In the 21st century, clean water and environmentally friendly industrial processes are more critical than ever. According to the World Health Organization (WHO), contaminated drinking water causes 485,000 diarrheal deaths every year. The U.S. Centers for Disease Control and Prevention also reports that the United States saw almost 7,000 deaths and 477,000 emergency department visits in 2017 because of the 13 most common water-related illnesses.

Fortunately, proper treatment can help commercial and industrial facilities become more environmentally responsible and compliant in their wastewater disposal.

What Is Wastewater and Wastewater Treatment?

Before we delve into some of the details of the main wastewater treatment methods, let’s first discuss what wastewater and wastewater treatment are.

What Is Wastewater?

What is wastewater? Wastewater is any kind of used water affected by domestic, commercial or industrial processes. The used water that flows out of a person’s house, a school or an office building is wastewater. So is the water produced during industrial operations like making paper or clothes or manufacturing chemical products.

The composition of most wastewater is about 99.9% water by weight — the remaining 0.1% consists of contaminants like organic matter, inorganic matter and microorganisms that require removal.

What Is Wastewater Treatment?

Wastewater treatment applies special processes to wastewater to remove organic matter and other contaminants. Wastewater treatment programs aim to make the wastewater clean and safe enough to discharge back into the environment without harming nearby residents or the local ecosystem.

A city might use a wastewater treatment plant to clean sewage and stormwater. An industrial production facility might have an on-site wastewater treatment plant to decontaminate its chemical-filled process water, or it might partner with nearby facilities for wastewater treatment.

The Four Types of Wastewater Treatment Systems

Wastewater treatment systems break down into four main categories. Below, we’ll discuss the different types of sewage treatment systems and other wastewater treatment solutions and explain how they handle different types of wastewater:


1. Sewage Treatment Plants (STPs)

Sewage treatment plants remove contaminants from wastewater. An example of an STP is the treatment plant you might find in a large American city. This plant would receive sewage waste from households and commercial buildings and sometimes get an influx of commercial wastewater as well. It would also receive rainwater and debris from storm sewers.

An STP such as this plays an integral role in keeping residents safe and healthy by cleaning their wastewater with a combination of physical, chemical and biological treatment before discharging it into the environment.

When wastewater reaches a sewage treatment plant, it first goes through a basic filtering procedure. It flows through screens and into settlement basins that can sluice out debris. Debris is incredibly hard on an STP’s tanks and equipment. It can cause excessive wear and tear and diminish the equipment’s life span, so it’s essential to remove as much of the dirt, grit and sand from the wastewater as possible.

This settling is often referred to as preliminary treatment because it occurs before the three more aggressive treatment phases. Many STPs are known as three-stage because they process their wastewater in three principal steps: primary, secondary and tertiary treatment.

Primary Treatment

During primary treatment, the wastewater flows into primary clarifiers. In these tanks, as in the settlement basins, wastewater flows sluggishly. The tanks’ design promotes settling — organic solids collect at the bottom of the tank, and lighter substances float to the top for removal.

The organic solids that settle to the bottom form what’s known as a primary sludge blanket. After a few hours in the clarification tanks, this primary sludge moves into aeration basins for the operation that forms the backbone of most STPs — the activated sludge process.

Secondary Treatment

Secondary treatment often consists of aerobic aeration. Aeration basins contain aerators — systems of pipes with discs or tubes attached. The discs or tubes — often made of ceramic or rubber membranes — have small holes for air to flow through. When air flows through the aerators, the small holes turn it into bubbles that mix into the water column. The interaction of oxygen with the bacteria in the sewage causes the bacteria to digest the organic matter that gives wastewater its characteristic appearance and smell.

This technique typically also removes most harmful chemicals from the wastewater, though it cannot remove nitrates. Some STPs have additional processes for nitrate reduction at this stage.

Once aeration is complete, the wastewater flows into the next series of tanks — the secondary clarification basins. In these basins, the bacteria spend a day or two settling to the bottom and forming a sludge blanket, which the plant can then pump out.

