Depaint and Alodining Wastewater Treatment and Recycle
Gordon B. Kingsley, P.E
Robert Clay
Radian
International, LLC
10389 Old Placerville Road
Sacramento, CA
95827
(916) 362-5332
(916) 362-2318 (fax)
INTRODUCTION
During alodining and paint stripping operations at DoD facilities, large volumes of contaminated water that require treatment prior to disposal are often generated. In addition, many of paint stripping activities at the facilities use methylene chloride, which is an EPA 17 chemical targeted for reduction by the Air Force (AF). This paper describes the methods employed to meet the primary goals of the AF with respect to the modification of the alodining and paint stripping processes. The primary goals were to convert existing processes to an environmentally friendly paint stripper and provide a cost effective wastewater treatment system that would maintain parts quality and meet effluent limitations for recycle or discharge to the sanitary sewer in the future. The costs associated with treatment of the wastewater from these processes and the use of additional makeup chemicals can be a significant cost of operation for this process. Therefore, a secondary goal was to minimize the size of the treatment system by using source reduction and waste minimization techniques.
The steps to meeting the AF goals included:
Overall review showed that the key steps that made this a successful project for the AF were the team’s commitment to switch chemical strippers early in the project and to minimize chemical and water usage, as opposed to an end of pipe design of the current process. These steps alone reduced the cost at least $750,000 to $1,000,000. This remainder of this paper presents the description of the above steps using McClellan Air Force Base (AFB) as a case study.
CASE STUDY
Background:
McClellan AFB is located near Sacramento, California. McClellan AFB is being closed as part of the Base Realignment and Closure (BRAC) process. By the year 2001 the workload at McClellan AFB will either be transferred to private industry and conducted on-site or to other existing AF maintenance facilities. One of the larger sources of contaminated wastewater is from depaint and alodining operations performed within Building 243. These operations support maintenance of weapons systems including F-111, A-10, KC-135 aircraft. The work load varies considerably in both volume, size, shape and type of equipment maintained. On any given day a variety of parts from small bolts to large wing sections can be observed in various stages of processing.
Physical and chemical methods are used in depaint operations. Plastic bead media blasting and wire brushes are used where practical. Many parts however require chemical strippers, especially those with crevices on which physical methods are ineffective. These parts are loaded into baskets or racks and submerged in an open top chemical stripper tank where the removed paint accumulates in the tank. The parts are then raised and chemicals are allowed to drain or drip back into the tank prior to immersion in an adjacent water rinse tank to remove remaining chemical stripper. The finished parts are then transported to other areas within the building for painting.
At McClellan AFB, alodining, pretreatment of aluminum parts for better adhesion of coatings, is conducted in the same room as the paint stripping. Parts such as wing sections are placed on carts and rolled into the room. Phosphoric acid is used to etch the surface. Deionized water is applied to remove the phosphoric acid prior to application of alodining solution containing chromates. Cloth rags are used in the final step to remove the remaining alodine solution and prevent chromates from being added to wastewater, which would then require treatment. Prior to implementation of waste minimization techniques, the rinse tank was replenished daily by flushing with fresh water until clear. This practice generates the majority of water requiring treatment. The combined volume of these waste streams at McClellan AFB was estimated to be as high as 40,000 gallons per week (gpw). Contaminants include N-methyl-pyrrolidone, methylene chloride, phenol, chrome, zinc, lead, cadmium, copper, and nickel. Water from these operations enters a common drain and flows by gravity to a central industrial wastewater treatment plant (IWTP). Eventually the IWTP will be closed. Each facility will, at that time, be required to meet effluent permit requirements where wastewater exits the building. In anticipation of this requirement McClellan AFB retained Radian International to design a wastewater treatment system. The goal is to provide a cost effective yet flexible system that will maintain parts quality and meet effluent limitations for recycle (closed loop) or discharge to the sanitary sewer in the future.
Approach
The approach to meeting the AF goals included:
At the on-set of the project McClellan AFB, Environmental Management (EM) and Radian International recognized the importance of forming a team with Building 243 operations (OP) personnel. Meetings were conducted to brainstorm practical methods for accomplishing the overall goals. Process water quality requirements were discussed and minimum standards that would meet part cleanliness standards were set.
The next step was to evaluate alternative paint strippers. Initially, methylene chloride and phenol (MCP) were being used. Methylene chloride is an EPA 17 chemical targeted for reduction by the AF. N-methyl-pyrrolidone in combination with ethanol amine (NMP) was identified as a potential substitute. Radian, EM, and OP contacted DoD facilities using NMP and arranged a site visit to evaluate the effectiveness of NMP as a paint stripper. Based on team observations, NMP was selected for testing at McClellan AFB. The paint stripper was tested over a six month period and it was determined that for most applications NMP met McClellan AFB paint stripping requirements. A small dip tank containing MCP was retained for parts that proved recalcitrant to NMP such as polysulfones.
