Since 2015, City West Water has been treating wastewater from its Altona Sewage Treatment Plant to supply high-quality recycled water to local customers for irrigating golf courses, green spaces, and for industrial use. Recycled water is a key factor in managing Australia’s water security requirements and a range of technologies are being utilised to reduce salt content, remove pathogens and produce fit for purpose water.
The water from the Altona Sewage Treatment Plant is too saline for its intended use without further treatment, but City West Water has optimised its salt-reduction process to allow the water to be sustainably recycled and to enable a higher and more cost-effective return on water treatment processes.
The Altona Sewage Treatment Plant produces two separate water products, each fit for its intended use.
“Recycled water ensures our customers are provided with fit for purpose water while reducing demand on Melbourne’s drinking water supply,” City West Water Process Engineer, Shanli Zhang, said.
“For example, approximately 2 billion litres of high-quality fit for purpose recycled water is supplied annually to plastics manufacturer Qenos for use in their boilers and cooling towers.”
The utility has successfully applied Ultrafiltration membranes and multiple pass reverse osmosis in treatment at its Altona Salt Reduction Plant (ASRP), and is utilising several complementary technologies to provide an optimised multi-barrier treatment.
“Reverse osmosis membranes are semi-permeable membranes that work by applying high pressure to feedwater, forcing it across the membrane while dissolved salts are rejected,” Mr Zhang said.
“Between 95 and 99 per cent of salt is rejected to produce high-quality water with low salinity, and a separate brine stream. Reverse osmosis also serves as a barrier to pathogens and is an important part of a multi-barrier treatment train.
“Reverse osmosis membranes provide effective treatment for producing recycled water but are susceptible to biological fouling (as well as other forms of fouling) which can impact the effectiveness of the membranes and shorten the asset life.”
Mr Zhang said to achieve optimised reverse osmosis performance, the ASRP sought to optimise its microbial control strategy and cartridge filters selection.
“Our feed water is effluent water from the Altona Sewage Treatment Plant, which means that naturally there will be some amount of organic residuals from the biological process.
“The reverse osmosis membrane is very good at reducing the salt content, but we needed pre-treatment to reduce the pollutants as reverse osmosis may be fouled quickly by the organic and biological substance.”
Novel microbial control strategy
Most recycled water plants adopt continuous chloramination as a method of microbial control to prevent biological fouling within the membranes.
At the ASRP, City West Water has adopted shock dosing (one hour per day) of a biocide (2,2-Dibromo-3-nitrilopropionamide (DBNPA)).
“We have found this method of microbial control reduces risk by eliminating the possibility of over-dosing chlorine which can damage reverse osmosis membranes. It is also a lower cost option than chloramination when done in shock dosing mode,” Mr Zhang said.
“It also has the added benefit of being easier to implement and control because it removes the need to actively monitor and manage the ratio of chlorine and ammonia that reaches the reverse osmosis membranes.”
According to Mr Zhang, the focus was on optimising the performance and lifespan of the reverse osmosis membranes because it represented the greatest benefit in terms of optimising performance and asset costs at the ASRP.
City West Water was also aware of the high cost of the propriety biocide used and the need to only dose what was enough to control microbial growth without wasting the biocide.
In order to achieve this, City West Water needed to consider and optimise the performance of the biocide across the treatment train.
The utility decided to use a microbial audit technology (LuminUltra) which provides almost real-time feedback on the levels of microbial presence in the treated wastewater.
“Last year, we applied some new technology based on Adenosine Triphosphate (ATP), which is present in all living cells. This new lab technology can extract the ATP from the living cell of the bacteria and then test and quantify the amount of ATP,” Mr Zhang said.
“This gives us an idea of total microbial condition in our water and provides fast feedback for how well we are controlling it.”
Using this broad spectrum technology enabled City West Water to optimise its microbial control regime and check it was effective.
Implementing trial outcomes
The ASRP is one of the few plants that has adopted this pre-treatment process, which made it difficult when initially conducting the technology trials to learn from previous experiences.
“Before we adopt a technology, we conduct research, reference check and undertake trials,” Mr Zhang said.
“There were not a lot of references for us to learn from. We tried different scenarios and initially it was hard to make an assessment for the results.”
Mr Zhang said that critical to the success of the project and the adoption of the microbial audit technology, was working in partnership with DOW and LuminUltra, companies that specialise in microbial control and monitoring.
“We worked together to develop and conduct a trial which involved a site audit, selection of sample points for testing microbial presence and adopting the test methodology (the Quench Gone Aqueous test method in this case).”
Mr Zhang said that one of the key lessons learned from the project is that biocide shock dose can be a feasible option to control biofouling in dual membrane plants as long as it is designed, operated and monitored properly.