3EPA P2 Concepts & Practices For Metal Plating & Finishing A Pollution Prevention Training Course for the “Common Sense” lnitiativeMetal Plating and Finishing SectorUnder the Environmental Technology lnitiative Developed through a partnership between the Office of Research and Development of the U.S. Environmental Protection Agency & The American Electroplaters and Surface Finishers Society
Wastewater Treatment Factors Important to Pollution Prevention
Noth The U.S. Environmental Protection Agency, through its Office of Research and Development, partially funded the preparation of these training materials under Assistance Agreement No. CT902900 to the American Electroplatersand Surface Finishers Society (AESF). It has not been subjected to the Agency’s peer and administrative review, and therefore does not necessarily reflect the views of the agency. No official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendationfor use.
Wastewater Treatment Factors Important to Pollution Prevention
By: Frank Altmayer, President, Scientific Control Labs., Inc. Technical Director, AESF
Slide 1 This lecture is part of a training course in Pollution Prevention for Metal Finishers, produced by AESF, under a grant from USEPA’s Environment Technology Initiative. AESF is very grateful to Ms. Teresa Harten and Mr. Brian Westfall of the Office of Research & Development, Risk Reduction Engineering Laboratory, Cincinnati, Ohio, for their assistance and support of this project. AESF also wishes to thank the more than 100 AESF volunteers, who devoted many long hours to peer review these materials and guide the authors. Acknowledgement is also given to Concurrent Technologies, Inc., CAMP, and NAMF for their support of this endeavor.
This lecture was authored by Frank Altmayer, President, Scientific Control Labs. Inc., Chicago, IL 60623 and Technical Director, AESF.
Conventional Wastewater Pretreatment System for Metal Finishing Cyanide Wastewater
AcidlAlkali Wastewater Filter Cake to 4 Disposal
Slide 2 Waste treatment system schematic Once a metal finisher has reviewed the needs of the facility, has reduced rinsewater flow rates, has reduced drag-out, and has recovered and recycled as much as is possible physically and economically, what is left over must be chemically treated to yield an effluent that complies with Federal and local regulations. This normally means the employment of a wastewater treatment system. Wastewater treatment systems vary to a significant extent from one plant to the next, depending upon management philosophy, financial resources available, the discharge limitations imposed by local authorities and EPA, available floor space, and the skill level of the personnel available to operate the equipment. Shown is a schematic of a conventional treatment system utilizing typical unit operations. Each unit operation, along with a few additional factors is critical to successful treatment. This lesson will discuss these critical operations and factors.
Unit Operations Used in Wastewater Treatment: 0
0 0 0
Cyanide Oxidation Chromium Reduction Special Treatments Initial pH Adjust Coagulation/Flocculation Clarification or Microfiltration Polishing Filtration Ion Exchange Sludge Concentration Final pH Adjust Slide 3
Unit Operations Wastewater treatment normally involves a number of unit operations, each designed to perform one or more tasks. The combination of these ultimately creates an effluent that can be legally discharged or water of suitable quality for re-use within the facility and a solid waste that must be legally treated and disposed or recycled off-site. The key to successfully operating a wastewater pretreatment system is to control each one of the unit operations within the system to the optimum control points. Failure to control all of the unit operations within specifications usually leads to a malfunction and can often lead to a violation of discharge standards. The wastewater treatment operator must make frequent visits to each unit operation and verify that the equipment is calibrated, clean, and functioning within the control settings. Many violations are caused by simple carelessness, such as allowing a reagent drum to go empty.
Segregation of Wastewater Streams
Cyanide Bearing Hexavalent Chromium Bearing Chelated Wastes Acid/Alkaline Wastes Concentrated Solutions/Dumps "Clean" Slide 4
Segregation While there are cases where compliance was achieved by simply routing "everything" into a tank and then treating it after it was filled, such cases are the exception, not the rule. It is highly desirable to segregate streams and treat them separately to avoid chemical unions that work counter to your desires, to improve the safety of waste treatment workers, and to make waste treatment more efficient. Each plant is a potentially unique case, so we can only give generally useful information. Typically, the rinsewaters are separated into the following streams: A. Cyanide Bearing B. Hexavalent Chromium Bearing C. Chelated D. Acidic/Alkaline E. Concentrated Wastes F. "Clean" rinsewaters The above is self explanatory, except for the terms concentrated wastes and "clean rinsewaters". Concentrated wastes include spent cleaners, acids, chromates, stripping solutions, and plating solutions that have been hopelessly contaminated. Clean rinses are rinses that routinely contain regulated pollutants at concentrations below regulated amounts. They usuaily are few in number, but there are plants generating them. An example might be the rinse after a soak cleaner for aluminum. Such a rinse will be close to neutral in pH and will contain aluminum, a non regulated light metal.
Calculating Retention Time:
Time = Tank Volume Flow Rate
= 10 Minutes
T = 50 gallons 5 gal/minute
T = 500 gallons 5 gal/minute
= 100 Minutes Slide 5 Retention Time Chemical reactions used in wastewater treatment do not happen “instantaneously”, especially when the reaction is between a liquid and a gas or a liquid and a solid. Each chemical reaction has a minimum amount of time it takes for the reaction to go to completion. Sometimes, you can get into trouble by providing too much reaction time! Retention time is the average residence time of a molecule of water within the reaction tank before it leaves the tank. The larger the tank, the longer it will take for a molecule of water to find the exit, especially if there is good mixing in the tank. Mathematically, you can obtain the retention time for a given reaction tank by dividing the tank volume by the flow rate: Time (minutes) = Volume Flow Rate Minutes = 500 aallons 25 gallons per minute Minutes = 20 Each wastewater treatment reaction will have a recommended retention time. This is usually a “guess” on the part of the designer based on experience. “Fine tuning’’ is most often required.
Poor mixing results in incomplete reactions
Mixing Methods: Prope1ler Recirculating pump Eductors Air
Slide 6 *
.Tank Agitation Proper mixing of reagents for wastewater treatment is crucial to success. There have been cases where a technician troubleshooting a malfunctioning system, discovered that a mixer in one of the treatment tanks was "turning", but the propeller had corroded off the end of the shaft. Excellent agitation of reaction tanks yields control meters that function better and reactions that go closer to completion (you get closer to the theoretical "residence time" of the reaction tank). To visualize the importance of excellent mixing, imagine a tank of water that is dyed green. If we introduce undyed water into the top of the tank in one corner and allow it to flow out the top at the other corner, you can imagine how long it would take for the entire tank contents to be free of dye. Theoretically it would take an infinite amount of time. Also, you can readily imagine that the water at the top of the tank would be relatively clear after some time, while the water in the corners and at the bottom would be dark green. If we repeated the experiment with a propeller mixer running, you would always have a homogenous mix of green and clear water. The corners, top, and bottom would be the same color. It would still take an infinite amount of time to get rid of all the green dye, but all of the dye and water would be at the same concentration all the time, in all parts of the tank.
