nicholas p. cheremisinoff, president scitech technical ...infohouse.p2ric.org/ref/29/28242.pdf ·...

NICHOLAS P. CHEREMISINOFF, President SciTech Technical Services, Inc.

Morgansville, NJ

everse osmosis is a pressure-driven process in which a salt, low molecular weight organic molecules, and R 'onic species of 1 0-3 meters in dia-

meter and less than 300 molecular weight are retained. The majority of reverse osmosis applications currently use thin-skinned anisotropic membranes. These differ from ultrafiltration membranes in that the pores in the top skin layer are so small that water passing through the membrane can be con- ceptually modeled by means of solution-dif- fusion within the membrane itself. Since ionic species are retained by the membrane, reverse osmosis processes are operated at relatively high pressures of 100 to 2,000 psi, 0.69 to 13.8 MPa to overcome osmotic pressure of the retentate and thus drive the permeate fluid through the membrane.

Reverse osmosis membranes retain salt or other ionic species and pass water. This process requires a membrane suitably dense to render the pores virtually non-existent. However, a membrane suitably dense for the reaction of ionic species would, if it were thick, also greatly retard the flux of permeate. Therefore, reverse osmosis membranes need to have a very thin, dense skin cov- ering a very porous structure that permits passage of permeate while providing structural support.

TYPES OF MEMBRANES There are six basic types of reverse

osmosis membranes: asymmetric microporous membranes, Loeb-Souri- rajan thin-skinned anisotropic membranes, composite membranes, dynamically-formed membranes, liquid membranes, and plasma-polymerized membranes. Because only three of these membrane types-Loeb-Sourira- jan, composite, and dynamically- formed membranes-are currently available for commercial application,

this discussion is limited to these three. Loeb-Sourirajan thin-skinned aniso-

tropic membranes used in reverse osmosis applications are essentially the same in manufacture and structure as those used in ultrafiltration. However, after the reverse osmosis membrane has been formed, it is annealed in hot water to tighten the membrane by increasing its density. This process causes an increase in salt reaction and a decrease in water flux with an increase in annealing temperature. Ultrafiltration membranes, which are not annealed, do not reject salt ions.

Reverse osmosis membranes are generally cast as flat-sheet stock or as hollow fibers. Reverse osmosis hollow fibers are cast with the thin, dense skin on the outside of the fiber so that the permeate passes into the lumen of the tube. This fact, coupled with the smaller cross-sectional area of reverse osmosis membranes compared to ultrafiltration membranes, makes them better able to withstand the compressional forces of the higher pressure reverse osmosis.

Composite membranes have a very thin dense membrane over a porous supporting substrate. These membranes are produced by first making a fine microporous membrane substrate by dissolving a non-water soluble polymer in an organic solvent and then casting this so,uiion in air or in water

Once the membrane substrate is formed, a thin film of material, such as cellulose triacetate or polyethylene imine, is applied to the supporting membrane surface. For such a composite membrane to work successfully, the supporting membrane pore diameter must be less than the film thickness (0.05 pm to 0.1 pm) to ensure that the entire membrane is film coated, and the supporting membrane must have a sufficiently high porosity to maintain the required water flux.

Fluid Systems Corporation recently introduced composite membranes on a

commercial scale that show promise for superior performance in reverse osmosis applications. The advantages of composite membranes include their ability to utilize thin films made of poly- mers that cannot be fabricated into Loeb-Sourirajan membranes, a high degree of control over film thickness and its reproducibility, and the membrane substrate can be fabricated of materials that exhibit greater resistance to membrane compaction under the pressures of reverse osmosis.

were first developed at the U.S. Depart- ment of Energy's Oak Ridge National Laboratory, primarily to treat pulp and paper mill effluent. These membranes are formed by coating a porous carbon, ceramic, or metallic tube with colloidal zirconium oxide, poly (acrylic acid), or with suspended material in the effluent waste stream itself. Such membranes tend to have higher flux and lower rejection than conventional reverse osmosis membranes but they can operate at the relatively high temperatures (65°C) of kraft pulp effluent.

D y i i a i ~ i i ~ l l y f a i f i d i~e i i i l j ia i ies

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18 The National Environmental Journal January/February 1993

GROUND WATER decide whether they are implementing the system for multiple sites/facilities or only one site. A system that will handle multiple sites will have broader requirements than a system that will handle data from only one facility. The input is determined by the sources of data and the types of data provided.

