de-ozonation for treated potable water 3

[c4eng]

We are taking potable water to a residence and laboratory for a fish hatchery from a supply which has been ozonated. I am trying to determine the acceptable ozone limits for potable water, and best methods (least costly and safe) to de-ozone the water prior to consumption. Some surfing has suggested that UV or activated carbon has been used but have not been able to get guidelines or details.

Any suggestions??

 

[mmenarry]

How low in O3 levels do you need to go? As far as I can remember, O3 has a very short residence time.
Simplest method may be to install a storage tank to give a residence time before use that drops the O3 level through natural losses.
If it's only potable sources that are being supplied, then a the tank may not need to be that large at all.
Michael

 

[EngrInfo]

As mmenarry states, ozone has a very short residence time and would speculate that litte to no ozone would be available at the two connections.

This is groundwater correct? Otherwise if you are supplying surface water to the resident, you have many other things to consider.

 

[waterexpert]

Ozone has very high toxicity especially to fish. Contrary to common belief, traces of O3 can persist for quite long periods of time (hours). GAC contactors will remove any traces. GAC or thermal destruction is usually used to remove O3 from off gases at the treatment station. UV/O3 is used for other purposes than O3 destruction.

Regards
Seán

http://www.expertise-limited.co.uk

 

[c4eng]

Water supply is from a surface source. I am not sure what other things need to be considered before consumption other than the usual parameters as noted in the guidelines for drinking water. Are there concerns about compounds formed by ozone which may affect quality of surface water for drinking.

 

[anylev]

Sean, that's a very interesting comment re persistence of trace ozone. I'd like to know more on that particlularly.

Re. removal of trace ozone, i've seen sodium bisulphite dosed at the outlet to an ozone contactor in an ozone/biologial activated carbon system.
A sample of the BAC output was going to a freshwater fish tank at the plant. unfortunately the fish didn't generally make it to old age - those who checked said that it was because of DO fluctuations in the feed to the tank. Some species allegedly are fairly intolerent to do fluctuation. Not being an aquarium kind of guy, i take their word for it. i don't think anyone checked out the detail as to residual ozone or excess bisulphate.
AnyLev

 

[EngrInfo]

c4eng

Keywords are "least costly and safe". Considering the surface water aspect, the thing to consider is water quality and source protection. What appears to be good one day may be a highly turbid water with embedded cryptosporidium or giardia the next day after a rain. Even ozonation of a particulate may not adequately inactivated an oocysts that is encapsulated within the particle. Therefore, most surface waters will require a multibarrier process to make it “safe”.

Ozone is just one barrier. The second barrier must be a physical removal process such as a filter or strainer. Considering only two connections, the least costly may be a point-of-entry device at each connection that is rated for turbidity removal (more specifically, giardia and cryptosporidium removal if labeled). If you have a lot of turbidity, the point-of-entry device may plug quite frequently making it intolerable to operate. In this case, a pre-filter or bag filter could be used to increase maintenance time. So the correct straining system depends on the water quality.

 

[c4eng]

The surface water quality is expected to be relatively good most of the time. We are proposing a 5micron filter and activated carbon filter ahead of a pressurized storage tank, at the point of use, based on the expectation that the filters will remove some turbidity, odour, colour, and any residual O3. The storage is included to provide additional time for 03 degredation.

 

[PUMPDESIGNER]

Our method works great, but we have a 150,000 gallon holding tank.

1. Dump all ozonated water at the back of a holding tank as far away from the delivery pump intake as possible. This gives the O3 maximum contact time and time to go through numerous half lives.

3. Dump water into the holding tank through multiple outlets to prevent stirring or tank circulation.

2. Water goes through a UV light sterilizer just before passing to the consumer. Use only UV lights that do not produce additional O3.

PUMPDESIGNER

 

[c4eng]

150,000 gal. storage sounds like a lot for our application of a small lab and two bedroom apartment. What sort of demands were involved in your application?

 

[PUMPDESIGNER]

120 gpm max. Typical was 20-40 gpm.
Overseas application, power was intermittent, all water had to be made on site by RO from seawater, critical to not run out of water so large storage capacity was needed. Chlorine was not permitted.

PUMPDESIGNER

 

[PUMPDESIGNER]

I see that there is quite a lot of knowledge on this board.
My suggested method was obviously not useable on a small system. However we always attempt simple (sometimes too expensive however) with no maintenance required.

How 'bout a smaller tank with a lot of tight baffling to create a torturous long path for the ozonated water. Dump water gently into one end of the tank, make that water travel long slow path through baffles to get to the pump intake? Add a little air bubbling if absolutely necessary Get a system like that to work and you have little or no maintenance.

PUMPDESIGNER

 

[c4eng]

Assuming a maximum consumption rate of approx. 8gpm and 30 min. contact time, we are inclined to think that 250 Igal. of storage should be adequate. We can do this by adding a section of 24" pipe of suitable length. If an activated carbon filter is used at the end of pipe, we think that any residual ozone will be stripped out, along with any seasonal colour & odour which may occur.

 

[BobPE]

c4eng:

the contactor is a great choice, just remember for it to work it has to be vented to the atmosphere to allow for the offgassing of the ozone. It sounds like your design may not consider this. Ozone break through for ozone is extremely fast, so using this technology may not be appropriate since EBCT's will be huge to allow any type of realistic run time life for the carbon.

BobPE

 

[c4eng]

To BobPE

Could you explain EBCT? I am not familiar with the acronym.

 

[BobPE]

c4eng:

sure, sorry I speak technalise to save time lol...It means empty bed contact time. It is the measurement of how much carbon you will need to treat your targeted chemicals. It allows you to size the carbon contactor.

BobPE