Site logo

Secondary Treatment in Wastewater: Understanding the Biological Process

Secondary treatment in wastewater is a vital process that follows the primary treatment stage to purify wastewater further before it is released into the environment or reused. Primary treatment involves the removal of large solids and sedimentation, which reduces the water’s organic load but does not eliminate dissolved organic matter or pathogens. This is where secondary treatment plays a critical role. It typically involves biological processes that utilize naturally occurring microorganisms to break down organic matter that remains after the initial phase of treatment.

Biological treatment methods, such as the activated sludge and trickling filters, leverage microbial communities to metabolize and convert organic pollutants into more stable substances. Aeration is critical in secondary treatment because it supplies oxygen to the microorganisms, enhancing their growth and activity. Following the biological phase, secondary clarifiers allow the treated solids to settle out, resulting in significantly cleaner water. Monitoring these processes is essential to ensure effective treatment complies with regulatory standards. Despite its effectiveness, this stage of therapy confronts challenges such as handling the varying nature of wastewater and integrating new technologies that improve efficiency and sustainability.

Key Takeaways

  • Secondary treatment is crucial for reducing organic matter and pathogens after primary treatment.
  • The process largely relies on biological treatment methods and aeration to degrade pollutants.
  • Effective monitoring and adherence to regulations are imperative to ensure treatment efficacy.

Overview of Wastewater Treatment

Wastewater treatment is an essential process that protects water quality by removing contaminants from used water before it is released into the environment or repurposed. This multi-stage process can generally be broken into primary, secondary, and tertiary treatment phases.

Primary Treatment:

  • In primary treatment, the focus is on separating solids from the liquid.
  • Large objects are removed through screens, and sedimentation allows heavier particles to settle out as sludge.

Secondary Treatment:

  • Secondary treatment is specifically designed for the biological degradation of dissolved organic matter.
  • Microorganisms play a crucial role in breaking down substances in a controlled environment.

The secondary stage involves critical processes:

  1. Activated Sludge: Air is pumped into large aeration tanks, mixing the wastewater with seed sludge. The pumped oxygen supports aerobic bacteria in consuming organic matter.
  2. Trickling Filter: Wastewater is distributed over a bed of stones or other material, where a biofilm of microorganisms metabolizes the organic content.
  3. Rotating Biological Contactors: Discs partially submerged in wastewater are rotated to allow microbial growth on surfaces for organic decomposition.

After these processes, the resulting effluent has significantly lower concentrations of organic contaminants, suspended solids, and pathogens. Yet, the water is not entirely pure; some dissolved chemicals and nutrients remain, often leading to the necessity for further tertiary treatment.

The Role of Secondary Treatment

Secondary treatment in wastewater management is pivotal to reducing pollutants before they enter the environment. It primarily targets organic matter using biological processes, ensuring that the effluent reaching the tertiary stages or being released into water bodies meets regulatory standards.

Biological Processes

Biological processes within secondary treatment leverage natural microbial action to break down organic compounds from wastewater. Two prevalent methods are the trickling filter and the activated sludge process. The trickling filter uses a bed of stones or other materials as a surface on which biofilms grow and metabolize the organic matter. The activated sludge process, in contrast, suspends microorganisms in wastewater, mixing air to encourage the breakdown of pollutants. These processes exploit bacterial activity to reduce organic content substantially, often eliminating around 85 percent of the organic matter.

Secondary Clarification

After the biological phase, the water moves to secondary clarification, where solids and microbial flocs can settle. Settling tanks or clarifiers are the main components in this step. They permit heavier particles to settle to the bottom while the more transparent water on top moves to the next phase or is discharged subject to local regulations. This stage is critical for separating the biosolids from the essentially purified water, signifying a successful diminution of the wastewater’s organic load.

Stages of Secondary Treatment

Secondary treatment, also known as the biological stage of wastewater treatment, focuses on breaking down organic matter using microbial organisms. This stage is crucial for reducing pollution before the effluent is released into the environment or moves on to tertiary treatment.

