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Pollution Prevention and Control Technologies for Plating Operations
Section 3 - Chemical Recovery
3.7 REVERSE OSMOSIS
3.7.1 Overview
Reverse osmosis (RO) is a separation process that has been employed in the metal finishing industry to purify raw water (e.g., city water) before use as rinse water, recover plating chemicals from rinse water, and polish wastewater treatment effluents (usually for reuse as rinse water). Of particular interest in this section of the report is the application of RO for chemical recovery, however; end-of-pipe applications are also covered. Use of this technology as a raw water treatment technology is not covered in this report.
As a recovery technology, RO has been applied to a range of processes, including: brass, chromium, copper, nickel, tin and zinc plating solutions (ref. 269, 348), with nickel recovery being the most frequent and successful (ref. 39). Of the 318 plating shops responding to the Users Survey, only six applications of RO chemical recovery were identified. The survey also included one application for end-of-pipe treatment and several applications of raw water purification. The infrequent use of RO for chemical recovery may be due to the limited number of baths to which it has been successfully applied and the availability of competing technologies (especially for nickel plating). Further, one of the competing technologies frequently used for nickel recovery is atmospheric evaporation, which has a very low capital cost.
Reverse osmosis is often referred to as a "crossflow filtration" process. This term, which also describes most ultrafiltration and microfiltration equipment used by the metal finishing industry, distinguishes these processes from surface barrier filtration, which operates in dead-end flow. As shown in Exhibit 3-53, in dead-end filtration, all of the feed solution is forced through the membrane by an applied pressure. With crossflow filtration, the fluid to be filtered is pumped across the membrane, parallel to its surface. Because the feed and concentrate of RO flows parallel to the membrane instead of perpendicular to it (i.e., dead-end flow), the process is termed crossflow. The pressure required to drive the process is determined by the specific nature of the feed solution and the membrane pore size (ref. 380, Osmonics file).
There are several key differences between RO and ultrafiltration/microfiltration: (1) only RO has the ability to concentrate dissolved salts (e.g., plating chemicals); (2) RO cannot tolerate significant concentrations of suspended solids, whereas the other two processes can, especially microfiltration; and (3) RO operates at higher pressures and usually requires a heavy gage stainless steel housing, whereas the other two lower pressure processes can be housed in plastic or lightweight stainless steel. It should be noted, that whereas RO is a distinctly unique filtration process, ultrafiltration and microfiltration are similar to one another and have overlapping definitions (ref. 380). Microfiltration, which has the largest pore size of the three technologies, is discussed in Section 4, as a method of alkaline cleaner maintenance. Also, both microfiltration and ultrafiltration are discussed in Section 6, as end-of-pipe polishing technologies. Ultrafiltration is also used by several survey respondents for the recovery of electrocoat (paint), an application not covered by this project. It is also used in the machining industry for the recovery of cutting oils.
RO theory is based on two physical processes: osmosis and ionic repulsion. Osmosis is related to diffusion, which describes the tendency of molecules in solution to move about until they are uniformly distributed. Osmosis is the tendency for diffusion to take place across a semipermeable membrane. It occurs when a water permeable membrane separates two solutions of different concentrations of dissolved solids. Pure water will flow into the concentrated solution until an equilibrium energy state is achieved. By applying pressure to the more concentrated solution, the normal osmotic flow is reversed and pure water is forced through the semipermeable membrane into the less concentrated solution. Suspended solids are blocked by mechanical exclusion and dissolved solids are chemically repulsed by the membrane surface. Multi-charged ions are rejected at rates exceeding 99 percent and single-charged ions have rejection rates in the range of 90 to 96 percent. RO will also reject neutral solutes, although no general efficiency data are available. Besides ionic charge, rejection efficiency is also affected by the concentration gradient. As the concentration gradient increases, the rejection efficiency decreases. The flow of water through an RO membrane (flux) is determined by the pressure differential across the membrane. Higher pressure differentials generally result in higher flow rates.
The RO process is designed to operate continuously. The RO membrane is enclosed in a pressure vessel and the feed stream is pumped through the vessel under pressure, 400 to 1,000 psig, where it is separated into a clean water permeate stream and a concentrated chemical stream by selective permeation. Three important parameters describe the performance of the RO process: recovery, flux, and rejection.
Recovery is defined as the percentage of the feed that is converted to permeate and it is usually expressed as percent. Flux is the rate at which the permeate passes through the membrane per unit of membrane surface area. Rejection is the ability of the membrane to restrict the passage of dissolved salts into the permeate, and is related to particular salt species (ref. 39).
There are different types of RO membranes used (tubular, spiral wound and hollow fiber), the selection of which depends mostly on the applications and in particular the plating bath chemistry. The most common RO membranes are the hollow fiber and spiral wound configurations.
Most reverse osmosis systems are designed with a single filtration stage operating below 700 psig. With a single stage system operating in this pressure range, the practical limit for concentrating plating chemicals in rinse waters is 15 to 20 g/l. Because this concentration is below that of most plating baths, a "solution volume" problem is sometimes created with an RO recovery application, in that there is insufficient head-room in the process tank for the return of the recovered chemical solution. This problem occurs especially with ambient to low temperature baths, where the surface evaporation rate is low. This condition limits the direct reuse of the RO concentrate stream in the plating tank. An evaporator can be used to further concentrate the solution or to supplement tank surface evaporation. However, the added capital and operating costs of an evaporator often make this approach less attractive than using an alternative recovery method.
Newer reverse osmosis technology includes multiple stage systems and higher operating pressures (800 to 1,000 psig). With the multiple stage design, the concentrate stream from the first stage is passed through a second stage to further concentrate the chemicals. This permits the direct reuse of some solutions that could not be directly recovered with the less effective single pass units.
The key attributes of RO as a recovery technology are: (1) it is an ambient temperature, low energy process; (2) it generates a permeate stream that is usually of sufficient quality that it can be reused for rinse water; and (3) for some applications, it has relatively low capital and operating costs as compared to other recovery technologies. The negative aspects of this technology are: (1) RO membranes can be fouled by precipitation products and/or suspended solids; (2) membranes have a fairly limited life-span; (3) this technology does not sufficiently concentrate the chemicals for direct return in some applications; and (4) similar to most other recovery technologies, RO returns both essential plating chemicals and unwanted impurities to the bath, unless some post-treatment is performed.