TABLE OF CONTENTS Membrane cleaning historically has not been the focus of major study vis-a-vis fouling even though it is a key element of long-term membrane life and operation. Membrane cleaning and sanitisation are required to restore membrane performance. Prompt and regular cleaning is necessary to prevent fouling from getting worse with long-term adverse effects. A typical flux vs. time curve for a UF membrane system in Figure 2.34 illustrates the effects of fouling and cleaning on flux. Flux decline slope values are used to judge the rate of colloidal fouling, and to determine when cleaning is required. [29] Figure 2.35 shows a severely fouled RO membrane that was not cleaned on time so that cleaning at that late stage became ineffective.
Cleaning is the removal of foulants and scalants from the membrane element to restore membrane flux and rejection as far as possible (flux is never restored to the new membrane condition). The main chemical and physical cleaning methods used are:
Chemical detergents and cleaner solubi-lises or disperses the foulants. Chemical cleaning efficiency increases with temperature since the rate of chemical reac-tion doubles with every 10 C increase in temperature.
Hydraulic energy in the form of high flow velocity in the feed channel. Ideally, cleaning should be done at turbulent conditions, i.e. Re 4 2,100, which is only possible with tubular membranes. Sometimes foam balls are used to scrape dirt...
