Huge amount of waste activated slud

Huge amount of waste activated sludge (WAS) is continuously generated from the wastewater treatment plants (WWTPs). The sludge treatment and processing consume up to 50% of the operation cost in WWTPs (Hii et al., 2014). Anaerobic digestion (AD) is a widely adopted and proven technique with relatively lower cost to stabilize sludge and recover energy (Pilli et al., 2014). However, the rate-limiting step of hydrolysis slows down the overall AD process and hampers the application of this technology (Tyagi et al., 2014).Various pretreatment methods have been developed to enhance the hydrolysis efficiency and subsequently to promote AD, such as thermal hydrolysis pretreatment (THP), chemical, mechanical and biological pretreatment (Gonzalez et al., 2018; Li et al., 2016; Zhen et al., 2017). Among them, THP has been successfully implemented commercially for a few years. In the THP process, sludge is heated up to around 165 °C under high pressure conditions prior to AD (Han et al., 2017). The merits of THP, such as enhanced biodegradability, improved dewaterability and reduced sludge viscosity, have been well documented recently (Bougrier et al., 2008; Ennouri et al., 2016). Up to 60% sludge solubilization could achieve with THP (Bougrier et al., 2008). This means abundant dissolved substances would be present in the liquid fraction of THP sludge (THP-L). Soluble organics are regarded as the readily usable carbon source for bacteria compared with solid particulates (Lu et al., 2018b). The digestion of THP sludge filtrate has been performed previously, and the digestion time is faster than that of THP sludge (7–9 days vs 12–15 days) (Li et al., 2017; Lu et al., 2018a; Xue et al., 2015). Thus, the solids fraction of THP sludge (THP-S) would have less readily biodegradable organic matters, which results in the less biogas generation and/or longer digestion period (meaning larger reactors). However, there is no literature so far to report the needs to digest the solids fraction of THP sludge and the contribution to biogas production from the solids portion is not clear.It is well known that sludge displays non-Newtonian flow behavior (Eshtiaghi et al., 2013; Zhang et al., 2016a). Its rheological characteristics are important parameters, which greatly affect sludge treatment processes, including dewatering, drying, mixing, pumping and transport (Chaari et al., 2003; Xia et al., 2009). With better flowability, the energy required to homogenize and transport the feedstock could be reduced due to the lower head loss and pumping power (Zhang et al., 2016a). The different sludge origin and solids content would affect the rheology of sludge (Feng et al., 2014). It is known that the dewaterability and flow behavior (such as yield stress and viscosity) of sludge could be significantly improved by THP treatment (Barber, 2016). The lower viscosity and yield stress usually correspond to the better dewaterability (Miryahyaei et al., 2019). However, such good THP sludge properties could not be maintained during the anaerobic digestion. The dewaterability of the THP sludge during digestion is generally worse than the original THP sludge (Zhang et al., 2018). Thus, it would make more sense to carry out the dewatering step prior to AD to separate the liquid and solids portions more efficiently. To date, it is lack of information on the feasibility and assessment on such separation.The main objectives of this study are to (1) investigate the AD performance of THP sludge, THP-L and THP-S, (2) study the rheological properties of different sludge fractions during AD, (3) propose an integrated system to handle both liquid and solids portions of THP sludge in an energy-efficient way. The preliminary energy analysis for the proposed process was performed.