Heavy Metals In Sludge Research Articles

Study on the removal of Cd from dewatered sludge by combined extraction agents: Peel extract and compound chemical extraction agent

The recycling of fruit peel resources is a current research hotspot. This study screened and configured a composite extractant consisting of peel and chemical extractant through extraction experiments and explored the heavy metals(HMs) release effect. The results showed that citrus reticulata(CR), citrullus lanatus (CL), 0.16 mol·L−1 nitrilotriacetic acid(NTA) with 0.04 mol·L−1 oxalic acid(NO5), and 0.12 mol·L−1NTA with 0.08 mol·L−1tartaric acid(NT4) had the strongest extraction ability. After the cross-combination was optimized, CR-NO5 (1:50, 6 h, 35 °C) and CL-NT4 (1:50, 36 h, 45 °C) had the highest extraction rates for Cd and Zn, which were 92.6 % and 98.4 %, respectively. The CL series increased the nutrient content of sludge (157.75–177.88 g·kg−1). The four combined extractants increased the proportion of soluble components of HMs in sludge (14–36 %). Therefore, the combined leaching agent will provide a valuable reference for the harmless treatment of HMs in sludge and the resource utilization of peel waste.

Insight into enhancing the performance of sludge dewatering using a novel flocculant CS–TA prepared through free radical-mediated conjugation

ABSTRACT Flocculation is one of the most significant conditioning methods for sludge dewatering. In the study, a novel flocculant CS–TA, prepared through free radical-mediated conjugation of tannic acid (TA) and chitosan (CS), was proposed to improve sludge dewatering. The characterisation using Fourier transform infra-red spectroscopy and X-ray diffraction analysis shows that the CS chain was the backbone of CS–TA, and the presence of CS–TA aromatic rings confirmed the conjugation of CS with TA. Moreover, the conditioning of CS–TA yielded the best dewatering performance at 30 mg g TS−1 with the water content of sludge cake by press filtration (Wsc) of 59.78% ± 0.3% and capillary suction time (CST) of 11.8s ± 0.35 s, compared to 98.2% ± 0.15% and 56.2 s ± 0.16 in raw sludge. The results of different influencing factors (e.g. pH and temperature) on flocculation efficiency indicated that CS–TA possessed the capacity for enhancing sludge dewaterability over a wide range of pH, and the optimal temperature was observed to be 35 °C. Furthermore, the increase of particle size and zeta potential implied the addition of CS–TA favoured the formation of larger particles charge neutralisation and adsorption bridging effect. In addition, extracellular polymer substances (EPS) analysis indicated that the decrease in the polysaccharide and protein contents in EPS after CS–TA addition could increase the relative hydrophobicity of sludge. Moreover, the contents of heavy metals in sludge and their leaching toxicity and environmental risk were reduced. This study provides comprehensive insights into the exploration of CS–TA for sludge dewatering and the maintenance of ecological security in an eco-friendly.

Research on the mechanism of copper removal during electromagnetic enhanced aerobic fermentation of sludge

The migration and transformation of copper during magneto electric assisted aerobic fermentation were studied by establishing a control group, a magnetic assisted group (1.4 mT), an electric assisted group (10 V), and a magneto electric assisted group. The results showed that during the aerobic fermentation process of magnetic assisted sludge, changes in pH, temperature, and microorganisms in the system promoted the dissolution of Cu2+ and its migration from solid sludge to fermentation broth. Due to the presence of an electric field, Cu2+ deposits on the cathode plate through anisotropic attraction, combined with the Lorentz force generated by the magnetic field, promoting mass transfer of copper ions and reducing the concentration of Cu2+ in the fermentation broth, resulting in a copper removal rate of up to 75 %. In addition, the COMSOL simulation results show a consistent overall trend with actual experiments, indicating that the external electric and magnetic fields have a significant inhibitory effect on the removal of heavy metals in sludge.

