Many processes eliminate or remove color from pulp and paper mill wastestreams, but which is best for your system?

Transcription

1 Many processes eliminate or remove color from pulp and paper mill wastestreams, but which is best for your system? By James K. Baird and Brian A. Dempsey ulp and paper manufacturing wastewater has a yellowish brown color that conventional secondary treatment processes do not completely remove. When discharged into a receiving stream, the residual color gives the impression that wastewater is improperly treated, and it affects the receiving stream's aesthetic value. However, various pollution prevention techniques and treatment processes are available that can reduce color to less than proposed federal regulatory requirements and, in some cases, remove more than 90 percent of color from bleach plant and total mill effluents. Sources of Color An object's observed color depends on the specific wavelengths of visible and invisible light absorbed or reflected by the object. Two types of color exist: apparent and true color. Apparent color is the measure of a sample as is, regardless of its ph and suspended and colloidal material content. (The apparent color concentration of pulp and paper wastewater increases as ph and turbidity increase.) True color is measured after the ph has been adjusted to 7.6 and turbidity The primary source of color in the pulp and paper industry is lignin, the glue that binds cellulose together in trees. Most color is generated during pulp production and bleaching because of the loss of black liquor and brownstock washing water to wastewater treatment. Wastewater also receives color from the pulp bleaching and extraction stages, as well as from dyes and pigments used to make colored paper. Researchers also have reported that 300 pounds of color are generated per air-dried bleached ton (ADBT) of pulp, where 1 Pt/Co color unit is the same as 1 mg/l of color. (Pt/Co refers to a standard solution of platinum and cobalt used to calibrate spectrophotometers.) About 200 pounds of color are generated per 51

2 ADBT in the bleach plant, of which 145 pounds comes from first-stage extraction. Of the remaining 100 pounds of color, 65 are generated by pulping, and the rest comes from the recovery and causticizing processes, as well as the wood yard and paper mill. Regulatory Requirements Existing regulations. EPA's National Pollutant Discharge Elimination System (NPDES) program does not regulate color in pulp and paper mill discharges, nor does the agency's October 1982 guidance for pulp and paper mill permitting. States authorized to administer the program, however, can impose more stringent requirements in NPDES permits than EPA dictates. The Pennsylvania Department of Environmental Protection (DEP), for example, regulates color in pulp and paper mill discharges under water quality-based effluent limits, which are used "to attain or maintain spe- cific water quality criteria in order to assure protection of a designated use." The Pennsylvania DEI' developed its color limits using mass balance, based on the allowable color in a stream after complete mixing between effluent and the stream Q7. flow value (a statistical value of the seven-day average low flow in a 10-year period). Without a best available technology (BAT) treatment standard for color, however, Pennsylvania's limits have been difficult, if not impossible, for treatment plants discharging into small receiving streams to achieve because they are below what is obtainable with existing technology. Proposed regulations. In December 1993, EPA proposed new federal regulations for the pulp and paper industry that would regulate end-ofpipe discharges and stack emissions and dictate which manufacturing processes the industry uses. Color is one of many parameters the agency would regulate if the proposed regulations are finalized as initially published in the Frdernl Register (58 Fed. Reg , Dec. 17, 1993). The proposed color limits for continuous discharges to surface waters and publicly owned treatment works is a monthly average of 76.3 kilograms of color per megagram of unbleached pulp produced at 10 percent moisture (air-dried metric ton) and a daily maximum of 120 kilograms per megagram for total chlorine free (TCF) or non-tcf bleaching processes. EPA's October 1993 guidance for developing NPDES permits under the proposed regulations states that: The process changes used as a basis for the proposed effluent limitations guidelines and standards will result in significant reduction of effluent color from: (1) More extensive delignification of chemical wood pulps through use of extended cook- 52 For More Information, Circle 39 on Reader Service Card

3 ing and oxygen delignification; (2) Effective brownstock pulp washing; (3) Addition of oxygen and peroxide to bleach plant caustic extraction stages; and (4) Effective pulping liquor management, spill prevention, and control. Accordingly, because color is a useful measure of the performance of these pollution prevention process technologies, the agency is proposing to regulate color. Although the 1993 guidance does not list a best available technology (BAT) for color removal, it says that, with best reported removal efficiencies of 30 percent, biological treatment is ineffective, and the agency therefore recommends the use of physicochemical processes for end- of-pipe color treatment. The 1993 guidance states that activated carbon adsorption can reduce color by 70 percent to 85 percent. It also says that lime precipitation or coagulation, enzyme pretreatment, magnesium and lime, alum coagulation and precipitation, ozone oxidation, resin separation, ion exchange, aluminum oxide, wood adsorption, membrane separation and MYCOR processing all can reduce color by at least 90 percent. However, it notes that these color treatment technologies are infrequently used because of high operating costs. Prevention Pollution prevention has become popular because pollution control costs are increasing and pollution that is not generated does not require treatment or disposal. Pulping and bleaching processes are the best areas to examine for colorrelated pollution prevention strategies because they are where most color is generated. Pulp and paper manufacturing is complex; process changes in one area can directly and indirectly affect many other areas. Increasing digester cooking time, for example, reduces the quantity of pulp produced. Extended pulp cooking times and oxygen delignification can reduce the amount of color in wastewater, but oxygen delignification systems can cost tens of millions of dollars to install. Altematives to using elemental chlorine for bleaching include chlorine dioxide, hydrogen peroxide, oxygen and ozone. Chlorine dioxide, a atio s (1988) "Controlling Wastew nt Inc. (NCASI;

