Author: Thomas J.M. Weaver President/CEO

Transcription

1 MEMBRANE 1992 Thomas J.M. Weaver Author: Thomas J.M. Weaver President/CEO PROSYS Corporation Abstract: As Environmental Compliance becomes a critical issue in the successful operation of a metal finishing organization, various treatment techniques are required to meet the compliance demand. Newer treatment technologies have been developed to allow the operators to concentrate on the quality of the product while maintaining discharge compliance. Evaporation, ion exchange, adsorption and numerous filtering methods are employed to aid the operation of the finisher. Clarification, sand filters, polymers and other devices are coupled together to meet the discharge limits. While these various methods are useful, often they do not produce the quality of discharge which will allow the operation to continue without fear of non-compliance. Membrane separation offers a reliable solution to the problem in many circumstances. Depending upon the physical size of the particle or molecule, and the separation required, various membranes exist with the correct pore size and chemical compatibility to successfully and reliably perform the technique of separation. Using membranes, the "art" of separation is reduced to a physical science. New solutions to old problems are being developed using membranes, as newer membranes, with different materials of construction, are gaining market acceptance. Although membranes have been on the market for many years, there are many misconceptions as to the applications and the types of membranes which may be used to solve a particular concern. This presentation will define the various types of membranes, evaluate the differences and introduce logical applications for membranes in the waste treatment arena. Definitions: (1) Pore Size: The size of the hole in the membrane through which the product can travel. This may be expressed in micron size, Angstroms or molecular weight cutoffs. (2) Flux: The nominal volume of clean water which can pass through the membrane as rated per square foot of surface area and time, as an example: Gallons per square foot per hour. (3) General Filtration: Filtration techniques such as filter presses, cartridge filters, bag filters, sand filters and other techniques. Typically, the nominal pare size of this filtration is 10 microns or larger. (4) Microfiltration: Filtration techniques with a membrane whose pore size is between.1 and 10 microns. (5) Ultrafiltration: Filtration techniques with a membrane whose pore size is between.001 and.1 microns

2 (6) Reverse Osmosis: Filtration techniques with a membrane whose pore size is less than -001 microns. (7) Crossflow Membrane Filtration: A phenomena of solid/liquid separation by a membrane employing simultaneously superimposed tanqential flow across the membrane surface. Goals of Wa6teueter Treatment: The basic goal of wastewater treatment, be it end-of-pipe or treatment at the source, is three fold. * DISCHARGE COMPLIANCE allows a company to continue to operate and discharge process water without the fear of environmental violations. * REUSE AND RECOVERY allows the operation to recover many of the valuable assets which would otherwise be lost to the discharge water and the hauling of sludge. Global trends dictate that water itself will become the most expensive resource. Finally, * ZERO DISCHARGE relieves the company from many of the liabilities associated with the generation of hazardous waste. Although complete recycling is near impossible and extremely expensive, many technical advances have been made on selected waste streams. INTRODUCTION: Methods Although compliance, recycling and zero discharge issues are -important, the most important goal of treatment is not the treatment itself, but the ability of the company to maintain consistent process quality for its product, while operating within the guidelines of the environmental laws. In an effort to achieve quality, yet'live within the discharge guidelines, more and more companies are developing long term plans for the treatment of the generated wastewater, the reuse of the byproducts and the recycling of water. Typically, the end result of this process is the implementation of many different technologies. Processes which earn profits are improved, while processes which do not generate operational margins are terminated. Proper control of drag-in and dragout, coupled with improvements in rinsing techniques are employed to reduce the amount of waste generated, both in chemical losses and water usage. Prior to any purchase of treatment equipment, a complete evaluation of the available techniques is conducted, complete with operational pilot studies. This detailed planning and survey results in the achievement of all the basic wastewater treatment goals. There are many different methods available for the treatment of wastewater. The methods may be roughly classified into three techniques: SEPRRATION, EXCHANGE and EVAPORATION. Each of these methods has applications in which it will operate more efficiently than other methods. Careful consideration of the positive aspects of the methods and the negative aspects of the methods are required to evaluate which method best fits the operational need of the company. It must be noted, that while capital expense is considered one of the most important aspects of the decision, evaluation of pros and cons will highlight other cost impacts which can easily modify a purchase decision. Evaporation techniques involve the removal of water and volatile organics while maintaining the contaminants for disposal. Typically, this 2 424

