Components of sludge from municipal wastewater treatment plants and its evaluation for land application in JiUJiang city of China

Abstract

Municipal sewage sludge has a potential for widespread application on land, but the implementation of new standards for the agricultural use of sludge in China support a cautious approach. The organic matter, nutrients, and heavy metals in sewage sludge from four municipal wastewater treatment plants in Jiujiang City of China were continuously monitored. The feasibility for its land use was analyzed from three aspects such as sludge nutrients, heavy metal pollution levels, and potential ecological risks in soil. The average content of Cd, Cu, Zn, Cr, Ni, Pb, Hg and As in the sludge of Jiujiang City are 1.85, 305.25, 764.48, 170.57, 42.45, 42.27, 0.59, 28.79 mg∙kg-1, which are less than or close to mean value of heavy metals in China (except As), and all meet the minimum standards of heavy metal content in the sludge according to Chinese Quality of sludge from municipal wastewater treatment plant (GB24188-2009). The maximum content of Cu, Zn, Cr, and As exceed the control standards of pollutants in sludge for agricultural use, and the maximum value of Cu and Cr are close to the limit of landscaping standards. The average contents of organic matter, total nitrogen, total phosphorus, total potassium, and total nutrients in sludge are 385.16, 34.14, 11.05, 7.87, 53.06 g∙kg-1, respectively. The organic matter content meets the limit value (no less than 200 g∙kg-1) of Chinese Control standards of pollutants in sludge for agricultural use (GB 4284-2018), and the total nutrients content meets certain content limit (no less than 40 g∙kg-1) of total nutrients in China’s organic fertilizer. The single-factor pollution index and Nemerow’s synthetic pollution index methods were used to assess the pollution of heavy metals. The heavy metal pollution indexes (PI) are in descending order: Cd > Cu > Zn > Cr > As > Ni > Pb > Hg. The PI shows that Cd and Cu are in strong pollution level, Zn is in moderate pollution level, and As, Ni, Pb, Hg are in absolute safe level. The potential ecological risk levels of heavy metal are ranked in the following order: Cd> Cu> As> Ni> Zn> Cr > Pb. The monomial potential ecological risk coefficients reveal that Cd is in extremely strong ecological risk, Cu is in medium ecological risk and other heavy metals are in slight ecological risk. The results indicate that Cd and Cu are the main contributors to the pollution and potential ecological risks of heavy metals in soil, to which more attention should be paid during sludge disposal. Being rich in organic matter and inorganic plant nutrients，sewage sludge in Jiujiang City may be widely used on soil, but availability of potential toxic metals exceeding the limit value often restricts its uses. Before being harmlessly treated, the sludge in Jiujiang City should not be used on agricultural land and may be applied with highly caution on landscaping and land improvement, but it can be used as cover materials for co-landfilling.