A Risky Conversation: Collignon & Khan.

I awoke this morning to find another response from Prof Peter Collignon in my inbox. I think it provides a very good opportunity for us to start discussing the issue of ‘risk’ in a little more detail. That’s where Peter started and I have tried to take it a little further in my ‘right of reply’ at the bottom of this post. I hope readers will find it interesting and feel encouraged to contribute to this public conversation. All points of view are welcome!

I’m also glad to post some of the detail on Peter’s thoughts about the specific IPR proposal for Canberra and his evaluation of its need. It would be great to receive some feedback from Actew on this...


From Prof Peter Collignon
Infectious Diseases Physician and Microbiologist Director Infectious Diseases Unit and Microbiology Department, The Canberra Hospital. Professor, School of Clinical Medicine, Australian National University.

Dear Stuart you wrote,

"This is Prof. Collignon’s interpretation of the ‘risk matrix’ provided in the Australian Drinking Water Guidelines (and in many other risk-based guidelines). The risk matrix requires risk assessors to consider the combined implications of the ‘likelihood’ and the ‘consequences’ of a potential hazardous event. Doing so provides a ‘risk rating’ of low, medium, high, etc..

One disadvantage of the risk matrix is that it is quite qualitative in the nature of the ratings that it provides and thus very open to interpretation or opinion. However, I feel that Prof. Collignon’s use of the risk matrix is overly simplistic and thus incorrect. His simplistic approach would be expected to deliver a risk rating of “high” or “very high” for any water source whatsoever. In other words, while the likelihood of something going wrong is very low, the consequences are potentially devastating. But this is not an appropriate use of the matrix (which is fortunate or cities may have to cease delivering potable water!).

The risk matrix should be used to evaluate risks posed by the occurrence of specific “hazardous events”. A hazardous event may be something such as a high rainfall period, a failure of reverse osmosis pump, a loss of electricity to the treatment plant, a cross-connection between potable and non-potable water supplies, etc. Only once such an event is identified can the risk matrix be used to evaluate it. Risks associated with hazardous events that are not assessed as ‘low’, require the institution of additional barriers or management practices until a ‘low’ risk can be determined. The risk matrix is a planning tool intended to ensure that planners and scheme operators identify the sources of risk in their systems and manage them effectively with multiple barriers."

Stuart how you would define the level of risk of recycling water from sewage into potable water supplies if you believe my interpretation of the risk as “high” is not correct? If we just take one part of a system (eg reverse osmosis as you suggest) then surely if that goes wrong for some reason ( a membrane rupture that is undetected for a period of time), then at least for faecal viruses the consequence could be potentially be considerable for a very large number of people would they not? Does that not make this "high" risk? Yes there are steps that can be taken after this such as a UV step after the RO, but if large numbers of viruses are present can we be sure that UV will cope? In any case does that still not make the use of RO as “high” risk? It is just then as you say, the planners and scheme operators after identifying the sources of risk in their systems need to be able to manage them effectively with multiple barriers.

I attach the tables from the Australian water guidelines which I hope you might post along with these comments. I would be interested to see what others think and how they would rank the "risk" in Canberra of the current “water2water” recycling proposal.





The trouble for the Canberra proposal is that there is not good data available from elsewhere on what is proposed for Canberra to make a quantitative risk assessment as no similar proposal has been used anywhere else. Therefore it is likely that only a qualitative risk assessment can be done. I still think it is "high" risk. This however does not mean we should never take a "high" risk, just we need to be sure that it is necessary to take this risk and that there are not other reasonable alternatives available. The drinking water guidelines are available from here.

