Problems with Chloramines and Higher pH

Problem

The pH of municipal water has been recently increased in some areas in anticipation of the newly proposed lead regulations. In instances when pH has exceeded 9, and the water contained chloramines, a decreased rejection of solutes by polyamide thin film composite membranes has been observed.

It is thought that the high pH causes chloramines to dissociate into ammonium and hypo chlorite ions. The ammonium ions, which are poorly removed by activated carbon, interact with the polyamide membranes, causing their rejection characteristics to deteriorate. The decrease in rejection can generally be reversed by lowering the pH of water supply.

NOTE: Most larger municipal water systems are now using chloramines to treat water (versus free chlorine). This dramatically reduces membrane performance (and lifetime).

Solution

At pH higher than 9, chloramines must be completely removed from water to eliminate the potential damage of RO membranes. The damage can be caused by dissociation of chloramines into ammonia and chlorine. Chloramines can be removed completely by proper sizing of the activated carbon system at 1.35 cubic Ft. per minute of flow rate (this gives required empty bed contact time of 10 minutes).

The effect is more pronounced when the water sits stagnant on the membrane surface. A permeate flush is recommended to prevent any membrane damage.