Sunday, November 26, 2006

Today RO, tomorrow FO?

Here’s a challenge for any budding scientists/inventors out there. It’s a promising concept for advanced water treatment. However, it is currently very much more a ‘concept’ than it is a ‘reality’ for large scale implementation.

Australians have been actively talking about municipal water recycling and desalination for a couple of years now. Many people have been interested enough to find out about one of the most touted technologies for these processes, being reverse osmosis (RO). For a very quick refresher on the general principal of RO you might like to skim through my previous post on seawater desalination.

RO has the potential to clean almost any water to practically any quality we may desire. However, there are two major limitations to RO. The first is considerable energy requirement to force saline water through a semi-permeable membrane against an osmotic pressure pushing in the opposite direction. The second is the production of large volumes of concentrated brine which must somehow be disposed of.

Researchers at Yale University (USA) are working on a novel potential answer to these limitations. Rather than working against the osmotic pressures of RO, Prof Menachem Elimelech proposes a concept that makes the osmotic pressure his ally. The process is called forward osmosis (FO).

Unlike RO, the driving force for FO separation is osmotic pressure, not hydraulic pressure. By using a concentrated solution of high osmotic pressure called the “draw solution”, water can be induced to flow from contaminated or saline water across a semipermeable membrane, leaving the dissolved contaminants behind. The (now diluted) draw solution can then be reconcentrated, yielding purified water and a “draw solute” ready to be recycled in a closed loop.

Identifying suitable draw solutions is currently the largest obstacle to the successful large-scale implementation of FO water treatment. Ideally, draw solutes should be those that can be easily chemically or thermally precipitated from solution for removal. Some researchers have proposed the use of dissolved gases that can be removed by thermal means, or the use of larger molecular weight solutes that can be separated by physical means.

An ideal draw solute would have a high solubility, a low molecular weight, and a way of easily being removed from the water. High solubility coupled with low molecular weight allows for the generation of large osmotic pressures which lead to high product water transport across the membrane and high purified water recoveries (ie. minimal brine volumes). The ability to be easily removed from the water is crucial since the overwhelming majority of the energy used in the FO process is for draw solute recovery.

Ideas for using FO for water purification processes have been around for some time and a small number of studies have been published since the mid-1970s. Furthermore, a number of patents have been awarded, but none of these have really matured or proven practical for implementation. Interestingly, the US space agency NASA is currently investigating FO for direct potable reuse of wastewater in advanced life support systems for space applications.

One draw solution currently being investigated is composed of ammonium salts which are formed when ammonia and carbon dioxide gases are mixed in water. Once the concentrated draw solution is used to effect separation of water from the feed source, the subsequently diluted draw solution may be treated thermally to remove the ammonium solutes, producing purified water as the primary product.

This thermal separation of draw solutes is based on the useful characteristic of these ammonium salts to decompose back into ammonia and carbon dioxide gases when the solution is heated. If the process is done under vacuum, the necessary applied temperature can be as low as 40°C, meaning that low grade “waste heat” from other industrial processes may be a viable energy source.

However, current methods for fully recovering the ammonia and carbon dioxide gases from the product water are still relatively inefficient and still render the overall process relatively energy-intensive. Therefore, further improvements are required for draw-solute recovery to improve the overall viability of FO for large-scale implementation.

Any good ideas?

Tuesday, November 21, 2006

Politics of IPR in Queensland

I don’t want to get too caught up in politics, but this has been an interesting week for water recycling in Queensland (and its only Tuesday!). I thought it would be useful to provide an update on where the major political players stand in that State. The issue, of course, is indirect potable water recycling (IPR)…involving using highly treated effluent to (intentionally and openly) supplement dwindling supplies in Brisbane’s Wivenhoe Dam.

QLD Premier Beattie has proposed a community poll on the issue, originally slated for 2008. However, Beattie now suggests a South East Queensland poll may be required as soon as next year.

