Reverse osmosis is a membrane separation process which has as its greatest application to date the desalting or purification

Transcription

1 ECONOMIC AND DESIGN FACTORS IN THE APPLICATION OF REVERSE OSMOSIS TO METAL FINISHING SOLUTE RECOVERY Abstract By Peter S. Cartwright, P.E., President C3 International, Inc W. County Rd. C, Roseville, MN Reverse osmosis is a membrane separation process which has as its greatest application to date the desalting or purification of water. This ability to remove both ionic and organic solute from relatively dilute streams make it a viable candidate in metal finishing waste treatment applications. In certain situations, reverse osmosis can be applied directly to the rinse water from a particular bath concentrating the salts enough to return them to the plating bath and the purified water back to the flowing rinse. Many factors affect the use of this technology in such "zero discharge" applications, and these are examined in detail. Economic considerations are also examined. New discharge regulations, coupled with more rigid enforcement of the older regulations, have required that many existing precipitation/clarification systems be revised or replaced. Reverse osmosis can be used to "dewater" or concentrate the mixed stream prior to the clarifier, thereby reducing the hydraulic load. Also, this technology can be used to "polish" the clarified effluent to ensure that the final stream will safely meet all requirements. Design and economic considerations are detailed. Backsround Alt.hough reverse osmosis has been commercially utilized since only about 1969, the basic concepts have been understood for years, and the origins of the process are based on osmosis, a fundamental action of nature. When a semipermeable membrane such as a living cell wall separates two solutions with differing concentrations of dissolved solids, pure water will flow from the solution containing the lower concentration of solute through the membrane into the solution containing the higher concentration of solute. This movement of water through the cell wall (semipermeable membrane) is a result of the fact that the solution containing less solute is at a higher energy state than the more concentrated solution. In order to attain an equilibrium of energy, the movement of water results. I

2 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, In the process of reverse osmosis, applied pressure is usually provided by a pump and amounts to adding energy to the more concentrated (lower-energy) side to cause the movement of water. Osmotic pressure is the difference between the potential * energy of any solution and that of pure water. It is a function of the specific solute and its concentration in water. In practical terms, it. is the minimum pumping energy required to produce the first drop of pure water from a solution of a given solute at a specific concentration. An extreme example of osmotic pressure can be found in the desalination of sea water. Typical sea water has an osmotic pressure of 400 psi, In order to get the first drop of potable water from sea water, the applied pressure must exceed 400 psi. On a practical basis, the pumping pressure should be in the range of 600 psi. Reverse osmosis (RO) specifically involves the separation of dissolved ionic materials from water. The exact mechanism of this separation, or "repulsion", is not fully understood, and disagreement exists between the leading theorists on this issue; but it is well-documented that the higher the ionic charge (valence) of an ion, the greater its tendency to be repelled from the surface of the memebrane. This means that monovalent salts such as sodium (Na+) and chloride (Cl-) will tend to pass through the membrane into the pure water side (permeate) at a higher rate than multivalent salts such as calcium (Ca++), nickel (Ni++) and sulfate (SO4--). The typical pore size of an RO membrane is on the order of 5 angstrom units (5 x 10-4 micrometers).

3 Figure 1 illustrates the mechanism of reverse osmosis. Figure 1, Reverse Osmosis Mechanism Although reverse osmosis can be discussed in terms of a "filtration" process, it does not involve the aspects of conventional (dead-end) filtration where the entire liquid is pumped through the filter media. HO utilizes a different process, known as "crossflow filtration" where the solution to be filtered flows tangentially (parallel) across the surface of the filter media, and, under pressure, a portion of this stream is forced through the filter media forming the permeate stream. The portion feed exiting without passing through the media is known as "concentrate" or "rententate." 3

4 0 Particle-free permeate I- Figure 2 illustrates these two filtration processes. I = Figure 2, Filtration Processes

5 Only certain plastic polymers have the properties to effectively perform as reverse osmosis membranes; these can be characterized as follows: POLYMER Cellulose Cellulose Thin Film Polyamide Acetate Triacetate Composite ph stability Oxidat ion resistance Poor Good Fair-Good Fair -Good Biological resistance Good Poor Fair-Good Good Temperature limit (C) 35 Typi cal reject ion of ionic species (8) > >90 The membrane can be "Packaqed" in several element configurations ("devices"j, each offering par ti cular advantages depending on the application. Tubular- Manufactured from ceramic, carbon, or a number of porous plastics, these tubes have inside diameters ranging from 1/8 inch up to approximately 1 inch. The membrane is typically coated on the inside of the tube, and the feed solution flows through the interior from one end to the other, with the "permeate" passing through the wall to be collected on the outside of the tube. ~ Hollow Fiber- Simi.lar to the tubular element in design, hollow fibers are generally much smaller in diamet.er and require rigid support.such as is obtained from the "potting" of a bundle inside a cylinder. Feed flow is either down the interior of the fiber or around the outer diameter. Spiral Wound- This device is constructed from an envelope of sheet membrane wound around a permeate tube that is perforated to allow collection of the permeate or filtrate. Plate and Frame- This device incorporated sheet membrane that is stretched over a frame to separate the layers and facilitate collection of the permeate. 5

