METALS REDUCTION IN WASTEWATERS FROM PIGMENT INK PRINTING OPERATIONS

Transcription

1 METALS REDUCTION IN WASTEWATERS FROM PIGMENT INK PRINTING OPERATIONS Tracy A. Nickelsburg, E.I.T., Earth Tech, Inc. W. Gilbert O Neal, Ph.D., P.E., Earth Tech, Inc. Introduction A corrugated container manufacturing and printing facility ( the Facility ) was issued a pretreatment permit requiring the reduction of copper and zinc concentrations in the Facility s effluent. Examination of the manufacturing processes and identification of the chemicals utilized in each individual process was conducted to identify the source of metals in the Facility effluent. The evaluation successfully identified a single manufacturing process, pigment ink printing operations, as the primary source of metals discharge. Bench scale treatability testing was conducted to evaluate methods for reduction of the concentration of copper and zinc in the Facility effluent. Treatment was targeted to achieve pretreatment standards of 3.02 mg/l copper and 1.67 mg/l zinc. An additional objective of the treatability testing was to evaluate the benefits of segregated treatment of the pigment ink printing wastestream. Data developed during the treatability study were utilized in the design of a pretreatment system. Methodology A preliminary site walk through was conducted to trace the origins of all Facility process wastestreams. All chemicals involved in and discharged from each manufacturing process were reviewed. Based on this evaluation it was determined that wastewater generated during washdown of pigment ink printing machinery was the primary contributor of metals to the Facility s final effluent. It was further determined that this wastestream could be readily segregated from the Facility s combined effluent and diverted for pretreatment. Samples of the Facility s wastewater were collected from two locations for preliminary evaluation. Sample A was collected from the Facility s final effluent. Since the washdown of pigment ink printing machinery is an intermittent activity at the Facility, Sample A was collected during a period when discharge from the pigment ink printing process was occurring. Sample B was collected directly from the pigment ink printing process wastewater discharge prior to combination with the non-metalcontaining wastewaters generated from the remaining manufacturing processes. These two samples were selected to confirm that the metal-containing pigment inks were the primary source of metals in the Facility effluent and to provide a comparison of treatability between the combined and segregated wastestreams. Wastewater characterization included analyses for parameters of concern in the Facility s pretreatment permit and parameters of concern in evaluating treatment feasibility. These parameters included copper, zinc, and total suspended solids (TSS). Testing was conducted using EPA Method for copper and zinc and EPA Method for TSS. In addition, apparent color (ADMI) of the 6-3

2 wastestreams was measured for use as an indicator of the presence of metal-containing pigment inks. Color was determined using EPA Method Pigments used in printing applications are inert, water insoluble, organic or inorganic substances (Larsen, 1962). Pigment particle sizes are relatively large with diameters as large as 1 micron. The pigment molecules are centered around a chemical group, called a chromophore, which gives rise to the molecule s color (AATCC, 1981). The pigment inks of most concern at the Facility contain an organic copper phthalocyanine chromophore. Metals are bound within the very stable crystalline structure of the phthalocyanine chromophore (Apps, 1961), therefore very little copper is present as free metal ion. Figure 1 shows this chromophore in a typical pigment, C.I. Pigment Blue 15. The large particle size of pigments makes the use of ultrafiltration feasible (Cheryan, 1986). Other proven wastewater treatment methods include sedimentation and coagulation (Nemerow and Dasgupta, 1991). Treatment technologies evaluated through laboratory testing in this study included chemical coagulation and precipitation, microfiltration, and ultrafiltration. Prior to conducting comprehensive treatability testing for treatment optimization and design, Earth Tech conducted a preliminary, visual screening of the effects of a variety of organic coagulants to ensure their ability to promote precipitation, flocculation, and settling of solids. Coagulants evaluated during the screening phase were chosen based on their documented Figure 1: Phthalocyanine Cromophore (adapted from AATCC, 1981) ability to cause the precipitation of pigments. Coagulants and flocculants screened included: alum, ferric chloride, anionic polymers, and cationic polymers. Coagulants typically used for free metals reduction, such as sodium hydroxide and sodium sulfide, were not evaluated due to the bound nature of the metal within the chromophore. All coagulation jar tests were conducted by placing 200 ml of Sample A or Sample B in a glass beaker under a variable speed paddle stirrer. The paddle stirrers were set for rapid mixing during coagulant addition. Following a period of rapid mixing, the variable speed paddle stirrers were slowed to allow evaluation of the precipitate. Coagulants that most readily resulted in precipitation were chosen for analytical evaluation and quantification of removal efficiency. 6-4