3 method has a high degree of operating cost and capital outlay. Yet, there are many critical plating baths where evaporation offers the most economical treatment. Exchange techniques involve the removal of a specific contaminant using resin or adsorption media. Provided the removal is limited to a single metal, the metal may be recovered. Care must be taken to ensure compatibility between the various plant chemicals and the resins or adsorption material used. Pretreatment and filtration are necessary to insure long term success with the Exchange techniques. Separation techniques are as varied as evaporation and exchange. The basic separation technique involves the precipitation of the contaminates and separation by gravity, filters and other devices. Membrane filters can achieve remarkable discharge levels while reducing the amount of generated sludge and providing water reuse capability. The proper selection of membranes result in a system which can meet compliance without reliance on balanced flows and waste stream concentrations. Membranes are typically more expensive than clarifiers and sand filters, however, the reliability of membrane systems justifies the expense. All treatment techniques have a common need for proper segregation and pretreatment. This operation ensures that the operation of the waste treatment system ia successful by allowing the system to operate within the design limits of the equipment. As newer treatment processes are developed, segregation allows the operation to implement the technologies with relative ease. Common chemicals should be gathered together for treatment. In SEPARATION, cyanides must be destroyed, hexavalent chromes reduced, and complexed metal bonds destroyed. These steps must be correctly accomplished or the waste treatment operation will fail. The company must maintain the system, calibrate controllers and ensure the proper operation of all equipment. The long term success of the waste treatment is dependent upon the preventative maintenance and the attention to detail in the operation of the plant. Daily discharge samples should be taken and analyzed to ensure that the system is fully operational. MEMBRANES: Range and Types As depicted on Figure 1, Range of Membranes, there are three basic membrane ranges available in the marketplace. Each range of membrane has a specific function in which it will operate repeatably and reliably. h"des Ma~r~mole~ules o*rrolvm Ions Figure 1: Ranqe of Membranes Membranes come in different configurations, materials of construction and various pore sizes within the ranges of applications. Each difference must 3 425

4 ~ ~ be evaluated prior to the selection of a system. Table 1: Membrane Data Sheet MF UF RO Pore size (microns).2 to 10 (Angstrom) >loo0.001 to <.001 < 10 (mol. wt) >1M 10K - 1M <10K Operating Pressure 30-5OP: # >loo# 1 Confisurations Hollow Fiber * Tubular xxx * Sheet xxx * Spiral Wound I MEMBRANES: Hicrofiltration xxx xxx XXX xxx The typical HF membrane used in the wastewater treatment industry removes, or rejects, particles whrch have a size of greater than.2 microns. For comparison, 40 micron particles are not visible to the human eye. Thus, the MF membrane has a cutoff 200 times smaller than a visible particle. Yet, this a relatively large hole with respect to the permeation of water. As such, the membrane has a high flux while the operational pressure of the membrane is low. The MF membranes are typically configured in tubular forms. Hollow fiber, flat sheet and spiral wound configurations are also available. In a tubular membrane, Figure 2, Tubular Membrane Illustration, a large number of membrane tubes are combined in a central housing. The combined membrane area in this bundle is used to determine the nominal flowrate of the xxx XXX membrane. It is important to evaluate the amount of area provided with a membrane system, as area results directly in the amount of fluid which may be processed through the membrane

