Abstract
Mixture of fly ash (FA) and sewage sludge (SS) has been used as substrates to plant vegetables and turf grasses. Most studies chemically analyzed physiochemical properties of leachates from FA/SS mixtures, but did not considered the effects of plants growing on leaching properties, and the toxicity of leachates on aquatic animals were not well understood. In the present study, FA and SS were mixed in different ratios and then used to cultivate Zoysia matrella and Cynodon dactylon. After one month, these artificial soils were irrigated with water and leachates were collected for three months. Chemical analyses revealed that, although the heavy metal concentrations of FA and SS almost conformed to the relevant national standards, the levels of Cr, Zn, Cd, Pb and Cu in leachates still exceeded the limits in the Chinese Standard of Surface Water Quality, displaying potential environmental hazards. The effects of leachates on reproduction of Daphnia magna were evaluated and the results showed that leachates from high proportion of FA or SS would both negatively affect D. magna, demonstrating an environmental risk. However, leachates from mixture with FA: SS ratio from 1: 2 to 1: 6 did not significantly affect D. magna. Overall, the present results suggested that applications of FA/SS mixtures as artificial soil potentially contaminated aquatic environments and should be monitored carefully.
Highlights
Treatment of sewage sludge (SS) is a major problem worldwide, which is produced in large amount daily
2.5 Statistical analysis Data were expressed as mean ± standard deviation (SD), and were analyzed using one-way analysis of variance (ANOVA) and SNK tests to detect differences among treatments
Compared with the threshed values in national standards (Table 2), the concentrations of heavy metals in SS were lower than the limited standards in USA (EPA 503 Biosolids Rule, 1993), EEC (86/278/EEC, 2000) and China (GB 4284-84) of SS for agricultural application, expect that the concentration of Zn (840 mg/kg) was little higher than the limited value (500 mg/kg)
Summary
Treatment of sewage sludge (SS) is a major problem worldwide, which is produced in large amount daily. Mixture of FA and SS would contain both organic matters and essential elements, which is a suitable artificial soil for plant growth. One shortcoming of FA/SS artificial soil in agriculture is the high level of heavy metals [3,4,5] and organic pollutants [6]. Chemical analysis alone could not accurately evaluate the toxicity of leachate, since potential interactions (synergistic or antagonistic effects) might exist among various heavy metals and organic pollutants [14]. The concentration of heavy metals in leachate and the effects of leachate on reproduction of the aquatic organism Daphnia magna were investigated. These esult would be useful to environmentally evaluate the application of FA/SS mixture as artificial soil
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