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^-1. The WTRs from plants using ferric chloride as a coagulant averaged 5.6 mg Mo kg^-1, significantly higher (p = 0.02) than the 1.6 mg Mo kg^-1 for utilities using alum. Differences were related to coagulant purity: Mo content in liquid ferric chloride was 10.0 mg L^-1 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 Al 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.