5 The primary task of a microfiltration membrane is to remove solids and emulsified oils from a waste stream. For this application, tubular membranes offer the highest degree of reliability. In this operation, the separation process is reduced to a science: if the contaminants are in particle form, the contaminants will not enter into the discharge stream. This statement should not be taken lightly. Membrane treatment is a science, it is not an art which depends on flowrates, sludge blanket levels and many other parameters. However, like the clarifier which depends on pretreatment, the membrane system will only produce compliant discharges if the pretreatment is correct. A typical process schematic is shown on Figure 3, Membrane Process Schematic. High flows from a centrifugal pump provide the operational Figure 3: Membrane Process Schematic pressure for the separation task. A differential pressure is placed across the membrane by means of a diaphragm valve or orifice plate. This resultant pressure gradient raises the trans-membrane pressure and induces the permeation through the membrane surface. As the trans-membrane pressure increases, the permeate flowrate increases. In many instances, as the flowrate of the pump increases, the permeate flowrate increases. As the system generates filtrate, the concentration of the contaminates in the Concentration Tank increases. Removal of sludge is accomplished by the use of a filter press. There are many factors which determine which MF membrane will operate the most efficiently in a given waste stream. Material compatibility, operating temperatures, ph, membrane configuration and the actual manufacturing process of the membrane are items which should be addressed Prior to the selection of a membrane. Manufacturers literature identifies the majority of the key parameters. With regard to the manufacturing of a membrane, there are two primary methods. First, the membrane is attached to a support structure by means 5 427

6 of solvents or other processes. In this process, the support structure is typically a different material than the membrane. This process yields a very thin membrane. Long term adhesion to the substrate is ensured by always pressurizing the membrane, that is the inner shell of the membrane tube, not the substrate. The second method is that the membrane is part of the substrate, that is, it is the substrate. This method of manufacturing allows the operating pressure to be on either side of the membrane tube. In wastewater applications, tubular microfiltration membranes foul. In fact, all membranes foul. The fouling of membranes reduces the flowrate of the system. When fouling reaches a level at which the wastewater cannot Figure 4: Tubular Flow 1 fillrat< be treated in the proper amount of time, the system must be chemically cleaned. As with human skin, cleaning restores the filtration rate of the membrane. There are various methods used to improve the mean-time-betweencleaning. Additions of lime, iron and carbon aid in the scrubbing of the membrane surface, yet it does not prevent the long term cleaning of the system. There is a physical law in fluid flow which explains why a scrubbing action is relatively ineffective as a method to extend the mean-timebetween-cleaning. As illustrated by Figure 4, Tubular Flow, there is no tangential flow at the boundary of the tubular wall due to a continuum of flow, that is, the tangential flowrate at the wall is zero. Yet, the overall tangential flow has long been proclaimed as the reason that the flowrates stay high for long periods of time. Given a high flowrate through the tube, the flow is turbulent. This turbulent flow greatly reduces the thickness of the boundary layer. However, careful examination of the area near the membrane wall indicates that the turbulent flow does not affect the wall. In fact, the only flow in the boundary layer is that of the filtrate passing through the membrane. This action is equivalent to a filter press. As the filtrate exits the membrane, it deposits the contamination on the wall of the membrane. A thin boundary layer of sludge accumulates, cutting the overall pore size of the membrane. This reduces the amount of water which may be passed at a given operating pressure. As such, the flowrate of the system decreases

7 In systems which have membranes which are also the support substrate, a reversal of flow disrupts the.boundary layer and allows the turbulent flow to remove the contaminants which foul the membrane surface. As illustrated in Figure 5, Backpulsing Operation, this action lifts the foulants into the flowpath of the waste. Many systems are equipped with Figure 5: Bacbulsinu ODe ration automatic pulsing units. When pulsing is used, the addition of lime, carbon and iron are not used, other than required to properly pretreat the waste stream for removal of the solids. Microfiltration membranes offer an excellent upgrade in applications were the operation of the clarifier and sand filter is not meeting discharge limits. These systems may be easily reconfigured with membranes -aqcp-lexels~s in Table 2, Post Retrofit Discharge Levels. The same Table 2: Job Shop Post Retrofit Discharue Levels CONCENTRATION ppm SUBSTANCE INFLUENT CLARIFIER cu Ni Sn Captive Plater Cu PCB Shop Ni Ni cu Pb pretreatment chemistry is required, however, co-precipitants and polymers are not required for the proper operation of the system. The basic precipitation of metals into hydroxides forms a particle which is large enough to successfully separate with the microfiltration membrane, thus, the removal of the particle is based on science. 7

