RECOVERY AND RECYCLE

Transcription

1 , RECOVERY AND RECYCLE OF AQUEOUS-BASED CLEANERS Mark S. Rizzone and Daniel F. Tumer Sanbom Technologies 25 Commercial Drive Wrentham, MA Paper presented at the seminar on "Metal Cleaning Altematives to l,l,l- Trichloroethane and CFC-113: Ozone Layer Protection," Dallas, TX- September 28, 1993.

2 . RECOVERY AND RECYCLE OF AQUEOUS-BASED CLEANERS ABSTRACT New products have been developed which recover and recycle aqueous-based cleaners. These products are based on crossflow membrane technology. Ultrafilters and microfilters have been used. These filters allow the cleaner to pass through but reject the oil and solid contaminants. Benefits include extended bath life, reduced waste volume, reduced cleaner purchases, less bath maintenance, improved parts cleaning, and less liability

3 INTRODUCTION With the use of l,l,l-trichloroethane and CFC-113 being phased out by the end of 1995, an increasing number of metal finishers will be converting from solvent-based cleaners to aqueous-based cleaners. These cleaners will remove dirt, oil and grease, metal fines, and other contaminants from the metal part by immersion in a recirculated cleaner or by spraying with a recirculated cleaner. Bath sizes can range from ,000 gallons. Alkaline cleaners are the most common aqueous-based cleaners. They consist of water, alkali, surfactants, and organic and inorganic additives. They have a ph usually between 9 and 13 and are at elevated temperatures to enhance cleaning. Temperatures can range from 105 F to 195 F. As the parts are cleaned, dirt, oil and grease, metal fines, and other contaminants are washed into the cleaner bath. These contaminants gradually accumulate in the bath due to continuous drag-in and recirculationheuse of the cleaner. Eventually, they reach a high enough concentration to reduce the effectiveness of the cleaner. At this time, the cleaner is disposed. The tank is cleaned and refilled with fresh cleaner. PROBLEM Spent aqueous cleaners will contain oil and grease, metal fines, and other organic and inorganic contaminants, both dissolved and undissolved. Figure 1 shows a typical spent cleaner analysis. The surfactants in a cleaner will contribute to the oil and grease value. Many of these surfactants are ionic in nature, typically anionic for oily wastes. The total hydrocarbon test is similar to the freon extraction oil and grease method except that silica gel is used prior to the extraction to remove ionic surfactants. Therefore, the hydrocarbon test is actually an indicator of true petroleum hydrocarbons. Limits exist for discharge of oily wastes to sewer, surface water, or ground. The limits for sewer discharge are less strict than for surface water which are less strict than for ground discharge. The limits are less strict for sewer discharge because a municipal wastewater treatment plant is downstream of the sewer. The discharge of the treatment plant must meet surface or ground limits

4 n FIGURE 1. TYPICAL SPENT CLEANER ANALYSIS Parameter Oil & Grease Result mg/l Hydrocarbons Phosphorous Fluorides Manganese Total Suspended Solids mg/l I mg/l mg/l mg/l mg/l Total Dissolved Solids ,000 mg/l PH 9-10 Temperature F - 4 -

5 Figure 2 shows a typical set of effluent limits for sewer discharge. If these effluent limits are exceeded, the dischargers are subject to heavy fines, plant shutdown and/or imprisonment. Spent aqueous cleaners will typically not meet sewer discharge limits because of the presence of oil and grease and other contaminants. SOLUTION The plant has a number of choices for solving the problem of oily waste disposal: 1. pay a waste hauler to haul the spent cleaner and other oily wastes to an appropriate disposal site 2. treat the cleaner and other oily wastes by an end-of-pipe waste treatment method 3. recover and recycle the aqueous cleaner The choice of solution will depend on a number of factors including capital cost, operating cost, payback, available space, effluent limits, operator skills required, amount of labor and maintenance required, effect on cleaning efficiency, and liability. BAULING Hauling is the easiest and simplest solution. A waste hauler simply picks up the waste and hauls it to an appropriate disposal site such as a waste treatment center. Hauling costs can vary depending on the nature of the waste, the region of the country, and the transport distance. Typical hauling costs are in the range $0.25-$5.00 per gallon. Hauling can be expensive. For example, if 5000 gallons of wastewater are disposed every month at $2.00 per gallon, the annual hauling cost would be $120,000 per year. Waste minimization methods exist which can reduce the hauling cost by 90% or more. These methods include chemical treatment, evaporation, and membrane filtration. These are end-of-pipe methods

