Water Treatment Sludge: Potential for Use as a Soil Ameliorant
This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.
Report No UWRAA 146
This report outlines the investigations conducted between 1993 and 1996 to identify the potential for water treatment sludges, which are subject to stringent EPA guidelines with respect to handling and disposal, to be used as a soil ameliorant in preference to dumping it in municipal landfill sites. Before the sludge can be utilised in an unregulated manner, however, it was necessary to establish whether it had any physical or chemical properties that might pose an environmental health hazard when released. While there was no evidence in the literature of any such problems, much of the literature was from overseas, and thus applicable to different soils and environmental conditions. It was considered necessary to establish up-to-date local evidence to govern the disposal of sludges generated in Australia. The four objectives of the project were therefore to 1) determine the magnitude of soluble aluminium in alum sludge in comparison with naturally occurring soils, 2) determine the magnitude of soluble aluminium in an acidic soil at the Waite Research Institute with and without sludge and lime in various proportions, 3) determine the effects of adding sludge to soils in different proportions on the establishment and growth of lawn grasses, and 4) evaluate the utility of sludge as an ingredient in commercial potting mixes.
In all laboratory, glasshouse and field experiments, the sludge was found to possess no serious detrimental properties, and in particular no soluble aluminium by comparison to other natural soils in Australia. On the contrary, a number of beneficial properties were identified, including low bulk density, high infiltration rate, plenty of available nitrogen, a neutral to alkaline pH, and a modest calcium carbonate equivalence. When mixed as a potting ingredient with sand and other materials, the sludge imparted favourable properties when compared with commercially available mixes. Plant yields from sludge and from sludge-based mixes were as high or higher than those in just soil. The main agronomic limitation of the sludge was its large P-fixing capacity, which necessitated large quantities of added P-fertiliser. From an environmental viewpoint, however, this could be regarded as an advantage, as the sludge might be used to remove P from contaminated water sources; further research to quantify its effectiveness is being planned. In short, once the sludge has been air dried, its disposal need not be stringently regulated and can be safely used in many ways, only a few of which have been identified here (eg. to raise soil pH of acidic soils, or as a potting mix ingredient).