Advice for Installing a Conventional Wastewater Treatment System by Frank Altmayer, CEF Scientific Control Laboratories Inc. 3158 Kolin Avenue Chicago, IL 60623-4889 Originally Published in: PLATING AND SURFACE FINISHING Journal of the American Electroplaters and Surface Finishers Society May 1993 Dear "Advice & Counsel," We are planning to install a conventional wastewater treatment system, to comply with federal pretreatment regulations, under category 403.33 (metal finishing). This treatment system would include the following unit operations: 1. Chromium reduction system 2. pH Neutralization system 3. Clarification system 4. Filtration system 5. Final pH adjustment system What advice can you provide that would make such a system easier and less troublesome to operate? Answer The following discussion will cover some design parameters and typical problems that have been experienced with the above unit operations: 1. Chromium Reduction System The chromium reduction system typically consists of: A. A single- or two-compartment tank Use a tank, either one- or two-compartment, that is made of cross-linked polyethylene or steel/PVC lined. We have seen tanks made of polypropylene, but these typically evidence stress cracks after a short period of time. Stainless steel can be used successfully, if the wastewater does not contain significant amounts of halogens, such as fluoride/ chloride. The first compartment is used to lower the pH of the chromium-bearing wastewater. The second compartment is used to perform reduction of hexavalent chromium to trivalent, by use of a reducing agent. The pH reduction and chromium reduction can also be carried out in a single compartment, simultaneously. A typical design parameter for tank size is 10-15 min of retention time in each compartment. If a single compartment is used, 20 min of retention time should be adequate. B. Mixers for each tank/compartment Mixer motor size typically is 1 HP for each 1,000 gal to be mixed, although smaller motors can be used, and the size of blades adjusted to adequately agitate the tank. The mixer shaft and prop should be made of 316 stainless steel, with the shaft and prop welded together, to avoid separation during use—the most common problem with mixers. C. A pH meter controller These devices are great sources of wastewater treatment headaches. The pH probe is sensitive to oil/grease contamination. The probe should be "self-cleaning," unless the operator is willing to clean it as often as every shift. Self cleaning can be accomplished through the use of electrically operated brushes or ultrasonic transducers, mounted on the probe. Automatic temperature compensation and an internal reference electrode is highly desirable. The pH probe, out of necessity, is either made of a flimsy glass, or is partially made of glass, so it is subject to breakage by rough handling. As much as possible, the probe should have a minimum amount of glass, be protected from impact by a "guard device," and whatever glass is involved should be as impact-resistant as possible. Given all that, the probe must be readily accessible for easy cleaning (even if it is "self- cleaning"), and it should be easy and quick to replace a broken or malfunctioning probe. If the distance between the probe and meter is greater than 6 ft. the probe should have a built-in pre-amp, to amplify the signal to the meter. The meter should come with two probes (one spare). Use either combination probes, or separate pH probe, plus reference probe. Combinations cost more, but take less effort to maintain. The pH meter controller should have a recording device to chart the pH for at least the previous 24 hrs. This will allow the operator to verify proper function, in the event that a problem occurs. The recording device should be water/moisture-resistant. The entire pH meter controller should be encased in a water/ moisture-tight, clear plastic case, to avoid breakdowns of the electronics caused by corrosive fumes and moisture entering the meter. The meter should be electrically isolated from stray currents and radio frequencies generated by nearby equipment. All electrical switches/contacts within the meter should be nickel gold plated. All wiring should be copper or better. The meter should have the ability to program high- and low-alarm setpoint(s). The meter should have visible alarms (red lights) for lost setpoint(s), and should allow for a plug-in audible alarm, to sound when the setpoint has been lost. Likewise, the meter should have a green light, to indicate that the setpoint is being maintained. The meter should electronically monitor the function of the temperature sensor and the pH probe, and indicate, through an alarm or display screen, when it is not functioning properly. An LED display is preferable over analog, because the latter can be affected by static electricity. D. An ORP