Page 243 - ro membanes

226 10. COMPARISON OF GRANULAR MEDIA AND MEMBRANE PRETREATMENT
Another waste stream that is associated only with membrane pretreatment is generated during the periodic chemical cleaning of the pretreatment membranes. Extended off-line chemical cleaning, often referred to as clean-in-place (CIP), during which membranes are soaked in a solution of hydrochloric and/or citric acid, sodium hydroxide, biocides, and sur- factants, is critical for maintaining steady-state membrane performance and productivity, and such cleaning is usually needed once every 1e3 months. The CIP cleaning generates addi- tional waste stream that is 0.03%e0.05% of the source water volume.
One key advantage of membrane pretreatment systems is that the waste filter backwash generated by them contains less or often no source water conditioning chemicals (e.g., coagulant, flocculant, and acid), and therefore, it is more environmentally friendly compared to the waste filter backwash stream generated by conventional granular media filtration pretreatment facil- ities. This benefit stems from the fact that typically coagulant dosage for source water pretreat- ment by membrane filtration is two to three times lower than that for granular media filtration.
In some cases, source water may not need to be conditioned with coagulant before membrane pretreatment and this spent filter backwash water could be disposed along with the RO concentrate without further treatment. For comparison, due to the high content of iron, the spent filter backwash from granular media filtration pretreatment would need to be treated by sedimentation and the settled solids would need to be dewatered and disposed to sanitary landfill. Otherwise, the high content of iron salt in the backwash water will cause the desalination plant discharge to have a red color every time, when a pretreatment filter is backwashed and the backwash is discharged with the plant concentrate. Fig. 10.1 depicts the reddish discoloration of the desalination plant discharge caused by the iron coagulant con- tained in the spent backwash water generated from granular media filtration system. After the addition to the saline source water, the iron coagulant (ferric chloride or ferric sulfate) is converted to ferric hydroxide, which is also commonly known as “rust” and has red color.
The waste streams generated during the CEB and the CIP membrane cleaning should be pretreated onsite in a neutralization tank before discharge. The additional treatment and disposal costs of the waste membrane cleaning chemicals should be taken under consider- ation when comparing membrane and granular media pretreatment systems.
FIGURE 10.1 Discharge of untreated backwash water from granular media filters.