This is what we want in our reaction tanks, the same thing happening all the time. By mixing, we cause the reactants to enter into a chemical reaction at essentially the same rate in all places inside the reaction tank. We then can use theoretical residence times with appropriate safety factors to yield a reaction time that will be as near to completion as is practical with the floor space and money available. When mixing fails, un-reacted wastewater can leave the tank and result in violations and the reaction will not be close to 100% completed in the residence time allotted. Mechanical methods for agitating a reaction tank include propeller ("prop") mixing, eductors, recirculating pumps, and air agitation. Of these, "prop" mixing is by far the method of choice because the others typically are inefficient for mixing large tank volumes and or release gases and mists.
Level of Mixing Depends Upon: Propeller size Numbedpitch of blades
1 s/, I
RPM/Horsepower of motor Shapeoftank Liquid viscosity
Slide 7 The level of mixing achieved with a prop mixer is related to: Propeller (Impeller) Size Number and Pitch of Propeller Blades RPM of the Motor Shape of the Reaction Tank Viscosity of the Liquid The larger the propeller, the more violent the mixing (and the larger horsepower required unless gearing is employed). The faster the motor tarns, the higher the mixing rate. The shape of the reaction tank impacts the efficiency of mixing by influencing the movement of the pushed water. A round tank, for example, has a tendency to create a vortex from the propeller (looks like a liquid tornado). Vortexes may look pretty, but they do not dissipate the mixing energy efficiently throughout the liquid, instead concentrating it locally at the vortex. Reaction tanks displaying vortexes, should either be modified (install baffles for example), or the mixing propeller (use two opposing pitch propellers) or the orientation of the mixer with the walls of the tank should be modified (increase the angle between the shaft and tank walls).
The shaft of the mixer typically goes about 2/3 of the way into the depth of the water. The shaft should be stainless steel or, in some cases, the shaft and prop should be plastisol coated to avoid corrosion of the proplshaft and to avoid stray currents being introduced into the water. Prop mixers are driven either directly by motors or through a set of gears that reduce prop speed but increase torque. Gear driven mixers yield high flow/low shear and are preferred for mixing liquids of higher viscosity. Direct drive mixers provide a high level of fluid motion and shear and are excellent for rapid mixing and dissolving solids into liquids.
The Role of the Equalization Tank
I 1 - b
High Flow Out
Low Flow EQUALIZATION TANK
High Flow 1111, l Low Flow
~ Moderate ~ Flow Out
Slide 8 Need for Flow Equalization Wastewater treatment systems operate much more effectively if the incoming waste stream is controlled in flow and composition. Chemical feed pumps, transfer pumps, and control meters require predictable flow rates and incoming pollutant concentrations. "Spikes" in flow or concentration of pollutant can cause upsets, malfunctions, and violations. A constant flow rate is obtained by routing the incoming flow to a holding pit or tank and transferring a fixed amount of wastewater to the unit operation. The variation in flow will then occur in the holding tank instead of the reaction tank. Obviously, one equalization tank is needed for each primary treatment unit, such as chromium reduction, cyanide oxidation, initial pH adjust, etc.
Chemical loading will also have a significant impact on the efficiency of the system. If the incoming flow contains concentration spikes, the chemical feed pumps will be unable to deliver enough reactant to react with the spikes. The partially treated waste can then cause problems elsewhere. Chemical overloading is most often caused by dumps of concentrated waste solutions to the waste treatment system. Provisions must be made to hold these dumps and slowly "bleed" them into the treatment system. The size of the equalization tanks depends on the variability of the flow and composition. The higher these are, the larger the equalization tanks should be. 11
Silver Silver Chloride Reference Electrode Reference Liquid Junction Glass Membrane
Slide 9 Control Instrumentation Proper control of pH and ORP is essential to successful wastewater treatment. Chemical reactions occur most effectively at a specific pH or within a given pH rar?ge and/or a specific ORP reading or range. Failure to adequately calibrate these controllers or control the pH of the reaction within specified limits, can result in serious violations and continuous operational problems. Each pH measurement probe utilizes a reference electrode to measure milivoltages against. The reference electrode is sometimes built into the sensing electrode. These are called combination electrodes. They are more expensive, but tend to be less troublesome. Glass electrodes work well in the presence of oxidizers and reducing agents, but may be affected at high pH values if the wastewater contains high concentrations of sodium. Electrodes designed for operation in high concentrations of sodium should be used in wastewater treatment, since sodium based compounds are commonly employed.
Problems with Probes pH: Temperature Finite Glass Life Locate Probes in Agitated Area Stray Currents Surface Contamination High Sodium Creates Errors ORP: Finite Life Fouling of Metal Disk
Slide 10 Problems with Probes pH values vary with temperature. Fortunately, most modern pH controller systems utilize probes that incorporate temperature sensors and compensation circuitry. Some pH probes may therefore have three sensing devices in the probe that goes into the wastewater: a reference probe, a pH measuring probe, and a temperature probe. Any failure in one of these, will cause the system to be inaccurate or can cause a major malfunction. Because the temperature probe sometimes looks like an unimportant metal rod, the operator may not clean it. Oil covered temperature probes, will yield erroneous temperature compensations. To verify proper operation of a temperature compensator probe, the entire probe can be immersed in tap water that has been allowed it to reach room temperature overnight and compare the pH of this water to the cold water flowing from the tap. The pH values should be identical. For systems that require the operator to adjust for temperature, the operator should monitor the temperature of the incoming wastewater and adjust the temperature compensation knob accordingly.
