Enhanced methane production from waste activated sludge by combining calcium peroxide with ultrasonic: Performance, mechanism, and implication
Graphical abstract
Introduction
As a biological wastewater treatment technology, activated sludge process has been widely used in the wastewater treatment plants (WWTPs) (Ni and Yu, 2008, Wang et al., 2018a). Although it is effective and has made world-famous contributions to environmental protection, large amount of waste activated sludge (WAS) as byproduct are produced (Li et al., 2016, Zhao et al., 2017a). Based on our communications with industry partners, a WWTP (Q = 100000 m3/d) is estimated to generate ∼50 tons of WAS per day. The treatment and disposal cost of WAS is high, accounting for up to 60% of the whole operation cost of a WWTP (Guo et al., 2013, Wang et al., 2018b). Moreover, WAS is rich in biodegradable organic matters (e.g. proteins and carbohydrates), which can be reused and recovery as energy (Zhao et al., 2015, Xu et al., 2018a). By using WAS as digestive substrate, methane could be obtained from anaerobic digestion, which not only effectively achieves sludge reduction, stability and reutilization, but also recovers energy and nutrient to reduce the operational cost of WWTPs (Liu et al., 2018a, Xu et al., 2018c). Considering the huge environmental and economic benefits, the production of methane from WAS anaerobic digestion therefore has been drawn extensive attention (Dai et al., 2017, Zhao et al., 2017b).
However, methane production from anaerobic digestion is often upset by the low-level disintegration of WAS, which is considered as the rate-limiting step (Yuan and Zhu, 2016, Yang et al., 2019). This is mainly because that the extracellular polymeric substances (EPS) and/or cell envelop can protect sludge cells, resulting in less extracellular or intracellular components release (Xu et al., 2017, Wang et al., 2019a). Therefore, to facilitate the disintegration of WAS and improve subsequent methane production, a lot of pretreatment technologies have been widely studied such as alkaline, thermal, ultrasonic, enzymatic and their combinations (Carrere et al., 2010, Ariunbaatar et al., 2014). Among them, ultrasonic (US) pretreatment has been proved to be an effective technology in the lab-scale and has been implemented in real-world situation (Pilli et al., 2011), because the cavitation bubbles and the instantaneous high temperature produced by US can effectively disrupt the EPS and cell envelop of sludge cells, accelerating the disintegration of WAS and the release of organic substrates into the liquid phase (Bao et al., 2015, Cheng et al., 2018). Moreover, this technology is easy to operate without secondary pollution. US therefore is regarded as a promising method for sludge disintegration.
US can be used not only as an independent pretreatment method for WAS, but also in combination with other methods, during which synergistic effect on disintegration of WAS could be obtained, causing more organic substrates release (Ushani et al., 2017, Ince, 2018). Meanwhile, those combined methods can make sludge treatment more energy-efficient and cost effective (Kavitha et al., 2016). For instance, Kim et al. (2010) reported that the 16.6% of synergetic effect could be achieved in sludge disintegration when US and alkaline pretreatment was combined. In addition, as complex heterogeneous substance, sludge not only is rich in organic matters but also contains large amount of refractory organic contaminants (e.g., bisphenol A, antibiotics and polycyclic aromatic hydrocarbons) (Matouq et al., 2008, Virkutyte and Rokhina, 2010). Although ultrasonic can produce some active free radicals which can degrade these contaminants to some extent, it is difficult to achieve satisfied degradation by solo US and the corresponding energy consumption is huge and uneconomical (Ince, 2018, Nikfar et al., 2016). Therefore, combination of US with other advanced oxidation processes (AOPs) is economically more attractive than using US alone (Mahamuni and Adewuyi, 2010, Khanal et al., 2007). For instance, Nikfar et al. (2016) reported that combined US/H2O2 achieved 98.65% degradation of bisphenol A, which was respectively 51.65% and 68.65% higher than that of solo US and solo H2O2. Considering the large amount of WAS generated per day and the concentrated refractory organic contaminants, any improvement in current solo US pretreatment, during which simultaneously enhance the disintegration of WAS and degradation of contaminants, would have important economic and ecological benefits.
