Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering ConferenceShanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Insight into deep dewatering of stocked sludge using composite conditionersof PFS and phosphogypsum: the mechanism and the fate of heavy metals
Xuyuan Zhao1, Zhichao Shen1, Xiaodan Mao1*, Jifu Yin1, Feixin Wang1
1National Engineering Research Center of Dredging Technology And Equipment, Shanghai, China

biological chain and threaten human health (Zhu, 2009, Zhang, 2014,Chen, 2015). Therefore, the stabilization of heavy metals during theprocess of deep dewatering of stocked sludge is the premise ofsubsequent disposal , and it also has important practical significance.

ABSTRACTIn this study, phosphogypsum (PG) and polyferric sulfate (PFS)composite containers were proposed to facilitate treatment of stockedsludge. Experiments were conducted to optimize the dosage of PFS andphosphogypsum. 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 throughadsorption bridging and electrical neutralization, while PG plays animportant role in the construction of skeleton structure in the process ofpressure filtration. Therefore, the combined containers of PFS and PG
have broad application prospects for stocked sludge treatment.

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 andlandfill 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 andbiological chain and threaten human health (Zhu, 2009, Zhang, 2014,Chen, 2015). Therefore, the stabilization of heavy metals during theprocess of deep dewatering of stocked sludge is the premise of
subsequent disposal , and it also has important practical significance.

KEY WORDS: Stocked sludge; Deep dewatering; Composite
conditioners; Heavy metals stabilization.

Researchers have been focusing on the research of deep sludge
dewatering and the stabilization of heavy metals. They put forward
many prospective research methods such as such as physicochemicaltreatment, hydrothermal treatment, ultrasonic treatment and biologicaltreatment (Chen, 2016, Cao, 2016, Saha, 2011), to enhance dehydration
performance by destroying the structure of sludge extracellularpolymerase and releasing the intracellular bound water. Compared withother methods, chemical conditioning method is widely used becauseof its engineering economy and because it is easy to implement. Atpresent, the research shows that fly ash, lime, biomass and other solid
waste materials can reduce the exchangeable content of heavy metal insludge and have strong binding capacity to heavy metals. In addition,these solid waste materials can form a skeleton for sludge dewatering.
PG is the waste residue of phosphate fertilizer and phosphoric acidindustry. There have been a lot of international reports on the study andapplication of PG as soil improver, but few studies have focused on therole of PG in sludge dewatering and heavy metal stabilization.

INTRODUCTION
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 ofbiochemical treatment of sewage. The daily average sludge production
reached 97500 tons/day in 2020 and will still maintain a high growthrate in the future (Wu, 2016, Liu, 2012, Feng, 2015). Heavy metals insludge mainly come from industrial sewage. Most of heavy metals insewage are transferred to sludge through microbial adsorption andinorganic salt precipitation (Lester, 1983). Above 80 percent of heavy
metals are adsorbed in the gel group, and the rest is soluble or otherforms 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 andlandfill 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 andIn this study, the combined containers of PFS and PG were constructedin the alkaline system to improve the dehydration capacity of stocked
sludge. The stabilization effect on heavy metals was studied. It
provided a theoretical basis for the new method of sludge compound
conditioning and deep dehydration.

945