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Front. Environ. Sci. Eng. 2021, 15(4): 71 https://doi.org/10.1007/s11783-020-1364-4 FEATURE ARTICLE High-solid anaerobic digestion of sewage sludge: achievements and perspectives 1 1 1,2 Ying Xu , Hui Gong , Xiaohu Dai (✉) 1 State Key Laboratory of Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Tongji University, Shanghai 200092, China 2 Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China HIGH LIGHTS GRAPHIC A BSTRA C T � High-solid anaerobic digestion (HS-AD) of sewage sludge (SS) is overviewed. � Factors affecting process stability and perfor- mance in HS-AD of SS are revealed. � HS effect and knowledge gaps of current research on the HS-AD of SS are identiﬁed. � Future efforts on addressing knowledge gaps and improving HS-AD of SS are proposed. AR TICL E I N F O ABSTRA CT Article history: Received 3 July 2020 High-solid anaerobic digestion (HS-AD) has been applied extensively during the last few decades for Revised 23 September 2020 treating various organic wastes, such as agricultural wastes, organic fractions of municipal solid wastes, and kitchen wastes. However, the application of HS-AD to the processing of sewage sludge Accepted 30 September 2020 (SS) remains limited, which is largely attributable to its poor process stability and performance. Available online 4 November 2020 Extensive research has been conducted to attempt to surmount these limitations. In this review, the main factors affecting process stability and performance in the HS-AD of SS are comprehensively reviewed, and the improved methods in current use, such as HS sludge pre-treatment and anaerobic co- Keywords: digestion with other organic wastes, are summarised. Besides, this paper also discusses the High-solid effect characteristics of substance transformation in the HS-AD of SS with and without thermal pre- Anaerobic fermentation treatment. Research has shown that the HS effect is due to the presence of high concentrations of Methane production substances that may inhibit the function of anaerobic microorganisms, and that it also results in poor Biodegradability mass transfer, a low diffusion coefﬁcient, and high viscosity. Finally, knowledge gaps in the current Sludge treatment research on HS-AD of SS are identiﬁed. Based on these, it proposes that future efforts should be devoted to standardising the deﬁnition of HS sludge, revealing the law of migration and transformation of pollutants, describing the metabolic pathways by which speciﬁc substances are degraded, and establishing accurate mathematical models. Moreover, developing green sludge dewatering agents, obtaining high value-added products, and revealing effects of the above two on HS-AD of SS can also be considered in future. © The Author(s) 2020. This article is published with open access at link.springer.com and journal.hep. com.cn increased. Consequently, the production of sewage sludge 1 Introduction (SS), a by-product of biological wastewater treatment, in wastewater treatment plants (WWTPs) has continuously With the increasing global population and the growth of increased in recent years (Xu and Dai, 2020). For example, cities, the quantity of municipal wastewater has rapidly by the end of 2019, SS production (deﬁned as having an 80% moisture content) in China had exceeded 50.0 million tons/year, and it is predicted to exceed 65.0 million tons/ ✉ Corresponding author year between 2020 and 2025 (Geng et al. 2020). Notably, E-mail: email@example.com 2 Front. Environ. Sci. Eng. 2021, 15(4): 71 SS contains large concentrations of toxic, harmful and (FAN)< 600 mg/L, the HS-AD of SS was satisfactorily perishable pollutants (i.e., biological protein, polysacchar- stable. Moreover, they found that although methane ides, microbial cells, and its secretion etc.), which may production and VS reduction by the HS-AD of SS were cause great environmental harm if not properly treated. similar to those by the LS-AD of SS, with the same solid Anaerobic digestion (AD) is one of the most popular and retention time (SRT), a much higher volumetric methane promising methods of treating the sludge, because it can production rate was achieved by the HS-AD system. reduce the amount of sludge, kill pathogenic microorgan- Hidaka et al. (2013) also reported that the AD of SS isms, and recover the bio-energy such as methane at the containing 10% TS can be successfully achieved under same time. mesophilic conditions, and highlighted that controlling The earliest application of AD is thought to have total ammonia concentration renders the HS-AD of SS th commenced in the 19 century, and it has gradually suitable for use in small facilities. These ﬁndings were become an accepted technology for the treatment of further conﬁrmed by Liao et al. (2014), who found that the biodegradable organic wastes (McCarty, 2001). For HS-AD of SS signiﬁcantly increased the volumetric biogas example, between 1995 and 2010, approximately 150– production rate and the treatment capability of digesters. 200 large-scale AD plants were established across Europe, These researchers also proposed that HS-AD offers an with a capacity increase of 6 million tonnes of biomass attractive option for treating dewatered sludge, which annually (Fagbohungbe et al., 2015). In general, non-SS could provide a new direction for the anaerobic treatment organic waste is categorised in terms of its total solids (TS) of SS. content, with low-solid AD (LS-AD) systems processing Figure 1 shows the annual numbers of publications on non-SS organic waste with a TS content< 15%, and high- ScienceDirect containing the terms “HS-AD of SS” and solid AD (HS-AD) systems processing non-SS organic “HS-AD”. Notably, the numbers of such publications have waste with a TS content>15% (Rapport et al., 2008; Li increased in the last 15 years, with more than twice as et al., 2011). As SS contains a large volume of water, many publications containing the term “HS-AD” than the conventional AD operates on SS with a low solid content term “HS-AD of SS” in this period. This indicates that HS- (0.2%–5%), Zhang et al. (2015 and 2016) explored the AD has been mainly applied to the processing of non-SS inﬂuence of sludge TS content on AD and diffusion organic wastes in the last 15 years, as conﬁrmed by recent behaviour, and concluded that a TS content of 6% studies (Li et al., 2011; Fagbohungbe et al., 2015; André et represented the boundary between the LS-AD and HS- al., 2018). In addition, a careful examination of the Fig. 1 AD of SS. HS-AD systems have also been applied to treat shows that the annual number of published papers non-SS solid organic materials, such as yard waste, food containing the term “HS-AD” did not increase from wastes, and organic fractions of municipal solid wastes 2006 to 2010, but sharply increased after 2011. From 2006 (OFMSW) (Li et al., 2011). However, the use of LS-AD to 2008, fewer than 10 papers contained the term “HS-AD systems to treat SS is not always feasible in small-scale of SS”, but this number rapidly increased from 2008 to WWTPs, or if SS has a low organic-matter content, i.e., 2017. It can be seen that all of the publications containing volatile solids (VS)/TS< 50%. For example, the LS-AD of the term “HS-AD of SS” between 2006 and 2020 were SS has not been well applied in China: up to 2019, only 70 research papers, and there has been no literature review of of China’s 5200 WWTPs incorporated LS-AD systems, the research on HS-AD systems used to process SS. and only 20 of these were operated routinely. The main Accordingly, the aims of this paper are to provide a reasons for this are poor management, economic limita- comprehensive review of the research progress in the tions, and inadequate planning, as well as the character- development of HS-AD systems for SS processing, to istics of SS (i.e., low VS/TS) in China. Speciﬁcally, in identify knowledge gaps, and to discuss