4 severely hazardous chemical, typically is produced onsite; a chlorine dioxide reactor can cost tens of millions of dollars, depending on its size. Systems for storing, handling and applying either oxygen or hydrogen peroxide, meanwhile, usually can be installed for less than $1 million. In order to use oxygen or hydrogen peroxide as a primary bleaching chemical, facilities must modify existing chlorine or chlorine dioxide bleaching stage hydraulic retention time and other specific design parameters, such as pressure, temperature and stock (pulp) consistency. Because pulp and paper mill manufacturing processes, treatment processes and final effluent color requirements vary, evaluations should be made on a case-by-case basis. Pollution prevention strategies may be more cost-effective if they can be easily incorporated into an existing manufacturing process, or if the manufacturing process already must be modified to meet other proposed cluster rule components, such as dioxin, furan or adsorbable organic halide (AOX) limits. For example, suppose a pulp mill that produces 100 ADBTs bleaches its pulp through a chlorine-caustic extraction-chlorine sequence and discharges 1 million gallons per day of wastewater treated through a conventional biological plant that removes 20 percent of color. If the mill produces 300 pounds per ADBT of color, it would generate 30,000 pounds per day (lb/day) of color and discharge 24,000 Ib/day of color. EPA's proposed cluster rule would limit color discharge to 15,260 lb/day. The mill could achieve that discharge limit by switching the bleaching sequence to chlorine dioxidehypochlorite-chlorine dioxide. Chlorine dioxide substitution would reduce bleaching-related color by 65 percent, to about 20 pounds per ADBT. Hypochlorite (caustic) substi- tution would cut color generated by extraction by at least 70 percent, to about 44 pounds per ADBT. Total color generated, therefore, would be 16,275 Ib/day. After treatment by the existing wastewater treatment plant, the effluent would contain 13,020 lb/day of color - about 2,000 pounds less than the agency's proposed limit. Treatment Many treatment processes, including coagulation, biological treatment, oxidation, activated carbon adsorption and membrane filtration, can remove color generated during pulp and paper manufacturing. Coagulants, such as lime, alum, ferrous sulfate, ferric chloride and polymers, are commercially available and can remove at least 90 percent of color from bleach plant effluent and total mill effluent. Conventional biological treatment processes remove up to 30 percent of Contact us now for a f@ 9085 Marshall Court Westminster, CO 8o FAX For More Information, Circle 38 on Reader Service Card 54 For More Information, Circle 8 on Reader Service Card

5 Massive Full scale International Caustic 290%; 1,000- Not $6.25/ Actual lime Paper Co., extraction total mill ton/d reported ADBT (20,000 Springhill, La. and decker >72% mill ($0.63/ mga) (2/70-8/71) lime Pacific Co., extraction bleached excluding ADBT (1,500- Woodland, and woodyard Kraft pulp, lime kiln ($0.69/ 3,000 mg/l Maine 4.3 mgd 13 of 15 mgd; $6,1 15,259 $2.67/ Actual Kraft and airdried neutral sulfite unbleached ton semichemical ($0.3 1 / 74,214,600 $2.49/ $8.34/ $5.24/ Actual airdried air-dried unbleached unbleached ton ton ($0.22/ Feasibility Total mill 80% 35 mgd $58,703,080 $1.93/ Estimated alum study Caustic extraction Surfactant cetol Bench scale Total mill 95% Not (chemical) Estimated reported $2.27/ color, but they can be enhanced by prior wastewater oxidation using ozone, ultraviolet light or hydrogen peroxide. Ozone, which is commercially available, is the most effective; it removes up to 100 percent of color. Ultraviolet light also can remove up to 100 percent of color, but it requires a hydraulic retention time of 30 minutes. Hydrogen peroxide reduces up to 60 percent of color at dosages of moles per liter and hydraulic retention times of three hours. Powdered activated carbon treatment (PACT) typically removes less than 40 percent of color from unbleached Kraft wastewater and up to 62 percent of color from bleached Kraft wastewater. Biological treatment with white rot fungus (Phneroclmete chrysoporium) can reduce 80 percent of color but is not self-sustaining and, therefore, impractical at this time. Powdered and granular activated carbon, which are commercially available, can remove more than 90 percent of color in caustic extraction stage effluent and total mill effluent. Most pilot studies have involved granular activated carbon (GAC) rather than powdered activated carbon. Powdered activated carbon treatment (PACT) can reduce color in bleached Kraft effluent up to 62 percent, based on a 1,000-mg/L dose. Commercially available mem- brane processes include ultrafiltration, nanofiltration and reverse osmosis, which can remove more than 90 percent, more than 99 percent and 100 percent of color, respectively. AI1 membrane processes require pretreatment by conventional filtration, and the harsh nature and chlorine content of bleaching wastewater can shorten membrane life. Cost Comparison Color treatment processes were compared on the basis of removal efficiency and capital and operating costs (see table). Costs were summarized and converted into 1994 dollars using consumer price indexes. 55

