Sewage sludge (mainly composed of excessive bio-sludge) is an inevitable by-product of biological wastewater treatment process and contains various toxic substances, such as pathogens, heavy metals, ...and organic contaminants. The production of sewage sludge may cause serious pollution risks without appropriate disposals. As the essential step of sludge treatment, dewatering plays significant roles in minimizing the sludge volume, facilitating the transportation, increasing the calorific value and even reducing the leachate production in landfill sites. This paper presents a comprehensive review on the issues related to dewatering of sewage sludge. Section 1 starts with the environmental implications of sludge dewatering. Section 2 deals with the concepts and challenges about differentiation of bound water fractions, and also reviews the recent progress of in-situ visualization of water occurrence states in bio-flocs. Section 3 discusses about how various physiochemical properties influence the sludge dewaterability, and the insufficiency in in-situ micro-characterization of sludge constituents is pointed out. Section 4 reviews the existing conditioning technologies for sludge dewaterability improvement, and the advantages/disadvantages of each technology in terms of applicable occasions, material consumption, energy consumption and environmental impacts are evaluated. The last section (section 5) specifically analyzes the feasibility of integrating sludge dewatering and re-utilization, and raises attention to the potential environmental risks of dewatering conditioning. Based on the above discussion, we propose that a unified theory for sludge dewaterability improvement remains to be established. Especially, how the molecular structures of sludge compositions affect the solid-water interface behavior requires to be deepened, which will further unravel the mechanism behind strong water-holding capacities of bio-flocs. Additionally, we believe that the key challenges for sludge dewatering is how to select the appropriate conditioning technique according to the physiochemical properties of target sludge. The reliable indicators for real-time control of conditioning operations are still deficient, e.g., dynamic dosage control of conditioning chemicals. Accordingly, the potential environmental risks of excessive conditioning chemicals should be taken into more consideration.
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•Sludge dewaterability improvement is reviewed from fundamentals to implementations.•Challenges and recent advances in water differentiation of bio-sludge are discussed.•Micro-mechanism analysis on water binding behavior of bio-sludge is insufficient.•Impacts of conditioning operation on sludge utilization should be seriously examined.
•A new perspective of phosphorus occurrence states in activated sludge was proposed.•Phosphorus mainly existed in cells, extracellular polymeric substances and minerals.•A targeted clean extraction ...approach by acidic cation exchange resin was developed.•Sludge acidification and metal adsorption led to successful phosphorus extract.
Waste activated sludge (WAS) is an important source of non-renewable phosphorus (P) recovery. Given the factor that the occurrence states of phosphorus in WAS determines its recovery efficiency, the spatial distribution and chemical speciation of phosphorus were comprehensively and simultaneously analyzed by in-situ and step-by-step extraction methods for the first time. It was confirmed that the phosphorus in solid phase of WAS could be mainly divided into three parts: polyphosphate in cells, extracellular polymeric substances (EPS)-bound P, and phosphate precipitated with metals (P-precipitates) in extracellular inorganic minerals. Among these forms, EPS-bound P (mainly orthophosphate, Ortho-P) and P-precipitates (mainly Ca-P, Fe-P, Al-P, and Mg-P) were the major forms of phosphorus in WAS, accounting for 65%-82% of total phosphorus (TP). Owing to the acid solubility of P-precipitates, acid extraction could be a potentially effective means for phosphorus recovery. However, the co-solution of metals may hinder the phosphorus recovery and the EPS-bound P cannot be recovered by acid extraction. To enhance phosphorus release from EPS and reduce metal interference, a targeted clean extraction technology using acidic cation exchange resin (ACER) was also developed. The results showed that a low dosage ACER could effectively extract EPS-bound P and P-precipitates, and the content of phosphorus in the extract exceeded 50% of TP. Compared with acid extraction, the release efficiency of TP increased by 13%-23%, and the dissolved metal content decreased by more than 90% in the extract by ACER. This was attributed to the acidification and metal capture by ACER. Finally, more than 90% of Ortho-P in the extract was recovered as calcium phosphate, which alleviated the depletion of phosphorus resources.
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Sediment is an important part of the aquatic ecosystem, which involves material storage and energy exchange. However, heavy metal pollution in sediment is on the increase, becoming an important ...concern for the world. In this paper, the state-of-art in situ remediation technology for contaminated sediment was elaborated, including water diversion, capping, electrokinetic remediation, chemical amendments, bioremediation and combined remediation. The mechanisms for these techniques to reduce/immobilize heavy metals include physical, electrical, chemical and biological processes. Furthermore, application principle, efficiency and scope, advantages and disadvantages, as well as the latest research progress for each restoration technology, are systematically reviewed. This information will benefit in selecting appropriate and effective remediation techniques for heavy metal-contaminated sediment in specific scenarios.