Co-pyrolysis of municipal sludge and papermaking sludge: Pyrolysis behaviors and heavy metals immobilizations

The co-pyrolysis of municipal sludge (MS) and papermaking sludge (PS) is beneficial for improving the quality of co-pyrolysis biochar and the stability of heavy metals in it. This study investigated the synergistic effect of co-pyrolysis of MS and PS, as well as the influence of different mass ratios (1:9, 3:7, 5:5, 7:3, and 9:1; MS:PS) and pyrolysis temperatures (400–600 ℃) on the physicochemical properties of pyrolysis char, chemical speciation of heavy metals, and their potential ecological risk index. Furthermore, thermodynamic equilibrium calculations were used to study the transformation and stabilization mechanisms of heavy metals during co-pyrolysis. The results indicated that the addition of PS increased the comprehensive pyrolysis index of MS (by 2.09–16.72 times) and the specific surface area of co-pyrolysis char. The inclusion of MS effectively promoted the transformation of heavy metals from unstable fractions to stable fraction, significantly reducing the potential ecological risk index of pyrolysis biochar (by 21.93–41.58 %). The combination of thermodynamic equilibrium calculations and experimental data demonstrated that heavy metals in sludge could be stabilized through the formation of stable heavy metal compounds or their encapsulation within the lattice of inorganic components during co-pyrolysis. These findings suggest that co-pyrolysis of MS and PS is a feasible and promising approach for producing high-quality pyrolysis char with low risk of heavy metals.

Stabilization of heavy metals in solid waste and sludge pyrolysis by intercalation-exfoliation modified vermiculite

Increasing amounts of solid waste and sludge have created many environmental management problems. Pyrolysis can effectively reduce the volume of solid waste and sludge, but there is still the problem of heavy metal contamination, which limits the application of pyrolysis in environmental management. The intercalated-exfoliated modified vermiculite (IEMV) by intercalators of sodium dodecylbenzene sulfonate, hexadecyltrimethylammonium bromide and octadecyltrimethylammonium bromide were used to control the release of Cd, Cr, Cu, Zn and Pb during pyrolysis process of sludge or solid waste. The retention of heavy metals in sludge was generally better than that in solid waste. The IEMV by octadecyltrimethylammonium bromide as the intercalator calcined 800 °C (STAB-800) was the best additive for heavy metal retention, and the retention of Cr, Cu and Zn was significantly better than that of Pb and Cd. Cr, Cu, Zn and Pb were at low risk, while Cd had considerable risk under certain circumstances. New models were proposed to comprehensively evaluate the results of the risk and forms of heavy metals, and the increasing temperature was beneficial in reducing the hazards of heavy metals by the addition of STAB-800. The reaction mechanism of heavy metals with vermiculite was revealed by simulation of reaction sites, Fukui Function and Frontier Molecular Orbital. Thermal activation-intercalated-exfoliated modified vermiculite (T-IEMV) is more reactive and had more active sites for heavy metals. Mg atoms and outermost O atoms are the main atoms for T-IEMV to react with heavy metals. The Cr, Cu and Zn have better adsorption capacity by T-IEMV than Pb and Cd. This study provides a new insight into managing solid waste and sludge and controlling heavy metal environmental pollution.

The sulfidation behavior of zinc during microwave hydrothermal sulfidation of heavy metal-containing sludge

ABSTRACT The heavy metals present in the sludge can undergo a reaction with sulfur, leading to their conversion into metal sulfides through hydrothermal sulfidation. Sulfur ions, possessing a strong sulfidation capability, can operate within a wider pH range at elevated temperatures. The high temperature environment promotes the sulfidation process of zinc within heavy metal-laden sludge. Increasing the temperature of microwave hydrothermal sulfidation and extending the sulfidation duration for heavy metal-containing sludge can enhance the growth of crystal size in the artificially synthesized zinc sulfide. Zinc sulfide predominantly takes the form of ZnS, which facilitates the subsequent flotation recovery of zinc.

