Chemistry in Industry- water treatment

[leah3000]

hello

I'm studying industry and the environment as part of my chemistry A level course. Water and water treatment falls under this.

I'm having trouble understanding what exactly is meant by a disinfectant residual in the treatment of water. Can someone please explain this.

Also why are chloramines more widely used as a disinfectant if it causes the growth of bacteria? I thought the point of disinfecting was to eliminate the bacteria.

 

[turbo]

Let's say you are using chlorine to disinfect waste-water. You inject chlorine (in some form) into the waste stream and the waste spends some time in a reaction vessel or a series of reaction vessels so that the chlorine has time to kill pathogens. If there is not a residual of chlorine in the waste-water when it leaves the last reaction vessel, that is an indication that all the chlorine was taken up, and you might not have killed all the pathogens. If there is a residual of free chlorine in the waste-water leaving the reaction vessel, that is a sign that you have done a thorough job killing the organisms that can take up chlorine. This is a rough guideline, and wastewater treatment plant operators fine-tune the required residual by running biological tests (typically E Coli tests) on the effluent. If E Coli is still present at a given chlorine residual, it will be necessary to increase the feed rate of the disinfectant.

That stage of treatment is usually called primary treatment. Secondary treatment generally involves running the treated effluent through an aeration tank of some kind, where aerobic bacteria can continue to break down the waste. For this reason, you don't want to send waste to the secondary stage with a very high concentration of disinfectant - you don't want to kill off too many of the "good" bugs.

 

[chemisttree]

...Also why are chloramines more widely used as a disinfectant if it causes the growth of bacteria?...

Here's where you logic is wrong. Chloramines don't cause the growth of bacteria. It is somewhat less effective than free chlorine but it certainly doesn't cause the growth of bacteria.

 

[leah3000]

this explanation helps alot...thank you

 

[leah3000]

Here's where you logic is wrong. Chloramines don't cause the growth of bacteria. It is somewhat less effective than free chlorine but it certainly doesn't cause the growth of bacteria.

I think I phrased it incorrectly. What I'm questioning is the ammonia part...isn't that a nutrient that aids in the growth of bacteria?

 

[chemisttree]

I think I phrased it incorrectly. What I'm questioning is the ammonia part...isn't that a nutrient that aids in the growth of bacteria?

3 ppm isn't going to grow many bacteria. If there is residual present, no growth is possible since they've been effectively killed.

 

[leah3000]

ok thank you for your help

 

[clustro]

Not to be contrary, but I thought the microbe they usually tested for was cryptosporidium, because it easily incubates (right word...?) if things in the treatment plant are not operating correctly, and is a very dangerous contaminant.

 

[turbo]

Not to be contrary, but I thought the microbe they usually tested for was cryptosporidium, because it easily incubates (right word...?) if things in the treatment plant are not operating correctly, and is a very dangerous contaminant.

Things may have changed since I let my license lapse ~25 years ago. We tested for coliforms then. It's a simple test and VERY sensitive.

 

[nhfs49]

The Chloramine is used widely because he has a less effect on taste and odor in water.

 

[chemisttree]

The Chloramine is used widely because he has a less effect on taste and odor in water.

I believe it is used because chlorine reacts with stuff to produce carcinogens. This doesn't happen with the chloramine. http://www.epa.gov/ogwdw000/mdbp/pdf/alter/chapt_6.pdf