For the benefit of your readers, I will outline what I understand is the Canberra sewage recycling proposal. It is going to start with recycling 9 GL per year of water from sewage and then go to 20 GL relatively quickly (I presume within a few years). Water will be pumped from the Molonglo sewage outflow after RO treatment etc to new artificial terraced wetlands in the upper reaches of the end of the valley where the creeks/streams that eventually enter the Cotter Dam (the Cotter Dam is lowest of the three Dams on the Cotter river). The Cotter dam is only very small at 3.8 GL. It also has to be kept at about 90% full at all times so that the breeding ground of threatened fish is not prejudiced. This has meant however that even in droughts after rainfall this Dam frequently overflows. There are plans to also build a bigger Cotter Dam (to 80 GL and which I think is a good idea). However this has still not been approved and completion will be quite a number of years after the recycling plant will go into operation.

I have been told that it will only be about 2 or 3 days before the recycled water pumped into the artificial terraced wetlands enters the Dam water. Because water in the dam runs the risk of overflowing, my understanding is that water will need to be pumped out almost as fast as it enters the reservoir. This means it will be pumped up to the treatment works at Stromlo and then put directly into the reticulated water system of Canberra. What is not used along the way in the city while it traverses the reticulated water system will be pumped into the Googong dam (which has over 100 Gl capacity but is on the other side of the city). However I presume this will mean, given that Canberra uses currently about 50 Gl of water per year, that if 20 Gl of water is recycled, then about 40% of our reticulated water may be recycled water. One can argue that we might access other sources of water from other Dams and this percentage will be lower, but if we do that why then produce such energy expensive recycled water? While the Cotter dam is so small, unless the added water is pumped out it will likely just overflow over the top of the dam and be lost from the catchment.

I would also appreciate people’s view on how this proposal should be defined. This is currently being called an “Indirect” potable water usage. However if it is really only takes 2 or 3 days before this water hits the small Cotter dam and then is also likely to be relatively quickly pumped into the reticulated water system, is this not really a “direct” potable use? (if not, does it not miss out on that definition by just a couple of days).

This rapid return of this recycled water to our reservoir and then into our reticulated water system seems to me to be taking short-cuts with a lot of normal biological and natural safety barriers that are usually in place, namely major dilution factors and long retention times in shallow wetlands, aquifers or dams.

As a last note, with level 3 water restriction in Canberra, we use 50 Gl or less from our dams per year for domestic/industrial consumption. In every one of the recent drought years, except for 2006, we have had much larger inflows than 50 GL into our dams (in 2005 it was over 100 GL’s). Indeed at the beginning of 2006 our dams’ levels were close to 70% despite the drought. In Canberra therefore, I can't see why we need to recycling of water from sewage into our drinking water, especially given the compromises to natural safety barriers that will occur if it is done before an enlarged Cotter Dam is built. Stuart I don’t know if you can attach graphs but this I think shows our situation.


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Stuart’s Response:

The risk matrix is a useful tool to assist in assessing the risks associated with specific, well defined ‘hazardous events’. The risks arising from such ‘hazardous events’ are described in terms of their ‘likelihood’ and ‘consequences’ in order to assign a ‘risk rating’ from ‘low’ to ‘very high’.

As mentioned in the previous post, a major limitation of the risk matrix is its rather qualitative nature and thus the fact that interpretations may be left largely open to opinion or even ‘gut feeling’. However, I find that it is useful to try to use quantitative (or at least semi-quantitative) assessments as much as possible. This can be a little cumbersome with the risk matrix, but there are a number of workable approaches.

Table A4 above gives ‘qualitative measures of likelihood’. The recent National Guidelines for Water Recycling (Phase 1) take this a step further by providing some semi-quantitative descriptions for assigning likelihood. These involve describing expected frequency of occurrence by the following descriptors. Rare: “May occur only in exceptional circumstances. May occur once in 100 years”, Unlikely: “Could occur within 20 years or in unusual circumstances”, Possible: “Might occur or should be expected to occur within a 5- to 10-year period”, Likely: “Will probably occur within a 1- to 5-year period”, Almost certain: “Is expected to occur with a probability of multiple occurrences within a year”. With such an approach, it is then possible to examine performance histories of other plants using similar technologies (or other predictive means) to make a reasonable judgement of the frequency (and thus likelihood) of specific hazardous events occurring.

