Wastewater is a downstream contaminated byproduct from any domestic, industrial, or commercial activity with various characteristics depending on its source, and wastewater requires treatment so that it can be either reused or disposed of safely.
Strict environmental regulations require that wastewater streams be treated before reuse or discharge to the environment and this generally involves a combination of physical, chemical, and biological processes.
In the outline below, you can see the various steps in the wastewater treatment process, followed by a detailed explanation of each process:
(click the image to enlarge)
Conveyance System – Collection Sewers
The wastewater treatment collection sewers are a network of channels, pipes and other equipment that collects the wastewater from various sources, such as industrial and residential wastewater, and conveys the stream to the treatment plant.
Bar Screening / Pumping System
The process of wastewater treatment starts with physical screening to remove large debris that may be present in the wastewater. This prevents damage to equipment during the next steps. The objects removed by screening are then taken to a landfill and the influent is pumped to the settling tank.
Primary Wastewater Treatment – Settling Tank
During this step, solids that sink to the bottom of the settling tank are known as sludge or organic matter. This sludge is pumped from the bottom of the tank for the sludge dewatering/processing area for disposal. The influent is then pumped to the aeration tank for further processing and treatment.
Primary Wastewater Treatment – Aeration Tank
Pumped air is introduced into the aeration tank to promote the conversion of ammonia (NH3) to nitrate (N0-3) and giving the bacteria oxygen so that they can continue to grow. The bacteria then break down the organic matter, reducing the biochemical oxygen demand (BOD) levels (the level of organic matter that can be biologically oxidized and broken down into smaller molecules). There are three types of aeration methods:
- Fine bubble diffusers: fine bubbles introduce air into the water, helping to transfer oxygen to the water and boosting air-water contact.
- Coarse bubble diffusers: coarse bubbles are used to make the wastewater mixing process more effective and this method is best for standard aeration.
- Surface aeration: used when a large supply of oxygen is required quickly, as this method creates a greater surface area, promoting gas exchange and increasing oxygen levels.
Secondary Wastewater Treatment – Biological Treatment
In the next step after the aeration tank, the treated wastewater is pumped to clarifiers for secondary treatment where fine solids (activated sludge) settle at the bottom of the tank. Some of the activated sludge then goes back to the aeration tank for further biological degradation. Excess sludge is disposed of and the remaining wastewater, with low organic matter levels, continues to the disinfection process.
Disinfection is necessary to make sure that the water returned to the environment does not contain levels of bacteria that are higher than permitted. The addition of chlorine is the most common and economical method of disinfection, however ozone and UV treatment are also sometimes used.
Tertiary Wastewater Treatment
This is the final step of treating the actual wastewater, also known as the polishing stage. Tertiary treatment is required to remove harmful bacteria, viruses, parasites, inorganics, nitrogen and phosphorus, so that it can be safely discharged back into the environment or reused for industrial or irrigation applications. Several methods of wastewater treatment may be applied, depending on its final use, including filtration, coagulation, chlorination, ozonation, UV treatment, activated carbon absorption and ion exchange.
Sludge is the organic and inorganic sediment accumulated during the wastewater treatment processes mentioned above.
Dewatering reduces the volume and weight by separating the sludge into liquid and solid forms so that it can be effectively disposed of by adding it to landfill or for use as fertilizer in agricultural applications. Sludge can also be mixed with other substances to produce biosolid compost for landscape mulch. Filtration plays an important role in dewatering by protecting the spray nozzles used during this step and keeping the system running efficiently.
The Benefits of Wastewater Treatment
As we explained in detail above, the various steps in the wastewater treatment process are used to eliminate pollutants, thus making the treated water safe for reuse and discharge back into the environment. Wastewater treatment benefits may vary based on the source of the wastewater and its final use. Here are some of the most important benefits:
1. Less environmental impact
Wastewater treatment removes harmful chemicals and bacteria, making it safe to discharge back into the environment.
2. Less demand on the freshwater supply
Water scarcity is becoming more and more severe as global populations continue to grow. By reusing treated wastewater for industry and agriculture, the demand on precious freshwater sources is decreased.
3. More energy efficient
The sludge produced during the wastewater treatment process contains biodegradable material that can be treated with anaerobic bacteria. This step produces a gas with high levels of methane which can then be collected and heated to generate electricity.
4. Compliance with local regulations
Effluent discharge guidelines and regulations for wastewater vary from country to country and even from state to state. Complying with regulations will save costly fees and protect the environment.
You can read more about how filtration solutions for tertiary wastewater treatment were implemented in this case study:
Containerized Tertiary Wastewater Treatment, Ireland
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