Page 134 - ro membanes

6.2 COAGULATION 117
At present, ferric chloride is more commonly used than ferric sulfate for saline water pre- treatment because of the lower application dosage and lower unit chemical costs. However, the use of ferric chloride could have a significant disadvantage if it is not of high purity and contains significant amount of manganesedi.e., accelerated fouling of the plant cartridge fil- ters and potential oxidation of the RO membranes by spontaneous generation of permanga- nate oxidant in the concentrate as it travels through the membrane vessels. In general, ferric chloride is cheaper coagulant because it is generated as a waste product of iron ore pickling. Because all iron ore naturally contains manganese, typically commercially available ferric chloride also has this metal impurity.
If a manganese content in ferric coagulant exceeds certain threshold, this manganese tends to precipitate on the cartridge filters downstream of the plant pretreatment facilities and ultimately on the RO membranes. As a result, the useful life of a cartridge filters is often shortened from a normal duration of 6e8 weeks down to 1e2 weeks and RO membranes are exposed to slow oxidation, which results in the permanent loss of membrane rejection within 1e2 years of their installation.
The most common approach to identify whether the ferric chloride used for coagulation is of inadequate quality (i.e., contains unacceptably high concentration of manganese) is to observe the color of the cartridge filters downstream of the desalination plant pretreatment facilitiesdexcessive content of manganese creates typical dark brown/black discoloration of the cartridge filters (Fig. 6.3).
RO membrane oxidation is caused by the spontaneous generation of permanganate in the concentrate from the magnesium in the ferric chloride as concentrate salinity increases along the length of the RO membranes. The process of permanganate generation is not well known and studied at this time. Nevertheless, operational experience indicates that permanganate oxidation has a slow but irreversible degrading impact on RO membrane salt rejection in seawater reverse osmosis (SWRO) desalination plants using low-cost/low-quality ferric chlo- ride which contains more than 50 micrograms of manganese per gram of ferric chloride.
FIGURE 6.3 Cartridge filters with dark brown/black discoloration as a result of the use of poor quality ferric chloride.