6 , FIGURE 2. TYPICAL EFFLUENT LIMITS FOR SEWER DISCHARGE Parameter Oil & Grease Limit mg/l BOD5 300 mg/l Total Suspended Solids 250 mg/l Cadmium Chromium (Total) Chromium (Hex.) Iron Nickel Phosphorous Zinc 2 mg/l 25 mg/l 10 mg/l 50 mg/l 5 mg/l 500 mg/l 15 mg/l PH 5-10 Temperature 150 F - 6 -

7 Chemical treatment utilizes chemicals such as acid and alum or polymer to break the oil-in-water emulsion. A sludge is generated along with clarified effluent. This method can treat low flows (as small as a drum a day) as well as high flows (hundreds of thousands of gallons per day). The chemical treatment process will reduce oil and grease, suspended solids, and sometimes dissolved heavy metals (by coprecipitation). However, it is subject to upset by variable flows and composition. If an upset occurs, the discharge limits can be exceeded. The process requires considerable attention to ensure proper chemical dosage. Evaporation utilizes heat from a boiler to evaporate water from the oily waste. An oily waste concentrate is left behind. The water vapor is normally discharged to the atmosphere. In some states, an air permit is necessary for discharge. The permit is not required if the vapor is condensed. Evaporation is usually used for low volumes of wastewater due to the large amount of energy necessary to evaporate water (approx BTUAb). It has a high operating cost. A high waste volume reduction can be achieved (on the order of 95%). The evaporator is simple to operate, requires little space and little chemical knowledge. However, it does have the risk of fire or explosion due to the introduction of low flash point liquids. It can also produce noxious odors. A crossflow membrane filter can be used to separate water from the waste. The quality of the water will depend on the nature of the waste and the type of membrane. Different membranes exist: microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. When treating oily wastes for disposal to sewer, an ultrafilter is typically used. If the discharge limits cannot be satisfied by an ultrafilter alone, a nanofilter or reverse osmosis unit might be used. Membrane filtration systems have been designed to handle low flows of wastewater (e.g., a drum a day) to high flows of wastewater (e.g., thousands of gallons per day). Scale-up is relatively easy since output is primarily a function of membrane area. The effluent quality is more consistent than in the case of chemical treatment since a physical barrier is used. A membrane system is less subject - 7 -

8 to upset by varying flow and composition. Chemicals are used only if ph adjustment is necessary or if a cleaning has to be performed. The manpower requirement is much less for a membrane system than a chemical treatment system. Compared to evaporation, a membrane system is less energy-intensive and has smaller capital and operating costs. A permit for discharge to air is not necessary but a permit for discharge to sewer, surface water, or ground is necessary. The objective of an aqueous cleaner recovery and recycling device is to remove the contaminants but not the components of the cleaner (e.g., the alkali, surfactants, and organic and inorganic additives). The cleaner is recovered and recycled back to the bath thereby purifying the bath. Contaminants can include coarse suspended solids, free oil (floatable and dispersed), colloidal solids, and emulsified oils. The choice of method for recovery and recycle of the cleaner will depend on the nature and concentration of contaminants which must be removed. For removal of coarse suspended solids, one or more of the following devices can be used: settling tank, strainer, bag filter, paper bed filter, hydrocyclone, or centrifuge. For removal of finer solids, an electrostatic precipitator can be used. Free floatable oil can be removed by overflow, decantation, or oil skimming. Mechanically dispersed free oil can be removed by a coalescer. Emulsified oils and colloidal solids can be removed by a crossflow membrane filter. A thermal oil separator has been used to thermally crack the emulsion but it requires high energy and is not as efficient at removing the oil as a crossflow membrane fiiter. EMBRA NE FILTRATI ON Crossflow membrane filtration will produce the highest quality, recyclable aqueous cleaner. It must usually be preceded by a coarse solids removal device such as a bag fiiter and a free oil removal device such as a skimmer or a coalescer. - a -