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•MMT-nano CaO2 was first developed to enhance dewatering of waste-activated sludge.•Montmorillonite has the catalytic capacity for generating OH from nano CaO2.•MMT-nano CaO2 ...exhibited synergistic effects of filtration aid and peroxidation.
Nanosized CaO2 was first synthesized using montmorillonite clay layers as templates for combining the positive effects of filtration aid and peroxidation on the dewatering of waste-activated sludge (WAS). Exchangeable Ca2+ compensating for negative charges of montmorillonite reacted with the added H2O2, affording nano CaO2 on the surface or in the interlayer regions of montmorillonite (montmorillonite-supported nano CaO2, MMT-nano CaO2). The catalytic capacity of montmorillonite for generating OH from nano CaO2 was verified by electron paramagnetic resonance (EPR), therefore, the radical oxidation induced by MMT-nano CaO2 can destruct the floc structure of WAS with the omission of Fe(II), the conventionally used catalyst for Fenton reaction. As a result, the reduction in specific resistance of filtration (SRF) by 62.2% was achieved with the MMT-nano CaO2 dosage of 100mg/g dry solid (DS) and the weakly acidic pH of 5. OH led to the reduction of hydrophilic protein in extracellular polymeric substances (EPS), and the swelling behavior of montmorillonite contributed to the formation of a porous structure as draining medium for water withdrawing. The above factors were the dominant mechanisms for enhancing dewatering performance.
Sediment is an important part of aquatic systems, which plays a vital role in transporting and storing metals. Due to its abundance, persistence, and environmental toxicity, heavy metal pollution has ...always been one of the hot spots in the world. In this article, the state-of-art ex situ remediation technology for metal-contaminated sediments is elaborated, including sediment washing, electrokinetic remediation (EKR), chemical extraction, biological treatment, as well as encapsulating pollutants by adding some stabilized/solidified materials. Furthermore, the progress of sustainable resource utilization methods, such as ecosystem restoration, construction materials (e.g., materials fill materials, partition blocks, and paving blocks), and agriculture use are reviewed in detail. Finally, the pros and cons of each technique are summarized. This information will provide the scientific basis for selecting the appropriate remediation technology in a particular scenario.
Here, we report a promising new strategy for achieving rapid and complete destruction of perfluorooctanesulfonate (PFOS) through the application of hydrothermal conditions (condensed water, 200–350 ...°C, 2–16.5 MPa) to solutions amended with NaOH. Initial screening experiments with a wide range of solution amendments (e.g., acids, alkalis, oxidants, reductants) revealed highly variable extents of PFOS defluorination, ranging from 0% to 80% after 90 min of reaction at 350 °C. The most reactive amendments, regardless of type, shifted solution pH to highly alkaline conditions (pH ≥ 9), suggesting a base-promoted mechanism. For NaOH-amended solutions, rates of PFOS degradation increased with temperature and followed a second-order rate law, −dPFOS/dt = k 2OH–PFOS, with k 2 = 0.052 ± 0.004 M–1 min–1 at 350 °C, and 19F-NMR measurements show complete conversion of C–F bonds to F– (2.5 g/L PFOS) within 40 min for the reaction with 1 M NaOH. Small quantities of short-chain perfluorocarboxylic acids (≤1.5% PFOSinit) were detected as transient intermediates, indicating that an initial OH–-catalyzed cleavage of the sulfonate headgroup is followed by rapid sequential decarboxylation reactions, eventually leading to complete mineralization. These findings suggest a promising technology for destruction of PFOS-containing wet concentrates (e.g., aqueous film-forming foam (AFFF) stockpiles, industrial waste, sorbent regenerate, and membrane reject waste streams).
