Wastewater treatment removes impurities from used water, often called sewage, before release into the environment. This process transforms contaminated water from homes, industries, and agriculture into an effluent safe for natural water systems like rivers, lakes, or oceans.
Untreated wastewater poses substantial risks, harboring disease-causing microorganisms and harming aquatic ecosystems by depleting oxygen and introducing toxins. Treatment protects public health, natural resources, and aquatic life.
Removing Initial Contaminants
Wastewater purification begins with preliminary and primary treatment, physically separating larger, heavier materials. Raw wastewater first passes through screens, removing large debris like rags and plastics that could damage equipment. This initial screening prevents operational issues and prepares water for subsequent stages.
Following screening, wastewater flows into grit chambers. Water velocity is controlled, allowing heavier, inorganic particles like sand and gravel (grit) to settle. This prevents premature settling of organic solids. Removing grit is important as these abrasive materials cause wear on mechanical components.
After preliminary treatment, water moves into primary sedimentation tanks. Flow rate is reduced, allowing suspended solids to settle by gravity, forming primary sludge. Lighter materials like grease and oils float to the surface, forming scum that is skimmed off. Primary treatment removes significant suspended solids and some organic matter, preparing water for biological purification.
Biological Purification
After larger solids are removed, wastewater enters secondary treatment, relying on biological processes. Microscopic organisms, primarily bacteria and protozoa, break down dissolved and suspended organic matter remaining from primary treatment. These microorganisms consume pollutants, converting them into less harmful byproducts.
The activated sludge process is a widely used method. Wastewater is introduced into aeration tanks where air or pure oxygen is supplied. This aeration provides oxygen for aerobic bacteria to thrive and metabolize organic compounds. Without sufficient oxygen, biological degradation would be slow.
As microorganisms consume pollutants, they clump, forming biological flocs. These flocs are aggregates of bacteria, protozoa, and other microbes embedded within a sticky, gel-like extracellular polymeric substance (EPS). Flocculation traps finer particulate matter and allows microbes to work effectively on dissolved organic material.
Dense, settleable flocs form. Once microorganisms consume organic matter, the mixed liquor flows into a secondary clarification tank. Here, flocs, laden with consumed pollutants, settle by gravity, separating from cleaner water. A portion of this settled material, “activated sludge,” is recycled back into the aeration tank to re-seed incoming wastewater, ensuring continuous active microbes.
Final Water Polishing
Following biological purification, water often undergoes further refinement in tertiary treatment for higher purity. This involves processes like filtration, which removes remaining fine suspended particles and other impurities. Water passes through media such as sand beds or activated carbon filters, physically trapping particulate matter. More advanced methods, including membrane filtration, separate even smaller contaminants.
After filtration, water is disinfected to eliminate harmful microorganisms before release. This protects public health and prevents waterborne diseases. Without effective disinfection, residual pathogens could contaminate natural water bodies and pose risks.
Several disinfection methods exist. Chlorination, a traditional approach, adds chlorine compounds to destroy pathogens. However, concerns about disinfection byproducts and dechlorination needs have led to alternatives.
Ultraviolet (UV) light disinfection is a physical process where water flows under UV lamps. UV radiation damages microorganism genetic material, rendering them unable to reproduce without adding chemicals. Ozonation, using ozone gas, is another powerful disinfectant that destroys viruses and bacteria, leaving no chemical residue.
Managing Treatment Byproducts
Wastewater treatment removes contaminants, concentrating pollutants into sludge. This byproduct, composed of settled solids, requires further processing to reduce volume and health risks. Sludge management, also called biosolids, is an integral part of the treatment cycle.
Sludge treatment typically begins with thickening, which removes excess water to reduce volume. This makes sludge easier and more cost-effective to handle. Thickening commonly occurs in gravity thickeners or through dissolved air flotation, using air bubbles to float solids for removal.
Following thickening, sludge undergoes digestion, a biological process stabilizing organic matter and reducing disease-causing microorganisms. Anaerobic digestion, a common method, takes place in sealed tanks without oxygen. Bacteria break down compounds, producing biogas (primarily methane) for energy. Aerobic digestion introduces oxygen, allowing bacteria to consume organic material.
The final stage is dewatering, which further removes water to decrease sludge weight and volume. This significantly lowers transportation and disposal costs. Techniques like centrifugation or filtration using belt filter presses are common. Dewatered biosolids are prepared for disposal or beneficial reuse. These nutrient-rich biosolids can be applied to agricultural land as fertilizer or soil amendments.