Study Notes on Sewage Treatment and Wastewater Management

Enduring Understanding and Learning Objectives

  • Enduring Understanding STB-3
      Human activities, including the utilization of resources, have significant physical, chemical, and biological consequences for ecosystems.
  • Learning Objective STB-3.N
      Describe best practices in sewage treatment:
      - STB-3.N.1: Primary treatment of sewage involves the physical removal of large objects, typically using screens and grates, followed by the sedimentation of solid waste at the bottom of a tank.
      - STB-3.N.2: Secondary treatment is a biological process wherein bacteria decompose organic matter into carbon dioxide and inorganic sludge, with the sludge settling at the bottom of a tank. Aeration is applied to enhance the breakdown rate of organic matter by bacteria.
      - STB-3.N.3: Tertiary treatment incorporates ecological or chemical procedures to eliminate any remaining pollutants in the water after primary and secondary treatments.
      - STB-3.N.4: Before discharge, the treated water is treated with one or more disinfectants (typically chlorine, ozone, or ultraviolet light) to eliminate bacteria.

Essential Knowledge

  • AP Environmental Science Unit 8: Aquatic and Terrestrial Pollution
      - 8.11: Sewage Treatment

Need for Wastewater Treatment

  • Reasons for Treatment:
      - Water used for industrial and municipal applications often gets polluted through the introduction of suspended solids, salts, nutrients, bacteria, and oxygen-demanding wastes.
  • Legal Requirement:
      - According to U.S. law, any water degraded due to industrial and municipal purposes mandates treatment before it can be reintroduced into the environment.
  • Treatment Methods:
      - Common across various methods is the employment of bacteria to decompose organic matter into carbon dioxide and inorganic compounds (like nitrogen and phosphorus). Pathogens are eradicated using chlorination, UV light, ozone, or other methods.

Wastewater Treatment in Rural Areas

  • Septic Systems:
      - Typically used in rural regions without access to municipal wastewater treatment facilities.
      - Consists of a septic tank and a leach field — the septic tank receives wastewater from a household and is usually buried underground adjacent to the house.

Septic Tank Dynamics

  • Layer Formation Over Time:
      - Three layers develop in a septic tank:
        1. Scum Layer: Anything that floats rises to the surface, forming the scum layer.
        2. Sludge Layer: Heavier materials, including numerous pathogens, sink to the bottom, creating the sludge layer.
        3. Septage: The middle layer consists of less turbid water, termed septage, containing abundant bacteria (both pathogenic and non-pathogenic) and inorganic nutrients such as nitrogen and phosphorus.

Septage Management

  • Drainage and Treatment:
      - Septage is drained from the septic tank via gravity flow into the buried leach field through a piping system.

Septic Leach Field Operation

  • Functionality:
      - Pipes in the leach field have perforations allowing fluid to seep out gradually across the leach field.
      - The surrounding and underlying soil absorbs and filters the septage.
      - Soil microorganisms typically degrade any pathogens present in the septage.

Pros and Cons of Septic Systems

  • Advantages:
      - Septic tanks can be the only practical solution in certain areas.
      - Require no electricity for operation.
      - Represent a natural means of treating wastewater.
  • Disadvantages:
      - Installation can be costly.
      - Require substantial land area for effective operation.
      - Care must be taken to limit what enters the system.
      - Septic tanks must be periodically pumped, and installation may not be allowed in all locations.

Installation Considerations

  • Percolation Tests:
      - Sufficient drainage must be confirmed through a percolation (perc) test prior to installation to ensure proper permeability exists.
      - Poor drainage in an unsatisfactory site can cause wastewater pooling on the surface.

Risks of Septic System Failure

  • Consequences of Failure:
      - Failure of a septic tank leach field can lead to groundwater and surface water pollution.
      - Proper maintenance and installation on well-drained soils are crucial for septic systems.

Centralized Sewage Treatment Plants

  • Urban Wastewater Management:
      - In urban areas, wastewater treatment is conducted at specialized plants that handle municipal sewage from homes, businesses, and industries.
      - Raw sewage is transported to the plant via a network of sewer pipes.

Purpose of Sewage Treatment

  • Goals of Treatment:
      - The primary aim of sewage treatment is to breakdown organic waste (BOD) and to eliminate pathogenic organisms.

Summary of Treatment Flow

  • Stages of Treatment:
      - Wastewater treatment involves clearly defined phases: primary, secondary, and optionally tertiary treatment.
      - Each stage is characterized by specific processes (such as sedimentation, aeration, clarification, etc.) and distinct nature of exit flow.

