Page 136 - ro membanes

6.2 COAGULATION 119
(e.g., their zeta potential is <20 mV), pH adjustment is not likely to have a measureable effect on the coagulation process.
Ferric chloride and sulfate are corrosive liquids and moderately toxic. Inhalation of their fumes could cause throat and respiratory tract irritation. It is important to keep in mind that when dry chemicals are diluted with water, the chemical reaction releases a large amount of heat.
In-line static mixers have lower energy and maintenance requirements and are relatively easy to install. They typically operate at a velocity range of 0.3e2.4 m/s (1e8 fps) and are designed to operate in a plug-flow hydraulics to provide uniform mixing within the entire pipe cross-section. The velocity gradient  contact time for such mixers can be determined by the formula:
.
GT 1⁄4 1212d ðDp L=QÞ0:5 (6.1)
where G, velocity gradient (s1); T, time (s); d, diameter of the static mixer (in.); Dp, differen- tial pressure through the mixer (psi); L, length of the static mixer (in.); Q, flow (gpm).
Although in-line static mixers are simple to install and significantly less costly, they have two disadvantages: (1) their mixing efficiency is a function of the flow rate because the mixing energy originates from the flow turbulence; (2) static mixers are proprietary equipment and the project designer would need to rely on the equipment manufacturer for performance pro- jections. Static mixers also create additional head losses of 0.3e1.0 m (1.0e3.3 ft), which need to be accounted for in the design of the intake pump station. Another important issue is to provide adequate length of pipeline (at least 20 times the pipe diameter) between the static mixer and the entrance to the pretreatment filters to achieve adequate flocculation.
Mechanical flash mixing systems consist of coagulation tank with one of more mechanical mixers and chambers. The coagulation tank is designed for velocity gradient GT1⁄4 4000e6000. The power requirement for the mechanical mixers is 2.2e2.5 horsepower/ 10,000 m3/day of processed flow. This type of mixing usually provides reliable and consis- tent coagulation, especially for desalination plants designed for significant differences in minimum and maximum plant production (e.g., more than 1:10).
6.2.2 Planning and Design Considerations
Overdosing of coagulants and their inadequate mixing with the source water are some of the most frequent causes for RO membrane particulate and colloidal fouling. When over- dosed, coagulant accumulates in the downstream pretreatment facilities and can cause fast-rate fouling of the cartridge filters (Fig. 6.4) following the pretreatment step and colloidal iron fouling of the RO membranes (Fig. 6.5).
The effect of overdosing of a coagulant (iron salt) on the silt density index (SDI) level can be recognized by visually inspecting the SDI test filter paper. In Fig. 6.6, the first two SDI test pads are discolored as a result of coagulant overdosing. The numbers below the pads are the SDI readings.
In such a situation, a significant improvement of source water SDI can be attained by reducing coagulant feed dosage or, in the case of poor mixing, modifying the coagulant mix- ing system to eliminate the content of unreacted chemical in the filtered seawater fed to the RO membrane system.