Contamination of the reference electrode solution will result in measurement errors as will residues deposited onto the electrodes. pH and ORP probes have a finite service life, even when properly cared for. A typical life is 6 months to 1 year. A complete set of spare elements (sensor, reference, temperature compensator) should be readily available and any spare should be replaced as soon as it is employed. pH controlling probes must be located in an area of the treatment tank where there is good mixing, but not close to the entry of the neutralizing reagents. You need to allow some time for the acid-base reaction to proceed. You also want to place the probe below the surface of the wastewater. The depth should be adjusted to a point at which the controller is most stable in its reading. Since probes sense millivoltages, they are very sensitive to stray currents that may be generated by mixers and feed pumps. If the meter reading is going crazy, chances are a stray current is at fault. The source of the current must be eliminated.
pH Meter Controllers: On-Off Proportional
Two set point control Four set point control Chart recorders recommended Calibration: Use two buffers spanning range of control
Operation of the pH Meter/Controller pH meters that simply measure the pH value are of little use in wastewater treatment. What we really want is a controller that sends out a signal which turns on a feed pump and then adds acid when the pH is too high and a different feed pump that adds an alkali when the pH is too low. Such controllers employ relay circuitry that perform just the tasks described. The controller has a high and low set point that corresponds to which level of pH the feed pumps turn on. These set points must allow for some "slippage" in the system. In other words, if you want to add alkali when the pH drops below a value of 5.0, then you probably need to set your low to a value around 5.5 or 6.0. By the time the feed pump kicks on at pH 5.5, more acid has entered the neutralization tank and has probably lowered the pH more. How much "lead" to give the system depends upon the concentration of the acid/alkali being used and the rate at which they are pumped. You also do not want to feed the neutralizing chemicals at such a fast rate that you swing the pH beyond the other set point. This requires some trial and error. Controllers are typically also set up to sound an alarm when the set [email protected]
) are exceeded. The alarm can be a belVhorn, a light that turns on, or both. More complicated controllers with more set points that feed different strengths of acid or alkali, depending on which set point is exceeded, are available.
Controllers that utilize proportional control are also available. These controllers send out a signal that varies in strength, depending on the level of variation from the set point. The stronger the signal, the more neutralizing agent is fed by a variable speed feed pump. Note: Chemical additions can be made by feed pumps or by gravity feed from storage tanks equipped with metering valves. In either case the feeding device is turned on and off by the pH meter-controller.
Calibration of pH Meters pH meters are calibrated by immersing the electrode into special solutions that are resistant to pH change, called buffer solutions. These solutions are normally made up from a weak acid and its conjugate sa!t (example: phthalic acid and sodium phthalate), but are most easily purchased in ready to use form or in concentrates. Buffer solutions are best when freshly made or diluted from concentrates. They should be discarded daily. Use two buffer solutions bracketing the range of pH values that need to measured/controlled. With the probe in the lower of the two buffers, adjust the calibrate or standard knob on the meter to the value of the buffer (allow a few minutes for the probe to stabilize). Remove the probe, rinse, and immerse in the sacond buffer. A well working meter should then read within 0.1 pH units of the second buffer without adjustment. Some meters have a "span" adjustment that compensates for some offset with the second buffer.
ORP Meter Controllers
Calibration: To verify proper meter function: .5g quinhydrone/20ml acetone + 200 ml pH 4 buffer = 220-250 mV Calibration (practical): CN-Oxidation: KI starch papers Chromium Reduction: Go by color and jar test Slide 12 ;
O'RP Meter Controllers Oxidation-Reduction control meters are used to measure and control the feed rate of either oxidizing agents or reducing agents, depending on which reaction we wish to carry out. Typical unit operations for each are oxidation for cyanide destruction and reduction for converting hexavalent chromium to trivalent. The ORP measuring probe has a metal band or disc at the tip that is made of platinum, gold, or a mercury amalgam (Mercury electrodes are outdated and pollution hazards). This metal band is the working end of the electrode. The larger the band or disc, the more sensitive the probe is. The ORP probe is also used with a reference electrode and potentiometer circuit to measure the potential of oxidation or reduction. For example, in the oxidation of cyanide, the ORP probe measures the millivolts generated by the battery formed by the two half cell reactions:
+ 2e- + 2CI-
The ORP millivolt measuring equipment is essentially identical to that for pH monitoring, except for the probe. ORP meter-controllers that utilize on-off control and proportional control (similar to pH controllers) are also available. Note, however, that ORP meters are not specific to any ion. They only measure the potential generated from combinations of half cell reactions. Therefore, other oxidizers or oxidizable ions present in the wastewater will affect the ORP potential generated. For this reason, each wastewater stream normally has it's own ORP "set point". Reliance on published ORP settings for specific wastes usually results in trouble.
Calibration: ORP controllers must be regularly calibrated in one of two major methods: the practical and the theoretical. 1. Practical Method: Immerse a cleaned ORP probe into a bucket containing raw wastewater and manually treat that bucket until you are certain that excess oxidant or reducing agent has been added (depending on which you are trying to accomplish). You can verify excess treatment as follows: a. For Cyanide Oxidation Use KI-Starch test papers to test and verify the point at which an excess of chlorine is in the bucket. Whatever reading the meter has at this point becomes your set point. b. For Chromium Reduction Add reducing agent until you believe you have added an excess (aquamarine blue color). Remove a small jar-full of this treated waste and neutralize to pH 9 (you can use sodium bicarbonate because an excess will never go above pH 9, or you can use any other alkali and a pH meter). Allow the pH adjusted waste to settle for around ten minutes and then look for any yellow coloration in the supernatant water. If yellow coloration is present, you have not reached an excess of reducing agent. If no yellow color is present, you either have added the right amount, or you have added too much. You can repeat the test, adding less reducing agent if you think you added too much. When the test results are acceptable, the ORP reading of the treated waste becomes your set point. You can now move the probe into the raw waste and the meter should turn on the feed pump. If the probe is moved to the bucket of treated waste, the feed pump should shut off.
2. Theoretical Calibration Method: ORP measuring systems may be checked by measuring a solution having a known ORP reading. One such solution is made by dissolving .5 grams of quinhydrone powder in 20 ml of acetone. This is then added to 200 ml of pH 4.0 buffer solution. The resulting mix has a theoretical ORP value of 220-250 mV. Other ORP standards can be made by mixing the same quinhydrone -acetone mix with other pH buffers: pH Buffer ORP Value 4.0 220 to 260 7.0 80 to 90 9.0 -26 to -39 For cyanide oxidation and chromium reduction, the set points will be in the "positive" millivolt range. If they are not, chances are the probe has been installed with reverse connections.
