Back wash water- with or without chlorine

[nilaypathak]

Hello,

Our water treatment plant has three filter. Plant design to pump back wash water from clear well after chlorine injection. Thus, plant had de chlor sytem prior to discharge to over land channel.

My question - if we pull filter water without chlorine, what would be pro and cons?

What would be the impact on the performace of filter?

Thank you for your time .

R

 

[QualityTime]

If you backwash with unchlorinated water you have contaminated the filtered chlorinated water. You basically have created a cross connection. Nobody backwashes a filter with unchlorinated water

 

[djs]

All of the plants I have been in, both water and wastewater, the filter is before the chlorination.

 

[ashtree]

Many plants backwash with unchlorinated water. In some cases unchlorinated water will actually improve performance because it allows some biological activity to develop in the filter medaiu. This can sometimes reduce the organic carbon removal efficiency.

Backwashing with chlorinated water is not uncommon though.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[QualityTime]

Many plants backwash with unchlorinated water. In some cases unchlorinated water will actually improve performance because it allows some biological activity to develop in the filter medaiu. This can sometimes reduce the organic carbon removal efficiency.

Backwashing with chlorinated water is not uncommon though.

Ashtree, I respect your contributions to this board but I have heard this argument before about allowing biological activity in the filter media to reduce organic content in the treated water.

My response is that water was chlorinated to remove pathogens etc. That is the reason why everyone stopped getting sick. Chlorination of water supplies probably started to take hold 100 years ago. It really is not that long ago. Researchers are trying to get their articles published with the next great discovery. Because of that, I think sometimes we go too far and can't see the forest because of the trees.

By allowing biological growth to grow in the filter media you are, in fact, building up live bugs. Live bugs shit and pee like you and me. How does this "water treatment" process distinguish between good and potentially bad biological organisms growing in the media? By using unchlorinated water for backwashing you are essentially using raw water to backwash; double jeopardy. Long story short is that something does not make sense. Sounds like we are contaminating our water supply again

 

[ashtree]

A couple of points to consider.

Filters remove bugs primarily filtering out the particles that the bugs are living on. Media filters do not remove much below about 10um give or take a bit and many bugs are less than that. Bacteria can range in size between 0.1 and 100um. Ecoli is about 1um long. So a media filter is not a 100% barrier anyway, but probably removes 99% or 99.9% of bugs that are applied. Some get through and that is one of the reasons that we disinfect. There are also many bugs in the system that pose no particular hazard to human health and may or may not be killed off with chlorine. Thats why chlorination is called disinfection not sterilisation.

Running filters at too high of filtration rate, or past the point of breakthrough will increase the passage of bugs.

Most of the bugs are removed from the filters during backwash. This is through two mechanisms. The bed expands and the water lifts the dirt particles from the bed and carries them away. Those dirt particled have various bugs in them and on them. Some bugs that are clinging to the filter media will be rubbed off by the abrasive action of the particles colliding and the stream of water running past the media. But some bugs may be left.

Backwashing with chlorinated water will kill off some bugs no doubt but the contact time is relatively short, no more than a couple of minutes probably and the chlorine residual if taken from the clearwell or a service reservoir is probably no more that 3mg/l if that. However the chlorine demand is likely to be quite high given that the filters have trapped all sorts of stuff including a heap of bugs during the last filter run. So some bugs may be killed off by the chlorine, some will be backwashed out and some may be left behind.

A Google search will soon pull up various research papers on the impacts of having a biologically active filter bed. Usually its considered to slightly reduce the potential to form disinfection biproducts. Whilst some plants report significant reductions others report negligible to nil reduction. Those that report the best reductions usually have unchlorinated backwash, however unchlorinated backwash is not neccessarily a guarantee of such removals. Having ozone ahead of filters with a layer of GAC over the media tend to work even better.

