Ultrasound technology as a pre-treatment for biofouling control in reverse osmosis (RO) system

Ultrasound technology as a pre-treatment for biofouling control in reverse osmosis (RO) system

The scarcity of pristine water resources has been a serious issue worldwide. This issue has resulted in the emergence of alternative water resources represented by seawater and wastewater. To treat these water resources, advanced membrane technologies such as Reverse Osmosis (RO) membrane technology are required. In spite of the capability of the RO technology in desalinating seawater and reclaiming
wastewater, the efficiency of this technology is significantly affected by the problem of RO membrane fouling. There are several types of fouling that can occur on RO membrane, biofouling was selected to be the focus of this thesis. In this study, ultrasound technology has been suggested to pre-treat the feedwater of an RO system
with the aim of decreasing the formation of biofouling on the membrane.

It has been reported by many studies that the mechanism of deactivating microorganisms through the use of ultrasound lies in the mechanical effects of acoustic cavitation, especially the pressure shock generated from bubble collapse. The intensity of the pressure shock depends on a number of parameters such as ultrasonic intensity, ultrasonic frequency, temperature of the treated sample, pressure of the treated sample and the initial size of the bubble. Therefore, the effect of these parameters on the intensity of collapse pressure of transient bubbles was
investigated. A 55 kHz horn type ultrasound batch configuration reactor was used in this study. The experimental ultrasound work, involved sonication, thermosonication and manosonication treatments for E.coli deactivation. The effect of different levels of ultrasonic intensity, temperature, pressure and treatment time on the ultrasonic deactivation for E.coli suspension was examined in sonication, thermosonication and manosonication experiments.

The experimental results showed that the optimum process parameters of ultrasound treatment for E. coli disruption are represented by thermosonication treatment at low sub-lethal temperature of E. coli of 45°C. The feed solution of the RO system was prepared by suspending E. coli with a concentration of approximately 4×106 CFU/mL in a MacCONKEY broth. A stirred cell UHP 90 with a maximum capacity of 600 ml was used to simulate the RO system in its batch configuration. The efficiency of ultrasound treatment in reducing the formation of biofouling on the RO membrane was assessed based on two criteria: i) the measured
amount of permeate flux of the stirre cell and ii) analysis of the developed biofilm on the RO membrane. Straining and epiflourescence microscopy techniques were
used in this analysis. The ultrasound treatment was found to be capable of eliminating 103 CFU/mL of the existing E. coli in the broth-based suspension and resulted in injuring more than 10% of the log survival of E. coli. As a consequence, E.coli was able to recover more than 0.1 L/m2.h of the permeate flux of the treated feed during fouling treatment lasting 60 hours. Moreover, the captured
microphotographs of the membrane used with untreated and treated feeds revealed that the treated E. coli built sparse biofilms on the membrane, while the developed
biofilm by untreated E. coli covered almost all the membrane area.

The ultrasound and RO experimental results obtained in this study showed that ultrasound technology can be an effective free-chemical method to control the
formation of biofouling in a RO membrane system. Further study in optimising the current batch reactor is needed; a flow configuration of the membrane system can be
the focus of future work.