THE NATURE OF AIR POLLUTION
Before we can discuss the nature of the equipment used to control air pollution, we must first define what constitutes air pollution, in order to understand the nature of the specific problem which control apparatus must be designed to meet.
For our purposes, air pollution consists of participates or impurities that are suspended in or conveyed by a moving stream of gas or air. This pollutant material may exist in liquid form (commonly described as, gaseous fume, or as solid particulate matter, including dust. It will generate result in at least one of four detrimental effects: loss of valuable products, at atmospheric nuisance or safety hazard, damage to the quality of the manufacture product, or mostly plant and equipment maintenance.
Particulates are formed and classified in either of two size categories: submicron or micron. Equipment design and collection efficiently are directly related to particle size. Particles 1 micron or larger in size (1 micron equals 1/25,400 in.) are generally easily collectable; smaller (submicron) sizes are more difficult. One exception is the gas molecule. Although extremely small (0.0 micron in size), it is usually easily removed by packed tower type methods discussed at length later in this chapter.
The first consideration in the proper selection of a collector is the nature of the pollutant materials to be collected.
Origin of Air Pollutants
Both the chemical and physical characteristics of all particles must be known in order to choose the proper collector(s). The origin of the particle may furnish a clue as to whether the pollutant constitutes a fume, dust, mist, or just an undesirable gas.
Particles formed by mechanical disintegration may generally be classed in the micron size group. These may be formed by pulverizing, crushing, and grinding. Generally, these particles are larger than 1 or 2 microns. But there are always exceptions to the rules. For example, a particle of dust which is subject to continuous "self-disruption," as in a roller grinding mill, may produce particles with distribution into the submicron size range.
Particles created by coarse disintegration such as sawing, jaw crushing, or tumbling (as in slow-moving rotary equipment) may be above the 15-micron range.
Particles created by a physical change of state (such as sublimation or chemical reformation) or from an intensive heating and melting operation are generally sub-micron in size.
Furnace operations, in both the steel and nonferrous industries, usually produce exhaust or combustion gases containing metallurgical submicron fumes. The generation of finer fumes is directly related to the heat intensity.
Two current areas which are known producers of extremely submicron metallurgical fumes are:
1. Oxygen injection-to raise melting temperature at increased speed
2. Electric furnace-to reach temperatures above 2000°F.
Pollutants having a composite combination of particles may be found in air which has been swept through the inside of a plant having various processes, or which has been trapped in the hoolding around operating equipment.
The secondary consideration in selecting a collector is the feasibility of available collectors to suit the specific needs of the user. Factors such as particulate or gas recovery, the area which is to be occupied by the collector, power usage, disposal problems, maintenance, replacement parts, and situations governed by the overall process operation weigh heavily on the best choice for the type of collector. Often a combination of these collectors will offer many advantages.
INFORMATION REQUIRED PRIOR TO EQUIPMENT DESIGN
Because each air pollution problem is unique, exact preliminary knowledge of pollutants is required in order to design compatible equipment. Particle size, gas temperature, and corrosive constituents are all factors influencing equipment selection and design. To acquire this knowledge, samples of dust and mist must first be collected.
Sulfuric acid mist (SO3) illustrates how the chemical properties of a pollutant can cause complications. Appearing as a visible fume with little odor, SO3 is a submicron aerosol mist which forms sulfuric acid upon combining with moisture. When this occurs on moist skin tissues, a tingling feeling or burning sensation results. To be scrubbed, SO3 must first be hydrated to convert it into the removable form of sulfuric acid. Wet conditioning prior to scrubber entry performs the hydration. This requires a dual operation at the scrubber-conversion plus energy of impaction.
Metallurgical plants have generally collected sulfuric acid with an electrostatic precipitator. More recently, plants manufacturing sulfuric acid have utilized venturi scrubbers (operating at 30 in. W.G. pressure drop) to make the characteristic whitish cloud of SO3 mist essentially disappear.
A similar collection problem is involved with the phosphorus pentoxide fumes, P2O5. Although simple collection methods are not sufficient for this submicron aerosol mist, venturi scrubbers operating at 30 in. W.G. pressure drop have proved efficient.
As a rule, the submicron mist must be reduced to approximately 1 or 1½ mg/scf to approach invisibility.
SPECIFYING APPROPRIATE TYPES OF COLLECTION EQUIPMENT
Manufacturers must take many factors into account when selecting the correct method of treating a specific air pollution problem.
Each type of control equipment embodies particular design peculiarities which influence installation, maintenance, and efficiency.
A manufacturer must also weigh the interrelated process and environmental factors. Frequently, the preferred solution to one or more side problems may indicate a choice other than that first apparent. Thus, the collection equipment eventually selected often represents the most desirable compromise. Alternately, a combination of equipment may be the answer.
Factors Affecting Equipment Specification
Location. Proximity of the gas collector to the gas source can determine the treatment required. As a general rule, a reasonably close location minimizes many side problems. Among problems alleviated are conveyance of dust particles which could settle in the ducts, and cool spots which cause condensation and ultimate corrosion. Close placement also minimizes pressure drop losses which occur when gas is drawn through a long distance.
Location is also influenced by the nature of the dust loading. If the dust is micron size or larger, in an essentially dry and ambient condition, the collector may be installed as close to the source as possible. However, if the gas loading is sub-micron in size, factors such as nuclei growth should be considered. Collection efficiency is somewhat enhanced when submicron particles have an opportunity to agglomerate with one another while traveling a long distance to a collector.
Space Requirements. The feasibility of a particular type of equipment may hinge on space factors. If the source is on the roof, for example, considerations of roof loading will affect collector design. These can involve size, as in extremely large bag collectors or precipitators. In the case of a wet scrubber, on the other hand, weight may be the determining factor, when recycle tanks are requisite. Foundation factors of support materials and design are therefore involved whether on the roof, near the ground, indoors, or outdoors.
Complicated Fumes. Fumes consisting of heavy particles, tar vapors, or organic solvents may create side problems varying from fire hazards to
dust accumulation on the walls. In this case, short duct systems are preferred. If impractical, consideration should be given to spraying the ducts, necessitating sloping the ducts (possibly 5°) to direct drainage toward the scrubber.
Temperature. When handling extremely hot gas, the collector must be placed at some distance from the source to allow cooling in transit. This is especially true in the case of bag collectors, where pretreatment of the gas, by cooling, improves collection efficiency and protects the filter fabrics. These fabrics affect the overall operation and cost of the equipment, and require specific temperature ranges for maximum performance and longevity.
Excessive temperature may contribute to duct warpage.
Sudden cooling often causes a "wet-dry line" condition, in which solids accumulate at the cooled duct walls.
Optimum temperatures for electrostatic precipitators range from 350 to 500°F. The presence of such acid compounds as SO2 and SO3 requires serious consideration. Mixtures of sulfur trioxide have a high dew point which can create a corrosive atmosphere.
The SO3 dew point should be considered at all times.
