Sunday, July 08, 2007

Desal Brine Disposal

Seawater desalination is clearly set to become a major component of potable water supply for many of Australia’s largest cities. Perth already has a large plant with capacity to supply 17% of the city’s needs and recently announced plans for a second plant of similar size. A smaller plant is under construction at Tugan and will contribute to water supplies for South East Queensland. Sydney has recently begun constructing a plant at Kurnell. Melbourne and Adelaide also have fairly definite plans.

Like all potential new water supplies, there are serious economic, social and environmental issues that should be carefully addressed and managed. Among the major environmental issues for seawater desalination is the need to sustainably manage the discharge of membrane ‘concentrate’ or brine.

Australian seawater desalination plants rely on a treatment process called reverse osmosis. Most regular readers are relatively familiar with the concept of reverse osmosis, but a general refresher is available from an earlier post. The process relies on forcing water through a very tight membrane. Usually about 50% of the water is passed through the membrane, leaving the vast majority of salt behind in the remaining 50% of the water. The desalinated water that is passed through the membrane is known as the ‘permeate’ and becomes the usable water supply. The remaining water, which now contains double the concentration of salt, is called ‘concentrate’, ‘membrane reject’ or ‘brine’. The volume of brine produced is roughly equivalent to the volume of purified water produced and needs to be disposed of.

In inland areas, where brackish groundwater may be desalinated, the need to dispose brine is the major limiting factor preventing wider implementation of membrane-based desalination. In coastal areas, the solution is normally just to discharge the brine back to where it came from. This may sound simple, but there are complications.

Australia has plenty of sewage ocean outfalls and plenty of experience in designing and operating them. Treated sewage is much less salty than seawater and often slightly warmer. So the solution has been to discharge the effluent via an outfall in as deep water as possible. Because the effluent is less salty (and often warmer) than seawater, it is less dense. Being less dense, it immediately begins to rise towards the surface, gradually dispersing and mixing with the ambient water as it does so. The more it mixes, the closer the density becomes to the ambient water. Ideally, the densities of the two waters will be roughly equalised before the plume reaches the surface and excellent mixing can be easily achieved.

But desalination brine is different to regular sewage outfalls. Because it is contains roughly twice the concentration of seawater, it is more dense, rather than less dense, than seawater. So instead of rising and dispersing, it naturally sinks towards the seabed and flows along deep ocean channels. This severely restricts the amount of mixing and can result in a ‘hypersaline’ layer of water on the seabed. The major concern is the effect that this may have on important seagrass habitats and benthic organisms. The issue is succinctly illustrated in the following image, which I borrowed from the "Ecological Assessment of the Effects of Discharge of Seawater Concentrate from the Perth Seawater Desalination Plant on Cockburn Sound", prepared by D.A. Lord & Associates Pty Ltd (2005).

The Perth desalination plant ecological assessment determined that all of the information available would indicate that the anticipated well-dispersed changes in salinity should have no deleterious impacts on marine fauna, including the sessile benthic fauna, of Cockburn Sound. However, the assessment did conclude that the concentrate discharge may have a minor effect on reducing high dissolved oxygen levels under conditions of normal mixing, which may have ecological implications. This reduced dissolved oxygen is assumed to result from increased stratification due to brine plume density. This stratification may reduce vertical mixing, which would otherwise relieve natural oxygen depletion (caused by respiration) in deep areas.

A marine ecological assessment for the Sydney desalination plant was undertaken by The Ecology Lab for GHD on behalf of Sydney Water Corporation. The authors reported that “no studies on the effects of toxicants in desalination plant discharge on benthic communities or species have been found to date” and that “the response of fish, fish larvae and other planktonic biota to fronts or plumes of concentrated seawater is also unknown”. The authors expected that “larger, mobile biota such as fish are likely to be able to avoid the zone of higher salinity in the immediate area of the discharge, but smaller invertebrates and some species of fish living in or near reefs and bottom sediments would be unable to escape its influence”. They concluded that “because the dense, hypersaline plume will tend to sink and disperse slowly, biota likely to be affected are bottom-dwelling or non-mobile species that live on or are physically attached to the reef. These include fan corals, sponges, stalked and sessile ascidians, anemones and attached algae. Little, if any information is available on the salinity tolerances of these species or their responses to chemicals contained in the discharge plume”.

The Ecology Lab suggested that further assessments should be undertaken:

“Because no specific information can be found on the effects of discharge for desalination plants on benthic or planktonic communities, it is essential that reef assemblages at Kurnell exposed to the plume are monitored and that toxicity tests be done using seawater concentrate surrogates on local species. Species tested should include members of benthic and planktonic communities known to be present in the area. Monitoring could involve sampling of sessile organisms (i.e. algae, attached invertebrates), large mobile benthic invertebrates (e.g. abalone, sea urchins and turban shells) and fish. Divers could sample assemblages of algae and sessile and mobile invertebrates using a combination of photoquadrats and random transects along the seafloor. Photoquadrats could be used to monitor any small-scale changes to assemblages of sessile fauna. Divers could monitor abundances of large, mobile, benthic invertebrates by counting them in fixed-length transects. Divers could sample fish using underwater video or underwater visual counts (UVCs). The most suitable method would be determined by pilot studies.”