I do process work for a Utility with 9 water treatment plants. We have two that use unchlorinated water and the rest chlorinated. One of the two that has unchlorinated backwash consistently has the lowest DBPs but there are so many things that influence DBP formation I don't think you could point to the unchlorinated backwash and say thats the answer. However it probably contributes.




Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[ashtree]

Just another point from an earlier post.

Backwashing with unchlorinated water does not create a cross connection because the filter outlet valve will be closed to allow the entry of backwash water.

The backwashed will come out of the top of the filter above the media somewhere, typically a back wash trough(this depends on the filter design and style).

Provided the filter is run to waste for a time after the backwash to allow the turbidity to return to normal then only filtered water should reach the clearwell.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[QualityTime]

I respect what you are saying Ashtree.

Plants that employ biogrowth in their filters are far and few inbetween

Water treatment plants typically prechlorinate the raw water. The typical minimum contact time from the point of raw water prechlorination to the filter inlet is at least 0.5 hours in a direct filtration plant at design full plant flow. Longer if you have sedimentation tanks in a conventional water treatment plant. Because of that there is nothing growing in the filters. Nothing equates to NO bugs AT ALL in the filter. No bugs means no chance of anybody getting sick.

Prechlorination cannot be practiced, when you are trying to achieve biogrowth in the filter, because the chlorine will kill the growth in the filters. That is why they use ozone (combined with de-ozonation upstream of the filters)and maybe UV instead of prechlorinating. Have you ever seen the growth in a backwash waste water holding tank after say one or two weeks? If you have it will start to make you think

Using unchlorinated backwash water to backwash a filter does not make sense...especially if is a standard non biological filter

To me, this business of growing bugs in a filter to eat the organic compounds really requires more explanation to the treatment plant design engineers

BTW a filter outlet valve is typically a butterfly valve and not a backflow preventer

 

[ashtree]

Another few points to consider.

"Water treatment plants typically prechlorinate the raw water."

There is no doubt some do but many don't. Prechlorination tends to increase the formation of DBPs and seriously increases the amount of chlorine required. Some prechlorinate for other reasons, eg: oxidation of iron or manganese.

You are correct that a butterfly valve is not a backflow prevention device. Whilst i would be certain that some plants have only a butterfly valve between the filter and the clearwell many do not. Some build the filters above the clearwell so that there is an airgap. Some have multiple valves between the filter and the clearwell. It is not unusual to have a rate control valve and a gate valve on the outlet , both of which will be shut during a backwash. There are many variations in the design of filter outlet and backwash pipework, some are good and some not so good.

Because of the range of plant designs, operating environments, and water quality it is difficult to be absolute definite about recommending any particular strategy.

However the OP wanted to know the Pros and Cons. Is that not what we have attempted to provide?

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[QualityTime]

DBPs such as THMs can be caused by chlorinating coloured (organics) water. The lower the chlorine dose applied to colored water the lower the THMs. There are many strategies to reduce THMs in colored water; two of which are:

[ol 1]
[li]Lightly prechlorinate; add chemicals to flocc and settle out the color; then top up the chlorine to ensure there is a residual going onto the filters[/li]
[li]Add chemicals to flocc and settle out the color; then add prechlorine to ensure there is a residual going onto the filters[/li]
[/ol]

Ashtree, it is interesting that out of the 9 water treatment plants you work at 7 use chlorinated backwash water. Without knowing the process do all 9 plants meet the DBP guidelines?

In the past what was found was that filters that did not have some level of chlorinated water passing through it experienced filter cracking and filter growth (due to organic bugs). That is why they went back to prechlorinating. With the advent of ozone and UV it will serve the same purpose as chlorine but there is no residual being carried onto the filters to keep it "clean".

 

[ashtree]

One plant that has never had chlorinated backwash and has never prechlorinated has also never had a THM exceedance, and does not experience cracking in its filters.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[bimr]

Continuous chlorination of filters is not common. Having said that, there may be a need for periodic disinfection of the filters.

 

[QualityTime]

I respectfully disagree with you bimr. Prechlorination of raw water is very common