What Is Tertiary Wastewater Treatment, And How Does It Work?

Wastewater treatment is a crucial process that aims to remove contaminants from used water to ensure it is safe for release into the environment or suitable for reuse. The treatment process is typically divided into three stages: primary, secondary, and tertiary treatment. While primary and secondary treatments are effective in removing large solids and organic matter, tertiary wastewater treatment is the final stage that eliminates remaining non-biodegradable pollutants, such as phosphorus, nitrogen, and other dissolved substances. This process is essential in refining water to meet stricter environmental standards and in preparing it for reuse in various applications.

In this article, we will explore tertiary treatment of wastewater, how it works, the methods involved, and its importance in modern wastewater treatment plants. We will also discuss the differences between primary, secondary, and tertiary treatments and highlight some of the key technologies used to achieve effective tertiary treatment.

What is Tertiary Wastewater Treatment?

Tertiary wastewater treatment is a post-treatment stage in the wastewater management process, following primary and secondary treatments. The main objective of tertiary treatment is to remove any remaining contaminants that were not removed during the earlier stages of treatment. This includes non-biodegradable substances like nitrogen and phosphorus, as well as fine particles, pathogens, and organic chemicals that require specialized removal techniques.

Tertiary treatment involves various chemical, physical, and biological processes. These methods can include filtration, disinfection, chemical precipitation, and nutrient removal. By the end of tertiary treatment, the water is significantly cleaner and can either be discharged into natural water bodies or, in some cases, recycled for reuse in industrial processes, agriculture, or even as potable water after further treatment.

The Stages of Wastewater Treatment

Before diving deeper into tertiary treatment of wastewater, it’s essential to understand the stages that precede it:

• Primary Treatment: This is the first stage of wastewater treatment and involves physical processes like screening, settling, and flotation to remove large solids and debris from the wastewater.

• Secondary Treatment: This stage uses biological processes to remove dissolved organic matter and reduce the concentration of oxygen-demanding substances in the wastewater. Activated sludge systems or trickling filters are commonly used for this purpose.

• Tertiary Treatment: The final stage focuses on polishing the water by removing remaining pollutants, including fine solids, nutrients (like phosphorus and nitrogen), and pathogens, using advanced techniques.

How Does Tertiary Wastewater Treatment Work?

Tertiary wastewater treatment is highly effective at achieving the required effluent quality standards, and it employs a combination of physical, chemical, and biological processes to treat the water. The specific methods used can vary depending on the type of pollutants present, the quality of water required for discharge or reuse, and the design of the wastewater treatment plant.

Key Methods of Tertiary Treatment

Here are some common tertiary wastewater treatment methods used to achieve the desired level of water quality:

1. Filtration

• Filtration is often the first step in tertiary treatment, used to remove remaining suspended particles or solids that were not removed in primary or secondary treatments. The filtration process can include sand filters, membrane filtration, or disc filters.

2. Chemical Precipitation

• One of the most common methods used in tertiary treatment is chemical precipitation, which involves adding chemicals like alum or ferric chloride to the wastewater. These chemicals react with dissolved phosphorus and other pollutants to form solid precipitates that can then be removed by clarification or sludge dewatering systems.

3. Nutrient Removal

• Phosphorus and nitrogen are two key nutrients that contribute to water pollution and eutrophication. Tertiary phosphorus removal typically involves chemical treatments, while tertiary nitrogen removal may involve processes like nitrification-denitrification or the use of biological filters.

4. Disinfection

• Disinfection is an important aspect of tertiary treatment, especially when water is intended for reuse. Methods such as chlorination, ozonation, or UV radiation can be employed to kill any remaining pathogens or bacteria in the water.

5. Advanced Oxidation Processes (AOP)

• Advanced oxidation processes like ozonation, UV-H2O2, or photocatalysis are used to treat organic pollutants in the water, breaking down complex organic compounds into simpler, less harmful molecules.

6. Membrane Filtration

• Advanced membrane filtration techniques, such as reverse osmosis or ultrafiltration, are also used for polishing treated water. These processes can remove microscopic particles, bacteria, and even dissolved salts, ensuring that the water meets the highest quality standards.

