Waste activated sludge pretreatment by Fe(II)-activated peroxymonosulfate oxidation under mild temperature

Sequential oxidation-composting-phytoremediation treatment for the management of an oily sludge from petroleum refinery

Oily sludges are generated in large quantities in petroleum refinery wastewater treatment plants. Given their complex composition, they are classified as hazardous waste. Selecting a single treatment technique for their remediation is challenging. This work aims to assess the extent of composting followed by phytoremediation on an oily sludge from an API separator unit, pre-treated by chemical oxidation with alkaline activated persulfate (PS). 18% of total petroleum hydrocarbons (TPH) were determined by IR spectroscopy. The aliphatic hydrocarbon content was 4714 ± 250 ppm by GC-FID, and aromatics were not detectable, suggesting a high amount of non-chromatographable complex hydrocarbons. The density of generalist and hydrocarbon-degrading populations of the oily sludge estimated by quantitative polymerase chain reaction (qPCR) evidenced an autochthonous microbiota with hydrocarbon-degrading capacity. The oxidative treatment with PS removed 31% of the TPH determined by IR after 20 days. The significant reduction of the native bacterial community was counterbalanced by coupling a composting treatment. Co-composting the sludge with goat manure and oat straw produced, after a year, a 96% reduction in TPH content, regardless of the oxidative pretreatment. Organic matter transformation was evidenced by the decrease of dissolved organic carbon (DOC) and the variation in E4/E6 ratio. The matrices obtained of composting were used as substrates for phytoremediation for 4 months. Ryegrass seeds were planted in both PS-treated and untreated sludge substrates. The presence of the plant grown in the pre-oxidised and composted substrate resulted in a higher aerial biomass of ryegrass (67%), an increase in enzymatic activities, and higher concentration of DOC, although without evidence of additional dissipation of TPH. The dynamics of the bacterial communities of the different substrates generated during the biological treatment were analyzed by Illumina NovaSeq DNA sequencing of 16S rRNA amplicons. The findings mirrored a succession compatible with that described in contaminated matrices, but also in other non-contaminated ones. According to these findings, an organic matter transformation process occurred, which included the complex hydrocarbons of the oily sludge, resulting in an active substrate that promoted the retention of nutrients and water and provided the necessary support for plant development.

Enhanced technology for sewage sludge advanced dewatering from an engineering practice perspective: A review

The increasing production of sludge poses significant environmental risks. Sludge disposal and transport are costly because of the high water content (WC). Reducing the WC of sludge is the most efficient strategy to decrease treatment costs. However, the sludge contains a large amount of hydrophilic organic matter, causing poor dewaterability. Therefore, research on preconditioning and mechanical dewatering has great significance for advanced sludge dewatering. In this study, the features of sludge, the advantages and disadvantages of preconditioning methods, and the action mechanisms (including physical, chemical, and biological preconditioning) are thoroughly described. In addition, the dewatering principle and engineering applications of mechanical dewatering techniques are introduced in this manuscript, especially the application of vacuum preloading as an in-situ dewatering technology in sludge. Finally, cost analysis of different conditioning and mechanical dewatering methods is conducted to explore their application feasibility. This manuscript provides new insights for engineering applications of preconditioning methods and mechanical dewatering technology.

Insight into sludge dewatering by advanced oxidation using persulfate as oxidant and Fe2+ as activator: Performance, mechanism and extracellular polymers and heavy metals behaviors

This study established a Fe²⁺/persulfate oxidation system to dewater sludge in WWTPs. Dewatering performance, persulfate consumption and the variations of sludge pH, TN and TP during dewatering process were monitored. EPS and ζ-potential behaviors for ameliorating sludge dewatering was investigated. Transformation, leaching toxicity and environmental risk of heavy metals in sludge during dewatering were determined. Results demonstrated that after treated by Fe²⁺/persulfate oxidation system with 0.6 mmol/g-VS of persulfate at Fe²⁺/persulfate molar ratio 0.6, WC decreased to 53.5% and SCST increased to 4.15, which implied an excellent improvement of sludge dewatering. The fast persulfate consumption, the decrease of sludge pH and the increase of TN illustrated the positive effects of Fe²⁺ in activating persulfate and the decomposition of EPS by the activation products, SO₄^•- and ^•OH. Another product (Fe³⁺) generated during persulfate activation could decrease the content of phosphorus-containing matter (released from EPS decomposition) through the precipitation reaction with PO₄^3-. The decrease of TOC and UV-254 happened in HPO-A, HPO-N and TPI-A organic substance of EPS (mainly contained in TB-EPS fraction) indicated that the destruction of hydrophobic organic matter of EPS would stimulate the release of bound water, which was beneficial to dewater sludge. The largest protein loss in TB-EPS (from 24.5 to 10.7 mg/L) indicated that the effective decomposition of TB-EPS could significantly ameliorate sludge dewatering. The increase of ζ-potential indicated the degradation of organic matter in EPS with negative charge. To sum up, the destruction of protein-like substances in hydrophobic organic matter of TB-EPS was the main mechanism for improving sludge dewatering by Fe²⁺/persulfate oxidation system. 3D-EEM fluorescence spectroscopy analysis proved that these protein-like substances were mainly tryptophan protein and humic acid. Moreover, due to the disruption of EPS, the contents of heavy metals in sludge, and their leaching toxicity and environmental risk were reduced. Therefore, Fe²⁺/persulfate oxidation system has potential and application prospects to improve sludge dewatering and optimize sludge management in WWTPs.