Effect and potential mechanisms of sludge-derived chromium, nickel, and lead on soil nitrification: Implications for sustainable land utilization of digested sludge

Increasing occurrence of heavy metals (HMs) in sewage sludge threatens its widespread land utilization in China due to its potential impact on nutrient cycling in soil, requiring a better understanding of HM-induced impacts on nitrification. Herein, lab-scale experiments were conducted over 185-day, evaluating the effect of sludge-derived chromium (Cr3+), nickel (Ni2+), and lead (Pb2+) on soil nitrification at different concentrations. Quantitative polymerase chain reaction and linear regression results revealed an inhibitory sequence of gene abundance by HMs’ labile fraction: ammonia-oxidizing bacteria (AOB)-ammonia monooxygenase (amoA)> nitrite oxidoreductase subunit alpha (nxrA)> nitrite oxidoreductase subunit beta (nxrB). The toxicity of HMs’ incremental labile fraction decreased in the order of Ni2+>Cr3+>Pb2+, with respective threshold values of 5.01, 24.03 and 38.42 mg·kg−1. Furthermore, extending incubation time reduced HMs inhibition on ammonia oxidation, mainly related to their fraction bound to carbonate minerals. Random Forest analysis, variation partitioning analysis, and Mantel test indicated that soil physicochemical properties primarily affected nitrification genes, especially in the test of Cr3+ on AOB-amoA, nxrA, nxrB, Ni2+ for complete ammonia-oxidizing bacteria-amoA, and Pb2+ for nxrA and nxrB. These findings underline the importance of labile HMs fractions and soil physicochemical properties to nitrification, guiding the establishment of HM control standards for sludge utilization.

Modulatory Role of Biochar Properties and Environmental Risk of Heavy Metals by Co-Pyrolysis of Fenton Sludge and Biochemical Sludge.

Recent studies have reported that Fenton sludge and biochemical sludge contain high concentrations of toxic substances and heavy metals (HMs), whereas improper treatment can pose serious threats to environmental safety. Pyrolysis is considered an efficient technology to replace conventional sludge treatment. This study investigated the pyrolysis and kinetic processes of Fenton sludge and biochemical sludge, revealed the physicochemical properties of sludge biochar, and highlighted the role of co-pyrolysis in sludge immobilization of HMs and environmental risks. Results showed that Fenton sludge and biochemical sludge underwent three stages of weight loss during individual pyrolysis and co-pyrolysis, especially co-pyrolysis, which increased the rate of sludge pyrolysis and reduced the decomposition temperature. The kinetic reaction indicated that the activation energies of Fenton sludge, biochemical sludge, and mixed sludge were 11.59 kJ/mol, 8.50 kJ/mol, and 7.11 kJ/mol, respectively. Notably, co-pyrolysis reduced the activation energy of reactions and changed the specific surface area and functional group properties of the biochar produced from sludge. Meanwhile, co-pyrolysis effectively immobilized Cu, Pb, and Zn, increased the proportion of metals in oxidizable and residual states, and mitigated the environmental risks of HMs in sludge. This study provided new insights into the co-pyrolysis properties of sludge biochar and the risk assessment of HMs.

Optimum design of eco-friendly high-strength cementitious matrix incorporating pottery sand derived from sintered heavy metal sludge

This paper explored the utilization of pottery sand (PS) sintered from heavy metal sludge (HMS) as fine aggregate in the high-strength cementitious matrix (HSCM), which can reduce the cost and facilitate the reutilization of HMS. To establish function expression for various parameters, D-Optimal Mixture Design (DOMD) method was adopted. Considering the comprehensive effects of multiple influencing factors, the optical mix proportion of HSCM with PS was ultimately proposed. Then, experimental verification was conducted on the proposed optimized mix ratio, indicating the accuracy of DOMD in calculating the mix ratio of the high-strength cementitious matrix with PS. Specifically, the optimal mix proportion exhibits superior performances in improving density, fluidity, mechanical properties, cost-effectiveness, and reducing carbon emissions. The appropriate inclusion of PS primarily serves to enhance the mechanical properties of the matrix and lower costs. The effects of PS on the porous structures and the characteristics of the interfacial transition zone (ITZ) between paste and fine aggregates were evaluated by scanning electron microscope (SEM), combining mercury intrusion porosimetry (MIP) and X-ray computed tomography (X-CT). It is found that the ITZ structure between paste and PS is much denser compared to that between paste and river sand. The appropriate content of PS is beneficial in reducing the mesoscopic porosity of HSCM.