Federal Mandates and Legal Considerations

  • Clean Water Act:
      - Mandates all U.S. municipal sewage treatment plants to implement primary and secondary treatment processes.
      - Many municipalities may further utilize tertiary or advanced wastewater treatment methods.
      - Provisions state that the discharge of any pollutant is unlawful.
      - The Environmental Protection Agency (EPA) requires publicly owned treatment facilities to provide uniform secondary treatment prior to discharge.

Tertiary Treatment Necessity

  • Contextual Requirement:
      - When secondary treatment alone does not suffice to maintain the quality of surface waters into which treated water is released, advanced treatment may be necessary, especially for rivers housing endangered fish species.
  • Correlation with Endangered Species:
      - Studies show a link between the density of wastewater treatment facilities and the distribution of threatened and endangered (T&E) species per watershed.

Primary Treatment Processes

  • Filtering:
      - Initial raw sewage is filtered through bar screens to remove large floating organic materials.

  • Sedimentation:
      - Sewage enters a grit chamber to have dense materials (sand, small pebbles) removed before moving to the primary sedimentation tank.
      - Here, remaining particulate matter settles into sludge, while the effluent stream exits lighter substances.

Sludge Management in Primary Treatment

  • Removal and Disposal:
      - The thickened sludge removed may be sent to a digester for further anaerobic processing or dried and disposed of through landfill, incineration, or agricultural use.
      - Primary treatment successfully removes about 30-40% of BOD by volume mainly in suspended solids and organic matter.

Secondary Treatment Overview

  • Biological Processing:
      - Primarily involves microorganisms to eliminate dissolved and suspended biological material, operating in a controlled setting.
      - Most systems utilize aerobic bacteria and protozoa to diversify the breakdown of remaining organic materials into carbon dioxide and inorganic nutrients (nitrogen, phosphorus).
  • System Types:
      - Fixed Film Systems: Bacteria grown on filters where water is passed through.
      - Suspended Growth Systems: Use 'activated' sludge where decomposing bacteria are directly mixed into the sewage, usually aerated for effective bacterial growth.

Activated Sludge System

  • Microbial Recycling:
      - Activated sludge encompasses aerobic bacteria that digest organic substances in wastewater.
      - A fraction of enriched activated sludge is recycled to mix again with incoming new wastewater in the aeration tank, maintaining a continual microbial population.

Trickling Filter System

  • Aerobic Degradation:
      - Trickling filters allow sewage to seep through a bed of crushed stones coated with bacteria and protozoa, facilitating aerobic degradation of sewage.

Anaerobic Digestion of Sludge

  • Digestion Process:
      - Remaining sludge from the final sedimentation tank typically goes to an anaerobic digester.
      - The ultimate objective is to treat sludge with anaerobic bacteria, resulting in carbon dioxide and methane (CH4).

  • Methane Utilization:
      - Methane, a byproduct of anaerobic digestion, can be used within the plant as fuel for equipment, electricity generation, or for heating and cooling needs, or may be simply flared off.

Disinfection of Remaining Wastewater

  • Pathogen Elimination:
      - Wastewater from the final sedimentation tank is disinfected, commonly through chlorination (using sodium hypochlorite).
      - Treated water is released into natural bodies (rivers, lakes, oceans) or used for agricultural irrigation.

Alternatives to Chlorination

  • Other Disinfectants:
      - Other methods including ozone, ultraviolet light, and hydrogen peroxide can be employed as alternatives to chlorination.
      - These alternative methods are typically more expensive and require detailed design and management.

Issues with Sludge

  • Toxic Contaminants:
      - Some sludge from treatment plants in urban and industrial areas may harbor toxic pollutants like heavy metals not removed in treatment.
      - Communities may require industries to pre-treat sewage to eliminate heavy metals prior to reaching the treatment plant.

Tertiary Treatment Processes

  • Process Complexity and Cost:
      - Generally rare due to high costs, tertiary treatment involves specialized chemical and physical methods to extract residual pollutants after primary and secondary treatments.

  • Specific Pollutants:
      - Primarily employed to reduce phosphorus and nitrates before water is discharged into surface waters.

Rationale for Tertiary Treatment

  • Eutrophication Prevention:
      - Technology to reduce phosphorus levels is effective in curbing potential eutrophic conditions in bodies of water.

  • Wildlife Protection:
      - Tertiary treatment may be vital for conserving wildlife in ecosystems receiving treated wastewater, especially when it’s intended for reuse in irrigation (food crops, golf courses) or recreational water bodies (lakes, estuaries) or even drinking water applications.