Calcium peroxide (CaO2), a sort of the versatile and safe inorganic peroxides, is considered as a “solid form H2O2” (Qian et al., 2013). When it is dissolved in water, H2O2, O2 and Ca(OH)2 will be slowly released (Zhang et al., 2015, Wang and Li, 2016). In recent years, in addition to the application in the in-situ degradation of contaminants in water or soil, CaO2 in pretreatment of sludge has also been studied. It was reported that 27% reduction of VSS and more than 80% removal of endocrine disrupting compounds (e.g., estrone, bisphenol A and 17b-estradiol) were simultaneously achieved when WAS was added to 0.34 g/g TSS CaO2 (Zhang et al., 2015). Free radicals (i.e. HO· and·O2−) and alkali produced by CaO2 play a major role in the above process (Zhang et al., 2015). Based on the laws of thermodynamics and arrhenius equation, the diffusion of free radicals and alkali as well as chemical reaction can be improved under instantaneous high temperature and high pressure produced by US. Hence, we hypothesized that synergistic effects on the disintegration of WAS and the removal of contaminants might be obtained by combining CaO2 with US pretreatment, increasing hydrolysis rate and methane production whereas decreasing the risks to the ecological environment than solo pretreatment. However, up to now, it is unknown whether synergistic effect occurs when CaO2 and US were combined.
Thus, the aim of this study is to explore the feasibility of the combination of CaO2 and US on improving the production of methane from WAS. Firstly, the performance of pretreatment of WAS using CaO2 (0.05, 0.1 and 0.2 g/g VSS) + US (0.5, 1, 2 W/ml) on methane yield was comprehensively evaluated. Then, the underlying mechanisms of how the improvement on methane yield was achieved by combining CaO2 and US pretreatment were explored through the analysis of combined effect on WAS disintegration and biodegradability of the organics released. Moreover, the hydrolysis rate and the biochemical methane potential (BMP) of WAS using un-pretreatment, solo pretreatment and combined pretreatment were evaluated and compared using model-based analysis. Finally, the effect of pretreatments on the removal of refractory organic contaminants enriched in sludge was also evaluated. The findings presented here enhanced the application of US in anaerobic digestion, benefiting to form a promising method for simultaneous energy recovery and contaminants removal during the sludge disposal.
Section snippets
The characterization of CaO2 and sludge
The CaO2 (98%) was purchased from Shanghai Macklin Biochemical Co., Ltd., China. WAS utilized in this study was taken from a municipal WWTP in Changsha, China. The sludge retention time (SRT) of the WWTP was approximately 20 d. The collected WAS was filtered from a 2 mm × 2 mm strainer and then placed in the 4 °C refrigerator for at least 24 h prior to use. The inoculum for the subsequent BMP tests were withdraw from an anaerobic digester operated in our laboratory. The main characteristics of
Determination of optimal condition for CaO2/ultrasonic pretreatment
The corresponding 27 sets of experimental results were shown in Table 2. It could be found that both CaO2 and US affect the methane yield. At CaO2 dose of 0.05 g/g VSS, methane yield increased significantly with increasing US density or US time. When CaO2 dose was increased to 0.1 and 0.2 g/g VSS, methane production at any US density or US time was higher than that of at CaO2 dose of 0.05 g/g VSS. However, the change in methane production was not very large with the increase of CaO2 dose, US
Conclusion
This study demonstrated CaO2 combined with US pretreatment as a feasible and novel method can effectively enhance the production of methane and simultaneously improve the removal of refractory organic contaminants contained in WAS. The optimal pretreatment condition was 0.1 g CaO2/g VSS combined with US (1 W/ml, 10 min), after which the CH4 cumulative production from anaerobic digestion was 1.36-fold relative to the control. Mechanism investigations revealed that the combined pretreatment
Acknowledgement
This study was financially supported by the National Natural Science Foundation of China, China (51508178, 51779089, and 51521006), Natural Science Funds of Hunan Province for Distinguished Young Scholar (2018JJ1002) and Planned Science and Technology Project of Hunan Province, China (No. 2017WK2091).
Notes
The authors declare no competing financial interest.
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