A 20-year shift in China's sewage sludge heavy metals and its feasibility of nutrient recovery in land use

Recycling resources from wastewater is even more important for developing a more sustainable society. Disposing sewage sludge, which accumulates most pollutants and resources in sewage, is the main challenge in wastewater pollution control and resource utilization. Heavy metals (HMs) are the greatest constraint limiting the application of sewage sludge to land as a sustainable use of this material. We conducted a meta-analysis of the concentrations of HMs in Chinese sewage sludge by combining data from studies published from 2000 to 2019 (N = 8477). Over this period, the reported concentrations of HMs in sewage sludge declined in three stages (a fluctuating stage, a slight decrease stage, and a rapid and stable decrease stage). The results revealed that economic development and environmental policy implementation were the main factors mitigating HM pollution in sewage sludge in China. Moreover, if environmental regulations were strengthened and HM pollution-mitigation strategies were made consistent, such that the proportion of sewage sludge applied to land in China could be increased from 18.6% to 48.0% (the proportion applied to land in the United States), the ecosystem services analysis showed that huge ecological–economic benefits could be realized (3.1 billion Chinese Yuan) and the use of fertilizers could be substantially reduced (the use of nitrogen fertilizers by 8.5% and the use of phosphate fertilizers by 18.1%). This review shows that China should formulate a unified policy and interdepartmental committee for sustainable application of sewage sludge to land and wastewater resource recycling management.

Heavy metals in sludge from the sewage treatment plant network: a tool to evaluate source and risks of heavy metals to land application

ABSTRACT The aim of the study was to evaluate heavy metals concentration based on the total and chemical fractions of sewage sludge, to identify the sources of heavy metals and to assess ecological risks. Thirty-six anaerobic mixed sludge samples were collected from six different types of sewage treatment plant network in Dhaka city, Bangladesh. Total and fraction concentration of heavy metals in sludge samples were analysed by inductively coupled plasma mass spectrometer following acid digestion. The mean ranges of chromium (Cr), nickel (Ni), copper (Cu), arsenic (As), cadmium (Cd), lead (Pb), iron (Fe), manganese (Mn) and zinc (Zn) in the sludges were 237.3–381.8, 108.5–220.2, 116.1–207.2, 14.9–56.1, 0.68–2.86, 72.4–172.0, 12055 –20,015, 64.8–170.6 and 117.5–178.5 mg/kg, respectively. The data of geochemical speciation showed that Fe, Zn, Ni, Cu and Mn were associated with the residual fraction, whereas As, Cd and Pb showed predominant proportion in the exchangeable fraction, indicating the severity of risk by As, Cd and Pb to the surrounding ecosystem. The findings of the study revealed that heavy metals posed moderate to considerable ecological risk. Anthropogenic activities including industrial wastes and agricultural chemicals are substantial sources for heavy metals in the sludge of STP network in the study area.