the future many areas of China, the VS content of sludge has been directions of research to improve the HS-AD of SS. found to be much lower (typically< 55% of TS) than that in developed countries (usually >70% of TS) (Duan et al., 2016; Xu et al., 2020a). HS-AD may be a viable way to 2 Overview of current research on the solve these problems, because this SS processing techni- HS-AD of SS que uses a smaller reactor volume, has lower energy requirements for heating, generates less wastewater, and 2.1 Factors affecting the HS-AD of SS has a higher volumetric biogas production rate than LS- AD. Recently, SS dewatering has been developed to reduce Although the HS-AD of SS was proposed more than 20 years ago (PWRI, 1997; Hidaka et al., 2013), it is still not a SS volume, which enhances the HS-AD process. For widely used technique. The instability of HS-AD perfor- example, Duan et al. (2012) proved the feasibility of HS- mance is a key bottleneck to its wide uptake. There are AD of SS under mesophilic conditions by using dewatered many factors that affect the stability of HS-AD of SS. SS (TS contents of 10%,15% and 20%), and also found According to the published literatures, the operating that with a concentration of free ammonia-nitrogen Ying Xu et al. High-solid anaerobic digestion of sewage sludge: achievements and perspectives 3 Fig. 1 ScienceDirect bibliometric study with the topics “high-solid anaerobic digestion (HS-AD) of sewage sludge (SS)” and “high- solid anaerobic digestion (HS-AD)” (June 2020). factors and the intrinsic factors are two main factors decreased. They attributed these ﬁndings to the fact that a affecting the HS-AD performance of SS. These main high TS concentration led to the rapid generation of a high inﬂuencing factors are summarized in Table 1. concentration of metabolites (i.e., volatile fatty acids (VFAs) and ammonia), which then accumulated rather than being transformed further. Zhang et al. (2015 and 2.1.1 The main operating factors affecting the HS-AD of SS 2016) demonstrated that the inhibition of mass transfer was Recently, Duan et al. (2012) proved that the semi- a non-negligible problem during the HS-AD of SS, as they continuous mesophilic HS-AD of SS with HS concentra- found that increasing TS from 6% to 15% without agitation tions of 10%,15%, and 20% was possible, which suggests led to sharp decreases in diffusion coefﬁcients. They that satisfactory stability of the HS-AD of SS can be proposed that solid concentration has a signiﬁcant achieved by adjusting the main process parameters. As inﬂuence on the mass transfer during HS-AD. Importantly, shown in Table 1, the main operating parameters that affect Liao and Li (2015) highlighted that the inhibition of mass the efﬁciency of HS-AD of SS — namely solid concentra- transfer in HS sludge can be relieved by improved tion, agitation, SRT, temperature, and pH — are thus agitation. They conducted a pilot-scale HS-AD of SS for regaining attention. 9.5 months using an enhanced stirring system, and found First of all, the most direct factor is the magnitude of the that the VS reduction and biogas production were similar HS concentration, which has been considered to limit the to those achieved by the LS-AD of SS, which was efﬁciency and stability of HS-AD of SS (Lay et al., 1997; consistent with the ﬁndings of Duan et al (2012). These Liao et al., 2014). For example, Lay et al. (1997) ﬁndings also suggest that the HS-AD of SS may need a investigated the effects of moisture content on the HS- special anaerobic digester with the enhancement of stirring AD of SS under mesophilic conditions and found that the impeller and mix system, which is different from the relative methanogenic activity decreased from 100% to conventional anaerobic digester. 50% with a decrease in SS moisture content from 96% to The SRT is a key parameter in the anaerobic treatment of 90%, which indicated that the HS concentration restricted SS. Typically, an appropriate SRT is crucial for balancing the mass and energy transport in the biochemical reactions hydrolysis-acidiﬁcation and methanogenesis in the HS-AD of the HS-AD process. These ﬁndings were further of SS. For example, it is well known that a long SRT leads conﬁrmed by Le Hyaric et al. (2011), who have reported to the increased removal of VS, especially from the slowly that a low water content in sludge decreased molecular degradable organic matters of SS (Kapp H., 1984; Young diffusivity and resulted in a substantial decline in et al., 2013; Jahn et al., 2016), while a long SRT decreases methanogenic activity. Liao et al. (2014) also found that SS treatment efﬁciency and thus increases SS disposal with an increase of TS from 4.47% to 15.67%, the slow costs (Young et al., 2013). Therefore, reducing SRT degradation period was prolonged and the biogas yield becomes an important way to improve the HS-AD of SS. 4 Front. Environ. Sci. Eng. 2021, 15(4): 71 Table 1 The main factors affecting the HS-AD performance of SS Factor Stability Description Microbial Activity Biogas/Methane Production VS Reduction References Solid The stability decreases with an Methanogenic activity Decrease with an increase of TS Degradation of VS Lay et al., 1997; Le Hyaric et al., 2011; concentration increase of TS content from 6% decreases from 100% to content from 4.47% to 15.67% is prolonged Liao et al., 2014; Zhang et al., 2015; to 15% 50% with an increase of TS Zhang et al., 2016 content from 4% to 10% Agitation The stability increases – Increase by improving agitation Increase by Duan et al., 2012; Liao and Li, 2015 by improving agitation improving agitation SRT The stability decreases with – Increase in biogas-production rate Increase with an Kapp, 1984; Nges and Liu, 2010; a decrease of SRT from 15 days with a decrease of SRT from increase of SRT Young et al., 2013; Jahn et al., 2016 to 10 days 35 days to 12 days Temperature The stability decreases with the The mesophilic HS-AD has The thermophilic HS-AD Increase with an Hidaka et al., 2013; Wang et al., 2014; temperature shift from a richer and more diverse shows better biogas production increase of temperature Jahn et al., 2016; Wu et al., 2020 mesophilic to thermophilic active microbial community than mesophilic process than the thermophilic process pH The process may fail at Methanogenic activity decreases –– Lay et al., 1997; Bitton, 2002; apH< 6.1 or>8.3 at a pH< 6.3 or>7.8 Gerardi, 2003; Xu et al., 2020a Ammonia/ The process may fail at FAN Methanogenic activity Decrease with an increase – Kayhanian, 1994; Lay et al., 1997; Ammonium concentration>600 mg/L and sharply decreases with an of TAN concentration Duan et al., 2012; Li et al., 2015b; stress TAN concentration>4000 mg/L increase of TAN concentration Li et al., 2017b from 4090 mg/L to 5550 mg/L VFAs The process may fail at VFA Methanogenic activity Decrease with the accumulation – Boe and Angelidaki, 2012; Duan et al., concentration>4500 mg/L decreases with the of VFAs 2012; accumulation of VFAs Zhang et al., 2014; Wang et al., 2018; Yin and Wu, 2019; Zhou et al., 2020 Toxic and harmful The stability decreases with the Decrease with an increase Increase at 100 mg/L of – Boráň et al., 2010; Luo et al., 2011; substances increase of concentration in concentration of antibiotic antibiotic residues; Decrease at Qi et al., 2011; Dai et al., 2014b; residues and PAM 500 mg/L of antibiotic residues; Dai et al., 2015; Litti et al., 2019; Decrease with the presence of PAM Zhi and Zhang, 2019 Rheological The stability decreases with an – Decrease with high viscosity – Kirby, 1988; Slatter, 1997; properties increase in the viscosity or an or low diffusion coefﬁcient Cheng and Li, 2015; Sajjadi et al., 2016; decrease in the diffusion coefﬁcient Zhang et al., 2016; Hu et al., 2018 Notes: HS-AD: high solid-anaerobic digestion; SS: sewage sludge; VS: volatile solid; TS: total solid; SRT: solid retention time; FAN: free ammonia-nitrogen; TAN: