Alternative Technology Section Toxic Substances Control Division California Department of Health Services

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INTRODUCTION

The Hazardous Waste Management Act of 1986 (SB 1500) requires the Department of Health Services (Department) to prohibit, by May 8, 1990, the land disposal of untreated hazardous wastes, and to adopt treatment standards establishing the level of treatment required prior to land disposal. Since the federal RCRA wastes are subject to federal prohibitions or treatment standards adopted by the federal Environmental Protection Agency (EPA), the Department is focusing its efforts on developing treatment standards for non-RCRA, California-only hazardous wastes.

Existing State regulations prohibit land disposal of certain types of hazardous wastes. These restricted wastes include the following liquid wastes: with a pH less than 2, high concentrations of heavy metals, cyanides, polychlorinated biphenyls and halogenated organic compounds containing wastes. Several federal solvent and dioxin containing wastes, and certain First Third and Second Third scheduled wastes are also restricted from land disposal by the Hazardous and Solid Waste Amendments of 1984 to RCRA.

Existing State regulations (Section 66699, Title 22, California Code of Regulations) provide the hazardous waste criteria for 18 toxic metal constituents by establishing the Soluble Threshold Limit Concentration (STLC) values. The metal-containing aqueous waste group includes ten metal constituents that are not regulated by EPA as characteristic wastes (Table 1). The Department believes that aqueous wastes with non-RCRA metals are the major portions of the California-only wastes. According to 1987 manifest data, 65,000 tons of metal-containing aqueous wastes were disposed of offsite. Of the 65,000 tons, the Department estimates that 27,000 tons are non-RCRA wastes.

The RCRA metal-containing aqueous wastes include characteristic wastes (D wastes) and the listed wastes (K and F wastes). The characteristic wastes are those containing any of the eight RCRA metals above the concentrations (EP Toxicity levels) identified in the Code of Federal Regulations (40 CFR Part 261.24). The F and K wastes are source-specific wastes and do not require any minimum metal concentration to be a RCRA hazardous waste. Table 1 identifies the RCRA waste codes that have aqueous wastes with metals, and associated effective dates for land disposal restrictions.

The Deparcmenc is proposing treatment standards for non-RCRA metal-containing aqueous waste. Information on the availability and effectiveness of amenable treatment technologies was collected from the technical literature, surveys of technology vendors and affected industry, EPA studies, and field visits.

The steps employed in developing the treatment standards for metal-containing aqueous wastes included the following:

- Assess waste volumes and affected industries.

- characterize the wastes by their chemical compositions.

- Identify waste treatability groups.

- Assess demonstrated treatment technologies and collect associated treatment data.

- Derive the Feasible Treatment Levels for each constituent, based on existing treatment data.

- Determine proposed treatment standards.

Our findings on waste treatability groups, technology assessment and derivation of proposed treatment standards are presented below.

Table 1: The RCRA Metal-Containing Aqueous Wastes

Waste Category

Characteristic Waste

RCRA Waste Codes

D004 D005 D006 D007 D008 D009 DO 10 DO 10

Description

Arsenic

Barium

Cadmium

Chromium (VI)

Lead

Mercury

Selenium

Silver

EPA's Land

Ban Effective Dates

1990 1990 1990 1990 1990 1990 1990 1990

Listed Waste K062 Spent pickle liquor August 8, 1988

generated by steel finishing operations of plants that produce iron or steel

F007 Spent cyanide plating July 8, 1989

bath solutions from electroplating opera-operations .

F009 Spent stripping and July 8, 1989

cleaning bath solutions from electroplating operations where cyanides are used in the process.

Sources: (1) 51 FR 19300 dated May 28, 1986; Schedule for Land Disposal Restrictions; Final Rule.

(2) 53 FR 31138 dated August 17, 1988; Land Disposal Restrictions for First Third Scheduled Wastes; Final Rule.

(3) 54 FR 26648 dated June 23, 1989; Land Disposal Restrictions for Second Third Scheduled Wastes; Final Rule.