9 There are basically four types of membrane filtration processes: microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. See Figure 3. They vary in terms of the sizes of particles they reject and the pressures needed for operation. In general, the order of decreasing particle size is microfiltratiom ultrafiltration> nanofiltratiom reverse osmosis Microfiltration will remove suspended solids and emulsified oils greater than the pore size of the membrane ( microns). Ultrafiltration will remove suspended solids, emulsified oils, colloidal solids, and high molecular weight organics greater than the pore size of the membrane ( microns, ,000 molecular weight) Nanofiltration will remove organics of intermediate molecular weight (typically, > molecular weight) such as soluble dyes and surfactants. It will also remove multivalent ions such as calcium and magnesium and heavy metals. Reverse osmosis will remove low molecular weight organics (>lo0 molecular weight) and monovalent ions such as sodium and chloride. The order of increasing operating pressure is microfiltration< ultrafiltration< nanofiltration-e reverse osmosis psi psi psi psi The above pressures are from the authors' own experiences on oily wastewaters. In the case of reverse osmosis, the pressure must overcome the osmotic pressure of the solution. Microfiltration and ultrafiltration have been applied to recovery and recycle of aqueous cleaners ( primarily alkaline cleaners). Nanofiltration and reverse osmosis are typically not applied to these cleaners since they would reject the components of the cleaner (e.g., surfactants and dissolved salts). These latter membranes could be used for recovery and recycle of rinsewaters. Microfilters and ultrafilters separate based on the relative size difference of the particles and the pores of the membrane. See Figure 4. Waste is passed tangentially across the membrane surface. Those species which are larger than the pores of the membrane get rejected by the membrane while those that are smaller than the pores of the membrane pass through the membrane. In crossflow membrane filtration of aqueous cleaners, it is desired to reject - 9 -

10 FIGURE 3. FILTRATION SPECTRUM

11 . FIGURE 4. Separatiqon By Size WASTE OUT I Porouscros sflow Membrane Filter I 0 F F I OIL 0 WATER Filtrate Out: WATER SURFACTANTS DISSOLVED SALTS WASYE IN

12 emulsified oils and suspended solid contaminants but to allow the cleaner components (i.e., water, alkali, surfactants, and other additives) to pass through as permeate. The permeate is retumed back to the cleaner bath. A microfiltration membrane is isotropic, that is, its structure is uniform throughout. The pore sizes are also uniform. Pore sizes of microfilters can range from microns. For recycling of alkaline cleaners, cornmon pore sizes are microns. An ultrafiltration membrane is anisotropic, that is, it consists of a thin skin supported by a porous support structure. The pores of an ultrafilter are smaller than the pores of a microfilter. Pore sizes can range from microns. Ultrafilters are usually characterized by molecular weight cutoff (that is, the nominal molecular weight of a solute that is 90% rejected by a membrane). Cutoffs for ultrafiltration membranes have ranged from ,000. For recycling of aqueous-based cleaners, cutoffs of 3 0, ,000 have been used. Choice of a microfiitration or ultrafiltration membrane will depend on several factors including the nature of the cleaner, the nature of the contaminants, the desired permeate quality, the desired permeate output, membrane cleanability, membrane compatibility with the cleaner, and the resistance of the membrane to oils and high temperature. A membrane should be chosen which adequately rejects the contaminants such as emulsified oils and suspended solids but allows passage of the cleaner components. Because of the larger pores in a microfilter, more surfactant should pass through than in an ultrafilter. However, too large a pore size could result in passage of emulsified oils. A typical analysis of permeate from an ultrafilter is shown in Figure 5. A membrane should be chosen which will yield a high, stable permeate output. The output is typically measured as a volumetric flow rate in gallons per minute. Membrane technologists usually convert this output to a flux, which is typically given in gallons per square foot of membrane area per day. Typical fluxes for crossflow membrane filtration of alkaline cleaners are gal/sq. ft./day. The flux will vary according to a no. of factors (to be discussed later)

13 FIGURE 5. FEED AND UF PERMEATE ANALYSIS Parameter Oil & Grease Hydrocarbons Phosphorous Fluoride Manganese Suspended Solids Dissolved Solids PH Temperature Feed mg/l mg/l mg/l mg/l mg/l mg/l ,000 mg/l F UF Permeate mg/l 6-15 mg/l mg/l mg/l mg/l 40 mg/l mg/l F

