Page 210 - ro membanes

9.2 THE MEMBRANE FILTRATION PROCESS 193
During CEB, the membranes are soaked for several minutes in chlorine and sometimes other cleaning chemicals (acids, alkali, or sodium bisulfite) and are than backwashed. The needed chemical dosages are a function of the predominant type and amount of foulants in the source water and of the type of membrane material and configuration.
Periodic membrane backwash and CEB do not completely eliminate membrane fouling and, therefore, the TMP needed to produce filtered saline water of target volume (flux) and quality increases over time. Once the TMP reaches a certain preset maximum level (typically 0.7e0.8 bar for submerged systems and 1.5e2.5 bars for pressurized systems), the membrane modules have to be taken offline and cleaned with chemicals that aim to reduce the TMP back to a reasonable level. Such a type of deep membrane cleaning aiming at the recovery of the mem- brane permeability, which cannot be achieved by CEB, is referred to as clean-in-place (CIP).
Membrane CIP is typically needed every 1e3 months and is performed using a combina- tion of low-pH solution of citric or sulfuric acid followed by a high-pH solution of sodium hydroxide and sodium hypochlorite. The cleaning chemicals are recirculated through the membranes for a period of 4e24 h and then the membranes are flushed and returned back to normal operation. Depending on the nature of the fouling, sometimes other cleaning chem- icals (biocides) are used to address specific fouling compounds (i.e., oil and grease, excessive biogrowth, etc.).
It should be pointed out that UF and MF pretreatment units have CIP system separate from that of the RO membrane racks, despite the fact that many components of the two CIP systems are similar.
Microbial biofouling is one of the most difficult types of fouling to remove from the UF and MF membranes and usually recovery of membrane permeability from such fouling requires very high dosage of chlorine (e.g., over 500 mg/L). If the membranes used for pretreatment of saline water of high biofouling potential have low chemical resistance and durability, and they can be damaged at chlorine dosages over 150 mg/L, while appropriate cleaning of the membranes requires chlorine dosage of 500 mg/L, then such membranes will lose permeability quickly and they will need to be cleaned in place very frequently (e.g., once every 1e2 weeks). For comparison, if more costly but more durable membranes that can withstand chlorine dosages of 500 mg/L or more are used to pretreat highly fouling source water, the membrane CIP frequency would typically be reduced to once every several months.
Typically, the lowest cost UF and MF membrane products available on the market also have the lowest strength and chemical resistance and therefore, the integrity of such mem- branes can be easily damaged by exposure to high dosages of membrane cleaning chemicals (oxidants, acids or bases). Therefore, such UF or MF membranes should only be used for pre- treatment of saline source water of low fouling potential.
9.2.3 Membrane Cleaning
Typically, inorganic foulants such as iron and manganese solids and colloids accumulated on the membrane surface are cleaned with citric, sulfuric acid, or hydrochloric acid. Fouling caused mainly by organic materials is treated by base such as sodium hydroxide, whereas biological and algal fouling is cleaned using disinfectants such as sodium hypochlorite or per- acetic acid, and surfactants. In all cases, the CIP is completed with heated cleaning solutions and RO permeate or filtrate. Using RO permeate for CIP cleaning is preferable.