In this study, complementary techniques, including ultrahigh-resolution Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) and X-ray photoelectron spectroscopy (XPS), were ...applied to characterize the transformation of nitrogen components during hydrothermal liquefaction (HTL) of sewage sludge. Results showed that 3–35% of nitrogen in sludge was transferred into the biocrude product when using HTL reaction temperatures ranging from 200 to 350 °C, with the remaining nitrogen partitioning to aqueous, solid, and gaseous coproducts. N-containing organics in biocrude shifted to structures with higher H:C ratios when HTL reaction temperatures were increased, whereas an opposite trend was observed for O:C ratios of N-containing chemicals in biocrude and aqueous products. Accordingly, weighted averages of double-bond equivalents (DBEw) and aromatic index (AImod,w ) of N-containing products in biocrude decreased as reaction temperature increased, while AImod,w increased for products in the aqueous phase. These trends are consistent with cyclization/aromatization reactions, promoting the formation of N-containing products that partition predominantly to the aqueous phase. XPS analysis revealed that the proportion of amine-N in biocrude increased sharply with reaction temperature, whereas heterocyclic-N (pyridinic-N and pyrrolic-N) and nitrile-N structures were mainly concentrated in solid residues. Improved molecular insights can serve as the basis for optimizing nitrogen management and recovery operations during HTL of sewage sludge.
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A novel absorbent, PVA/SA-CFZ, was developed by jointly enveloping nanostructured hydrated ferric-zirconium binary oxide and clinoptilolite in the cross-linking of polyvinyl alcohol ...(PVA)/sodium alginate (SA). As a result, the cation exchange capacity of clinoptilolite and electrostatic attraction capacity of hydrated ferric-zirconium binary oxide can be integrated for the simultaneous removal of nitrogen and phosphorus from wastewater. The adsorption behavior of PVA/SA-CFZ for the co-existing ammonium and phosphate was primarily investigated by batch experiments. Over a wide pH range (4 ~ 11), PVA/SA-CFZ always showed the excellent adsorption performance both for the low-concentration ammonium (<5 mg/L) and phosphate (<0.5 mg/L), which reflected the well adaptability towards unfavorable environmental conditions. The kinetic analysis indicated that the pseudo-second-order kinetic model could quantitatively describe the adsorption process optimally. The mechanistic investigation revealed that ammonium was captured through the cation exchange reaction with clinoptilolite. The electrostatic attraction, inner sphere complexation with Fe/Zr-O-OH groups, and formation of phosphate precipitation jointly contributed to the fixation of phosphate by PVA/SA-CFZ. Generally, it is believed that PVA/SA-CFZ can serve as a promising composite absorbent for the simultaneous deep removal of nitrogen and phosphorus, and support for the advanced treatment of wastewater containing low-level nutrients.
Dam construction interfered with the original environment of the river system and greatly affected the geochemical behaviors of trace metals. Thus, a set of toxic metals of Cr, Ni, Cu, Zn, As, Cd, Pb ...and Hg in soil/sediment of the Three Gorges Reservoir (TGR) during the period of 2008–2020 were analyzed and summarized. The results showed that levels of trace metals (except Cr) were apparently higher than the soil background in the TGR and China, in which Cu, Zn, As, Cd, Pb and Hg corresponded to the moderately to highly contaminated grade. As expected, most trace metals (except Ni and As) were observed an evident increase after the full impoundment stage of 2008–2014, suggesting the dam construction of the TGR that promoting the sediment adsorption effects for trace metals. For spatial patterns, metal levels largely depended on the sampling sites, that intensive anthropogenic activities might well be the primary contributors. Main stream with higher concentrations of trace metals in comparison with tributaries reflected the larger loads of metal pollution. In the water-level-fluctuating zone, hydrological regime induced by damming played a critical role on the redistribution of trace metals through eroding soil/sediment particles or bedrocks and altering the physiochemical characteristics and vegetation coverage of soil/sediment. Finally, submerged sediment seemed as a major sink of trace metals that had greater concentration than that in the water-level-fluctuating zone.
Bottom ash is an inevitable by-product fi＇om municipal solid waste （MSW） incineration plants. Recycling it as additives for cement production is a promising disposal method. However, the heavy metals ...and chlorine are the main limiting factors because of the potential environmental risks and corrosion of cement kilns. Therefore ,investigating heavy metal and chlorine characteristics of bottom ash is the significant prerequisite of its reuse in cement industries. In this study, a correlative analysis was conducted to evaluate the effect of the MSW components and collection mode on the heavy metal and chlorine characteristics in bottom ash. The chemical speciation of insoluble chlorine was also investigated by synchrotron X-ray diffraction analysis. The results showed that industrial waste was the main source of heavy metals, especially Cr and Pb, in bottom ash. The higher contents of plastics and kitchen waste lead to the higher chlorine level （0.6 wt.%-0.7 wt.%） of the bottom ash. The insoluble chlorine in the MSW incineration bottom ash existed primarily as AlOCl, which was produced under the high temperature （1250℃） in incinerators.
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