Insight into the improvement of dewatering performance of waste activated sludge and the corresponding mechanism by biochar-activated persulfate oxidation

A novel activator, corn biochar, was produced to activate persulfate to dewater waste activated sludge (WAS). Results demonstrated that the biochar-activated persulfate oxidation can effectively improve the dewatering performance of WAS. After treating WAS by biochar-activated persulfate oxidation (biochar dosage: 2.1 g/L, persulfate concentration: 7.5 mM) at the original WAS pH, standardized-capillary suction time (SCST) increased to 4.21 times and moisture content (MC) decreased to 43.4%, indicating an excellent performance of WAS dewatering. The decrease of residual persulfate with the increasing biochar dosage during WAS dewatering process illustrated that the role of persulfate in improving WAS dewatering was because of the biochar activation. The behaviors of extracellular polymers (EPS) proved that the protein in tightly bound EPS (TB-EPS) linked to WAS dewatering, and its content significantly reduced to 10.5 mg/g-volatile solids (VS) after WAS treatment. Three-dimensional fluorescence spectroscopy for EPS once again proved that the disintegration of tryptophan protein and humic acid (hydrophobic organic substances in EPS) was responsible for the improvement of WAS dewatering. To sum up, the biochar-activated persulfate oxidation was a feasible application in improving WAS dewatering.

Insight into dewatering behavior and heavy metals transformation during waste activated sludge treatment by thermally-activated sodium persulfate oxidation combined with a skeleton builder-wheat straw biochar

This work focused on dewatering performance and heavy metals (HMs) transformation during waste activated sludge (WAS) treatment by employing thermally-activated sodium persulfate (SPS) oxidation combined with a biochar made of wheat straw (WS-BC). Results demonstrated that the combined treatment was an efficient way to improve WAS dewatering, especially when SPS and WS-BC dosages were adjusted to 120 and 150 mg/g-volatile solids (VS), respectively. After being treated at 70 °C without pH adjustment, standardized-capillary suction time (SCST) was increased to 5.03, centrifuged weight reduction (CWR) was increased to 86.8% and moisture content (MC) was decreased to 42.6%, indicating an excellent dewatering performance. The underlying mechanism identified were: (1) SPS oxidation disintegrated extracellular polymers (EPS) at high temperature, thereby releasing EPS-bound water; (2) WS-BC acted as a skeleton builder conducive to form porous structure and provide free water release channels in WAS, which was favorable to improve dewatering properties of WAS; (3) thermally-activated SPS oxidation significantly improved the solubilization and reduced the leaching toxicity of HMs in WAS; and (4) the following WS-BC treatment further reduced the leaching toxicity of HMs in WAS. Therefore, the combined technology might be a promising strategy to improve WAS dewatering and reduce HMs risks in WAS.

Transformation of heavy metals and dewaterability of waste activated sludge during the conditioning by Fe2+-activated peroxymonosulfate oxidation combined with rice straw biochar as skeleton builder

This work investigated the improvement performances and mechanisms of waste activated sludge (WAS) dewaterability and the transformation behavior of heavy metals (HMs, including Cu, Zn, Pb, Cd and Cr) by jointly conditioning of Fe²⁺-activated peroxymonosulfate (PMS) oxidation and rice straw biochar (RS-BC). Experimental results showed that at original WAS pH of 6.5, the joint conditioning was the most effective when PMS dosage was 0.6 mmol·(g-volatile solids (VS))^-1, Fe²⁺/PMS molar ratio was 0.6 and RS-BC dosage was 120 mg·(g-VS)^-1. Under this condition, the lowest moisture content (MC) was 38.5% and the standardized-capillary suction time (SCST) was as high as 8.74. For the improvement mechanism, Fe²⁺-activated PMS oxidation can significantly disintegrate the extracellular polymeric substances (EPS) composing WAS to release EPS-bound water, and the RS-BC was helpful to form porous structures to improve WAS compressibility, facilitating the subsequent dewatering. In addition, Fe²⁺-activated PMS oxidation can obviously improve the solubilization and reduce the leaching toxicity of Cu, Zn, Pb, Cd and Cr, which was further enhanced by RS-BC. Therefore, the joint application of Fe²⁺-activated PMS oxidation and RS-BC can be a feasible way to improve WAS dewaterability and reduce HMs risk during WAS dewatering.

Improved dewaterability of anaerobically digested sludge and compositional changes in extracellular polymeric substances by indigenous persulfate activation

In this study, an indigenous activation of persulfate by iron-bearing minerals was examined using peroxymonosulfate (PMS) and peroxydisulfate (PDS) for the dewaterability and extracellular polymeric substances (EPS) composition of anaerobically digested sludge (ADS). Iron minerals originally present in the ADS seemed to operate as an initiator of the persulfate activation, which was indicated by an increase in the total dissolved iron in the supernatant. The PMS showed higher performance in improving the ADS dewaterability compared to the PDS, with more reduction in the average microbial flocs size. The extracted EPS consisted of three different fluorescent components of tryptophan-like (C1), humic-like (C2), and fulvic-like (C3) components. In the tightly bound (TB)-EPS, two humic-like components (C2 and C3), which were resistant to degradation, exhibited a strong linkage with dewaterability; whereas, in both the PMS and the PDS systems, the C1 was preferentially degraded by the radical-based oxidation, with a greater extent for the PMS-based treatments. The results showed that prior to the actual EPS degradation, the produced radicals were initially involved in cell disruption. The SEC results during the oxidation clearly demonstrated the degradation of a large-sized biopolymer fraction, followed by the production of relatively small sized molecules. They also revealed in the TB-EPS a close association between the ADS dewaterability and the degradation of humic substances.