Chemical, microbial and physical properties of manufactured soils produced by co-composting municipal green waste with coal fly ash
Introduction
In cities in most parts of the developed world municipal green waste, originating from parks and reserves and domestic gardens, is routinely collected separately from other wastes. The material is usually composted (either alone or with other organic wastes) for use as a soil amendment/garden compost (Bradshaw et al., 1996, Manser and Keeling, 1996). However, in Australia the main market for the material is as “manufactured soil” which is used for landscaping purposes in place of natural topsoil. For the latter market the material is either co-composted with inorganic additives (e.g. coal fly ash, sand, subsoil) or these are blended-in after composting (Haynes and Belyaeva, 2009). The inorganic component typically makes up only 10–20% by volume of the final product. Visual observations suggest that the bulk of such manufactured soil is composed of slowly-decomposable woody material which remains after composting. The product is considerably cheaper than excavated natural topsoil and is therefore commonly used by landscaping contractors and landscape divisions of local councils and State Governments. A characteristic of the material is that under field conditions it slowly decomposes and therefore looses its volume over time.
Natural topsoil differs enormously in content from green waste-derived, manufactured soil. Indeed, topsoil is typically composed of 95% mineral matter derived from its parent rock and only about 5% organic matter (Brady and Weil, 2004). The mineral component is dominated by silicate clay minerals while the organic component is mainly composed of semi-stable humic substances. It has been suggested that the use of a greater percentage of inorganic material in green waste-based manufactured soil would result in a more stable product with properties more similar to topsoil (Haynes and Belyaeva, 2009). However, in order to produce an economically viable product, a suitable, freely available, inorganic waste material is required. Coal fly ash may be such a product. The bulk of electricity in Australia is produced in coal-fired power stations and fly ash is the major by-product/waste produced. About 20% is used in cement manufacture, and most of the remainder is disposed of in sites surrounding power stations (Heidrich, 2003). Coal fly ash has been used successfully as a soil amendment to supply essential plant nutrients and boost plant growth (Jala and Goyal, 2006).
The purpose of this study was to investigate the feasibility of co-composting coal fly ash with municipal green waste to produce manufactured soil material as well as to optimize the proportion of fly ash to be mixed with green waste to get best quality compost for landscaping.
Section snippets
Materials and composting
Municipal green waste, originating from the Brisbane City Council, was collected from a pile at Phoenix Power Recyclers, Yatala, Queensland, soon after it had been mechanically shredded. It had a particle size distribution of: >5 mm = 55%, 2–5 mm = 26% and <2 mm = 19%. Recently-deposited fly ash was collected from the fly ash disposal lagoon at Tarong Power Station, 80 km west of Brisbane. Particle size distribution of the ash was: 100–200 μm = 9%, 50–100 μm = 36%, 2–50 μm = 52% and <2 μm = 3%.
The compost treatments
Properties of materials
The green waste had a pH 7.6, EC = 0.98 mS cm−1 and organic C = 316 g kg−1 and corresponding values for coal fly ash were 6.8, 0.73 mS cm−1 and 6.3 g kg−1. Total elemental content of the green waste and fly ash are presented in Table 1. The TCLP leaching procedure showed all measured elements were well below the USEPA standards for classification of a material as an inert substance. Concentrations were: Cu, 0.068 and 0.005; Zn, 0.357 and 0.091; Cr, 0.058 and 0.011; Cd, 0.056 and 0.005; Pb, 0.007 and
Discussion
Municipal green waste is often left in piles prior to shredding and remains in stockpiles prior to transport to the sites of the composting contractors. During these storage periods, much of the “soft” green waste (grass clippings, leaves and green stems) partially decomposes. As a result, composting of the remaining material (without addition of more decomposable organic material) is rapid and a thermophilic stage (⩾50 °C) was only reached for a very short period. This occurs despite the C/N
Conclusions
The bulk of composted green waste consists of woody material which is not decomposed greatly during composting. This gives the product its bulk which enables it to be used as a topsoil substitute. Addition of fly ash to green waste greatly increases the available water holding capacity of the manufactured soil product; an outcome of substantial importance if the product is to be used in field landscaping. Additions of 50% or 75% ash tended to give the product a pasty consistence and 25% or less
Acknowledgements
We thank Paul Vievers of Tarong Energy for supplying the fly ash, Sean Kemble of Phoenix Power Recyclers for supplying the shredded municipal green waste, Katherine Raymont for assistance with C and N analysis and David Appleton of the School of Land, Crop and Food Sciences Analytical Services Group for analysis of metals and mineral N.
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