USE OF BIOSOLIDS AS A TOP SOIL SUBSTITUTE IN URBAN MARKETS – THE MWRDGC EXPERIENCE
The Metropolitan Water Reclamation District of Greater Chicago (District) has a dynamic biosolids marketing program dedicated to cost effective beneficial use. A growing component of this program is the use of biosolids in local urban reclamation projects. In most of these projects,
the demand for the biosolids arises from their ability to substitute for expensive topsoil or for other soil substitutes such as composts.
The use of biosolids traditionally involves low to moderate application rates for the purpose of fertilizing agricultural crops or turf, or conditioning
soil at reclamation sites. In this context, the biosolids act to enhance the fertility and/or physical properties of the soil, which constitutes the bulk of the matrix in which plants are subsequently grown.
However, when biosolids are utilized in urban reclamation projects, they are
typically utilized as fill and to cover less suitable soil materials to provide a root zone for vegetation. This includes covering the impermeable layer in the final closure of landfills, covering compacted clays or fill materials that may be strewn with stones and rocks, or covering steel
mill slags at brownfield sites prior to establishment of golf courses, parks, athletic fields, or natural habitat.
Unlike traditional land application, in these innovative urban uses the biosolids, rather than soil, make up the bulk of the matrix in which plants are grown. For this reason
the District has initiated a research program to examine and demonstrate the suitability of biosolids to substitute for topsoil as a medium for plant growth. This paper presents some of the results of this program.
Agronomic properties of biosolids are compatible with its use as a topsoil
in the Chicago metropolitan area. The texture of District biosolids, silt loam, is superior to that of typical top soil, silty clay or silty clay loam, for providing better tilth for plant growth. As expected, organic matter content is higher in biosolids (approximately 38 percent volatile
solids) 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 NH4-N and NO3-N content, and soluble salt content are 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.
Major challenges to the use of biosolids as a soil substitute were identified and discussed including regulatory challenges, production of biosolids with favorable chemical and
physical properties, and the need to find agronomic innovations to overcome limitations due to excess nutrients and soluble salts at some sites.
Mean electrical conductivity of saturated paste extracts of air-dried District biosolids was 9.6 dS/m as compared to 1 to 3 dS/m for topsoil
indicating that biosolids have much higher soluble salt content than typical top soils. The predominant cationic species in District biosolids following anaerobic digestion is ammonium, while the predominant anionic species is alkalinity (bicarbonate). However, following centrifuging, lagoon
aging and air-drying the predominant cationic species remained ammonium while the predominant anionic species became sulfate. The District's standard processing train was demonstrated to reduce volatile solids content by 41.7 percent, TKN content by 64.5 percent, NH3-N content
by 81.6 percent and soluble salt content by 93.7 percent following anaerobic digestion. Because the biosolids contain significantly higher levels of soluble salts than topsoil, even after reductions due to processing, and because ammonium is the predominant cationic species, biosolids were
also evaluated for their ability to support germination and growth of turf grass and native plant species.
Twenty varieties of turf grass were screened, in a greenhouse trial, for their growth rate in biosolids. Under simulated irrigated conditions, growth rates in biosolids for perennial
ryegrass, tall fescue, red fescues, alkaligrass, Kentucky bluegrass, bentgrass, and red top were greater in biosolids than their growth rates in topsoil. Under conditions simulating mild drought, growth rates in biosolids for perennial ryegrass, tall fescue, red fescues, and alkaligrass were
greater than their growth rates in topsoil; and growth rates of Kentucky bluegrass, bentgrass, and red top in biosolids were less than their growth rates 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.
Another component of our assessment of the suitability
of biosolids as a substitute for topsoil was the screening 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 as compared with 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 as
in topsoil despite the higher level of soluble salts.
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