- Cleaning of MF and UF membranes is currently performed by a combination of water and air in either the forward or backward direction. When the above-mentioned cleaning methods are not effective enough to restore the flux to an acceptable level, it is necessary to clean the membranes chemically. During chemical cleaning, membranes are soaked in a solution of strong acids and bases such as hydrogen chloride (HCl) or sodium hydroxide (NaOH), or disinfection agents such as hypochlorous acid (HOCl). As a result of the effective chemical cleaning, the initial flux is restored and the membrane deemed as amenable for further operation. The shadow side of the process is an alteration of the membrane surface, which under some forced cleaning conditions results in formation of holes in the membrane skin layer. Since such a chemical cleaning shortens membrane lifetime, the next logical step is to evaluate to what extent chemical cleaning alters the membrane's surface. The parameters under investigation included various cleaning agents and their concentration, time of clean-in-place treatment and frequency of cleanings. As was found during the current study, higher dosages of cleaning agents result in complete restoration of the initial flux at the first step, but lead to more severe fouling, thus requiring faster clean-in-place operations in the long term. Another finding of the current research was that the character of the bonding between the foulant and membrane surface changes between suspended and adsorbed stages. Albeit the electrostatic character of BSA-PES surface interactions, the flux was not restored with the application of NaOH at virtually any concentration.