Investigation of polycyclic aromatic hydrocarbons (PAHs) formed in three-phase products from the pyrolysis of various wastewater sewage sludge

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Highlights

  • The effect of sludge source on the formation of the different molecule’s PAHs was evident.

  • Most of 16 target PAHs were mainly distributed in pyrolysis oil and gas.

  • PAHs yielded most in the bio-oil produced by PDSS at 850℃, which was 51.25 mg kg−1.

  • The PAHs distribution in bio-oil was largely dependent on pyrolysis temperature.

  • The types and concentrations of PAHs formed in gas were less than that in bio-oil.

Abstract

This study investigated the presence and concentration of 16 U.S. EPA priority controlled PAHs in gas, bio-oil and residues from the pyrolysis of different sewage sludges. We studied the temperature as a key influential factor for the formation of 16 targets PAHs and the effect of sludge source on the distribution of different molecules’ PAHs were analyzed. Results showed that most of the 16 PAHs were formed during sludge pyrolysis and mainly ended up in bio-oil and gas. The distribution of PAHs in bio-oil was mostly dependent on pyrolysis temperature. With the increase of pyrolysis temperature from 450℃ to 850℃, it has been observed an increase of PAHs concentration in the bio-oils as follows: 16 % (ISS), 1.3 % (food manufacture wastewater sludge, FSS), 194 % (printing and dyeing wastewater sludge, PDSS), 334 % (DSS). 2, 3 and 4-ring PAHs dominate, and their total mass proportion is over 70 %. In gas, the types and concentrations of PAHs were less than in bio-oil. PAHs yield in solid was very low, and a trace content of PAHs of 0.0161 mg kg−1 was detected from the solid after the pyrolysis of DSS, while PAHs in solid for ISS and FSS are even non-existent and would cause fewer environmental problems.

Graphical abstract

PAHs content distribution and TEQ values in pyrolysis liquid products of sludge from different sources

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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.

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