Chemically-Treated Composted Biosolids Enhance Water Conservation and Quality on Urban Landscapes

Water conservation is a priority in San Antonio, Austin, Dallas, and other major Texas cities that routinely face water shortages and drought. Soil management is fundamental to water conservation efforts. Incorporation of composted biosolids (CB) in low quality soils can enhance water conservation and provide organic carbon and nutrients that improve growth of vegetation and limit sediment loss (McCoy 1998). The CB produced by Texas cities could be a valuable soil amendment for water conservation on drought-prone landscapes. For example, the San Antonio Water System (SAWS) requires a 10-cm depth of topsoil on constructed soils to enable survival of established turf grasses for up to 60 d without rain or irrigation. Yet, not one of 25 turfgrass species established on a 10-cm depth of topsoil survived a 60-d summer period without rain or irrigation during 2006 (Chalmers et al., 2006). Incorporation of a volume-based rate of CB could enhance water infiltration and storage and turfgrass persistence in the 10-cm soil depth mandated under SAWS guidelines. Schnell et al (2007) demonstrated up to 53% greater turfgrass coverage of the surface during establishment and 49% greater soil water content at harvest for sod grown in soil mixed with CB (0.25 m m) than for sod grown in soil alone. In addition, the volumebased rate of CB reduced soil bulk density 34% and sod weight 19% at harvest compared to sod without CB. The contribution of CB to improvements in water conservation and soil properties can be offset by potential impacts on water quality. Volume-based CB rates can increase extractable P concentration in soil, which contributes to increased concentration and mass loss of dissolved P in runoff and drainage from CB-amended soils (Schnell et al., 2007). The challenge is to develop and evaluate practices that will immobilize P in CB and preclude detrimental effects of volume based CB rates on water quality. Both CB and soil chemical properties need to be considered during CB management. Increasing clay concentration increases potential sorption of dissolved P incorporated in soil with CB. In an effort to further limit dissolved P loss from CB amended soil, Alum [Al2 (SO4)3 · 18 H2O], calcium hydroxide [Ca(OH)2], and other chemical agents have been incorporated with CB byproducts of anaerobically-digested sludge to immobilize P and NH4-N (Huang and Shenker, 2004). Yet, the ratio of Alum or Ca(OH)2 in CB must be managed carefully to limit P solubility without large changes in pH. In addition to managing the CB source, application sites must be evaluated to achieve potential benefits to soil and water conservation while minimizing detrimental impacts on water quality.