Assessing the ‘consequences’ of a hazardous event can be undertaken in a similarly quantitative manner. Considering Peter’s example of viruses, we can consider the ‘consequences’ of a hazardous event in terms of the impact it has on the number of infections. For a given virus, this is directly related to the level of human exposure to the virus. So a useful approach is to assess the consequences of a hazardous event in terms of its impact on exposure.

To quantify the impact of a hazardous event on exposure, we first need to have a quantitative description of ‘normal’ exposure. This will be defined in terms of virus concentrations (and variability) in the source water and how well it is removed at each of numerous subsequent barriers (eg. microfiltration, reverse osmosis, advanced oxidation, environmental residence, drinking water flocculation, chlorination). Again, these removals (or ‘decimal reductions’ or ‘log reductions’) can be informed by experience with existing plants using the same or comparable technologies. By considering a large number of hazardous event scenarios, we can examine the impact that they can be expected to have on the removal of the virus, and thus on human exposure to it.

All significant international health organisations (eg the WHO), accept some concept of ‘tolerable risk’. For viruses, this is typically defined in terms of infection likelihood (or more recently, a concept called DALYs). Advanced water recycling schemes will be designed to achieve sufficient removal of viruses, such that accepted infection likelihoods will not be exceeded. The multiple barrier approach ensures that it must be achieved with significant safety margins of numerous orders of magnitude.

Consideration of a hazardous event by its impact on human exposure and thus by impact on infection likelihood, now allows us make a reasonable quantitative assessment of risk.

Peter has asked me “Stuart how you would define the level of risk of recycling water from sewage into potable water supplies if you believe my interpretation of the risk as “high” is not correct?”. However, it was not his assessment of “high” that I necessarily disagreed with. I was really commenting on the simplistic reasoning that was used to arrive at it. I hope that after the above very brief introduction to the concept of quantitative risk assessment, readers will appreciate that I can not provide a simple answer to how I would rate the risks associated with a particular IPR scheme without access to significant plant design information. Even once that is available, it is a formidable task to systematically consider possible hazardous events and assess them in terms of their likelihood and consequences.

Nonetheless, I will go as far as to say that it is not necessarily the case that a ruptured RO membrane presents a significant risk. Even if we ignore the ‘likelihood’ for a moment and just consider ‘consequences’, as we have described, in terms of impact on human exposure. A typical RO plant will not have just one RO membrane module, but hundreds of them installed in parallel (and even in series in some cases). If one RO membrane out of 100 has a complete rupture, that would represent a 1% increase in exposure. A complete rupture would be instantly noticed by a dramatic loss of pressure, so a much smaller fraction of this 1% is actually more realistic. This may mean that the schemes overall performance for reducing virus concentrations may be reduced from 10 log units to 9.9 log units [Note: this calculation is incorrect. See comments below]. For a safely designed scheme, this would result in an elevated level of exposure, but one that was still well within tolerable risk levels.

The issue of ‘indirect potable recycling’ and ‘direct potable recycling’ is one that we have dealt with a few times on this blog (eg. see the long discussion between ‘Mark’ and myself here). I see that it really is a sticking point for many people!

If you want to go by the accepted definitions, the scheme proposed for Canberra is an indirect potable recycling scheme. This is because the water is not plumbed directly from the advanced water treatment plant to the distribution system. Instead, it discharges to a reservoir and then will be treated (with other source water in the reservoir) at a drinking water treatment plant. Only after the drinking water treatment plant will it be plumbed to the distribution system.

However, before we go down that path again, I wish to state that simply being named or defined as an ‘indirect potable recycling scheme’ doesn’t automatically make a scheme safe (a rose by any other name…). In order to be judged as ‘safe’ it is necessarily for each scheme to be carefully considered in terms of its actual design attributes…not just its name.