meter controller The same guidelines as described for the pH meter controllers apply to ORP meter controllers, because the meters are similar. The ORP probe, however, must not contain or use any mercury or mercury containing compounds. The probes generally are not made of glass, so those particular requirements do not apply. A typical operational problem is finding the right "setpoint." Do not set the meter to a published ORP reading. Use jar tests, instead, to determine the correct reading for your operation. E. Chemical feed pumps (two) Chemical feed pumps are used to add acid for pH reduction, and the reducing agent for chromium reduction. The chemical feed pumps are crucial to the continuous function of the entire system, so it is recommended that you have at least one spare for each type of chemical to be fed (acid, alkali, reducing agent). Some companies stock spare parts, so that common repairs can be made quickly, but they usually discover that they are in violation during the repair (sometimes far longer). The chemical feed pumps should be adjustable, from slightly more than 0 gpm, to 200 percent above the anticipated chemical feed. The materials touching the liquid being pumped must be highly resistant to chemical attack by those chemicals. The pumps must be self-priming, with adjustable stroke and speed. All electronics must be in a moisture-proof housing. It would be great to have a sensor in the discharge line of each chemical feed pump, to sound an alarm when no liquid flow is in the line, because the most common problem with these pumps is that they have broken down, or that the supply tank is empty and the pump is pumping "air," instead of the chemical. It is a good idea to color code the chemical feed pumps, so that "green" is the reducing agent pump, etc. This allows for rapid identification of a problem pump. The head, fittings, diaphragm, valve seat and balls must be made of materials that can withstand rigorous use and chemical attack. If a slurry of lime or magnesium hydroxide is used as the neutralizing agent, the feed pumps must be specifically designed for use with these slurries. Be sure to specify the slurry concentration to the pump supplier. Also, slurries must be pumped through piping and/or tubing that has no low spots for solids to accumulate, and as few elbows (preferable none) as possible, that could prevent/reduce the tendency for these to clog. Some slurry pumps have automatic back-flushing capabilities (recommended). The slurry piping should have back-flushing capabilities, too. Chemical feed pumps should have circuit breakers and over-temperature sensors, for fire prevention. When tripped, an alarm should sound, to alert the operator to a dead pump. F. Day mixing tank This is typically a stainless steel tank, with a mixer for blending the reducing agent with water. The tank can be eliminated if the reducing agent is pre-dissolved when purchased. Sodium bisulfite and sodium meta-bisulfite are typical reducing agents used in the reduction of chromium. Sulfur dioxide can also be used, in which case, the mixing tank is replaced by a compressed gas cylinder and ancillary safe storage devices. Another reducing agent, hydroxyl ammonium sulfate, can be used as a reducing agent. The advantage is that it does not yield the noxious/toxic fumes created by the others mentioned, and it works at neutral or acidic pH. The disadvantage is that it costs significantly more to buy. 2. pH Neutralization System Typical elements of a pH neutralization system are: A. A single-compartment tank Provide a tank, sized to hold 15 min of flow, if sodium hydroxide is the neutralizing agent; 30 min of flow, if lime is the neutralizing agent; and 60 min of flow, if magnesium hydroxide/oxide is the neutralizing agent. This assumes that the net total flow will be acidic. If the net total flow is alkaline, 15 min of retention is adequate for any acid used to neutralize an alkaline wastewater stream from metal finishing operations. The tank can be made of polypropylene, Koroseal-lined steel, stainless steel or polyethylene. B. Mixer See discussion for chromium reduction tank. C. pH meter controller See discussion for chromium reduction tank. The problem we have discovered most frequently with neutralization systems is that the probe tends to become coated with the metal hydroxides formed by the neutralization reaction. The issue of probe cleanliness is, therefore, even more important here. D. Chemical feed pump See discussion for chromium reduction tank. 3. Clarification System The biggest problem with wastewater treatment systems that "don't work" is that they have a clarifier that is too small. Clarifiers for treating metal finishing wastewaters fall into two major categories: Lamella types and Gravity. Both types of clarifiers work well when properly sized and operated to manufacturers' specifications. There