Wednesday, June 14, 2006

Ozone and UV

Many people will have noticed the inclusion of ultraviolet (UV) radiation treatment for the proposed Toowoomba scheme and ozone treatment for the proposed Goulburn scheme. Since Australians generally do not have much experience with UV or ozone for water treatment, I thought it would be useful to instigate some discussion.

UV treatment and ozone treatment can have two distinct purposes. The most common is disinfection. That is, inactivating microorganisms such as viruses, bacteria and protozoa. The second use is ‘advanced oxidation’, which involves breaking down (oxidising) organic chemicals in the water.

The disinfectants that we are more familiar with in Australia are chlorine-based. Usually either chlorine gas or chlorine dioxide are added to drinking water (or to wastewater prior to disposal). Chlorine is very toxic and also reacts with other naturally occurring chemicals in water to form a wide range of byproducts. Some chlorine disinfection byproducts called trihalomethanes (THMs) and haloacetic acids (HAAs) are known or suspected to cause cancer at sufficient lifetime doses. As a way of minimising the formation of THMs and HAAs, it is common to react the chlorine with ammonia to form ‘chloramines’. These chloramines are much weaker disinfectants than chlorine, but are also less toxic. They are known to form fewer THMs and HAAs, however recent studies have shown that another extremely potent cancer-causing byproduct (NDMA) can be formed while forming chloramines.

Now that I have made chlorine sound like a gift from Satan, you may wonder why we use it at all. Well, chlorine has a number of advantages:

1) Its effective for many microorganisms
2) Its cheap
3) It stays in drinking-water while it is being delivered to our homes, thus continues to disinfect organisms that might turn up in the distribution pipes.

UV disinfection and ozone disinfection are generally more expensive than chlorine disinfection. Furthermore, these agents don’t stay in the water once it has left the treatment plant, -so there is no ongoing disinfection in the distribution pipes. On the other hand, they do have a number of advantages:

1) They are more effective than chlorine for many organisms
2) They are believed to form fewer toxic byproducts (under appropriate conditions)
3) No trace of ozone (or UV radiation) is left in drinking water

A number of countries in Europe rely on UV or ozone for disinfection of their drinking water supplies. They don’t use chlorine at all, primarily because of its toxicity and the toxicity of byproducts. However, such an approach may be more difficult in Australia due to our warmer climate. The warmth encourages bacteria to grow in the distribution pipes and ‘residual’ chlorine (or chloramines) is required to prevent it.

Traditionally, Australian sewage treatment plants have also disinfected effluents with chlorine before discharging them to waterways. However, because of the toxicity of chlorine towards fish and other organisms, many have begun to switch to UV disinfection.

Toxicity towards the environment and the problem of disinfection byproducts would be the main reasons that chlorine has not been selected as the primary disinfectant at Goulburn and Toowoomba (however, it would be added later at the drinking water treatment plant). An additional benefit is the enhanced effectiveness of UV and ozone for many organisms (including Cryptosporidium and Giardia).

But UV and ozone can also provide a secondary benefit of “advanced oxidation”. Normally, this requires higher doses of UV or ozone to be applied, along with some additional chemical (usually hydrogen peroxide). This combination can form very reactive species called ‘free radicals’. (I have just realised how much panic I will probably create by using that term! Oh well), however the free radicals are very short-lived and will be long-gone before the water leaves the treatment plant. The free radicals are very powerful for quickly breaking down large organic chemicals to smaller, simpler molecules.

These smaller molecules are often much more biodegradable than the larger molecules from which they came. Therefore, they can be quickly removed in the environment or by processes such as ‘biological activated carbon’ (BAC). If advanced oxidation is applied to very pure water (such as post-reverse osmosis), it can be possible to apply sufficient dose to mineralise any remaining chemicals, -that is, turn them into carbon dioxide and other non-organic species.

I should mention that in some places, ozone can also form a nasty byproduct called ‘bromate’. However, the formation of bromate relies on the presence of bromine in the water, which is only typical in coastal areas where there is seawater-intrusion into water sources. This is unlikely to be an issue in most Australian circumstances.

As you may have detected, I like ozone and UV disinfection so much that I would be happy to say goodbye to chlorinated drinking-water for ever. If only we could find an alternative solution to control bacterial growth in our drinking-water distribution pipes!

Any good ideas?

12 comments:

Anonymous said...

This is scare tacticts to say chlorination of water can cause cancer. If it was true, we would be dropping dead like flies!!!