WASTE TREATABILITY GROUPS

All metals except hexavalent chromium can be directly precipitated and removed from an aqueous media. Hexavalent chromium must be reduced to the trivalent state prior to metal removal because hexavalent chromium does not form a precipitate.

1. Liquid Wastes Containing Metals Other Than Chromium (VI).

Liquid wastes containing metals other than chromium (VI) require direct precipitation. After precipitation, the metal precipitates are separated by gravity settling, clarification, and/or filtration. The sludges are subsequently dewatered before disposal.

When the metals exist in the complexed form, it is necessary to break up the complexes and transform the metals to "free" form amenable to chemical precipitation. Metal complexes consist of a central metal ion surrounded by a group of other organic or inorganic ions or molecules. Examples of complexing molecules are ammonia, citrates, and ethylene diamine tetra acetic acid (EDTA).

Wastes containing complexing molecules such as EDTA can be treated by a coprecipitation with ferrous sulfate, ferrous chloride or dithiocarbamate, which is used in conjunction with the regular precipitant, such as sodium-hydroxide. The treatment scheme requires two reaction vessels. Sulfide precipitation can be also used for complexed metals treatment.

Wastes that do not contain complexed metals are precipitated by the regular précipitants such as lime, sodium hydroxide, or sodium carbonate.

Other waste characteristics affecting precipitation performance are the presence of other metals in the waste, oil and grease or surfactants. The presence of other metals is important because the optimum pH giving minimum solubilities of metal hydroxides or sulfides is different depending on the metal (Figure 1) . The presence of oil and grease or surfactants in the waste affects settling characteristics of the sludges by creating emulsions requiring a long settling time. Removal of these constituents (by adding a demulsifier and skimming oil from the liquid surface prior to the chemical precipitation step) should eliminate this problem.

High influent concentrations do not adversely affect treatment since the residual metal concentrations depend on the solubilities of metal precipitates. However, high metal concentrations often indicate that the metals are complexed in solution and complexed metal compounds, if not dissociated, could have an adverse effect on treatment as mentioned above.

2. Liquid Wastes Containing Hexavalent Chromium

Liquid wastes containing hexavalent chromium require reduction of chromium to the trivalent state prior to metal removal. Commonly used reducing agents are sodium metabisulfite, sulfur dioxide, ferrous sulfide, and other ferrous ions (ferrous sulfate, ferrous chloride, or electrochemically generated ferrous ion). All of these reagents create some form of chromium sludge, which must be separated and dewatered before disposal.

FIGURE 1 Solubility of Metal Hydroxides and Sulfides as a Function of pH. (Source: EPA publication, EPA-600/2-82-011C, 1981)

Precipitation Separate Substances

DEMONSTRATED TREATMENT TECHNOLOGIES

The Department evaluated information on treatment technologies collected from the literature, a survey of treatment technology vendors, site visits to selected treatment facilities in California, and an industry survey. There are two major groups of demonstrated technologies for aqueous wastes with metals: chemical precipitation, and chemical or electrochemical reduction.

1. Chemical Precipitation

Chemical precipitation refers to both the primary steps of forming the insoluble metal precipitates from dissolved metal ions by the precipitant(s), and follow-up operations that separate the solid precipitates from the liquid.

There are two treatability groups of dissolved metals for chemical precipitation, complexed and non-complexed metals. Non-complexed metals can be removed by a direct precipitation with such a chemical as lime (Ca(OH)j), caustic (NaOH), sodium sulfide (Na2S), ferrous sulfide (FeS), or sodium carbonate (Na2COj). Complexed metals require coprecipitation with ferrous sulfate (FeSO^), ferrous chloride (FeCl2), or sodium dimethyl dithiocarbamate (DTC) in addition to a regular precipitant such as caustic or lime. Electrochemically generated ferrous ion is also effective in removing a wide variety of heavy metals, including hexavalent chromium.

