Biofilm solution trialled for cost-effective drinking water treatment
Issues with the effective treatment of taste and odour in drinking water are expected to rise with global warming, but one group of researchers is piloting the large-scale application of a biofilm-based solution that removes unwanted compounds more cost-effectively, providing benefits and savings at every level of the treatment train.
University of Queensland (UQ) researchers – in partnership with Seqwater and AnoxKaldnes (Veolia Water Technologies) – have been working on an approach applying the moving bed biofilm reactor (MBBR) technology at the front end of the typical drinking water treatment train.
UQ’s Australian Centre for Water and Environmental Biotechnology’s Dr Gilda Carvalho said the project is focusing on more cost-effectively treating taste and odour and potentially eliminating a manual handling risk to water treatment operators.
“Taste and odour compounds caused by cyanobacteria are problematic for drinking water treatment systems, particularly in Australia, where we harvest 90% of our drinking water from surface water reservoirs, rivers or lakes,” she said.
“Global warming will likely exacerbate these issues. The rise in temperatures gives rise to more frequent occurrences of these microorganisms. Like anything biological, higher temperatures encourage growth.
“But more frequent and intense rain promotes growth, too. Nutrients carried by runoff make their way into the reservoirs and rivers, and typically algae blooms are more frequent when there are excessive nutrients.”
One of the guidelines that drinking water treatment systems have to comply with, Carvalho said, is making sure water is aesthetically compliant, including colour, taste and smell. That is why there is a lot of effort put into tackling taste and odour compounds.
“The taste and odour issues often result from the organic compounds that are produced by cyanobacteria. There are two that are typically most prevalent, which are geosmin and MIB [methylisoborneol]. These compounds are harmless, they are not toxic,” she said.
“But they do give an earthy, musty smell, which may cause concern – people don’t know if there is something wrong with their drinking water. Aesthetics is important. Water that tastes weird, smells bad or has colour causes great distrust from consumers.
“And this can and does become a health problem. If people don’t drink water because it smells, they won’t necessarily buy bottled water. It might be cheaper to buy soda.”
“Our approach represents a retrofit to existing processes and has the potential to drastically reduce the chemical dosing costs and manual handling, and improve climate resilience, while ensuring the production of high-quality, safe drinking water.”
A new approach
When there are events of taste and odour compounds in Australia, utilities add an adsorbent such as powder activated carbon (PAC) to the water as a typical response, but this ad hoc response is expensive and, in some particularly extreme instances, not completely effective, Carvalho said. Moreover, the manual handling of this fine carbon powder has associated risks to the water treatment operators.
“What we propose instead is the use of a biological process. The use of biology to remove these taste and odour compounds is not novel. But we suggest bringing that process into play at the very start of the treatment train,” she said.
Typical treatment plants have a series of steps to physically and chemically remove organics and nutrients from the water, including coagulation, settling, filtration and chlorination. But Carvalho said the new approach proposes a change in order in which biology can be introduced in the treatment train, helping the process become more efficient.
“The filtration step can be biological – biofiltration – where leftover nutrients help the growth of beneficial bacteria on the filter media that remove organics still left in the water, including taste and odour compounds. This can be used as a biological polishing step,” she said.
“But when the coagulation step comes before filtration, most of the nutrients that these beneficial organisms could survive on have been removed. It’s not a very effective application of biodegradation to remove taste and odour compounds.
“What we propose is to move the biological process to the very start of the treatment train to give the beneficial microorganisms the nutrients they need to grow and make them much more effective.”
One of the key benefits of using MBBR is that it helps mitigate clogging in this upfront biological step, Carvalho said.
“A biofilter typically consists of a column of materials packed in tight. If you filter water straight from a lake, it clogs very quickly. The cleaning of those filters at the start of the treatment train would not be viable,” she said.
“So instead of using a packed filter column, we use a fluidised system using carriers. These suspended carriers provide a surface for the bacteria to attach and be retained within the reactor.
“We are trialling putting this treatment process at the front end of the drinking water treatment train. We want to make the most of the nutrients to provide a suitable environment for the development and growth of beneficial microorganisms that biologically remove taste and odour compounds.”
While adding an MBBR to the front of the treatment train is a change in a single unit of the process, Carvalho said the benefits derived from the new approach are many.
“We have tested at three different locations in South East Queensland, at different times of year, and we got up to or over 80% removal of taste and odour compounds using our approach,” she said.
“So the immediate benefit is that you remove the taste and odour issues, or at least most of the issues, in a much more effective way, with less or no PAC application, lowering the associated costs and handling risks for the operators.
“But the beneficial organisms don’t just remove taste and odour compounds, they consume a range of easily biodegradable organics. This means that the coagulation step, which comes second now, that’s in place to remove the rest of the organics.
“Coagulation is still a necessary step, but because microbiology takes care of some of the organics, this is enough to reduce the chemicals needed in the coagulation step. This can reduce up to 60% the chemical requirements, and also reduce the cost of dealing with the coagulation sludge because less of it is formed.”
The third benefit is that the technology is also capable of removing manganese, which, while not hazardous, can cause staining and taste issues and requires higher doses of chlorine to manage effectively.
“Manganese occurs naturally in many reservoirs, and we found our tech can remove it via oxidation. In a typical system, when manganese is present, it requires a higher dose of chlorine to be effectively removed,” Carvalho said.
“Overall, our technology provides savings in PAC, chemicals used for coagulation and subsequent sludge production, and chlorination. There are benefits in every step in the treatment train.”
At the moment, the research team is operating a 1000-litre pilot, and has been awarded an Australian Research Council Linkage Grant to implement two larger pilot plants, one in Queensland and one in South Australia, in collaboration with Seqwater, SA Water, AnoxKaldnes (Veolia Water Technologies) and Queensland Health.
“The idea is to investigate and optimise the operation of the technology in two very different geographic scenarios in terms of temperature and water quality,” Carvalho said.
“This MBBR technology is already very well established for wastewater treatment. Biological processes are the bread and butter of wastewater treatment. But the carriers have not yet been implemented to remove taste and odour or manganese from drinking water.
“This gradual upscaling process is necessary to figure out the best design and operation conditions to maximise the technology as we scale it up.”
“We are planning to do a model that links these parameters with performance and at the end we want to do a proper cost-benefit analysis so that we have a good tool to provide to industry.”