14 The membrane output decreases with time and eventually must be cleaned to restore its flux. Different membranes will vary according to their ability to be cleaned due to their different materials of construction and their different structures. A membrane should be chosen which can be easily cleaned. It should not be irreversibly fouled. Otherwise, it would have to be replaced. The membrane material should be compatible with the components of the cleaner. Many times in new applications, a cleaner is tested for its effect on the physical dimensions and properties of the membrane. If these dimensions or properties after exposure are much different than normal, then the membrane is not recommended for the application. The membrane material should be resistant to oils and should have good thermal resistance. In conventional filtration, flow is perpendicular to the filter surface. If membranes were operated in this manner, they would rapidly foul due to a build-up of accumulated material on the filter surface.. Instead, crossflow filtration is used to minimize fouling along the membrane surface. Crossflow involves flow which is tangential to the membrane surface (see Figure 6). A strong sweeping action is created along the surface of the membrane. The general crossflow membrane filtration schematic is shown in Figure 7. Feed enters at pressure, P1; concentrate leaves at pressure, P2; and permeate leaves at pressure, P3. The pressures decrease in the following order: P1> P2> P3. The crossflow pressure drop, Pcf, is defiied by Pcf= P1- P2 (1) The average transmembrane pressure drop, Ptm, is defined by The transmembrane pressure drop varies along the length of the membrane. It is highest at the feed end and lowest at the concentrate end

15 w N 3 0 e, w 0 4 cn H

16 n Q) t M e

17 The output of either a microfilter or an ultrafilter is a function of a no. of factors including crossflow pressure drop, average transmembrane pressure drop, concentration of emulsified oil, foulants, temperature, and ph. The greater the crossflow pressure drop, the greater the sweeping velocity, and the higher and more stable the output. The increase in stability is due to the decreased tendency to foul from the increased sweeping action. The greater the transmembrane pressure drop, the greater the output up to a point. As the concentration of emulsified oil increases during the concentration cycle, the output decreases. Initial oil and grease concentrations can range from 50 mg/l-5000 mg/l. Final concentrations can be 20-50% emulsified oil or higher. Foulants can cause a crossflow membrane filter to rapidly decline in output. Typical foulants include free oil and coarse suspended solids. A good pretreatment scheme can help prevent these foulants from entering the membrane system. For example, free oil can be removed by overflow, decantation, skimming, and/or coalescence. Coarse suspended solids can be removed by settling, straining, filtration, hydrocyclones, and/or centrifugation. Foulants which accumulate on the membrane surface are usually removed by a hydraulic chemical cleaning. Cleaning agents include detergent, permeate, or fresh alkaline cleaner. The hardness of the make-up water to the cleaner can affect the output of a membrane filter. If the water is hard, a precipitate can form which will reduce the output. Usually deionized or soft water is recommended for make-up to the alkaline cleaner. Temperature affects the output of a crossflow membrane filter. As temperature is increased, the fluid viscosity decreases. Therefore, the pore resistance to flow decreases and the flux increases. The temperature limit of the membrane must not be exceeded since the membrane structural properties might be affected, which would affect flux and permeate quality. In addition, solubility is affected by temperature. If a temperature is chosen which causes precipitation, output will most likely go down. The ph of the cleaner can have an effect on output. A ph should be chosen which will tend to keep the oil emulsified and will tend not to form precipitation. Also, the ph limit of the membrane should not be

18 exceeded since it might cause changes in physical properties of the membrane which would cause a change in output. A variety of membrane geometries exist: tubular, hollow fiber, spiral wound, and flat sheet. All types have been used to recycle alkaline cleaners. Performance varies depending on pretreatment, pressure profile, membrane material, and configuration. A description of each type and its advantages and disadvantages follow. The tubular membrane has the simplest design (see Figure 8). The membrane is cast onto the inside surface of a porous tube. The cleaner is fed under pressure through the center of the tube. Permeate exits radially and collects in the annulus between the porous tube and the plastic housing. Tubes can be up to10 ft. long and 1/2 to1 inch in diameter. Single tube and multitube configurations are available. Tubular membranes have a high tolerance to suspended solids and a good resistance to fouling. Prefiltration is minimal. However, they have a low surface area-to-volume ratio and permeate production per unit volume is low. They tend to require larger floor space than the other configurations. Membrane cost is moderate to high but membrane life is relatively long. The hollow fiber membrane has a small inside fiber diameter, typically 45 mil (0.045 inches), for alkaline cleaner applications. The cleaner is fed through the inside of the fiber while the permeate exits radially. The membrane skin and its support structure are spun as one integral piece during production. This feature enables the membranes to be backflushed during operation. A no. of these hollow fibers are arranged in the form of a bundle in a hollow fiber cartridge (see Figure 9). The arrangement is similar to a shell-and-tube heat exchanger. A typical cartridge is 43 inches long and 3 inches in diameter. The cartridges are arranged in parallel to form a module. The hollow fiber cartridge has a relatively high area per unit volume and requires less floor area than a tubular system. The membrane can also be backflushed to rernove solids. However, these membranes are susceptible to pluggage and require extensive prefiltration. They are also limited in operating pressure to about 25 psi. Tubes can withstand higher pressures (typically, 70 psi)