Inspect ORP probes for deposits/crusts on the metal tip. If present, the probe needs cleaning. ORP electrodes are cleaned by wiping with a clean, soft, wet cloth. Oily deposits can be removed by adding a dilute dish-washing liquid to the water that the cloth is soaked in. If metal hydroxide crust is on the probe, immerse in dilute acid
Types of Pumps Used in Wastewater Treatment
Correct pH Range for Pump Materials
Specific Gravity >l=Motor Overload
Slide 13 Pump Types Materials used for contact with pumped liquid and seals and gaskets must be considered. Most plating solutions and many chemicals are corrosive, and pump materials must resist chemical attack. Corrosion-resistant materials include chlorinated polyvinyl chloride (CPVC) which withstands temperatures to 2000F (somewhat higher at low pressures), Polypropylene (not as versatile as CPVC), Teflon, and Kynar (for highly corrosive solutions). Corrosion-resistant exotic alloys are also available. The centrifugal pump is used mostly for transfer purposes. Centrifugal pumps have an advantage of being inexpensive. The discharge can readily be controlled by valving. Centrifugal pumps transferring corrosive liquids often have the motor and drive shaft isolated from the impeller by the installation of magnetic couplings. The pump motor horsepower needs to be increased in direct proportion to the specific gravity of the pumped liquid for a direct-drive pump. For a magnetic-coupled pump, trim the impeller to pump higher specific gravity liquids. In any match-up of pump and motor, make sure the piping is adequate. A too-narrow pipe can starve the pump and ruin efficiency. Positive displacement pumps are used for chemical metering to supply pressure to filter presses and ,in some cases, for the transfer of liquids that may be chemically corrosive, viscous, or contain abrasive solids. Positive displacement pumps are more expensive and maintenance oriented. The discharge rate from a positive displacement pump is constant, so it must be controlled either at the motor or a bypass must be installed on the discharge from the pump.
3 Cyanide Oxidation Methods: Alkaline Chlorination lnsitu Chlorination Ozonation Electrolytic
Slide 14 Treatment of cyanide-bearing wastes The method of choice (by far) for cyanide destruction is alkaline chlorination. The EPA chose this method of destruction as "BDAT" (best demonstrable, achievable technology), based on the vast amount of experience and data available on the method. For this reason, we will spend the major portion of this lecture on chlorination. Ozonation and electrolytic methods have been used successfully in various applications, but do not have the broad-based acceptance in industry. Included in the category of "others" are methods of destruction that have limited applicability or success, such as peroxide, activated carbon, bio-destruction, radiation, and acidification (dangerous!). Since the overwhelming majority of cyanide destruct systems used in the U.S. utilize alkaline chlorination, this lecture will cover only alkaline chlorination.
A IkaIine ChIor ination 1st Staae Reaction, (pH = 10-11): CI, + NaCN + 2NaOH + NaCNO + 2NaCI + H,O 2.7 Ib. CI, per Ib. CN required, 3-4 Ib. if Cu or Ni is present Reaction Time = Approx. 20 minutes
2nd State Reaction b H = 8-91: 3CI, + 4NaOH + 2NaCNO + 2C0, + N, + 6NaCI + 2H,O 7.5 Ib. CI, required per Ib. CN for two stages, 8-8.5 Ib. CI, required if Cu, Ni present Reaction Time = Approx. 60 minutes
Slide 15 AI kaline chlorination of cyanide-bearing wastes utilizes a chemical class of reactions called "oxidation". The goal of the oxidation reaction is to ultimately convart the cyanide into two non-toxic gases: carbon dioxide and nitrogen. There is some confusion as to the exact reactions involved in alkaline chlorination. The following is a general description of the process.
Chlorine (from compressed gas cylinders, from chlorine gas dissolved in sodium hydroxide...sodium hypochlorite, or from a solid called calcium hypochlorite) reacts with cyanide to form cyanate (CNO), which is generally non-regulated. There is some suspicion, however, that the reaction to cyanate is reversible to an extent that makes meeting low discharge limits very difficult. The second reaction converts cyanate to carbon dioxide and nitrogen gases and is, therefore, irreversible (because gases are free to leave the reaction tank). Reaction I is carried to completion in about 20 minutes, while reaction 2 takes about 60 minutes.
The cyanide in plating rinses or in diluted cyanide solutions reacts rapidly in the first stage, according to the equations shown, but cyanide complexed with copper, nickel, and precious metals reacts more slowly and requires an excess for complete destruction. The cyanide in very stable complexes, such as iron, nickel, and cobalt, is highly resistant to reaction with chlorine under normal conditions. The second stage chlorinates the waste from the first at a pH of 8-9. This must be done very carefully, by adding dilute acid (usually sulfuric) in very small increments into the most turbulent part of the mixing zone of the mixer. If too much acid is added, the acid is added too rapidly, or if mixing is not adequate, toxic gases can be released. For this reason, ventilation of the oxidation tanks is highly recommended. The second reaction step proceeds very slowly. In fact, it is so slow that complete destruction of cyanide by high pH chlorination may not be possible in a flowthrough system. By lowering the pH below 10, the ionization of cyanides increases and the activity of the chlorine accelerates, resulting in more efficient cyanide and cyanate destruction. Above pH 10, the oxidation tends to stop at the conversion of cyanide to cyanate and usually leaves some undestroyed cyanide behind. Cyanide solutions/wastes containing concentrated amounts of non-complexed (free) cyanide should not be chlorinated, because the reaction can be quite violent with the emission of toxic gases. In general, wastewater treated by alkaline chlorination should contain less than 500 ppm of uncomplexed cyanide. Note that ORP meters do not "sense" whether cyanide is present or absent. They sense the presence or absence of oxidizable ions. These ions could be cyanide, sulfite, nitrite, monovalent copper, and numerous others. The ORP meter is functioning properly when it turns off the hypochlorite pump to the first stage reaction tank when there is an excess of 50 ppm chlorine, and when the excess has been reduced to 10 ppm or less. The millivolt readings these excess chlorine readings correspond to are the "setpoints".
Use of Potassium Iodide-Starch Test Papers
Note Excess Chlorine Can Cause Clarifier Problems! Slide 16 Use of potassium iodide-starch test papers Chlorine levels can be monitored by use of a swimming pool test kit or potassiumiodide-starch test papers. To use these papers, dip one strip into the wastewater for 5 seconds and observe the color obtained: Faint blue: Slight excess of chlorine (2-5 ppm) Sky blue: Normal amount of excess chlorine (5-10 ppm) Dark blue: Above normal amount excess chlorine (20-50 ppm) White with blue stripe: High excess (loo+ ppm) Note: If the paper stays completely white, the most likely case is that you do not have enough chlorine to adequately destroy the cyanide and you must adjust your system accordingly. Because a very strong excess of chlorine (10,000 ppm) can also turn the paper white (through bleaching action), however, you should first dilute the wastewater 1O:l then re-test. If the re-test stays white, you most likely need to increase the chlorine addition rate. If the diluted wastwater tests blue, you are adding too much chlorine and need to make a major cut-back. The second stage of the cyanide-destruct system should have only a slight excess of chlorine (l-5ppm). Too much chlorine can cause floatation of solids in the clarifier. In many plants, a small amount of ferrous sulfate is fed into the discharge compartment of the second stage tank to eliminate excess chlorine and reduce cupric copper (which doesn't settle well in the clarifier) to cuprous copper. The ferrous sulfate also tends to solidify dissolved complexed cyanide and creates more efficient settling by acting as a flocculating agent.