6 Figure 3 illustrates these devices. PLATE AND FRAME ROLLTO SPIRAL WOUND POROUS SHEET MEMBRANE CONCENTRATE OUT PERMEATE OUT TUBULAR CONCENlRATE POROUS TUBE MEMBRANE - - c PERMEATE OUT HOLLOW FIBER PERMEATE SIDE BACKING MATERIAL WITH MEMBRANE ON EACH SIDE AND GLUED AROUND EDGES TO CENTER TUBE Fiyure 3, MeiiIbrdi,e Llriiierit Conf iguratioris For a given membrane polymer, the total volume occupied by the elements is dependent upon the area of membrane contained in each, or "packing density". The ability of the membrane element to resist fouling from suspended or precipitated solids is an extremely important property, as this is the greatest single cause of element failure. These characteristics are summarized for each element in the following table: Element Packing Fouling Configuration Density* Resistance Tubular Hollow fiber Spiral wound Plate and frame Low High High Low Good Poor Fair Good * Membrane area per unit volume of space required 6

7 Because of the propensity of suspended or precipitated materials to settle out on the membrane surface and plug the membrane pores (fouling), turbulent flow conditions must be maintained (Reynolds numbers in excess of 2100). For most waste treatment applications, this requires recycling a percentage of the concentrate back to the feed side of the pump. The addition of this concentrate stream into the feed solution obviously increases the dissolved solids concentration further increasing osmotic pressure. Svstems Desiun Considerations In order to treat an effluent stream, it must be thoroughly analyzed for the following properties: Total solids content Suspended (TSS) Dissolved organic (TOC) Dissolved ionic (TDS) Specific chemical constituents Oxidizing chemicals Organic solvents Operating temperature Usually, the goal is to "dewater" the feed stream as much as possible; that is, to remove water to facilitate either reuse or removal of the concentrated solute. Of importance also is the possible reuse of the purified water. These two considerations are significant in.determining both the process and membrane device to be used. 7

8 Figure 4 illustrates a general schematic for the reverse osmosis process. Note that the term "recovery" is defined as the percentage of feed volume that is pumped through the membrane and comes out as permeate (purified water). FEED >TREAY 'F cf 2- c... \.\..\ PERMEATE STREAM QP CP > CCNCENTAATE STREAM LJF Feed flow rete CF - holute coocentrstion in faad up - PerneaZe flor rete Cp - 5oluta concsntrotlon In permeare Uc - Concentrata flo- veta CC - Solute concentration In concentcmte Figure 4 namorene ~roccssinq Schemetic In virtually all applications, it is desired to otain as high a recovery as possible. Two factors limit recovery - osmotic pressure and permeate quality. As defined earlier, osmotic pressure increases as concentration increases. The force available to pump permeate through a membrane can be related to osmotic pressure as follows: Net driving force (psi) = pump pressure - osmotic pressure From a practical standpoint, the maximum pump pressure is usually 1000 psi.

9 No membrane is perfect in that it rejects 100% of the solute on the feed side, and this solute leakage through the membrane is known as "passage." Expressed as "percent passage," the total quantity of solute which passes through the membrane is a function of the concentration of solute on the feed side. Under high recovery conditions, the concentration of solute on the feed side is increased and therefore the actual quantity of solute passing through the membrane also increases. Because most effluent applications demand that, in addition to a minimum concentrate volume, the permeate quality be high enough to allow reuse or meet discharge regulations, the "Catch-22" predicament of permeate quality decreasing as recovery is increased can impose design limitations. Zero Discharge Applications Figure 5 illustates the layout for a reverse osmosis system installed on the 1st countercurrent rinse in a standard electroplating line. Under the right conditions, the concentrate flow will equal the evaporation rate from the plating tank, and since this usually accounts for the only loss of bath volume, the plating salts which are concentrated from the first rinse by the reverse osmosis system are almost all returned to the plating bath. The permeate from the reverse osmosis unit is usually pure enough to be directed to the last rinse for total recycle. Make-up water, in the same volume as that lost though evaporation from the plating tank, is also added to the last rinse. This is usually tap water purified by deionization or reverse osmosis, and typically amounts to less than 108 of the feed flow to the zero discharge reverse osmosis unit. 1 PL4TlNG TANU V., REVLRSE PERME4TE?