19 I WASHER 7 1 I MEMBRANE -/ LEPOXY REINFORCED FIBERGLASS SUPPORT TUBE FIGURE 8. Tubular Membrane MANIFOLD ADAPTER fl POLYSULFONE OR PVC SHELL ANNULAR SPACE \ (PERMEATE COLLECTION AREA) / MEMBRANE FIGURE 9. Hollow Fiber Cartridge

20 Hollow fiber cartridges are moderate in cost but membrane life tends to be shorter than with tubes. The spiral wound membrane contains two layers of membrane separated by a porous, woven fabric support. Three edges of the membranes are sealed to form an envelope around the porous support. The fourth edge of the membrane envelope is attached to a perforated tube. A sheet of plastic mesh is placed on one side of the membrane envelope. The membrane envelope and mesh are wrapped around the central perforated tube in spiral fashion. The mesh separates membrane layers and serves as a turbulence promoter. See Figure 10. The spiral wound element is inserted into a cylindrical pressure vessel. Elements are available in diameters of 2.5 inches, 4 inches, and 8 inches and a length of 40 inches. Flow channel spacing can be in the range of mil. The spiral wound membrane has a high area per unit volume, a low cost to manufacture, and can tolerate extremely high operating pressures, typically as high as 600 psi in waste treatment. Spirals can be arranged in series to reduce flow requirements. The membrane elements are replaceable. Those in tubes and hollow fiber cartridges are not replaceable; the whole housing has to be replaced in these cases. Spirals must generally be prefiltered because of the size of the flow channel. Flat sheet membranes are normally arranged in the form of a plateand-frame module (see Figure 11). The module consists of membrane support plates and spacers that are stacked alternately "sandwich-like" one above the other and held together by a central bolt. The membrane support plates, which have a membrane mounted on both sides, not only provide a rigid backing for the membranes, but also provide flow channels that collect the permeate from the two membranes and carry it to an outlet tube. Spacer plates direct the feed uniformly over the membrane surfaces. Plate-and-frame modules have a reasonably high tolerance for suspended solids. They also allow for visual inspection of the membranes without destroying the membrane. However, they are susceptible to fouling because of the small flow channels. In addition, they have a moderate area per unit volume and are relatively high in cost

21 PERFORATED COLLECTION TUBE 7- k ANTI-TELESCOPING DEVICE e CONCENTRATE Dd PERMEATE j e CONCENTRATE e MEMBRANE CARRIER MATERIAL I CHANNEL SPACER PERMEATE FLOW AND CONTROLLED - BYPASS SPACER FIGURE 10. Spiral Wound Membrane Support Plate L I Spacer FIGURE 11. Plateand- Frame Module

