Delivering clean water
From your toilet to your tap, recycled water could soon become a reality in Australia with the development of a national validation framework for treatment processes and technology.
A report prepared by the Australian Academy of Technological Sciences and Engineering (ATSE) in 2013 concluded that returning highly treated sewage directly into the drinking water supply of Australian cities and towns could have considerable economic and environmental benefits.
“In a water-challenged country like Australia it just makes sense,” says Associate Professor Stuart Khan from the School of Civil Environmental Engineering.
Stuart was the primary author of the ATSE report – which was also supported by the Australian Water Recycling Centre of Excellence (AWRCE) – and has been a long-time proponent of direct potable reuse (DPR). This is where treated waste water is returned directly to the drinking supply, and differs from indirect potable reuse (IPR). Water in the second scenario is pumped into ‘environmental buffers’ such as rivers and lakes for temporary storage.
“The real driver for considering DPR is sustainability,” remarked Stuart. “Hypothetical DPR schemes for cities like Sydney and Brisbane would be much less energy-intensive than comparable indirect potable reuse or seawater desalination schemes. There would also be savings on construction and operational costs, as pipelines to environmental buffers aren’t needed.”
Water recycling for irrigation and a select number of other non-drinking applications has already begun, but before recycled water can start flowing from our taps, a national framework for validating treatment processes and technologies must be developed.
The Australian Guidelines for Water Recycling (2006) specified performance targets that equipment and processes must meet. These focus on measuring how well a system removes harmful pathogens and toxins, and are intended to protect the health of people and the environment. Yet the guidelines make no mention of how validation should be carried out.
At present, there is no unified national approach, says Stuart. This means validation of identical technologies is being repeated numerous times across different states and territories, slowing down the commissioning of water recycling schemes which increases the overall cost of implementation, and increases the workload for regulators.
It also means different operators could be using the same technologies but getting different validation results.
All of these factors can serve to discourage the uptake of water recycling systems, particularly among smaller, regional utilities or private scheme operators.
A new national framework
Stuart is playing an important role in developing the new National Framework for Validating Water Recycling Technology. This is being spearheaded by the AWRCE and has so far involved extensive consultation between researchers, urban and regional utilities, state and territory regulators, technical experts and technology providers.
It will create consistency across Australia, which will benefit recyclers, regulators and technology providers alike, Stuart said. It will also reduce the cost of validation for specific treatment technologies, as this will only need to be done once.
Stuart is leading the group looking at validation protocols for a multi-barrier approach in water recycling. This type of approach doesn’t just focus on treating water once it leaves a reservoir, but addressing water quality risks at multiple stages along the supply chain.
“Validation approaches for water recycling schemes have tended to consider each process one step at a time and do not sufficiently integrate between process steps to quantify the benefits of synergies and multiple barrier reliability,” stated Stuart.
What this means is that multiple conservative assumptions are often compounded, which leads to a requirement for additional treatment steps, resulting in a greater overall system cost.
By establishing a comprehensive validation strategy, Stuart can help ensure that water recycling schemes are realised in the safest, most cost-effective way possible.
Innovative applications for rice husks
Each year, more than 580 million tonnes of rice is produced, largely by developing nations in Asia. Yet a quarter of this production mass consists of rice husks – hard, inedible shells that protect the grain. It also creates a significant waste problem for growers.
There has been considerable research devoted to uncovering innovative applications for these husks so they don’t end up in landfill. According to Scientia Professor David Waite from Civil and Environmental Engineering, one exciting possibility is water treatment.
When burned, the ash from rice husks has a high concentration of silica, which is thought to be an excellent supporting material for ultra-fine silver nanoparticles.
These nanoparticles have become well-known for their anti-bacterial properties and applications in water treatment, but at the size where these particles are most effective (diameters less than 20 nm), they have a tendency to aggregate, which decreases their disinfecting potential.
David is leading a team working at the intersection of nanotechnology, materials science and environmental engineering, developing new composite materials made from silver nanoparticles anchored onto the low-cost silica from rice husk ash (RHA). In addition to preventing aggregation of the silver nanoparticles, the rice husk ash support slows down the release time of dissolved silver, enhancing the long-term anti-bacterial applications of the particles.
Over the last two years, Tata Corporation in India has begun developing water filtration systems for households that use these innovative particles. The devices, which cost about US$40, appear to be more cost-effective than similar domestic-scale units, which use ultra-violet or reverse-osmosis technologies. However, little is known about the nature of the silver nanoparticle-impregnated RHA, or how well the nanoparticles work.
David says the technology is ideal for supplying clean, affordable drinking water to remote communities, and for doing so following a disaster or emergency, but also says there’s considerable room for improvement.
“Production costs of these units are still too high and the efficacy of the technology has not been demonstrated robustly.” explains David. “As such, the market for the technology outside of India, where the original units are manufactured, is currently quite small.”
But it doesn’t have to be. David and his team are investigating how these particles work, to understand and optimise their properties for water treatment applications.
Investigating which ash type best cleans water
In a recent study published in the journal Environmental Science and Technology, David and his team documented the synthesis of silver nanoparticles anchored onto black and white rice husk ash – the type of ash is determined by the conditions in which the rice husks are burned.
The end goal of providing clean, affordable drinking water to the developing world is worth the effort
They were looking specifically at the surface chemistry, preparation modes, the anti-bacterial properties against specific targets like Escherichia coli, and the mechanisms governing silver dissolution. David says their results indicate “that the precise nature of the rice husk ash to which the silver nanoparticles are attached has a major impact on both the toxicity and longevity of the product as does the composition of the water being treated.”
There is still a lot of work to be done to understand the mode of disinfection, and to refine the preparation method, but the end goal of providing clean, affordable drinking water to the developing world is worth the effort.