Chlorine, ozone, or UV light: How is wastewater disinfected?

Disinfection is the last and one of the crucial steps of wastewater treatment. Why? Read our article.

Wastewater is often used for irrigation. Singaporeans even drink it. Before it is released back to the ecosystem, wastewater must be thoroughly treated and disinfected. Today, we are going to cover the disinfection part.

Wastewater means any used water discharged into sewers.

The good news is that there are wastewater treatment plants which treat the water and renew its original quality in order to be reused.

Treated wastewater can be re-used for irrigation, cleaning of public space, fire extinguishing, or industrial cooling. Singaporeans even drink it.

Naturally, only the water causing no harm is released back to the system.

Disinfection and its purpose

How can you remove all the residual harmful substances from the wastewater after the removal of the strongest contamination? The answer is disinfection. It is the last and one of the crucial steps of wastewater treatment.

At this stage, all harmful viruses, bacteria, fungi, or other pathogens are inactivated or killed.

If they remained in water, the irrigated soil could be damaged or, even worse, they could cause serious health problems, such as diarrhea, vomiting, typhus, hepatitis, or salmonella.

disinfection of wastewater

And no one wants this to happen. Therefore, the water gets disinfected to renew the its original quality.

Chlorine as the safest method

There are three methods of wastewater disinfection. The most frequently used method is chlorine that enters the water in liquid form.

This chemical exterminates a large amount of pathogens. Even if they are not killed, they are inactivated to such extent that they are no longer able to reproduce.

The benefit of chlorination is that the water remains disinfected long after the chlorine application, making this method highly effective.

Wastewater treatment plants also use chlorine on a large scale for pragmatic reasons: it is very easy to dose and also cheap when compared to other disinfection alternatives.

disinfection of wastewater by chlorine
Chlorine is stored under high pressure and enters the wastewater in gaseous form.

On the other hand, the disadvantages must be also considered. The chlorine residues in the wastewater are very toxic. This means that the chlorine could kill the microorganisms after the water is re-used (e.g. for irrigation).

Furthermore, some viruses are beyond the capability of chlorine so some harmful substances still remain in the water.

Chlorination poses a huge risk both underwater and out of water. Since chlorine is a toxic caustic, it must be stored and transported with utmost caution.

Ultraviolet disinfection

The second method we will cover is the ultraviolet disinfection which is being increasingly used to treat municipal wastewater.

Unlike chemical treatment with chlorine, UV disinfection is a physical process with higher safety standards.

When the water lands in a designated spot, it is exposed to ultraviolet irradiation which kills undesirable microorganisms, leaving them unable to reproduce.

ultraviolet disinfection of wastewater

What’s even better, the UV light mostly inactivates viruses and bacteria, with no residues of harmful substances. The process is very simple and does not require a large area for irradiation.

Given that it is a safe and ecological alternative, it is no surprise that UV light is growing in popularity.

However, to be fair, we also have to mention what speaks to its disadvantage. Just like chlorine, the ultraviolet light is also not capable of destroying all the dangerous substances in the water.

Furthermore, it takes more time to kill some of the microorganisms. The entire process is prolonged and requires more doses of UV light.

Compared to chlorine, the UV light treatment is significantly more expensive. The WWTP operators think twice before choosing the most suitable method.

Ozone removes both viruses and turbidity

E. coli is a bacteria commonly found in the lower intestine. Some E. coli mutations are not vital since they are harmful. For instance, an undesirable E. coli strain causes diarrhea, pneumonia, or infection of the urinary tract.

As you might think, this bacteria can also be commonly found in the contaminated wastewater. Disinfection, however, suffices for its removal. Ozone, for instance, kills up to 99% of the harmful E. coli bacteria.

wastewater disinfection by ozone

It percolates into the water in gaseous form to do two things: treat the water and remove all harmful microorganisms.

Ozone is quite good at it. The great benefit of ozone is its fast spread. However, it requires higher concentration, and thus more ozone, to inactivate some of the bacteria and viruses.

Some of you might be discouraged by the costs of this method. In addition, considering the higher dosage, the ozone treatment results in much higher costs than the chlorination.

If the WWTP operators care more about the water quality than their expenses, ozone is a very good choice since it removes both harmful viruses and turbidity, neither chlorine or UV light can.

Ozone disinfection takes only up to 30 minutes so the water is cleaned in a short time.

If you need to design a wastewater treatment solution tailored to your needs within your price range, feel free to contact us. The Hydrotech experts will be happy to help.

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