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10.5 CHEMICAL USE 227 10.5 CHEMICAL USE
Typically, the cost of chemical conditioning of source water for granular media filtration is in a range of 4%e6% of the total annual O&M costs for production of desalinated water. Granular media pretreatment systems use 50%e100% more source water condition- ing chemicals (iron salts and sometimes polymer) than membrane pretreatment systems for the removal of particulate and colloidal foulants (Prihasto et al., 2009). Practical experience to date shows that most membrane pretreatment systems do not use coagulants or only apply them intermittently during periods of severe algal blooms or heavy rains.
Granular media pretreatment systems do not use any chemicals for filtration media clean- ing (except for an occasional addition of chlorine). In contrast, membrane pretreatment sys- tems use a significant quantity of membrane cleaning chemicals for CEB and CIP, which in terms of total annual chemical costs may be comparable to the total costs of source water con- ditioning chemicals used by granular media filters. The cost of these cleaning chemicals should be considered in the costebenefit analysis of the plant pretreatment system.
Another factor that should be accounted for in the overall plant chemical use and cost anal- ysis is the RO system cleaning frequency, and therefore, the RO membrane cleaning costs. These costs may be reduced by using membrane pretreatment due to the typically better solids and silt removal efficiency of this type of pretreatment. However, if microbial fouling (e.g., biofouling) is the predominant type of RO fouling that occurs at a given desalination plant, membrane filtration typically does not offer any significant advantages to granular me- dia pretreatment and in some cases may accelerate the biofouling rate due to enhanced algal cell breakage/release of easily biodegradable organics under the high pressure/vacuum needed for filtration.
Under such circumstances, the accelerated biofouling caused by the breakage of algal cells by the membrane pretreatment system would result in more frequent (rather than reduced) RO membrane cleaning and elevated CIP chemical costs as compared to the use of gravity granular media filtration system for source water pretreatment.
A significant difference between the two types of pretreatment systems is the amount of chlorine used for filtration media maintenance. To control pretreatment filtration rate and RO biofouling, granular media filters are occasionally fed with chlorinated source water, which contains 1.5e5.0 mg/L of chlorine. This, so-called “shock” chlorination is typically completed once per month for 4e6 h at a time.
For comparison, chemically enhanced backwash with chlorine dosages of 20e200 mg/L is performed on all pretreatment membranes a minimum of once per day for a period of 20e30 min. Because the CEB process of most membrane pretreatment systems involves aire water backwash, some of the chlorine is stripped into the surrounding air and can cause corrosion of nearby equipment and unprotected structures. Therefore, the use of protective filter cell struc- ture and equipment coatings and of suitable corrosion resistant materials is of critical importance.
In addition, some of the applied chlorine is soaked into the pretreatment membranes and they may leach chlorine into the RO system feed for 20e40 min after CEB. Therefore, dechlo- rination of the filtered source water with sodium bisulfate after CEB cleaning is very impor- tant and the additional cost of dechlorination chemical should be taken into consideration when comparing granular media and membrane source water pretreatment.