3 Samples from selected coagulant jar tests were collected while the sample remained fully mixed in order to analyze the TSS concentration of the treated wastewater. These values were utilized in determining solids production volumes and in designing sludge handling facilities. After removal of sufficient mixed sample for the TSS analysis, the paddle stirrers were stopped, and flocculated solids were allowed to settle. The resultant supernatant was carefully decanted for analytical testing to determine copper and zinc removal efficiency. Prior to filtration treatment, untreated samples were glass fiber filtered to remove larger particulate matter. Microfiltration testing of Sample A and Sample B was conducted using 0.45 p mixed cellulose ester filters. Ultrafiltration was performed using mixed cellulose ester molecular weight cut off (MWCO) membranes having cutoffs of 100,000, 10,000 and 500 Dalton. Resultant filtrate was analyzed to determine copper and zinc removal efficiency. Results Both Sample A and Sample B proved readily amenable to precipitation using either alum or ferric chloride. Based on visual assessment of the rate and extent of pigment ink precipitation during the screening process, optimal dosages of both alum and ferric chloride were chosen for further analytical testing. For Sample A, dosages of 1,500 mg/l alum and 1,000 mg/l ferric chloride were required to promote substantial floc development and rapid settling. For sample B, dosages of 1,200 mg/l alum and 1,200 mg/l ferric chloride were necessary. Results of wastewater characterization and treatability testing for Sample A are outlined in Table I. The combined untreated effluent averaged 16.1 mg/l Copper and 0.52 mg/l Zinc. The ADMI color of the sqmple was approximately 164,000 ADMI units. The copper concentration was successfully reduced to beiow the 3.02 mg/l pretreatment permit requirement by all treatment methods except 0.45~ filtration. The zinc concentration for all untreated and treated samples was below the pretreatment requirement of 1.67 mg/l. ADMI Color was reduced significantly for all treatments as a result of the a1 of pigment ink particulate. Table I: Sample A - Combined Effluent Treatability Summary Treatment Copper Zinc Color mg/l mgk ADMI 1 164;6000 Untreated (Average) 1500 m a Alum ::):, 1000 mg/l Ferric Chloride Filtration ,000 MWCO Filtration ,000 MWCO Filtration MWCO Filtration

4 ~ Untreated Sample A had a TSS concentration of 1,660 mg/l. The addition of alum and ferric chloride coagulants increased the TSS concentration of the unsettled solution to 4,760 mg/l and 5,240 mg/l respectively. Results of wastewater characterization and treatability testing for Sample B are outlined in Table 11. The segregated washwater from pigment ink printing averaged 65.5 mg/l Copper and 0.15 mg/l Zinc. The ADMI color of the sample was approximately 101,000 ADMI units. Copper concentration was successfully reduced to below the 3.02 mg/l pretreatment permit requirement by all treatment methods except 0.45~ Filtration. The zinc concentration for untreated and treated samples was below the pretreatment requirement of 1.67 mg/l in all cases. ADMI Color was reduced significantly for all treatments as a result of the removal of pigment ink particulate Table 11: Sample B - Segregated Washwater from Pigment Ink Printing Treatability Summary Treatment Copper Zinc Color mg/l mg/l ADMI Untreated (Average) , mg/l Alum mg/l Ferric Chloride p, Filtration ,000 MWCO Filtration I ,OOO MWCO Filtration MWCO Filtration 2.24 ~ Untreated Sample B had a TSS concentration of 850 mg/l. The addition of alum and ferric chloride coagulants increased the TSS concentration of the unsettled solution to 3,200 mg/l and 3,480 mg/l respec ti vel y. Results of treatability testing clearly indicated that chemical coagulation using alum or ferric chloride was effective in coagulating pigment inks and concurrently removing metals complexed in the ink matrix. Coagulation effectively reduced the concentration of copper and zinc well below the pretreatment standards of 3.02 mg/l copper and 1.67 mg/l zinc for both Sample A and Sample B. Further investigation was conducted to determine the impact of adding high molecular weight emulsion polymer to the wastewater following coagulant addition to enhance flocculation of solids and to improve the dewatering characteristics of the sludge. The addition of cationic emulsion flocculant significantly enhanced flocculation and settling of the alum treated sample, while the ferric chloride treated sample was more responsive to the anionic emulsion flocculant. MWCO membrane filtration also successfully reduced metals concentrations to within pretreatment permit requirements, but due to the apparent presence of relatively soluble, low molecular weight forms of copper, was less effective than coagulation in reducing the copper concentrations. --

