Sunday, February 11, 2007

Iemmanomics of IPR 'vs' Desal for Sydney

Let’s talk about money for a bit. Let’s put on a purely economic rationalist hat (or perhaps suit and tie) and think solely about cold hard cash for a few minutes.

If I had a dollar for every time I’ve been told on this blog that ‘money doesn’t matter’, -well I might be able to afford a two litre bottle of Evian mountain spring water by now. However, the uncomfortable fact is that money does matter. Money that we spend to produce and deliver drinking water where we could have produced and delivered the same volume and quality at lower cost by other means, is money that is not available for other things (think schools and hospitals). The economists amongst us (G’day Snow) call it ‘opportunity cost’ and the social implications are worth considering carefully.

So putting aside factors such as environmental impacts of desalination brine, it behoves us to ask “which costs more, -desal or recycling?”. As I will try to explain, the answer is frustratingly complex, highly scheme-specific, city specific and depends on who’s doing the sums! However, I expect that taking a closer look at the question might help us to identify some of the snags along the way.

A story in the Daily Telegraph this week reported that “NSW Premier Morris Iemma says adding treated sewage to Sydney's drinking water supply would cost at least twice as much as his planned $1.9 billion desalination plant.” Iemma apparently stated that adding “treated sewage” to Sydney’s drinking water supplies would come at a capital cost of $4 billion to $5 billion (what's an extra billion between friends?).

Even our friends at 4350 Water suggested that Iemma might perhaps be out on a bit of a limb here. Indeed, Sydney Water published a cost-comparison just under a year ago (March 2006). This comparison estimated overall capital costs for a 500 megalitre-per-day desalination scheme at $2.5 billion and a 500 ML/day indirect potable recycling (IPR) scheme at $3.8 billion.

Iemma’s desalination discount (from $2.5 billion to $1.9 billion) is possibly explained by the fact that the scheme will now initially be built with a capacity of only 125 ML/day (expandable to 500 ML/day at a later date). How IPR went from $3.8 billion to $4-5 billion has not been explained, but presumably accounts for the massive rate of inflation currently applied to major engineering projects.

We have previously discussed why reverse osmosis treatment of seawater is always going to be more energy-intensive, and thus more expensive, than reverse osmosis treatment of municipal sewage effluent. I think that point is inarguable in the absence of some significant technological advances. However, reverse-osmosis treatment alone constitutes neither a seawater desalination scheme nor an IPR scheme, -both have many other components and are much more complex than just a shed full of membranes.

A 500 ML/day IPR scheme would indeed be a relatively expensive development in Sydney. The greatest difficulty lies with the fact that our major sewage treatment plants are at sea-level on the coastline (North Head, Bondi and Malabar), while our drinking-water reservoirs are inland and elevated. The concept of IPR requires some form of environmental detention, usually in a river, reservoir or aquifer. To transport water from the treatment plants to a drinking-water reservoir in Sydney would require pumping large volumes westwards to higher altitudes.

A second complication is that the term ‘treatment plant’ is somewhat of a misnomer for the plants at North Head, Bondi and Malabar. I think ‘sewage transfer station’ would be a more appropriate term since their function is primarily to transfer water from the sewage system to the ocean. The only treatment that takes place is rapid ‘primary treatment’. This involves collecting a few of the heavier solids by scraping them off the bottom of a tank that the water flows through on its way to the ocean outfall. Most cities would calculate the cost of a water recycling scheme starting with the assumption that the available sewage effluent would be at least secondary-treated. However the cost of treating the water from these three Sydney plants for recycling needs to factor-in the additional cost of preparing the water by secondary sewage treatment.

Sydney Water’s comparative cost analysis suggested that the overall capital costs of building a 500 ML/day seawater desalination scheme or an IPR scheme of the same size were differentiated predominantly by costs associated with transporting and connecting the water to the potable supply system. This assumed that desalinated seawater could be transferred from Kurnell to Waterloo (practically on the coast), while recycled water would need to be transferred from Malabar all the way up to Nattai (in the lower Blue Mountains, elevation 160 m) above Warragamba Dam. It seems obvious to me that Prospect Reservoir would be a closer, lower altitude and more practical location to transport the water to and I have no idea why this was not considered.