Source reduction methods to minimize dragout were considered. Many parts have cavities that can retain up to gallons of chemical stripper when the parts are removed from the chemical tank. The placement of parts, chemical draining times, and use of compressed air to remove surface stripper were determined to have the greatest benefits. Allowing longer draining times and using compressed air was found effective for the chemicals clinging to the surface of parts. However, chemicals retained in cavities could not be removed by these methods. A potential solution was to position the parts so the stripper could drain from the cavaties. Also, a rotating basket with a hinged lid was evaluated to enhance draining.
Further reduction of wastewater volume was achieved through waste minimization techniques. The most significant reduction was obtained by using multiple water rinse tanks in series. Originally, a single tank was used in the rinsing system. To conserve water, an a second rinse. Parts were soaked in the first tank followed by rinsing in the second tank. When the first tank became cloudy the water was pumped out and replenished with water from the second tank. The second tank was then filled with fresh water. In addition, all hoses used for wash down or rinse operations were equipped with spring loaded valves. Meter readings were recorded to determine use for depaint and alodining. Over time, water use was reduced from a high of 27,000 gpw to as low as 400 gpw, a 98% reduction. Figure 1 shows the water usage over time.
Following waste minimization, water treatment technologies were evaluated. A water flow and contaminant balance was developed. Average and instantaneous flow rates were determined and a design basis set. A literature search was conducted and viable technologies were selected for screening. Vendors were then contacted along with users and the list was condensed to the following:
Each technology was evaluated and advantages and disadvantages listed. Scores of 1 to 5 were assigned based on technical feasibility, cost, ease of operations and ability to treat all contaminants to within effluent limits or parts quality requirements and were weighed according to their relative importance (technical feasibility [.4], cost [.3], ease of operation [.2], and treatment of secondary streams [.1]). The highest rankings were UVO in combination with carbon and ion exchange polishing. In addition a skimmer was selected for the primary rinse tank to remove surface oil.
Selection of the specific UVO unit for pilot testing and demonstration was based on life cycle costs. Of the manufacturers evaluated, Purifics® was judged to provide the best over-all value. The Purifics® UVO is unique in that activated titanium dioxide (TiO2) is used as a catalyst. Using the catalyst, electrical energy requirements are lower, UV lamp life is longer, and suspended solids do not effect efficiency to the degree of other systems evaluated. Heavy metals such as chrome and lead may also tend to plate out on the surface of the catalyst providing the potential for recovery.
For the testing, a pilot unit was transported to the site. Small batch tests were conducted on site to determine optimum flow rates, hydrogen peroxide addition, pH and other operating parameters. In addition to the wastewaters from Building 243, water containing methylene chloride and phenol and water containing benzyl alcohol were treated. After batch testing was complete, continuous testing was started. Initial results indicated that a light, fluffy solid (attributed to the intermediate breakdown products of NMP) was forming that coated the catalyst and reduced efficiency and increased hydrogen peroxide and electrical energy requirements. This material accumulated with time and was not observed during batch testing. An assessment by Purifics® determined that the system flow rate needed to be reduced prevent the accumulation of this solid. The system was modified, and additional continuous tests were conducted. Analysis of the effluent indicated that the system should be operated using a two-pass mode of operation. Initially a "low flow-high treatment" mode would be used followed by a second pass at higher flow. Based on the testing, NMP concentrations were reduced from 360 parts per million (ppm) to 52 ppm in one test and from 740 ppm to less than 0.01 in a second test. Treatment time was approximately 20 minutes for each test and the reaction rates were found to be first order. In addition, the surface of the TiO2 was scanned at 1,000 and 10,000 magnification using electron microscopy to determine potential plating of metal. Metals were detected on the surface. X-ray fluorescence analysis were conducted and quantities from 1 to 3% chromium, 5 to 8% iron, and 8 to 12% lead were determined to be present. These studies support similar research by R. Tanaka and others (Maillard, 1994) (Hermann, 1988).
Additional equipment to remove oil and filter solids was also tested. An oil skimmer was mounted in the first rinse tank. Mineral oil used as a seal in the chemical strip tank and oil and grease from parts accumulates on the rinse water surface. A continuous revolving polysulfone motorized belt selectively (hydrophobic) removed oil from the surface. A knife scraped the oil from the surface of the belt and directed it to a pipe terminating at a bucket. Although the skimmer was relatively simple and inexpensive it proved highly effective. A filter element was also installed within the rinse tank connected to the suction of an air diaphragm pump. Particulate was thereby confined to the rinse tank. As a backup, a 25 micron filter with combination oil absorbing and solids removal bag was installed ahead of the UVO.
