FACTORS INFLUENCING MOLYBDENUM CONCENTRATION IN WATER TREATMENT RESIDUALS

A survey of the molybdenum (Mo) concentration in water treatment residuals WTR throughout Pennsylvania was conducted to establish the level of Mo in residuals generated at water treatment plants and interpret the findings in the context of land spreading of WTR. The average Mo value of 32 WTR samples was found to be 3.7 ppm, with 78% of the samples having Mo concentrations less than 5 ppm Mo. The findings of this study suggest that, in general, Mo levels in WTR will be well below the state regulatory land application limit of 18 ppm. However, Mo was present at concentrations above 10 ppm for three of the samples analyzed, with a maximum Mo value of 26.4 ppm. These results imply that the potential exists for Mo to limit land application of WTR in Pennsylvania. Coagulant type was found to be a primary factor influencing the Mo content of WTR. WTR from plants using ferric chloride averaged 5.6 ppm Mo, which was significantly higher (p = 0.05) than the 1.6 ppm Mo average for the WTR from plants using alum. This was a reflection of the Mo content of the coagulants since ferric chloride was found to have 10.0 ppm Mo while the Mo levels in alum were below detection limits for most samples.WTR from the Clarion Water Treatment Plant, were found to contain the highest Mo levels (26.4 ppm). A detailed study of this facility was undertaken to identify the source of this high Mo. Solids generated during coagulation contained 5.5 ppm Mo, whereas filter backwash solids contained 44.2 ppm Mo. This high Mo level is thought to be due to a combination of factors: (1) an elevated Mo level (7.6 ppm) in the river sediments, (2) enrichment of Mo on the surface of finer particles captured during filtration, and (3) erosion of anthracite coal media containing Mo during backwashing. This research indicates that several factors influence the Mo content of WTR. In the plants studied, treatment chemicals, water sources, and treatment processes were all found influence the Mo concentration in WTR.