When the sludge blanket has settled, it becomes return activated sludge (RAS). The RAS goes back into the primary clarification tanks, where the bacteria it contains will assist with breaking down organic matter in the incoming sewage.

Once RAS has cycled through the primary and secondary clarification basins several times, it becomes waste-activated sludge (WAS). Rather than going back to the primary clarification tank to break down organic matter, WAS moves on to covered tanks called aerobic sludge digesters. In these tanks, instead of digesting the organic matter in wastewater, the bacteria in the activated sludge begin to digest one another. In time, most of the sludge will disappear.

Finally, the last dregs of the sludge move to the dewatering facility. In dewatering tanks, the plant uses belt presses to squeeze any remaining water out of the remnants of the activated sludge.

Tertiary Treatment

So far, we’ve traced the steps of primary treatment, or settling, and secondary treatment, or aeration and activated sludge processing, which occur in most plants. Tertiary treatment follows these processes and combines mechanical and photochemical methods into one final step. Not all wastewaters require advanced tertiary treatment — this process is particularly helpful for sanitary sewage with microorganism contaminants that require disinfecting.

In tertiary waste treatment, the wastewater flows through sand filters, which remove any remaining fine particulate matter. The water then flows under banks of ultraviolet (UV) lights, which irradiate bacteria and viruses and remove their infectious capabilities. The UV lights don’t always kill these microorganisms, but they render them sterile and safe.

Once the wastewater in an STP has undergone the three treatment stages, it can flow into the environment as effluent. It is clean and healthy enough for wildlife, plants and humans to come into contact with. They won’t notice strange smells or odors or become sick.


2. Effluent Treatment Plants (ETPs)

We’ve seen that sewage treatment plants provide initial treatment primarily for municipal and domestic wastewater. Effluent treatment plants, on the other hand, typically clean industrial wastewater. They treat industrial effluent — the wastewater that flows out as a byproduct of these industries.

ETPs are often useful in sectors with high likelihoods of extensive chemical contamination in their wastewater — for instance, pharmaceuticals manufacturing and chemical industries like textile and dye manufacturing. Plants cannot discharge these chemical contaminants into the environment without first treating them thoroughly. Different companies may contend with varying wastewater compositions and require slightly different ETPs.

Like STPs, ETPs typically involve preliminary, primary, secondary and tertiary treatment of the wastewater. The processes used may vary in response to the characteristics of the wastewater in question — for instance, its suspended solids concentration, temperature, color, hardness or alkalinity, or the presence of contaminants like metals, agricultural chemicals or microorganisms. Here’s a more detailed look at each stage:

  • Preliminary treatment: Preliminary treatment for ETPs often consists of physical techniques like screening, sedimentation, filtration, flow equalization and clarification. The goal is to reduce the amount of solid physical waste present before sending the effluent on for further treatment.
  • Primary treatment: Primary treatment in ETPs targets the removal of additional solid waste and organic matter. Primary treatment methods often involve adding chemicals that can break up solid and chemical waste. Techniques may include chemical coagulation, chemical precipitation, dissolved air flotation, flocculation and the addition of sodium carbonate or hydrochloric acid to control pH levels.
  • Secondary treatment: Secondary treatment for ETPs typically consists of removing suspended particles and biodegradable organic matter. Standard techniques at this stage include many of the chemical approaches used in primary treatment, as well as biological processes that can help biodegrade organic waste. Common biological processes in secondary ETP treatment include the suspended-growth method or the attached-growth or fixed-film technique. These two processes use beneficial digestive microorganisms, either suspended freely in the water or attached to fixed media.
  • Tertiary treatment: Tertiary treatment in ETPs uses a combination of physical, chemical and biological techniques to remove any remaining solids and contaminants from the industrial effluent. This step incorporates many of the processes from primary and secondary treatment and tailors them to the more advanced removal of smaller particles.

ETPs are beneficial for large operations, but they can pose challenges for smaller ones. They are large and expensive to build and maintain, and they often require specialized technicians for operations and maintenance. Though larger facilities can easily supply these needs, smaller ones look to alternative solutions.