In a groundwater program, input data might include well location and construction details, field data from sampling events, and laboratory data from sample analysis. These data are presented in essentially two ways, in printed paper reports or in electronic form on computer diskettes. Nearly all field data come from handwritten log books. But because printed data must be transcribed, or entered, into the computer manually, which is labor- intensive and a major source of errors, it should be kept to a minimum.

For samples sent offsite, most laboratories will produce analytical data in electronic form for a nominal added cost at the time of initial testing. Design- ing the system so that it can directly import laboratory data from diskettes reduces labor costs and improves the quality of the results dramatically. The system designers should work closely with laboratories to define a format that the laboratory can produce and the system can import efficiently.

output The desired output is determined by

the objectives of the system and by the data's application. The system should be able to provide basic statistics to help users interpret groundwater monitoring information, along with reports providing statistical summaries, such as numbers of samples, percentages of detects, maximums, minimums, and averages, are useful for groundwater data analysis.

Graphics are also an important analytical tool, by revealing relationships over space and time and among many parameters. Spatial graphics include bubble maps, icon maps, and contour maps, and plot data as lines, symbols, or colors on a site map, highlighting the extent or movement of plumes and the locations of sources. Plotting data against time in trend, overlay, and control charts reveals changes in the data. Through proper scaling, the causes of the changes, such as varying detection limits, seasonality, and sampling frequency, can often be identified. Control charts are particularly useful in groundwater monitoring programs, by helping to determine whether

apparent changes in the data are significant or are the results of normal fluctua- tions, such as laboratory variation. Graphics also form a basis for effective communications with management and regulators who may be less familiar with the details of conditions at the site. The system design should provide graphics directly or produce output in an electronic format that can be imported into spreadsheets, modeling, and graphics programs for further analysis.

Implementing the System There are essentially three ways to

implement a computerized data management system: off-the-shelf software, semi-custom software, and custom software. The decision about which method to use involves balancing cost and flexi- bility against existing capabilities, since there are many subtle challenges and trade-offs specific to the implementation of a groundwater database. Because capabilities such as the correct handling of significant digits, non-detects, and multiple names for parameters and wells are not common to general data manage-

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ment systems, it is often best to consult with designers who have specific expe- rience with environmental data manage-

ensures short-term success and long- term benefit from the system.

Using the System The purpose of developing a

groundwater data management system is to provide a foundation for important decisions regarding past, present, and future events. With a flexible query system, users can obtain information on the effectiveness of a remediation system, plan cost-effective investiga- tions that provide new and useful information, and plan new activities to control environmental impacts. Of course, to obtain the maximum value from the system, it must be used. The database should be the first resource consulted for questions related to the information that it stores.

Richard Sands is an associate specializing in the computerization of environmental data for Dames & Moore

ment systems. Proper implementation . _ _ _ _

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(Los Angeles, CA). 0

DELTA VANGUARD@ AIR STRIPPERS REMOVE VOCs FROM GROUNDWATER

Delta Vanguardm Air Strippers purify groundwater by effectively removing fuel/gasoline hydrocarbons, chlorinated hydrocarbons, solvents, and certain other volatile organic chemicals (VOCs). Pre-piped and pre-wired skid mounted systems are available for economical low cost installation. Pre-assembled systems are also factory tested for trouble free start-up. Iron removal, chemical cleaning, air emission and other factory assembled and tested equipment packages, as well as pilot test units, are also available. Delta-Pak! structured packing is an ultra low pressure drop mass-transfer media which efficieiiiiy r e i i i ~ e ~ most ~~i i i i i iu i l curiiarriiriariis, including some considered difficult, if not impossible, to strip while providing maximum resistance to fouling. Various other types of packings, as well as different column materials of construction, are also available.

DELTA COOLING TOWERS, Inc. 114 Clinton Road Fairfield, N.J. 07004-2970

Outside NJ 1 -800-BUY DELTA (201) 227-0300

I FAX (201) 227-0458 AD1

Circle 117 on card.

The National Environmental Journal January/February 1993 17

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1 MGD R.O. at Otay Municipal Water District for reclaiming municipal secondary effluent.

SYSTEM OPERATION Reverse osmosis and ultrafiltration

have several similarities: they are both pressure-driven processes and they both separate a feed stream into a product stream and a react stream, usually using a thin-skinned anisotropic membrane. The differences in these two processes include the following.