(1) Lime or Caustic Precipitation

Lime or caustic precipitation is the most commonly used method for treating aqueous wastes with non-complexed metals. Alkaline reagents such as lime, or caustic raises the pH of the wastewater and causes metals to precipitate out of the solution as metal hydroxides. For many metals there is a specific pH at which the metal hydroxide is least soluble, as shown in Figure 1. Because several metals co-exist in a waste in most cases, it is not possible to operate a treatment system at a single pH value that is optimum for all metal removals. As a compromise, a pH between 9.5 and 12.0 is maintained for typical mixed metals removal. A typical precipitation reaction is as follows:

(2) Sulfide Precipitation

Two sulfide precipitation methods currently used are the soluble sulfide precipitation (SSP) and insoluble sulfide precipitation (ISP) processes. Soluble sulfides and insoluble sulfides are used to precipitate dissolved metals as metal sulfides. Metal sulfides have lower solubilities than the hydroxides, resulting in lower residual metal concentrations in the treated water (Figure 1). Also, sulfides can be used to precipitate complexed metals in the presence of complexing agents such as ammonia, citrate and EDTA and over a broader pH range than possible for the hydroxide precipitation.J A typical sulfide precipitation reaction is as follows:

(3) Coprecipitation with Ferrous Sulfate, Ferrous Chloride or Dithiocarbamate

Wastewaters containing complexed metals with a strong complexing agent such as EDTA, ammonia, or citrates require a two step precipitation for the metal removal. A continuous process using ferrous sulfate or ferrous chloride is as follows:

Step 1: Acidification of the wastewater is needed in the first vessel to break up the complexes and transform the metals to a "free" form. When the pH of the waste is lowered to about two or less, in the case of EDTA - metal complexes, the bond between EDTA and the metal is broken and the free acid of EDTA is formed as follows:

To prevent recomplexation of EDTA, a reducing agent such as ferrous sulfate or ferrous chloride is added in the first vessel at the low pH. A reduction-oxidation reaction takes place between ferrous ion and the metal and the free metal ions are reduced to their lowest balance state where they are least likely to recomplex. The ferrous ion is oxidized to ferric ion which, in turn, forms a ferric-EDTA complex of low toxicity.5

Step 2: In the second reaction vessel, the pH is raised with caustic or lime. As the pH is increased, the free metals in the water precipitate as metal hydroxides.

The ferrous ion is a reducing agent, therefore, ferrous sulfate or ferrous chloride coprecipitation method can reduce hexavalent chromium to the trivalent state and subsequently precipitate it along with other metals.

In case of the dithiocarbamate scheme, the pH in the first vessel is raised by lime or caustic and all non-complexed metals precipitate as metal hydroxides. Dithiocarbamate is added in the second vessel at the same high pH, where the complexes are broken up and the rest of the dissolved metals precipitate.11

2. Chemical or Electrochemical Reduction

As discussed, the aqueous waste with hexavalent chromium requires reduction of chromium to the trivalent state prior to metal removal because hexavalent chromium does not form a precipitate. Demonstrated reducing agents are sodium metabisulfite (NajS205), sulfur dioxide (SOj) , ferrous sulfide (FeS), and other ferrous ion (ferrous sulfate, ferrous chloride, or electrochemically generated ferrous ion). The treatment processes using these are described below.

(1) Reduction With Sodium Metabisulfite or sulfur dioxide

Sodium metabisulfite or bisulfite is a commonly used chemical for chromium reduction. The metabisulfite hydrolyzes to sodium bisulfite, and the bisulfite in turn dissociates to sulfurous acid, which reduces the hexavalent chromium at a pH of 2-3.

