What Is Removed During Secondary Wastewater Treatment

What Is Removed During Secondary Wastewater Treatment?

Introduction

Wastewater treatment is an essential process for maintaining public health and environmental integrity. It involves multiple stages to ensure the safe return of water back into the ecosystem. Secondary wastewater treatment is a critical phase designed to substantially degrade the biological content of the sewage, which is derived from human waste, food waste, soaps, and detergents. Understanding what is removed during this stage and the mechanisms behind it offers insight into the importance of secondary treatment and its role in pollution control.

Overview of Wastewater Treatment Stages

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Before delving into secondary treatment, it’s important to understand the general steps of wastewater treatment:

1. Preliminary Treatment: This stage involves the removal of large objects and debris using screens and grit chambers. The aim is to protect mechanical components from damage and to prevent blockages in the subsequent phases.

2. 

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   Primary Treatment: In this phase, the wastewater is held in a sedimentation tank where heavy solids settle at the bottom and lighter materials like oils and greases float to the top. The goal is to remove these particles to reduce the load on secondary treatment facilities.

3. Secondary Treatment: The focus here is on biological degradation of dissolved and suspended organic matter. Microorganisms play a pivotal role in this stage, converting harmful constituents into safer forms.

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4. Tertiary Treatment: This optional stage further purifies water through chemical, biological, or physical methods to remove any remaining substances that may be detrimental in the environment.

The Role of Secondary Treatment

Secondary wastewater treatment primarily targets the removal of organic matter that remains after primary treatment. Organic matter is a broad category encompassing various forms of living and dead plant and animal material, which are potential contributors to environmental pollution if left untreated.

The secondary stage uses biological processes to convert dissolved and suspended organic matter into simpler end products, primarily carbon dioxide and water, with the help of microorganisms. The primary mechanisms include aerobic processes, like activated sludge and trickling filters, as well as anaerobic processes, which are used in specific conditions or systems.

Key Components Removed During Secondary Treatment

1. Biochemical Oxygen Demand (BOD)

Biochemical Oxygen Demand (BOD) is a measure of the amount of oxygen that microorganisms will consume while decomposing organic material over a set period. High BOD levels in discharged wastewater can deplete oxygen in natural water bodies, harming aquatic life.

• Removal Process: Secondary treatment effectively reduces BOD by introducing aeration and microbial digestion processes. Aerobic bacteria metabolically digest organic material, thereby reducing BOD and producing simpler byproducts such as CO2.

2. Suspended Solids

Suspended solids refer to small particles that remain dispersed in water, including organic and inorganic matter. These can contribute to turbidity, affecting water quality and aquatic ecosystems.

• Removal Process: In activated sludge systems, the aerated mix is passed into a secondary clarifier where microbial flocs formed by the bacteria settle out by gravity, effectively removing suspended solids from the water.

3. Pathogenic Microorganisms

While not all pathogens are removed, secondary treatment reduces the concentration of disease-causing organisms in the wastewater.

• Removal Process: The action of bacteria and other microorganisms consumes a significant number of pathogens as well. Certain pathogenic microorganisms are reduced simply by competition for nutrients and predation by other microorganisms.

4. Nutrients (Nitrogen and Phosphorus)

These are not fully removed in all treatment systems during secondary treatment and can lead to eutrophication in water bodies if present in large quantities. However, many treatment systems have adapted processes to target these nutrients.

• Removal Process: Some facilities utilize specialized bacteria that can remove nitrogen through processes like nitrification and denitrification, while phosphorus removal can be achieved through biological uptake in bio-P systems.

5. Organic Compounds and Toxins

Certain organic chemicals and toxins inadvertently enter the wastewater stream through household and industrial waste.

• Removal Process: Biodegradable organic compounds are broken down by microorganisms. However, not all organic toxins are fully removed at this stage, necessitating additional treatment steps if they are present.

Types of Secondary Treatment Systems

1. Activated Sludge System

The activated sludge system is one of the most common forms of secondary treatment. It involves aerating wastewater in the presence of a biological floc composed of bacteria and protozoa.

• Process Details:
  • Aeration Tank: The wastewater is mixed with a small amount of recycled sludge, known as returning activated sludge (RAS), and then aerated to encourage the growth of oxygen-loving bacteria.
  • Settling Tank: Wastewater flows to a settling tank, or clarifier, where biomass is allowed to settle. A portion of this biomass is returned (as RAS) to the aeration tank to maintain an effective population of microorganisms.
  • Result: The process effectively reduces BOD, suspended solids, and some level of pathogens and nutrients.

2. Trickling Filter System

Trickling filters use a bed of rocks, gravel, or plastic media to support microbial films that digest organic material in the water as it trickles over the surface.

• Process Details:
  • Media Bed: Wastewater is distributed over the surface of the media, creating a biofilm where microorganism colonies thrive.
  • Biological Action: As the water trickles down, microbes metabolize dissolved and suspended organic matter.
  • Result: This process is effective for BOD reduction and removing some suspended solids.

3. Rotating Biological Contactors (RBCs)

RBC systems consist of a series of closely spaced circular disks mounted on a rotating shaft placed in a tank partially submerged in wastewater.

• Process Details:
  • Rotation: The slow rotation exposes the biomass attached to the disks to air and the organic material in wastewater alternately.
  • Biological Action: Microbial activity on the disks processes organic materials in the wastewater.
  • Result: RBCs effectively reduce BOD and suspended solids while also providing some pathogen reduction.

4. Sequencing Batch Reactors (SBRs)

SBRs are a type of activated sludge process that treat wastewater in batches rather than continuously.

• Process Details:
  • Fill and React: The reactor is filled with wastewater, followed by aeration to promote microbial digestion.
  • Settlement and Decant: Following aeration, the process allows the biomass to settle before decanting clear effluent.
  • Result: SBRs are known for their flexibility and effectiveness in dealing with varying flow rates and treatment loads.

Advantages and Limitations of Secondary Treatment

Advantages:

• Efficiency in Organic Matter Removal: Secondary treatment significantly reduces the concentration of organic matter in wastewater, thus lessening BOD and COD levels.
• Pathogen Reduction: It reduces the presence of pathogenic organisms, contributing to public health safety.
• Aerobic and Anaerobic Options: Provides diverse treatment approaches tailored to specific requirements and conditions.
• Scalability and Adaptability: Systems can be scaled up or down and adjusted to handle different flow rates and pollution loads.

Limitations:

• Nutrient Removal Limitations: Standard secondary processes do not completely remove nutrients like nitrogen and phosphorus, potentially necessitating tertiary treatment.
• Energy Usage: Processes like activated sludge systems have high energy demands due to mechanical aeration requirements.
• Effluent Quality Variations: Variations in influent characteristics can affect the efficiency and output quality from these systems.

Conclusion

Secondary wastewater treatment is a crucial step in the wastewater treatment process dedicated to reducing organic pollutants and other key constituents in municipal and industrial effluents. Through biological processes involving various forms of microbial digestion, secondary treatment systems, such as activated sludge systems, trickling filters, RBCs, and SBRs, effectively lower BOD levels, reduce suspended solids, and to some extent pathogens and nutrients. Despite certain limitations, these systems form the backbone of modern wastewater treatment infrastructure, ensuring that discharged water meets environmental safety standards and contributes less to pollution. The evolution and optimization of secondary treatment methods continue to play a significant role in sustainable water management and environmental protection.