Tertiary Treatment of Wastewater Diagram

A tertiary treatment diagram typically illustrates the various stages involved in treating wastewater after primary and secondary treatment. This includes:

• Pre-filtration or coarse filtration

• Chemical dosing (for phosphorus and nitrogen removal)

• Membrane filtration or fine filtration

• Disinfection (UV light, ozone, or chlorine)

• Sludge treatment (including sludge dewatering and clarification)

The diagram highlights how each step works to remove specific types of pollutants, leading to the purification of wastewater.

Tertiary Treatment Examples

1. Phosphorus Removal: In a wastewater treatment plant, chemical precipitation using aluminum or iron salts can remove most of the phosphorus. Alternatively, biological processes such as enhanced biological phosphorus removal (EBPR) can be employed.

2. Nitrogen Removal: Tertiary nitrogen removal often involves biological nitrification-denitrification processes. In this system, aerobic and anaerobic conditions are created, allowing nitrifying bacteria to convert ammonium to nitrate and denitrifying bacteria to reduce nitrate to nitrogen gas.

3. Disinfection with UV Light: UV disinfection is commonly used in the tertiary treatment stage to eliminate pathogens that might remain in the water after secondary treatment.

4. Membrane Filtration for High-Quality Effluent: In some cases, reverse osmosis is used for water intended for reuse as drinking water or industrial applications, ensuring removal of even dissolved contaminants.

What Are Tertiary Wastewater Treatment Protocols?

The protocols for tertiary wastewater treatment depend on the specific contaminants in the wastewater and the desired quality of the effluent. These protocols generally include:

• Chemical dosing for phosphorus and nitrogen removal.

• Filtration and membrane processes to remove fine particles and pathogens.

• Disinfection to ensure pathogen-free water, especially for water reuse.

• Sludge dewatering techniques, including the use of screw press dewatering or belt filter press, to manage waste produced during the treatment process.

The design and implementation of these protocols depend on the local regulatory requirements, as well as the needs of the treatment plant.

Primary, Secondary, and Tertiary Treatment of Wastewater

The difference between secondary and tertiary wastewater treatment lies in the specific contaminants they target and the processes involved:

• Primary treatment focuses on the removal of large solids and debris from wastewater through screening, settling, and flotation.

• Secondary treatment is a biological process that removes dissolved organic matter and reduces the level of oxygen-demanding substances.

• Tertiary treatment, on the other hand, aims to remove the remaining dissolved nutrients, fine particles, and pathogens using advanced methods such as filtration, chemical precipitation, and disinfection.

What is Tertiary Produced Water Treatment?

In the context of industrial wastewater, tertiary produced water treatment refers to the treatment of water that has been used in industrial processes, such as oil and gas extraction. Produced water often contains high levels of salts, organic compounds, and heavy metals. Tertiary treatment in this context involves the use of advanced filtration, chemical treatments, and disinfection processes to make the water suitable for discharge or reuse.

Tertiary Treatment of Wastewater Jobs

The implementation of tertiary wastewater treatment methods creates a demand for skilled professionals in various roles, including:

• Wastewater treatment plant operators who oversee the operation of tertiary treatment systems.

• Wastewater treatment plant jobs that involve monitoring and maintaining equipment such as screw press dewatering, belt filter presses, and membrane filtration systems.

• Environmental engineers who design and optimize tertiary treatment processes.

Individuals interested in these roles often need a wastewater treatment plant operator certificate and knowledge of various treatment technologies.

Conclusion

Tertiary wastewater treatment plays a vital role in modern wastewater treatment plants by refining the quality of treated water and ensuring that it meets strict environmental standards. It employs a range of advanced processes, including chemical precipitation, membrane filtration, disinfection, and nutrient removal, to effectively treat remaining pollutants such as phosphorus, nitrogen, and fine particles. Tertiary treatment methods are critical for water reuse, protecting natural water bodies from pollution, and supporting sustainable water management practices.

As the need for clean, reusable water increases, particularly in urban areas and industrial applications, the importance of tertiary treatment of wastewater will continue to grow. The development and implementation of advanced treatment protocols, coupled with skilled workforce development, will be essential in meeting the challenges of modern water management.