3Na2S2 05 + 3HZ0 - SNaHSOj

4H2Cr04 + 6NaHS03 + 6H2S04 - 2Cr2(S04)3 + 6NaHS04 +10H20

Sulfur dioxide is another commonly used chemical for chromium reduction. The reduction occurs when sulfurous acid, produced by the reaction of sulfur dioxide and water, reacts with chromic acid as follows:

3S02 + 3H20 - 3H2SOj

(2) Reduction and precipitation with ferrous sulfate or ferrous sulfide

Ferrous sulfate (FeS04) or ferrous sulfide (FeS) reduces and precipitates hexavalent chromium in one step. This eliminates the two-step reduction and precipitation required when sulfur dioxide or sodium bisulfite is used.

(3) Electrochemical Reduction Unit

A patented electrochemical unit that uses sacrificial iron electrodes to generate the ferrous ion is effective in removing hexavalent chromium as well as other heavy metals. In the electrochemical cell, a direct current is conducted through the cell containing a number of carbon steel plate electrodes. This generates the ferrous ion (Fe**) and hydroxyl ion (OH"). The ferrous ion reduces hexavalent chromium to the trivalent state as follows:

Na2Cr207 + 6 Fe(OH)2 + 7H20 - 2Cr(OH)3 + 6Fe(0H)3 + 2NaOH

Metals other than chromium are removed through a process of adsorption and coprecipitation within insoluble ferrous ion matrix that is formed from the electrodes.38 Since the ferrous ion is a good chelate breaker, the electrochemical unit is also applicable for removal of complexed metals.

FEASIBLE TREATMENT LEVELS AND PROPOSED TREATMENT STANDARDS

The Department collected data on treatment of the aqueous wastes with metals from six major sources; the Department's analysis of samples collected from operating facilities, responses from the Department's Industry Survey, EPA evaluation of commercial facilities, EPA's pilot test evaluations, EPA's Development Documents for Effluent Limitations Guidelines, and the literature. Table 2 summarizes information on the treatment data sources used for the treatment levels calculations.

The Department has established feasible treatment levels ("levels of treatment which are achievable using the best demonstrated available technologies") for 13 regulated metals (Table 3). The feasible treatment levels are lower than or equal to the STLC values. Since the aqueous waste with any of these metals below the STLC level is not a hazardous waste, the Department is proposing treatment standards for these 13 metals at the STLC levels. The Department has determined that the remaining five metals are also treatable below the STLC levels, even though the feasible treatment levels could not be established because of insufficient treatment data.

EPA's treatment standards identified in Table 3 have lower concentrations than our feasible levels. This is because EPA's standards are for the source specific wastes, where as the Department's feasible levels apply to all metal-containing aqueous wastes.

The proposed treatment standards for metal-containing aqueous wastes are summarized in Table 4. The Department is proposing an immediate effective date, after the adoption of the regulations based on a finding that there is sufficient offsite commercial treatment capacity in California.

Tabic 2: Sunmary of Treatment Oata Sources

Manufacturing Treatment Technology Data Data Source

Procès» Type Ref.No. Description

Electronic o Na0H-FeS04 copreclpl-Copponents tatfon, settling l>

filter press; batch unit for complexed metals.

o NaOK-FeSC>4 coprecipi-tat ion, settling 1 filter press; batch unit for high concentration non-conplexed metals.

o Na0H-fe$04 coprecipi-tatlon, ultrafiltration & filter press: continuous unit for low concentration metals.

•Industry Survey, Site Visit

Industry Survey, Site Visit o NaOH precipitation, clarification i filter press o Electrochemical metals reductI on

F H Industry Survey

F 1$ Oata Senary

Metal o Chemical precipitation F 16 EPA's Dev. Doc.

