Abstract
Sewage sludge and livestock (chicken, swine and cattle) manure samples were collected from the Yanmenguan Cattle Herbivorous Livestock Area to compare the potential ecological and human health risks caused by heavy metals contained in them. In this study, the Class II level of Quality Control of Imported Organic Fertilizers is selected as the limit standard value of heavy metals. Based on the mean content values, no heavy metal in cattle manure was higher than the limit standard value; the content of Cu in swine manure was higher than the limit of Cu; the content of Zn in sewage sludge, chicken manure and swine manure were all higher than the limit of Zn; and the content of Cr in sewage sludge and chicken manure were all higher than the limit of Cr. Results indicated that sewage sludge and livestock manure all had high contents of Zn, Cu and Cr. The mean pollution index (PI) suggested that Cu, Zn, As and Cr in sewage sludge and livestock manures all induced potential ecological risks. According to the mean Nemerow’s synthetic pollution index (PN) values, swine manure had the highest potential ecological risk for agricultural use. Daily exposure to Cu, Zn and Cr was higher than other heavy metals from sewage sludge and livestock manures, and heavy metal exposure was always higher for children than adults, with ingestion as the main pathway. Non-carcinogenic risk was caused mainly by Cu and Cr, based on the higher hazard quotient (HQ) values for adults and children. There was no non-carcinogenic risk for all people, except exposure of Cu from swine manure for children, which was 1.76 times higher than the threshold value of 1. According to the mean hazard index (HI) values, only swine manure had a non-carcinogenic risk for children. As the carcinogenic risk index (Risk) values were continuously greater for As than Cd, As had a higher carcinogenic risk than Cd. There was no carcinogenic risk for any single heavy metal, although As exposure from sewage sludge was found to have an inapparent carcinogenic risk for both adults and children. Regarding the RISK value, sewage sludge had an unacceptable carcinogenic risk for adults and children, and swine manure had an unacceptable risk for children only. In general, for both non-carcinogenic and carcinogenic risks, ingestion was the main pathway, and children were more sensitive than adults. Comparing the four kinds of organic waste, cattle manure was the safest for agricultural use in terms of ecological and human health risks. In multiple comparisons, swine manure was significantly different regarding potential ecological risk and non-carcinogenic risk, and sewage sludge was significantly different regarding carcinogenic risk.
Highlights
Sewage sludge and livestock manure are rich in nutrients, such as organic matter, nitrogen, phosphorus, and potassium, which are necessary for plant growth and improved crop yields [1,2]
The results show that the potential ecological risk of sewage sludge and livestock manure mainly involved Zn and Cr
The mean pollution index (PN) value for swine manure was higher than three, indicating that pig manure had heavy pollution for agricultural use; the mean PN values for sewage sludge and chicken manure were higher than two, showing that these had moderate pollution; and for cattle manure, the value was lower than 0.7, showing that it was safe for land use. These results suggested that sewage sludge, chicken manure and swine manure all had dangerous levels of heavy metals and should undergo more scrutiny for agricultural use
Summary
Sewage sludge and livestock manure are rich in nutrients, such as organic matter, nitrogen, phosphorus, and potassium, which are necessary for plant growth and improved crop yields [1,2]. In addition to the advantages of the nutritious substances contained in sewage sludge and livestock manure that can increase plant yield and improve soil properties, the toxic materials they contain, especially heavy metals, enter into the soil [6,8,9,10]. Due to its non-biodegradability, persistence and toxicity, the agricultural use of sewage sludge and livestock manure causes heavy metal pollution of the soil environment [14,15,16,17]. Heavy metals in sewage sludge and livestock manure will enter into the soil, and other ecosystems, such as the atmosphere, groundwater, surface water and biosphere, as all ecosystems are mutually connected [1,18,19,20]. Heavy metal has become a critical factor in the agricultural use of sewage sludge and livestock manure [22]
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