Generally, extremely hot gases are best handled by wet collectors, which effectively utilize several cooling techniques. Adiabatic cooling to saturation is one means of reducing the temperature. For best scrubbing results, gas temperatures above 450°F should be saturated prior to entry into any wet scrubber.
The oldest known method for removing dust from an airstream is the cloth filter. It is versatile and highly efficient for collecting solid particulate matter in a wide range of sizes.
As a general rule, efficiency increases in direct proportion to the amount of cloth area in the filter. Maximized cloth area delivers lower pressure drop requirements, greater reserve capacity for surges or expansion, and longer media (fabric) life. Thus, efficient design of fabric dust collectors involves a compromise between the ideal of large bag filters and their considerable cost. To maximize filter surface per unit volume, designers frequently determine size by selecting the highest filter rate (velocity through the media expressed in feet per minute) consistent with good operation.
Filter fabrics The actual fabric of the filter is chosen on the basis of its ability to withstand temperatures and stresses inherent in the process, as well as its compatibility with the pollutants to be collected. The fabrics are fibrous material, either natural or man made, in woven or felt form. In the case of woven fabric, the material serves as a base for the accumulation of a porous layer known as a filter cake. This cake heightens the Filter's cleaning efficiency by screening out submicron particles. With felt materials, cleaning is accomplished as the gas travels through the maze of fibers in the fabric itself.
Factors affecting fabric filter performance These include the fineness and size distribution of particulate matter, particle shape, agglomeration tendencies, static charge or tendency, physical properties such as adhesion or sublimation, and chemical properties such as crystallization and polymerization reactivity. System factors include gas constituents, loading, media limitations, temperature, humidity, desired differential pressure, turbulence, and dust origin.
Increased temperature, for example, requires more cloth, probably because of an attendant gas viscosity increase. This is eventually counterreacted by reduced density. Dust load is also a major consideration, requiring more cloth area at higher loadings (more frequent cleaning or precleaning). Certain continuous automatic collectors can handle more material without lowering cfm per square foot when loading surpasses a certain point, normally in excess of 100 grains/cu ft. This is probably due to total saturation of the air with dust, thus giving bag surfaces a saturation-limited rate of accumulation per unit time.
Specialized application factors fall into three basic categories. The first of these is nuisance venting, which includes relief of transfer points, conveyors, and packing stations. The second, product collection may involve air conveying-venting mills, flash driers, and classifiers. The third, process gas filtering, ranges from spray driers to kilns and reactors.
Sudden contraction Less pressure is lost in a sudden contraction than in a sudden enlargement, as seen in the formula
in which Kc is a function of the ratio of the cross-sectional area of the smaller to the larger pipe. The following table gives the value of Kc for various area ratios:
Values of Kc for Sudden Contraction*
a2/a1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Kc 0.362 0.338 0.308 0.267 0.221 0.164 0.105 0.053 0.015
* O'Brien and Hickox.
Venturi or orifice type scrubbers are employed in installations requiring high-energy collection of submicron particles.
The venturi's basic construction and principles of operation are noncomplex in nature. Since there are no internals, workings are accessible from the unit's exterior.
Figure 10: Various resistances to flow within the collector cause a loss of pressure.
The venturi is the most accurate of fluid meters, since it contains no moving parts to impede the airflow. The unique shape of the venturi offers 98 percent velocity head (power consumption) recovery, thereby allowing efficient introduction of fluid to meet the gas crossflow in the throat region. Atomization and impaction occur as the injected liquid is shattered by the unscrubbed gas into minute droplets which collide with and carry away minute particles.
Construction materials Because of the equipment's simplicity, designers can choose from a full range of construction materials to handle problems of corrosion, abrasion, or both, depending on the nature of the emission. Among materials frequently specified are stainless steel or Hastelloy. Lead linings may be employed for protection against concentrations of sulfuric acid. Brick (ceramic) linings are used for protection against high temperatures and/or excessively abrasive particles. Rubber linings provide protection against fluorine and phosphoric acid fume emissions. To minimize deterioration and eliminate maintenance, current designs frequently use plastics such as fiber glass and PVC (for nitric acid).
General principle Atomization of the scrubbing liquid takes place in the throat of the venturi. Here, the liquid is introduced at relatively low pressure and is shattered into minute droplets by the onrushing gas flow. For coarse particles, such as those found in the lime kiln gases and flue gases from the powdered coal furnaces, efficient collection may be attained with lower velocities and water rates than those needed for the collection of submicron particles.
As a general rule, a higher liquid rate is usually more advantageous to use than a higher gas velocity. Figuring 3 gal/1,000 cfm, to take an extreme example, the combination of high velocity and low liquid rate would probably create an unwetted void through the middle of the venturi. If we were to look down the venturi, we would see an area in the middle where wetting action does not take place. Hence, it is always preferable to lean toward a higher liquid rate to ascertain a proper liquid-to-gas impaction level. At the other extreme, a problem can occur where a low gas velocity exists with too high a liquid rate (12 gal/1,000 cfm, for example). Under these conditions, liquid shattering may be reduced to such a point that the scrubber starts to operate like an "ejector," which means that very poor collection efficiencies are achieved. In summary, as a general rule, the ideal operating range of the venturi type scrubber varies from 5 to 8 gal of liquid/1,000 cu ft, so that maximum and minimum scrubber contact velocities of 140 fps at 300 fps are attained.
Basic advantages Even though venturi scrubbers occupy less ground space than most bag collectors and precipitators, they can also clean gas volumes within a minimal equipment area. The venturi's lack of internals, however, eliminates the need for work stopping checks for plugging and deterioration.
An effective firestop in applications where extremely fine, dry, or combustible materials are involved, the venturi is capable of handling hot gas, some noxious gases, sticky dust, and moisture, with no secondary dust problems.
Venturi operation costs may be low if the collected sludge can be disposed of without clarification. However, sludge that requires clarification and collection can raise the capital cost of the venturi plant to that of the electrostatic system.
Basic limitations The basic disadvantage of the venturi scrubber is the rise in operating cost-usually 50 to 60 percent above that of electrostatic cleaning-primarily because of the high power cost for fans and water pumping. As mentioned above, further processing of the wet sludge poses a costly auxiliary problem.
Continuing overhead also includes fan cleaning, which must take place at frequent intervals to prevent mud from collecting on the blades, causing the fan to go "out of balance." In systems handling saturated gases, equipment design must make full allowance for corrosive gas handling to avoid costly maintenance.
Steam Plume. In terms of psychological community relations, the venturi (as well as any wet scrubber) is under a disadvantage in that the stack emits a steam plume, especially in cold weather. Although recent innovations in cooling towers have made it possible practically to eliminate the plume, it may give the impression that cleaning is less satisfactory than in actuality.
Other Types of Wet Scrubbers
Spray chambers are the most elemental of wet scrubbers. These are empty towers utilizing liquid introduced via a bank of spray nozzles at the top. Gases passing countercurrent to the falling drops are scrubbed clean of particulars matter of larger than micron size. These towers may also be used as coolers or as primary cleaners. Though these units were acceptable in prior years, recent developments have surpassed their performance.
