9.3 Wastewater Collection and Treatment

Wastewater management is crucial for protecting public health and the environment. This section covers the types of wastewater, collection systems, and treatment processes that remove pollutants before discharge.

From domestic sewage to industrial effluents, wastewater requires careful handling. We'll explore how gravity-fed sewers and treatment plants work together to clean water, making it safe for release back into nature.

Wastewater Types and Sources

Domestic, Industrial, and Storm Water Runoff

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• Wastewater categorized into three main types (domestic, industrial, and storm water runoff) each with distinct characteristics and sources
• Domestic wastewater originates from residential areas including water from toilets, showers, sinks, and laundry facilities
  • Contains organic matter, nutrients, and pathogens
  • Examples of nutrients (nitrogen, phosphorus)
  • Examples of pathogens (E. coli, Giardia)
• Industrial wastewater generated by manufacturing processes
  • May contain chemicals, heavy metals, and other pollutants specific to the industry
  • Examples of industrial pollutants (lead, mercury, organic solvents)
• Storm water runoff flows over impervious surfaces, collecting pollutants from roads, parking lots, and other urban areas
  • Examples of collected pollutants (oil, grease, sediment, pesticides)

Infiltration, Inflow, and Composition

• Infiltration and inflow (I/I) contribute to wastewater volume
  • Infiltration occurs through cracks in pipes
  • Inflow comes from illegal connections or storm drains
• Wastewater composition varies significantly based on its source
  • Influences treatment requirements and potential environmental impacts
  • Example variations (BOD levels, suspended solids concentration, pH)

Wastewater Collection Systems

Gravity Flow and Network Components

• Wastewater collection systems utilize gravity flow in most cases
  • Pipes designed to maintain minimum velocity to prevent solids deposition
  • Minimum velocity typically 2 feet per second (0.6 meters per second)
• Collection network consists of lateral sewers, trunk sewers, and interceptor sewers
  • Increase in size as they converge towards treatment facilities
  • Example sizes (8-inch diameter laterals, 36-inch diameter trunk sewers)
• Manholes placed at regular intervals and at changes in pipe direction or slope
  • Facilitate inspection and maintenance of the sewer system
  • Typical spacing 300-500 feet (90-150 meters)

Lift Stations and System Types

• Lift stations employed when gravity flow not feasible
  • Use pumps to move wastewater from lower to higher elevations
  • Examples of pump types (centrifugal pumps, submersible pumps)
• Combined sewer systems collect both sanitary sewage and storm water in same pipes
• Separate systems keep sanitary sewage and storm water flows isolated
  • Advantages of separate systems (reduced treatment volume, improved water quality)

Design and Maintenance

• Hydraulic design of sewer systems considers peak flow rates, pipe materials, slope, and capacity
  • Ensures efficient and reliable wastewater conveyance
  • Example design considerations (Manning's equation for flow calculation, minimum pipe slopes)
• Sewer system monitoring and maintenance involve various techniques
  • Closed-circuit television (CCTV) inspection
  • Smoke testing
  • Regular cleaning to prevent blockages and overflows
  • Example cleaning methods (high-pressure jetting, root cutting)

Wastewater Treatment Stages

Preliminary and Primary Treatment

• Preliminary treatment removes large debris and grit
  • Uses screens, comminutors, and grit chambers
  • Protects downstream equipment and improves treatment efficiency
  • Example of screen sizes (coarse screens 1-2 inches, fine screens 0.25-0.5 inches)
• Primary treatment utilizes sedimentation tanks
  • Removes suspended solids and floating materials through gravitational settling
  • Reduces biochemical oxygen demand (BOD) by 25-40%
  • Example of sedimentation tank detention time (2-3 hours)

Secondary and Tertiary Treatment

• Secondary treatment employs biological processes
  • Activated sludge or trickling filters remove dissolved and colloidal organic matter
  • Typically achieves 85-95% BOD reduction
  • Example of activated sludge process parameters (MLSS concentration, F/M ratio)
• Tertiary treatment (advanced treatment) uses various processes
  • Removes specific pollutants like nutrients, pathogens, or trace contaminants
  • Nutrient removal targets nitrogen and phosphorus
    • Uses biological nutrient removal (BNR) or chemical precipitation
    • Example BNR processes (anaerobic-anoxic-oxic process, modified Ludzack-Ettinger process)
  • Disinfection eliminates pathogenic microorganisms
    • Methods include chlorination, UV irradiation, or ozonation
    • Example UV dose for disinfection (40 mJ/cm²)

Sludge Treatment and Emerging Technologies

• Sludge treatment and disposal integral parts of wastewater treatment
  • Involves thickening, stabilization, dewatering, and final disposal or beneficial reuse
  • Example of sludge stabilization method (anaerobic digestion)
• Emerging technologies in wastewater treatment include:
  • Membrane bioreactors (MBRs)
  • Anaerobic digestion for energy recovery
  • Advanced oxidation processes for micropollutant removal
  • Example of MBR advantages (high effluent quality, smaller footprint)

Wastewater Discharge Impacts

Environmental Effects

• Untreated or inadequately treated wastewater discharge leads to eutrophication
  • Causes algal blooms and oxygen depletion in receiving water bodies
  • Example of eutrophication indicators (chlorophyll-a concentration, dissolved oxygen levels)
• Heavy metals and persistent organic pollutants from industrial wastewater bioaccumulate
  • Affects aquatic ecosystems and food chain
  • Example bioaccumulation factors for mercury in fish (1,000 to 100,000)
• Nutrient loading from wastewater effluents disrupts aquatic ecosystems
  • Leads to loss of biodiversity and habitat degradation
  • Example of nutrient loading effects (dead zones in coastal areas)

Public Health Concerns

• Pathogenic microorganisms in wastewater pose significant public health risks
  • Can cause waterborne diseases if discharged without proper treatment
  • Examples of waterborne diseases (cholera, typhoid fever, hepatitis A)
• Inadequate wastewater management contributes to spread of antimicrobial resistance
  • Growing global health concern
  • Example of antibiotic-resistant bacteria found in wastewater (MRSA, VRE)

Mitigation and Monitoring

• Proper wastewater treatment significantly reduces environmental and health risks
  • Supports sustainable water resource management and public health protection
  • Example of treatment efficiency (99.99% reduction in fecal coliform bacteria)
• Monitoring and regulatory compliance of wastewater treatment plants essential
  • Ensures consistent effluent quality
  • Minimizes negative impacts on receiving environments
  • Example of effluent monitoring parameters (BOD, TSS, pH, residual chlorine)