10 In studying this application, it soon becomes apparent that those plating baths which operate at higher temperatures are the best candidates for zero discharge reverse osmosis treatment. Watts nickel baths are excellent in this regard, and with the high value of these plating salts, the investment in the recovery equipment can usually be paid off within 18 months, The chemistry of the bath can play a major role in the performance of the membrane, and to date, those baths with high oxidizing chemistry present chemical compatibility problems for the available membrane polymers, Because of its high packing density and relatively high resistance to fouling, the spiral wound element configuration is used in virtually all existing systems. Plating baths operating at temperatures below 50 degrees C (120 degrees F) offer a multitude of problems because of their low evaporation rates. If reverse osmosis is used directly, it may be impossible to attain the recovery required because of the high osmotic pressure effect. This effect can be significant when considering that the increase in salts concentration between recoveries of 95% and 98% is a factor of 2.5. to 1. To minimize this problem, techniques such as air agitation of the plating bath, partial evaporation of the concentrate stream, directing the concentrate stream to an air scrubber and bleeding part of the bath directly into the reverse osmosis unit have been used with success. Figures 6 Through 10 illustrate the relationship of recovery to osmotic pressure for rinse streams from various baths. These rings contain approximately 3000 mg/l of plating salts. Athough the osmotic pressure increases with increasing recovery for all bath chemistries, there is considerable variation in the curves. IO LO REC)VER'I, I

11 LOO &O LOO MCOVERI. I F I GURt 7 ttftci OF PLCOVLIlY ON OSWTIC PRLSSURt : ZINC CYAVIDL RISE WATER

12 EFFT.CT OF RECOVERY 01 OSWTIC PRfSSVRE : BRACS CIAVIOL RIRSC VATLR " U Id 20 XJ WCOVERI. r i v d I)ECORATI\'E CHROr HARU CHI(OXE

13 Fiqure 11 summarizes test data on zero discharge applications for 6 common bath rinses. RO Test Data For Six Common Plating Bath Rinses Walts Ni #.tiw baul Copper cyanlde Zinc cyanide Brass cyanlde Oecoratwe Cr Hard Cr Bath 1.mpmhrre. 'CPF 60f140 60/140 27/80 27/80 43/110 55/130 Toxk contamlnmt' Ni" CU' CNa* CNa* cu' CN- Cr" Cr" Pucull n)rctkn" g9t m 'Rlnse flow adjusted to glve a tot61 salt concentration of approximately 3000 mg/l. "Rejectlon of toxfc contamlnant in permeate calculated as follows: [l-(conc. In PermeaWConc. in Feed)] 100. The following have been installed in North America to date: Bath Treated Nickel Acid Copper Acid Zinc Copper Cyanide Hexavalent Chrome Number of Existing Systems Feed TDS: mg/l Typical feed rate: gph "End of Pipe" Mixed Effluent Treatment Because of its ability to concentrate all dissolved solids out of water solutions, reverse osmosis has found application in "dewatering" effluent streams, both before and after conventional waste treatment systems. Figure 12 illustrates the application of reverse osmosis to dewater mixed rinses prior to chemical precipitation/ clarification. This approach offers the advantage of reducing hydraulic loading to the precipitation/clarification process, thereby conserving space when a treatment system requires expansion. It also offers the advantages of producing pure water that can be recycled to the process for rinsing, boiler feed, or other purposes. /3

14 ..nt.ratc s 1 anl-tldr clarif icr Figure 12 t o I.uxif I I I Figure 13 illustrates the applications of reverse osmosis to clarified effluent. This is particularly useful when a change in discharge regulations, process chemistry or an upset in the precipitation/clarification system results in unacceptable concentrations of hazardous wastes remaining in the clarified effluent. Also, there are situations where the high concentration of sodium and calcium salts from the clarifier preclude either reuse of the treated water or discharge to the POTW (Publically Owned Treatment Works). By treating the clarified effluent with reverse osmosis, purified water can be obtained representing 90-95% of the feed flow, with the concentrate portion accounting for only 5-10%. 15-LO? Figure 13 "E"* ti-..tlld"t

15 As with zero discharge applications, osmotic pressure and permeate quality are the limiting factors for maximum recovery. Usually, the goal is to dewater the stream as much as possible in order to have a minimum quantity of concentrate for further treatment or disposal. The intended use of the permeate will typically dictate its quality requirements, and this, along with osmotic pressure considerations, will set the maximum recovery for each application. Figure 14 illustrates the approximate capital costs of the reverse osmosis systems as a function of feed flow. These uninstalled costs apply to high recovery systems for both zero discharge and total effluent dewatering applications. 3. * -ljure 14 fccd Rate lgphl I \,tr, I

16 Operating costs are a function of the following factors: electrical consumption membrane element life cleaning frequency pre-filter maintenance other system maintenance The most significant operating cost factors are electrical usage and membrane replacement; at an electricity cost of 3.06 per kwh and a two year life for the membrane elements, (Typical- operating costs are $1.50 to $2.00 per 1000 gallom of feed. Conclusion The purpose of this paper has been to illustrate the versatility of reverse osmosis for effluent treatment in metal finishing applications. As the advantages of this unique separation technology are more thoroughly understood, it. will find greater application in the metal finishing industry,