QLD Liberal Leader Dr Bruce Flegg says forget the poll and get on with water recycling as a matter of urgency.

Labor and Liberal members of Brisbane City Council voted today to support any poll, but Liberal Councillor Jane Prentice agrees with Dr Flegg that such a referendum is a waste of time and Beattie should just bight the bullet.

Brisbane (Liberal) Lord Mayor Campbell Newman is a strong supporter of IPR and was prepared to say so long before most.

The State Nationals Leader Jeff Seeney says it is “the Coalition’s vision to ensure all of Queensland’s waste water was recycled to put an end to ocean outfalls”. However, Seeney’s support is for a pipeline to carry recycled water for industry and agricultural use. He has stopped short of supporting IPR except as a “a worst-case scenario”. He says “there would be no decision on whether to support the recycled water referendum until the question's wording was revealed”.

Democrat Senator Andrew Bartlett has been a tireless campaigner for water recycling as an alternative to building new dams in Queensland.

The Queensland Greens have a formal policy supporting IPR as a component of overall urban water management.

So there you have it. In South East Queensland at least, the political differences appear largely to have moved on from support or opposition to IPR. Of course, not everyone supports it but the main differences now seem to revolve around details like whether a poll is appropriate or an unnecessary delay, and just how urgent things need to get before action should be taken.

Friday, November 17, 2006

Ask a stoopid question...

In June last year Toowoomba City Council (TCC) submitted a proposal to the Commonwealth Government for co-funding of a planned indirect potable recycling scheme to supplement that city’s dwindling water supply.

The Commonwealth Government would provide one third of the scheme costs through the National Water Commission (NWC). The Queensland State Government would also contribute one third on the basis that the NWC funding was forthcoming. TCC would be responsible for funding the remaining third. Early indications were that the NWC would support the proposal and even the local member of Federal Parliament (Ian MacFarlane) publicly expressed support.

A month later, Mr MacFarlane received a community petition opposing the co-funding of TCC’s proposal. The petition carried the weight of around 7000 signatures and MacFarlane quickly changed his tune. He claimed to have been misled about the details of the proposal and the established safety of (planned) potable water recycling.

MacFarlane responded by asking Malcolm Turnbull to make any NWC approval of co-funding conditional on a positive outcome of a community poll. MacFarlane’s request was approved and the City of Toowoomba went to the polling booths on 29th of July 2006. The question posed was simply:

“Do you support the addition of purified recycled water to Toowoomba’s water supply via Cooby Dam as proposed by the Water Futures Toowoomba Project?”.

The answer was a resounding “No” (62% of formal votes). The only other option provided was “Yes” and the whole process provided no viable solution.

As far as I am aware, Toowoomba is the only city on Earth to have been given the opportunity to vote (directly) on its potable water supply source. But MacFarlane and Turnbull appear to have set a precedent. Queensland Premier Peter Beattie has since indicated that he intends to call a similar planned potable water recycling poll for Brisbane in 2008. A few individuals are now demanding a poll in Goulburn (NSW). But is this an effective approach for addressing imbalances in a city’s water supply and demand?

Even Toowoomba’s prominent “No” campaigners expressed dissatisfaction in the limited options provided for in the “Yes” or “No” poll. (Almost) all members of the Toowoomba community appear to agree that new water management strategies are required, but no opportunity was provided to properly evaluate, compare or express preferences for alternative strategies. What’s the point in rejecting one option without identifying an alternative? Ironically, Malcolm Turnbull should be as aware as anyone of the perils of an inadequate referendum question.

Planned potable water recycling has much to recommend it, but it makes little sense when considered in isolation. All water supply options come with costs (economic, social and environmental). Once a specific proposal was announced, seawater desalination became extremely contentious in Sydney and similar community sentiments are now unfolding on the NSW Central Coast. Peter Beattie’s recent announcement of plans to build a new dam to supply Brisbane was met with community outrage. Historical plans for a new dam for Sydney (on the Shoalhaven River) have been shelved after extensive investigations have revealed significant environmental, social and economic costs, coupled with the long term inadequacy of such a dam.