Cyclonic spray scrubbers combine the spray technique with the mechanical principle of centrifugal force. The cylindrical tower contains a central manifold, from which droplets are sprayed into the airstream, which enters through a tangential duct. Centrifugal force created by the gas rotates the droplets at high velocities, enabling them to collide with and carry particulate matter to the scrubber walls.
Normally designed for specific installations, cyclonic spray scrubbers can accommodate gas entrance velocities up to 200 fps, with low-energy pressure drops ranging from 2 to 6 in. W.G. They have been utilized for cleaning micron-sized particles created from mechanical disintegration.
The packed tower type of gas scrubber is a prime method of scrubbing a true gas. In use since the 1800s, packed towers are used for the removal of gaseous fumes, noxious gases, and entrained droplets in gas cleaning installations for various chemical processes.
General principle Basically, the packed tower is a vertical vessel in which various fill material is wetted. Surface area provided by the various packings offers a basis for inducing interaction between the liquid and gas phases. The air or gas enters the bottom of the tower and receives a preliminary washing as the scrubbing liquid drains in an opposing flow from the packed, irrigated bed. This liquid, which is pumped into the top of the tower, flows down over the packing bed. En route, it covers the surface areas of the packing with a liquid film that accomplishes the major work of collection. Finally, the airstream passes through a mist eliminator section (a dry packing pad) before it is permitted to exhaust.
Variations in design Spray towers with baffles or slotted plates are also used to produce a circuitous path for the contaminated airstream. As in the case of packing, these cause the contaminant to contact the water film. High-pressure nozzles maximize air and water contact.
Because of the complexities involved in packed towers, air pollution control specialists normally design equipment for each specific industrial situation, extrapolating data from laboratory and/or pilot plant studies to accommodate the variables inherent in the particular gas flow.
Factors in specifications The following guidelines will help to determine the best uses of the packed tower.
Velocity. To obtain maximum throughput in a vessel so that there will be sufficient contact between the gas to be collected and the liquid media, the gas flow, or velocity, should range between 3 and 6 fps.
Selection of Packing. Packing should be specified to provide maximum surface area along with minimum void area.
Maximum surface is a prerequisite for covering the packing with a liquid film sufficient to contact and react with the gas film. Obviously, the packing is not wettable during the absorption cycle, this vital interface reaction is reduced. In this case, more packing is required. Similarly, if a tower has a very large packing bed with minimal surface exposure, the liquid film created may be minimal. This contact will be limited to the gas and droplets which exist between the packing, thereby reducing interaction of the gas and the liquid film on the packing surface.
Generally, the smaller the packing size, the more contact offered, thereby requiring more energy (pressure drop) to pass the gas through the tower. At the same time, the smaller packing requires less height in the tower.
However, larger quantities of packing with large void space have proved to be equal in contact efficiency to smaller packing with greater surface area. Experience has shown that a nominal packing size ranging from 1 to 2 in. is optimum for the capacity of gas handled as well as for the factors of grillage availability, water distribution, and pressure drop consumption.
Packing Materials. The tower's packed "bed" may contain any of a variety of materials in an assortment of shapes, each specifically chosen for its compatibility with the constituent in the particular gas stream. Conventional packings include Berl saddles or Raschig rings, which are commonly constructed in chemical stoneware. Porcelain and carbon are also frequently used. Specialized packings marketed today include those made of polyethylene and propylene. These packings are commercially known as Maspac, Pall rings, or Tellerettes. Different packing materials will have different pressure drop which in turn result in various horsepower consumption.
Packing Height. Calculation depends on such factors as the absorption coefficients, the partial pressure of the gaseous contaminant, and the cross-sectional area of the tower. Usually effective irrigated packing heights range between 4 and 8 ft with a 9 in. high mist eliminator bed located above the spray region.
Pressure Drop. Pressure drop is normally described in terms of inches water gage (W.G.). Increased water gage refers to the power consumed in order to move the gases through the equipment. This power is generally consumed by the air moving device, which may be the blower or the fan. As a rule of thumb, a 64 percent mechanically efficient blower consumes 1 hp/1,000 cfm of gas when developing approximately 4.3 in. W.G. Approximately 408 in. W.G. is equivalent to 14.7 psi, or 27.4 in. W.G. is equivalent to 1 psi. As a general guide a low pressure drop through a packed bed is associated with a high percentage of free or void space.
Table 2 offers variables of the many packings that may be used in the general equation for pressure drops.
Purpose of application Packed towers are greatly influenced by the solubility of the gas to be collected.
Simple gases are defined as having "gas film controlling" whereas difficult gases are defined as having "liquid film controlling." Simple gases which are readily water soluble (gas film controlling) are as follows: NH3, HCl, SO3 in strong H2SO4, SO2 by alkali and NH3 solution, H2S in caustic solution, also the evaporation and condensation of liquids. Those gases which are not readily soluble (liquid film controlling) are CO2, O2, H2 by water, the absorption of CO2 in alkali solution, and the absorption of chlorine in water.
Difficult, or readily insoluble, gases require special considerations which are beyond the scope of this discussion. Furthermore, their products are not generally associated with the air pollution nuisance purpose.
The general design of packed towers is dependent upon whether the gas contains any solid particles. Where the solid particles are in concentrations higher than 5 mg/cu ft, a packed tower is not recommended.
Treating water soluble gas In a very simplified manner, we have listed those gases which are readily removed using water on a once-through basis. There are also times when the concentrations in water allow recirculation with maintained optimum removal. Such a concentration is largely dependent upon the gas temperature leaving the tower as well as the concentration of the contaminant in the gas stream. At high temperatures, gases have a tendency to revaporize from the surface. Accordingly, the recirculation cycle becomes limited as the gas vapor pressure above the liquid surface increases. Those gases which are ideal for water contact are as follows: ammonia (NH3), fluorine (F2), hydrogen fluoride (HF), hydrochloric acid (HCl). Gaseous tetrafluoride (S1F4) is also effectively removed, provided the packing does not receive gas concentrations above 5 mg/scf as F. Upon initial wetting S1F4 converts partially to the gelatinous S1O2, which will foul and plug the packing. Where the gases need additional treatment to convert them to the soluble form, solubility is influenced by a neutralizing additive. Gases which need such additional neutralizing treatment are sulfur dioxide (SO2), hydrogen sulfide (H2S), and nitric-nitrous oxides (NO-NO2).
The packed tower type scrubber further lends itself to the handling of gases which may have entrained droplets as from plating or various metallurgical operations. These droplets may consist of corrosives such as sulfuric, phosphoric, and nitric acids. Caution must be exercised here (particularly when dealing with sulfuric and phosphoric acid) to ensure that the droplets are true, and not a decomposition product existing as an aerosol mist. These "aerosol mists," as they are termed, occur as a white cloud and cannot be effectively removed by a packed tower scrubber.
Description of a packed tower may be further clarified in that the tower uses a support grid to hold the packing or fill. The grid has an open pattern to permit an upflow of gas through it with minor resistance to gas flow.