3. Activated Sludge Plants (ASPs)

Activated sludge plants are similar to sewage treatment plants. They treat sewage wastewater using primarily activated sludge to digest biological contaminants. Though an STP may also use activated sludge in its secondary treatment, activated sludge takes on more of a primary role in an ASP.

Activated sludge processes use dissolved oxygen to promote the growth of clumps of organic matter known as biological flocs. The biological flocs then help break down contaminants in the wastewater. They trap particulates and can also clean the wastewater by converting ammonia to nitrites and nitrates and eventually into harmless nitrogen gas.

How does the process in an ASP work? During activated sludge treatment, the ASP first uses blowers to blow air into raw, unsettled sewage. The air breaks up the solid matter and turns the raw sewage into a soupier liquid. The biological components in the mixture then digest much of the organic matter and contaminants.

The first step of blowing air into the sewage must occur over a sustained period in a digestion chamber so the dissolved oxygen can work properly. Once that step is complete, some of the sewage liquor undergoes discharge into a clarification chamber. Here, the sewage liquor sits and settles. The live bacteria in the mixture settle to the bottom.

The dead bacteria rise to the top of the chamber, where they form a crust on the sewage liquid. In the middle, a clear liquid forms. This liquid is clean water that the plant can discharge into a soakway or watercourse for further treatment. The live bacteria left at the bottom of the clarification chamber make up activated sludge — “activated” because they are live and ready to digest organic matter, and “sludge” because they form a filmy, sludgy substance.

Once the layer of activated sludge has formed, the plant transfers it back into the digestion chamber, just as in a standard STP. The bacteria in the activated sludge then break down organic matter in the flow of new raw sewage entering the tank.

ASPs differ slightly from standard STPs in their construction. Most STPs contain a primary settlement chamber that requires regular emptying. Instead of forming activated sludge, these settlement chambers form anaerobic sludge that cannot aid in organic matter breakdown and instead require removal. ASPs deal only with activated sludge, so they do not contain these settlement chambers and do not require the removal of anaerobic sludge.

Unlike anaerobic sludge, which can give off a foul smell, activated sludge has less odor. It is easier to work with in less-isolated settings where smell is a concern.

ASPs offer a few other benefits compared to standard STPs:

  • Simpler process.
  • More dependable results.
  • Easy ability to reseed the digestion chamber with beneficial bacteria.
  • Fewer unpleasant smells.
  • Longer intervals between tank emptying.
  • Less unwanted anaerobic sludge to remove.
  • Heightened economical maintenance.
  • No moving parts that might break or become fouled.
  • Convenient upkeep of not having to top off the tank with extra bacteria.

They are often useful in situations that prioritize advantages like convenience, cost-effectiveness and simplicity.

4. Common and Combined Effluent Treatment Plants (CEPTs)

Common and combined effluent treatment plants exist to help smaller wastewater generators treat their effluent. Smaller manufacturing companies and other operations that produce wastewater — tanneries, for instance — cannot always afford to run their own effluent treatment plants. They do not have the budgets to maintain these large, complex structures and operate them safely and effectively.

CEPTs offer a solution. CEPTs allow several different wastewater-generating facilities in an industrial cluster to pool their effluent in a centralized plant for treatment. All the small facilities that send their wastewater to the CEPT share the expenses of maintaining and operating the plant. In return, they get clean, compliant wastewater without the hassle of running multiple full-scale plants.


Make the Environmental Recovery Corporation Your Trusted Choice for Wastewater Management

To gain the peace of mind of being responsible and environmentally conscious in your wastewater treatment and disposal, work with ERC. We are industry leaders in all types of waste disposal, and we can provide you with customized, turnkey solutions that let you spend less time on waste management and more time focusing on your business’s core aspects.

Contact us today to take advantage of our industrial wastewater treatment, chemical waste management or sludge and solvent waste removal services. Or, learn more about what proper waste management entails by visiting our blog.