Secondary treatment plays a crucial role in wastewater management, effectively reducing pollutants and contaminants to protect the environment and public health. Following primary treatment, where solids are separated from wastewater, secondary treatment focuses on breaking down and removing organic matter, nitrogen, and phosphorus. There are several types of secondary treatment systems, each with its own method and benefits. This guide will walk you through the main options, helping you understand how each system works and when it may be best suited for specific applications.

1. Activated Sludge Systems

Activated sludge systems are among the most widely used secondary treatment methods in wastewater treatment plants (WWTPs). These systems rely on aerobic microorganisms to break down organic matter in wastewater. In this process, wastewater is mixed with “activated” sludge, which contains a rich community of bacteria and other microbes.

• How It Works: The mixture is aerated in a tank, promoting microbial growth and allowing bacteria to consume organic pollutants. This process produces “flocs,” or clusters of microorganisms and waste particles, which can be settled out in a secondary clarifier.
• Benefits: Activated sludge systems are highly effective at reducing biochemical oxygen demand (BOD) and removing suspended solids. They are commonly used in municipal and industrial wastewater treatment plants due to their efficiency.
• Considerations: This system requires significant aeration, leading to high energy costs. Proper management and control of sludge levels are essential to prevent overgrowth and maintain system efficiency.

2. Trickling Filters

Trickling filters are a type of biological treatment system where wastewater is distributed over a bed of media, typically rocks or plastic, which provide a surface for microbial growth. As wastewater trickles down, the microbes break down organic matter.

• How It Works: Wastewater is sprayed or dripped over the filter media, where it comes into contact with a biofilm of bacteria, fungi, and protozoa. These organisms metabolise the organic pollutants, cleaning the water before it moves to a secondary clarifier.
• Benefits: Trickling filters are simple to operate and maintain. They are energy-efficient, as they don’t require the intense aeration that activated sludge systems do. They’re suitable for smaller communities and rural areas.
• Considerations: Trickling filters may produce odours and can be less efficient than activated sludge systems at removing BOD and total suspended solids (TSS). They require regular cleaning to prevent clogging and can be impacted by cold weather.

3. Rotating Biological Contactors (RBC)

Rotating Biological Contactors, or RBCs, are another type of biological treatment that uses a series of rotating discs to support microbial growth. These discs are partially submerged in wastewater, allowing microbes to break down organic pollutants as they rotate through the wastewater.

• How It Works: The rotating discs expose the biofilm on their surface to both wastewater and air. As the biofilm moves through the water, it consumes organic materials, achieving secondary treatment.
• Benefits: RBCs require low energy input and produce minimal odours. They have a smaller footprint compared to activated sludge systems, making them ideal for smaller treatment plants.
• Considerations: The initial cost of RBC systems can be high, and regular maintenance is essential to prevent fouling of the discs. They are also sensitive to temperature changes, with efficiency dropping in colder conditions.

4. Sequencing Batch Reactors (SBR)

Sequencing Batch Reactors (SBRs) are a form of activated sludge treatment that operates in batches rather than continuously. This allows for a flexible and controlled process, making SBRs suitable for varying wastewater inflows.

• How It Works: SBRs operate in cycles, including fill, react, settle, and discharge phases. During the react phase, aeration encourages microbial activity to break down pollutants. The sludge settles during the settle phase, and the treated water is discharged.
• Benefits: SBRs are highly effective at removing BOD, TSS, and nutrients. They are ideal for smaller treatment plants and facilities with variable flows.
• Considerations: Due to their batch processing nature, SBRs require careful control of cycle timing and aeration. They also require higher operator skill levels compared to continuous systems.

5. Membrane Bioreactors (MBR)

Membrane Bioreactors combine biological treatment with membrane filtration, creating an efficient secondary treatment method. MBRs are particularly effective at producing high-quality effluent suitable for reuse applications.

• How It Works: In an MBR system, wastewater is treated biologically through activated sludge. Afterward, it passes through a membrane, which removes suspended solids and other particles. This results in highly purified effluent.
• Benefits: MBRs are excellent for wastewater reuse, as they produce a clean effluent with low pathogen levels. They are compact and require a smaller footprint.
• Considerations: MBRs have high operational and maintenance costs, mainly due to the need for periodic membrane cleaning and replacement. They are also sensitive to fouling and require precise control.

6. Constructed Wetlands

Constructed wetlands use natural processes to treat wastewater by simulating the conditions found in natural wetlands. They are particularly effective in rural or eco-friendly projects.

• How It Works: Wastewater flows through a wetland area containing plants, soil, and other materials that encourage microbial activity. These plants absorb nutrients, while microbes in the soil break down organic pollutants.
• Benefits: Constructed wetlands are eco-friendly, require minimal energy, and provide additional benefits, such as habitat creation for wildlife. They are ideal for small communities and environmentally conscious projects.
• Considerations: Constructed wetlands require more land than other systems, which can be a limiting factor in urban areas. They are also affected by seasonal weather changes, which can impact efficiency.

7. Biofilters

Biofilters are similar to trickling filters but use different media, such as sand, gravel, or specially designed plastic, to host microbial communities that treat wastewater.

• How It Works: Wastewater is passed through the biofilter, where microorganisms on the media surface break down pollutants. The treated water is then collected at the base of the filter.
• Benefits: Biofilters are compact and relatively easy to operate. They have low energy requirements and can achieve high levels of BOD and nutrient removal.
• Considerations: Biofilters can become clogged if not maintained properly, especially if there is high organic loading. They also require periodic replacement of the filter media.

Choosing the Right Secondary Treatment System

Selecting the most appropriate secondary treatment system depends on factors such as the size of the treatment facility, budget, local regulations, and specific treatment goals. Here’s a quick guide to help with choosing:

• For Small, Rural, or Eco-Friendly Applications: Consider trickling filters, constructed wetlands, or biofilters for their simplicity and low energy usage.
• For High-Efficiency or Reuse Goals: Membrane Bioreactors (MBR) and activated sludge systems are ideal for producing high-quality effluent suitable for reuse.
• For Municipal or Industrial Facilities: Activated sludge systems, Sequencing Batch Reactors (SBR), or RBCs are well-suited for larger-scale applications requiring high treatment efficiency.

Each secondary treatment system has its own strengths and limitations, so choosing the right one is essential for effective wastewater management. Investing in a well-suited secondary treatment system benefits the environment, protects water resources, and contributes to a sustainable future.