Saturday, August 04, 2007

DALY Dalliance

One of the Toowoomba-based anti-potable water recycling blogs posted an article this week titled ‘Recycled water - giving 1 person in 1000 the shits’. While I admire the creative title, the blog post revealed a need for more discussion and improved community understanding of microbial water risks.

Most people are not familiar with the concept of a ‘DALY’. This is understandable since it is reasonably complex and only quite recently introduced to international water safety guidelines. So let’s take a look at DALYs and their relevance to recycled water.

DALY is the acronym for a Disability-Adjusted Life Year. The use of DALYs is an important feature of the World Health Organisation (WHO) Guidelines for Drinking Water Quality. They are used to help us define a reference level of risk in order to define safe water.

Practically all drinking water supplies contain very very low concentrations of toxic chemicals. Among the most obvious are byproducts that are formed from chlorine disinfection. The chlorine reacts with natural organic matter (present in all natural waters) to form chlorinated chemicals such as chloroform and dichloroacetic acid. In high concentrations, these chemicals would be quite toxic. However, we understand from toxicology that low levels of exposure have very low risks of doing us harm. Accordingly, if we can somehow put a number on the level of risk that we –as a society- are prepared to tolerate, then this will help us to define acceptable concentrations of these chemicals in drinking water.

We have been associating numbers with acceptable risks of chemical exposure for many decades. For chemicals which are known or suspected to cause cancer, the acceptable lifetime exposure is defined in the WHO Guidelines for Drinking Water Quality as that which will result in one excess cancer case per 100,000 people. The Australian Drinking Water Guidelines are stricter and set limits based on one excess cancer case per million people.

However, applying numbers to microbial risks (pathogens) has always been a more challenging task. This is because infections with different microbial organisms (viruses, bacteria, protozoa, etc) can have vastly different health outcomes, which are difficult to compare. Some outcomes are nasty (eg diarrhoea) and some are really severe (eg typhoid). So the risk of acquiring one cannot be simply equated with all others. Furthermore, any one pathogen may have multiple diverse effects or variable effects on different people.

The idea of a DALY was developed in order to be able to objectively compare water-related hazards and the different health outcomes with which they are associated. The basic principle of a DALY is to weight each health effect for its severity from 0 (normal health) to 1 (death). This weight is multiplied by the duration of the effect, which is defined as the time in which the disease is apparent. When the outcome is ‘death’, the ‘duration’ is the remaining life expectancy. The product is then multiplied by the number of people affected by a particular outcome. It is then possible to sum the effects of all the possible outcomes due to a particular hazard.

Accordingly, the DALY is the sum of years of life lost by premature death and years of healthy life lost in states of less than full health (ie years lived with a disability, which are standardised by means of severity weightings).

The World Health Organisation has defined a tolerable level of risk of 1 millionth of a DALY (one microDALY) per person-year. This is equivalent to the loss of one DALY per million people per year. This could be interpreted as one person in a million losing one year of life, or it could be interpreted as a larger number of people suffering less severe impacts. For a pathogen causing watery diarrhoea (but rarely causing death), this reference level of risk has been determined to be equivalent to around 1 in 1000 annual risk of diarrhoea to an individual, -or about 1 in 10 risk of contracting diarrhoea over a lifetime.

Of course, the outcomes of these risks would not actually be observable. For example, the normal reported rate of diarrhoeal illness in Australia is about 0.8-0.9 cases per person per year. In other words, the average Australian gets (and reports) diarrhoea just slightly less often than once per year.

Now that this ‘benchmark’ risk level has been defined, concentrations of specific hazards, which are consistent with this benchmark can be determined. Based on an understanding of source water quality (and variability), this information can inform us about the minimum level of treatment that is required to make the water safe for consumers.

DALYs are used in the Australian Guidelines for Water Recycling in a manner that is entirely consistent with the WHO Guidelines for Drinking Water Quality. Most of the discussion of how they are used is included in Phase 1 of the Guidelines (recycling for non-potable uses). However, they apply equally to the draft Phase 2 Guidelines (recycling for drinking water). DALYs are also expected to be included in the next revision of the Australian Drinking Water Guidelines.

While the guideline risk benchmarks are the same for recycled water as for drinking water, this doesn’t mean that the actual risks are the same. Indeed, a well managed comprehensive advanced water recycling scheme can be expected to achieve much better water quality for most hazards, compared with simply meeting the drinking water guideline.

6 comments:

Anonymous said...

Stuart:

Cr Snow Manners was invited to a National Health and Medical Research Council workshop in Canberra to discuss the draft guidelines for recycled water for drinking. At the workshop he was advised that the Toowoomba Water Futures plan would not meet the proposed guidelines. Do you have any idea why not?

Stuart Khan said...

Hello Anonymous,

Your information is not quite accurate.

The workshop was facilitated by Dr Norman Swan (ABC Radio National Health Report). Norman’s role was to ask provocative and probing questions to stimulate discussion, -that’s what he was being paid for.

It was Norman –and nobody else- that offered the suggestion that Toowoomba Water Futures may not have met the requirements of the draft guidelines. He made this provocative suggestion directly to Snow in response to a comment or question from Snow. Snow declined to respond to it and that was the end of the conversation. It was not discussed more widely and it certainly can not be considered to have been the consensus from the meeting. Any suggestion that it represents an authoritative or consensus view of the workshop participants is a misrepresentation of the discussions that took place.

John said...

Regardless of what went on on Canberra, Toowoomba's failed project should not be revived. It would never cost $68 million as the Toowoomba City Council continues to maintain. Also, they are still stuck for anywhere to send the RO waste stream - there is no commitment by any third party to take it - so add on the evaporation ponds at additional cost.

Any new Federal funding application should make interesting reading. Some Councillors think they can somehow get a recycled water plant built by next March and it will save them from being tossed out at the ballot box. Good luck with that.

ps - I thought the blog article used 'sh*ts' rather than 'shits'.

pps - saw you on the Catalyst program last week - have you lost weight? All that conference organising?

john said...

ppps everyone IS interested in more discussion and improved community understanding of microbial water risks. One of Toowoomba City Council's great failures was their intensive campaign to ram recycled water down people's throats labelling anyone who merely raised a question about recycled water as a 'flat-earther'.

Stuart Khan said...

G’day John,

The two issues you raise (construction costs and concentrate disposal) are indeed key factors in determining the viability of any water recycling scheme. I agree with your point that they would require careful revision to take into account changed circumstances from a year or two ago.

The Canberra meeting was primarily focused on safety and health risks of indirect potable reuse and the implications of the draft national guidelines. It did not really involve significant discussion of costs or environmental impacts. I always think that all water management schemes should be assessed in terms of a triple bottom line, -carefully considering environmental, economic and social impacts.

PS. Well, yes, however I didn’t feel that I needed to be so precious as to censor a word that appears in most respectable dictionaries and is widely used throughout the English-speaking world. I apologise if I caused any offence to anyone…not my intention and all that.

PPS. Ha! Actually, I think the conference had the opposite effect…that footage was filmed quite a few weeks back…

PPPS. I know that you are correct and that it is true that there is considerable community interest in increased knowledge and understanding regarding a diverse range of water management issues. The existence of this blog is essentially an acknowledgement of that. I fully agree that labelling people ‘flat Earthers’ simply for questioning the risks associated with IPR is inappropriate. I think I said as much here.

john said...

Must be going for the 'PG' blog rating rather than the 'G' rating!

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