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Pollution Prevention and Control Technologies for Plating Operations


Section 3 - Chemical Recovery

3.7 REVERSE OSMOSIS

3.7.5.1 Capital Costs

The capital costs of reverse osmosis units are best expressed in terms of membrane surface area, where the required area for a given chemical recovery application will depend on the flux rate and the percent rejection. Flux is the volume flow of permeate per unit of membrane area, usually expressed as gal/ft2/day or gfd. The percent rejection is defined as follows (ref. 348):

% Rejection = [(feed concentration - permeate concentration)/(feed concentration)] x 100%

Higher percent rejections will result in better quality (i.e., higher purity) permeate and a higher concentration of the plating chemicals. The permeate is typically reused for rinsing and the concentrated chemicals are typically returned to the bath.

In turn, these two factors are affected by the design of the RO equipment and the operating conditions of the system. More specifically, this includes: the type of membrane used, the applied pressure of the RO unit, the number of RO stages, the flow rate of the feed stream, the chemical concentration of the feed stream, and the required concentration of the product returned to the bath.

The feed stream flow to the RO unit is the rinse water, as shown in Exhibit 3-54. The flow rate of this stream and its chemical concentration will depend on the bath concentration, the drag-out rate, the required quality of rinse water in the final rinse and the rinsing configuration employed. For example, if drag-out recovery rinsing is practiced, the RO membrane surface area will be proportionally reduced by the percentage of drag-out returned to the bath by the recovery rinse.

Unfortunately, there are insufficient data available to account for the various parameters that impact RO system sizing and cost. Articles and conference papers that describe this technology fail to provide all of the needed data on system components such as membrane area, flux rates, drag-out rates, etc. Therefore, at this time, capital costs are simply presented in terms of the feed stream rate (Exhibit 3-60). The multi-pass data include systems with feed stream rates of 5 and 10 gpm and corresponding membrane surface areas of 597 ft2 and 1,194 ft2. The single pass data include systems with feed stream flows of 5 and 200 gpm. These units are applicable to end-of-pipe polishing. No corresponding membrane surface area data were provided for the single pass units.

Most RO units are sold as packaged systems that include a set of membrane modules, feed and recirculation pumps, prefilters, controls, and internal tanks.

 

Exhibit 3-60. Capital Costs for Single and Multi-Pass Reverse Osmosis Systems

3.7.5.2 Operating Costs

The most significant operating costs include O&M labor, energy, chemicals (cleaning) and membrane replacement. Estimates of RO operating costs for single and multi-pass systems are shown in Exhibit 3-61.


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