The use of chloramines has become a standard pretreatment method for biofouling control in water reuse RO. When used in the 2 – 3 ppm range, chloramines have been known to cause only minimal losses in salt rejection, and the benefits are generally believed to far outweigh the risks. However, in recent years, significant irreversible declines in permeability have been observed at some plants, with no major foulants found on the membrane surface or incoming feed water. These declines were often attributed to irreversible fouling caused by invisible layers of unknown organic compounds in the wastewater effluent feeding the water reuse RO systems. A thorough autopsy and feed water analysis of relatively new water reuse RO plant that was showing an especially severe irreversible decline found no evidence of any type of organic fouling. A review found several research papers identifying a relationship between dichloramines and loss in membrane permeability. However, at one plant using membranes from 3 different manufacturers and operating at pH ? 7, trains containing certain models saw an irreversible permeability loss of more than 50% while other trains fully recovered with a high pH CIP. Based on the Cl2:NH4 ratios and the operating pH, it was unlikely that any dichloramines were forming at this plant. Virgin membrane sheet from various manufacturers were evaluated in a cell-test apparatus. The membranes were operated with only NaCl, a bicarbonate buffer, and chloramine at varying pH levels. It was observed that all membranes exhibited a decline in permeability coupled with an improved salt rejection. However, upon cleaning, certain membranes would fully restore to their original performance, while others would show only a temporary improvement and then return to a continued decline in performance. Streaming potential analysis found that membranes exposed to chloramines had a significantly altered zeta potential as compared to virgin membranes of the same model. A stepwise method combining oxidation with a proprietary chemical CIP was found to fully recover lost membrane permeability, and when placed back in operation, losses in permeability due to chloramine exposure occurred at a far lower rate.
This presentation is available to AMTA Members only.
- Mo Malki
- American Water Chemicals, Inc. / Water Repenishment District of Southern California
- AMTA/AWWA Membrane Technology Conference, Las Vegas
- AMTA/AWWA Membrane Technology Conference
- Chloramines, Reverse Osmosis, Pretreatment