Methods of liquid distribution The "weir box" is a unique liquid distribution vehicle with V notches which permit uniform liquid distribution. It is subject to reasonable leveling within the tower. The primary disadvantage of the weir box device is that the operator cannot tell whether a reasonably uniform distribution exists and is effectively compensated for with variations in gas flow. V notches in the weir box having spacing beyond 3 in. do not offer full coverage at the upper edge of the packing, resulting in loss of effective packing height. V notches spaced at 3 in. may lose up to 2 ft of packing height. Also, weir boxes are very costly and add excessive weight to the tower.
Spray Assemblies. In place of weir boxes, the spray pipe assembly method has recently been demonstrated to be superior as it allows positive and adequate distribution throughout, and fuller use of packing height, since distribution exists at the upper edge of the packing. Further, removal of the spray headers from the exterior is allowed. Disadvantage of the spray assembly occurs when recirculated water contains particles that may plug the spray nozzles. But careful choice of nozzles will help prevent such a happening.
Use of plastics-such as FRP or polypropylene for the shell housing-has increased in recent years. The packing made of polypropylene and the pipe headers of PVC and spray jets of Teflon has proved to be ideal construction. Plastic is also advantageous as it is lightweight, noncorrosive to any of the fluids mentioned above, easily repaired and assembled, and requires no outside painting. Of course, there are some temperature limitations where PVC is employed, but this situation is primarily restricted to the pipe header and does not prove to be a problem in most instances.
Basic limitations Although small quantities of fine micron sized particles can be scrubbed by the packed tower, it is not generally recommended as a primary apparatus for the collection of micron particles, because such particles can close the space between the tower's packing and cause it to become clogged, rendering the equipment inoperative. A good rule of thumb is to allow a maximum of 0.2 grain/scf of dust to enter a packed tower. Otherwise a serious problem of blocked packing may result.
The steam plume is a very important psychological consideration which may affect the overall equipment selection and design.
A wet type scrubber or wet type electrostatic precipitator which is doing an excellent job will show a steam discharge plume upon becoming saturated with moisture. Although the steam will have no deleterious effects oil the surrounding area, its appearance may cause concern with the novice that the air cleaning equipment is inadequate or malfunctioning.
Cause Steam plumes are the result of rapid cooling of gases or air carrying moisture to below the saturation temperature. Obviously, gases from wet scrubbers will quickly show a steam plume at the stack discharge. During the colder winter months, the gases are subjected to a colder atmosphere. They are thus air-condensed sooner and have shorter plume trails compared with equivalent gases cooled by the summer atmosphere.
As a guide, saturated gases which discharge from the stack below 105°F will have a negligible appearance and will not create a questionable steam plume. At 105°F the volume of moisture content is less than 7 percent, whereas at higher saturation temperatures, the percentage of moisture by volume is as follows: 130°F, 15.0; 160°F, 32.5;180°F, 51.0.
Side effects In addition to appearance, steam plumes have other side effects which include:
1. SO2 (or other corrosive gases) may become aggravated through steam plume condensation when SO2 is absorbed by the newly formed droplets into sulfurous acid and then falls on homes and industrial sites.
2. In some cases odoriferous constituents may be entrapped by falling droplets to increase odor at ground elevation.
Methods for steam plume minimization
Indirect Cooling of Hot Gases. A quick inspection of the psychrometric reveals that the saturation, easily reached by all wet type scrubbers, is dependent upon the initial moisture content (Ib moisture/lb dry air) and the initial hot gas temperature. Reduction of either or both of these conditions ultimately decreases the saturation temperature.
In the example in the chart, point A is at an initial temperature of 500°F and moisture content of 0.11 Ib moisture/Ib dry air; therefore, the saturation temperature will be 150°F at point B. Point C is designated a reduced initial temperature at 140°F and the same moisture content, 0.11 Ib moisture/Ib dry air. Now the saturation temperature will be 131°F.
The 17°F reduction at the final saturation represents a reduction from 25.2 per cent water vapor to 15.3 percent water vapor content (approximately a 40 percent water vapor content reduction).
Figure 19: Adiabatic saturation lines and percentage saturation curves at temperatures ranging from 0 to 500ºF at a pressure of 29.921 in. Hg.
Cooling is effected within a continuous S shaped duct with sufficient surface exposure for radiation and cooling by atmospheric air surrounding the ducts. Often the ducts may be arranged essentially vertically with U bends returning upward and downward. The bottoms of these U bends should be furnished with cleanout doors and hoppers to permit intermittent dust removal.
Sensible or Direct Cooling of Saturated Gases. This technique cools the already 100 percent saturated gases with cool water. Sufficient coolness is required to dehumidify or condense water vapor down to the desirable lower saturation.
Thus we see, in Fig. 18, point B at 150°F saturated has a heat content of 267.0 Btu/lb dry air cooling to 105°F saturated; the heat content is 73.6 Btu/lb air. Heat to be removed equals 267.0 - 73.6, or 193.4 Btu/lb dry air.
By using a standard spray tower where available, cooling water at 70 to 85°F may be introduced at 25 psig. Droplets in the range 500 microns in diameter will fall counterflow to the gas passage and carry away latent heat of the water vapor and sensible heat of the dry gas. As a general rule, the cool water leaving a properly designed tower will approach within 10 or 15° F of the entrance gas temperature. Therefore, pounds of 80°F water needed equals 193.4/(150) - (80+ 15) or 3.51 Ib water/lb dry gas or 0.42 gal/lb dry gas.
Use of a tower filled with drip-point grid tiles offers a method to obtain benefit of maximum heat transfer with an approach of approximately 5°F or less ( between the gas and liquid discharging).
Cooling by Mixing with Atmospheric Air. In some special cases sensible cooling may be obtained by the addition of atmospheric air having a low dew point temperature. However, this method becomes impractical where already large saturated volumes of gas having high saturation temperatures are involved.
Treatment to Reduce Disposal
Wastes are generated by virtually every industrial enterprise. These wastes can be solids or liquids. For example, the pharmaceutical industry generates waste solvents when purifying ethical drugs and discarding manufactured products or raw materials that do not meet specifications. Paper mills generate waste from wood pulping, trimmings from the paper-making machines, and from roll ends. Dry cleaners generate waste oil, dirty filters used to recycle solvents, and spent solvents. The list and variety of waste products is virtually endless.
There are three main methods of waste disposal: land disposal, which has been and continues to be predominant; waterborne disposal with eventual drainage out to sea; and dispersal to the atmosphere. Treatment before disposal is desirable for reducing the toxicity, mobility, or volume of the waste.
Techniques are available for recycling parts of these wastes and developing new, marketable products. To be commercially viable, these products must meet minimum standards. For instance, to be saleable, recycled lubricating oil must meet standards for lubricating quality and temperature stability set by the Society of Automotive Engineers. Recycled solvents must meet commercial criteria for industrial solvents, such as cleaning effectiveness and corrosion inhibition. Recycled paper must be suitable for cardboard manufacture, writing paper, or cellulosic insulations. If these commercial constraints are not met, the recycled product itself will be discarded as waste.