Size of the molecule retained by the membrane differs between the two processes with the actual boundary depending upon the specific membrane manufacturer. Generally, reverse osmosis membranes have a molecular weight cut off of less than 300, while ultrafiltration membranes have a 300 to 300,000 molecular weight cut off. Reverse osmosis is generally used to separate ionic species from a solution. This results in formation of an often considerable osmotic gradient across the membrane due to the

differential ionic concentrations of the fluids on each side of the membrane. Osmotic pressure of seawa- ter is generally about 350 psi (2.4 NPa) and is even higher in those portions of reverse osmosis modules where ionic separation has resulted in an appreciable concentration of brine. Osmotic pressure is a negligi- ble consideration in most applications of u!fr~f!!f!!frgtj~n syst+ma. Operating pressures are consider- ably higher in reverse osmosis than in ultrafiltration systems. Therefore, the thin membrane skin is on the outside of the fiber and pressure is applied on the outside so that the fiber is subject to compression rather than tension. The actual membrane function is different in reverse osmosis than in ultrafiltration. In reverse osmosis, the membrane serves primarily as a dif- fusive transport barrier, and as such, the actual chemistry of the mem-

The National Environmental Journal January/February 1993

brane plays a key role in determining the rejection of various molecules. Although molecular screening may play a role in reverse osmosis membrane retention, it is the prime mech- anism in ultrafiltration processes. As in ultrafiltration processes, the

formation of a boundary layer of solutes next to the membrane by means of concentration polarization is a potentially

sis, Polarization determines the design of modules and systems to maintain a high level of flux. In terms of operational consequences of concentration polarization to reverse osmosis applications, it should be noted that:

As the solute concentration increases at the surface of the membrane, the increased concentration gradient results in an undesirable increase in solute flux across the membrane. The increased solute concentration at the membrane surface also increases the osmotic pressure at

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the surface, resulting in an undesirable reduction in water flux. At high solute concentrations, a solute precipitate can form, thus fouling the membrane surface. Fouling of membranes, either by pre-

cipitated solutes in the boundary layer, micro-organisms, or particulates in the feed stream, tends to reduce membrane flux and to eventually render membrane irreparably inoperative. Membrane life can be extended by cleaning with chemical agents or physically by backflushing, scouring, or by feeding high velocity fresh water through the system. Care must be taken with chemical cleaning agents so as not to use those that might damage

the membranes and with physical meth- ods so as not to rupture the membranes with excessive pressure.

Reverse osmosis membrane performance in industrial applications is a function of two major factors, the membrane material and the configuration of the membrane module. Three types of commercially available membranes module are cellulose acetate, aromatic polyamide, and composite membranes variously fabricated.

Cellulose-acetate membrane performance is particularly susceptible to annealing temperature with lower flux and higher rejection rates at higher temperatures. Such membranes are

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prone to hydrolysis at extreme /xes--"' sure, are subject to compaction at operating pressures, and are sensitive to free chlorine above 1 .O ppm. Cellu- lose acetate membranes generally have a useful life of 2 to 3 years.

Composite membranes of poly (ethedamide) generally exhibit a higher water flux and salt rejection than cellulose acetate. They are so sensitive to free chlorine, however, that the Fluid Systems Corporation product literature recommends complete removal of free chlorine from the feed prior to osmosis.

Of the reverse osmosis membrane module types, the majority of industrial applications use either spiral-wound modules fabricated from sheet stock or hollow-fine fibers. Hollow-fine fibers are particularly prone to fouling and, once fouled, they are hard to clean. Thus, applications which employ these fibers require a great deal of pretreatment to remove all suspended and colloidal material in the feed stream. Spiral- wound modules, due to their relative resistance to fouling, have a broader range of applications and are currently used in about 50 percent of the world- wide desalination applications. A major advantage of the hollow-fine fiber modules, however, is the fact that a hollow- fine-fiber module can pack 5,000 square feet (465 square meters) of surface area in a 1 cubic foot (0.028 cubic meter) volume, while a spiral wound module can only contain 300 square feet (27.9 square meters) in the same volume.

The advantages of reverse osmosis membranes are primarily related to economics and to the relatively high rates of flux and ion rejection found in most reverse osmosis membranes in commercial use. Water desalination by reverse osmosis is generally more eco- nomically favorable than distillation processes, although electrodialysis appears to be competitive with reverse osmosis at least within certain salinity ranges.