I suggest that communities forget about “Yes or No” polls and consider alternative forms of effective participation in decision making. It is thoroughly inadequate to vote on a question that allows for the possibility of no viable solution to be the preferred outcome.

A better approach is to provide a choice between a range of fully evaluated options. Allow the community to identify the options and provide a clear indication of the social, environmental and economic consequences of each one. Its likely that none of the options will attract an outright majority of the vote, so a preferential voting system will be essential.

If the issues involved are highly complex (which in many cases they will be), an effective approach may be to engage the services of a ‘stakeholder jury’ (or ‘citizen’s jury’).

A stakeholder jury process replicates a court-room procedure. Stakeholder juries typically comprise 10-15 representative stakeholders who consider a particular issue and decide for or against a proposal. Alternatively, juries may be tasked to identify the most favourable of a defined list of proposals. The community should be given the opportunity to nominate jury members (or even elect them).

The jury hears or reads evidence from expert witnesses and jury members are able to question the witnesses directly. The process may last several weeks until the jury reaches an informed decision. The jury may then prepare a short report summarising the debate leading to the decision reached.

A major benefit of stakeholder juries is that they allow participants to select and pursue their own lines of enquiry. They support detailed consideration of key issues or sticking points and may help identify relative levels of concern about specific issues. Members of the public may be invited to make detailed submissions for the jury to consider before making their decision.

There are many options for effective community participation in identifying solutions to Australia’s current water woes. Not only is such participation appropriate, it has the potential to deliver the optimum solutions to what is becoming an exceedingly difficult problem with long-term repercussions. Has the jury reached a verdict?

Friday, November 10, 2006

A closer look at UV treatment

I have been catching up on some recent research about the use of UV radiation for water treatment and thought I would share some of it with interested readers.

Most of this research comes from Associate Professor Karl Linden and his research group at Duke University in North Carolina. I have made an effort to cut out some of the technical jargon without over-simplifying the findings. I hope you will find it readable and worthwhile. For those who are interested in the detail of the studies, I have included a number of references at the end. These are generally not freely available on the internet, but if you ask me politely (okay…even if you choose to abuse me), I’ll try to get you a copy of the papers that you are interested in. You’ll find my email address by clicking on my profile, above. Okay, here goes…

Ultraviolet (UV) radiation is an effective disinfectant in water and is used widely for this purpose in many developed countries including Australia. The disinfecting properties of UV radiation are due to the ability of certain wavelengths to penetrate the cell walls of microorganisms and be absorbed by the nucleic acids (components on DNA and RNA). The effect may be to either cause the death of the cell or to prevent it from replicating. The portion of the UV radiation band that is most effective for inactivating microorganisms is between about 220 and 320 nm. This is known at the ‘germicidal band’.

UV disinfection is generally more effective than chlorine for inactivation of most viruses, spores and cysts. However effective inactivation some of these organisms require higher UV doses than is used for some disinfection systems aimed primarily at bacterial organisms.

UV lamps for water treatment come in two main types known as ‘Low Pressure’ and ‘Medium Pressure’. Low pressure UV lamps generate radiation at mainly a single wavelength (254 nm), which is close to the most effective germicidal range. Medium-pressure lamps generate radiation at numerous wavelengths within the germicidal band. Only about 7 to 15 per cent of the output is near 254 nm, however the total germicidal output can typically be 50 to 100 times that of low pressure lamps.

In addition to microbial disinfection, UV radiation can also be used (directly) to break down organic chemicals in water [1; 2]. Organic chemicals are affected by UV radiation at various wavelengths in the 200-300 nm range, with degradation generally dependant on the presence of aromatic rings or double bonds or triple bonds.

In 2000, The Orange County Water District in California detected small concentrations (nanogram per litre) of the carcinogenic chemical nitrosodimethylamine (NDMA) in their Water Factory 21 scheme. The problem was quickly resolved by the addition of a UV treatment process, which rapidly destroys NDMA.

