Biofouling results in increased energy and chemical costs, loss of water production, and reduced membrane life. Optimal operation of the increasing number of reverse osmosis (RO) membrane plants in Australia is essential to reducing the large quantities of expensive conventional chemical cleaning agents used, chemicals that may also cause environmental issues with their disposal. Stored membranes also incur costs because the standard preservation solution (sodium bisulfite) used to prevent biofilm growth on them is not stable and needs to be frequently replaced.
Assess the effectiveness and benefits of free nitrous acid (FNA)—alone or in combination with hydrogen peroxide—as a novel, low cost, non-oxidising cleaning agent for preventing and removing biofouling, as well as for scale control in RO membranes for wastewater recycling and seawater desalination.
Over a three month testing period FNA was shown to be a reliable and stable membrane preserving reagent and wouldn’t require replacement or top up as with sodium bisulfite.
Bacteria activity tests suggested the FNA concentration used significantly affects its biocide effect. At 10 mgN/L the biomass activity was completely suppressed, which is comparable to the finding obtained with those tests using 1% SBS as the preserving reagent. At 5 mgN/L the biomass activity was not able to be detected in the first two weeks, but gradually recovered to 5% of its original activity at the end of the tests. At 1 mgN/L the biomass activity gradually reduced over the three month testing period from 100% in week 1 to around 20% at end of the test.
The results of FNA cleaning under static conditions demonstrated that it does have an effect on bacteria viability. Cleaning trials conducted with 50 mg/L NO2–-N showed the best cleaning efficiency at pH 3 and 4 (29 and 6 mg/L HNO2-N, respectively) and exhibited similar removal to that of a commonly used cleaning solution (NaOH). However, FNA had higher killing efficiency than the NaOH solution (76% dead cells with FNA versus 61% with NaOH). The proportion of dead cells increased with FNA concentration.
The collaboration with the partners will continue with plans to conduct further cleaning tests in dynamic conditions using bench-scale cleaning cells on used membranes from a desalination plant.
Total Value: $1,015,404 (cash and in-kind contributions)
Principal Investigator: Professor Zhiguo Yuan
Title: Green chemicals for effective biofouling removal from and preservation of reverse osmosis membranes
Length: 35 months
Personnel: 11 collaborators
Related Project: Development of cleaning guidelines for desalination membrane users