total ammonia-nitrogen; VFAs: volatile fatty acids; PAM: polyacrylamide. Ying Xu et al. High-solid anaerobic digestion of sewage sludge: achievements and perspectives 5 Nges and Liu (2010) shortened the SRT from 35 days to 12 prone to inhibition by ammonia. They also found that the days in the HS-AD of SS under both thermophilic and mesophilic HS-AD of SS was performed by a substantially mesophilic conditions, and found that this increased the richer and more diverse active microbial community than biogas-production rate and volumetric methane productiv- the thermophilic HS-AD of SS, which may explain why ity, and decreased the VS-reduction efﬁciency. They also the thermophilic HS-AD of SS is a more unstable process. stated that the short SRT operation usually means more These ﬁndings were supported by Wang et al. (2014), who sludge can be treated and time is saved while utilizing the achieved the thermophilic HS-AD of SS with an initial TS same facility (Nges and Liu, 2010). Jahn et al. (2016) content of 9.5%, conﬁrming the feasibility of this process. studied the inﬂuence of SRT on the HS-AD of SS by As pH can inﬂuence enzymatic activity, it is another key performing 3.0 L semi-continuous experiments under parameter affecting the HS-AD of SS. It has been shown mesophilic conditions. They found that the HS-AD of SS that the activity of methanogens decreases at a pH< 6.3 with an initial TS of 6.7%–7.8% was possible, as long as a or>7.8 (Bitton, 2002; Xu et al., 2020a), and Gerardi minimal SRT of 15 d was ensured: the HS-AD of SS with a (2003) determined that most anaerobic bacteria perform 10 d SRT resulted in an unstable process. These results well at a pH range of 6.8–7.2. However, these ﬁndings were supported by the DWA (2014), who provided the were obtained under LS-AD conditions, so it is not clear design recommendations for digesters, including a mini- whether this pH range is the most suitable for anaerobic mum SRT of 15 d. Moreover, STR usually correlates with bacteria in the HS-AD of SS. Based on calculations, Lay the organic loading rate (OLR), and it is critical to the et al. (1997) proposed that a high rate of methane stable operation of HS-AD that the OLR is maintained in production during the HS-AD of SS was obtained at a the range at which the rate of microbial decomposition of pH of 6.8, and that the process may fail at a pH< 6.1 or solids and metabolism of organic molecules is sufﬁcient to >8.3. In their study, the lag-phase time for methane prevent the accumulation of inhibitors (i.e., acids and free production at pH 6.8 reached a minimum in the HS-AD of ammonia). SS with an initial TS content of 4%–10%. Unfortunately, Temperature is another important operational parameter the optimal pH value for the HS-AD of SS with an initial for the HS-AD of SS. Mesophilic and thermophilic TS content >10% has been rarely reported. temperature conditions are the most widely used for AD. Although thermophilic AD reduces VS more than 2.1.2 The main intrinsic factors affecting the HS-AD of SS mesophilic AD, it is a more expensive process. In addition, microorganisms in thermophilic AD prefer a narrow Although the HS-AD of SS is a promising technology that temperature range, and thus temperature shifts can rapidly can be performed in smaller reactors and consumes less destabilise the process (Jahn et al., 2016). However, it is heating energy than LS-AD, the high concentration of not clear which temperature is most favourable for the HS- potentially inhibitory substances (i.e., ammonia, VFAs, AD of SS. Hidaka et al. (2013) studied the performance of and toxic and harmful substances) released from HS SS mesophilic and thermophilic HS-AD of SS in laboratory- and poor rheological properties of HS SS may deleter- scale continuous reactors for 600 days, and found that the iously affect the efﬁciency of the AD process, as shown in performance of mesophilic HS-AD of SS with an initial TS Table 1. content of 10% was stable at a total ammonia-nitrogen It has been reported that the FAN concentration can (TAN) concentration of 3000 mg N/L and achieved a 60% reach up 600 mg/L in a stable system in which the HS-AD VS removal, similar to the performance of LS-AD of SS. of SS was occurring (Duan et al., 2012), which is Conversely, the thermophilic HS-AD of SS with an initial signiﬁcantly higher than the inhibition threshold (200 TS content of 7.5% failed when the TAN concentration mg/L) of conventional systems which perform the LS-AD was>2000 mg N/L, and a few weeks were required for the of SS (Yenigün and Demirel, 2013; Liu et al., 2016b). It is methanogenic activity to recover from inhibition. Hidaka notable that the HS-AD of SS is stable when the TAN is et al. (2013) also revealed that the high viscosity of HS 4000 mg/L, which is signiﬁcantly greater than the sludge created a challenge to its mesophilic processing, inhibition threshold (1000 mg/L) for the HS-AD of other and that a more careful operation would be required for the organic wastes (Kayhanian, 1994; Duan et al., 2012). This stable thermophilic HS-AD of SS. In other work, Wu et al. is presumably attributable to the special micro-ecosystems (2020) also recently described a comparative study of operating in the HS-AD of SS. It is clear that the mesophilic (37°C2°C) and thermophilic (53°C2°C) overproduction of ammonium can inhibit methanogenesis, HS-AD of SS with an initial TS content of 10% at an OLR and this is a decisive factor that may signiﬁcantly of 4 g VS/(L$d) for 170 days, and found both the imbalance microbial community structure in an HS-AD mesophilic and the thermophilic HS-AD of SS were stably process (Kayhanian, 1994; Li et al., 2015b; Li et al., maintained, although the thermophilic HS-AD was slightly 2017b). For example, Lay et al. (1997) found that inhibited by ammonia. They reported that the thermophilic methanogenic activity decreased by 10% with an increase HS-AD of SS showed better VS reduction and biogas in TAN concentration from 1670 mg/L to 3720 mg/L, and production than the mesophilic process, but was more decreased by 50% with an increase in TAN concentration 6 Front. Environ. Sci. Eng. 2021, 15(4): 71 from 4090 mg/L to 5550 mg/L. Moreover, methanogenic adding magnetite (Fe O ). It has been reported that adding 3 4 activity was lost when the TAN concentration was>5880 scrap iron and nano zero-valent iron to a HS-AD system mg/L. Li et al. (2017a) explored the effects of ammonium processing SS accelerates the conversion of VFAs into stress on metabolic pathways in bacterial and archaeal methane (Zhang et al., 2014; Zhou et al., 2020). For communities in the HS-AD of SS. As depicted in Fig. 2, example, Zhang et al. (2014) reported that the scrap iron three main methanogenic pathways including acetoclastic, with ferric oxides on the surface can induce the microbial hydrogenotrophic, and methylotrophic pathways were iron reduction, which accelerated the conversion of VFAs. identiﬁed and the acetoclastic methanogenesis was the Zhou et al. (2020) found that with the addition of nano dominant pathway in the HS-AD of SS with the effects of zero-valent iron the propionic acid was more easily ammonium stress. Li et al. (2017a) also revealed that in the decomposed in HS-AD of SS. This result is consistent HS-AD of SS, the expression of only 6 of the total 22 with the ﬁndings of Yin and Wu (2019), who revealed that ammonium-related genes was upregulated, and that the the addition of conductive materials in AD system can expression of some amino-acid-related genes decreased effectively accelerate the degradation of propionate and under ammonium stress, thereby resulting in an accelera- butyrate via enhancing the DIET. In addition, Lv et al. tion of the syntrophic acetate oxidation reaction. For (2020) proposed that enriching syntrophic associations to example, the acetate kinase (AckA) and phosphate synchronously enhance their ecological function