5 Discussion The results indicated that conventional chemical coagulation and precipitation provide optimum metals removal for both the combined wastestream and the segregated wastestream. Reduction of metals by coagulation was in all cases superior to removals obtained by 100,000, 10,000, or 500 Dalton MWCO filtration. Conventional chemical treatment is also economically favored over ultrafiltration. A major complication in the use of filtration technologies for metals treatment is associated with the treatment and disposal of the resultant concentrated wastestream. Not only would a treatment system. based on ultrafiltration be costly, it would still require chemical treatment of the resultant concentrate to facilitate solids dewatering. While the metals reduction for both alum and ferric chloride was similar, alum treatment generated approximately 8% fewer solids than coagulation using ferric chloride. Due to superior treatment performance combined with lower solids production, alum was selected as the coagulant. Sludge conditioning prior to dewatering in the recessed plate filter press will be conducted through cationic polymer emulsion addition. In order to project the volume of solids to be produced by chemical treatment and to determine the characteristics of the sludge resulting from recessed plate filter press sludge dewatering, a 28.5 liter (7.5 gallon) bench scale alum coagulation treatment trial was initiated using fresh segregated pigment ink wastewater from the Facility. Results from this larger scale test confirmed prior TSS, copper, and zinc results. The treated sample was utilized in bench scale pilot testing performed by the filter press manufacturer. Results of the pilot test indicated that the alum treatment in combination with polymer conditioning resulted in an acceptable filter cake of approximately 33 percent solids. In order to optimize filter press operation, a diatomaceous earth (DE) precoat of the filter cloths was incorporated. This precoat operation allows controlled coating of the filter cloths and eliminates the pass through of very fine precipitated particles at the start of the dewatering cycle. Based on the TSS concentration of 3,200 mg/l obtained in alum treatment of Sample B and a cake density of 1,140 kg/m3 (71 Ib/ft3) as determined during the pilot test, a projection of approximately 0.17 m3 (6 ft3) filter cake generated per day was calculated. Permeate generated during the filter press pilot test was analyzed for copper and zinc concentrations to determine if the permeate would be acceptable for direct discharge to the Facility effluent. The copper concentration was found to be mg/l, and the zinc concentration was mg/l. Both values are well below the pretreatment permit limits. Benefits of Wastestream Segregation Total wastewater flow from the facility is approximately 76,000 liters per day (20,000 gpd). Of this total flow, approximately 19,000 liters per day (5,000 gpd) is produced intermittently by washdown of pigment ink printing machinery. The copper concentration in the segregated wastestream from pigment ink printing operations is approximately 65.5 mg/l. This concentration is diluted to approximately 16.1 mg/l after combination with the non-metal-containing wastewater from the remaining processes. Since

6 the metals are concentrated in one wastestream, the benefits of isolation of this stream and treatment prior to combination with the final effluent were evaluated. The analytical results show that optimum alum treatment performance on the combined wastestream occurs at a dosage of 1,500 mgl. This translates to I 14 kg (250 Ib) alum per day. The treatability information further shows that the segregated wastestream alum demand was 1,200 mgl, or just 23 kg (50 lb) alum per day. Preliminary treatment of the segregated wastestream would provide both a 75% reduction in hydraulic capacity of the treatment system and an 80% reduction in the chemical demand of the system. Segregation of the pigment ink wash water wastestream also precludes the precipitation of nonmetal-containing solids that are not of interest in this pretreatment application. TSS concentrations following alum treatment of Sample A were 4,760 mgl versus a concentration of 3,200 mg/l for segregated Sample B. Dry solids generated from a full stream treatment system would total approximately 361 kg (795 Ib) per day, while solids production from the segregated stream would be only 60 kg (I 33 lb) per day. This amounts to over 80% reduction in required sludge disposal. Treatment System Design Based on analytical data and accounting for benefits derived from implementing treatment on the segregated wastestream, an alum coagulation treatment system was designed to pretreat the segregated wastestream from pigment ink washdown operations. The system consists of two treatment tanks, an alum feed system, a ph control and caustic feed system, a polymer feed system for sludge conditioning, and a recessed plate filter press for sludge dewatering. Level, ph, and chemical feed systems are controlled by a Programmable Logic Controller (PLC) with touchscreen Operator Interface Panel. A schematic flow diagram is presented in Figure 2. The system was located in a small area within the existing manufacturing facility in a location with ready access to the wastewater lines from the pigment ink printing process. To minimize space requirements, the system design incorporated a two-tiered layout. A steel platform was installed to serve as a work area for treatment tank equipment maintenance and jar testing activities as well as a central location for the PLC and Operator Interface. In order to simplify operator control of the coagulation process, the system was designed to incorporate two batch chemical treatment tanks operating sequentially. Tanks were designed to be of sufficient volume to accumulate the projected daily discharge of 19,000 liters (5,000 gallons) wastewater. Wastewater flow is directed into one of the treatment tanks until an Operator input fill level is reached as indicated by signals from an ultrasonic level sensor. The PLC then directs the flow to the second of the two batch tanks through manipulation of automatic valves to allow sequencing from one tank to the next. The PLC initiates a Tank Full alarm to alert the Operator that the batch tank is full and awaiting chemical treatment. The Operator performs jar tests to determine the required dosages of coagulant and polymer and inputs the dosage information into the Operator Interface Panel. Upon an Operator initiated

