Utilization of coal fly ash and municipal sewage sludge in agriculture and for reconstruction of soils in disturbed lands: results of case studies from Greece and China
Coal fly ash (CFA) and municipal sewage sludge (MSS) management is a great concern worldwide. An alternative gaining high interest, is their use in agriculture and for reclamation of degraded lands. The purpose of this paper was to present very briefly the results of some case studies carried out in China and Greece related to land reclamation and agricultural use of CFA and MSS separately or combined. An experiment in Platanoulia area, central Greece showed clearly that CFA applied together with MSS at appropriate rates increased substantially wheat grain and biomass yield and improved soil quality (increased soil pH, organic matter content, total nitrogen, available P and boron). In a long-term experiment carried out in Huaibei city, Anhui province, China with a reconstructed soil in a subsided land by using CFA, it was found that physicochemical characteristics (infiltration rate, bulk density, total nitrogen, available P and extractable K) tended to be improved over time. In another experiment in Rodia area central Greece, MSS application improved soil quality of limestone mining spoils from bauxite mining activities. Several other experiments with MSS in Greece showed a clear positive effect on cotton and maize yield and on soil quality.
KeywordsCoal fly ash Municipal sewage sludge Soil quality Land reclamation
Coal fly ash (CFA) is the finer ash produced in coal-fired power station, collected by using electrostatic precipitators. It comprises about 85% of the total ash produced. In terms of their potential to neutralize acidity, fly ashes can be broadly grouped into two classes: a) class C (produced by lignite and sub-bituminous coal) which is high in CaO content (> 20%), alkaline, and contains crystalline compounds, but low in silica and b) class F (produced by bituminous and anthracite) with modest content of CaO (< 20%), lower proportions of crystalline components, and pozzolanic properties. Since the total amount of CFA produced worldwide annually is huge, amounting to about 750 million ton (Shaheen et al. 2014), its safe management is of high importance. Some properties of CFA both physical and chemical are interesting from an agronomic point of view and thus CFA in some cases may be used as a soil amendment (Yunusa et al. 2012). Municipal sewage sludge (MSS) is the byproduct (liquid or solid) resulted from the biological treatment of domestic wastes. Based on its management MSS may be considered as “waste” and it is land filled or incinerated or as a byproduct and may be applied in the soils. In the later case MSS is also called biosolid.
Coal fly ash utilization options include wasteland reclamation (for improving nutrients’ conditions and promoting revegetation of disturbed/degraded lands), agriculture (for improving soil pH, aeration, percolation, providing nutrients, and enhancing growth and yield), waste stabilization (by increasing adsorption of metals, color from waste water, by reducing of pathogens and odor from sludge, and by neutralizing of acid mine drainage) and bricks construction, supplement of cements/concrete, roads and embankments construction, and production of new products such as zeolites, ceramic filters etc. (Malik and Thapliyal 2009). CFA use in agriculture has no in all cases beneficial effects. Constraints related to agricultural use of CFA are referred to the content of inert material that may reduce water holding capacity, and the usually high heavy metal content (Malik and Thapliyal 2009), and organic pollutants (Sahu et al. 2009).
The properties of MSS which are significant when we use it in agriculture as soil amendment are physical (solid content, organic matter content), chemical (pH, soluble salts, plant nutrients, trace elements, and organic chemicals) and biological (pathogens such as viruses, bacteria, protozoa, helminthes etc.).
The purpose of the present paper is to summarize the experimental results of some case studies carried out in Greece and China, aiming at using CFA and MSS as amendments of agricultural soils, for reclamation of disturbed land and reconstruction of soils in disturbed land due to mining activities.
2.1 Some research results of CFA and MSS use in soil and land of Greece and China
2.1.1 Concurrent use of CFA and MSS
2.2 CFA use as filling material for reconstructed soils in subsided coal mined land
In the frame of the same research, a laboratory experiment was conducted to investigate the possibility of toxic substances leaching. In plastic columns 50 × 70 cm, various mixtures of CFA and soil and CFA and coal wastes from Pingdingshan, Henan Province, China, were put and flushed with deionized water. In the effluents, heavy metal concentrations were determined with ICP. The results showed that among the strongest leached elements were B, K, Mo, Cr and P, As, Cu, Ni, Fe, Co, Cd, and Pb. In another experiment aiming at the investigation of Mycorrhizae (G. mosseae και G. Versiforme), effects in reconstructed soils using CFA and a calcareous soil from Pingdingshan, Henan Province was mixed with CFA in different ratios and planted with corn. After 8 weeks, the plants were harvested and biomass yield as well soil properties were determined. The results showed that Mycorrhizae enhanced plant growth obviously due to the increase of nutrients release (Tsadilas 2003).
2.3 MSS use for reclaiming a mined land
Similar results of the influence of MSS application in mined lands were reported by others. Brofas and Varelides (1997) used sewage sludge for improvement of calcareous bauxite mining spoils with application of 0, 40, 80 and 120 tones dry weight per hectare. They found that significant increase of organic matter, N, P, Ca, K, Mg, and CEC resulted from MSS application. In addition, pH decreased and heavy metals Pb, Cu, Zn and Mn increased, but their concentrations were within the range of natural soils occurrence. Water holding capacity and available spoil moisture were significantly increased. The number of the plants per square meter, above ground biomass yield and canopy cover were also significantly improved by MSS application. Brofas et al. (2000), in a similar field experiment on calcareous bauxite mine spoils with dried aerobically digested sewage sludge applied at seven rates (0, 10, 20, 40, 60, 80 and 120 Mg/ha), found that MSS significantly increased the available water capacity, concentrations of organic matter, total N, extractable P (Olsen), exchangeable Mg2+, and DTPA-extractable Cu, Mn, Zn, and Pb of mine spoils. Total N and extractable P concentrations decreased over time after sludge application. Plant biomass, plant density, and foliar cover significantly increased with treatment rates in the first and fourth growing seasons but decreased over time. Fiddleneck and burner were the species favored by the high rates of sludge application.
2.4 MSS use as soil amendment of agricultural soils
Coal fly ash and municipal sewage sludge separately or together may be used for reclamation of disturbed lands due to mining activities and as soil amendments in agricultural soils. This management looks to be among the most beneficial and environmentally sustainable strategies.
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