Investigation of polycyclic aromatic hydrocarbons (PAHs) formed in three-phase products from the pyrolysis of various wastewater sewage sludge
Introduction
Disposal of dewatered sewage sludge as a by-product after wastewater treatment plants is a significant environmental concern throughout the world. The quantity and types of contaminants contained in sludge pose a challenge for its effective disposal. To dispose of a large amount of produced sludge, its reuse technology in an environmentally friendly way has attracted much attention [Syed-Hassan et al., 2017; Folgueras et al., 2013; Zhou et al., 2014; Wang et al., 2016]. As a result of the significant calorific value(5000–6000 KJ/Kg)of dried sewage sludge in comparison with wood (15000–17000 KJ/Kg), the use of thermochemical conversation technologies provide an interesting disposal way to produce different chemicals using sewage sludges as starting materials. The physical and chemical properties of sewage sludge are influenced by the origin of wastewater and its treatment processes they result from. The chemical reagent and wastewater treatment processes used in different wastewater treatment plants also impact the chemical properties and components of the resulting sludge.
In China, except for domestic municipal wastewater, industrial wastewater treatment is being one production source of the massive amount of sewage sludge due to the rapid development of modernization and industrialization. The production of municipal sludge in cities and industrial sludge was 43.82 million tons and 40 million tons in 2018, respectively. The dispose of sludge in landfills has been completely banned in China and its stabilization, harmlessness and resource treatment and disposal are therefore being encouraged. Pyrolysis was reported as an economical and environmentally acceptable disposal way for the treatment of sewage sludge, which can reduce more than 50 % of sludge volume and stabilize the organic components [Chan and Wang, 2018]. An additional positive outcome is that such treatments can produce solid materials with adsorption property [Patryk et al., 2014], high heating value syngas and valuable chemicals from the obtained bio-oil [Magdalena and Patryk, 2016; Tsai et al., 2009a; Shen and Zhang, 2003].
The pyrolysis bio-oil typically consists of a complex chemical mixture of organic compounds, with the composition and yield changing with sludge source and used pyrolysis process conditions [Shen and Zhang, 2003; Tsai et al., 2009a, 2009b]. In the process of pyrolysis of sludge, some persistent organic pollutants were also be formed, such as PAHs (polycyclic aromatic hydrocarbons). There are 16 kinds of PAHs that appeared on the US EPA (Environmental Protection Agency) priority controlled pollutants list. Their release is drawing great public attention due to their substantial toxicity and carcinogenic, teratogenic, and mutagenic characteristics. The thermal chemical process of fuel and biomass and is a public issue in China influence the extensive emission of PAHs. Investigations on the formation of PAHs during the thermal treatment of sewage sludge in the environment has extensively studied by some researchers [Chen et al., 2014; Deng et al., 2009; Waqas et al., 2014]. Researchers [Shen and Zhang, 2003; Tsai et al., 2009a, 2009b] studied the distributions of PAHs in pyrolysis liquid from different types of sewage sludge. It was noted that PAHs formation mostly depends on its thermal conditions. However, there is not much available information about the influence of sludge source on the generation of PAHs in the resulting pyrolysis products.
Sewage sludges from various industrial and municipal wastewater treatment were sampled to comprehensively study the generation mechanism of 16 PAHs in the pyrolysis process of different types of sludges. The sampled sludge was pyrolyzed to collect bio-oils, gas and solid residues. In this study, the 16 USEPA PAHs were quantified regarding their generation and distribution in the gas, bio-oil, and solid residues. The temperature as a key influential factor was determined, and indications of the different aromatic molecules PAHs produced were obtained. The yield of each phase and PAHs distribution influenced by temperatures were analyzed to help with understanding generation and their transport mechanism of PAHs. The obtained results could also be significant in achieving suitable methods of various sludges disposal environmentally friendly.