Life cycle assessment of AnMBR technology for urban wastewater treatment: A case study based on a demo-scale AnMBR system

This study aims at assessing the environmental performance of a projected full-scale anaerobic membrane bioreactor (AnMBR) treating urban wastewater (UWW) at ambient temperature. To this aim, data from an AnMBR demonstration plant equipped with commercially available equipment, including industrial hollow fiber and degassing membranes, was used for projecting a full-scale facility. The use of real operation data allows to obtain robust results that contribute to improve the knowledge of the environmental performance of this technology, pointing out its strengths and the challenges that still need to be addressed. Life cycle assessment (LCA) was applied by means of Ecoinvent data base and ReCiPe2016 methodology considering 1 kg of removed COD as functional unit. Additionally, sensitivity and uncertainty analysis were conducted. Energy balance showed AnMBR performing as energy producer (net energy surplus up to −0.688 kWh·kg CODrem−1) and carbon sink (emissions credit up to 0.223 kgCO2eq·kgCODrem−1). Results also showed energy recovery, heavy metals in sludge, dissolved methane in the effluent, and effluent nutrient content as the most important aspects affecting LCA outcome. Construction phase affected some impact categories significantly (e.g., 51–71% in mineral resource scarcity, 18–27% in fossil resource scarcity, 21–28% in water consumption), therefore its exclusion should be carefully evaluated. CHP efficiency, dissolved methane recovery, filtration productivity, membrane scouring, reactor mixing, HRT and SRT appeared most influencing parameters. Finally, actions leading to increase the recovery and valorization of dissolved methane and/or of nutrients through, for instance, fertigation, improve the environmental performance of AnMBR for UWW treatment.

Enrichment characteristics and environmental risk assessment of heavy metals in municipal sludge pyrolysis biochar

Pyrolysis treatment of municipal sludge is a promising sludge treatment technology. However, due to the presence of heavy metals in sludge, the practical application of sludge pyrolysis products, especially biochar, is greatly affected. In this paper, the enrichment characteristics of different heavy metals in municipal sludge and its pyrolysis biochar and their leaching characteristics in the environment were studied. The environmental risks of municipal sludge and biochar were also assessed. The experimental results revealed that pyrolysis would promote the enrichment of different heavy metals in sludge in pyrolysis biochar. And with higher pyrolysis temperature, the enrichment is stronger. But for metals with high volatility such as Hg, Cd, Se, increasing the pyrolysis temperature will reduce their enrichment. Pyrolysis can inhibit the heavy metals in sludge leaching, and the effect of pyrolysis temperature on different kinds of heavy metals is different. The leaching rates of Se, As and Cr first increased and then decreased as the temperature of pyrolysis increased. The leaching rate of Cd increased as the temperature of pyrolysis increased. Considering the leaching rate of each heavy metal, the leaching toxicity of heavy metals in pyrolysis biochar is the lowest at the pyrolysis temperature of 400 °C. The risk assessment of municipal sludge and biochar revealed that the most harmful heavy metals in the original sludge were Hg, Cd and Se, followed by Cu and Zn. Sludge pyrolysis into biochar can reduce the content of Hg, Cd and Se. The use of copper and zinc water pipes should be reduced for sewage pipe laying to decrease the contents of Cu and Zn in municipal sludge.

Study on preparation and properties of the leather sludge wrap shell aggregate by non-burning method

The leather sludge was used as raw material to prepare non-burning sludge wrap shell aggregate by the non-burning method. The effects of different processes on the cylinder compressive strength, water absorption rate, and frost resistance of the non-burning wrap shell aggregate were discussed, through the research on the wrap shell amount, core component, and curing age of the non-burning wrap shell aggregate. The entropy method was used to determine the best process of sludge wrap shell aggregate, and the solidification effect of the shell wrapping process on sludge heavy metal Cr+6 was evaluated and analyzed. The results show that with the increase of curing age, the cylinder compressive strength of wrap shell aggregate increases first and then stabilizes, the water absorption rate decreases gradually, and the frost resistance is significantly enhanced until the 28d of curing age. The more the wrap shell amount and the less the sludge content in the core component is, the higher the cylinder compressive strength, the lower the water absorption, and the better the frost resistance of the wrap shell aggregate are. The entropy method, finally determined the optimal shell wrapping process to be sludge: cement: slag = 50%: 25%: 25%, the wrap shell amount was 25g/100g, and the curing age was 28d. The composite cementitious system and the shell wrapping process can inhibit and effectively reduce the leaching of heavy metals Cr+6 from the sludge wrap shell aggregate by more than 80%.

