Abstract
Abstract: Anaerobic digestion (AD) plant is an effective way to treat animal manures, which has been largely applied in China. As the main by-product of biogas, more than 1 billion tons of liquid digestate (i.e., digested slurry) have been produced by AD plants. Applying digestate to land is the most attractive and effective option in terms of environmental issues, because it allows nutrients (such as nitrogen and phosphorus) to be recycled and, hence, increases the organic matter in soils. However, high levels of heavy metals, such as copper (Cu), zinc (Zn), arsenic (As), lead (Pb), chromium (Cr) in swine manure could result in high risk of land application. The AD plant is conservative for heavy metals, which results in high total concentrations of heavy metals in digested slurries. Furthermore, heavy metals may be highly leachable and soluble because of their inorganic forms during AD treatment; they can even shift from stable fractions to more bioavailability/toxic fractions after water addition and decomposition, especially for variable valence metalloid As. In addition, AD treatment effectively degrades the organic matter and causes obvious variations in physical and chemical properties, such as water content, pH value, oxidation reduction potential (ORP) and microbial activities. These changes may influence the chemical fraction of heavy metals, which is a critical factor in predicting their mobility and eco-toxicity (especially for As). However, there was no study dealing with the distribution of heavy metal contents and chemical fractions in digested slurries and follow-up processing steps, e.g. multilevel sedimentation pond, aquatic plant pond, etc., from farm-based AD plant. In this study, we chose an intensive swine farm in southern part of Jiangsu Province as a model to study the changes of total and dissolved contents of Cu, Zn, As, Pb, Cr in primary digested slurry and follow-up processing effluent. The objective of this study was to provide scientific support for safety investigate concentrations of the main heavy metals and their morphologic changes characteristics in wastewater during the processes of treating the wastewater (anaerobic digestion, multistage precipitation, aquatic plant treatment) in an intensive swine farm typical of the region of South Jiangsu in winter and summer. Results showed that the total heavy metal content was increased after anaerobic fermentation. The total contents of Cu, Zn, As, Pb and Cr in the primary digested slurry were 4 024.9, 6 656.0, 22.9, 193.8, 319.6 μg/L, respectively in summer, and 6 490.3, 1 1687.9, 89.3, 152.0, 351.7 μg/L respectively in winter. Except As in summer, all of the heavy metals were above national farmland irrigation water quality standard. The total heavy metal concentrations were significantly decreased after the last treatments. The concentrations of Cu, Zn, As, Pb and Cr from the biological pond were 15.7, 60.7, 8.8, 13.7, 34.8 μg/L in summer, respectively; while those were 10.9, 80.4, 6.0, 13.6, 43.2 μg/L in winter, respectively, which met the national farmland irrigation standard. The multistage precipitation wastewater treatment system could remove total Cu, Zn, As, Pb, Cr with 99.5%, 99.2%, 62.2%, 88.7%, 91.6% in summer and 99.8%, 99.0%, 91.0%, 90.1%, 87.9% in winter, respectively. The changes of dissolved heavy metal concentrations were complex, while in general, multilevel sedimentation combined with aquatic plant treatment could reduce the bio-availability to the heavy metal, while the effects in summer was lower than that in winter. The relationship analysis showed that total and dissolved heavy metals negatively correlated with the pH values of wastewater, except the total As in summer; whereas the positive correlation with EC was found in wastewater. Multilevel sedimentation combined with aquatic plant treatment was a meaningful treatment for the intensive swine farms, which had limited land resources around the farm.
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