Automated Destruct Systems Are Not Truly "Automatic" You Should Check: Mixers Feed Pumps Reagent Levels Probes Every 8 Hours Conduct Jar Tests Daily
Slide 17 Flow-through chlorination system tanks are also equipped with pH meters, mixers, and chemical feed pumps so that everything is "automatic". Many wastewater treatment operators assume that everything is controlled by meters so that they won't have to monitor the system very often. This is usually a big mistake. All system components, including pumps, mixers, meters, and chemical supply, should be checked twice per shift (more often in some cases). The pH in the second stage is critical to the success of the process. Frequent cross checks with hand-held pH meters or pH papers should be made. Meters without automatic temperature compensation should be adjusted to reflect the temperature of the wastewater (colder in winter than in summer).
Factors in Treatment of Chromium Important to P2 pH of Reaction Reaction Time Choice of Reducing Agent Choice of Neutralizing Agent Level of Mixing Clarification pH Presence/Absence of Strong Oxidizers Following Treatment Slide 18 The treatment of chromium-bearing wastes Hexavalent chromium can not be removed from rinsewater no matter what pH adjustment is used. We need to first convert the hexavalent chromium to the trivalent valence because trivalent chromium forms insoluble chromous hydroxide at neutral pH. Important to successful treatment are: The pH of the reduction reaction must be sufficiently acidic to completely convert the chromium to the trivalent state. A pH value above 3, tends to yield poor results. pH values below 2.0 are preferred. While the reaction between most reducing agents and hexavalent chromium tends to be very fast, it is generally accepted that a minimum of 10 minutes is required to assure completion. While all reducing agents can successfully perform the reaction, some create more sludge upon neutralization, than others. The choice of neutralizing agent can impact the volume of sludge produced, as some of these create more sludge than others. The pH level, at which the trivalent chromium is clarified, has a direct impact on the solubility. If strong oxidizers are allowed to come into contact with the treated waste, some of the trivalent chromium may be re-oxidized to hexavalent.
Reducina Aaents: Ferrous Sulfate [FeSOJ 6FeS0, + 2H2Cr0, + 6H2S04-+ Cr (so& +3Fe2(so& + 8H20 16 Ib. FeS0,07H20, 6 Ib. H2S04per Ib. Cr+6 9.5 Ib. Ca(OH,) or 7.2 Ib. NaOH per Ib. Cr+6to neutralize 25.5 Ib. Sludge Produced
Sulfur Dioxide Gas ( SO2) SO2 + H20+ H2S03 Cr,(SO,), + 5H20 3H2S03 + 2H,CrO, 1.85 Ib. SO, per Ib. Cr+6 2.5 Ib. Ca(OH), or 2.0 Ib. NaOH per Ib. Cr+6to neutralize 6 Ib. Sludge Produced
Reducing agents- ferrous sulfate, sulfur dioxide gas Ferrous sulfate is a pale green crystalline material that readily dissolves in water without producing a large amount of noxious fumes or odors. It is the least expensive reducing agent, based on purchase price, but becomes significantly more expensive when factors such as amount required per pound of chromium reduced and amount of sludge produced per pound of chromium are taken into account. It takes 16 Ib. of ferrous sulfate (FeS04 -7H20) and 61b. of sulfuric acid to convert one pound of hexavalent chromium to the trivalent state. Once reduced, it typically takes 9.5 Ib of lime to neutralize the acidic waste and convert the chromous sulfate and ferric sulfate to the respective hydroxides. One advantage in using this reducing agent, is that the iron can act as a coagulant to assist clarification in wastewaters that are low in iron content. Ferrous sulfate reduces chromium in acidic (pH less than 2) conditions, so acid must be added before or during reduction. Sulfur dioxide gas is the least expensive reducing agent that can be used. It takes about 2 Ib of this gas per pound of chromium, and produces 6 Ib of sludge in the process. The gas also does not require pre-acidification of the waste stream since it makes its own acid when added to water. The major disadvantage is the safety hazard the material poses in the use and storage of compressed gas cylinders. If sulfur dioxide is used at a pH far below 2.0, it tends to give off high concentrations of noxious fumes.
Chemistry of Sodium BisuIfite/Metabisulfite Reduction of Chromium Na2S20,+ H20% 2NaHS0, 3NaHS0,
+ 2H2Cr04+ 3H2SO4 + Cr2(S04)+ 3NaHS0, + 5H,O 2.8 Ib. Na2S20,per Ib. Cr+6 1.5 Ib. H2S04per Ib. Cr+6
2.5 Ib. Ca(OH), per Ib. Cr4 6 Ib. Sludge Produced
Slide 20 Sodium bisulfite/metabisulfite These reducing agents are white crystalline powders and are There may be price differentials, in which case the lower cost material can be used, because it takes about 3 Ib of either one to reduce one Ib. of hexavalent chromium under acidified conditions and either one produces 6 Ib of sludge per pound of chromium. A disadvantage to the use of these materials is the noxious sulfur dioxide fumes
they form when they are mixed with water. This can be avoided, however, by purchasing the reducing agent in liquid (dissolved in water) form.
Precipitation of Heavy Metals 100
PPm Dissolved Metal
6 7 8 9 10 11 12
Precipitation of metal salts in wastewater Conversion of dissolved metals to undissolved metal hydroxides is commonly called "neutralization." The term means that the pH of acid solutions is raised toward neutrality, or the pH of alkaline solutions is lowered to the same condition. Metal salts from simple inorganic compounds will tend to become insoluble in the neutral pH range, but not all metals will precipitate on neutralization, and not all metals will precipitate at the same pH point and to the same extent. In view of the low discharge limits, with regard to soluble metal content of a metal-finishing effluent, the initial problem in separation may be to decide what pH to aim for to reach the most complete precipitation of the metals present in the waste. Some of the metals that may be present are amphoteric and, therefore, are soluble at alkaline pH. Examples of such metals are aluminum and zinc. Some other metals may require a relatively high pH to reach minimum solubility. This would include vickel and copper. With mixed waste streams, the best pH for the most complete separation will almost always be a compromise. Published solubility charts, such as that shown in the slide, should only be used as rough guidance, as they were produced on pure solutions in the laboratory. Each facility must develop its own optimum neutralization pH for maximum metal hydroxide insolubility.