Survey on the quality of outlet biosolids and sludge heavy metals in the wastewater treatment plants of Mazandaran province

Background: Sewage sludge is rich in fecal coliforms, Salmonella, parasite eggs, and heavy metals that can cause disease in humans. The aim of this study was to evaluate the quality of outlet biosolids and sludge heavy metals in the wastewater treatment plants (WWTP) in Mazandaran province and to compare them with the Environmental Protection Agency (EPA) standards. Methods: This study was carried out on the sludge of the treatment plants of Sari, Joybar, Babol, Babolsar, Nowshahr, and Nowshahr-Chalus sludge compost plants. In this study, physical, chemical, and biological properties of sludge samples including retention time, pH, total solids (TS), color, heavy metals (copper, zinc, nickel, cadmium, and chromium) as well as the density of fecal coliform, Salmonella and parasite eggs were determined and reported. Results: Biosolids of Joybar, Babol, Babolsar, and Nowshahr-Chalus WWTP in terms of parasite eggs were in class A. Biosolids of Sari WWTP in summer after drying off naturally were located in Class A, and in summer and spring, were located in Class B based on the US EPA criteria. The average concentration of heavy metals (copper, zinc, nickel, cadmium, and chromium) was lower than the EPA standard. Conclusion: The results showed that mostly the treated sewage sludge in Mazandaran WWTPs were in Class B of the EPA guideline. The concentration of heavy metals in the biosolids of these WWTPs is lower than the EPA regulation and the use of these biosolids in agriculture is not limited for these metals.

Behavior of heavy metals in municipal sludge during dewatering: The role of conditioners and extracellular polymeric substances

Heavy metals, the main harmful substances in the sludge, are easily enriched, have adverse effects on the treatment and disposal of the sludge. In this study, two conditioners (modified corn-core powder, MCCP, and sludge-based biochar, SBB) were separately added and jointly added into municipal sludge to enhance sludge dewaterability. Meanwhile, diverse organics, such as extracellular polymeric substances (EPS), were released under pretreatment. The different organics had different effects on each heavy metal fraction and changed the toxicity and bioavailability of the treated sludge. The exchangeable fraction (F4) and carbonate fraction (F5) of heavy metal were nontoxic and nonbioavailable. When MCCP/SBB was used to pretreat the sludge, the ratio of metal-F4 and -F5 decreased, indicating that MCCP/SBB reduced the biological availability and ecological toxicity of the heavy metals in the sludge. These results were consistent with the calculation of the modified potential ecological risk index (MRI). To understand the detailed function of organics in the sludge network, the relationship between EPS, the secondary structure of the protein, and heavy metals was analyzed. The analyses revealed that the increasing proportion of β-sheet in soluble EPS (S-EPS) generated more active sites in the sludge system, which enhanced the chelate or complex function among organics and heavy metals, thus reducing the migration risks.

The Fate of Heavy Metals and Risk Assessment of Heavy Metal in Pyrolysis Coupling with Acid Washing Treatment for Sewage Sludge.

Pyrolysis is an emerging and effective means for sludge disposal. Biochar derived from sludge has broad application prospects, however, is limited by heavy metals. In this study, the fate of heavy metals (HMs) in pyrolysis coupling with acid washing treatment for sewage sludge was comprehensively investigated for the first time. Most of the HMs redistributed in the pyrolyzed residues (biochar) after pyrolysis, and the enrichment order of the HMs was: Zn > Cu > Ni > Cr. Compared with various washing agents, phosphoric acid presented a superior washing effect on most heavy metals (Cu, Zn, and Cr) in biochars derived at low pyrolysis temperature and Ni in biochars derived at high pyrolysis temperature. The optimal washing conditions for heavy metals (including Cu, Zn, Cr, and Ni) removal by H3PO4 were obtained by batch washing experiments and the response surface methodology (RSM). The total maximum HM removal efficiency was 95.05% under the optimal washing specifications by H3PO4 (acid concentration of 2.47 mol/L, L/S of 9.85 mL/g, and a washing temperature of 71.18 °C). Kinetic results indicated that the washing process of heavy metals in sludge and biochars was controlled by a mixture of diffusion and surface chemical reactions. After phosphoric acid washing, the leaching concentrations of HMs in the solid residue were further reduced compared with that of biochar, which were below the USEPA limit value (5 mg/L). The solid residue after pyrolysis coupling with acid washing resulted in a low environmental risk for resource utilization (the values of the potential ecological risk index were lower than 20). This work provides an environmentally friendly alternative of pyrolysis coupling with acid washing treatment for sewage sludge from the viewpoint of the utilization of solid waste.

