Lea, I think your question is a multi-million-dollar one that can not be answered like a snap as you seem to want. First it depends on your treatment targets. When the effluent is treated with Ozone what are the targets or discharge standards? You probably already know that toxicity is indeed a function of dose and exposure time. The amount of Ozone used to create a certain effect say to obtain a certain discharge standard will vary depending on the exposure time. The reverse is also true. I think you could answer this question quite easily yourself. Test the effluent you are dealing with for the parameters that the Ozone is used to reduce. Treat that same effluent with Ozone in batches for different cycle periods. The Cycle length that gives you the equivalent parameters as your anticipated effluent will be your required exposure time.

Dear Ms. Tan,

Your question cannot be answered as you do not have a mass rate production from your ozone generator in your question. We would need g or mg/min to answer the question. Then we would really also need the ozone demand of samples you are ozonating.

Assuming the value in the title of your question means you are able to add 0.3 mg of ozone to each 1L per minute, your ozone production is 3 mg/min or 180 mg/h?

If so, I would suggest you may want to run your ozonation for at least an hour and see what happens in the toxicity removal.

Hans van Leeuwen, Iowa State University

Dear,

In fact, the generation capacity of your generator is of minor importance. What matters, is how fast and efficient the ozone is transferred to the water phase. These properties will determine the ozone exposure. In the end, ozone exposure, and not ozone production, is the value that counts. E.g. it is perfectly possible that you have a high capacity ozone generator but you only have a poor transfer.

I would highly recommend you to calculate the mass balance of ozone in the gaseous phase. In fact, you have to measure your initial gas concentration and the O3 concentration in the offgas. This will allow you obtain information on how much ozone is actually transferred to the water phase. This transferred ozone will either react with the water matrix or will give rise to aqueous ozone building up. As such, I would also recommend you to measure the aqueous ozone concentration. Together with the mass balance, these data will allow you to have a very detailed picture of the ozone exposure.

Regarding your question related to optimal contact time:

A lot will depend on 1) if you want to either disinfect or oxidize and 2) which is the reactivity of your water matrix. For disinfection, often one requires a ct calculation (which is the integral of the liquid ozone concentration vs time, with unit mg L-1 s). But if you have highly loaded water (e.g. the DOC and concentration of other ozone reactive species is high) it will be difficult to obtain liquid ozone with a small capacity generator, especially if the transfer effeciency is low. However, you will still have disinfection but you will not be able to calculate ct.

To conclude, the capacity of your generator is of minor importance. All depends on how effecient your transfer is (e.g. how small your bubbles are, what's the height of your reactor), what you want to study (disinfection/oxidation) and what the ozone consumption speed (i.e. reactivity towards ozone) of your water matrix is (e.g. surface water/drinking water will consume less ozone compared to secondary wastewater effluent).

I agree with Ronald: it is necessary to know what is the objective of the treatement.

If the ozone is inyected as gas (if it not previously mixed with water i.e. using a venturi injector) also is very important to know the gas-liquid mass transfer kinetic (KLa). The mass transfer is strongly dependant of the gas bubble size and liquid column height (because the real contact time is the elapsed for the bubble to go from the bottom to the top of the reactor). In case of ozone, mass transfer also may be affected by the pH (by OH* radicals formation). The KLa must be determinated (or estimated) for each experimental system. Some systems are controlled by the mass transfer and others by the reaction kinetics. The control depends of the slower one. Thus, the KLa is very important when the reactor operates in mass-transfer controled regime.

So, if you want an answer based on calculations you need to know: 1-the chemical kinetics between ozone and your target parameter/s (it may be affected also by all the ozone consuming compounds present in the liquid) 2-Mass transfer rate of your system. Both data are difficult to be correctly determinated or estimated.

In my opinion, if you don´t have the above information, the best way is just start to try and adjust the optimal contact time experimentally as Ronald suggest.

Depends on the ozone dosage needed, and the mass transfer efficiency of ozone into the water which you are using.

I'd agree with everything mentioned above.

As usual with questions here there is often key information missing in order to answer the question clearly.

Experiment Unknowns:

1. Why are you treating with ozone?

2. What are you treating with ozone (water or wastewater or soil or sludge) ,

3. What are you trying to oxidize?

4. What do you consider a successful test?

Since you mentioned reactor, I will have to base my comments on you treating waste water. You also mentioned toxicity, so your either disinfecting or your trying not to destroy the activated sludge at best guess.

Your best chance of determining the optimal contact time is to treat with ozone and sample at regular intervals to test against your required parameters.

Not only is mass transfer based on the surface area of the gas bubble, but also pressure/pH/temp. Ozone's mass transfer maximizes at 90psi, there's also an optimal temp and ph and both to my memory are low (below 5.0pH and below 75F), but I cannot remember them exactly.

You made no mention of your bench sample being under pressure so that option is probably not on the table for you.

So I would suggest bench testing at least three batches, making sure to test for your parameters and values prior to ozone exposure, then same and test at 30 and 60 minute intervals. Also make sure your bench testing in glass or steel as even the most resistant plastics will absorb ozone and possible sqew the results.