6 atme process Full scale Continental Total mill 95% 13 of 15 mgd; $91 7,289 <$1.34/ Actual polymer - Forest industries Kraft and- ai r-d r ied dissolved (Stone Container], neutral sulfite unbleached ton air flotation Hodge, La. semichemical (<$O. 15/ [ present) pulp 1,500 ai r-dried ton Polymer Pseudomows full scale Proctor & Total mill >40% 925!on, d $267,053/yr Estimated oerugrnosa Gamble Cellulose Ltd., Grand Prairie, Alberta bleached Kraft pulp and paper mill ($0.08/ ozone (30-40 p ozone bleached reported ($0.45/ (70 mg/l; Kraft pulp * 8,750 Ib/d) and paper mill II 78% 16 mgd Not $8,3 d bleached reported ($0. Kraft pulp 1,000 and paper mill Ozone tab scale Chlorination 84% Bleached Not $0.05/ Estimated (5.4 mg/l and caustic Kraft reported * at 20-min extraction pulp mill hlorination 96% (32.4 mg/l and caustic Kraft reported * 3t 20-min extroction pulp mill H RT) Powdered Be <do% Unbleached xtivated carbon treatmen (600-1,000 mg No cost estimates were reported in the literature for ozone treatment processes, so a cost estimate was made using a value of $1.70 per kilogram of ozone generated. If no flow value was reported, a value of 10,000 gallons per ADBT of pulp was used to calculate the cost per lons, because the amount of wastewater generated can range from 5,000 gallons to more than 20,000 gallons per ADBT of pulp. Total mill effluent. According to the literature, PACT is the least expensive treatment for total d effluent, at an operating cost of $0.02 to $0.04 per lons. PACT can be incorporated into existing activated sludge treatment systems, so capital costs are relatively low, depending on the speclfic modifications required. Ozone has the second lowest operating cost, except for Pseudomoms aeruginosn, which, according to the literature, only reduces color by 40 percent. Capital costs are higher than for PACT, because an ozone generator and a contact reactor are required. However, because the reaction time would be 20 minutes or less, only a small reactor is needed. Because of wide variations in reported costs, operating costs for chemical coagulation and GAC adsorption were found to be similar. Coagulation reportedly costs between $0.15 and $2.37 per lons of total mill wastewater, while GAC reportedly costs between $0.29 and $4.59 per lons. However, chemical coagulation has lower capital costs because it can be incorporated into existing primary or secondary clanfiers. Bleach plant effluent. Treating bleach plant effluent separately allows for the treatment of a high 56

7 Powdered activated carbon Bench scale Total mill 32%-62% Bleached Not $0.02- Estimated Kraft mill reported 0.04/ GAC Pilot scale St. Regis Caustic 94% 1.25 mgd $6,035,680 $1 8.79/ Estimated Paper Co., extraction (1,000 ADBT Pensacola, Fla. ADBT) ($15.03/ lime and GAC Bench scale Total mill 96% 500 ton/d $2,538,170 $2.1 1 / Estimated unbleached air-dried Kraft, 5 mgd ton ($O.21/ E Ultrafiltration Pilot scale Caustic 87% extraction Not $1 1.99/ Estimated reported ADBT ($4.54/ Pi1 0 $35,652/d Estimated ($28.52/ ultrafiltration Feasibility study Total mill 80% 35 mgd $272,788,800 $6.95/ Estimated Treatment cost in 1995 dollars. Treatment cost expressed in 1994 dollars, except where noted. HRT = hydraulic retention time. GAC = granular activated carbon. ADBT = airdried bleached ton concentration, low volume of colored wastewater. An ozone system can effectively treat bleach plant effluent less expensively than it can treat total mill effluent. It is the most cost-effective alternative if used prior to biological treatment. The literature review indicates that chemical coagulation is the second least expensive option for treating bleach plant effluent. Two chemical coagulation processes are needed. Acidic chlorination effluent is mixed with enough caustic extraction effluent to adjust the ph to 4.5 before alum addition, rapid mix, flocculation and sedimentation. The remaining caustic effluent is treated using stoichiometric lime addition, rapid mix, flocculation and sedimentation. The two effluents are then combined, ph adjusted (using lime kiln stack gas or sulfuric acid if the combined effluent ph is greater than 9 or sodium hydroxide if ph is less than 6), clarified and discharged into the mill s wastewater treatment plant. Future Studies Additional treatment processes that should be evaluated for color removal from pulp and paper wastewater include anaerobic treatment processes, enzyme pretreatment, polymeric adsorbents, ion exchange resins, electrochemical processes, commercial ultraviolet treatment systems and application of genetically engineered microorganisms. James K. Baird is a staffenvironmental engineer nt the Spring Mill, Pa., facility of Appletotr Papers lnc. (Appleon, Wid, and Brian A. Dempsey is an associate professor of environmental engineering at Penn State University s University Park cnmpiis. 57