22 NEW RE CYCLING PRODUCTS New products based on crossflow membrane technology have been developed to recover and recycle aqueous-based cleaners. Capacities range from a few hundred gallons per day to a few thousand gallons per day. One such device is shown in Figure 12. Most operate in the so-called topped-off batch mode; see Figure 13. In this mode, the cleaner is withdrawn from the cleaner bath by a transfer pump and fed to the process tank in the recycler. A recirculation pump in the recycler circulates the oil-containing cleaner to the membrane filter and back to the process tank. The oil-free permeate is withdrawn and sent back to the cleaner bath. The level in the process tank decreases as the permeate is removed. Eventually, the level reaches that of a level switch in the process tank. The switch then signals the transfer pump to refill the process tank. When the process tank is filled, the level switch tums the transfer pump off. As time progresses, the emulsified oils and the suspended solids concentrate in the process tank. The rate of concentration can be calculated by dividing the permeate rate by the size of the process tank. For example, the rate of concentration for a 180 gdl/day microfilter with a 60 gal process tank would be 3X per day. Initially, the oil-and-grease concentration would be 1X in the process tank (e.g., 0.1%). At the end of the first day, the concentration in the process tank would be at 4X (i.e., 0.4%). At the end of the second day, it would be at 7X (i.e., 0.7%). As the concentration increased, the output from the crossflow membrane filter would decrease. Eventually, the oil-and-grease concentration would reach a maximum level whereby the system would be shut down and the concentrate disposed. The unit would then be cleaned. The membrane recycling device should have pretreatment to remove free oil and coarse suspended solids. Pretreatment can involve a holding or equalization tank; decantation or skimming for removal of free floating oil; filtration for removal of coarse suspended solids; and a coalescer for removal of mechanically dispersed free oil. See Figure

23 FIGURE 12. Crossflow Membrane Filtration Product for Aqueous Cleaner Recycling

24 FIGURE 13. TOPPED-OFF BATCH OPERATION BAG RLTER 100 MICRON PI2 COMPRESSED AIR!- r--- I I ----I TRANSFER PUMP b PROCESS TANK 60 GAL V18 1 OR 2 MEMBRANES MlCROFlLTER \ Ef? 10 MESH u ClkCULATlON.PUMP 1-

25 FIGURE 14. Pretreatment Went Wastes Segregate at Source Heavy Solids Metal Fines Grit --% Free Oil TO MEMBRANE 1) Settle Out 2) coarse Filtration 1) Belt Skimmer 2) Caalescer 3) Manual Gravity Draw Off

26 A crossflow membrane filtration product can be used on a single bath to remove emulsified oils and suspended solids or can be moved from bath to bath. In the former case, the recycling device would continuously remove contaminants and the bath would be purified continuously. See Figure 15. In the latter case, the bath would be purified intermittently when a maximum oil and grease concentration was reached. For the continuous case, the membrane filter must remove oil as fast or faster than the rate at which it is brought into the bath. Otherwise, oil would accumulate in the bath. The average rate at which the bath is contaminated by oil can be estimated by the following equation: rate in= cmaxw (3) where cmax= maximum concentration of oil (i.e., at time of disposal) V= volume of cleaner bath disposed over a given time period The average rate at which the oil is removed by the membrane filter can be estimated by rate out= c.q (4) where c= concentration of oil in bath at steady state after membrane filtration Q= feed rate to membrane filter= permeate rate The required permeate rate would be Q= (cmax/c)w (5) For example, a 5000 gallon immersion bath is disposed every month due to oil contamination. The concentration of oil in the bath at time of disposal is 5%. It is desired to keep the oil concentration to less than 0.5% in the bath on a continuous basis. The cleaner emulsifies the oil in the bath. The plant operates 20 days per month. What size membrane filter is necessary?

27 r! 0.d u T \ I / a%

28 , Using Equation (3, along with the given information yields Q= (5%/0,5%)*5000 gal/mo (6) = 50,000 gal/mo = 2500 gallday (0.5 bath turnover per day) If the membrane product is used intermittently (when the bath reaches its maximum oil concentration), then the following equation would apply (if the perrneate is returned to the cleaner bath): c= cmax*exp(-t*q/v) (7) where c= concentration of oil in bath at time, t cmax= maximum concentration of oil in bath (at time of disposal) exp= exponential function Q= permeate rate V= volume of cleaner bath Equation (7) assumes that no oils are brought into the bath during its cleanup. Solving for Qyields Q= (-V/t)*LN (c/cmax) (8) where LN= natural logarithm For example, if the previous bath of 5000 gal is cleaned on an intermittent basis (when the bath reaches 5% oil), the required permeate flow to reduce this concentration to 0.5% in a day's time would be Q= (-5000 gal/l day).ln (0.5%/5%) (9) = 11,500 gal/day This is equivalent to a tumover rate of 2.3 turnovers per day