However, for the purpose of breaking down chemicals, UV radiation is more commonly used to promote the initial formation of hydroxyl radicals (HO.). Hydroxyl radicals are about the most powerful oxidising agents known and are much more effective at breaking down chemicals than direct UV radiation. Processes that promote the formation of hydroxyl radicals are known as Advanced Oxidation Processes (AOPs).

UV-AOPs can be achieved by a number of methods including ‘photocatalysis’ with titanium dioxide (TiO2). However the most common method is by reaction of hydrogen peroxide (H2O2) [3-5] by the following reaction:

H2O2 + UV (200-280 nm) → 2HO.

Oxidation of organic chemicals by hydroxyl radicals is very non-specific and all organic chemicals are ultimately susceptible if sufficient dose is applied [6]. Because of this considerably broader oxidation potential, AOPs can be used to degrade trace amounts of recalcitrant organic chemicals in highly treated effluents.

Once generated, hydroxyl radicals can attack organic molecules by a number of mechanisms. Under suitable conditions, the reaction of hydroxyl radicals with organic compounds may proceed to complete oxidation to produce final products of water, carbon dioxide and salts. This process is known as mineralisation.

The overall extent of oxidation for any AOP is dependant on the contact time and the concentration of ‘scavengers’ in the water (ie non-target oxidisable species). Typically, dissolved organic carbon (DOC) and carbonate/bicarbonate are the most important scavengers in drinking waters or recycled waters. However, pre-treatment processes such as granular activated carbon (GAC) or reverse osmosis significantly reduce DOC concentrations, thus enhancing oxidation efficiency.

In 2004, Linden’s research group published a paper investigating the degradation of some endocrine disrupting chemicals (bisphenol A, ethinyl estradiol, and estradiol) by direct UV photolysis and UV-AOPs [3]. This study included both low pressure UV lamps and medium pressure UV lamps. It revealed that without enhanced hydroxyl radical formation, medium pressure lamps are required for the degradation of these contaminants. However, regardless of which type of lamp was used, the EDCs were much more effectively degraded using UV/H2O2 advanced oxidation. The chemical degradation rate constants for the AOP process were on the order of 10,000,000,000 /M/sec. This means that given suitable initial water quality (achieved by pre-treatment) and sufficient UV/H2O2 dose, these AOP processes can be an extremely effective barrier for these chemicals in water recycling schemes. Some results from the study are shown below (click for a larger image).

[1] Rosenfeldt, E. J., Melcher, B. and Linden, K. G. (2005) UV and UV/H2O2 treatment of methylisoborneol (MIB) and geosmin in water. Journal of Water Supply Research and Technology-Aqua, 54(7), 423-434.
[2] Shemer, H., Sharpless, C. M. and Linden, K. G. (2005) Photodegradation of 3,5,6-trichloro-2-pyridinol in aqueous solution. Water Air Soil Poll., 168(1-4), 145-155.
[3] Rosenfeldt, E. J. and Linden, K. G. (2004) Degradation of endocrine disrupting chemicals bisphenol A, ethinyl estradiol, and estradiol during UV photolysis and advanced oxidation processes. Environ. Sci. Technol., 38(20), 5476-5483.
[4] Shemer, H. and Linden, K. G. (2006) Degradation and by-product formation of diazinon in water during UV and UV/H2O2 treatment. J. Haz. Mat., 136(3), 553-559.
[5] Shemer, H., Kunukcu, Y. K. and Linden, K. G. (2006) Degradation of the pharmaceutical Metronidazole via UV, Fenton and photo-Fenton processes. Chemosphere, 63(2), 269-276.
[6] Shemer, H., Sharpless, C. M., Elovitz, M. S. and Linden, K. G. (2006) Relative rate constants of contaminant candidate list pesticides with hydroxyl radicals. Environ. Sci. Technol., 40(14), 4460-4466.