Precipitation of Heavy Metals: pH Adjustment
Retention Times: 10 Minutes (NaOH), H,SC),, HCI 30 Minutes (Ca(OH),) 45 Minutes (Mg (OH),) Slide 22
Neutralizing reagents alkali Several materials can be used to neutralize acidic wastewater containing the reduced chromium, including sodium hydroxide, calcium hydroxide, sodium carbonate, magnesium hydroxide, and combinations of these alkalies. Sodium hydroxide, sodium carbonate, and magnesium hydroxide do not add to the sludge volume by forming additional metal hydroxides, while calcium hydroxide (lime) precipitates as calcium sulfate and un-reacted hydroxide, thereby adding to the sludge volume generated. There is evidence, however, that sludges formed by neutralization with calcium and/or magnesium hydroxides have lower leachability than others, so sometimes blend of alkalies are used. Calcium and magnesium hydroxides also have a disadvantage in that they are not water soluble, so a "slurry" must be fed, creating pump and tubing clogging problems. Magnesium hydroxide has limited alkalinity, so you can not adjust to a pH greater than 6-7 with this material. Both calcium and magnesium hydroxide react slower with acids than sodium hydroxide, so additional retention time is required to react acid with these materials. The chromium is precipitated by adjusting the pH of the wastewater containing the reduced chromium to a value around 8-9.
Neutralizing reagents-Acids Typical acids used for pH adjustment are sulfuric and hydrochloric. Acids react very quickly. 10 minutes of retention time is adequate.
Special Treatments: Sulfides: Sodium or Ferrous Sulfide 0
Insoluble Starch Xanthate Sodium Dithiocarbamates
Proprietary (John Deere)
Treatments for Chelates 0
Addition of CalciumhAagnesium salts under acidified conditions
Slide 23 Formation of alternate metal precipitates Metals can be converted to other compounds that have lower solubility than metal hydroxides. This additional reaction is usually performed after conversion of most of the dissolved metals to hydroxides (for economical reasons). It almost always is more complicated and more expensive. Some of the more common alternate metal precipitate forming compounds are sulfides (sodium sulfide, ferrous sulfide, starch xanthate, and a variety of carbamates), and phosphates. Chelate Breaking Compounds Metals that are heavily chealted, require special treatments to render them insoluble. Chelate treatments include: Addition of calcium or magnesium salts to acidified wastes. Sodium Hydrosulfite treatment Sodium borohydride treatment
Treatment of Concentrates Specific Gravity of Floc = 1.05 Specific Gravity of Water
Agglomeration increases density Gravity Settling: Most Economical
Before, After Treatment Slide 24 Gravity separation of precipitated metal salts Gravity settling is the most economical and simplest way to effect separation of flocculated metal insolubles. This is normally performed by using a clarifier. Efforts must be made to cause sufficient increase in flocculent-particle size and weight to make gravity separation possible. To best separate the solids from the water, the particles must be given enough time to float to the bottom of the clarifier. This sinking will be affected by turbulence in the water flow-through the clarifier (among other things). Clarifiers that are too small will yield too much turbulence and only the largest particles (if any) will sink. The rest will float out the top and cause metal violations. When neutralized, concentrated wastes make a liquid that essentially is totally "gelled" with solids. Such a liquid would take an eternity to create any significant amount of separation between the water and the solids. Solids loading is, therefore, very important to proper operation of any clarifier. They are typically designed to treat only dilute wastewater containing up to a few thousand parts per million of solids. Concentrated solutions must therefore be stored and slowly "bled" into the wastewater treatment system.
Precipitation of Heavy Metals
Water soluble long chain hydrocarbons Branches with negative charge = Anionic Branches with positive charge = Cationic Branches with no charge = Nonionic Most metal hydroxides require Anionic (jar test)
Coagulation/flocculation Some of the metal precipitates form very good hydroxide floc, capable of removing other solids by adhesion. When the waste stream is very dilute, or when some content of the waste has to be removed because of its colloidal nature, or the organic content of large molecules that cannot be separated because they are soluble, coagulants are added to the waste stream to provide additional flocculant precipitate. Most often such salts as ferrous sulfate, ferric chloride, or aluminum sulfate are used in the range of 100-300 mg/L as coagulant additions to the waste stream. Coagulant additions can greatly increase the amount of sludge to be handled as a final solids removal step. The usual metal content of a metal-finishing effluent stream is far less than the quoted quantities of metal that are fed as coagulant to be subsequently precipitated and settled with the metal that is present. As the metal salts precipitate, the anion becomes free, therefore, the pH tends to be lower when coagulants are added. Either the initial pH should be high enough to allow for this depression of the pH, or additional alkali has to be provided. With these coagulant additions, large volumes of total flocculant precipitate are created. If the mixing of the coagulant into the waste stream for subsequent flocculation is not designed carefully, all the benefits that were to be gained thereby can be easily lost.
Flocculation Flocculation is further agglomeration of insoluble particulates in an aqueous medium, caused by the addition of an organic "flocculant", (aka "polymer" or "polyelectrolyte"). Most metal finishing wastewaters form metal hydroxides that respond well to anionic polymers. Jar testing is often necessary, however, to determine if this is truly the case and which commercial anionic polymer works best. These large agglomerates formed by a flocculant look much like snowflakes and cause the solids to settle faster because of a slight increase in density (analogous to taking snowflakes and compacting them slightly). Flocculants are added in a special reaction tank equipped with a low-speed, lowshear agitator (usually under 100 rpm and adjustable down to 25 rpm). The low shear is very important, as mixing at too high a speed can break up larger flocs, eliminating the benefit of the process. Commercial units that automatically dilute concentrated liquid polymer and feed it into the flocculating tank, equipped with the proper mixer, are available. An addition of 5-10 mg/L polyelectrolyte is used on metal-finishing effluents. Excessive amounts of polyelectrolyte can cause solids to float instead of sink. Ideally, flocculants should be added based on solids loading. However, since this is not practical, they are added based on flow, instead. This often leads to over or under flocculation resulting in poor clarifier performance, unless the operator performs frequent jar tests and adjusts the level of addition accordingly. 5-10 minutes should be allowed for flocculation to occur.