is no known formula for sizing a Camera, but experience has shown that, typically, they can comfortably handle about one-half the flow for which they are rated. A gravity clarifier is sized according to "rise"—that is, the speed at which the liquid level in the tank rises. A typical sizing parameter is a rise of one-quarter in./min. Gravity clarifiers with rises greater than this will usually need to use a flocculent to adequately settle the metal hydroxides from the pH neutralization system. Clarifiers that use flocculating agents are sensitive to the kind and concentration of flocculent used: Too much causes flotation of solids; too little causes the same problem. The flocculent should be added according to the ppm of suspended solids in the incoming wastewater stream. But this is too difficult to measure, so it is added according to flow instead. The result is that you rarely add the right amount. The most successful clarifiers are those that operate with little or no help from flocculates. Note: All lamella-type clarifiers must use flocculates to work. Lamella clarifiers should be operated with extremely close control over flocculent concentration. The Camera should have a flocculent mixing tank and a flocking compartment, where the incoming wastewater is gently blended with the flocking agent. The speed of blend and feed rate should be adjustable over a wide range. Lamella clarifiers require periodic cleaning of the plates inside, so provision should be made to allow for this to be done as easily as possible. Allow enough overhead space to remove the plate packing, and provide a tank into which the plates can be placed and cleaned. Having a spare set of packing plates would allow for cleaning of dirty ones without shutting down the operation for very long . Lamellas also require continuous removal of solids collected at the bottom, into a "thickening tank" that is routed back to the entry of the clarifier, but should by-pass the floccing tank, to avoid floc's concentrating in the clarifier. There should be a "window" installed in the Camera to allow the operator to see the level of 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 Cameras have a series of "taps" that allow sludge-blanket-level verification. These taps end up getting clogged with sludge and stop working well. The Camera should be made out of non-corroding materials, if possible, but are usually made of steel to reduce cost. We have seen gravity clarifiers made out of stainless steel. Corroding clarifiers are subject to catastrophic rupture, because of corrosion at some point during their service life. It would be a good idea to have a flow meter on the outgoing flow of the clarifier, so that the operator can verify that he/she is operating within the limitations of the hardware. All valves on clarifiers should be oversized, so that sludge will not readily clog them. If a 2- in. valve will do the job, use a 3-in. one, etc. The clarifier should have provision for oil-skimming at both the entry and the exit. Consideration should be given to substituting a microfiltration system for the clarifier/polishing filter described in this column. Such a system costs more to install and operate, but is more "foolproof" in operation and, based on experience, has a much better record of compliance—mainly because of the elimination of "upsets," caused by floating solids. In a microfiltration system, floating solids are filtered out of the wastewater stream, along with non-floating solids. 4. Filtration System A modern wastewater pretreatment system 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, or mixed-media types, although one manufacturer makes a "floating media" filter that we have found works well also. The straight-sand-type filter works least well, and most often is undersized, in terms of soil-holding capacity and suspended-solids loading. The higher the suspended solids a filter can comfortably handle, the better it is. A clarifier that works well typically produces suspended solids around 10-20 ppm. A "burp," caused by dissolved gases, or improper flocculent control, can shoot this value to 200 ppm or more, easily. Provision must be made to backwash the filter easily (automatically is preferred), 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 tanks] can be supplied by others, but should be sized by the filter manufacturer). It would be wise 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. 5. Final pH Adjustment System See neutralization system above. Notes: 1. It is helpful to the operator to have a control panel that displays the status of each unit operation, as long as the operator does not take what he/she sees as "Gospel." 2. Bench space should be provided near the wastewater treatment system, so that the operator can perform jartests and "quickie" analyses, in order to verify that all is working well. u