Land co-applications of Alum-Based Drinking Water Treatment Residuals (Al-WTRs) and biosolids: Effects on heavy metals bioavailability and bioaccessibility

Two Lysimeter experiments were conducted in Egypt to: explore possible effects of land-applying Al-WTRs and /or biosolids on the environment, and recommends ways to minimize human and animal impacts. The specific objectives were to (1) determine the co-application effects on Diethylene Triamine Penta Acetic acid (DTPA)-extractable heavy metals in relation to their accumulation in plant, (2) assess the effectiveness of WTRs in reducing bioavailability of heavy metals in the soils amended with different rates of biosolids, and (3) quantify the optimum application ratio of WTRs to biosolids in relation to the reduction of plant metal accumulation. Thus, in these lysimeter experiments, the WTRs and biosolids were obtained twice in 1999 and 2008. The used soil was classified as Typic torrifluvent. Treatments in both experiments consisted of the combination of WTRs (0, 20, 40, 80, and 160 Mg.ha-1) and biosolids (0, 25, and 50 Mg.ha-1, DW) by fixing one rate of biosolids and with varying the rate of WTRs. The results showed that land application of biosolids increases the accumulation of toxic metals in corn tissues in slightly alkaline soils. However, WTRs-application of (20, 40, 80 and 160 Mg.ha-1) to the soil amended with (0, 25 and 50 Mg.ha-1) of biosolids decreases significantly the DTPA-extractable metal concentrations. The reduction in DTPA-extractable metals resulting from the application of WTRs to biosolid-amended soils can be explained by formation of metal-sulfate, low solubility product, and the floc-adsorption and the co-precipitation processes, in which the formation of a mixed solid phase by the incorporation of metal ions into the crystal lattice of another precipitating solid phase is expected. The combined studies clearly demonstrate that Al-WTRs should have no negative impacts on the environment when appropriate rates are land applied. Thus, Al-WTRs are safe soil amendments to control heavy metals contamination in soil and water bodies.