29 L ECONOMICS Use of a crossflow membrane filtration system for recovering and recycling aqueous cleaners can lead to significant cost savings. Because of the extended bath life, cost savings result from reduced waste volume due to reduced disposal costs, reduced cleaner purchases, and reduced bath maintenance. However, there will be additional costs due to membrane replacement, membrane pumping energy, and labor and chemicals for membrane cleaning. An economic analysis is shown in Figure 16 for a 5000 gal bath. The original bath was disposed every month. The annual cost for disposal was $120,000 per year (based on a hauling cost of $2.00 per gallon). The membrane filtration system extended the bath life from 1 month to 6 months. As a result, less bath has to be disposed (10,000 gal per year from the 2 changeouts and 5000 gal per year from the membrane concentrate disposal). The overall reduction in waste volume is 4X from 60,000 gal per year to 15,000 gal per year. The disposal cost savings is $90,000 per year. Of course, this calculation assumes that the 10,000 gal of cleaner cannot be disposed to sewer and must be hauled away with no volume reduction. If this 10,000 gal of cleaner can be disposed to sewer then the overall reduction would bel2x from 60,000 gal per year to 5000 gal per year. The disposal cost savings would then be $1 10,000 per year. Reduction in new cleaner purchases will be from $37,500 per year to $6250 per year, a cost savings of $31,250 per year. These results assume a cleaner cost of $1.00 per pound of concentrated cleaner and a cleaner concentration of 10 oz. per gal. Labor for bath maintenance will be reduced since instead of changing the bath every month the bath would be changed every six months. The labor time is assumed to be 10 hours per bath change and the labor rate is assumed to be $65 per hour. The cost savings due to reduced bath maintenance is $6500 per year. There will also be a cost savings from reduced energy costs for heating the bath. These energy savings are not included here

30 t FIGURE 16. ECONOMIC ANALYSIS GAL BATH MPENSE ANNUAL COST WITHOUT MEMBRANE FILTRATION ANNUAL COST WITH MEMBRANE FILTRATION ANNUAL COST SAVINGS Disposal Cost New Cleaner Purchases $1 20,000 $30,000 $90,000 $ $6.250 $ Labor for Bath Maintenance $7.800 $1.300 $ I b c e m e n t NIA I $7.040 <$7040> Labor for Membrane N/A $6.240 c$6240> Cleaning Energy for Membrane Pump Chemicals for Membrane Cleaning N/A $1,609 c$1609> N/A $615 <$615> Notes: 1. Bath life extended from 1 to G months. 2, Payback on $40,000 membrane system is 4 months.

31 The membranes must be cleaned at the end of every concentration cycle or when they become fouled. A once-a-week cleaning frequency is assumed. Usually two hours of labor are required for a chemical cleaning of the membranes. The annual labor cost for cleaning the membranes almost balances the labor cost savings from reduced bath maintenance. Chemicals are used for cleaning the membranes. A 1% concentration is assumed. The concentrated cleaner is assumed to cost $23.63 per gallon. The cleaner tank size is 50 gallons The annual cleaner cost for the membranes is $615. In some cases, permeate has been used to clean the membranes, thereby saving on cleaner cost. In a crossflow membrane filtration system, a pump must circulate the cleaner through the membranes. The pumping energy cost is estimated to be $1609 per year. This assumes a 5 HP motor and an electricity cost of $0.075 per kwhr. Operation is 24 hours per day, 20 days per month, 12 months per year. The highest operating cost for the membrane system is the replacement cost for the membranes. The membranes are assumed to be replaced every year. The annual replacement cost is $7040. The total cost savings for a 5000 gal bath would be $112,246 per year. The cost of a coalescer, bag filter, and crossflow membrane filtration system would be about $40,000. The calculated payback would be just over 4 months. The following case histones show equipment paybacks less than a year. A company which cut and stamped metal components for the automotive industry was originally dumping alkaline wash tanks every two weeks. The alkaline cleaner was used to remove oils from the components after stamping. Three tanks each 9400 gallons were used. The contaminated cleaner was hauled off-site at a cost of $250,000-$300,000 per year. An ultrafiltration unit was installed in It generated 7900 gal per day of permeate. For an initial washwater concentration of 1.1% oil, the ultr&ilter permeate was 0.02% oil (200 mg/l oil). The permeate which contained water, alkali, and emulsifiers was recycled back to the wash tanks. The concentrate was hauled away