was a acetyltransferase (PTA), which are involved in the useful solution for alleviating VFAs accumulation. In reversible reaction of acetate conversion, enriched from related work, Nguyen et al. (2019) developed an 8670 and 6858 hits to 10004 and 7120 hits under intermittent oxidation-reduction potential (ORP)-con- ammonium stress, respectively, suggesting that the active trolled micro-aeration system to prevent the accumulation syntrophic acetate oxidation performance with the effect of of VFAs in the HS-AD via regulating facultative hetero- ammonium stress. Although how to effectively control trophs. Speciﬁcally, they used micro-aeration to precisely ammonium stress in the HS-AD of SS is rarely reported, control the ORP in AD and found that the VFAs were various strategies for recovering the AD performance of rapidly consumed by the facultative heterotrophs. They other organic wastes from ammonia inhibition have been concluded that the intermittent ORP-controlled micro- studied (Rajagopal et al., 2013). For example, adding the aeration system could enrich the facultative heterotrophs biochars, zeolites, and activated carbon into the AD can and conserve crucial anaerobic niches for methanogens, reduce the ammonium level via adsorption reactions and it was a useful tool for recovering the anaerobic (Mumme et al., 2014; Cuetos et al., 2017; Poirier et al., digester on the verge of failure, which is due to the 2017). In addition, Liu et al. (2020a) reported that the accumulation of VFAs. Although this ﬁnding is not based carbon- and iron-based additives (i.e., zero valent iron) can on the HS sludge system, it has an important reference for play important roles in accelerating microﬂora acclimation alleviating VFAs accumulation in the HS-AD process of to tolerate ammonia stress. These strategies could provide SS. some important references for relieving ammonia inhibi- Toxic and harmful substances in SS may also affect the tion in the HS-AD process of SS. efﬁciency of the HS-AD of SS, typically in a concentra- The accumulation of VFAs is another problem with the tion-dependent manner. For example, Zhi and Zhang HS-AD of SS. For example, Duan et al. (2012) reported (2019) investigated the effects of antibiotic residues (i.e., that the VFAs concentration in a reactor in which the HS- residues of oxytetracycline, sulfadimethoxine, sulfa- AD of SS was performed was as high as 4500 mg/L, and methoxazole, enroﬂoxacin, ciproﬂoxacin, oﬂoxacin, and that this led to the failure of the HS-AD process. Boe and norﬂoxacin) on the methane production and microbial Angelidaki (2012) also reported that at high OLR activity during the HS-AD of SS. They found that low anaerobic digester is susceptible to failure due to the concentrations of antibiotics (10 mg/L) had no obvious accumulation of VFAs, which is caused by an imbalance effect on methane production; that medium concentrations between syntrophic bacteria and methanogens. Typically, (100 mg/L) signiﬁcantly stimulated methane production; HS-AD has a high OLR and a low rate of diffusion of and that high concentrations (500 mg/L) inhibited methane intermediate metabolites, with the former favouring the production at the initial stage of the HS-AD process, but production of VFAs and the latter disfavouring the enhanced methane production after recovery at the late metabolism of VFAs. This therefore results in the stage of the process. They also revealed that these accumulation of VFAs, which can directly cause the antibiotics affected the archaeal community, but did not instability of the HS-AD of SS. Wang et al. (2018) found signiﬁcantly affect the bacterial community. In related that the conventional process of interspecies hydrogen research, polyacrilamide (PAM), a refractory and common transfer (IHT) between anaerobic microorganisms was ﬂocculant used in SS dewatering, has been found to be often inhibited in the HS-AD of SS, but that promoting the ubiquitous in HS sludge (Boráň et al., 2010; Qi et al., 2011; direct interspecies electron transfer (DIET) pathway in HS- Dai et al., 2015; Litti et al., 2019). Dai et al. (2014b) AD effectively prevented the accumulation of VFAs. Thus, determined that the biodegradation of PAM in SS they enhanced the DIET pathway in the HS-AD of SS by processed via HS-AD was usually accompanied by the Ying Xu et al. High-solid anaerobic digestion of sewage sludge: achievements and perspectives 7 accumulation of high concentrations of toxic acrylamide conﬁrmed by Litti et al. (2019), who reported that the monomers (AMs), which inhibited microbial activity (Luo addition of PAM led to decreased methane production in et al., 2011). They also revealed that PAM can be the HS-AD of SS. The researchers attributed this to the hydrolysed at different position of its carbon-chain back- formation of large ﬂocs and the consequent suppression of bone, and that the hydrolysed PAM fragments combined mass transfer. Unfortunately, the migration and transfor- with tyrosine-rich proteins to form colloid complexes mation processes of these toxic and harmful substances during the HS-AD of SS. These ﬁndings were further during the HS-AD of SS remain unclear. However, these Fig. 2 Hit numbers of genes involved in the relevant methanogenesis pathways in HS-AD process of SS without and with ammonium stress (Reprinted from Li et al., 2017a, Copyright (2017), with permission from Elsevier). (The acetoclastic pathway is shown in red, the hydrogenotrophic pathway is marked in blue, and the methylotrophic pathway is represented by the green dashed line). Abbreviation: FdhA, glutathione-independent formaldehyde dehydrogenase; EchA, hydrogenase subunit A; FmdA, formylmethanofuran dehydrogenase subunit A; FTR, formylmethanofuran-tetrahydromethanopterin N-formyltransferase; MCH, methenyltetrahydromethanopterin cyclohydrolase; MTD, methylenetetrahydromethanopterin dehydrogenase; MER, coenzyme F420-dependent N5, N10-methenyltetrahydromethanop- terin reductase; MtrA, tetrahydromethanopterin S-methyltransferase; MtaA, [methyl-Co(III) methanol-speciﬁc corrinoid protein]:coenzyme M methyltransferase; McrA, methyl-coenzyme M reductase alpha subunit; AckA, acetate kinase; ACSS, acetyl-CoA synthetase; PTA, phosphate acetyltransferase; HdrA, heterodisulﬁde reductase subunit A; CdhC, acetyl-CoA decarbonylase/synthase complex subunit beta. 8 Front. Environ. Sci. Eng. 2021, 15(4): 71 ﬁndings suggest that although high concentrations of limit the widespread adoption of AD processes. Inhibition metabolism-inhibiting substances may be present or of mass transfer, poor diffusion of intermediate metabo- formed during the HS-AD of SS, the tolerance of the lites, and high sludge viscosity are also a problem. Pre- system to these may be high, as long as its operating treating sludge has been reported to be effective in parameters are effectively controlled. improving its biodegradability and hydrolysis via AD It has been reported that the rheological properties of SS (Xu et al., 2020a). Many physical, chemical and biological play an important role in its performance in anaerobic pre-treatments (and combinations thereof) for LS sludge, digesters, especially in the design, selection, and operation prior to AD, have been reported (Neumann et al., 2016; of anaerobic digesters (Baudez et al., 2011; Baroutian Gonzalez et al., 2018; Xu et al., 2020a), but only the et al., 2013; Dai et al., 2014a). Moreover, the rheological thermal and alkaline methods have been used to pre-treat properties of normal SS with a TS content of 0.2%–4% HS sludge prior to HS-AD (Jolis, 2008; Li et al., 2015a; have been well studied (Lotito et al., 1997; Ruiz-Hernando Guo et al., 2016; Liao et al., 2016). Aside from economic et al., 2013; Dai et al., 2014a). However, the rheological reasons, these two pre-treatment methods may be favoured properties of HS sludge are different from those of the due to their substantial improvement of