In a waste reduction project, the first goal is to eliminate the waste as early in the production process as possible. It is, however, rarely feasible to optimize a production process to the extent that no waste is produced. "Zero discharge" is a laudable goal, but it is seldom practical. However, even when the volume of waste from a production process is minimized, waste reduction opportunities do not end. The waste can still be processed to recover useful materials or treated to reduce its volume, toxicity, or mobility, thus reducing its impact on human health and the environment. This chapter discusses and presents examples of a few of the treatment technologies for reducing the environmental and health effects of materials that are disposed of as waste.
POSSIBLE USES OF TREATMENT
Treatment can be used to reduce the volume of waste requiring disposal, to render a hazardous waste nonhazardous, or to recover a useful product or some other resource. The following examples illustrate potential uses of treatment for waste reduction.
Volume reduction can be used to reduce treatment costs or to reduce the handling and disposal costs for residues remaining after treatment. Volume reduction can be accomplished by using a variety of methods, including:
• reuse of treated wastewater or other wastes
• treatment modifications to reduce the generation of solid residues
• segregated treatment to reduce hazardous waste mixtures
In addition, incineration can be used to reduce waste volume or to render a hazardous waste nonhazardous.
The most extreme example of wastewater reduction is a zero-discharge wastewater management system. In such a system, wastewater is treated to sufficient quality that it can be reused in place of raw water within the facility that generated the wastewater. Typically, wastewater that cannot be reused in the original process is evaporated or used for some other beneficial use, such as spray irrigation.
Zero discharge has been practiced for some time in locations where water is scarce. Zero discharge may involve the use of technologies for removing suspended solids, such as clarification and filtration, and technologies for removing dissolved solids, such as reverse osmosis and evaporation. The actual technologies required depend on the quality of the wastewater as well as the water-quality requirements of the process that reuses the treated water. Complete demineralization is relatively expensive; however, in some cases, wastewater discharges can be reduced significantly using less-expensive selective removal technologies.
Case Study 1 : Zero Discharge at an Integrated Manufacturing Facility
Background and Objectives. A corporation located a new manufacturing facility in a rural area. This facility was to be highly integrated, with many product components manufactured onsite. Minimizing the impact of the facility on the local environment was an important consideration; in addition, local water supplies and wastewater disposal capacity were limited.
Because of these factors, the company decided to investigate the feasibility of in-plant recycling of wastewater. The ultimate goal was zero discharge of wastewater from the site.
Water Management Computer Model. To assist in evaluating water and wastewater management alternatives, a computer model was developed of the manufacturing facility's water use and wastewater production. The water management model included water usage and quality requirements for more than 50 manufacturing processes in 9 separate manufacturing areas, taking into account the chemical constituents and pollutants added to the water. In addition to manufacturing processes, 20 water treatment processes were included in the model.
Approaches to Zero Discharge. Approaches to achieving zero discharge include at-source treatment and end-of-plant treatment and recycle, or some combination of these in an integrated water management system. At-source treatment and recycle was appealing because it would treat waste streams in small, possibly modular, systems, and would be designed for only those parameters not meeting water reuse requirements. The primary disadvantage was that the large size of this facility would necessitate multiple treatment and recycle systems. An end-of-pipe treatment system would have to be extensive and contain many unit processes in order to accommodate the various flows at the facility. An integrated approach combined the best features of at-source and end-of-pipe treatment.
Recommended Water Management System. The water quality model was used to analyze numerous configurations of an end-of-plant wastewater treatment facility. An end-of-pipe treatment system was recommended to receive all process effluents, treat the wastewater, and recycle the treated water back to the manufacturing units. This was supplemented with some at-source treatment to reduce the number of unit processes required in the end-of-source treatment to reduce the number of unit processes required in the end-of-pipe facility. An estimate of the capital costs for this approach is presented in Table 1.
Case Study 2: Wastewater Reuse at a Coal-Fired Power Plant
Background and Objectives. An electric utility located in the eastern. United States operates a large coal-fired power station. Recently, state regulators required the station to meet new stringent discharge limits for metals. Meeting the limits would require very expensive treatment of the entire discharge from the station.
Wastewater Management Planning. The client contracted CH2M HILL to develop a wastewater management plan for the station. CH2M HILL conducted in-plant sampling and flow monitoring to identify flow and pollutant sources. This information was used to develop a mass-balance diagram of the power station. Significant sources of flow include cooling-tower blowdown, ash-hopper overflow, coal pile runoff, and miscellaneous waste streams. Significant concentrations of metals are present in enough waste streams so that end-of-pipe treatment will be required.
Scope allowance (30% of subtotal) 11,341,177
TOTAL ESTIMATED CONSTRUCTION COSTS 49,145,099
Engineering (15% of construction costs) 7,371,765
Contingency (10% of construction costs) 5,651,686
TOTAL CONCEPTUAL LEVEL CAPITAL COSTS 62,168,550
Recommended Treatment Process. The power station currently generates an average of 7 million gallons per day (mgd) of wastewater. The required size of an end-of-pipe treatment faculty for this volume of effluent is approximately 9 mgd. Many of the waste streams are contaminated with suspended solids. Settling of solids from these waste streams can produce an effluent having a water quality rivaling that of the raw river water used at the facility. These waste streams represent more than 50% of the wastewater currently discharged from the facility. The settled wastewater can be reused as cooling-tower makeup water, ash-hoppers makeup water, and for other power plant processes. The client and CH2M HILL reached a consensus that average wastewater discharge flows could be reduced to approximately 3 mgd.
Table 2: presents order-of-magnitude capital costs for the end-of-pipe treatment facility with and without wastewater reuse. As shown, reuse of
Table 2: Comparison of Capital Costs for Treatment at a Coal-Fired Power Station
With Wastewater Without Wastewater
Treatment Process Flow Reduction Flow Reduction
Settling basins 1,150,750 1,430,630
Pump station 136,730 143,630
Flocculation tanks 82,590 106,680
Clarifiers 1,144,250 1,669,000
Filter package 900,490 1,016,230
Sludge handling 792,670 1,077,520
Building 268,530 399,140
Chemical feed systems 70,250 80,700
Subtotal 4,546,260 5,923,530
General conditions 248,000 323,100
Finishes 165,300 215,400
Misc. undefined mech 826,600 1,077,000
Electrical/l&C 1,239,900 1,615,500
Sitework 413,300 538,500
Yard piping 826,600 1,077,000
Subtotal 8,265,900 10,770,100
Contingency 2,479,800 3,231,000
Subtotal 10,745,700 14,001,100
Engineering 3,761,000 7,900,000
Mobilization/insurance/bonding 537,300 700,100
Total Capital Cost (s) 15,044,000 19,601,200
wastewater can save the client $4.6 million in treatment costs. The estimated capital cost of the piping modifications required to reuse wastewater is approximately $1 million.
