Molybdenum Content of Water Treatment Residuals

Molybdenum (Mo) content of water treatment residuals (WTRs) was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) to evaluate suitability for land application under state regulatory policies that limit Mo to 18 mg kg-1. Samples of WTR were collected from 32 Pennsylvania facilities that employ aluminum salts, ferric chloride, and/or polymers for coagulation. The mean Mo content of all samples was 3.1 mg kg-1, with 78% having Mo levels <5 mg kg-~. The WTRs from plants using ferric chloride as a coagulant averaged 5.6 mg Mo kg-~, significantly higher (p = 0.02) than the 1.6 mg Mo kg-~ for utilities using alum. Differences were related to coagulant purity: Mo content in liquid ferric chloride was 10.0 mg L-~ but below detection by ICP-AES for alum. The initial sample from one facility, collected from the filter backwash basin, contained 26.4 mg Mo kg 1. Elevated Mo in backwash solids was attributed to filtration capture of extremely fine, Mo-enriched Ai hydrous oxide particles and erosion of anthracite filter media during backwashing. Combined backwash and coagulation solids from this facility’s storage lagoon averaged 6.3 mg Mo kg-1, underscoring the need for consistent sampling procedures. The mean Cu to Mo ratio in these WTRs was >100, well above the minimum dietary ratio (2:1) considered protective of grazing animals. A . recently as 1971, more than 90% of water treat~ment plants used direct discharge into rivers and lakes for disposal of residuals produced during purification of drinking water (American Water Works Association, 1990). As environmental concerns over this practice increased, land-based disposal methods gained popularity as an alternative for WTRs. Potable water supplies and certain treatment chemicals contain potentially toxic trace elements and these constituents must be considered in managing WTRs by land application (Elliott et al., 1990). The Federal Standards for the Use or Disposal of Sewage Sludge (USEPA, 1993), codified as 40 CFR Part 503, provided national rules for recycling biosolids produced in municipal wastewater treatment. While the federal standards specifically exclude WTRs (USEPA, 1996), some states use Part 503 for regulating WTR land application. Prior to the federal rules, state regulations typically imposed limits on heavy metals like Cd, Cu, Ni, Pb, and Zn in land-applied wastes. The inclusion of Mo in Part 503 focused attention on this element and its management in land-based waste disposal and recycling. The major issue with Mo is its potential effect on animals grazed on, or fed, forages from application sites. Relatively small concentrations of Mo in feed can induce Cu deficiency in livestock if the Cu level is below the minimum recommendation of 10 mg kg-1. This Moinduced Cu deficiency is called molybdenosis and is Agricultural and Biological Engineering Dep, Pennsylvania State Univ., University Park, PA 16802. Received 30 Nov. 1999. *Corresponding author ([email protected]). Published in J. Environ. Qual. 29:1835-1839 (2000). generally managed by insuring an adequate Cu to Mo ratio in forage and livestock feeds. The minimum dietary Cu to Mo ratio for cattle and sheep is set at 2:1 (National Research Council, 1996). In addition to monitoring the Cu to Mo ratio in the vegetation, it is important to evaluate this ratio in land-applied materials, since ruminants can consume significant soil material along with forage during grazing (Thornton and Abrahams, 1983). The original Part 503 defined an upper limit of 18 mg Mo kg-~ for exceptional quality biosolids. This was based on assessment of the risk associated with ruminant animals consuming forage grown on biosolids-amended soil. This value came under attack by biosolids producers on the grounds of insufficient supportive research. Litigation followed, and the USEPA rescinded the 40 CFR Part 503 Table 3 Mo value until more field data could be gathered. However, permits issued in Pennsylvania for land application of WTRs contain a ceiling limit of 18 mg Mo kg-1. Since published analyses suggest low (<5 mg kg-1) Mo concentrations in WTRs (Schmitt and Hall, 1975; Elliott and Singer, 1988; Peters and Basta, 1996), sporadic reports from commercial laboratories of WTRs containing 200 to 300 mg Mo kg-~ were puzzling. We could find no published comprehensive study of Mo in WTRs. Therefore, our purpose was to determine Mo concentration in WTRs generated in our state and representative of utilities in the Northeast. The trace element content of WTRs is inextricably related to the purity of coagulant chemicals added during water treatment (Elliott et al., 1990), thus we surveyed utilities using both aluminum and ferric salts. The ultimate goal was to establish the level of Mo in residuals generated at coagulation-filtration water treatment plants and to interpret the findings in the context of land spreading of WTRs. MATERIALS AND METHODS Treatment residuals were collected in March 1998 from lagoons or sedimentation basins at 32 water treatment plants throughout Pennsylvania. As there are no major softening plants in Pennsylvania, all facilities sampled were typical coagulation-filtration plants designed to remove turbidity, pathogens, color, taste-causing, and odor-causing compounds from the water supply and produce potable water (Fig. 1). Water treatment residuals consist primarily of the precipitated hydroxide of the coagulant [e.g., AI(OH)3 for alum] along with the material removed from the raw water (sand, silt, clay, bacteria, color-forming compounds). Various other additives (chlorine, lime, polymers, filter aids, fluoride, corrosion inhibitors, activated carbon) improve process performance or finished water quality, but typically have little influence on WTR quantity or composition. Facilities provided information on coagulant usage, source water, and WTR disposal method. Abbreviations: ICP-AES, inductively coupled plasma atomic emission spectroscopy; PAC, polyaluminum chloride; WTR, water treat-