Inclined Plate Clarifier
Flocculation Tank v
Drawing: Courtesy of the Parkson Corp., Fort Lauderdale, FL 33340
Slide 26 Clarification For solid particles to settle in a conventional clarifier, they must fall a great distance before they become part of the mass at the bottom, called the sludge blanket. The clarifier must therefore operate at very low flow velocity to not disturb the particle on its way down to the blanket. Once a part of the sludge blanket, the particle becomes physically trapped in the mass and bonded by slight ionic charges as well. Inclined plate clarifiers can handle much higher flow rates than conventional clarifiers taking up the same amount of floor space, because they give the particle an artificial "floor" to fall onto (the plates or tubes), thereby reducing the distance the particle must fall before it becomes a part of the sludge blanket. The angle at which the plates or tubes are placed, the number of tubes or plates, and the distance between the tubes or plates determines the efficiency of the lamella clarifier. The tube diameter or plate spacing is typically on the order of 2 inches. The inclined plate clarifier operates by separating the feed, sludge, and effluent and allowing the sludge particles to slide down a 5 5 O incline and fall through a quiescent zone into a sludge hopper in the bottom. The sludge may be further thickened in the hopper by use of a plate vibrator before being withdrawn by pumping or gravity. The effluent flows out the top through flow-control orifices, which evenly distribute the flow between plates. The clarifier typically is supplied as a self- contained, skid-mounted system with flocculation compartment included.
Design/Maintenance of Inclined Platenube Systems Flow rates typically 0.4-0.5 gpm/ft2 plate area Plates too close or wrong angle causes problems Sludge blanket MUST BE MAINTAINED Annualhwice per year plate cleaning Add windowhaps Add flow meter on exit
Slide 27 Lamella-type clarifier design and maintenance considerations The entire horizontal projected surface area of all the plates in the lamella becomes the effective settling area. The result is a significant reduction in the size of the equipment compared with conventional or tube clarifiers. Lamella clarifiers are typically designed for a flow rate of 0.4-0.5 gal/min per square foot of projected settling area. If the plates are too close together for the flow, there is too much turbulence for the sludge to settle and slide down the plate or tube into the bottom. If the angle of the plates is too low the sludge blanket doesn't slide off well and increases in size to a point where the flow picks it up and lifts it off. If the tube angle is too high, the sludge slides off the plates/tubes too fast, also lifting it up intoathewater flow.
Lamella clarifiers must always have a sludge blanket at a controlled level to function at peak efficiency, because the inflow is directed into this blanket to enhance agglomeration. Lamellas also require continuous removal of the solids collected at the bottom into a "thickening tank" that the filter press is connected to.
The overflow from the thickening tank is routed back to the entry of the clarifier, but should by-pass the floccing tank to avoid artificially increasing the flocculant concentration in the clarifier. There should be a "window" installed in the lamella that will allow the operator to see the level of the sludge inside. If the level gets too low, the sludge does not settle well. If the level gets too high, the sludge tends to float out the top. Some lamellas have a series of "taps" that allow sludge blanket level verification. These taps often become clogged with sludge and stop working well. Lamella clarifiers require periodic cleaning of the plates inside, so provision should be made to allow for this to be performed as easily as possible. Allow enough overhead space to remove the plate packing and provide a tank that the plates can be placed into and cleaned. Having a spare set of packing plates would allow for cleaning of the dirty plates without shutting down the operation for very long. The lamella should be made out of non-corroding materials, if possible, but are usually made of steel to reduce cost. It is a good idea to have a flow meter on the outgoing flow of the clarifier so that the operator can verify he/she is operating within the volume limitations of the hardware.
Conventional Gravitv Clarifier I
Conical Bottom Clarifier
Flat Bottom Clarifier
Slide 20 Gravity clarifiers The depth of a “conventional” gravity clarifier is important because good depth allows a floc blanket (cloud of small flocculant particles) to persist and grow, slowly settling toward the bottom of the clarifier tank, without being affected by the flow conditions near the surface of the clarifier. As the sludge content of the clarifisr increases, the floc blanket, which can be 4-5 feet deep, comes increasingly closer to the surface of the clarifier until the top of the floc blanket may be disturbed by the flowing stream. As soon as this condition occurs, suspended solids are carried out of the clarifier, indicating that sludge should be removed from the bottom of the clarifier to allow sufficient room for the sludge and the floc blanket to remain safely below the effluent stream. Gravity clarifiers are essentially huge tanks that hold the wastewater flow long enough and without turbulence so that the solids have time to settle to the bottom. The principle is the same as that used to settle/clarify sewage at publicly owned treatment works (POTWs). For flows less than 15 gpm, a conical bottom clarifier can be used. These clarifiers eliminate the need for a “drag chain” and take up less floor space. They are subject to “rat-holing”, however, which can affect their efficiency.
Conventional Gravity Clarifier Waste In
Oil Skimmer & Weir
Sludge Collection Trough
A gravity clarifier is sized based on "rise" and retention time. Rise is the speed at which the liquid level in the tank rises. A typical sizing parameter is 1/4" per minute rise. Gravity clarifiers with rises greater than 1/4" will usually need to utilize a flocculant to adequately settle the metal hydroxides from the pH neutralization system. Clarifiers that use flocculating agents are sensitive to the kind and concentration of flocculant used. Retention time is based on the volume of the clarifier. A 12,000 gallon clarifier will have 4 hours of retention time at a flow rate of 50 gpm. Typical design criteria for the sizing of the clarifier would provide for 48 hours retention time for the wastewater and 1 weeks' retention time for wet sludge accumulation. The longer the retention time, the better the performance of the clarifier (assuming proper "rise" is utilized). The clarifier should have provision for oil skimming at the entry and the exit. Any and all valves on clarifiers should be oversized, so that sludge will not readily clog them. If a 2" valve will do the job, use a 3"etc
Sludge Thickening Lamella Sludge Thickening Tank
Photo: Courtesv of the Parkson Coro., Fort Lauderdale, FL 33340
Slide 30 SoIids concentration, Sludge thickening Because the sludge level in the lamella clarifier must be maintained at an optimum level, it must be continuously removed. A sludge thickening tank is utilized to contain the sludge and pre-thicken to 2-4% solids prior to processing it with a filter press. This also functions to improve the efficiency of the filter press. A sludge thickening tank is also a good idea for a wastewater treatment system using a conventional clarifier, because of the improved performance of the filter. Subsequent handling of the sludge accumulated and removed from the clarifier requires the removal of additional quantities of water, because the sludge has only 1-2 percent solids directly from the clarifier. The importance of sludge thickening can be perceived more clearly in terms of the volume of sludge generated in metal finishing. A dry weight content of 1-2 percent means 50-100 times the weight in the water to be moved when disposing of sludges from metal-finishing operations.