In Sydney Water’s analysis, the infrastructure costs for water treatment were also estimated to be slightly higher for IPR than for desalination. This arises largely because of the need to build a secondary sewage treatment plant (on prime real estate) at Malabar.

Furthermore, Sydney Water’s costings include the assumed need to pump water from both Bondi and North Head, all the way across the Sydney coastline to Malabar for treatment. The reason why this would be required (to produce 500 ML/day) is not clear since Malabar produces a dry weather flow of 456 ML/day and Bondi 130 ML/day. So assuming 80 % efficiency and a bit of wet weather, water from across the harbour at North Head should not be required.

Even once the plants are constructed, Sydney Water’s calculations assume a slightly higher annual operating cost for the IPR scheme ($175 million/year) compared to seawater desalination ($165 million/year). However, again these costs for IPR arise largely from the cost of secondary sewage treatment (which any other developed city would take for granted) and pumping water all the way to up to Nattai, above Warragamba Dam.

The Sydney Water cost comparison does not canvas alternative IPR options or variations. However, it does include the following paragraph towards the end:

“Finally, it is noted that the costs of desalination and IPR at a scale of 100ML/day are likely to be far more comparable. This is because at this scale there would be two sources of cost savings for an IPR project. First, at this smaller scale wastewater for an IPR project could be sourced from treatment plants in Western Sydney. These treatment plants produce a higher quality wastewater that would require less treatment for IPR. Second, due to the shorter distance from Western Sydney to the Warragamba Dam, the transfer cost would be lower than they would be for the 500 ML/day coastal plant”.

Sydney Water’s cost estimations have been revisited by the private company, Services Sydney. Even including the cost of secondary sewage treatment and pumping to Warragamba Dam, Services Sydney calculated reduced capital and annual operating costs for an IPR scheme compared to Sydney Water. While the IPR capital costs still exceeded the desal capital costs, annual operating costs were lower for IPR. So, by considering capital and operating costs over a 50 year period (with a 4% depreciation rate), Services Sydney came up with an IPR cost close to $6.1 billion and a desalination cost of $7.4 billion.

Certainly, the fact that Services Sydney have a clearly stated financial interest in water recycling should not be overlooked. However, I think this demonstrates that the figures ultimately calculated depend largely on who’s doing the sums. They also depend significantly on the assumptions made, as well as specific characteristics of the scheme and city being considered.

What figure does your calculator give?


njta said...

Hi Stuart. I remember hearing back in the early '90s that Sydney was going to introduce secondary and teriary treatment to those coastal plants as a result of public pressure, and environmental drivers.

Did this never happen? Are Sydney still effectively pouring scantly treated sewage into the ocean?

Stuart Khan said...

Hello njta,

You have a good memory. Much of the public pressure culminated in around 1989 with the huge ‘Turn Back the Tide’ concert on Bondi Beach. Up to that time, sewage pollution was rather horrendous at many of Sydney’s beaches (hence the term ‘Bondi cigar’) and a number of scientific reports were surfacing demonstrating significant toxic chemical accumulation in fish caught off Sydney.

At that time, the Bondi, North Head and Malabar treatment plant were discharging primary-treated effluent just off their respective headlands. Ironically, the only component removed from the sewage, ie the ‘sludge’, was subsequently dumped at sea!

The public pressure in the late 80s / early 90s was aimed at cleaning up the beaches. Upgrading the plants to secondary treatment was widely touted as a good approach. However, the NSW Government resolved instead to maintain the plants at primary treatment and build three deepwater ocean outfalls (2.2 – 3.7 km long and 60 – 80 m deep). Furthermore, they ceased dumping the sludge at sea. It is undeniable that the deepwater outfalls have had a huge effect on cleaning up Sydney’s beaches. However, the term ‘out of sight-out of mind’ does come to mind. As Sydney continues to grow, we continue to dump ever increasing volumes of poorly treated sewage into the ocean. Instead of reducing discharges, we are now planning on adding desal brine as well.

Dr Sharon Beder (now at University of Wollongong) wrote a great social history of sewage management (or perhaps, ‘mismanagement’) in Sydney. Its called “Toxic Fish and Sewer Surfing”, published by Allen & Unwin (1989). Its fairly hard to find now, but well worth tracking down -especially for Sydneysiders. Interestingly, one of her major conclusions was an identified need for authentic and formalised processes for public participation in decision making.

Anonymous said...