At the completion of pilot testing a summary report was prepared with recommendations and conclusions and design drawing and specifications for the system. Carbon and metals polishing equipment was included to supplement the UVO. Two beds of carbon in series ensure that water will meet existing and future effluent limits. The carbon beds were sized for several months continuous operation each using conservative estimates of loading. When breakthrough occurs the first bed will then be replaced with the second and a fresh carbon unit added. Metals polishing consists of using two beds of deionized resin in series. The units would be leased from a company that recovers the metals and provides a certificate of reclaimation, thereby eliminating the generation of a hazardous waste. The final schematic of the treatment system is shown in Figure 2.
Benefits
The benefits of the system include:
The water treatment system design provides high purity water that meets the requirements for processing parts. Water entering the UVO appeared to be dark brown and relatively opaque. Water exiting the system was clear.
Chemicals, solids wastes and water use are expected to be reduced substantially. Careful positioning of parts to allow drainage of chemicals, increased drain time and removal of residual chemical on the surface of parts using compressed air should reduce chemical use by over 50%. Water conservation measures have already demonstrated reductions of up to 99%. Once the treatment system is operational and closed loop mode is implemented water consumption is expected to match evaporation losses, further reducing fresh water use. Solids wastes will likewise be reduced. Chemicals currently are treated at the IWTP and generate sludge that is disposed of as a hazardous waste. The UVO forms (OH-) radicals that react with organic chemicals to form carbon dioxide, water and elemental gases. The only solids produced from the system are paint chips from the rinse tank, filter bass and a small amount of used catalyst. It is expected that the metals and the TiO2 catalyst will be reclaimed thereby eliminating this source of hazardous waste. Likewise the dissolved metals removed by the deionized beds will also be recycled.
Based on pilot testing at the site, the system is expected to be reliable, cost effective, flexible and safe. The UVO pilot unit was tested over a one month period to refine the system and reduce uncertainty. Critical process parameters have been defined, modifications implemented and the system demonstrated the ability to produce water of acceptable quality. A PurificsÒ model 5 DL rack, 11 kilowatt, unit complete with all piping and automation has been selected for this service. Improvements have been implemented to provide a high degree of reliability and requiring minimum operator interface. The proposed system includes the two mode flow described previously, provisions to prevent vapor locking of hydrogen peroxide metering pumps, and improved microprocessor design based on other field applications. This system has been sized to treat a batch of 2,000 gallons of wastewater containing 2,200 ppm of organic contaminant over a seven day period. The average expected wastewater volume and organic load is less than 50% of the design. A hazop study has been conducted and recommendations incorporated in final design in accordance with the methodology developed by the American Institute of Chemical Engineers (AICHE) center for Chemical Process Safety and compliance with Military Specification 882 C. The cost of the complete water treatment system is $250,000 to $350,000 including the cost of waste minimization and treatment equipment. Operation and maintenance is estimated to be $250 per batch (Purifics, 1996).
CONCLUSIONS AND RECOMMENDATIONS
Source reduction and water conservation studies have shown potential for significant reduction of chemical and fresh water used at McClellan AFB in depaint and alodining operations. It is important to conduct source reduction and water conservation prior to setting a design basis for water volume and contaminant levels. Once the volumes have been reduced, UVO in combination with activated carbon and deionized resin can economically provide water quality to allow closed loop recycle. The system should be pilot tested to ensure critical performance parameters are defined and implemented in final design. The step wise procedures followed at McClellan AFB for the improvement of these processes should reduce risk and provide a cost effective, safe design that can serve as a template for other DoD facilities.
REFERENCES
Maillard, Catherine, Chantal Guillard and Pierre Pichat. "The degradation of nitrobenzene in water by photocatalysis over TiO2; Kinetics, and products; simultaneous elimination of bromide orphanol or Pb 2+ cations". Journal of Chemistry Vol 18 No. 8-9, 1994.
Hermann Jeane Marie, Jean Disdier and Pierre Pichat. "Photocatalytic Deposition of Silver on Powder Titania: Consequences for the Recovery of Silver. Journal of Catalysis 113, 72-81, 1988.
PurificsÒ Environmental Technologies Inc. 1996. "Pilot Scale Test Report prepared for Radian Corp. Pilot Scale Photocatalytic Treatment System for the Treatment of Process Rinse Water of McClellan Air Force Base. Rev 3. 5. July.