Biofilm Systems

In biofilm systems, microorganisms grow on a stationary surface where wastewater passes over it. Trickling filters and rotating biological contactors (RBCs) are common biofilm system types. Trickling filters use a bed of porous material over which wastewater is distributed to trickle down as a biofilm forms on the media, absorbing and consuming organic matter. RBCs have a series of discs mounted on a rotating shaft partially submerged in wastewater; as they turn, the biofilm alternately absorbs organic matter from the wastewater and oxygen from the air.

Suspended Growth Systems

Suspended growth systems rely on microorganisms suspended in the wastewater to treat it. The most prevalent example is the activated sludge process. This process involves aerating the wastewater to encourage the growth of microbes that consume the organic matter in suspension. The mixture then moves to a settling tank where the biomass is separated from the treated water. Variations like the oxidation ditch and sequencing batch reactors modify the activated sludge principles to suit different scales and treatment needs.

Aeration in Secondary Treatment

During the secondary treatment of wastewater, aeration is a crucial process. It involves the introduction of air into the wastewater, allowing microorganisms to thrive and degrade organic matter aerobically. This process significantly reduces the biological demand for oxygen in the treated water.

Critical components of aeration systems include:

  • Diffusers or Aerators: Devices that introduce oxygen into the water.
  • Aeration Tanks: Basins where the wastewater is mixed with microorganisms.
  • Blowers: Equipment that supplies air to the diffusers or aerators.

The aeration process can be segmented into several stages:

  1. Mixing: Wastewater is mixed continuously to keep microorganisms in suspension.
  2. Oxygenation: Oxygen is supplied to the microorganisms to foster their growth and the subsequent breakdown of organic pollutants.
  3. Settling: Post aeration, the mixture is allowed to settle, separating the biomatter from the treated water.

Efficiency is vital in aeration systems, as it impacts the overall effectiveness of wastewater treatment. Dissolved oxygen levels are maintained to ensure the microorganisms are active, optimizing the breakdown of waste.

The design and operation of aeration systems must be carefully managed. Too little oxygen can lead to an incomplete treatment, while excessive aeration can be cost-prohibitive. Aeration is a tailored process adapted to the specific needs of each wastewater treatment facility.

Benefits of proper aeration include:

  • Reduction of noxious odors.
  • Breakdown of organic matter.
  • Lower concentrations of harmful byproducts.

It is important to note that the management of aeration systems requires continuous monitoring to ensure treatment objectives are consistently met.

Biological Treatment Methods

In wastewater treatment, biological processes are crucial in breaking down organic matter and nutrients that remain after preliminary treatments. These processes harness the metabolism of microorganisms to achieve a cleaner effluent ready for further treatment or discharge into the environment.

Activated Sludge

The activated sludge process involves aeration tanks that mix wastewater with a suspension of aerobic microorganisms. These microorganisms consume organic matter and form flocs, which can then settle out in a secondary clarifier. This method is widely regarded for its efficiency in reducing biochemical oxygen demand (BOD) and ammonia levels. The activated sludge process variations are tailored to different plant capacities and water qualities.

Trickling Filters

Trickling filters represent a fixed-bed biological reactor where wastewater is evenly distributed over a media bed, providing a large surface area for biofilm growth. As the wastewater percolates through the bed, microorganisms consume the organic material. Trickling filters are known for their simplicity, durability, and ability to handle fluctuating loads.

Rotating Biological Contactors

Rotating Biological Contactors (RBCs) consist of large, closely spaced, rotating discs partially submerged in wastewater. The discs are covered with microorganisms that absorb and break down pollutants as the disc rotates. RBCs are particularly suited for small to medium-sized communities because of their low energy requirements and ease of operation.

Secondary Clarifiers

Secondary clarifiers play a crucial role in wastewater treatment by removing suspended biological matter following the secondary treatment process. These clarifiers are essential for ensuring that the treated water has significantly reduced concentrations of organic matter.

Design Considerations

In the design of secondary clarifiers, engineers focus on maximizing efficiency and ensuring compliance with environmental discharge regulations. The typical design involves large tanks where settled water from the top is collected while the settled sludge is removed from the bottom. Key design parameters include surface loading rate, retention time, and depth, which must be carefully calculated based on the expected inflow and treatment capacity requirements. An effective secondary clarifier design ensures optimal settling of solids occurs, thus improving the quality of the effluent.

