Wastewater treatment is crucial for protecting our water resources. It involves removing pollutants from used water through physical, biological, and chemical processes. The goal is to produce clean water that can be safely returned to the environment or reused.

Water quality management ensures our lakes, rivers, and oceans stay healthy. It includes setting standards, monitoring water bodies, and regulating pollution sources. This helps prevent contamination and maintain water suitable for drinking, recreation, and supporting aquatic life.

Wastewater Treatment Stages

Primary Treatment

• Removes large solids, grit, and grease from wastewater through physical processes such as screening and sedimentation
• Screening removes large debris like rags, sticks, and plastic using bar screens or mesh screens
• Grit removal settles out sand, gravel, and other heavy inorganic materials in grit chambers
• Primary sedimentation allows suspended solids to settle out in clarifiers forming primary sludge (biosolids)
• Removes about 50-60% of suspended solids and 25-35% of the biochemical oxygen demand (BOD)

Secondary Treatment

• Removes dissolved and suspended organic matter using biological processes involving microorganisms
• Aerobic bacteria consume organic matter, converting it into carbon dioxide, water, and more microorganisms
• Common methods include activated sludge process, trickling filters, and rotating biological contactors
• Activated sludge process mixes wastewater with microorganisms in aeration tanks, then separates the treated water from the microbial flocs in secondary clarifiers
• Removes up to 85-90% of BOD and suspended solids

Tertiary Treatment and Disinfection

• Advanced treatment steps that further remove contaminants, nutrients, and pathogens
• Can include processes like filtration, adsorption, ion exchange, and membrane technologies (reverse osmosis, ultrafiltration)
• Removes specific pollutants like nitrogen, phosphorus, heavy metals, and micropollutants (pharmaceuticals, pesticides)
• Disinfection inactivates pathogenic microorganisms using chlorination, UV irradiation, or ozonation
• Tertiary treatment and disinfection produce high-quality effluent suitable for reuse or discharge into sensitive water bodies

Wastewater Treatment Processes

Biological Oxygen Demand (BOD)

• Measures the amount of oxygen required by microorganisms to decompose organic matter in wastewater
• Indicator of the organic pollution load in wastewater and the efficiency of treatment processes
• BOD test involves incubating a wastewater sample for 5 days at 20°C and measuring the oxygen consumed
• High BOD levels indicate significant organic pollution and can lead to oxygen depletion in receiving waters

Activated Sludge Process

• Widely used secondary treatment method that relies on aerobic microorganisms to remove organic matter
• Wastewater is mixed with activated sludge (flocs of microorganisms) in aeration tanks with ample oxygen supply
• Microorganisms consume organic matter, forming new cells and carbon dioxide
• Treated water is separated from the microbial flocs in secondary clarifiers, with some sludge recycled back to the aeration tank
• Produces high-quality effluent with low BOD and suspended solids

Filtration in Wastewater Treatment

• Used in tertiary treatment to remove remaining suspended solids, particulates, and microorganisms
• Common methods include granular media filtration (sand, anthracite), membrane filtration (microfiltration, ultrafiltration), and disc filters
• Sand filtration passes wastewater through a bed of sand or sand-anthracite mix, trapping particles in the pore spaces
• Membrane filtration uses semi-permeable membranes to remove particles based on size exclusion (microfiltration removes bacteria, ultrafiltration removes viruses)
• Improves effluent quality, reducing turbidity and pathogen levels before disinfection

Water Quality Management

Water Quality Standards and Regulations

• Set limits on pollutant concentrations and other parameters to protect water bodies for designated uses (drinking, recreation, aquatic life)
• Established by regulatory agencies like the U.S. Environmental Protection Agency (EPA) under the Clean Water Act
• Include criteria for physical, chemical, and biological characteristics (temperature, pH, dissolved oxygen, toxins, pathogens)
• Wastewater treatment plants must meet effluent standards based on the receiving water body's quality and designated uses
• National Pollutant Discharge Elimination System (NPDES) permits regulate point source discharges, setting effluent limits and monitoring requirements

Water Quality Monitoring and Assessment

• Regular sampling and analysis of water bodies to evaluate compliance with standards and identify pollution sources
• Monitoring parameters include temperature, pH, dissolved oxygen, nutrients, bacteria, and specific contaminants
• Wastewater treatment plants monitor influent and effluent quality to optimize processes and ensure compliance with permits
• Watershed-scale monitoring assesses the overall health of rivers, lakes, and coastal waters, guiding management decisions
• Biomonitoring uses aquatic organisms (macroinvertebrates, fish) as indicators of long-term water quality and ecosystem health
• Water quality data informs total maximum daily load (TMDL) development, pollution control strategies, and restoration efforts