Irresponsible!!!

Den said...

Chlorine

Stuart Khan said...

Thanks Den,

I think we can consider that site to be ‘slightly hysterical’. While there are real health risks from the chlorination of drinking water, as always, we need to weigh-up the relative risks and benefits. The number of lives saved by chlorination during the last century is in well in the millions. The comparatively minor cancer risk is easily justified. Nonetheless, it would be a positive thing to reduce the risk even further if suitable alternative disinfection processes could be identified.

Anonymous said...

But why do we need chlorination, isn't recycled effluent the future, even thou there is no impact study prove it's safe?

Scientist should just leave our water supplies alone, and stick to making fools of themselves by stating something is safe, then 10 years later saying it's not.

Stuart Khan said...

G'day Anonymous,

I think it would be fair to say that recycled water is the past, present AND future. As I pointed out in the post, we still rely on chlorination in Australia, largely to prevent microbial regrowth in water distribution systems. We may find a better solution one day.

Your point about science always continuing to develop is an excellent one. We should never forget disasters like asbestos and thalidomide! However, for all of its flaws and limitations, I would still suggest that 'science' is a more reliable means of understanding the world than any of the alternatives (however, I accept that you may have a different view on that!).

Anonymous said...

I have a headache!! Bloody chorine!!

Anonymous said...

stuart,

Scientist have always stated something to be safe only to do more study to find out it's not. It is not funny when a family member or anyone else dies from one these studies.

Scientist need to understand this is people's lives, and we are all smarter now and understand a hell of lot more then we did 30 years ago, possibly thanks to the Internet. So is very hard for people to put trust in something they say is safe.

For years now I used Teflon pots,,, not anymore since they says it causes cancer, or does it? There are just too many uncertainties these days, and people will be afraid of new technology, especially when it comes to everyday use.

I will not live an area that uses recycled effluent for drinking, I just think it’s a last resort for such towns as Toowoomba, and the TCC only have this to fall back on because they left it FAR to late to do anything else.

Stuart Khan said...

Hello again Anonymous,

I think it is useful to take a step back and consider what our average life-expectancies may have been without science compared to what they are with science. I politely suggest that health risks associated with Teflon frying pans provide an absolutely insignificant factor in this consideration.

But you have raised a very good example. Like potable water, Teflon pans are high in our conciseness. As a result we tend to over-identify the significance of the risks compared to other risks (and benefits) that we are more inclined to take for granted in life.

Anonymous said...

But why do it, it's a simple matter that recycled water should not be used for drinking, then this would not be needed, and the energy required to do this would be saved.

I mean by the electricity it uses. The amount of energy would use more water (for cooling) then it would produce. So to me, it is a waste of money, time and energy, which we can not aford to use.

People don't realise, the more engery we expend the more water is required to keep the electricity turbines cool, out weighing the use for recycled water to a drinking standard.

Jaun said...

Stuart Khan :
I think anonymous was trying to make a point, that scientist get it wrong, and that we don't see the bigger picture until it's too late. We are smarter these days, and I don't think we live longer because of scientist, that would be an understatment. Evolution has a big part in that factor, not scientist.

Messing around with a cities water supply is something people will not take lightly and scientist should not be doing it. CHM2 Hill are partly to blaim for drying up a 100Sq mile lake in the US, and now have to spend $80Mil to fix the problem. This is why I don't trust anyone when it comes to our water supplies, not even our council.

Stuart Khan said...

Hello Anonymous,

Regarding the above quote:

“The amount of energy would use more water (for cooling) then it would produce”.

I commend you on your holistic approach! This type of full-cycle thinking is urgently required in Australia. You have identified a very important trade-off that affects all high-energy water production methods.

However, in this circumstance, it would be inaccurate to state that more water would be required to produce the electricity for RO than could be produced by RO. For example, Delta Electricity (in NSW) report the use of around 1.14 megalitres of water to produce one GWh of power (slightly more if the water comes from the river and slightly less if it comes from the town supply).

My back-of-an-envelope calculation suggests that close to a million times as much water could be produced by RO treatment of tertiary effluent for one GWh of electricity (though I would be happy to be corrected on this).

Ryan said...

Risk is all relative...
Chlorine, recycled water or whatever.
The fact of the matter is although chlorine or recycled water might be more of a chronic health risk than say UV, the risk of drinking a beer is more likley to cause chronic health problems, including cancer, but I'm not giving up beer!

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