Aging Effects on Reactivity of an Aluminum-Based Drinking-Water Treatment Residual.

Several studies have shown that drinking-water treatment residuals (WTR) could be used to control mobility of excess phosphorus (P) and other oxyanions in poorly sorbing soils. Presently, only “aged” WTRs (those left or manipulated to dewater) are land applied. However, when demand for WTR increase in the near future, freshly-generated WTRs could possibly be considered for land application. To our knowledge, few studies have examined the reactivity and equilibration time of freshly-generated alum-based WTR (Al-WTR). A laboratory thermal incubation study was therefore conducted to determine the extractable Al forms in Al-WTR as a function of WTR “age”, and the time required for freshly generated Al-WTR to stabilize. Freshly-generated Al-WTR samples were collected directly from the discharge pumps of a drinking-water treatment plant, and thermally incubated at 52 ^oC either with or without moisture control, for ≤24 wk. Additional Al-WTR samples of various ages (2 wk- to 2 y old) were included in the study. Various methods of extractable Al [total-, oxalate (200 and 5 mM), and Mehlich 1 extractants]  were utilized to assess Al extractability over time. Freshly-generated Al-WTR samples were potentially more reactive (greater 5 mM oxalate extractable Al concentration) than dewatered Al-WTR samples stockpiled for ≥ 6 mo. Aluminum reactivity of the freshly-generated Al-WTR decreased with time. At least 6 wk of thermal incubation (corresponding to ≥ 6 mo of field drying) was required to stabilize the most reactive Al form (5 mM oxalate extractable Al concentration) of the Al-WTR. Although no adverse Al-WTR effects have been reported on plants and grazing animals (apparently because of low availability of free Al³⁺ in Al-WTR), only dewatered (≥ 6 mo old) Al-WTRs should be land applied to minimize overall potential ecological Al risk.