Section snippets
Material
Four kinds of sewage sludge were collected from different wastewater treatment plants of Zhejiang, China, including domestic wastewater sewage sludge (DSS) in Linan, Printing and Dyeing wastewater sewage sludge (PDSS) in Xiaoshan, industrial mix wastewater (ISS) in Hangzhou, and food manufacture wastewater sewage sludge (FSS) in Fuyang, respectively. The raw sewage sludge underwent mechanical dewatering, but no aerobic digestion before sampling. It was dried in a lab-scale air convection oven
Results and discussions
In this study, results regarding concentration distribution and mass proportion of PAHs generated during the pyrolysis were expressed as mg PAH of 1 kg target sample and % of the total target PAHs, respectively. The data regarding the 16 PAHs are displayed and classified by the number of the benzene ring and their distribution profiles.
The high-ring PAHs means more than five rings PAHs, including benzo(b)fluoranthene (BbF), benzo(k)fluoranthene (BkF), benzo(a)pyrene (BaP),
Conclusions
The formation of the 16 PAHs in three-phase products from the pyrolysis of different wastewater sewage sludge was characterized. The temperature as a key influential factor was investigated and the effect of sludge source on the formation of the different molecule’s PAHs produced was characterized. The results indicate that most of 16 US EPA PAHs were generated during sludge pyrolysis and mainly ended up into the resulting pyrolysis bio-oil and gas.
Among all considered sludges, DSS contained
Author contribution section
Yanjun Hu was the project leader, finished the design of the experiment, data analysis, conclusion, and paper writing.
Yuanyuan Xia was in charge of the sampling of three-phase pyrolysis products, part of PAH analysis and other experiments, and paper writing.
Francesco Di Maio was in charge of data analysis and language improvement.
Fan Yu did the measurement of PAH using GC–MS and data analysis.
Wenjing Yu was in charge of part of PAH analysis and PAH measurement using GC–MS.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors want to appreciate the project of the National Natural Science Foundation (Grant No. 51576178) for providing financial support for this work.
References (26)
- et al.
Characterization of sludge for pyrolysis conversion process based on biomass composition analysis and simulation of pyrolytic properties
Waste Manag.
(2018) - et al.
PAH and heavy metals in the sewage sludge from six wastewater treatment plants in Beijing, China
Chemosphere
(2007) - et al.
Formation of PAHs during the pyrolysis of dry sewage sludge
Fuel.
(2014) - et al.
Emission characteristics of dioxins, furans and polycyclic aromatic hydrocarbons during fluidized-bed combustion of sewage sludge
J. Environ. Sci. China (China)
(2009) - et al.
Influence of sewage sludge treatment on pyrolysis and combustion of dry sludge
Energy.
(2013) - et al.
Investigation into the distribution of polycyclic aromatic hydrocarbons (PAHs) in wastewater sewage sludge and its resulting pyrolysis bio-oils
Sci. Total Environ.
(2014) - et al.
Effect of catalysts on the distribution of polycyclic-aromatic hydrocarbon (PAHs) in bio-oils from the pyrolysis of dewatered sewage sludge at high and low temperatures
Sci. Total Environ.
(2019) - et al.
Distribution of 16 EPApriority polycyclic aromatic hydrocarbons (PAHs) in sludges collected from nine Tunisian wastewater treatment plants
J. Hazard. Mater.
(2010) - et al.
Toxic equivalency factors (TEFs) for polycyclic aromatic hydrocarbons (PAHs)
Regul. Toxicol. Pharmacol.
(1992) - et al.
Effect of pyrolysis temperature on the composition of the oils obtained from sewage sludge
La Rev. Des Sci. Gest. Dir. Gest.
(2009)
Thermochemical processing of sewage sludge to energy and fuel: fundamentals, challenges and considerations
Renewable Sustainable Energy Rev.
Characterization of bio-oil from induction-heating pyrolysis of food-processing sewage sludges using chromatographic analysis
Bioresour. Technol.
Levels of polycyclic aromatic hydrocarbons in the bio-oils from induction-heating pyrolysis of food-processing sewage sludges
J. Anal. Appl. Pyrolysis
Cited by (43)
Comparison of PAHs behaviors in the pyrolysis products of oily sludge with different ash contents
2023, Journal of Analytical and Applied PyrolysisStudy on characteristics, hydrogen-rich gas, bio-oil production, and process optimization of co-pyrolysis of sewage sludge and wheat straw
2023, Case Studies in Thermal Engineering