasses all used water flowing from homes, businesses, and sometimes stormwater in combined sewer systems. This includes water from toilets, showers, sinks, and industrial processes, containing solids, chemicals, and microorganisms. Treating this water is paramount to protecting public health and preserving the environment by safeguarding aquatic life and natural ecosystems.
From Drain to Plant
Sewer water begins its journey by flowing from individual drains into a complex network of underground pipes, known as sewer lines. These pipes are designed to transport wastewater, primarily relying on gravity to move the water downhill towards a treatment facility. The system includes smaller pipes that connect individual buildings to larger main sewer lines running under streets.
For areas where gravity flow is not feasible due to topography or long distances, pumping stations are integrated into the network. These stations lift the wastewater to higher elevations, allowing it to continue its gravitational flow toward the treatment plant. While most sewer systems are sanitary sewers, carrying only wastewater, some older municipalities use combined sewers that collect both wastewater and stormwater runoff in a single pipe. In such combined systems, heavy rainfall can sometimes overwhelm the system, leading to overflows of untreated wastewater into waterways.
The Treatment Process
Upon arrival at a wastewater treatment plant, sewer water undergoes a series of sequential processes designed to remove pollutants. This multi-stage approach ensures the water meets stringent quality standards before being discharged or reused. Each stage targets different types of contaminants, progressively purifying the water.
Preliminary Treatment
Preliminary treatment focuses on removing large debris that could damage equipment or interfere with subsequent steps. Wastewater first passes through screens, which capture objects like rags, plastics, and other bulky items. Following screening, the water enters grit removal chambers where its flow is slowed, allowing heavier inorganic materials like sand, gravel, and small stones to settle. These collected materials are sent to a landfill.
Primary Treatment
After preliminary treatment, wastewater proceeds to primary treatment, a physical process separating suspended solids and floating materials. The water enters large sedimentation tanks, also known as primary clarifiers, where it sits for several hours. Gravity causes heavier organic solids to settle, forming primary sludge, while lighter materials like oils and grease float as scum. Mechanical scrapers collect the settled sludge and skim off the floating scum. This stage can remove 50-65% of suspended solids and 30-40% of biochemical oxygen demand (BOD).
Secondary Treatment
Secondary treatment utilizes biological processes to remove dissolved organic matter remaining after primary treatment. Wastewater moves into aeration tanks where it is mixed with microorganisms, primarily bacteria and protozoa. Air is continuously bubbled into these tanks, providing oxygen that supports the microorganisms’ growth and activity. These organisms consume and break down dissolved organic pollutants, converting them into byproducts.
Following aeration, the mixture flows into secondary clarifiers, where the heavier microorganisms, often clumped together as activated sludge, settle. A portion of this settled sludge, rich in active microorganisms, is recirculated back to the aeration tanks to continue the biological process. The cleaner water then flows from the secondary clarifiers, having had over 90% of remaining suspended solids removed.
Advanced or Tertiary Treatment
Tertiary treatment, also known as advanced treatment, is an additional stage that further purifies the wastewater beyond the levels achieved by secondary treatment. This step removes specific contaminants like nitrogen and phosphorus, which can contribute to excessive algae growth in waterways. Common methods include advanced filtration to capture fine particles.
Disinfection is another component of tertiary treatment, targeting any remaining pathogens such as bacteria, viruses, and parasites. This is often achieved through chemical or physical methods. UV light damages the genetic material of microorganisms, rendering them unable to reproduce or cause infection. After tertiary treatment, the water is of high quality, suitable for safe discharge or reuse.
Where Treated Water and Solids Go
After undergoing the extensive treatment process, the outputs from the wastewater treatment plant are carefully managed. These consist of purified water, known as effluent, and processed solid materials, referred to as biosolids or sludge. Both have designated pathways for return to the environment or beneficial reuse.
Treated Water (Effluent)
The cleaned water, or effluent, is discharged into local waterways such as rivers, lakes, or oceans. Before discharge, this effluent must meet strict environmental quality standards set by regulatory bodies to protect aquatic ecosystems and public health. These standards regulate levels of pollutants like biochemical oxygen demand, suspended solids, and nutrients. In some regions, due to increasing water scarcity, highly treated effluent is being considered for water reuse applications. This can include irrigation for agriculture or landscapes, industrial processes, or replenishing groundwater supplies, conserving freshwater resources.
Solids (Biosolids/Sludge)
The accumulated solids, or sludge, from the primary and secondary treatment stages undergo further processing to become biosolids. This involves digestion, where microorganisms break down organic matter, and dewatering, which removes much of the water. The resulting biosolids are nutrient-rich organic materials. Common management methods include land application as a soil amendment or fertilizer for agricultural land, forests, or disturbed lands. Other disposal options include landfilling or incineration, with their management and reuse governed by regulations to ensure safe handling and application.