Investigating the capability of pulsed ultrasound technology in improving the performance of surface water treatment systems

Investigating the capability of pulsed ultrasound technology in improving the performance of surface water treatment systems

Surface water is an important resource for drinking water production. Due to increasing contamination caused by floods and urbanisation, the quality of surface water continues to deteriorate. This problem is currently addressed by increasing the chemical additives used in water treatment processes. This practice introduces
health-related problems such as the formation of disinfection by-products (DBPs) and the increase of Al residues. The application of physical rather than chemical
treatments is a logical solution to the above problems. When evaluating various physical treatments, ultrasound appears a sensible choice for improving contaminants
removal from surface water.

While numerous studies have addressed the application of ultrasound technologies in water treatment, most of these studies used single contaminant synthetic water samples which does not present a realistic assessment. The key focus
of this study is using natural water samples with a representative mixture of contaminants.

The main challenge cited regarding ultrasound application is high energy consumption. To tackle this issue, the use of pulsed ultrasound has been proposed in this study. An improved calorimetric technique has been developed and tested to provide a fair evaluation of energy conversion in ultrasonic reactors. Sonochemical efficiency (SE) based on •OH and H2O2 measurements for the ultrasonic system
along with the new calorimetric technique were employed.

The optimal location of pulsed ultrasound technology within the surface water treatment process was identified using natural water samples with different carbon origins. The optimal location was found to be prior to the coagulation stage. The operation of pulsed ultrasound in this location was optimized with regards to power, treatment time and pulse format. Total coliform, DOC and UV-vis measurements
were applied along with chemical fractionation techniques to establish the optimal operating conditions. Overall, ultrasound treatments resulted in 10-70% of microbial
removal and 7-15% of DOC removal with a decrease in the aromatic hydrophobic DOC and a marginal increase in the hydrophilic DOC. The effectiveness of these conditions in promoting further removal of contaminants with alum coagulation was also examined.

Turbidity, DOC removal and residual Al were measured for alum coagulation with and without pulsed ultrasound pre-treatment. Response optimization showed
that pulsed ultrasound pre-treatment increased turbidity and DOC removal and reduced residual Al. Analysis of downstream effects revealed that pulsed ultrasound
pre-treatment increased total coliform removal, decreased trihalomethane formation potential (THMFP) and improved the settling of coagulation sludge.

A large-scale laboratory magnetostrictive ultrasonic reactor operated on square-pulse waveform was designed and tested. It was observed that scaling up to 15 L improved sonochemical efficiency of ultrasonic reactor. Using a reactor tank with 45 inclined sides further improved sonochemical efficiency. The larger design was more energy efficient in removing water contaminants compared to the small scale. This study confirmed that pulsed ultrasound is an effective tool for improving surface water treatment system, however further investigations required using other
coagulants and monitoring of subsequent effects on other DBPs. It is also important to evaluate the performance and economic viability of the large scale reactor in continuous systems.