8 ~ In MEMBRANES: Ultrafiltration' Eighlights Ultrafiltration membranes are tighter than microfiltration. The pore size of UF membranes is between.001 to.1 microns. As a result, the flux rates are not as high per square foot of membrane and the operational pressures to achieve the flowrates are higher than that of the MF membrane. It physically takes more membrane surface area to produce a given flowrate than that of the MF membrane. For this reason, UF membranes are typically spiral wound or Hollow Fiber. UF membranes remove free oils, many surfactants and sub-micron particles. Zyglo oils, dyes and other components may be rejected by the membrane. Hollow Fibers are similar to tubular except that the operating diameter of the tube is extremely small, resulting in different flow characteristics within the tube. As a result of either the spiral or fiber membrane module configuration, UF systems will not operate with a high degree of solids in the waste stream. If the membrane module configuration is tubular, solids may not be a problem, however, as noted above, for a given flowrate, there will be many modules. In addition to solids, UF membranes in the spiral wound configuration do not operate as efficiently in the presence of emulsified oils. Both the oils and solids tend to block the inlet to the membrane, reducing the transmembrane pressure of the module. This reduces the maximum amount of flow which the membrane will produce. The UF membranes operate in a similar fashion to the MF membranes, except, UF membranes remove components which pass through a MF membrane. wastewater applications, it may be necessary to pulse UF membranes to maintain the operating flowrates required. AS with the MF applications, membrane manufacturing processes must be understood prior to the implementation of pulsing. With regard to hydroxide removal, there is little difference between a.1 micron tubular UF membrane and a.2 micron tubular membrane. Both membranes remove particles which are greater than.45 microns, the filter size used to determine Total Suspended Solids. MEMBRANES: Reverse Osmosis Eighlights The Reverse Osmosis (RO) membrane is the tightest of all membranes. The membrane removes salt. It is typically produced as a spiral wound membrane. This configuration provides the maximum surface area available in a membrane module. Typically, RO membranes produce drinking water or serves as the pretreatment for a De-Ionization process. In waste treatment, RO systems are used to concentrate fluids for reuse. The key in this process is to reduce the amount of salt in the stream. If the process uses city or well water, the RO systems will not only concentrate the desirable components, but it will concentrate the contaminants and salts as well. Pretreatment of the fluid prior to the RO system i s necessary to prevent scaling and unrecoverable fouling of the membrane. Often, MF systems serve as the primary pretilter to the RO system. Utilization of MF waste treatment and RO systems to recover reusable rinse water is a proven application. 8

9 I I The Proceedings of the 79th I AESF Annual Technical Conference... I June 22=25,1992 Atlanta, Georgia The American Electroplaters and Surface FinishersSociety, Inc. (AESF) is an international, individualmembership, professional, technical and educational society for the advancement of electroplating and surface finishing. AESF fosters this advancement through a broad research program and comprehensive educational programs, which benefit its members and all persons involved in this widely diversified industry, as well as government agencies and the general public. AESF disseminates technical and practical information through its monthly journal, Plating and Surface Finishing, and through reports and other publications, meetings, symposia and conferences. Membership in AESF is open to all surface finishing professionals as well as to those who provide services, supplies, equipment, and support to the industry. According to the guidelines established by AESF's Meetings and Symposia Committee, all authors of papers to be presented at SUWFIN@have been requested to avoid commercialism of any kind, which includes references to company names (except in the title page of the paper), proprietary processes or equipment. Statements of fact or opinion in these papers are those of the contributors, and the AESF assumes no responsibility for them. All acknowledgments and references in the papers are the responsibility of the authors. Published by the American Elecboplaters and Surface Finishers Society, Inc Research Parkway Orfando, FL 3282m98 Telephone: 407/ Fax: Jq Copyright 1992 by American Electroplaters and Surface Finishers Society, Inc. All rights reserved. Printed in the United Stales Of AmoncaIbswb liwtion may not bg roproaucca. storcd,n a reti.g.ai sqstom. or tnnsm.tied In HnO.0 or part. in any lorm or Dy LWL means. e o c t r o n m.wi. ohotocopy,ng, recomng. or C ~EM.SO wilnout the prior wnron pom:ss;on 01 AESF Rosoarcn h h a y Orlando FL? !39 Printed by AESF Press SUWFIN@is a registered tmdemarkol the Ameiican Electroplaten and Surface Finishen Society. Inc.