Loading capacity of sewage sludge for forestry application in Chinese provincial capital cities

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 tha-1y-1 in 31 PCCs, while the variations ranged from 7 to 91 tha-1y-1 among different PCCs. The priority control thresholds (Smins) of 1/2 PCCs were higher than 30 tha-1y-1 (CJ-T 362-2011). The Smin values of Lanzhou, Tianjin, Hohhot, Shanghai, and Yinchuan were above 55 tha-1y-1, but Smin values of Kunming and Changsha were below 10 tha-1y-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 ty-1, which was 125 times higher than the total quantity of the dry SS (404 x 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.