Operational Factors

Proper operation of secondary clarifiers is integral to maintaining treatment plant performance. Factors such as sludge blanket depth and return activated sludge (RAS) rate are closely monitored. Operators must adjust the RAS rate to balance the system, ensuring enough microorganisms are returned to the aeration basin without causing excessive sludge accumulation. Maintaining a vigilant watch over these parameters under different loading conditions can prevent common issues such as sludge escape or poor settling, which directly impact the efficiency of the secondary clarification process.

Monitoring Secondary Treatment

In the critical phase of secondary wastewater treatment, consistent monitoring ensures process control and quality assurance, which is vital for meeting regulatory requirements and preserving environmental and public health standards.

Process Control

Process control in secondary treatment involves close surveillance of the biological processes that reduce organic matter in the wastewater. Aeration tanks, for instance, require precise measurements of oxygen levels since aerobic bacteria play an essential role in breaking down organic compounds. The effectiveness of these tanks is often evaluated by monitoring parameters such as:

  • Biochemical Oxygen Demand (BOD): Indicates the amount of organic matter bacteria can oxidize.
  • Suspended Solids: Help to determine the concentration of solid particles remaining in the treated water.

Regular sampling and real-time data acquisition from sensors installed in treatment facilities, like trickling filters and clarifiers, provide actionable insights to optimize operations.

Quality Assurance

Quality assurance in secondary wastewater treatment confirms the process meets effluent quality standards. This involves:

  • Routine Testing: Schedules for testing the treated water’s various chemical and biological parameters to ensure compliance.
  • Calibration of Instruments: Maintenance checks and calibrates instruments like pH meters and dissolved oxygen probes to ensure accuracy.

Data from quality tests is compared against established standards from agencies overseeing wastewater management to ensure the treated water is safe for discharge or reuse. Quality assurance efforts might include employing water quality monitoring equipment for tracking parameters such as Total Organic Carbon (TOC), which provides a holistic measurement of organic pollutants in water samples from equipment and processes that are industry-leading and grounded by decades of experience.

Compliance with TOC, nitrogen content, and pathogens is regularly checked. If deviations occur, corrective actions are promptly implemented. This adherence to stringent standards showcases the industry’s commitment to environmental stewardship and public health protection.

Challenges in Secondary Treatment

In wastewater management, secondary treatment faces specific challenges affecting efficiency and effectiveness. These challenges require careful management to maintain the quality of treated water.

Handling High Load

The secondary treatment stage can become overwhelmed when wastewater treatment plants experience high inflow volumes, particularly during peak usage or storm events. This can result in inadequate treatment of the biological demand of incoming wastewater, leading to potential regulatory noncompliance and environmental impacts. Facilities must have sufficient capacity and contingency plans to handle such variations in load.

Process Stability

Maintaining process stability in secondary treatment systems is crucial for consistent performance. Factors such as fluctuations in temperature, pH levels, or microbial composition can undermine the efficiency of biological treatment processes, leading to sub-optimal removal of organic matter and nutrients. Regular monitoring and adaptive management strategies help maintain a stable and robust treatment process.

Recent Advances in Secondary Treatment

Recent developments in secondary wastewater treatment focus on enhancing efficiency and environmental sustainability. These involve innovations in aeration systems and biological nutrient removal techniques.

Innovations in Aeration

Aeration, a critical process in secondary treatment, has seen significant technological progress. Systems now employ Fine Bubble Diffusion, which has improved oxygen transfer efficiency and reduced energy costs. Innovations like the Membrane Aerated Biofilm Reactor (MABR) technology, as noted by Syracuse University, offer substantial energy savings and a smaller carbon footprint by optimizing oxygen delivery to the wastewater microbes.