Finishing 1 clarlfIcatlon o Lime precipitation F 20 Electroplating t CAC Dev. Doc o Ferric chloride pre- F 20 Electroplating clpltatlon & CAC Dev. Doc.

o FeS precipitation, P

clarification t Ion Exchange o Chrome reduction by S02 F

o Na2S205 reduction 1 F FeS04 - hydroxide coprecipitat Ion; batch unit

22 EPA Test Run

35 Site Visit

37 Industry Survey

MotejF Füll Scale Oata P • Pilot Scale Data B ■ Bench Scale Data

Page 1 of 3

Natals Removed

As Ba Cd Cr(VI) Pb Hg Se Ag Sb Be Cr(!II) Co Cu Mo Ni Th V Zn

Capaci ty Conplexed t/yr, (gpm) Metals

65800 (30)

3,364,550 (1530) 2000 gal/batch

Data from various plants

Data fron various plants

Manufacturing Treatment Technology Data Data Source

Process Type Ref.No. Description

Manufacturing Treatment Technology Data Data Source

Process Type Ref.No. Description

AS

Ba

Cd

Cr(VI)

Metal Finishing

o Electrochemical reduction, clarification li fiIter press

7

Technical Paper

X

o Electrochemical redjction, clarification & filter press

17

Technical Paper

X

o Electrochenical reduction, clarification I fiIter press

15

Data Suimary

X

X

o Na2s-Fes04 Coprecipl-tation, clarification, & filtration

P

24

Technical Report

X

CamercUt TSDs

o Llme-SulfIde Precipitation 1 vacuus filtration

25

EPA's Sanpling 1 Analysis at Envlrite

X

X

o Lime precipitation I CAC

26

EPA's Sampling A Analysis at Frontier Chemical

X

o NaOK-Alua Copreclplta-tlon i clarification

27

Case Study in a Book

X

X

X

Non-ferrous Metala

o Ma2S*Lime precipitation o Line precipitation

6, 33 20

Tech. Paper EPA's Dev. Doc.

X X

X X

o Llme*Na2S precipitation

B

34

Paper

X

X

Note:F • Fut I Scale Data

CD 00

Page 2 of 3

Metels Removed Pb Hg Se Ag Sb 8e Cr(lïl) Co Cu Ho Hl Th V Zn

Capacity Cooplexed Notes t/yr, (gps) Metsls

Osta fron 4 plants

Data fron 5 elec-troless plating plants

Data from various plants

Receives hard-to-treat wastes

*2 Feed punp capacity

Manufacturing Treatment Technology Data Data Source

Procesa Type Ref.No. Description

lead Battery

0

Ferrite copreclpitat Ion

F

9

EPA'a Dey.

Doc.

0

Hydroxide precipitation

F

9

EPA'a Oev.

Doc.

0

Ume-FeS precipitation

F

9

EPA'a Dev.

Ooc.

o

Lime-Na2S precipitation

F

9

EPA'a Dev.

Doc.

o

Limc-MaOH precipitation

F

9

EPA'a Dev.

Doc.

Zinc Battery

o

Lime precipitation I filtration

F

9

EPA'a Dev.

Doc.

o

Sulfide precipitation

F

9

EPA'a Dev.

Doc.

Steel making, EA.F Furnace

o 0

Hydroxide precipitation Carbonate precipitation

P P

23 23

Paper Paper

0

FeS precipitation

P

23

Paper

Galvanizing

0

time-Fe2S precipitation I filtration

P

32

Paper

Photo Processing

o

Ma2S precipitation I filtration

P

31

Paper

Chem Cleaning

0

Hydroxide precipitation

F

36

Comnents

letter

Pesticide Mfg. Runoff Water

0

Electrochemical Reduction/Removal

P

16

Paper

Pilot/Lab Test

o

Electrochemical reduction/removal

B

18

Paper

0

Electrochemical reduction/removeI

B

19

Paper

0

Insoluble Starch Xantate precipitation

B

29

Paper

0

MgO precipitation

B

30

Paper

o

Electrochemical reduction/removal

P

38

Commenta

letter

Mote:F > Füll Scale Data P « PIlot Scale Data B - Bench Scale Data

Page 3 of 3

Ketala Removed

Capacity Conplexed t/yr. (gpm) Hetala

30 gal. reaction Vessel

Batch Unit Batch unit

CD CD

TABLE 3: FEASIBLE TREATMENT LEVELS FOR AQUEOUS WASTES WITH METALS

EPA's Feasible Treat.

