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What Is Secondary Wastewater Treatment and How Does It Work?

By: Tom Frankel
Post Date: November 21st 2022

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What Is Secondary Wastewater Treatment and How Does It Work?

The secondary treatment phase is a crucial part of the wastewater treatment process because it further purifies water to remove more waste following the primary phase. After the primary treatment phase removes suspended solids using physical processes and gravity, secondary treatment systems use biological processes to continue treating water.

Secondary wastewater treatment introduces microorganisms to wastewater, and these microorganisms remove waste from the water through metabolic processes. The three types of secondary treatment are aerobic, anaerobic and anoxic treatment. Each method differs in its biological process, but each effectively removes impurities from water.


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Why Is Wastewater Treatment Important?

Wastewater treatment is important for the environment, human health and water preservation. Treating wastewater removes harmful materials that can kill or harm animals, plants and habitats when it’s released back into a natural source, protecting the environment from potential damage. It protects human health by purifying water that humans may come into contact with and alleviates water scarcity by making water safe for reuse.

Commercial and industrial wastewater discharges can contain dangerous pollutant levels that interfere with publicly-owned treatment works (POTWs). It is crucial for wastewater plants to implement efficient water treatment systems to properly purify wastewater before returning it to natural or public sources.

What Is Secondary Wastewater Treatment?

Secondary wastewater treatment occurs following the primary treatment phase. It further purifies the primary effluent from the pretreatment and primary phases to biologically remove impurities from wastewater. The secondary treatment of wastewater is crucial because it removes solids that remain in the water following primary treatment.

How Does Secondary Wastewater Treatment Work?

The secondary wastewater treatment phase removes impurities from water using bacterial processes. This phase biologically removes contaminants from water using microorganisms. This occurs after the pretreatment and primary treatment phases remove many suspended solids from water using physical processes and gravity. Secondary treatment removes over 90% of the remaining suspended solids from wastewater.

Wastewater treatment plants implement either aerobic, anaerobic or anoxic treatment methods to treat water during secondary treatment. Each method utilizes a different type of bacteria colony.

Aerobic and Activated Sludge Treatment

Aerobic treatment systems convert organic contaminants to end products such as water, carbon dioxide and additional microorganisms. This treatment method uses microorganisms with metabolic processes that require oxygen. Aerobic treatment systems use aeration to supply wastewater with oxygen, which feeds microorganisms that then consume waste from the wastewater.

Activated sludge treatment is a common aerobic process. Using both aeration and flocculation, this process uses a sludge blanket made up of clumped biosolids known as flocs. These flocs form during the aeration process and settle at the bottom of the water tank.

Aerobic and Activated Sludge Treatment

During the activated sludge treatment process, water treatment plants use secondary clarifiers to mix settled sewage with raw or primary sludge. They then use air compressors to add compressed air to the mixture and pump the flocs back into the water inside the aeration tank, allowing the microorganisms in the return sludge to break down more waste. Treatment plants use different processes and equipment for this treatment option:

1. Surface Aerators or Diffusers

Some treatment plants mix air into the water using surface aerators in lagoons. Other plants use ceramic diffusers or rubber membrane diffusers in aeration tanks.

An aeration tank pumps air through a tube or disc-shaped diffusers containing several small perforations. Air flows through the perforations and enters the aeration tank as tiny bubbles. These bubbles rise through the water tank to transfer oxygen and facilitate aerobic digestion.

2. Media Filters

Some treatment plants facilitate aerobic digestion using media filters. The moving bed biofilm reactor (MBBR) system is a common media filter system. This type of system uses thousands of small plastic media pieces inside a basin. A biofilm forms on the media pieces when bacteria attach to them.

The media pieces comprise approximately 50% to 70% of the basin’s volume and provide surface areas for bacterial growth. The pieces maximize space by suspending throughout the water, which is possible due to their density. Each intricate media piece resembles a wheel with several small spokes, providing maximum surface area for bacterial growth. The optimal shape and density of media pieces allow them to reach as much waste as possible and efficiently digest it to reduce hydraulic retention time.

Anaerobic Treatment

Anaerobic processes work without oxygen to convert organic contaminants to biofuel gas. This process works without any form of oxygen and often occurs in covered digestion lagoons. Inside the lagoons, anaerobic bacteria break down organic waste. The anaerobic process uses less energy than the aerobic process because it does not require equipment to pump oxygen into the wastewater.

Anaerobic water treatment typically produces biogas byproducts such as water vapor, carbon dioxide and methane. Wastewater treatment plants often reuse the resulting methane to fuel their plants. Anaerobic treatment is ideal for plants that treat water containing concentrated amounts of biodegradable materials such as food waste, animal excrement or municipal waste.

Anoxic Treatment

Anoxic treatment treats water using microbes with metabolic processes that do not require oxygen. This process occurs without free molecular oxygen, but it can happen in the presence of some oxygen in the form of sulfates, nitrates or nitrites. Plants often use this process to denitrify wastewater containing high nitrogen content.

Anoxic denitrification uses a suspended growth system or a trickling filter to convert nitrogen to nitrate. The wastewater plant then introduces certain microbes to consume the nitrogen content in the nitrate and leave only oxygen molecules behind. Treatment plants usually tightly seal reactors to prevent molecular oxygen interference.

Contact SSI Aeration, Inc. for All Your Secondary Treatment Needs

Contact SSI Aeration, Inc. for All Your Secondary Treatment Needs

Treating wastewater helps preserve the environment, protect human health and alleviate water scarcity. Secondary wastewater treatment uses biological methods to purify water further following the physical primary treatment process. This phase in wastewater treatment uses microorganisms to consume and remove waste, preparing the water for the tertiary treatment phase before it returns to a natural water source.

Secondary wastewater treatment is an essential part of the water treatment process because it removes smaller impurities that the primary phase leaves behind. SSI Aeration, Inc. provides high-quality water treatment equipment and services. SSI Aeration designs and manufactures equipment and systems for wastewater treatment plants globally.

SSI’s long-lasting and efficient systems save treatment plants money on energy and maintenance. As a global leader in water treatment equipment, SSI Aeration features innovative solutions and an expert engineering team. From design to installation, SSI is dedicated to maintaining product quality, sustainability, integrity and client satisfaction. Contact SSI Aeration to learn more about high-quality water treatment solutions and how innovative equipment can increase your water treatment plant’s efficiency.