Reducing Generation of Solid Residues
Modifying waste treatment practices can reduce the amount of solid residues requiring disposal. This can be achieved by modifying treatment chemistry to produce less sludge, or by removing water from the sludge to produce a drier cake.
Modification of Treatment Chemistry to Reduce Sludge
In conventional treatment of a mixed-metal waste containing hexavalent chromium, the pH of the waste stream is reduced to below 3 with a mineral acid (usually sulfuric). A reducing agent (sulfur dioxide or sodium metabisulfite) is then added to convert hexavalent chromium to the reduced trivalent state. After the reaction is complete, lime is added to raise the pH of the combined wastewater to approximately 9.5 in order to precipitate heavy metals as hydroxides. A disadvantage of this treatment scheme, however, is that it produces large quantities of sludge, in excess of the metals targeted for removal. The sludge is produced because calcium carbonate and calcium sulfate are precipitated out of the wastewater, particularly when treating a hard alkaline wastewater.
In research sponsored by the Air Force, it was found that chrome reduction could be accomplished at neutral to slightly alkaline pH using a combination of ferrous sulfate and sodium sulfide as reducing agents and sodium hydroxide for precipitating heavy metals. The study found that the resulting iron hydroxide precipitate was effective in removing other heavy metals, such as cadmium and nickel, at a neutral to slightly alkaline pH. The advantages of these modifications include eliminating an acidic chrome reduction step, eliminating the addition of acid and reducing the need for lime, and precipitating metals without also producing calcium sulfate or carbonate solids.
This process has been adopted at the Tinker AFB industrial wastewater treatment plant to remove chrome, copper, nickel, cadmium, and other metals from a 1-mgd combined industrial waste stream. Operating at a pH of 7.5 to 8.5, the treatment system achieves the same chrome reductions as the old process operating at a pH of 2.5 to 3, while reducing the sludge volumes by two-thirds. The Air Force estimated that this process modification would save $1,000 per day in chemical usage and sludge disposal costs.
In a similar treatment process, ferrous iron is generated electrochemically using sacrificial iron electrodes. Equipment has been installed at a Navy ordnance plant in Pomona, California, to remove copper and traces of chromium and nickel. The pH of the reactor is held to a range between 6 and 9 using caustic soda (sodium hydroxide). The precipitated metals are settled out in a clarifier and dewatered in a filter press. The process was reported to produce 75% less sludge when compared with acidic chrome reduction and lime precipitation.
Combined industrial wastewater treatment facilities typically use hydroxide precipitation for removing toxic metals. The metal hydroxide solids are usually removed by clarification. Metal hydroxide sludges withdrawn from a clarifier typically have solids concentrations ranging from 1% to 2%. A pound of copper precipitated with hydroxide produces 1.54 lb of solid copper hydroxide. In a 1% sludge, this pound of copper produces 154 lb of sludge. Disposal of such a large volume would be expensive, even if permitted (liquid waste disposal in hazardous waste landfills is banned). Therefore, most waste treatment plants dewater their sludge before disposal.
Use of a thickener can be useful before the sludge is dewatered. Adding a thickener after or as part of the clarifier provides additional sludge storage volume in addition to increasing the sludge solids to as much as 5 to 6%. (In our example, this equates to reducing the 154 lb of copper hydroxide sludge to 30 lb.) Increasing solids by thickening assists subsequent sludge dewatering in two ways. First it reduces the time required for dewatering, and second, it usually results in a drier sludge cake after dewatering.
Sludge dewatering has traditionally been accomplished in open sand drying beds, especially in areas of the country with warm climates, low rainfall, and cheap land. However, because sludge from industrial wastewater treatment plants is frequently hazardous, sludge drying beds have all of the design (and, potentially, the regulatory) requirements of a hazardous waste landfill, with collection and treatment of leachate required. Therefore, mechanical dewatering is most frequently used because of regulatory and cost advantages.
Three types of mechanical dewatering devices are typically used for treating sludge from industrial wastewater treatment plants: vacuum filters, belt presses, and plate-and-frame filter presses.
Vacuum filters are used infrequently because they produce the least-dry cake of the three mechanical methods. Vacuum filters frequently employ a precoating process, using diatomaceous earth, to improve dewatering. This is a disadvantage because some of the precoat is scraped off during the process, adding to the volume of solids to be disposed of. In addition, vacuum filters generally require higher energy use than the other mechanical dewatering processes. In some applications (for example, in dewatering aluminum hydroxide sludge), vacuum filters are preferred, because they are gentler to gelatinous sludges and can produce a dry cake without blinding the filter cloth, as occurs with the other filters.
Capital costs for the smallest belt press are higher than for the smallest plate-and-frame press. However, belt presses can be economical for large waste treatment plants. Operating costs are lower because belt presses operate continuously, as opposed to plate-and frame presses, which operate in a batch mode. Belt presses generally produce a cake having a range of 20% to 30% solids content.
Plate-and-frame filter presses are generally used for facilities that produce a small volume of waste, because they are simple to operate and are available in a broad range of sizes. Sludge from the thickener or clarifier is pumped, typically using an air diaphragm pump, to the chambers of a filter press. The solids are retained by the synthetic cloth filter media, and the liquid flows through the media and is returned to the wastewater treatment plant for retreatment. After the pressure required to pump sludge to the press reaches a maximum value, the hydraulic press that holds the plates together is released, and a filter cake is discharged. Plate-and-frame filter presses generally produce the driest sludge cakes (30% to 40% solids), which is an advantage when disposal is based on weight or volume.
Filter press operation generally requires little operator attention except at the beginning and end of a cycle. Automatic plate shifters greatly reduce the manual labor required to remove the filter cake. Adequately dewatered sludge will literally fall out of the press when it opens.
The major maintenance cost of a mechanical sludge filter is replacing filter cloths, which can be frequent when handling abrasive wastes having an extreme pH. However, metal hydroxide sludges generally have a moderately alkaline pH and are nonabrasive.
Typically, mechanical dewatering can increase the solids content of a sludge from 30% to 40%. Using the 1 lb of copper as an example, an unthickened 1% sludge weighing 154 lb would be reduced to 5 lb of filter cake (30% solids). This filter cake still contains 3.5 lb of water In addition, even though this sludge appears to be dry on the surface, free water typically will escape during shipment. Landfill operators may then either reject the sludge (because of a ban on landfill disposal of free liquids) or require that cement kiln dust be added to react with or soak up the excess moisture.
Further drying of the filter cake to 80% to 90% solids content is feasible: The process reduces the weight of our hypothetical sludge from 5 to 2 lb (80% solids). eliminating the need for adding kiln dust to prevent the generation of free water during shipment.
SEGREGATED TREATMENT TO REDUCE HAZARDOUS MIXTURES
In the past, operating a centralized treatment plant for industrial waste has been more cost-effective than providing individual treatment systems for each production unit in a large manufacturing facility. However, hazardous waste regulations have classified the residues from the treatment of wastes from certain industrial operations (i.e., electroplating or chromate-conversion coating of aluminum) as listed hazardous wastes, regardless of their actual composition or toxicity. This is complicated by the RCRA mixture rule, which states that mixing any quantity of a hazardous waste with a nonhazardous waste renders the entire mixture hazardous.
