Abstract
The application of sewage sludge (SS) in forestry is considered a viable option. However, the long-term application of SS potentially leads to metal accumulation, posing an environmental risk. Understanding the loading capacity of SS for forestry application is therefore of great significance. We used data from published studies and statistical bulletins across 31 provincial capital cities (PCCs) in China to calculate the loading capacity (LC) of SS for forestry application for each PCC. The results are as follows: (1) the mean value of the priority control threshold was 33 t·ha−1·y−1 in 31 PCCs, while the variations ranged from 7 to 91 t·ha−1·y−1 among different PCCs. The priority control thresholds (Smins) of 1/2 PCCs were higher than 30 t·ha−1·y−1 (CJ-T 362-2011). The Smin values of Lanzhou, Tianjin, Hohhot, Shanghai, and Yinchuan were above 55 t·ha−1·y−1, but Smin values of Kunming and Changsha were below 10 t·ha−1·y−1. (2) Cd was the priority control metal in most of the PCCs (27/31), with the exception of Shanghai and Guangzhou (Cu), Beijing (Hg), and Tianjin (Zn). (3) The total loading capacity was 507 million t·y−1, which was 125 times higher than the total quantity of the dry SS (404 × 104 t) for the 31 PCCs. Our results have important practical significance for the use of urban sludge forest land in China and suggest that SS disposal policies need to be tailored to specific regions. We provide a scientific basis to guide the development of national and provincial forestry policies.
Highlights
Sewage sludge (SS) is a residue of domestic and industrial wastewater treatment [1,2,3], and global sewage production exceeded 300 billion t·y−1 in 2010 [4,5]
Most of the provincial capital cities (PCCs) with lower Cd levels were located in industrial zones of such as Beijing, Shanghai, Tianjin, Guangzhou, Harbin, Changchun, Zhengzhou, and Chengdu
The Cd concentrations in Xi’an, Shijiazhuang, Kunming, and Hangzhou ranged from 6.0 to 7.5 mg·kg−1; PCCs with Cd concentrations from 4.5 to 6 mg·kg−1 and 3 to 4.5 mg·kg−1 accounted for one-third of all PCCs
Summary
Sewage sludge (SS) is a residue of domestic and industrial wastewater treatment [1,2,3], and global sewage production exceeded 300 billion t·y−1 in 2010 [4,5]. In another study, dried urban sewage sludge residues were applied to a sugar maple (Acer saccharum Marsh.) and yellow birch (Betula alleghaniensis Britton) forest in Quebec at a dose of 38.6 3 t ha−1·y−1, and Zn, Cd, Cu, and Pb concentrations were below the detection limits [14] These studies have shown that the leaching of major and trace elements into the soil is generally low at low SS quantities [11,12,13,14], most likely because a high percentage of elements may be retained in the humus layer by metal complexation to the surface of organic matter [15,16,17,18]. Sewage sludge-based Bacillus thuringiensis production, followed by its use in forest and agro-crops for pest control, is an acceptable disposal practice [28]
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