Doing this gives you your before and your after and the math needed to determine what the per minute and per hour ozone oxidation rates are.

On a secondary not, if this ozone generator is old enough, your probably best to replace it with a newer unit that has been recently calibrated to determine gas production. Otherwise your working with one arm behind your back as a loose or broken wire, ages capacitor or a dirty/worn electrode in the corona tube can prevent it from producing optimal and even ozone rates.

Best of luck with your experiment!!

Thank you everyone for all your comments and advice. I will take everything here and try to apply it to my bench experiment as I can.

Regarding the information:

1. I will be treating effluents from secondary treated municipal wastewater (from different material MBRs and conventional AS).

2. I don't have a specific target compounds as I am doing whole toxicity through bioassays but I guess I am targeting micropollutants such as PPCPs.

3. Ozone concentration is at 2mg/L and it is transferred through a rubber tubing that is immerse in the water.

Unfortunately, I am not able to fully explore the kinetics of my water samples with the ozonation. I was hoping to figure out the general rule of thumb or the commercially advice contact time with ozone and water samples. My master's research deals with exploring different reclamation system series to pair with different secondary treated effluents and which shows the most effective method/treatment series to reduce toxicity to be use as raw drinking water source. I was only looking into membrane filtration before but was suggested to include chemical process as comparison to physical.

Because your using air oxygen to generate the ozone, you may not find enough research to support your research, most of the studies I've read over the years are using pure oxygen to maximize the grams per hour production.

Several of the top manufactures of ozone generators have formula's for grams per pound of BOD or COD, so that might help give you a little of what you are after.

I'm not aware of a M.B.R. that will give you drinking water, only R1 or R2. Perhaps I'm not understanding your research but in all influent to effluent treatment technology's, each require chemical processes in some form. At minimum in the form of ferric or alum addition and at maximum, chlorination. Then there's always an R.O. filter in there somewhere if your trying to get to a drinking water standard.

So I might need to look up some things, but no one takes secondary waters to drinking without putting a tertiary step in there somewhere before going to ozone, UV or R.O. filtration.

I'm studying M.B.R. right now so it sounds like a great research project to me!!

One more thought, remember that most M.B.R. utilize a cleaning cycle utilizing 5mg/l of chlorine to help with back flushing or sloughing off buildup on those membranes, so they are at their heart physical and chemical.

GE has a bunch of great data to run your brain through for the Zeweed MBR.

Thank you Jay for the additional information.

In my research, I am not aiming for drinking water standards but rather, the raw drinking water source use to make the drinking water. Instead of storing the reclaimed water for sometime for augmentation then re-use, my research aims to investigate what treatment is the most feasible to treat reclaimed water to be equal to raw drinking source and to be able use 100% reclaimed water directly to the drinking water treatment plant as the influent. At least, that's my idea. My research only targets whole toxicity effluent and only biological-base response so it's a bit limited.

You want to maximize the mass transfer and the contact time. The mass transfer is maximized through the a large interface area which is obtained by have very small bubbles through a porous diffuser. The contact time is the time that the bubbles are in the liquid and one of the ways to maximize is to have a tall column or to stack columns in series. Without having detailed knowledge of your equipment these are the simplest design factors that should help.

I have been developed a new eco-friendly treatment solution for hydrophobic saturated contaminants (which are the most hard-to-decompose in the water medium) by using ozone/argon/ultrasound hybrid methods.

Please have a look at my papers. Maybe you will get some idea to solve your problem.

Data Free fatty acids degradation in grease trap purification (EPJAP)

Not sure how EPA laws differ in Japan, but here in the old US of A we aren't allowed to move water that quickly from toilet to tap. San Fransisco has an amazing storage vault system that allows for 6 month holding prior to being influent for drinking water. Not aware of Japan gaining any new land lately so I don't see that being a below ground option.

Something you may wish to experiment with is ozonation thru venturi introduction. This helps you increase mass transfer since the micro bubbling allows for the higher surface area OTE, and it can be done earlier in the treatment cycle without the heavier expenses that come with introducing it into the main aeration stream. The sooner you get it in the better the breakdown of complexities is before you go after them with other technology. In the field, micro bubble aerators are starting to gain ground because their lowered energy requirements..... so you could get a little ahead of the curve on your research.

Also don't forget about the possibility of utilizing hydroxial radicals. Whenever you use ozone this should be a definite considered since you can use FeIII to facilitate coagulation and precipitation of TDS and TSS, its already in your influent from the wastewater process..... if your going from wastewater processing directly to drinking processing you can utilize this rather than it be wasted out completely during de-watering of the sludge. There's a small cost savings there.

Lastly, electrocoagulation. Again because you've already utilizing Fe.

Obviously Fe addition will up the toxicity but it can't be ignored when its in the wastewater effluent and can be very useful in the influent stage of drinking water processing.

I know your only doing bench tests, so sometimes these things aren't practical, but I'm making my suggestions to you from my field work and how I've seen amazing oxidation rates achieved.

If you guys could just figure out a way to hook up old godzilla to the grid, you'd have all the power you need to run all the ozone and electrodes you'll need for your research. He's just always so damn cranky though!