I am in shock after reading this. I cant believe that you still dump primary treated sewerage into the sea. You people disgust me!!

[george]twba said...
This comment has been removed by the author.
[george]twba said...

Let me rephrase my previous post.

As a side issue, are people shocked at the people of sydney or Australia as a whole. I would suspect that the different between primary and secondary treatment for ocean outfalls is lost on most people.

However, I must admit that primary treatment prior to discharge does seem a bit "rough". At the time im sure this met the relevant engineering and social standards. Things have changed a lot since.

Stuart Khan said...

George, you sound confident that at the time “this met the relevant engineering and social standards”. But remember that “at the time” was only 15 years ago that the State Government was investing millions of dollars in these deepwater ocean outfalls. I don’t know that the relevant engineering and social standards were so different in 1990 than what they are today. If they do/did meet relevant standards, then I suggest that the standards are/were fairly low.

The main argument put forward for not upgrading to secondary treatment was the amount of land that would be required. Sharon Beder’s book shows maps released by the Water Board in 1987 indicating the land (including buffer zones) that would be needed. As Beder convincingly argues, these were enormously exaggerated, but even still, the necessary land is significant, encroaching on many existing developments and some of Sydney’s best water-frontage real estate.

However, one thing that has changed in 15 years is the development of membrane bioreactors (MBRs). MBRs offer an alternative processes of secondary treatment with a much smaller ‘footprint’. There is evidence that Sydney Water has begun flirting with the idea, having installed a small experimental MBR at North Head about a year ago and another at Malabar just before Christmas. These MBRs process only a very small proportion of the water through the plants, but the treated water that they produce is to be used for various non-potable recycling applications. I am involved in a research project that is examining the quality of the water from the MBRs and I think it is a very exciting development.

While the outfalls are obviously here to stay for some time yet, I am sure that someway down the line we will eventually make them redundant.

Gordon (student) said...

Good blog. I learn a lot. If they clean the sewage outflows where would the waste go? Does it not have to go somewhere.

Stuart Khan said...

Hi Gordon,

Thanks for your excellent question (and welcome aboard!),

The answer is...well...yes and no!

Back in the late 80s / early 90s, most of the people who were arguing for the sewage to be better treated (secondary treated) were mainly concerned about the pollution. Secondary treatment is a major improvement to water quality compared to primary treatment. So at the time, most people would have been satisfied with the secondary treated water continuing to be discharged into the ocean.

However, during the last few years, in addition to ‘pollution prevention’, the focus of water management has shifted very much towards preserving (or supplementing) drinking water supplies. So if secondary treatment (or better) was introduced at these plants today, the water would be considered much too valuable to be discharged to the ocean. Serious efforts would be made to identify alternative uses for it as ‘recycled water’. One good approach would be to look for industrial or irrigation uses that currently use tap water and replace the tap water with recycled water. However, the NSW government and Sydney Water argue that 70% of water use in Sydney is by households and thus there simply aren’t enough irrigation or industrial uses within a reasonable distance. The other option would be to treat the water so that it is clean enough to drink, and pump it back to our dams… that brings us up to the issues discussed in the current post (as well as many many other issues that have been discussed on this blog over the past few months!).

Stuart Khan said...

Sorry Gordon,

Now that I have re-read your comment, I think I may have misunderstood your question. I think perhaps you meant ‘the waste’ as in all of the stuff that gets taken out of the sewage in order for it to be cleaned, right? Also a great question!

The process that produces most of the waste is actually primary treatment (which already takes place). Primary treatment involves collecting all of the heavy solids that fall to the bottom of the tank. This mixture (which is still mainly water) is called ‘sludge’. The sludge is treated by heat and chemicals to remove some of the water, break down some of the chemicals and kill most of the bacteria and other disease-causing organisms. Then it is transported by truck to western NSW where it is applied to land as a fertiliser. This process returns many of the important nutrients back to the agricultural lands.

Secondary treatment also produces some ‘sludge’, but much less than primary treatment. Usually the waste ‘secondary sludge’ is simply mixed with the primary sludge for treatment.

If we were to treat the water up to a drinking standard, we would almost certainly use a membrane process called ‘reverse osmosis’. Again, this process would produce a concentrated waste stream. There are many ways that this could be processed, but the most likely would simply be to ‘return it to the front of the plant’, -that is, mix it back in with the incoming raw sewage. An alternative would be to mix it with the rest of the sludge and treat it that way. Ultimately, it would depend on the efficiency of the treatment processes, which would determine the relative volumes (and concentrations) of the various waste streams.

