Predatory bacteria could replace chlorine in purifying drinking water

Today, the final stage of purifying drinking water is almost always the addition of chlorine.

Sweden removed chlorine in 2020

However, in Varberg, Sweden chlorine was removed in 2020 following the installation of ultrafiltration, a filter that uses an extremely fine-meshed sieve to prevent harmful microorganisms from slipping through from lakes and groundwater. 

The researchers then analyzed water samples over a period of 6 months and a year and identified all the bacteria in them. 

“It’s as if a new restaurant had moved in. Chloramine kills certain bacteria, but we could see it was feeding other bacteria. During the period we could see how the bacterial buffet changed,” said Paul.

What happened?

Without chlorine present, some types of bacteria starved while others grew and thrived. In the third chlorine-free month, certain bacteria had drastically decreased in number, but one special type of bacteria had increased: the predatory bacteria Bdellovibrio.

“We have not seen this exact type of bacteria in previous studies of this drinking water network. It has probably been lying concealed in the biofilm but was now given an opportunity. It’s totally harmless for us humans,” said Paul.

The researchers concluded that chlorine became superfluous when the ultrafilter was introduced.

Next, the researchers asked one crucial question: could it perhaps be possible to add a mix of bacteria instead of chlorine?

“More studies are required in order to better understand how nature works within urban and built environments. I would really like to know how all the drinking water bacteria affect us. They are not harmful, but could they even be good for us?” said Paul.

“Our conclusion is thus that it is possible to have safe and clean drinking water without chlorine. We observed no increased risk, which of course was very important for the water company and their customers.”

The researchers also pointed to other ways to purify water without chlorine, such as UV light or biofilters.

“Each method has its advantages and disadvantages. UV light is an effective method, but one disadvantage is that the lamps use a lot of energy. Biofilters often don’t require any energy at all but take up a considerable amount of space. Ultrafilters are expensive. Many drinking water treatment plants in Sweden purify water using a combination of methods. However, our study shows that chlorine is not essential if you have other strategies to deal with, and monitor, bacteria,” concluded Paul in the statement.