32 As a result of installation of the ultrafilter, there was a 90% reduction in waste, a 50% savings in cleaner consumption, and a 20% recovery of oil. These benefits translated into an annual cost savings of $380,000. Payback was less than a year. CASE HISTORY (2 ) A leading manufacturer of refrigeration equipment for trucks was dumping a steel cleaner bath every month. Steel frames were being cleaned in an immersion tank. The cleaner had a ph of 12 and a temperature of F. The cleaner had the following composition: COD mg/l, BOD- 68 mg/l, oil and grease- 43 mg/l, and total suspended solids-100 mg/l. The cleaner was an emulsrfyrng cleaner. The tank size was about 2500 gallons. The cost to dispose of the cleaner off-site was $0.89 per gal. The disposal cost was estimated to be $26,700 per year. In addition, new cleaner had to be purchased to makeup the new bath. The annual cost of the cleaner was estimated to be about $23,850 based on a concentration of 12 oz. per gal and a cost of $1.06 per lb. Therefore, the total cost for disposal and cleaner purchases was $50,550 per year. In May, 1993, this manufacturer purchased a crossflow microfiltration product for recycling the alkaline cleaner. In addition, a coalescer and a 100 micron bag filter were purchased to remove free oil and coarse suspended solids ahead of the microfilter. The microfilter had a 180 gallon process tank to concentrate the emulsified oils and the fine suspended solids. The average output from the microfilter has been about 1 gal/-. The system is operated for 2-8 hour shifts per day, 5 days a week. A detergent cleaning is performed once a week. The coalescer removes about 3-5 gallons per week of free oil which must be disposed. After 3 months of operation with the microfilter, the cleaner bath has not been changed. The bath life is expected to be even longer. Assuming a 3 month bath life with the microfilter versus the original 1 month bath life, the cost savings on disposal and new cleaner purchases would be about $33,700 per year. Payback on the coalescer, bag filter, and microfilter would be less than a year

33 In March, 1993, a remanufacturer of auto transmissions installed a crossflow microfiltration system for recycling cleaner. Prior to installation, the 1000 gallon flow through spray washer and the 4 smaller cabinet washers (2-500 gallons, gallons) were dumped approximately once a week due to buildup of dirt and oil. The hauling cost was $0.42 per gallon. The ph of the cleaner was 13 and the temperature 165 F. The recycling treatment scheme included a 3000 gallon equalization tank, an oil skimmer, bag filtration (100 micron), a coalescer, a 1000 gallon process tank, and the microfilter. The system operates topped-off batch during the first day and batches down on the second day. The output of the microfilter is 1000 gal/day over a hour day. The permeate is retumed to the cleaner tanks. The final concentrate volume is gal. The reduction in waste volume is 10-13X. The payback has been less than 7 months. CASE HISTORY (4) An automotive assembly plant has installed an ultrafilter to extend the bath life of its second stage immersion bath. The bath size is about 72,000 gallons. The cleaner has a ph of 12 and a temperature of 120 F. The ultrafilter averages about 4 gal/min output. The permeate from the ultrafilter is sent back to the second stage but the concentrate is bled back to the first stage spray wash (4500 gallons). The ultrafilter actually operates as a single stage with a concentration factor of 4X. The immersion tank has not been dumped in over a year

34 In summary, crossflow membrane filtration has been shown to be a viable method for recovery and recycle of alkaline cleaners. The process is simple, easy to operate, and has a short payback on the equipment. Benefits include reduced waste volume, reduced cleaner purchases, reduced bath maintenance, and improved parts cleaning. Crossflow membrane filtration is a cost-effective pollution prevention technique well-worth investigating. I. REFERENCES,. 1. Davies, R., R. Curtis, and R. Laughton, "Recycling Metal Stamping Plant Wastes," Water and Polllatipn Com 1 (Sept./Oct.,1985). 2. Donahue, K., Presentation at Seminar on "Metal Cleaning Alternatives to l,l,l- Trichloroethane and CFC-113: Ozone Layer Protection," Milwaukee, WI (August, 1993). 3. Osmonics, Inc., Technical Bulletin, Minnetonka, MN (1990). 4. Raycheba, J., m a n e Tec-v Reference Guia e, Ontario Hydro (1990). 5. Rizzone, M.S. and P.S. Downing, "New Products for Waste Treatment," plectroco-, Cincinnati, OH (1992)