Industrial Waste Program

Molybdenum Study 2007-2010

Department of Natural Resources and Parks

Wastewater Treatment Division

Table of Contents 1.0 Background ..............................................................................................................1

2.0 Introduction ..............................................................................................................1 3.0 The fate of molybdenum ..........................................................................................2

3.1 Influent data 3.2 Biosolids data

4.0 Industrial user cooling tower survey ......................................................................2

4.1 Cooling tower data 4.2 Oil/Water separator data

5.0 Industrial user data..................................................................................................3

6.0 Recommendation......................................................................................................3 7.0 Implementation ........................................................................................................4

Prepared by Jim Sifford

December 5, 2011

1.0 Background

Concentrations of regulated metals in King County biosolids are consistently below the most stringent state and federal standards for land application. However, certain standards for molybdenum are under reconsideration by the U.S. Environmental Protection Agency (EPA). Following EPA’s promulgation of Code of Federal Regulations (CFR), Title 40, Part 503, “Standards for the Use or Disposal of Sewage Sludge,” the EPA was petitioned by numerous groups to reconsider the molybdenum limits for biosolids to be land applied, in light of concerns as to the technical basis utilized to develop the limits. In 1994, EPA suspended all molybdenum concentration limits, with the exception of the “ceiling limit” for land application pending further review. The current ceiling limit established for molybdenum in land application is 75 milligrams per kilogram (mg/kg) (40 CFR 503.13). A modified risk assessment to establish new molybdenum standards for land application of biosolids was proposed in 2001. In 2006, EPA notified publicly owned treatment works (POTWs) of its intent to assess the need and appropriate level for a numerical standard for molybdenum in biosolids and suggested a pollutant concentration of 40 mg/kg; however, there has been no announcement to date. New molybdenum limits for biosolids may be re-established by EPA. Depending on the final pollutant concentrations, molybdenum could become the most limiting factor for King County meeting all exceptional quality biosolids standards. With this in mind, the King County Industrial Waste Program (KCIW) has looked at identifying sources and corrective measures necessary to reduce the quantity of molybdenum being discharged into the collection system.

2.0 Introduction

Typical commercial and industrial sources of waste molybdenum include pigments, lubricants, catalysts, fertilizer, and corrosion control chemicals. According to EPA, the amount of molybdenum in wastewater has increased since 1990 as commercial and industrial operations began to replace chromium with less toxic molybdenum, used as a corrosion inhibitor in cooling waters.

Studies throughout the U.S. have identified the use of molybdate-based corrosion control chemicals in building heating and cooling systems as a potentially significant source of molybdenum loadings to wastewater treatment plants. The water contained and circulated in these heating and cooling systems typically contains residual corrosion control chemicals, which is periodically discharged to the municipal sanitary sewer during routine system purging events, “blow downs” or draining of the system, in preparation for scheduled inspection and maintenance activities.

KCIW Molybdenum Study December 5, 2011

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Based on the results of a major industrial user survey in 2006 and sampling data collected in 2007, KCIW has identified the use of molybdate-based corrosion control chemicals in industrial cooling towers as a significant source of molybdenum discharged to King County wastewater treatment plants. These same molybdate-based corrosion chemicals are also widely used in commercial building heating and cooling systems throughout the County.

3.0 The fate of molybdenum

Once molybdenum reaches the treatment plant, about 80 to 90 percent of the molybdenum detected in the influent passes through the treatment process and is discharged in the effluent. It is expected that less than 10 percent is removed and contained in the biosolids.

3.1 Influent data

In 2007, the analytical method detection limit for molybdenum was lowered to 0.026 micrograms per liter (µg/L), allowing for more detectable results of the daily influent concentration of molybdenum. Data from 2007 to 2010, indicates that the average daily influent concentration of molybdenum at West Point Treatment Plant (WPTP) is 7.0 µg/L (5.0 pounds per day) and at South Treatment Plant (STP) is 6.0 µg/L (4.0 pounds per day).

3.2 Biosolids data

Data from 2007 to 2010 shows that the average concentration of molybdenum in biosolids from both WPTP and STP is 10 mg/kg (dry weight basis). These levels are well below the current regulatory ceiling limit of 75 mg/kg established for molybdenum used for land application.

4.0 Industrial user cooling tower survey

Sodium molybdate is used as a corrosion inhibitor in solutions of antifreeze, paints, and finishes and in the treatment of water for industrial cooling towers. The most likely source of molybdenum in wastewater systems has been determined to be from cooling towers used to supplement standard air conditioning and refrigeration systems and not from other industrial contributors.

In 2006, KCIW conducted a limited survey of permitted industrial facilities with cooling towers. KCIW sent the survey to 128 significant industrial users, which included questions about product use, cooling tower size, chemical suppliers, pollution prevention practices, and maintenance programs.

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Thirty-three facilities completed and returned the survey (26 percent). Survey data showed that 22 of these facilities operated cooling towers (67 percent of respondents) and eight of those with cooling towers used corrosion control chemicals containing molybdenum (36 percent of respondents). 4.1 Cooling tower data

In 2007, KCIW selected six of the largest industrial cooling tower systems using molybdate-based corrosion control chemicals from the survey and collected grab samples from their discharge sumps. Results show that the concentration of total molybdenum in these samples ranged from 38 to 19,400 µg/L.

4.3 Oil/Water separator data

Molybdenum disulfide is used in many industrial grade lubricants to significantly reduce wear, friction, and sustain lubrication of machine components.

In August 2006, KCIW collected grab samples from 11 randomly chosen facilities with industrial oil/water separators to see if there was a relationship between molybdenum and used motor oil or petroleum products. The types of facilities sampled included solid waste transfer stations, vehicle washing operations, pressure washing operations, and metal recyclers. Results show that the concentration of total molybdenum in these samples ranged from less than 20 to 2,310 µg/L.

5.0 Industrial user data

KCIW also reviewed process wastewater data of industrial users to determine the contribution of molybdenum discharged to King County sewers by permitted industries. It is estimated that industrial users have the potential to discharge up to two pounds per day of molybdenum to each treatment plant.

6.0 Recommendation

At this time, KCIW has determined that levels of molybdenum would be better controlled through source management strategies and best management practices (BMPs) rather than by developing numeric pretreatment limits.

This nonregulatory strategy would seek the voluntary cooperation of industry representatives, building managers, and product manufacturers to develop BMP programs on their own to reduce the discharge of molybdenum, including replacement of their molybdate-based corrosion control chemicals with molybdate-free alternatives.

KCIW Molybdenum Study December 5, 2011

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Should the pollutant concentration limits for molybdenum in biosolids be revised and become more stringent, KCIW may re-evaluate this voluntary approach and seek other measures to control the levels of molybdenum in our system.

7.0 Implementation

KCIW will make this recommendation available to the public through electronic media sources and publications such as the KCIW Web pages and the KCIW Pretreatment Newsletter.