the rheological normal SS, and little relevant information is available on properties and organic solubilisation of HS SS, which this topic (Kirby, 1988; Slatter, 1997; Cheng and Li, 2015). enhances the HS-AD of SS. For example, Zhang et al. Therefore, it is necessary to reveal the characteristics of (2017) explored the effects of low- and high temperature rheological properties of HS sludge. It is well known that thermal pre-treatments on HS SS (TS content of 14.2% and an increase on SS concentration can result in an 18.2%). They found that with increasing treatment time, exponential increase in the viscosity of SS and an organic solubilisation increased logarithmically and the exponential decrease in its diffusion coefﬁcient (Zhang elastic modulus in the linear viscoelastic regime of HS et al., 2016). Cheng and Li (2015) found that HS sludge sludge decreased logarithmically, leading to a signiﬁcant with a TS content of 7%–15% exhibited thixotropic decrease in the SS viscosity, which was conducive to the properties. They also found that when the TS content subsequent HS-AD of the SS. The above results are also increased to 6%,8%,10%, and 12%, the viscosities of the consistent with those reported by Xue et al. (2015) and corresponding SSs increased by 5.0, 9.1, 25.7, and 24.9 Liao et al. (2016). The former showed that thermal pre- times, respectively. Moreover, Zhang et al. (2016) reported treatment is an effective method of increasing the organic that the diffusion coefﬁcient of SS decreased sharply as the solubilisation and decreasing the viscosity of HS sludge. TS content increased from 6% to 12%, and decreased The latter investigated the effects of low temperature gradually with an increase of TS content from 12% to 15%. thermal pre-treatment on the HS-AD of SS, and found that The high viscosity and low diffusion coefﬁcient of HS SS this increased the quantity of accessible substrates, can lead to non-uniform and non-ideal ﬂow conditions (i. decreased sludge viscosity, and even resulted in an increase e., incomplete mixing, short circuiting, and an increase in of biogas production in the HS-AD of SS. Alkaline pre- inactive and stagnant zones) in anaerobic digesters (Sajjadi treatment has also been used. Li et al. (2015a) proposed et al., 2016), which may further lead to the accumulation of that HS sludge may be more amenable to such pre- VFAs and FAN, and thereby destabilise the HS-AD of SS. treatment than LS SS, as the same extent of SS To improve the rheological properties of HS sludge, many disintegration could be achieved in HS SS as in LS SS, studies have been conducted. For example, Feng et al. but with less alkali. After alkaline pre-treatment of 8%– (2014) used thermal hydrolysis (170°C, 60 min) treatment 12% TS SS (30 min treatment with 0.05 mol/L NaOH), signiﬁcantly decreased the viscosity, shear stress, and they found that methane production was slightly increased viscoelasticity of HS sludge, which is further conﬁrmed by in the subsequent HS-AD process, while the digestion time the ﬁndings of Urrea et al. (2015) and Zhang et al (2017). was substantially decreased (24%–29%). In addition, the Liu et al. (2016c) investigated the effects of microwave- positive effects of thermal and alkaline pre-treatment of SS H O pretreatment on the rheological properties of HS have been combined in thermal-alkaline pre-treatment. For 2 2 sludge, and found that this method improved the sludge example, Guo et al. (2016) explored the effects of thermal- ﬂowability and decreased the viscoelasticity. Furthermore, alkaline pre-treatment (105°C–135°C and 5–35 mg NaOH/ Hu et al. (2018) conducted a ﬂow-ﬁeld investigation and g TS) on the HS-AD of SS, and concluded that this pre- proposed that multilayer impellers arranged abreast could treatment signiﬁcantly increased organic solubilisation and enable the more efﬁcient mixing of HS sludge, given its methane production during HS-AD. However, although rheological properties, and thus prevent the accumulation the thermal-alkaline pre-treatment is conductive to the of VFAs and ammonia and any subsequent inhibition of hydrolysis of sludge organic matter and more bio-methane HS-AD. can be further transformed by the organic solubilisation and hydrolysis, some organic pollutants and toxic 2.2 Pre-treatment features of the HS-AD of SS substances will also be released during this pre-treatment process. The concentrations of organic pollutants and toxic The poor biodegradability and slow hydrolysis rate of SS substances released from the HS sludge could be far higher Ying Xu et al. High-solid anaerobic digestion of sewage sludge: achievements and perspectives 9 than those released from the LS sludge, and these that the optimum mixing ratio (VS basis) of SS and CW pollutants and toxic substances also could affect the was 3:7 with an initial pH of 9.0, yielding a maximum subsequent HS-AD. Unfortunately, current researches on VFAs production of 98.33 g/kg TS and methane produc- the thermal-alkaline pretreatment of HS sludge prior the tion of >120.0 L/kg TS. They also highlighted that CM HS-AD of SS have ignored this point. increased the relative abundances of bacteria and archaea and the degradation of organic matters under the optimum 2.3 HS anaerobic co-digestion of SS and other organic conditions. wastes 2.4 Characteristics of substance transformation in the HS- Exploiting the characteristics of different organic wastes AD process of SS for energy recovery and waste disposal can optimise resource utilisation. As such, HS anaerobic co-digestion 2.4.1 Organic humiﬁcation in the HS-AD of SS (HS-AcD) of SS and other organic waste is a promising method of the utilisation and management of organic Organic humiﬁcation has been widely considered an waste. Notably, HS-AcD may be a more stable method important index of stabilisation in the treatment of SS than HS-AD of processing SS, due to its dilution of (Bernal et al., 2009), and can be realised in the AD of SS inhibitory substances, improvement of nutrient balance, (Provenzano et al., 2016; Xu et al., 2020b). Therefore, the and creation of synergies between microorganisms (Dai humiﬁcation of SS organic matter must be explored to et al., 2013; Aichinger et al., 2015; Lee et al., 2019). Lee et improve stabilisation during the HS-AD of SS. To reveal al. (2019) successfully conducted a long-term HS-AcD of the underlying mechanisms of humic formation and SS, food waste (FW) and yard waste (YW), achieving an transﬁguration in the HS-AD of SS, Tang et al. (2018) average VS reduction of 38% and methane production of monitored the aromaticity degree of humic-like substances 186 mL/g VS. They investigated the effects of the ratio of and the phytotoxicity of the digestate during a 48-day HS- substrate to inoculum (S/I), the mixing ratio of co- AD of SS. They found that there were signiﬁcant substrate, the inoculum source, and the alkalinity sources repolymerisation of aromatic substances and a positive on HS-AcD performance, and found that the highest correlation between digestate phytotoxicity and the degree methane production was obtained using the mixture of of substance aromaticity. They also proposed that the NaHCO and oyster shells as alkaline sources with the S/I aromatic repolymerisation of humics regulated the phyto- ratio of 1 (VS basis). Moreover, the presence of FW led to toxicity of digestate via reducing excessive salinity. Based a 1.43-fold higher methane production than from the HS- on these ﬁndings, they further investigated the effect of AD of SS. They also proposed that the mixing ratio of co- humiﬁcation on extracellular polymeric substances (EPS) substrate, the S/I, and the inoculum source are important in the HS-AD of SS (Tang et al., 2020), and found that the operational factors for the successful long-term perfor- hydrolysis and decomposition of extracellular protein mance of HS-AcD. Similarly, Dai et al. (2013) compared generated