Feed Water Source

Measurable Process Parameters. The RO process is relatively simple ia design. It consists of a feed water source, feed pretreatment, high pressure pump, RO membrane modules, and ia some cases, post-treatment steps. A schematic of the RO process is shown ia Figure 2a. 

If the concentrate is discharged to the same body of water as the feed water source, both the intake and outfall should be located so as to not to interfere with one another. The presence of any other local desalination or wastewater discharge is an important factor, as these may increase the salinity of the receiving water or reduce its quality, thereby reduce the compatibility of the concentrate. 

NOTE Generally speaking, the higher the TDS of the feed-water source to the RO, the higher the applied pressure required to produce a constant permeate water TDS. Also, for practical reasons, the rate of permeate recovery tends to decrease with increase in source water TDS. 

In consideration of some operational difficulties, like biofouling and corrosion, directly related to the feed-water source, more reliable water sources have been evaluated by the Evides Industriewater that owns and operates this plant, like anaerobic groundwater and sweet tertiary wastewater. 

From the 2007 treated secondary communal wastewater (effluent from the communal wastewater treatment plant in the city of Terneuzen) is used as the new feed-water source [16]. 

In some cases, the actual design permeate flow rate of the RO system may differ from the "name plate" flow rating. In most of these situations, the RO system is de-rated by design due to a poor feed water source or as a natural result of low feed water temperature. 

Table 9.1 Recommended flux rates as a function of feed water source, as adapted from Dow Water and Process Solutions and Hydranautics.1-...

Feed Water Source Silt Density Index Average Flux, gfd Conservative Flux, gfd ... 

Feed water source also influences the design array of the RO unit. This is because the feed water flow and concentrate flow rates are also determined based on feed water quality. Higher feed water quality allows for higher feed flows and lower concentrate flows to be employed. Higher feed water flows and lower concentrate flows reduce the number of membrane modules required in the RO system. 

Tables 9.2 and 9.3 list the recommended feed water and concentrate flow rates, respectively, as functions of feed water source quality.1 Higher feed water flow rates result in water and its contaminants being sent to the membrane more rapidly, leading to faster rates of fouling and scaling. As Table 9.2 shows, an RO operating on a well water source can have a feed flow rate as higher as 65 to 75 gpm per pressure vessel, while a surface water source RO should not exceed 58 to 67 gpm per pressure vessel. The well water RO would require 12% fewer pressure vessels than the surface water RO.

Feed Water Source Maximum Feed Flow Rate for 365 ft2 Modules gpm Maximum Feed Flow Rate for 400 and 440 ft2 Modules, gpm... 

The recommended concentrate flow rate is a function of feed water source, as described in Chapter 9.1. The "cleaner" the water source, the lower the concentrate flow may be, resulting in smaller systems and lower overall cost of operation. 

Flux determines the overall size of the RO system in terms of membrane area required to achieve the desired separation. As discussed in Chapter 9.1.1, the water flux for a given application should be based on the feed water source. "Cleaner" source water allows for higher flux, which, in turn, means less membrane area is required to achieve the desired separation. 

The starting point in any RO design is the water flux. The desired water flux should be selected by the designer based on the feed water source and quality (see Chapter 3.4 and Table 3.3). 

Other data that should be entered into the screen shown in Figure 10.8 includes the "Water Type." Clicking on this button will open up a box that allows the designer to select the feed water source (see Chapter 9.1 for details about the importance of the source of the feed water to the RO system). This box also lists the recommended percent salt passage increase per year and fouling factor that corresponds to the type of feed water selected. (These guidelines should be entered... 

The first input screen is the water analysis screen, shown in Figure 10.12. This screen is where the designer inputs the water analysis, either as ppm ion or ppm calcium carbonate. The screen also has inputs for iron, SDI, hydrogen sulfide, and turbidity. There is a drop-down menu where the feed water source is input. The bottom of the page lists the scaling indices. 

More recently electrodialysis is being used in combination with mixed-bed ion-exchange resins for the production of ultra pure water. In this application electrodialysis is used as a pre-treatment step and is in direct competition to reverse osmosis which has the advantage to remove also neutral components in addition to the salts. For certain feed water sources, however, electrodialysis is preferred for economic reasons. 

Once the feed water source has been determined, analysis of the feed water composition is necessary before a treatment system can be designed. Feed water constituents that must be analysed prior to designing a RO/NF membrane system as per ASTM Designation D4195-88 Standard Guide for Water Analysis for Reverse-Osmosis Applications are discussed in Chapter 6. Typical water treatment methods are summarised in Table 2.2. 

Hydrogen sulphide is considered as a suspended sohd because it is readily oxidised to form colloidal sulphur it reacts instantaneously with chlorine to precipitate sulphur at the pH of typical feed water sources. Hydrogen sulphide is found almost exclusively in well water make-up sources. Reaction with dissolved oxygen in water precipitates sulphur almost as rapidly as exposure to chlorine. Colloidal sulphur is difficult to remove from the membrane surface. 

Figure 2.27 Membrane flux characteristics of a spiral wound RO module for various feed waters. Source Film-Tec membrane catalogue. For non-pure water solutions the flux reaches a constant, steady state value at higher pressures. For optimal performance, the system should be run below the "critical flux" region, which is the mid-point of the curved part of the curve.

The feed water source is a seawater infiltration well located in porous volcanic rock about 100 m from the shorehne. The well is dosed with sodium bisulphite (SBS) for disinfection... 

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