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8 Start command, the tank mixer is activated and the required coagulant dosage is automatically added to the tank. The untreated wastewater enters the tank at a ph of approximately 8.3 Standard Units. As alum is added to the chemical treatment tank, the ph of the wastewater is reduced. PLC driven caustic feed is designed to maintain a ph setpoint designated by the Operator. The optimum ph for alum precipitation of the metals was determined to be approximately 7.0 standard units based on both laboratory and field observations. Although solubility information indicates that typical alum precipitation systems will be optimized at ph levels slightly higher than this, the particular characteristics of this pigment ink wastewater resulted in significantly better precipitation of solids at the lower ph. Following alum addition and ph adjustment, the PLC initiates polymer addition at the dosage selected by the Operator based on jar testing results. After a brief flocculation mixing stage, the PLC annunciates an alarm to alert the Operator that chemical treatment has been completed. The Operator may then visually assess the flocculation achieved by the system and may make any manual alum or polymer additions as necessary. Provisions are incorporated into the design to allow for an Operator designated settling stage following flocculation mixing. Following settling, the supernatant may be decanted directly to the Facility effluent with the use of a manually operated telescoping valve. The telescoping valve doubles as a tank overfill device to prevent accidental overflow of wastewater into the manufacturing area. The concentrated solids are then transferred to the filter press for dewatering. Transfer of treated wastewater to the filter press for dewatering is preceded by precoating of the filter cloths with diatomaceous earth (DE). A pre-mixed solution of DE and water is cycled through the filter press, and filtrate is returned to the DE precoat tank. This cycling continues until the resultant filtrate runs clear, indicating that the filter cloths are adequately precoated and solids pass-through is eliminated. Wastewater from the selected chemical treatment tank is then transferred to the recessed plate filter press for dewatering. Treated filtrate generated during filter press operation is recycled to fill the 100 gallon DE precoat tank for use in the next dewatering cycle, the remaining filtrate is discharged as pretreated wastewater to the Facility effluent. During chemical treatment and sludge dewatering of wastewater in one chemical treatment tank, pigment ink wastewater flow is being directed automatically to the second chemical treatment tank. The PLC logic allows for sequential operation of the two batch tanks. Conclusions Implementation of pretreatment technologies can be made significantly more efficient through thorough evaluation of process wastestreams. In this particular case, hydraulic requirements of the pretreatment system were successfully reduced by 75% and chemical demand was reduced by 80% as a result of segregating the wastestream of greatest concern. This reduction in required capacity permitted the installation of the system in an existing storage area rather than necessitating expansion of the manufacturing facility.

9 The conventional chemical coagulation wastewater treatment system continues to demonstrate reduction of metals concentrations to well within the proscribed pretreatment standards. Treatability testing, design, installation, and start-up of the system were successfully coordinated to meet an extremely aggressive compliance schedule requiring that the system be fully operational within four months of design initiation. While the expedited design and construction schedule allowed the Facility to maintain compliance with pretreatment permit requirements, the short time frame allotted for treatability testing placed constraints on representative sample collection. During start-up of the system, the concentration of pigment ink solids in the segregated wastestream was found to be significantly higher than observed during laboratory studies. This did not impede the performance of the treatment system as designed, but did result in an increased volume of total solids and resultant filter cake. In order to accommodate the increased solids generation, additional cycles of operation for the filter press are required on a daily basis. This occurrence emphasizes the need to accurately characterize wastestreams during preliminary treatability studies. While several samples were analyzed and thought to be representative of typical wastewater conditions, the variability of printing operations from day to day could not be adequately captured in the limited study time available. Waste management practices to minimize the discharge of pollutants are critical to efficient wastewater treatment system operation. In some cases, the presence of a wastewater treatment system may lead to diminished scrutiny of waste generation practices by equipment operators. Operators may develop a false sense of security that the wastewater treatment system will be capable of handling excessive discharges that would otherwise require additional time and effort to avoid. This could result in overload and failure of the treatment system. In this particular case, it is possible that waste inks which were previously manually removed in paste form are now being washed down the drain since a treatment system is in place. Care should be taken to ensure that operating and maintenance practices are not modified as a result of implementation of wastewater treatment. References AATCC, American Association of Textile Chemists and Colorists (198 I ) Dyeing Primer, AATCC, Research Triangle Park, NC. Apps, E. A. (1961) Printing Ink Technology, Leonard Hill Limited, London. Cheryan, Munir (1986) Ultrafiltration Handbook, Technomic Publishing Company, Inc., Lancaster, PA. Larsen, L. M. (1962) Industrial Printing Inks, Reinhold Publishing Corp., New York, NY. Nemerow, N. L., and Dasgupta, A. (1991) Industrial and Hazardous Waste Treatment, Van Nostrand Reinhold, New York, NY.