changes in the highly cross-linked structures of the stability and performance of the HS-AcD of SS and FW humics in EPS, resulting in the exposure of humic aromatic with those of the HS-AD of SS, and found that the addition groups and binding sites. Based on analysis of the electron of FW improved both system stability and volumetric exchange capacity and the metabolic activity of methano- biogas production. This was primarily attributed to the genesis, they proposed that structural changes in EPS dilution of ammonia and sodium ions. The researchers also proteins promoted the catabolism and anabolism of revealed that the mixing ratio of SS and FW determined the anaerobic microorganisms, and that the products of this performance of their HS-AcD system. This ﬁnding was metabolism, such as humic groups and active protein conﬁrmed by two further studies by Liu et al. (2016a) and derivatives, were beneﬁcial to EPS reconstruction in the Latha et al. (2019). Liu et al. (2016a) found that the HS-AD of SS. processing of LS sludge (VS/TS 41.6%) via HS-AcD was optimal with a 1:1 SS:FW mixing ratio (VS basis) in a 2.4.2 Characteristics of organic transformation in the HS- weak alkaline environment (pH 7.5–8.5), as this led to the AD of SS with and without thermal pre-treatment best synergetic effect. Latha et al. (2019) determined that the optimum mixing ratio of SS and FW in their HS-AcD As is well known to all, with the increase of TS content, the process was 1:3 (TS basis), with intermittent biogas physical, chemical and even biological reactions in the HS- recirculation, and that this mixing strategy increased the AD process of SS could be changed because of the blocked synergy of CO acidiﬁcation with high VFAs production. mass transfer, poor diffusion, and high viscosity. As a Cattle manure (CM) is another organic waste that is often result, the transformation of sludge organic matter would used for AcD with SS, because CM is unstable to AD, due inevitably be affected in the HS-AD process, and under- to the low C/N ratio of CM (Li et al., 2009). Dai et al. standing this transformation process is necessary to (2016) investigated the HS-AcD of SS and CM and found improve the HS-AD of SS. Han et al. (2017) investigated 10 Front. Environ. Sci. Eng. 2021, 15(4): 71 the organic transformation process during the HS-AD of HS-AD of SS being greater than that in the LS-AD, leading SS with and without thermal pre-treatment, by monitoring to the increased precipitation of heavy metal sulﬁdes. 3) variations in the chemical oxygen demand, and in the Due to the inefﬁcient hydrolysis of sulphurous proteins production of methane, carbohydrates, VFAs, and other during the HS-AD of SS, concentrations of sulphur- substances in SS containing nitrogen, sulphur and containing compounds remain low (Dai, 2016; Liao, 2016; phosphorus, as described in Fig. 3. They found, for Han et al., 2017). Another interesting phenomenon example, that thermal pre-treatment signiﬁcantly enhanced involving sulphurous substances was reported by Li et the biogas production rate during the HS-AD of SS, and al. (2020), who found that the thermal pre-treatment of SS resulted in increased methane content in the biogas. made HS-AD able to directly promote the transformation Without thermal pre-treatment, the biogas production rate of organic sulphur (OS) into volatile sulphur compounds and methane content yielded by the HS-AD of SS were (VSCs). They found that methyl mercaptan (MM), similar to those yielded by conventional AD. The above dimethyl sulﬁde (DMS), dimethyl disulﬁde (DMDS), and ﬁndings were further supported by Chen et al. (2018), who H S were typical VSCs, and that MM was converted into found that thermal pre-treatment shifted the methanogenic DMS (18%), DMDS (4%), and H S (78%) in biogas pathway from strict acetoclastic methanogenesis to generated from the HS-AD of SS with an initial TS content acetoclastic/hydrogenotrophic methanogenesis. In addi- of 10%, when SS had been thermally pre-treated. They tion, Han et al. (2017) found that thermal pre-treatment led also revealed that thermal pre-treatment increased the to more than 50% of the particulate nitrogen being activity of reductases such as adenine phosphate sulfate converted to a liquid-state during HS-AD, and that the reductase and sulﬁte reductase. These ﬁndings supported TAN concentration was increased to 3.57 g/L. However, those of previous researchers (Sommers et al., 1977; there was little effect on the transformation of phosphorus: Higgins et al., 2006; Moestedt et al., 2013; Dai et al., regardless of pre-treatment, 32%–35% of total phosphor- 2017), and the conversion pathway of sulphur substances ous (organic phosphorous and polyphosphate) was con- during the HS-AD of SS with thermal pre-treatment is verted to phosphate, primarily by the hydrolysis of depicted in Fig. 4. For example, the initial OS content of polyphosphate. One possible reason given by Liu et al. sludge decreased from 96% to 90% with thermal pre- (2020b) is that the thermal pre-treatment can improve the treatment, speciﬁcally, the initial methionine and cysteine release of phosphate, but this phosphate was subsequently contents of sludge decreased from 61% and 35% to 59% converted into a solid state during the HS-AD process by and 31%, respectively. However, the conversion pathway precipitation as struvite (NH MgPO $6H O), precipitation of sulphur substances during the HS-AD of SS without 4 4 2 with high concentrations of heavy metals, and adsorption pre-treatment has rarely been reported, although knowl- into microbial cells via the synthesis of adenosine tripho- edge of this is important for establishing a theoretical sphate (ATP). They also proposed that the neutralization of system for the HS-AD of SS, to enable improvement of the release of phosphate and redeposition makes it HS-AD processes. reasonable that the HS-AD process of SS with thermal pre-treatment has little effect on the transformation of phosphorus. 3 Knowledge gaps of current research on It has been reported that the H S content of biogas the HS-AD of SS generated by the HS-AD of SS with or without thermal pre-treatment is far lower than the H S content of biogas 3.1 The deﬁnition of HS sludge is not standardised generated by the LS-AD of SS. For example, Han et al. (2017) found that in their HS-AD of SS that the maximum As yet, there is no standard deﬁnition of HS sludge. For H S content in biogas was 168.019.2 mg/L, which was example, some researchers have reported conducting HS- far lower than that from the LS-AD of SS (approximately AD of an SS, and yet the TS content of the SS they used 1500 mg/L). This result was also conﬁrmed by Liao was only 4% (Lay et al., 1997). Chen et al. (2019) studied (2016), who found that the H S content of biogas the HS-AD of SS with an initial TS of 5%, while others generated by AD decreased with an increase in TS content, have used SSs with an initial TS of 10%,15%, and 20%, and that a maximum H S content of approximately 45 mg/ respectively (Duan et al., 2012), These different deﬁnitions L was present in biogas generated by the HS-AD of SS of HS sludge mean that it is difﬁcult to evaluate the with a TS of 20%. This phenomenon was attributable to the feasibility and applicability of the HS-AD of SS with effects of pH, heavy metals, and the hydrolysis of different TS contents, especially as the properties of SS from different WWTPs are also different. Even more sulphurous proteins, as follows. 1) As H S is an acidic importantly, without a standard deﬁnition of HS sludge, gas, it is consumed at a high pH; notably, the pH was 8.0 many research results can neither be effectively compared during the HS-AD of SS, which was greater than that nor function as references for establishing a theoretical during the LS-AD of SS (pH 7.0–7.5). 