Hot water sanitisation

Sanitisation means disinfection for reducing microbial contamination down to an acceptable level, whereas sterilisation implies the inactivation of all microorganisms. Cleaning always precedes sanitisation. When a membrane system (membrane elements, pressure vessels and piping) has been infected by bacteria or mould, the system requires chemical sanitisation. Hot water sanitisation is used in pharmaceutical membrane systems. 

HWS is commonly used to kill microorganisms. RO membranes reject bacteria and other microbes. However, due to defects in the membrane surface, the rejection is not 100% or absolute so that microorganisms can pass through the membrane into the permeate side of the membrane where they can multiply. Hence, sanitisation of a membrane system is essential for the production of US Pharmacopeia (USP) water and water for injection to meet the very low bacterial limits [57-59]. HWS of RO membrane (and EDI) systems is a relatively recent development made possible by the development of TFC membranes and membrane equipment capable of handling hot water at 85° C for a short time membrane elements are manufactured with special adhesives, permeate tubes and connectors to withstand elevated temperatures. These membranes also make it possible to sanitise and protect the RO membranes from bio-fouling especially when the use ofbiocides is not acceptable. Typical membrane manufacturers specifications of hot water sanitisable TFC membranes are given in Table 2.14. 

Table 2.14 Hot water sanitisable operating conditions for a TFC-SW-RO membrane eiement ...

It is not necessary to operate the RO-EDI system in a USP water plant continuously. This wastes water and there is no benefit in microbiological control. Similarly, hot water sanitisable RO and EDI systems are no better at controlling bacteria. 

Feed water temperature, hardness, alkalinity, free chlorine and pH must meet the EDI quality requirements given in Table 2.3. RO pre-treatment is essential. In critical applications where bacteria control is important, a bacteria destruct 254 nm UV light is installed to reduce the bacteria count in EDI feed water. EDI modules are sanitised with chemicals or preferably hot water at 80°C. 

Frequent backwash of carbon media filters is more effective than hot water or steam sanitisation since the microbial load reduction by sanitisation is temporary due to the rapid recolonisation of the sanitised biofihn in the carbon bed. 

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