The combination of these two regulations makes it imperative that a manufacturer investigate whether combined treatment of all waste streams makes sense. It is often economical to provide treatment for hazardous waste separately from all other wastes, especially when the hazardous waste is only a small portion of total waste flow in the facility or produces only a small portion of the total sludge when treated.
Following is an example in which an aerospace manufacturing facility plans to provide separate treatment (and recycle) of the wastewater from chromate conversion coatings operations. Segregating the wastes will result in the industrial waste-water treatment plant sludges becoming classified as nonhazardous. Reducing the amount of waste to be disposed of justifies the construction and operation of two separate treatment plants.
Case Study 3: Separate Treatment of Waste From Chromate Conversion Coating
Background and Objectives. CH2M HILL performed a waste reduction study for a major aerospace manufacturing facility in the western United States. The study identified rinsewater from chromate conversion coating (Iriditing) of aluminum as the main target for waste reduction efforts because it is responsible for the sludge from the entire industrial wastewater treatment plant being classified as hazardous. This rinsewater (approximately 15,000 gal/day) is mixed with other industrial wastes (212,000 gal/day) for treatment at a central industrial wastewater treatment plant.
The U.S. EPA lists "wastewater treatment sludges from the chemical conversion coating of aluminum" as hazardous under the classification F019, specifically because they typically contain hexavalent chromium (RCRA, Appendix VII). In addition to being used in the chromate conversion coating process, hexavalent chromium compounds are used at the facility to deoxidize surfaces (remove oxide surface coating) when, preparing parts for chromate conversion coating. Table 3 shows the amounts of chromium discharged to the waste treatment plant from the three locations that employ chromate conversion coating at the facility. These data were estimated from composition of the process baths involved and from approximations by company personnel of the production loads on these processes and typical drag-out rates.
170 tons of dewatered sludge was produced at the industrial wastewater treatment facility. The waste did not contain toxic concentrations of heavy metals, but because it was a listed waste, it was disposed of as hazardous at a cost of $210,000. Thus, less than 1 lb of chromium discharged to the treatment plant resulted in more than 1,000 lb of sludge per day being classified as hazardous. The cost of disposal was a significant incentive for eliminating chromium-containing wastewater from the industrial treatment plant.
Table 3: Chromium Discharges from Conversion Coating Facilities
Chem Mill Facility 0.90
Remote Location A 0.05
Remote Location B 1.01
Eliminating the F019 classification of the industrial wastewater treatment sludge could be achieved only by attaining zero discharge of rinse waters and baths from the chromate conversion coating and the deoxidation processes. To achieve this objective, a closed-loop rinsewater system would have to be installed, with all residues being hauled off the site for disposal as hazardous waste.
Discussion of Alternatives. Four unit processes were considered necessary for a closed-loop system:
1. rinsewater recycle
2. chemical recovery
3. volume reduction
4. bath purification
Rinse water recycle is necessary because rinse water is usually maintained at a low concentration of contamination to effect good rinsing. The existing operation produced approximately 15,000 gal of rinsewater waste per day (125,000 lb) in the process of removing the 1 lb of chromium per day. Alternatives considered for rinsewater cleanup were:
- ion exchange (IX)
- reverse osmosis (RO)
- ion transfer membranes
- electrodialytic processes
Recovering chromium from the rinse water would eliminate the major toxic constituent from this waste stream and simplify disposal of any blow down stream. Alternatives considered for process chemical recovery were:
- electrodialytic processes
Volume reduction was considered necessary for reducing the size of the rinsewater recycle system and the quantity of blowdown required to prevent buildup of salts in the system. Alternatives for volume reduction were:
- innovative rinsing
Bath purification is necessary if process chemicals are to be recovered from the rinsewater and returned to the bath, inasmuch as such systems will return contaminants and useful chemicals. Otherwise, impurities are concentrated in the bath until the bath is no longer functional, necessitating disposal of a large quantity of hazardous material. It makes little sense to recover 1 lb of chromium per day if this recovery necessitates the disposal of a 20,000-gal tank of Iridite solution containing more than 800 lb of chromium. Also, because the concentration of impurities in the process solutions is much higher than that in the rinsewater, bath purification is easier than rinsewater cleanup. Furthermore, cations are removed in the bath and are not carried over to the rinsewater; therefore, the volume of cation regenerant is reduced significantly. Alternatives for purification of chrome-containing process solutions were:
- electrodialytic processes
- porous pot
The alternative technologies were evaluated for their potential for waste reduction, effects on production, and relative cost. Existing users of the technologies were contacted to discuss their operating experiences. Sites were visited to collect information on the most-promising technologies.
Recommended System. IX was recommended for rinsewater cleanup. IX is an established technology and can also segregate chromic and nitric acid from cations and concentrate them for reuse. Deox solutions contain high concentrations of nitric acid. Reduced rinse flows are accomplished by providing some counter flow rinsing, so that four individual recycle streams are used for the seven tanks. An electrodialytic bath maintenance unit was recommended for the principal deox bath. Finally, evaporation was recommended for concentrating IX regenerant solutions, either to enhance recovery or to reduce cost of disposal.
Contaminated rinsewater is passed through a cartridge filter to remove particles that could plug the IX resin. A cartridge filter was recommended because suspended solids loading is low in these acidic rinsewaters. Also, a cartridge filter does not require backwashing, so less wastewater is generated. Cation exchange is used for removing metallic impurities such as trivalent chromium, aluminum, sodium, and zinc. Then an anionic exchange resin is used to remove hexavalent chromium and nitrate ions. Finally, the rinsewater is treated with activated carbon to remove traces of organics that could foul IX resins if allowed to build-up in the closed-loop rinsewater system. The carbon is placed after, rather than in front of, the IX resin, because carbon removes hexavalent chromium, reducing the potential for chromium recovery.
The cation exchange resin is regenerated with sulfuric acid, removing the metals and returning the resin to its acid form. The volume of this acidic regenerant solution is reduced by evaporation and disposed of off the site.
Figure 2: Iridite and deoxidizer bath maintenance system.
The anion exchange resin is regenerated with caustic soda (sodium hydroxide). The resulting regenerant solution is a caustic mixture of sodium chromate, sodium nitrate, and sodium hydroxide. This regenerant is then concentrated by evaporation.
Because of the low production of contaminants at Remote Sites A and B, it is not cost-effective to provide these locations with completely independent demineralizer units. It is more cost-effective to provide these locations with portable IX units, which are returned periodically to the Chem Mill facility for regeneration.
Estimates of Segregated Treatment. Projected capital and operating costs for the IX systems (a central system with regeneration facilities and two remote systems) are provided in Tables 4 and 5. The cost of installing this system is approximately $294,000 and would result in a savings of $148,300 per year, with a resulting payback period of less than 2 years for this investment.