Phytotoxicity of sewage sludge passivated by modified potassium feldspar and its effect on ryegrass growth

ABSTRACT Effective passivation of heavy metals in sludge is the key to achieve its land use. Potassium feldspar was modified to obtain G×K900, with a lower crystallinity, a higher specific surface area and cationic exchange capacity. G×K900 was chosen to passivate heavy metals in the sludge, and the best passivation conditions were 30% addition and a 10-day passivation period. The passivated sludge extract had no adverse effects on ryegrass germination and promoted radicle growth. The pot experiment revealed that adding 10% passivated sludge encouraged ryegrass growth while having no appreciable impact on the contents of Cu, Cr, Zn and Pb in the roots and above-ground parts compared to control treatment. The transfer factors of the four metals were all less than 1. Passivated sludge was used as a growing medium for landscaping plants, which provided a theoretical foundation for sludge land use and a resource outlet for potassium feldspar minerals.

Temporal variation of heavy metals in sewage sludge in typical cities in Gansu Province, northwest China

To investigate the temporal behaviors of heavy metals in sewage sludge in typical cities of industrial, industrial-agricultural, agricultural, or energy focused. Samples were collected every 10days for a period of 1year in four types of cities of Lanzhou, Tianshui, Qingyang, and Zhangye. The average annual values for all four cities were Cd (1.59-3.16mg/kg), Pb (41.9-55.1mg/kg), Cr (63.8-92.0mg/kg), Cu (75.7-92.6mg/kg), Zn (498-612mg/kg), and Ni (3.66-4.25mg/kg). The highest values were observed in June for Cd, Cr, and Zn, at Lanzhou and Tianshui. At Qingyang and Zhangye, the Cd, Cr, and Zn contents were stable throughout the year. There was a similar monthly change among the four cities regarding the levels of Ni content, and it was far below the background value. The monthly fluctuations in Cd, Pb, Cr, and Zn are mainly due to street dust effect. For cities with a developed industry, the impact of street dust during the first rains of the year on sewage sludge's heavy metal content must be highlighted as being of particular importance.

Spatial Distribution Characteristics and Potential Risk Assessment of Heavy Metals in Sludge of Shanghai Sewage Treatment Plant: A Case Study

Sewage treatment is an important indicator of the urban environment and an important method of sustainable development. In this paper, the spatial distribution characteristics of 8 heavy metals in sewage sludge from 44 sewage treatment plants in Shanghai were analyzed. The pollution degree and potential ecological risk of heavy metals in sludge were evaluated by the Nemerow comprehensive pollution index and Hakanson potential ecological risk index. First, the single factor pollution index of heavy metals was ranked. The Nemerow comprehensive pollution index showed that five sewage treatment plants in Nanxiang, Dongqu, Liantang, Songjiang west, and Xinhe were heavily polluted. Second, the single factor potential ecological risk index of each heavy metal was analyzed. Finally, it is concluded that in the monitoring and control of sewage treatment, Shanghai should pay attention to the sewage treatment plants with high potential ecological risks and gain an excellent performance in the reduction in the emissions of pollution sources, so as to improve environmental quality and reduce the potential ecological risks. This paper provides the basis for studying the characteristics of heavy metal pollution of sewage sludge.