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SUITABILITY OF BIOSOLIDS FOR USE AS A TOPSOIL SUBSTITUTE IN URBAN RECLAMATION PROJECTS

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A growing component of the biosolids marketing program of the Metropolitan Water Reclamation District of Greater Chicago (District) is the use of biosolids as a substitute for the more expensive topsoil used in local urban reclamation projects. The success of using biosolids as a topsoil substitute depends on their ability to enhance the fertility and/or physical properties of the soil, and sustaining the growth of plants in a biosolids-soil matrix in which biosolids constitute a significant portion. For this reason the District has initiated a research program to compare the properties of District's biosolids with that of the typical topsoil available in the Chicago metropolitan area, and determine whether the biosolids produced at the District can be used in lieu of topsoil. The germination and growth of various species of turf grasses, forage legumes, wild flowers, and prairie grasses were studied in biosolids and topsoil.

The texture of biosolids was determined to be silty loam and silty clay or silty clay loam for topsoil, indicating that biosolids would provide better tilth for plant growth. The organic matter content of biosolids (approximately 38 percent volatile solids) was higher than in topsoil (ranging from 0.5 to 10.0 percent volatile solids). In addition, the cation exchange capacity, total N content, available P content, water soluble species and NO3-N content, and soluble salt content were higher in biosolids than in topsoil. Moisture content at field capacity was 85 percent (g water/ g dry solids) for biosolids and 27 percent for topsoil.

The mean electrical conductivity of saturated paste extracts of air-dried District biosolids was 9.6 dS/m as compared to 1–3 dS/m for topsoil. The predominant cationic species in anaerobically digested sludge is ammonium, while the predominant anionic species is bicarbonate (alkalinity). However, the predominant cationic species in the air-dried biosolids derived from the District's sludge processing train (SPT) remained to be ammonium, whereas the predominant anionic species were found to be sulfate. Volatile solids, TKN, NH4-N, and soluble salt content destruction through the SPT was found to be 41.7, 64.5, 81.6, and 93.7 percent, respectively. In spite of the significant reduction in soluble salt content through the SPT, air-dried biosolids contain significantly higher levels of soluble salts and NH4-N than topsoil. Hence, the germination and growth of turf grass and native plant species in biosolids were also evaluated.

In a green house study, twenty varieties of turf grass were screened for their growth rate in biosolids. Under simulated irrigated conditions, growth rates for perennial ryegrass, tall fescue, red fescue, alkaligrass, Kentucky bluegrass, bentgrass, and red top were greater in biosolids than in topsoil. Under conditions simulating mild drought, growth rates for perennial ryegrass, tall fescue, red fescues, and alkaligrass were greater in biosolids than in topsoil. The growth rates were lower for Kentucky bluegrass, bentgrass, and red top in biosolids than in topsoil.

The concentrations of all essential nutrient elements were found to be sufficient for turf grasses grown in biosolids. Leaves of turf grasses grown on biosolids were found to have significantly higher P, Ca and Mg content than leaves of turf grasses grown on fertilized topsoil, indicating that biosolids have superior nutrient supplying power than fertilized topsoil.

The suitability of biosolids as a substitute for topsoil was also determined for the growth of turfgrass, forage legumes, wildflowers, and prairie grasses. Over fifty turf grass varieties, representing 16 species; ten forage grass varieties representing 6 species; ten species of forage legumes; and over 30 species of wild flowers and prairie grasses were screened for their ability to germinate in biosolids and topsoil. In general, biosolids were observed to exert a mild inhibition of germination in many species, presumably due to higher soluble salt content initially present in biosolids as compared to topsoil. However, this soluble salt effect is expected to be transient and short lived in the field due to nitrification of ammonium and dissipation of soluble salts. Species of turf, forage, prairie grass and wild flowers were identified that germinated as well or better in biosolids than in topsoil despite the higher level of soluble salts.
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Document Type: Research Article

Publication date: 2001-01-01

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