Distribution and toxicity of polycyclic aromatic hydrocarbons during CaO-assisted hydrothermal carbonization of sewage sludge

Highlights

• PAH yields decreased by 13.61% at 160 °C compared to the original sludge.

• PAH yields increased as the HTC temperature increased.

• PAHs were redistributed during HTC, and over 90% of PAHs were found in hydrochar.

• 3–9% CaO decreased PAH and TEQ yields by 5.55–15.98% and 2.88–3.54%, respectively.

• Influence of high temperatures on PAH formation was lessened by CaO introduction.

Abstract

Hydrothermal carbonization (HTC) of sewage sludge (SS) with and without calcium oxide (CaO) introduction was conducted at 160–240 °C, and the yield and distribution of polycyclic aromatic hydrocarbons (PAHs) were evaluated for the first time. PAHs (2972.99 μg/kg) and toxic equivalent quantity (TEQ) (373.09 μg/kg) yields in SS decreased by 13.61% and 14.65%, respectively, after treatment at 160 °C and substantially increased as temperatures increased. More PAHs were distributed in the hydrochar than in the aqueous products. Hydrochar yields decreased linearly with temperature, thus increasing PAH concentration in hydrochar; 6221.98 μg/kg of PAHs in hydrocar at 240 °C exceeded agricultural use standard limits. PAH and TEQ yields at 200 °C decreased by 5.55–15.98% and 2.88–3.54%, respectively, when 3–9% CaO was added, which could be ascribed to CaO inhibition in the free radical reaction for PAH generation. Additionally, 6% CaO addition substantially weakened the acceleration effect of high temperatures on PAH formation; the decrease of PAH yield at 240 °C was 22.14%, which is higher than that at other temperatures. Consequently, the PAH concentration in hydrochar declined by 2.33–22.37%. PAH content in hydrochar obtained from CaO-assisted HTC of SS fell within agriculture use standard limit and exhibits potential for use as a soil conditioner. However, condition with a CaO amount of 15% would significantly increase TEQ yields. Considering both PAH and TEQ yields and the ecological risks of PAHs in hydrochar derived from HTC of SS, the appropriate reaction conditions were found to be 200 °C with 3–6% added CaO.

Introduction

As solid waste from wastewater treatment, sewage sludge (SS) is composed of pathogens, heavy metals (HMs), and organic matter, such as dioxins and polycyclic aromatic hydrocarbons (PAHs), which negatively impact the environment (Tasca et al., 2019). In China, the amount of SS has gradually increased with the rapid development of wastewater plants and the increasing capacity of wastewater treatment. It is estimated that nearly 4000 tons (wet) of SS was produced in 2018 (Qu et al., 2019). Consequently, the increasing amount and complex characteristics of SS in China complicates its disposal. In addition, the low efficiency and secondary environmental pollution resulting from traditional disposal methods including sea dumping, sanitary landfills, and land applications, highlight the need for more sustainable and environmental techniques to manage SS (Cieślik et al., 2015, Yang et al., 2015).

In recent years, hydrothermal carbonization (HTC) has been considered a promising technology for SS disposal because of its low cost and high efficiency (Wang et al., 2019). HTC of SS is generally conducted at relatively low temperatures (150–350 °C) without requiring that SS is dried. The resulting hydrochar is a value-added material, which can be used as a solid fuel (Lin et al., 2015, Wang et al., 2016), absorbent (Liu et al., 2017), or soil conditioner (Chu et al., 2019, Zhang et al., 2014). In particular, the use of hydrochar from HTC of SS as a soil conditioner has received increasing attention owing to its potential as an alternative to fertilizer, primarily because that the major nutrient substances P and N and microelements Ca and Mg remaining in the solid products supply nutrients for crop generation and improve crop yield (Tasca et al., 2019). Additionally, some research has found that adding hydrochar decreases NH₃ volatilization and nitrogen runoff from soil, and promotes N use efficiency by rice (Chu et al., 2019).

However, toxic substances in hydrochar such as HMs and organic pollutants, negatively affect plant growth and have potential ecological risks, leading to increased public concern (Huang & Yuan, 2016). In the past, the distribution and toxicity of HMs in hydrochar attracted significant attention due to their variety and high concentrations, while organic pollutants in hydrochar, such as PAHs and dioxins, have received less attention (Huang and Yuan, 2016, Wang et al., 2016). In particular, PAHs comprise a group of organic pollutants that are highly carcinogenic, mutagenic, and toxic. PAHs are abundant in China’s agricultural soil, and their concentrations can reach as high as 14,722 μg/kg (Shao et al., 2015, Zhang and Chen, 2017). Although PAHs in soil can be removed naturally by microbial degradation, soil erosion, and air-to-soil exchange, PAHs are more likely to be absorbed by crops and vegetables. PAHs then accumulate in living organisms, posing a risk to human health due to their lipophilic nature (Sun et al., 2017). Consequently, PAH formation and reduction in the HTC of SS treatment should be emphasized to mitigate the negative effects of PAHs on plant growth and human health, prior to utilizing hydrochar as a soil conditioner.

Generally, PAHs are mainly produced from the incomplete combustion of fossil fuels, biomass, and other thermochemical processes. With the application of combustion, pyrolysis, and gasification to SS disposal and recovery, the formation and distribution of PAHs during these thermochemical processes has gained attention (Dai et al., 2014, Ko et al., 2018, Konczak et al., 2019). Dai et al. (2014) found that the total PAHs from SS pyrolysis were low at <500 °C and exceeded 1000 mg/kg at 950 °C. In contrast, Park et al. (2009) reported that the total PAH concentration resulting from SS combustion averaged 6.103 mg/kg. Some research related to PAH formation and distribution during biomass HTC treatment has indicated that the potential for PAH formation during HTC, although its reaction conditions are much gentler than pyrolysis, gasification and combustion (Lang et al., 2019a, Peng et al., 2017, Wiedner et al., 2013a, Wiedner et al., 2013b). Lang et al. (2019a) found that the HTC of manure at 180 °C resulted in a 50% increase in PAH content compared to raw manure, while a reduction in PAH content was observed with increasing temperatures from 200 °C to 220 °C. Peng et al. (2017) investigated PAH formation during HTC of municipal solid waste and revealed that increasing the temperature to 200 °C or higher, significantly increased PAH concentration and toxic equivalent quantity (TEQ) in hydrochar. A study by Wiedner et al. (2013b) found that PAH content in hydrochar was strongly dependent on biomass type, and the PAH concentration in hydrochar derived from HTC of SS was 12 μg/kg, which was much higher than that from other biomasses. Consequently, more attention should be paid to PAH formation and distribution in sludges because hydrochar from HTC of SS can be used as a soil conditioner.

Calcium oxide (CaO) is usually used as a conditioner for SS, and its effects on chemical reactions during thermal processes and the resulting products have been discussed (He et al., 2015, He et al., 2016). In addition, it was found that CaO had a positive effect on PAH reduction during manure HTC (Lang et al., 2019a). Therefore, it is expected that adding CaO to HTC of SS treatment may inhibit PAH formation and reduce TEQ.

At present, PAH formation and distribution during HTC of SS processes assisted by CaO have not been investigated, and the effects of CaO and its synergistic effects with temperature on PAH formation have not been revealed. In this study, HTC treatment of SS at different temperatures with varied CaO additions was conducted. Additionally, the PAH content and TEQ in solid hydrochar and aqueous products were analyzed. The principal objective of this study was to develop an efficient method that can facilitate riskless SS application to soil.