Abstract
Activated persulfate oxidation has been proven to be an efficient advanced sludge treatment technique to improve sludge dewaterability. This study investigates the influence of persulfate on the transformation of phosphorus (P) and heavy metals (HMs) during the hydrothermal treatment of sewage sludge. The hydrothermal temperature, time, and persulfate concentration are optimized by a Box-Behnken design to obtain the best sludge dewaterability, which is expressed by capillary suction time (CST). The highest CST reduction efficiency is 90.5% at the optimal hydrothermal temperature, time, and concentration of persulfate, which are 145°C, 2h, and 150mg/g dry sludge (DS), respectively. The distribution and transformation of P and HMs with different persulfate concentrations (100-200mg/g DS) during the hydrothermal process are investigated. Results show that more than 90% of the P and HMs in the sludge are retained in sludge cakes after the hydrothermal treatment. The addition of SPS can make the P in the sludge cakes transform into more stable P species according to the extraction capacity of sequential extracts. It can be found from the ecological risk indexes of the HMs that the addition of SPS during the hydrothermal treatment of sludge can reduce the environmental risk of HMs. This study provides insights into the P and HM distribution and transformation during hydrothermal treatment with persulfate, providing a reference for sludge recovery strategies.
Highlights
With the wide application of biological sewage treatment technology, a considerable quantity of sewage sludge is produced as a by-product during wastewater treatment (Neyens et al, 2004)
This study investigated P and heavy metals transformation during the hydrothermal treatment of sewage sludge to improve sludge dewaterability
Different concentrations of persulfate were added to explore its effects on the P and heavy metals distribution and transformation during hydrothermal treatment with sewage sludge
Summary
With the wide application of biological sewage treatment technology, a considerable quantity of sewage sludge is produced as a by-product during wastewater treatment (Neyens et al, 2004). Sewage sludge is a useful waste containing valuable resources, such as organic carbon, phosphorous (P) and nitrogen (N), along with hazardous substances, such as pathogens, heavy metals (HMs) and persistent organic pollutants (Zhang et al, 2009, Świerczek et al, 2018, Mahmoud et al, 2018). To reduce the costs of sludge transportation and disposal, a reduction of the sludge volume by solid-water separation is an essential step prior to sludge disposal (Li et al, 2018). Hydrothermal treatment has proven to be an effective pretreatment method to enhance sludge dewaterability owing to the solubilization of organic matter (Kepp et al, 2000). Hydrothermal treatment can reduce sludge toxicity, control HMs, and recover resources (e.g., P and N) and energy from biosolids (Dewil et al, 2007; Funke and Ziegler, 2010)
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