Metals mg/1 mg/1 mg/1

Arsenic

5

.0

4.0

Barium

100

.0

1.00

Cadmium

1

.0

0.70

Chromium(VI)

5

.0

0.6 hexavalent

0.32,

total

2.4 total

Lead

5

.0

0,

.04

0.5

Mercury

0

.2

0.

.03

0.06

Selenium

1

.0

1.0

Silver

5

.0

0.8

Antimony

15

.0

See

Note (2)

Beryllium

0.75

See

Note (3)

Chromium(III)

560.

.0

See Chromium (VI)

Cobalt

80.

.0

1.6

Copper

25.

.0

2.6

Molybdenum

350,

.0

See

Note (2)

Nickel

20.

.0

0.

44

13.4

Thallium

7.

.0

See

Note (3)

Vanadium

24.

.0

See

Note (2)

Zinc

250.

.0

4.1

Notes: (1) These concentrations are the compilation of treatment standards for various "listed" wastes, K062, K071, F007, F008, F009, F011, F012, P029 and P074.

(2) The treatment data are insufficient to establish a feasible treatment level. The constituent, however, should be treatable below its STLC value.

(3) Staff has no treatment data. The constituent should be treatable below its STLC value, based on solubility of the metal precipitate.

TABLE 4: Proposed Treatment Standards for metal-contalnlne aqueous wastes Metals Proposed Treatment Standards, mp/1

Antimony

15,

.0

Arsenic

5.

.0

Barium

100.

.0

Beryllium

0,

.75

Cadmium

1.

.0

Chromium (VI)

5.

.0

Chromium (III)

560.

.0

Cobalt

80.

.0

Copper

25,

.0

Lead

5,

.0

Mercury

0,

.2

Molybdenum

350,

.0

Nickel

20,

.0

Selenium

1,

.0

Sliver

5,

.0

Thallium

7,

.0

Vanadium

24,

.0

Zinc

250,

1. U.S. EPA (1981), Treatability Manual Volume_UJL_Technologies_for

Control/Removal of Pollutants. EPA-600/2-82-001c, Washington, D.C.

2. Staff Report on proposed Treatment Standards for metal-containing aqueous wastes. California Department of Health Services, September, 1988.

3. U.S. EPA (1986), Hazardous Waste Treatment Technology, EPA/600/0-86/006, Cincinnati, Ohio.

4. U.S. EPA (August, 1987), Notice of Availability and Request for Comments.

5. U.S. EPA (July, 1986), Facility Test Report for Chemical Processors, Inc., Cincinnati, Ohio.

6. Bhattacharyya, D.C., Sund-Hagelberg, K. Schwitzgebel, G.M. Blythe, and F.B. Craig, (1980), "Removal of Heavy Metals, Arsenic, and Fluoride from Smelter Effluents by Sulfide-Lime Precipitation," Proceedings of the Industrial Wastes Symposia, Las Vegas, N.V.

7. G. Horner and J. Duffey (1983), "Electrochemical Removal of Heavy Metals from Wastewater," American Electroplaters Society Seminar, Indianapolis, Indiana.

8. Deleted.

9. U.S. EPA (1984), Development Document for Effluent Limitations Guidelines for the Battery Manufacturing Category, Volume I, Washington, D.C.

10. Deleted.

11. Memtek Corp. (1987), Response to the Treatment Technology Survey by the Department of Health Services, State of California.

12. Hewlett Packard Company (1988), A Response to the Questionnaire by the Department of Health Services, State of California.

13. Deleted.

14. CTS Electronics Corp. (1988), A Response to the Industry Survey by the Department of Health Services, State of California.

15. Andco Environmental Processes, Inc., "Andco Heavy Metal Removal Systems, Actual Performance Results."

16. U.S. Environmental Protection Agency, Development Document for Effluent Limitations Guidelines and Standards for the Metal Finishing Point Source Category. EPA 440/1-83/091. June 1983.