Biological Nutrient Removal

Biological Nutrient Removal (BNR) has been refined to tackle nitrogen and phosphorus effectively. One notable advancement is the Anaerobic Ammonium Oxidation (Anammox) process, which provides a cost-effective method for nitrogen removal without the need for carbon sources. The University of Florida’s Extension discusses how advanced treatment plants incorporate multi-stage BNR systems, integrating processes like anoxic and aerobic digestion to adhere to stringent discharge regulations efficiently.

Regulatory Aspects of Secondary Treatment

Regulatory bodies enforce strict guidelines to ensure that wastewater treatment plants (WWTPs) effectively reduce pollutants before releasing water into the environment. Secondary treatment, a vital phase of this process, is governed by several regulations.

The United States Environmental Protection Agency (EPA) standards mandate that secondary treatment must remove a significant percentage of organic matter and suspended solids. Typically, this means achieving:

  • Biological Oxygen Demand (BOD5): 85% reduction
  • Total Suspended Solids (TSS): 85% reduction

Regulations often require WWTPs to perform routine monitoring and reporting to demonstrate compliance with these benchmarks.

Regulatory Benchmark Required Reduction
BOD5 85%
TSS 85%

Furthermore, secondary treatment practices are subject to the Clean Water Act (CWA), which sets forth the National Pollutant Discharge Elimination System (NPDES) permits. NPDES permits stipulate specific discharge limits on pollutants and outline monitoring and reporting requirements.

In addition to federal regulations, state and local governments may impose more stringent rules to address local environmental needs. These additional rules can range from tighter effluent quality standards to mandates for advanced treatment technologies if secondary treatment alone does not meet water quality objectives.

WWTPs must adhere to these regulations, as noncompliance can lead to fines, legal action, and increased scrutiny. Facilities must stay informed and adapt to any updates in regulatory measures to remain compliant and protect public health and the environment.


Secondary wastewater treatment is crucial in the multi-step process of ensuring water is safe for discharge into the environment or further treatment. It primarily involves biological processes that reduce organic matter and pollutants from the effluent after primary treatment, which is mainly a physical process.

Steps in Secondary Treatment:

  1. It begins with the activation of sludge, where aerobic bacteria digest organic matter.
  2. The process continues in aeration tanks, which increase the oxygen content, accelerating bacterial growth and degradation of waste.
  3. Then, water moves to secondary clarifiers or sedimentation tanks where biological floc or biomass settles.
  4. The clear effluent is ready for discharge or tertiary treatment, and the settled biomass is returned to aeration tanks or disposed of.

Efficiencies achieved through secondary treatment are a testament to modern engineering and environmental protection. High-quality effluent helps preserve aquatic ecosystems and supports the sustainability of water resources.

Facilities play a pivotal role in water pollution control and public health protection by employing secondary treatment. The continual advancements in this domain ensure better quality treated water while minimizing environmental impact.

Frequently Asked Questions

What are the different types of Secondary Treatment processes in wastewater management?

Secondary treatment involves processes such as activated sludge systems, fixed-film systems like biofilters and trickling filters, and suspended growth systems that utilize bacteria to break down organic waste.

How does Secondary Sedimentation work in the wastewater treatment process?

In secondary sedimentation, gravity helps to settle biological flocs or sludge formed after the aeration step of secondary treatment, allowing clarified water to move to the next stage of treatment.

What distinguishes secondary treatment from tertiary treatment in wastewater management?

Secondary treatment mainly focuses on organic and biological waste reduction, while tertiary treatment targets the removal of nutrients, such as nitrogen and phosphorus, and further disinfection.

What are the standard regulations for Secondary Treatment in wastewater?

Standard regulations for secondary treatment typically require the removal of 85% to 90% of the biochemical oxygen demand (BOD) and suspended solids in the effluent before it is released into the environment.

Can you describe an example of a Secondary Treatment system in a wastewater treatment plant?

An example of a secondary treatment system is the activated sludge process, where aerated wastewater allows microorganisms to digest organic pollutants, forming flocs that settle out as sludge in secondary clarifiers.

How does the Secondary Treatment of wastewater integrate with the overall stages of sewage treatment?

Secondary treatment integrates with the overall stages of sewage treatment by following primary treatment, which removes solids and precedes tertiary treatment, where further pollutant removal occurs for water to meet specific standards.