Two of the major operating costs of reverse osmosis plants are electrical power and membrane replacement. The high operating pressures of reverse osmosis (100 to 2,000 psi, 0.69 to 13.8 MPa) require relatively large amounts of electrical energy. Reverse osmosis membranes must be replaced relatively frequently due to loss of water flux over time as a result of compaction; fouling of membranes or modules to the extent that chemical or physical cleaning can no longer restore sufficient levels of water flux; and actual degradation of the membrane due to hydrolysis, extremes of pH, or the action of free chlorine.

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TYPICAL REVERSE OSMOSIS APPLICATIONS Salt separations, including chloride, sodium, calcium, fluoride, sulfates, phosphates,

Organic removal Waterreuse Pretreatment for deionizers Bacteria, pores, virus, and algae removal

* Boiler feed Humidifier feed

* Car wash rinses

* Reclamation of rinse waters and complete water reuse

b Upgrading wastewater to meet state and federal standards m lowering total dissolved solids in wastewater

nitrates, carbonates, aluminum, gold, silver, nickel, copper, etc.

Separation of plating salts

Reclamation of valuable metals from plating and photography

Lowering BOD and COD in wastewater Removing detergents Removing radioactive elements

m Removing pesticides

APPLICATIONS There are only three major commer-

cial applications for reverse osmosis: potable water production from substan- dard sources such as brackish water and sea water; production of process water for industrial use; and treatment of

industrial effluents or other sources c wastewater. The most widespread application of reverse osmosis appears to be in the production of potable and industrial process water from substan- dard sources, particularly in desalting brackish or sea water and potential in treatment of groundwater. 0

We do. That’s who. And we’ve been making these machines since 1889.

EXTRUDERS FEEDERS WASTE CHOPPERS PUGMILLS and for mixing, reduce lumps to 2 fine chop filter cakes HIGH SHEAR MIXERS agglomeration and and feed uniformly and other soft, sticky for intimate mixing of densification of from very dry to solids . sludges, solids a d waste materials sludge-consistency liquids

DISINTEGRATORS to crush and size soils, clays and other tough materials

J.C. STEELE & SONS, INC.

Circle 119 on card.

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The National Environmental Journal January/February 1993 21

I TECHNOLOGY UPDATE t **

ETHANOL FUELS USE A waiver has been granted of 1 Ib/in2

Reid Vapor Pressure (RVP) to ethanol, thus enabling it to compete with methanol as a fuel additive in the reformulated-gasoline market. In 1991, some 35 parties representing all aspects of the fuel industry, including the ethanol industry, came to a negotiated agree- ment that settled on a maximum RVP of 7.8 for reformulated gasoline. But since gasoline blended with straight ethanol cannot meet the 7.8 limit-whereas methanol can-prospects for using ethanol in reformulated gasolines were all but shut out. Now, the waiver is expected to allow ethanol to capture 30 percent and 20 percent of northeast and southern U.S. markets.

NEW MEMBRANE PROCESS Pertraction, a membrane-based sol-

vent extraction process is claimed as effi- cient as activated carbon from removing organic contaminants in industrial wastewater according to TNO (Apeldoorn, Netherlands), Tauw Infra Consult (Deventer, Netherlands), and Hoechst Celanese Corporation (Chatham, NJ). Unlike the case with activated carbon, the captured organics can be distilled and recycled. The process reverses conventional flow regimes by pumping the extractant, an aliphatic hydrocarbon mix- ture, through hollow fiber polypropylene membranes, while the wastewater flows perpendicularly over the membrane bun- dle. This configuration cuts power con- sumption and boosts the membrane module’s mass transfer coefficient to four times that of conventional ones operating with parallel flow. The first 300-UH pilot plant was commissioned in January in a Dutch chemical plant for removing such chlorinated hydrocarbons as per- chloroethylene, chloroform and trichloro- ethane. Removal efficiencies will be between 99.0 and 99.9 percent, based on pollutant inlet concentrations of 100 ppm to 0.1 ppb, respectively.