Effluent Polishing Systems
Photo: Courtesy of lndustliai Filter & Pump Co., Cicero, IL
Single Media Sand Filter
Photo: Courtesy of the Parkson Cow., Fort Lauderdale, FL 33340
Slide 31 Effluent polishing systems A modern wastewater pretreatment system using a clarifier should include a polishing filter to remove pin-flocs and other floating particulates at suspended solids concentrations as high as 500 ppm. Such filters are typically sand, mixed media, or "floating media" types. The higher the suspended solids the filter can handle, the better it is. A wellworking clarifier typically produces a suspended solid around 10-20 ppm. A "burp" caused by dissolved gases, or improper flocculant control, can shoot this value to 200 ppm or more, easily. Provisions must be made to backwash the filter easily (preferribly automatically) and the media must be easy to replace in the event of a serious clog. The associated tanks, piping sensors, etc. for automatic backwash should be supplied by the filter manufacturer (the [email protected]
) can be supplied by others but should be sized by the filter manufacturer). It is a good idea to install a turbidity monitor on the filter discharge to sound an alarm when the filter is malfunctioning. Sand filters that operate on systems using lime for neutralization are subject to "calcification" if they are not continuously operated. This can create a solid mass of "concrete"formed with the lime residuals and the sand.
Sump In Service Discharge
Critical to Success: Filtration, Choice of Resin(s) Slide 32
Ion exchange for polishing treated wastewater Some metal finishing facilities have added ion exchange systems to remove traces of dissolved metals that the conventional treatment system was unable to remove from the waste stream. These ion exchange systems range from simple strong acid resin cation exchangers to more sophisticated systems employing cation and anion exchange, prior to recycle of the water back to the process line. Key to successful operation of an ion exchange system is filtration ahead of the resin. If even small amounts of solids are allowed into the resin bed, the resulting fouling may cause loss of the resin, which may cost upwards of $300/w for replacement. Use of the correct resin for the job is also key to success. The resin should have a high selectivity for the ions to be removed and a low selectivity for ions that need not be removed, for the desired result. There are a variety or resins available, and testing should be performed to find the resin that most economically achieves the desired goal.
Summary Waste Treatment Facilities Consist of Unit Operations Each Unit Must be Optimized to Maximize Ef f iciency Choices of Equipment, Reagents, Operating Conditions Have an Impact on Discharge Quality, Amount of Sludge Generated, and Character of Sludge ..
Slide 33 In summary, the choices a facility makes in equipment, chemicals and operating conditions, have a direct impact on the ability of the system to deliver a compliant discharge, and a solid waste that is low in volume and hazard characteristic. Each unit operation in a wastewater treatment system must be continuously monitored, fine tuned, and modified, if necessary, in order to achieve consistently good results.
Reference: Additional information on wastewater treatment is available from American Electroplaters and Surface Finishers' Society, through purchase of course materials for their Wastewater Treatment and Control Course. The course is held twice per year.
Case Study: P2 in Anodizing Ion exchange as Polishing in Anodizing Wastewater filtration chelating
n w 1
Slide 34 Pollution Prevention Case Study, Anodizing Process Recovery/Reuse The following case study was originally published by Kirman, Lyle and J. Kovach, “Recycling Anodizing Rinsewater Using Ion Exchange,’’ Plating and Surface Finishing, Vol. 78, (April 1991), pp. 65-67. An anodizer in Northern Ohio adopted ion exchange technology to meet a local restriction of 0.5 mg/L for copper discharge. This limit had been reduced from 1 mg/L. The anodizer was unable to comply with the new lower limit despite utilizing existing neutralization and dilution techniques. A study was initiated to eliminate these discharge problems.
The facility’s discharge averaged 40 gal/min containing 1 mg/L of copper. Copper was coming from rinse tanks used in the anodizing lines. Most of these tanks were single station, each with its own fresh water feed. The conversion to a multi-station counterflow type was found to be undesirable because of downtime during the change-over, new space requirements for multi-stations and increased product processing time with the additional stations. Ion exchange technology was believed to be a possible solution. By using this system, water usage would be reduced and the quality of water being used would also be improved. This technology would avoid the copper discharge problem as well. ..-”-
Case Study: P2 in Anodizing Recovery & Recycling Conductivity averaged 880 ohms, allowing the frequency of regeneration for the cation and anion units to be 1,500 gallons. Regeneration analysis showed the chelating resin was able to selectively remove copper and zinc from the other wastewater components. Slide 35 A 2 gal/min flow of untreated wastewater was used for the test runs. Water was processed until resistivity fell below 10,000 ohm-cm. The system ran for four months, with a total of six cycles on the chelating resin, six cycles on the cation resin and 16 cycles on the anion resin. Even after long runs, the facility discovered that the capacity for removing copper on the chelating resin was not exhausted. The inlet conductivity was recorded throughout the study. The conductivity fluctuated with work processed, the average being 880 ohms. With this average inlet, the frequency of regeneration for the cation and anion units was found to be 1,500 gallons. The spent regenerate was low in copper and zinc as a result of the chelating resin’s effectiveness.
There was not any apparent decrease in resin performance or any sign of cumulative fouling throughout the test runs. Analysis of the regeneration showed that the chelating resin was able to selectively remove copper and zinc from the other wastewater components. The chelating resin was never fully exhausted with these metals because of the short duration of the test. There was a small amount of copper bleeding through the chelating resin (which increased with time). It is believed that a hydraulic load less than 2.5 gal/min per cubic foot would reduce copper leakage.
Resin regeneration was found to be low in copper and zinc. The regeneration of the three ion exchange units was done with 5% sulfuric, 4% hydrochloric and 4% sodium hydroxide, respectively. The pH in the ion exchange system was maintained at 3.0 or less.
Cost Comparison 30 GPM Ion Exchange vs. Conventional Treatment Ion Exchanae* Conventional* $123,340 $9,120 $1,100 $3,522 $14,728 $20,138 $4,778 $53,378 $21,100
Installation cost Operation & maintenance Labor Electricity Water & sewer Acid Caustic Waste disposal Total O&M Net savings
$95,000 $9,120 $1,600 $51,793 $0 $9,165 $2,800 $74,478
*Includes neutralization, pumps, prefiltration, ion exchange & waste holding tank. *'Includes equalization tanks, two-step neutralization,flocculation clarifier, sludge thickener and filter press.
Slide 36 Shown is a cost comparison of this system versus the conventional treatment. This analysis is based on a 30 gal/min ion exchange recovery system that includes a holding tank, pH adjustment, filtration, and a three-column system. The ion exchange system's capital and chemical costs are higher. The ion exchange system contributes significant savings in water and sewer costs, however. Therefore, it is significantly cheaper to operate. It would also be possible to use electrowinning technology to remove copper and zinc from the chelating ion exchange regenerants. In addition, it allows the neutralization and discharge of this stream. If this were accomplished, the system would not generate any hazardous waste and the difference in operation costs would favor the ion exchange alternative by an even wider margin.
Benefits and/or Problems Water reuse and copper removal on the anodizing line were the most significant benefits from this application.