2) An increase in TS system of the HS-AD of SS. content results in the concentration of heavy metals in the Fig. 3 The diagram of the substance transformation ratios in the mesophilic/thermophilic HS-AD process of SS with and without thermal pre-treatment: (a) the diagram of the transformation ratios of COD, protein and carbohydrate; (b) the diagram of the transformation ratios of nitrogen, phosphorus and sulphur (Reprinted from Han et al., 2017, Copyright (2017), with permission from Elsevier). 12 Front. Environ. Sci. Eng. 2021, 15(4): 71 Fig. 4 Diagram of the conversion pathway of sulphur substances during the HS-AD process of SS with thermal pre-treatment ((Reprinted from Dai et al., 2017, Copyright (2017); Li et al., 2020, Copyright (2020) with permission from Elsevier). Ying Xu et al. High-solid anaerobic digestion of sewage sludge: achievements and perspectives 13 3.2 Migration and transformation of pollutants in the HS- to the application of large-scale HS-AD of SS (Duan et al., AD of SS is unclear 2012; Liao et al., 2014), and it is very important to evaluate and predict the process stability and performance of HS- It has been reported that SS contains the inert organic AD of SS. Mathematical modelling has been widely pollutants (e.g., microplastics, benzene, chlorophenol, regarded as an important tool to assess and predict process polychlorinated biphenyls, polychlorinated dibenzofurans performance (Mendes et al., 2015). The anaerobic etc.), inorganic pollutants (e.g., heavy metals), and digestion model No. 1 (ADM1) has been used to evaluate microbial pollutants (e.g., enterovirus, bacillus coli, and predict the HS-AD of SS for more than 15 years, but it protozoan, parasites, and their eggs) (Li et al., 2018; is better suited to predicting the AD of wastewater or LS Száková et al., 2019; Souza et al., 2020). Although the sludge (TS 0.2%–2%) than that of HS sludge (Abbassi- concentrations of these pollutants are low in the normal SS Guendouz et al., 2012; Mendes et al., 2015). In recent with a TS content of 0.2%–2%, these concentrations may years, with the development of solid-state AD process, the increase with the increase of TS content and the pollutants mathematical model of solid-state AD of organic waste are environmentally persistent and potentially toxic. More have been widely proposed and studied (Xu et al., 2015; seriously, based on the limited detection method, it is Wang et al., 2016), however, due to the large differences in difﬁcult to determine the migration and transformation of the structure and properties of SS compared with other organic pollutants in sludge, which also restricts the organic wastes, it is difﬁcult to apply the mathematical understanding and development of HS-AD process of SS model of solid-state AD of organic waste to describe the to a certain extent. Therefore, enough attention should be HS-AD of SS. Consequently, the use of HS-AD of SS is paid to the migration and transformation of these pollutants hindered by the inability to accurately predict its stability in the subsequent treatment. However, it is unclear that the and performance. Therefore, to improve the HS-AD of SS, migration and transformation of these pollutants in the HS- a relevant mathematical model is urgently needed. AD process of SS up till the present moment. 3.3 Metabolic pathways of organic matter in the HS-AD of 4 Future perspectives on HS-AD process of SS are not known SS Although the metabolic pathways of organic matter in AD Although HS-AD has been successfully applied to treat are well known (Pavlostathis and Giraldo-Gomez, 1991; organic waste, there have been comparatively few studies Batstone et al., 2002), they have been determined mainly of its utility in treating SS. Indeed, the above overview based on the AD of complex composite particulate waste shows that research on HS-AD of SS is in its infancy, and (CCPW), which can be assumed to be homogeneous many questions remain open. (Batstone et al., 2002). This approach is suitable for As mentioned, there is no standard deﬁnition of HS SS, describing the fate of CCPW in systems such as waste- and the optimal TS content of SS for AD treatment is water or waste-activated sludge (TS 0.2%–2%), because undetermined. The development of sludge dewatering the inherent properties of CCPW can be effectively means that high TS contents of SS can be easily obtained. maintained by retaining sufﬁcient water in these systems. Thus, a deﬁnition of HS sludge and the optimal TS content However, with increasing solid concentrations, the water of SS for AD should be explored. An increase in TS content of the CCPW decreases and the adjacent micro- content will, however, inevitably alter the structure and environment of the CCPW changes, which profoundly properties of SS; thus, it would be reasonable to deﬁne HS alters the adjacent ionic strength. This leads to changes in sludge according to its structure and properties. For the micro-interfaces between the CCPW and water, and example, there is no signiﬁcant difference in sludge subsequent changes in the interfacial structure and proper- structure within a certain range of TS content. Therefore, ties of the CCPW, ultimately causing a substantial a TS content that causes signiﬁcant changes in sludge difference in the metabolic pathways operating during structure should be used as a basis for a standard deﬁnition the HS-AD of CCPW vs those operating during the LS-AD of HS sludge. Similarly, a TS content that causes of CCPW. Therefore, the speciﬁc metabolic pathways of signiﬁcant changes in certain key properties of sludge, CCPW during HS-AD of SS must be investigated to enrich such as its diffusion coefﬁcient or viscosity, should also be the theory of AD and provide a direct theoretical basis for considered in making such a deﬁnition. Moreover, in order improving the HS-AD of SS. However, information on this to reduce digester volume and improve sludge treatment has rarely been reported. efﬁciency, the HS-AD is proposed via increasing the TS content of sludge. This means that it should be possible to 3.4 The mathematical model for the HS-AD of SS is substantially increase the TS content of SS without inadequate affecting the AD treatment efﬁciency of SS. However, due to the unique semi-rigid structure of sludge and the Process stability and performance are two key bottlenecks variety of potential inhibitors within sludge that may 14 Front. Environ. Sci. Eng. 2021, 15(4): 71 inhibit anaerobic microorganisms, a high TS content will application to modelling the HS-AD of SS, due to the lead to the formation of multi-material cross-linking substantial difference in the structure and properties of structures that are resistant to biodegradation and an other organic wastes compared with those of SS. However, increase in inhibitor concentration. It therefore likely that it is logical and it should be feasible to develop there is an optimal TS content for the HS-AD of SS, and mathematical models for the HS-AD of SS that are based this should be determined by exploring the effect of on the ADM1 and the reported mathematical tools for the various TS contents on HS-AD of SS. HS-AD of organic wastes. For example, retarded hydro- The migration and transformation of substances under- lysis of the substrate and poor microbial access to the pin the HS-AD of SS, and must be understood at a substrate are two key problems in the processing of both mechanistic level to enable the overall process to be SS and other organic wastes. Hence, developing an enhanced. There have been some studies on the migration understanding of the interfacial properties of and the and transformation of substances in SS, but these have mass transfer that occurs in the solid, liquid, and gas phases focused on the transformation of readily biodegradable in the HS-AD of other organic wastes will inform the organic matter (e.g., proteins, carbohydrates, and VFAs) development of mathematical models for the HS-AD of and nutrients (i.e., nitrogen, sulphur, and