In this study, phosphogypsum (PG) and polyferric sulfate (PFS) composite containers were proposed to facilitate treatment of stocked sludge. Experiments were conducted to optimize the dosage of PFS and phosphogypsum. The results showed that the combined containers had good effects on deep dewatering and heavy metals stabilization. PFS promotes the transformation from bound water to free water through adsorption bridging and electrical neutralization, while PG plays an important role in the construction of skeleton structure in the process of pressure filtration. Therefore, the combined containers of PFS and PG have broad application prospects for stocked sludge treatment.
With rapid economic development and increasing demand for domestic and industrial water, sweage discharge is increasing year by year in China. The number of urban sewage treatment plants has increased from 35 in 1978 to 10113 in 2020. Sludge is the by-product of biochemical treatment of sewage. The daily average sludge production reached 97500 tons/day in 2020 and will still maintain a high growth rate in the future (Wu, 2016, Liu, 2012, Feng, 2015). Heavy metals in sludge mainly come from industrial sewage. Most of heavy metals in sewage are transferred to sludge through microbial adsorption and inorganic salt precipitation (Lester, 1983). Above 80 percent of heavy metals are adsorbed in the gel group, and the rest is soluble or other forms in sludge.
Landfill has the advantages of large disposal capacity and low operation cost, and accounts for 60% of sludge disposal capacity in China (Kim, 2016, Li, 2014, Luo, 2018). However, leachate and landfill gas produced by stocked sludge with high moisture content will have a serious impact on the surrounding ecosystem. In particular, heavy metals hidden in the stocked sludge have high toxicity, easy migration and high bioaccumulation, which will accumulate in soil and biological chain and threaten human health (Zhu, 2009, Zhang, 2014, Chen, 2015). Therefore, the stabilization of heavy metals during the process of deep dewatering of stocked sludge is the premise of subsequent disposal, and it also has important practical significance.
