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Secondary Treatment Of Wastewater Involves

Secondary Treatment of Wastewater: A Comprehensive Overview

Wastewater treatment is a critical process in modern society, aimed at removing impurities and contaminants from water before it is released back into natural water bodies or reused. The treatment process is generally divided into three main stages: primary, secondary, and tertiary treatment. Each stage serves a specific function in the purification process, addressing different types of pollutants and ensuring that the treated water meets environmental and health standards. Secondary treatment, in particular, plays a pivotal role in breaking down dissolved organic matter and significantly reducing the biochemical oxygen demand (BOD) and suspended solids in wastewater. This article delves into the intricacies of secondary treatment, discussing its importance, mechanisms, methodologies, and advancements.

The Importance of Secondary Treatment

Secondary treatment is essential because primary treatment alone is insufficient for meeting the stringent requirements for discharge or reuse. While primary treatment removes large solids and particulate matter through processes like screening and sedimentation, it does not adequately address dissolved organic materials, pathogens, and fine suspended solids. These impurities pose significant environmental hazards, including:

  1. Oxygen Depletion: Organic materials decompose in water bodies, consuming dissolved oxygen. This can lead to hypoxic or anoxic conditions, adversely affecting aquatic life.
  2. Pathogen Spread: Untreated wastewater can carry harmful microorganisms that cause diseases in humans and animals.
  3. Eutrophication: Nutrients in wastewater, such as nitrogen and phosphorus, can promote excessive growth of algae in water bodies, disrupting ecosystems.
  4. Aesthetic and Odor Issues: Discharged wastewater with high organic content can be unsightly and produce foul odors.

Secondary treatment tackles these issues by employing biological processes to degrade organic matter and reduce impurities, thus protecting ecosystems and human health.

Mechanisms of Secondary Treatment

The primary mechanism of secondary treatment is biological degradation. Microorganisms, such as bacteria, protozoa, and fungi, consume organic matter in the wastewater as a food source. The metabolic processes of these organisms convert complex organic molecules into simpler compounds, primarily water, carbon dioxide, and biomass (sludge).

Key Processes in Secondary Treatment

Secondary treatment can be broadly categorized into aerobic and anaerobic processes, depending on the presence or absence of oxygen. Both categories encompass a variety of treatment methodologies, each with its design and operational characteristics. The main aerobic processes include:

  1. Activated Sludge Process (ASP)
  2. Trickling Filters
  3. Rotating Biological Contactors (RBCs)
  4. Membrane Bioreactors (MBRs)
  5. Sequencing Batch Reactors (SBRs)
  6. Oxidation Ponds and Lagoons

1. Activated Sludge Process


The Activated Sludge Process (ASP) is one of the most widely used methods for secondary treatment. It involves the aeration of wastewater in an aeration tank where microorganisms thrive and form flocs (clusters). These flocs consume organic matter, producing carbon dioxide, water, and new microbial cells.


  • Aeration Tank: Where the biological activity takes place. Air or oxygen is supplied to support microbial metabolism.
  • Clarifier: A sedimentation tank where biomass solids settle out. The settled biomass (activated sludge) is partly recycled back to the aeration tank to maintain microbial population.
  • Return Activated Sludge (RAS): Portions of settled microorganisms are returned to the aeration tank to maintain an adequate concentration of biomass.
  • Waste Activated Sludge (WAS): Excess biomass is removed from the system to prevent overpopulation and is subjected to further treatment and disposal.

Operational Parameters

  • Mixed Liquor Suspended Solids (MLSS): Concentration of suspended solids in the aeration tank, including microorganisms.
  • Sludge Retention Time (SRT): Average time that microorganisms remain in the system.
  • Dissolved Oxygen (DO): Level of oxygen in the aeration tank, crucial for aerobic microbial activity.

2. Trickling Filters


Trickling filters are fixed-bed systems where wastewater is distributed over a bed of media (rock, gravel, or plastic). Microorganisms grow on the media’s surface, forming a biofilm that degrades the organic material as wastewater trickles through it.


  • Distribution System: Sprays or distributes wastewater evenly over the filter media.
  • Filter Media: The surface on which the biofilm forms.
  • Underdrain System: Collects treated effluent and removes excess biomass.