17. Scull, G. , Uhrich, K. , "Electrochemical Removal of Heavy Metals in the Presence of Chelating Agents," Inhouse Report, Andco Environmental Processes, Inc.

18. Duffey, J., Gale, S., and Bruckenstein, S., "Electrochemical Removal of Chromates and Other Metals," Cooling Towers, pp. 44-50.

19. Andco Environmental Processes, Inc., "Laboratory Test Results on Electrochemical Metal Removal."

20. U.S. EPA (1979), Development Document for Existing Source Pretreatment Standards for the Electroplating Point Source Category, EPA 440/1-79/003.

21. Sampling and Analysis by the Department of Health Services; Site Visit to Hewlett Packard Company.

22. Grosse, D (1987), "Treatment of Aqueous Metal-Bearing Hazardous Wastes," 8th AES/EPA Conference on Pollution Control for the Metal Finishing Industry.

23. Brantner, K. , and Cichon, E. , (1981) "Heavy Metals Removal; Comparison of Alternative Precipitation Processes." Proc 13th Mid-Atlantic Industrial Waste Conf., 13:43-50.

24. U.S. Army Belvoir Research & Development Center (1984), "Pilot Plant Demonstration of a Sulfide Precipitation Process for Metal Finishing Wastewater Treatment."

25. U.S. Environmental Protection Agency, Office of Solid Waste. Onsite Engineering Report of Treatment Technology Performance and Operation for Envirite Corporation. Prepared for EPA under EPA Contract No. 68-01-7053, December 1986.

26. U.S. Environmental Protection Agency, Office of Research and Development. Facility Test Report for Frontier Chemical Waste Process, Inc., Prepared by Metcalf & Eddy, Inc., under EPA Contract No. 68-03-3166. November 1985.

27. Martin, E., and Johnson, J., (1987), Hazardous Waste Management Engineering, pp. 214-218, Van Nostrand Reinhold Co., NY.

28. U.S. Environmental Protection Agency, Development Document for Effluent Limitations Guidelines and Standards for the Nonferrous Metals Point Source Category, Volume III. EPA-440/1-83/019-6. March 1983.

29. Wing, R. , and Rayford, W. (1976), Starch-Based Products Effective in Heavy Metal Removal, Proc. 31st Ind. Waste Conf., Purdue Univ., pp. 1068-1079.

30. Schiller, J.E., and S.E. Khalafalla (1984) Magnesium Oxide for Improved Heavy Metals Removal. Mining Eni;. . Vol. 36, No. 2, pp. 171-173.

31. LaPerle, R.L. (1976) The Removal of Metals from Photographic Effluent by Sodium Sulfide Precipitation. Soc. Motion Picture and Television Eng. J. , Vol. 85, No. 4, pp. 206-216.

32. Fender, R.G., A MacGregor, and K.E. Patterson, "Sulfide Precipitation Investigation and System Design for Zinc-Laden Foundry Wastewater," Proc. 14th Mid-Atlantic Industrial Waste Conf.. 14: 268-277, (1982).

33. Bhattacharyya, D. , A Jumavan, G. Sun and K. Schwitzebel, "Precipitation of Sulfide: Bench-Scale and Full-Scale Experimental Results," AICHE Symposium Series. Water-1980. 2Z(209): 31-42, (1981).

34. Bhattacharyya, D., A.B. Jumawan, and R.B. Grieves, "Separation of Toxic Heavy Metals by Sulfide Precipitation," Sep. Sei. Technol.. 14: 441-452, (1979).

35. McClellan Air Force Base (1988), "Draft Operation Plan, Industrial Waste Treatment Plant 2."

36. Dowell Schlumberger (1988), Comments Letter on the Department of Health Services' Draft Staff Report.

37. Embee Plating (1988), A Response to the Industry Survey by the Department of Health Services.

38. Andco Environmental Processes, Inc., (1988), Comments Letter on the Department of Health Services' Draft Staff Report.

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