Phosphorus adsorption/desorption of water treatment residuals and biosolids co-application effects

Abstract

Alum [Al2(SO4)3•14H2O] is commonly used in the municipal water treatment process to destabilize colloids for subsequent flocculation and water clarification. Water treatment residuals (WTR) can be classified as a waste material from these treatment plants. Concerns over land application of WTR are due to its postulated reduction of plant available P and potential plant Al toxicity with increasing WTR rates. Co-application of WTR with biosolids may benefit municipalities with biosolids inherently high in P concentrations and in terms of a cost savings by landfill avoidance. In a greenhouse study, I investigated the efficacy of co-application of WTR and biosolids to the native shortgrass steppe species blue grama (Bouteloua gracilis H.B.K. Lag) and western wheatgrass (Pascopyrum smithii (Rydb.) A. Love) using a factorial and a randomized complete block design. In a laboratory study, I studied the WTR's biosolids-borne P adsorbing capacity and the P adsorbing mechanism using batch studies, x-ray diffraction analysis (XRD), and electron microprobe analysis using wavelength dispersive spectroscopy (EMPA-WDS). The greenhouse factorial study showed that increasing WTR rate, averaged over biosolids rate, resulted in a decrease in blue grama P concentration, an increase in Al concentration, and a decrease in western wheatgrass P and Mo concentrations. Increasing biosolids rate, averaged over WTR rates, significantly affected most blue grama and western wheatgrass constituents. With only WTR addition (no biosolids), I observed an increase in blue grama Al concentration and uptake, a decrease in Mo concentration, and a decrease in western wheatgrass Mo concentration and uptake. The randomized complete block design greenhouse study showed a positive linear relationship between increasing WTR rate and yield and a negative linear relationship with shoot P and Al concentration with blue grama. With western wheatgrass, increasing WTR rate produced a negative quadratic effect on shoot Al concentration (p<0.10). The adsorption study indicated that co-mixing WTR and biosolids at ratios of 8:1 will adsorb all soluble biosolids P. The batch experiments suggested solid octacalcium phosphate formation as the P adsorbing mechanism. The XRD and EMPA-WDS results suggest that surface P chemisorption as an amorphous surface mineral phase might occur.