Biosolids Decomposition after Surface Applications in West Texas

ABSTRACT In a semiarid environment, climate is a critical factor in the decomposition of surface‐applied biosolids. This study examined the effect of 2‐ to 7‐yr exposure times on the composition of single applications of New York, NY biosolids in western Texas. Exposure time effects on organic matter, N, P, S, Cu, Cr, Pb, Hg, and Zn were studied near Sierra Blanca, TX. Due to organic matter decomposition, total organic C decreased from 340 g kg−1 in fresh biosolids to 180 g kg−1 in biosolids after 82 mo of exposure, whereas the inorganic ash content of the biosolids increased from 339 to 600 g kg−1 Total N decreased from 50 to 10 g N kg−1 and total S decreased from 12 to 6 g S kg−1 Bicarbonate‐available P in the biosolids decreased from 0.9 to 0.2 g kg−1 Successive H2O extractions yielded soluble P concentrations consistent with dicalcium phosphate (dical) for fresh biosolids and tricalcium phosphate (trical) for biosolids exposed for 59 months or more. Sparingly soluble phosphates, such as dical and trical, potentially yield >0.5 mg P L−1 in runoff waters for extended periods after biosolids applications, especially after multiple applications. Selective dissolution of the biosolids indicated that as much as 66 to 78% of P exists as iron phosphates, 16 to 21% as Fe oxides, and 5 to 12% as insoluble Ca phosphates. Chemical analyses of ash samples suggest that Cu and Zn have been lost from biosolids through leaching or runoff and no losses of Pb, Cr, or Hg have occurred since application.