DIOXIN’S EFFECTS AGAIN UNDER SCRUTINY

In another reversal of scientific opin- ion, the interim results of a major dioxin study being conducted by the U.S. Envi- ronmental Protection Agency (EPA) indicate that dioxin’s health threat may be more serious and broader than currently believed. In a 1988 review, EPA concluded that dioxin is a “proba-

ble” carcinogen, but since then other studies have found that while dioxin is carcinogenic at high levels, its carcinogenicity at low levels is difficult to pin down. Besides carcinogenicity, EPAs study shows a significant trend emerg- ing from the work performed, the impor- tance of studying non-cancer health effects, such as reproductive, behavioral and immune system changes. In fact, some data suggest that these effects may be happening at or near current background levels. After the study is completed in the summer of 1993, EPA will decide whether dioxin should have a new classification (i.e., “suspected carcinogen,” or “known carcinogen”).

LIGHTWEIGHT FILTER MEDIUM A filter medium of polypropylene

foam fragments is the secret behind the efficiency of an upflow biological filtration system developed by Japan’s NKK Corp. Pilot plant tests have typically shown removal efficiencies as high as 95 percent for biological oxygen demand (BOD), 82 percent for chemical oxygen demand, 60 percent for suspended solids, and 70 percent for turbidity. Per- formance is equivalent to, or better than, conventional anthracite or shale filter media. The polypropylene foam fragments (specific gravity, 0.2) float in upward-flowing wastewater, so there is no channelling, and wastewater is treat- ed uniformly. In the wash cycle, because air is bubbled to flush suspended parti- cles from the filter medium, the amount of washwater needed is at most 5 percent of the volume of the filtered effluent. The system works best when the wastewater has a BOD of 300 ppm or less.

PHOSPHATE WASTE Each year, some 40 million tons of

phosphogypsum-mainly calcium SUI- fate-are generated as a waste in the U.S. when sulfuric acid is poured over apatite ore to make phosphoric acid. Now, Science Ventures Inc. (San Diego, CA) has a way to recover an estimated $1 billion worth of sulfur from the waste, and has already tested the route in a 15- Ib/h pilot plant. In the process, called Flasc, for Flash Sulfur Cycle, phosphogypsum is mixed with coal and pyrite, and sprayed into a furnace at 1,340”C with oxygen-enriched air. The CaS04 is reduced to CaO and SOn; CO is also generated. The gases contain about 14 percent SO2, more than adequate for making H2S04. Also, the glass slag pro-

duces traps any natural radioactive materials in the phosphogypsum, decreasing their threat. Low sulfur prices make the route marginal for now in the U.S., but it may be attractive in India, China, and Saudi Arabia.

RECOVER SOLVENTS FROM SLUDGES

A combined evaporator and dryer developed by Bayer AG (Leverkusen, Germany) requires 80 percent less heat-exchange area than conventional units to recover organic solvents from sticky industrial sludges, such as paint residues. The system recovers more than 95 percent of the entrained solvents in a single pass, compared with about 20 percent for standard systems, which must use recirculation to boost recovery. In the unit, preheated sludge at 10 to 20 bar is decompressed rapidly in a coiled pipe, producing a fast-moving three-phase sludge stream. Centrifugal forces keep the solids and liquids along the tube wall, and vapor in the tube’s center. This generates high shear forces among the vapor-liquid-solid interfaces, enhancing turbulence and ensuring good heat and mass transfer. The mixing also prevents the buildup of sticky solids on the tube walls when the sludge viscosity peaks at about 1,000 Pa.s. The 5 to 10 percent of solvent remaining in the sludge is removed in a scraped- wall evaporator at 100-300°C. The system’s capital costs are about 70 percent of those of normal dryers, and operating costs are comparable. A 600-kg/h plant is planned for 1993.

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ORGANICS BIOTREATMENT ABB Environmental Systems, Inc.,

(Wakefield, MA) is developing a bioreactor that uses the synergies of two micro- organisms to cleanup groundwater contaminated with chlorinated hydrocarbons. UniiKe traaitionai fixed-iiim reac- tors, the bioreactor features between 50 and 100 rotating polyethylene discs. Each disc can support a different micro- bial colony, so the treatment is optimized for a given mix of contaminants. The microbes include a methanotrophic strain that thrives on oxygen and methane to produce an enzyme that oxidizes tough chlorinated hydrocarbons. A second line of aerobic bacteria consumes the byproducts, leaving behind carbon dioxide and water. Up to 0.5 percent of methane is supplied to keep the methanotrophic bacteria thriving. 0


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