Secondary Treatment of Wastewater

What is the Secondary Treatment of Wastewater?

After Preliminary and Primary Treatment of Wastewater, Secondary Treatment of Wastewater is carried out. Secondary Treatment of Wastewater involves the removal of biodegradable organic matter using microbial community. The microbial biomass present in the wastewater reproduces in certain ways and during the process, they utilize organic matters, minerals, and various other growth-related substances from the wastewater. Thus, resulting in decreasing the number of biodegradable solids. This purification system is often termed Natural/ Self- Purification. Secondary Treatment of Wastewater is known as Biological Treatment of Wastewater.

Secondary Treatment of Wastewater is classified based on the primary metabolic pathway utilized by the dominant microbial species.

  1. Depending upon the availability and utilization of Oxygen. Aerobic, Anoxic and Anaerobic
  2. Depending upon the growth condition. Suspended Growth Processes and Attached Growth Processes.

Depending upon the availability and utilization of Oxygen

Before proceeding with aerobic, anoxic, and anaerobic processes one must be familiar with the concept of electron transfer. For the generation of energy microbial cell utilizes the process of respiration (Oxidative catabolism) or fermentation (Fermentative catabolism). Catabolism is defined as the breakdown of organic compounds into simpler forms. While breaking down the energy is released which is utilized by the microbes. During the process of breakdown, the oxidized substance can be organic matter or reduced inorganic compound, both acting as electron donors. The electron then passes through a sequence of biochemical reactions to another organic compound called an oxidizing substance which acts as an electron acceptor and having an oxidation state as reduced.

Aerobic Processes

Aerobic secondary treatment of wastewater occurs when various types of electron acceptors are present in the medium then the bacterial cells will utilize the one that produces the highest quantity of energy. Thus aerobic processes will utilize oxygen first. The processes are most metabolically active and produce more residual solids in the form of cell mass.

A typical aerobic system. Courtesy: Samcotech

Anoxic Processes

If the nitrates are present then the microorganisms which are capable of utilizing nitrogen will prevail. during respiration, they will convert nitrate to nitrogen gas (Denitrification) and the processes are collectively called anoxic processes.

Anaerobic Processes

When nitrates are consumed then anaerobic processes start to prevail and other nutrients such as sulfates are utilized. During this process, the sulfates will be converted to sulfites and Carbon-d i-oxide to methane.

Typical Anaerobic, Anoxic and Aerobic Processes. Courtesy: Hossein Barzinmehr

Depending upon the growth condition inside the reactor.

Depending upon the growth condition inside the reactor the secondary treatment of wastewater steps can be of suspended growth or attached growth type.

Suspended Growth Process

In this growth process, the microorganism which is responsible for the conversion of waste organic matter is maintained into suspension of the liquid phase. Aerobic suspended growth processes include Activated Sludge Processes, Aerated Lagoons, and Sequential Batch Reactors. While, anaerobic processes include Bag Digesters, Plug Flow Digesters, Stirred Tank Reactors, and Baffled Reactors.

Activated Sludge Process. Courtesy: Paweł Frącz

Attached Growth Processes

In attached growth processes the microorganisms are attached to the surfaces (such as stones, inert materials) or are self immobilized on flocs or granules in the system. Aerobic Attached Growth Processes include Trickling Filters, Roughing Filters, Rotating Biological Contractors, and Packed Bed Reactors. Anaerobic Systems include Up Flow Packed Bed Reactor, Down Flow Packed Bed Reactor, Anaerobic Rotating Biological Reactors, Anaerobic Fluidized Bed Reactors, Up Flow Anaerobic Sludge Blankets (UASB), and Various Hybrid Anaerobic Reactors.

A Layout of Up Flow Anaerobic Sludge Blanket Reactor (UASB). Courtesy: VIRIDIS Engineering Sdn Bhd


  1. Volume 2: Basic Principles of Wastewater Treatment- Marcos von Sperling
  2. Wastewater Treatment- Biological- Shaikh Ziauddin Ahammad, David W. Graham, Jan Dolfing
  3. Wastewater Treatment Processes- Dr. Arun Kumar
  4. Evaluation Criteria Guide for Water Pollution Prevention, Control, and Abatement Programs

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