Clean Water Act & NPDES Permits

Legislation and Standards

• The Water Quality Act of 1965 was an early federal effort to guide clean water strategy
• It was strengthened by the 1972 Federal Water Pollution Control Act Amendments
• The goal was to make surface waters "swimmable and fishable" again
• The Federal Water Pollution Control Act was amended again in 1977
• This redefined EPA priorities and strengthened enforcement powers
• The Federal Water Pollution Control Act is referred to as the Clean Water Act (CWA)
• The National Pollution Discharge Elimination System (NPDES) implements the strategy of reducing pollution from point sources
• All municipal or industrial treatment facilities discharging wastewater effluents must obtain an NPDES Discharge Permit
• This permit is from the EPA or a delegated state agency
• NPDES permits specify the allowable amounts of pollutants a facility can discharge
• The NPDES permit program effectively set effluent standards for water pollution control

Treatment System Overview

• Screening, grit removal, and sedimentation (settling) are primary treatment processes
• Secondary treatment usually involves biological processes and additional settling
• Not all sewage treatment plants need tertiary (or advanced) treatment

Effluent Limits and Treatment

• Effluent limits are based on conventional technology, combining primary, secondary, and sometimes tertiary treatment
• Most U.S. treatment plants use secondary treatment
• Secondary treatment removes at least 85% of BOD and suspended solids from wastewater

Treatment Efficiency

• Treatment efficiency is the ratio of pollutants removed to the amount of pollutants in raw wastewater
• PIN = pollutant concentration flowing into the treatment system
• POUT = pollutant concentration flowing out of the system

Pretreatment of Industrial Wastewater

• Industrial wastewater often contains toxic chemicals like cyanide and lead
• Pre-treatment removes pollutants from industrial sewage before discharge to a municipal system to reduce toxicity
• The National Pretreatment Program has two sets of rules
  • Categorical pretreatment standards: industry-specific requirements
  • Prohibited discharge standards: substance-specific prohibitions

Primary Treatment

• Wastewater treatment processes are biological, chemical, or physical/mechanical
• Primary treatment removes pollutants from wastewater with physical processes (screening, comminution, grit removal)
• Untreated/raw wastewater flows continuously under gravity

Preliminary Treatment Systems

• Preliminary treatment systems include
  • Screens: remove large floating objects
  • Comminutors: mechanical cutting/shredding devices
  • Grit chambers: remove sand and gritty material from sewage

Sedimentation in Primary Treatment

• Wastewater still contains suspended organic solids after preliminary treatment
• These solids can be removed by sedimentation
• Settling tanks receiving sewage after grit removal are primary clarifiers
• Primary treatment combines preliminary processes and gravity settling
• CWA requires BOD and TSS removal efficiencies of at least 85% for sewage treatment plants
• Therefore, at least one additional treatment process must follow primary treatment
• The next step is generally secondary treatment

Secondary Treatment

• To remove suspended solids that didn't settle in primary tanks AND dissolved BOD unaffected by physical treatment
• A common minimum standard is 85% BOD and TSS removal efficiency
• In the United States, secondary treatment systems almost always comprise biological processes

Biological Treatment

• Biological sewage treatment involves microorganisms
• Microbes (bacteria and protozoa) consume organic pollutants as food
• They metabolize biodegradable organics into carbon dioxide, water, and energy for growth/reproduction

Biological System Must-Haves

• Microorganisms in a biological sewage treatment system must have:
  • Enough oxygen
  • Adequate contact with organic material in sewage
  • Suitable temperatures and favorable conditions

Common Biological Treatment Systems

• Trickling filter: fixed-growth system
  • Microbes remain fixed/attached to a surface as wastewater flows over it
  • Flows over the surface to provide contact with the organics
• Activated sludge process: suspended-growth system
  • Microbes are thoroughly mixed and suspended in the wastewater
  • Microbes do not attach to a particular surface

Trickling Filter

• Consists of a crushed rock layer/bed, ~2 m (6 ft) deep, typically circular, up to 60 m (200 ft) diameter
• Primary effluent is sprayed over crushed stone bed and trickles downward to an underdrain system
• Trickling filter effluent is collected in the underdrain system and sent to a secondary clarifier

Secondary Clarifier

• Similar to the primary clarifier
• There are differences in detention time, overflow rate, weir loading, etc

Trickling Filter Flow Rate & Effluent

• To maintain a uniform flow rate through the trickling filter AND to keep the distributor arm rotating during low sewage flow, wastewater may be recirculated
• A portion of the effluent is pumped back to the trickling filter inlet so water will pass through the stones more than once

Recirculation

• The amount of recirculation can vary
• Recirculation is characterized by a recirculation ratio
• That ratio is recycled flow to raw wastewater flow
• The recirculation ratio R, is generally 0.0 to 3.0

Hydraulic Load

• The rate at which wastewater flow is applied to the trickling filter surface
• Measured in cubic meters per day per square meter of surface area (m³/m²·d)

Organic (BOD) Load Rate

• The rate at which organic material is applied to the trickling filter
• Either an organic load or BOD load measured in kg BOD/m³·d or lb BOD/ft³·d
• OLR does not include the BOD added by recirculation
• A typical value for the organic load on a trickling filter is 0.5 kg/ m³·d

Efficiency Notes

• The BOD removal efficiency generally increases with
  • Decreasing organic load
  • Increasing recirculation
  • Increasing temperature

Activated Sludge System

• The basic components of an activated sludge sewage treatment system:
  • Aeration tank
  • Secondary settling basin or clarifier
• Primary effluent is mixed with settled solids AND solids recycled from the secondary clarifier
• Mixer is introduced into the aeration tank

Activated Sludge Treatment details

• Compressed air is injected continuously into the mixture through porous diffusers
• That's done along one side of the tank
• Microorganisms in the tank grow and multiply
• This forms an active suspension of biological solids
• The aerobic microorganisms consume dissolved organic pollutants as food
• The combination of activated sludge and wastewater in the aeration tank is called the mixed liquor
• In the basic/conventional system, a tank detention time of ~6 hours is required for thorough stabilization

Post Aeration

• After ~6 hours of aeration, the mixed liquor flows to the secondary clarifier
• There, activated sludge solids settle by gravity
• The clarified water near the surface, the supernatant, is discharged over an effluent weir
• The settled sludge is pumped out from the bottom of the tank
• Not all sludge can be put back into the aeration tank

Sludge

• The settled sludge is in an active state
• Microbes are well acclimated to the wastewater
• Microbes will readily absorb and decompose more organics by their metabolism
• Excess sludge, called waste-activated sludge, must eventually be treated and disposed of with primary tank sludge

Food-to-Microorganism Ratio

• Food is measured in kilograms (pounds) of BOD added to the tank per day
• Suspended solids in the mixed liquor are mostly living microorganisms
• Suspended solids concentrations are used as a measure of the number of microorganisms in the tank
• This concentration is called the mixed liquor suspended solids (MLSS)
• It's the indicator of the organic load on the system with respect to the amount of biological solids in the tank
• For conventional aeration tanks, the ratio is generally 0.2 to 0.5
• F/M = Q × BOD / MLSS × V