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How Waste Water Treatment Plants Work 

November 4, 2020

As mentioned in previous posts about Critical Infrastructure,  the industry is made up of 16 sectors which are considered vital to the day-to-day operations and overall security of the United States.

Of these 16 sectors designated as essential, the Water and Wastewater Systems Sector is considered uniquely critical because it facilitates most, if not all, of the other sectors functionality2

For example, it is well-known that water is a necessity for all life. However the process behind treating water and, specifically, wastewater to ensure it is safe for consumption and the environment. According to the Cybersecurity & Infrastructure Agency (CISA), "there are approximately 153,000 public drinking water systems and more than 16,000 publicly owned wastewater treatment systems in the United States. More than 80 percent of the U.S. population receives their potable water from these drinking water systems, and about 75 percent of the U.S. population has its sanitary sewerage treated by these wastewater systems.2"

Because so much of this essential service relies on the publicly owned wastewater treatment systems and because of the physical nature of the service which is supplemented by automated processes, this sector is vulnerable to attacks potentially leading to a contaminated water supply. These attacks could result in consumers of the water becoming ill, a public health crisis, or other commercial users of the water supply such as firefighters and hospitals not being able to perform their vital jobs. 

This sector's vulnerability to deliberate attacks and accidental issues caused by natural disasters, are a great concern to emergency services such as firefighters and hospitals and the other CI sectors that rely on a consistent water supply "such as Energy, Food and Agriculture, and Transportation Systems, [which] would suffer negative impacts from a denial of service in the Water and Wastewater Systems Sector.2"  

One of the most important processes undertaken by the water and wastewater sector is the cleaning of sewage, which has to meet standards set by the U.S. Environmental Protection Agency.

Wastewater treatment plants "remove solids and pollutants, break down organic matter and restore the oxygen content of treated water" so the clean water can safely be sent back to the environment by sending the untreated wastewater through "four sets of operations: preliminary, primary, secondary and sludge treatments.3"

1. Pretreatment Phase

During the pretreatment phase, large items are removed such as trash and natural debris using a rake mechanism. Some plants also utilize "basins and grit chambers of various types [to] regulate the rate of water inflow so that stones, sand and glass settle out.3" Other plants take this time to remove grease and oil, while some choose to save this until the next step.

2. Primary Treatment

During primary treatment, basins and tanks collect the water where it sits to allow sediment to sink and separate. The sunken sediment it scraped into equipment designed to hold small debris and any oil missed during the first step. 

3. Secondary Treatment

After primary treatment, air is introduced to the water in the basins to introduce microorganisms which break down organic matter. This produces sludge that is broken down in a variety of ways including running the water over biofilm, creating recyclable sludge by mixing organism laced water laced with waste material, introducing the water to wetlands which are known for breaking down organic material, or using "membrane bioreactors and biological aerated filters."3 No matter which method is used, the end product of this step is sent to a secondary clarifier tank.3

4. Sludge Treatment

In this final phase, once again gravity is used to separate water from "heavier grit, which can be deposited in a landfill" while "the remaining primary sludge passes to a thickener, where it is centrifuged and fed to digesting tanks containing anaerobic bacteria."3 This step produces two main byproduct:

1. Methane gas, which is often recycled into the plant as a power source; and

2. "Stabilized sludge" which is used to fertilize soil3.

After these byproducts are removed, "the water is treated to remove phosphorus, nitrogen and other nutrients, disinfected with chlorine, ozone or ultraviolet light and then returned to the water supply."3 

The equipment necessary to clean sewage, are controlled with Operational Technology (OT). OT is the "hardware and software that detects or causes a change, through the direct monitoring and/or control of industrial equipment, assets, processes and events."4

OT and the energy industry are essential to the functioning of the U.S. Apply for one of Capitol Technology University’s degrees in Critical Infrastructure to gain experience in Information Technology (IT), OT, and Cybersecurity learn about their applications in the 16 critical infrastructure sectors.  


  1. American Fuel & Petrochemical Manufacturers. (2020). How Refineries Work. Retrieved from
  2. Cybersecurity & Infrastructure Security Agency. Energy Sector. Retrieved from
  3. Sciencing. (2017, April 24). How Does a Waste Water Treatment Plant Work?. Retrieved from
  4. Gartner. Operational Technology (OT). Retrieved from