Logically, more research has been undertaken in areas that have been considerably more impacted by seawater desalination brines such as the Mediterranean Sea, Red Sea, and Persian Gulf. In particular, Mediterranean Posidonia grasslands and their associated ecosystems appear to be highly sensitive to even very small increases in salinity. Furthermore, echinoderms appear to have been severely impacted in an area close to a Mediterranean desalination discharge. However, the direct applicability of these studies to Australian cases is unknown and highly questionable.

I’m not suggesting that this is an unmanageable issue. While some environmental degradation is assured, that is a decision we accept every time we build anything at all. Despite the efforts of various journalists to have me suggest on camera that desal brine “will kill everything”, this is not an issue that I want to be alarmist about. However, like everything, there are good ways and not-so-good ways of going about things. If we don’t pay close attention to our marine environments, we risk causing unnecessary damage. So let me tell you about how carefully a city can manage desal discharge if they can be bothered investing the time, money and effort.

In my opinion, the best example of how to do things properly is right here in Australia. The Perth desalination plant has probably been subject to more environmental consideration and care than any other plant anywhere. As such, it is an example that all other cities should pay careful attention to.

For the Perth desal plant, a series of models – including a one dimensional box model and three dimensional hydrodynamic models – and tests were used to ensure the plant would meet required strict mixing criteria set by the state environment agency. Increased certainty was achieved by running various scenarios and different models. Tank tests were also undertaken during the diffuser design and an expert review of the design was undertaken prior to installation.

Pilot field measurements indicated that during calm periods, near-bed dissolved oxygen levels naturally decrease in Cockburn Sound. As a result and because of the semi-enclosed nature and topography of Cockburn Sound, a detailed study was undertaken to consider the extension (if any) of any natural stratification and associated dissolved oxygen issues that may result from brine discharge. This study concluded that any additional effect on dissolved oxygen levels would be infrequent and minor. However, it recommended that because of the uncertainty of predictions for long calm periods, a monitoring program should be implemented as part of an adaptive management plan.

The two photos below belong to Water Corporation and show the continuos dissolved oxygen monitoring activities and the transmitter used to relay the data.

The Perth desalination plant outlet is 1.2 m in diameter and has a 160m long, forty port diffuser where the ports are spaced at 5 m intervals with a 0.22 m nominal port diameter, located 470 m offshore, at a depth of 10 metres, adjacent to the plant in Cockburn Sound. The diffuser incorporates a discharge angle of 60 degrees. This design was adopted with the expectation that the plume would rise to a height of 8.5 m before beginning to sink due to its elevated density. It was designed to achieve a plume thickness at the edge of the mixing zone of 2.5 m and, in the absence of ambient cross-flow, 40 m laterally from the diffuser to the edge of the mixing zone.

The operating licence for the Perth desalination plant requires that certain dissolved oxygen levels are met in order for the plant to operate. Furthermore, a minimum of 45 dilutions must be achieved at the edge of the mixing zone, defined in terms of a 50 m distance from the diffuser. Extensive real-time monitoring is currently being undertaken in Cockburn Sound for the first year of operations to ensure the model predictions are correct and that the marine habitat and fauna are protected. This includes monitoring of dissolved oxygen levels via sensors on the bed of the Sound.

Visual confirmation of the plume dispersion was achieved by the use of 52 litres of Rhodamine dye added to the plant discharge. The expulsion of the Rhodamine dye from one of the plant diffusers is shown below. The dye was reported to have billowed to within about 3 metres of the water surface before falling to the seabed and spilling along a shallow sill of the Sound towards the ocean. The experiment showed that the dye had dispersed beyond what could be visually detected within a distance of around 1.5 kilometres, -well short of a protected deeper region of Cockburn Sound about 5 kilometres from the diffuser. The environmentally benign dye experiment was first commissioned in December 2006 and repeated in April 2007 when conditions were calm. These photos were taken by (and belong to) The West Australian newspaper.

I’ll be interested to see whether other cities consider it necessary to go to such lengths to ensure the protection of their own marine environments. I also reckon a few studies on the (possible) impacts of increased salinity levels on Australian marine organisms wouldn’t go astray.


Anonymous said...

Great photos!!!

Anonymous said...

You may like to check the website of Geo-Processors a company originating from Australia, now operating also from USA with presence in Middle East and where ever else that salinity control and water reclamation are serious challenges. Geo-Processors is a cleantech company with proprietary zero discharge technologies for sustainable management of brines (including desal concentrate) through recovery of useful mineral products.

Stuart Khan said...

Thanks for this information Anonymous,

I am actually somewhat familiar with Geo-Processors technology. Well… I haven’t actually seen it in operation, but have done a reasonable amount of reading on it. I agree that it is an extremely promising technology, particularly in cases where the additional (considerable) energy requirements can be justified. Currently, my impression is that this is limited largely to inland brackish water desalination processes. It would be a mighty task to achieve full salt recovery from a large seawater desalination plant (although I think one plant in Israel comes close?).

I certainly support the concept of zero liquid discharge (ZLD) and agree that Geo-Processors are leading players in achieving that. I know that their technology/processes are of great current interest in the USA.

I may write a blog post on the various approaches to ZLD that are being developed and promoted sometime in the future.

Sam said...

Cheers Stuart, It is something that appears to be largely ignored in the Kurnell (Sydney) case. I think it is simplistic to argue that as there is dispersal the problems are mitigated.
Also, I am concerned about the potential for backwash chemicals used in the RO process to be disposed of along with the brine - it is permissable if they can show that it will do 'limited' harm.

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