Operational Considerations

  • Hydraulic Loading Rate: Volume of wastewater applied per unit area of media per unit time.
  • Organic Loading Rate: Amount of organic matter applied to the filter per unit area of media per unit time.
  • Recirculation Ratio: Portion of treated effluent recirculated to the input to enhance treatment efficiency.

3. Rotating Biological Contactors (RBCs)


RBCs consist of a series of closely spaced, rotating disks mounted on a horizontal shaft. The disks are partially submerged in wastewater. As the disks rotate, microorganisms form biofilms on the disk surface and degrade organic material.


  • Disks: Provide surface area for biofilm growth.
  • Shaft and Motor: Facilitate continuous rotation of the disks.
  • Clarifier: Separates treated effluent from the biomass.

Key Parameters

  • Surface Area per Volume: The amount of disk surface area available for biofilm formation per unit volume of wastewater.
  • Rotation Speed: Affects the oxygen transfer and biofilm thickness.

4. Membrane Bioreactors (MBRs)


MBRs combine biological treatment with membrane filtration. The biological process occurs in a bioreactor, while membranes separate treated water from suspended solids and biomass.


  • Bioreactor: Houses the microbial population.
  • Membrane Module: Filters out suspended solids and microorganisms, producing high-quality effluent.

Pros and Cons

  • Advantages: Produces high-quality effluent, small footprint, effective pathogen removal.
  • Disadvantages: High operational cost, membrane fouling requires periodic cleaning and replacement.

5. Sequencing Batch Reactors (SBRs)


SBRs perform biological treatment in batch mode rather than continuous flow. Wastewater is treated in cycles that include filling, aeration, settling, and decanting phases.


  • Reactor Tanks: Where treatment phases occur.
  • Aeration System: Supplies oxygen during the aeration phase.
  • Decanter: Removes treated effluent after settling.

Operational Cycle

  1. Fill Phase: Wastewater enters the reactor.
  2. Aeration Phase: Microbial degradation of organic matter.
  3. Settle Phase: Biomass settles to the bottom.
  4. Decant Phase: Treated effluent is removed.

6. Oxidation Ponds and Lagoons


Oxidation ponds and lagoons are simple, cost-effective systems that use natural processes for wastewater treatment. They typically involve a series of shallow ponds or lagoons where microbial activity, sunlight, and oxygen work together to treat wastewater.


  • Facultative Ponds: Include aerobic upper layers and anaerobic bottom layers.
  • Aerated Lagoons: Use mechanical aerators to supply oxygen.

Key Parameters

  • Detention Time: Time wastewater spends in the pond or lagoon.
  • Pond Depth and Area: Affect oxygen transfer and microbial activity.

Anaerobic Processes

Anaerobic processes, while less common in secondary treatment, can be highly effective for specific applications, particularly in treating high-strength industrial wastewater.

1. UASB Reactors


Upflow Anaerobic Sludge Blanket (UASB) reactors treat wastewater through anaerobic digestion. Wastewater flows upward through a dense bed of granular sludge, where microorganisms degrade organic matter.


  • Reactor: Vertical tank housing the sludge blanket.
  • Gas-Liquid-Solid Separator: Separates biogas, treated effluent, and sludge.


  • Low Energy Consumption: No need for aeration.
  • Biogas Production: Generates methane for energy recovery.

Challenges and Innovations in Secondary Treatment


  • Energy Consumption: Aeration in aerobic processes is energy-intensive.
  • Sludge Management: Handling and disposal of excess biomass.
  • Pathogen Removal: Ensuring complete pathogen elimination.
  • Nutrient Removal: Reducing nitrogen and phosphorus beyond secondary treatment’s capabilities.


  • Advanced Aeration Systems: Improve oxygen transfer efficiency, reducing energy use.
  • Integrated Membrane Systems: Combine biological treatment with advanced filtration.
  • Microbial Fuel Cells: Harness microorganisms to generate electricity from organic matter.
  • New Bioreactor Designs: Enhance treatment efficiency and reduce footprint.


Secondary treatment is a cornerstone of wastewater management, offering robust solutions for removing dissolved organic matter and reducing environmental impacts. Through various aerobic and anaerobic processes, it addresses critical issues like oxygen depletion, pathogen spread, and nutrient loading. While challenges persist, ongoing innovations promise to make secondary treatment more efficient, sustainable, and capable of meeting future demands. By understanding and advancing these processes, we can continue to protect water resources and promote public health.