phosphorus) in SS. the solid, liquid, and gas phases. The related transforma- In addition, the effects of sludge dewatering agents on tion mechanisms and metabolic pathways of these the HS-AD of SS should be further explored, because most substances in the HS-AD of SS are rarely reported. For HS sludges contain a certain amount of dewatering agents. example, the differences between the quantitative trans- The underlying relationships between the dewatering agent formation of proteins, carbohydrates, and lipids are and the HS-AD of SS should be established and the unknown, as are the differences between the metabolic development of readily biodegradable dehydrators that do pathways of proteins, carbohydrates, and lipids in the HS- not degrade to molecules that may inhibit anaerobic AD of SS. More deeply, the underlying mechanisms of microorganisms should be considered. With the HS effects, electron transfer (i.e., DIET) between bacteria and archaea the concentrations of high value-added products (HVAP, e. via the transformation of these substances in HS-AD of SS g., lactic acid, biological protein, polyhydroxyalkanoates, are still blank and need to be further revealed in future. and poly-β-hydroxybutyrate) within sludge increased with Moreover, knowledge of the migration and transformation the increase of TS content, which suggests that attentions of toxic and harmful pollutants (i.e., inert organic should also be paid to the studies of obtaining the HVAP pollutants, inorganic pollutants, and microbial pollutants) from HS-AD process of SS and the effects of HVAP on in sludge will aid in their elimination and in the HS-AD of SS in future. stabilisation of SS treatment. Two key questions warrant attention, as follows: 1) Which toxic and harmful 5 Conclusions pollutants in sludge are degraded and what are the pathway and extent of this degradation in the HS-AD of SS? 2) What are the distributions of these toxic and harmful The HS-AD of SS has been proposed as an attractive pollutants in the solid, liquid and gas phases during the HS- option for SS treatment, as it requires only small reactors AD of SS? Future research must establish the fundamental and has a high volumetric biogas productivity and low metabolic transformations of organic matter in SS and the heating-energy demand. This paper comprehensively migration and transformation characteristics of toxic and reviews the current research on the main factors affecting harmful pollutants in the HS-AD of SS, to afford a strong process stability and performance, the improvement knowledge platform for further understanding the HS methods including pre-treatment and HS-AcD of SS and effect and improving the HS-AD of SS. other organic wastes, and the characteristics of substance To overcome the key bottlenecks to the application of transformation. The results of current studies indicate that HS-AD of SS, a relevant mathematical model that can the poor stability and performance of the HS-AD of SS are accurately predict the stability and performance of this the main bottlenecks to its wide application. These process should be established as soon as possible. To date, bottlenecks are due to the HS effect, manifested in the the HS-AD of SS has usually been operated empirically, as HS-AD of SS by high concentrations of substances that there is no mathematical tool available for improving may inhibit anaerobic microorganisms, and in poor mass parameter control, explicating mechanisms, and predicting transfer, low diffusion coefﬁcients, and high viscosity. The process performance. It is time-consuming to determine main knowledge gaps are the absence of a standard deﬁnition of HS sludge, insufﬁcient knowledge of the and optimise experimental parameters for the HS-AD of migration and transformation of pollutants, particularly the SS, and the resulting parameters and their values are not speciﬁc metabolic pathways involved, and a lack of universally applicable. This underscores the need for mathematical models. Attention should be paid to addres- accurate mathematical models. Whilst mathematical mod- sing these knowledge gaps in future work in this important els for the HS-AD of other organic wastes are well area. In addition, future efforts on developing the green established (Xu et al., 2015), these are not suitable for Ying Xu et al. High-solid anaerobic digestion of sewage sludge: achievements and perspectives 15 dependence of sludge dewatering efﬁciency on amount of ﬂocculant. sludge dewatering agents, obtaining the high value-added Resources, Conservation and Recycling, 54(5): 278–282 products, and revealing their effects on HS-AD process of Chen R, Wen W, Jiang H, Lei Z, Li M, Li Y Y (2019). Energy recovery SS can also be considered. potential of thermophilic high-solids co-digestion of coffee proces- Conﬂict of interest The authors declare no competing ﬁnancial interest. sing wastewater and waste activated sludge by anaerobic membrane bioreactor. Bioresource Technology, 274: 127–133 Acknowledgements This work was ﬁnancially supported by the National Chen S, Li N, Dong B, Zhao W, Dai L, Dai X (2018). New insights into Natural Science Foundation of China (NSFC) (Grant Nos. 51978498 and the enhanced performance of high solid anaerobic digestion with 51538008), the Support Program of Postdoctoral Innovative Talents dewatered sludge by thermal hydrolysis: Organic matter degradation (BX20190239), the China Postdoctoral Science Foundation (2020M671227), and the National Key R&D Program of China and methanogenic pathways. Journal of Hazardous Materials, 342: (2019YFC1906301). 1–9 Cheng Y, Li H (2015). Rheological behavior of sewage sludge with high Open Access This article is licensed under a Creative Commons solid content. Water Science and Technology, 71(11): 1686–1693 Attribution 4.0 International License, which permits use, sharing, adaptation, Cuetos M J, Martinez E J, Moreno R, Gonzalez R, Otero M, Gomez X distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the (2017). Enhancing anaerobic digestion of poultry blood using Creative Commons licence, and indicate if changes were made. The images activated carbon. Journal of Advanced Research, 8(3): 297–307 or other third party material in this article are included in the article’s Creative Dai L (2016). The characteristics and the sulphur control technology of Commons licence, unless indicated otherwise in a credit line to the material. the high solid anaerobic digestion in WWTP. Dissertation for the If material is not included in the article’s Creative Commons licence and your Master Degree. 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Effects of humic matter Dr. Xiaohu Dai received his B.E. and M.S. on the anaerobic digestion of sewage sludge: New insights from degree in Environmental Science and sludge structure. Chemosphere, 243: 125421 Engineering from Tongji University, Xue Y G, Liu H J, Chen S S, Dichtl N, Dai X H, Li N (2015). Effects of China (1987) and Ph. D. degree from thermal hydrolysis on organic matter solubilization and anaerobic Ruhr-Universität Bochum, Germany digestion of high solid sludge. Chemical Engineering Journal, 264: (1992). He is currently a full professor 174–180 and the Dean of College of Environmental Yenigün O, Demirel B (2013). Ammonia inhibition in anaerobic Science and Engineering in Tongji Uni- digestion: A review. Process Biochemistry, 48(5–6): 901–911 versity. His research interests mainly are Yin Q, Wu G (2019). Advances in direct interspecies electron transfer on the resource utilization of organic wastes, especially sewage and conductive materials: Electron ﬂux, organic degradation and sludge and sewage pollutants.
Frontiers of Environmental Science & Engineering – Springer Journals
Published: Aug 1, 2021
Keywords: High-solid effect; Anaerobic fermentation; Methane production; Biodegradability; Sludge treatment
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