Gordon said...

Thanks. I was asking about the real waste. Good to know.

[george]twba said...

Hi Stuart

Obviously I am not totally familiar with Sydney Waters decisions to construct the outfalls. My comment regarding engineering and social standards stems from my undergraduate engineering degree in the early 90's. At this time we were taught that minimalistic treatment (whilst not perfect) was a viable option for water treatment.

Possibly the debate over water recyling may lead to the outfalls being scaled back.

Stuart Khan said...


Yes, the more I think about it, the more I have to accept that the 90s really was a period of significant change. The state-based EPAs were created during this period (or just prior to it) and these had a significant impact on defining what was acceptable in terms of pollution discharge to the environment. As I suggested above, prior to then, concerns were largely limited to the protection of public health only, -rather than any real concept of environmental protection for its own sake.

Greg said...

What about the cost of the original water Stuart. Dams or the ocean shouldn't that be included in the costs. Unless you're going to stop the population from growing and tell mother nature we need just enough rain to sustain our recycling effluent plan forever without building new dams!

Stuart Khan said...

G’day Greg,

Sydney Water undertook their analysis to compare the costs of providing Sydney with an additional 500 ML/day of potable water from two potential sources. Naturally enough, they calculated the costs in terms of the sources as they are currently available: primary treated sewage effluent and saline seawater.

Your suggestion makes a lot of sense in terms of calculating the overall costs associated with providing our cities with water. However, it is not necessary for this more simple comparison of two options.

I agree that the extra 500 ML/day may not be sufficient to sustain Sydney’s future population forever. However, I think it’s a worthy short-to-medium term goal.

Murray Scott said...

I have been searching the web and my map collection for data on the heights above sea level of various reservoirs to estimate pumping energy requirements. Could you point us to such data, particularly for Warragamba and Prospect? Also can you suggest a source for data on pumping losses in Sydney Water's network?

I am puzzled by the requirement to transfer recycled water to a reservoir at all. The existing distribution network, from base-load (Warragamba) and peak-load (Prospect and smaller reservoirs) can accommodate demand variation, so why would it not also accommodate variation or shutdown of an input closer to demand sites? The risk of this occurrence appears to be acceptable for the proposed desalination plant. I presume that the difference reflects greater caution about the quality of recycled sewage or stormwater output but I am not sure why. Both processes involve pumping through reverse osmosis membranes with presumably similar reliability. Though the consequences of leakage would be worse than for seawater, sewage treatment should be more reliable since there is much less pressure drop accross the membrane.

Can you enlighten us?

Murray Scott.

Stuart Khan said...

Hello Murray,

The only reliable source of elevation data that I know of is the Bureau of Meteorology weather stations database.

There is a weather station called “Prospect Dam” (No. 067019) with an elevation of 61 m.

There is also one called “Warragamba” (No. 067027) with an elevation of 185 m. However, I’m not sure of the precise location of this station relative to Warragamba Dam. As you would know, this is relatively steep terrain and the dam wall is very large. So it would be necessary to identify the location of the station more precisely to know whether it is an accurate indication of the lift required to back-fill the dam.

As for pumping losses (I assume you mean water losses from the distribution system), I think you would need to go to Sydney Water for that. The oft quoted number for the whole system is 10%, but I don’t know the origin of that figure or how accurate it is. Perhaps you are interested in more specific regional information? If you find it, I would also be interested…

Your suggestion (commonly referred to as direct potable recycling) is certainly technically feasible. As you indicate, that is the manner in which the Sydney seawater desalination plant will be connected to the distribution system. However, it is not the preferred approach to water recycling since the environmental buffer (dam, aquifer, river, etc) is considered by many to be an important barrier for a number of reasons including:
- the effect that it may have on water quality.
- the opportunity to take action if water quality is determined to be sub-standard
- the important ‘barriers’ at the drinking water treatment plant (filtration, chlorination, etc)

The incorporation of the environmental buffer is thus consistent with the overall philosophy of a multiple barrier system.

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Stuart Khan said...


Drop one over to UNSW sometime and I’ll test it out over lunch.

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