Installing a bath maintenance system on Chem Mill Tank 3 (deox tank) would cost approximately $16,000. This would decrease cation loads to the demineralizer system by approximately 90% from this source, reducing overall cation loading to the demineralizer system by approximately 80%. The annual savings from installing a bath maintenance system for this tank are provided in Table 6. The table shows an annual savings of $4,400, for a projected payback period of approximately 3.6 years.
An additional production benefit not included in this analysis is improved consistency and increased operating life of the process solution, which reduces the need for disposal. The nonquantified production benefits (and low capital cost) were sufficient to convince management to adopt this improvement, despite the relatively long period projected for payback.
RECOVERY OF A USEFUL PRODUCT
Many treatment technologies concentrate waste materials into smaller volumes or convert wastes into less mobile forms (such as metal hydroxide precipitation from wastewater). Useful products can be recovered from waste streams. In some cases, that product is a raw material that can be used as feed stock. In other cases, the recovered product is energy, such as when wastes are used to fuel a power-generation facility.
Case Study 5: Recovery of Copper at a Printed-Circuit-Board Facility
Background and Objectives. An aerospace manufacturer operates a circuit-board fabrication facility in New England. CH2M HILL was contracted to design a waste treatment plant for this facility. The principal objective was to minimize the volume of sludge generated by the treatment plant.
Effluent from the treatment plant discharges to a publicly owned treatment works. The effluent is required to comply with federal and local pretreatment discharge limits. Wastes treated in the facility consist mainly of continuous overflow rinses from scrubbing, cleaning, electroless plating of copper, electroplating of copper, photoresist processes, and etching. These wastes contain a mixture of regulated metals (principally copper and lead) and complexing agents from electroless copper plating and etching processes.
The current maximum flow is approximately 90 gal/min. The treatment facility is designed for 50 gal/min with a maximum hydraulic capacity of 90 gal/min. The presence of complexing agents renders conventional hydroxide precipitation treatment ineffective. One effective technique for treating this type of waste has been the addition of large doses of ferrous iron, which displaces copper and lead from their complexes, allowing their precipitation as metal hydroxides. This process, however, produces voluminous sludge because of the precipitation of large quantities of ferrous hydroxide.
Process Description. To minimize hazardous waste, CH2M HILL recommended treating the combined rinsewater waste by chelating IX, and recovering elemental copper from the spent regenerant solution using electrowinning.
Rinsewaters are pumped directly to a pH adjustment tank. Concentrated acidic and caustic wastes are pumped to holding tanks to be metered into the mixing tank to prevent slug loading of acids, alkalis, or metals to the treatment system. In the pH adjustment tank, the waste is adjusted to a pH between 4 to 5 using sulfuric acid or caustic soda (sodium hydroxide), because this pH range is optimal for selective IX removal for copper.
The waste is then pumped through duplex cartridge filters. It is estimated that weekly replacement of filters will be required. The effluent from the filter passes through two activated carbon vessels in series. Activated carbon adsorption removes organic compounds that could foul the IX resins, reducing the resins capacity and resulting in more frequent replacement. Two carbon adsorption columns are provided in series, thus allowing complete use of the carbon in the first vessel, with the second providing polishing. Following complete breakthrough of organic compounds from the first carbon column, that carbon should be replaced and the roles of the two vessels reversed. It is estimated that annual replacement of carbon will be required. The carbon vessels will be backwashable, with the backwash water to be returned to the initial pH adjustment tank.
The wastewater then flows through dual IX columns, operated in series, using a chelating cationic IX resin. Following exhaustion, the lead IX column resin is regenerated with sulfuric acid, rinsed with city water, and returned to service as the lag, or polishing, unit. The rinsewaters are returned to the rapid-mix tank for treatment. The waste regenerant is piped to a holding tank for copper recovery by electro-winning.
The effluent from the IX units discharges to the final rapid-mix tank for pH adjustment with sulfuric acid or caustic soda before discharge to the city sewer.
The spent IX regenerant, electroless copper plating bath growth, and sodium persulfate etching solutions are stored in separate holding tanks. These solutions are pumped to a low-surface-area (LSA) parallel-slate, electrowinning unit for recovery of copper by electroplating onto flat stainless steel plates. The electrowinning unit is batch operated. After a batch is treated, the effluent is pumped to the acid or alkaline holding tank and bled into the influent pH adjustment tank for additional treatment. This effluent is expected to contain 1 or 2 grams of copper per liter.
A high-surface-area, high-mass-transfer (HMT) electrowinning system is being considered for use instead of an LSA system. The advantage of an HMT system is its reported ability to reduce effluent copper concentrations to a few milligrams per liter. This equipment would reduce the load of copper on the IX system resulting in less frequent regeneration. Disadvantages of the HMT are increased power consumption and lower rates of metal recovery (resulting in longer electrowinning times).
Another alternative to HMT electrowinning is an atmospheric evaporator, which would increase the concentration of metal in the IX regenerant prior to LSA electro-winning. Benefits include volume reduction, improved electrowinning efficiency from increased metal concentration, resulting in reduced frequency of IX regeneration, and potential reuse of IX regeneration acid following dilution to the original acid concentration. This reduction in volume benefits operation of the system in that fewer batches are required to be electrowinned. The required electrowinning cathode area and the time for electrowinning are unchanged because they are dependent on the mass of metal to be removed rather than hydraulic volume throughput. Disadvantages of evaporation include the requirement for heating an acidic solution, safety problems associated with handling a heated concentrated acid, and additional power and maintenance requirements.
Cost Estimates. Two systems were considered for installation at this facility: LSA electrowinning with an atmospheric evaporator and HMT electrowinning. Order-of-magnitude cost estimates for installing the two systems are provided in Table 13. Operating costs are listed in Table 14.
Thermal Treatment for Resource or Energy Recovery. Thermal processes can be used for recovering minerals from waste streams and for recovering energy in the form of steam or electricity.
Chlorinated organic compounds, when burned, will generate HCl gas and a small amount of chlorine gas. The HCl can be absorbed in a multistage absorption tower to manufacture hydrochloric acid at strengths varying from 6 to 24% HCl. These acid streams have been used for pickling acid in steel manufacture.
Waste containing more than about 20% sulfur can be burned. The sulfur dioxide can then be catalytically oxidized to sulfur trioxide and absorbed to manufacture sulfuric acid. An alternative process can be used to reduce the sulfur dioxide to make elemental sulfur.
The previous examples are of well-established industrial processes that have been modified to recover commercial products from wastes. The use of these methods has been limited, thus far, to situations where the waste streams are generated in a chemical production plant, and the furnace has been added to the production process for waste treatment or air pollution control.
Recently, work has been done on the feasibility of treating ash produced by solids incinerators to recover iron from the ash. These methods have not yet proved fruitful because of the low price of iron. However, reclaiming noble metals from the manufacture of printed circuit boards and electronic parts is being actively considered as an alternative to disposal. As with all recycling/reclamation projects, the attractiveness depends on the value of the reclaimed product and the cost of reclamation.