Combined hydrothermal and free nitrous acid, alkali and acid pretreatment for biomethane recovery from municipal sludge

Highlights

• Individual and combined chemical and HTP pretreatments have been investigated.

• COD solubilization for combined pretreatment was not notably higher than the HTP.

• The highest methane yield was achieved for combined HTP with FNA.

• Reaction Curve and First-Order models were applied and found to fit the experimental data.

• Preliminary economic analysis indicated that combined HTP with FNA is feasible.

Abstract

This study focused on investigating the effect of combined chemical and hydrothermal pretreatment (HTP) on the anaerobic digestibility of thickened waste activated sludge (TWAS). Three different combined pretreatment conditions of HTP + free nitrous acid (FNA), HTP + Acid, and HTP + Alkaline were applied to TWAS. To control and compare the effect of combined pretreatments and a single pretreatment, Acid, Alkaline, FNA and HTP pretreatments were applied done prior to AD. The results of this study revealed that combined pretreatments have higher potential to improve methane production yield and rate but not in the solubilization of COD. The highest methane yield of 275 mL CH₄/g TCOD _added was achieved for the combined pretreatment with FNA and HTP. HTP + FNA pretreatment was found to produce higher methane yields compared to the combination of other typical acid and alkaline reagents with hydrothermal pretreatment. Methane yields of 594, 527, and 544 L CH₄/g VSS added, were achieved for HTP + FNA, HTP + ALK, and HTP + ACID pretreatments, respectively. The preliminary economic analysis showed that out of the combined pretreatment, only combining HTP with FNA is economically feasible.

Introduction

Recent statistics show that the EU, North America and China are producing 12, 8 and 3 million dry tonnes of sludge annually, respectively, and this continues to increase steadily (Kor-Bicakci and Eskicioglu, 2019). Typically, sludge is disposed via land application, landfilling, incineration, composting, recycling for fuel and construction, and anaerobic digestion (AD) (Gude, 2015, Kor-Bicakci and Eskicioglu, 2019). The management of sewage sludge (primary and activated sludge), including disposal, can contribute from 20% to 60% of the operating costs of wastewater treatment plants (WWTP). Increasing costs of sludge management is driving the efforts to reduce sludge production, increase treatment efficiency, and enhance the sludge reuse/valorization (Zhang et al., 2017). Changing legislative requirements in many countries and the global shift towards promoting a circular economy (Gherghel et al., 2019) are important forces of change.

AD is a well-known and longstanding technology for sludge treatment and valorization in the form of biogas production. However, the application of this technology is limited by the biodegradability of the substrate during the first stage of AD, hydrolysis (Appels et al., 2008). The bound water content of waste activated sludge, an assemblage of complex aggregates of microorganisms and extracellular polymeric substances (EPS), imposes challenges in treatment that would require long retention times and low yields and rates biogas production (Chen et al., 2018, Zhen et al., 2017).

Pretreatment technologies have been studied extensively to improve hydrolysis during AD of sludge. Various methods include biological approaches (enzymatic, temperature phased AD, electro-methanogenesis), thermos-mechanical methods (ultrasonic, microwave), and chemical treatments (acid, alkali, ozonation, Fenton). Research has also been conducted on combinations of these pretreatments to improve the cost effectiveness of treatment, environmental impact, biogas production and solids destruction (Zhen et al., 2017).

Hydrothermal pretreatment (HTP), also referred to as thermal hydrolysis, has emerged as a well-established technology in sludge management. HTP disintegrates the EPS and the microbial cell walls in activated sludge to convert the bound water into free water and release intracellular organic matter (Yuan et al., 2019). The effects of HTP on sludge include reduced viscosity, reduced pathogen content and increased biodegradability (Jeong et al., 2019).

HTP involves subjecting substrates to high temperatures between 60 and 275 °C at high pressures for contact times ranging between 10 min and a few hours (Ahmad et al., 2018). The application of higher temperatures or extended contact time in HTP can lead to the production of refractory compounds and toxic materials such as ammonia at inhibitory concentrations which reduce biodegradability of the sludge (Barber, 2016) . As such, optimal temperatures and retention times to improve the biodegradability of sludge while avoiding reactions that produce compounds of low degradability are reported in literature in the range of 160 to 180 °C and 20 to 40 min, respectively (Barber, 2016). Table 1 (part a) lists recent studies on the application of HTP to sludge management.

Combining HTP with chemical pretreatment can produce synergistic effects of the individual pretreatments that enhances the disintegration and biodegradability of sludge (Li et al., 2017). Such combinations reduce the input chemical, temperature and residence time required while improving biodegradability. Published studies suggests alkali combined with HTP appears to be one of the most common approach for sludge pretreatment. On the other hand, HTP has also been combined with acid, ultrasonic, and enzyme pretreatments to enhance the biodegradability of other recalcitrant substrates like papermill sludge and algae (Bayr et al., 2013, Cheng et al., 2019). Table 1 part (b) lists recent studies on combined hydrothermal and chemical pretreatment.

Free Nitrous Acid (FNA or HNO₂) is a biocidal chemical that exists in acidic conditions in equilibrium with nitrite. FNA is particularly attractive for advancing the circular economy since it can be produced from partial nitritration of digestate liquor, which is rich in ammonium, and pH control (Law et al., 2015). FNA pretreatment disintegrates EPS and cell membranes which improves solubilization, digestibility of sludge and methane production. Studies (Table 1 (part c) have showed up to 50% improvement in hydrolysis rate, 60% improvement in solubilisation, and 37% enhancement of methane production, while applying FNA pretreatment using concentrations ranging from 0.15 to 6.1 mg HNO₂-N/L for 4 to 24 h (Duan et al., 2020, Meng et al., 2020, Wang et al., 2013, Zahedi et al., 2018). There has also been some evidence that FNA and its derivatives react with refractory solubilized material making them more biodegradable (Qilin Wang et al., 2013b). FNA pretreatment of WAS at 1.8 mg HNO₂-N/L has also been shown to reduce sludge viscosity which increases treatment plant capacity, improved dewaterability of the digested sludge leading to reductions in cost, and up to 2.1 log pathogen reduction (Wei et al., 2017). However, FNA pretreatment of sludge for AD is not sufficient to produce class A biosolids. Pretreatment will need to be combined with other technologies or further treatment required to achieve such high quality. For example, in combination with a low temperature of 55 °C, FNA led to 4.5 log inactivation of fecal coliforms (Wang et al., 2014). Another limitation of FNA pretreatment is the long contact time required as it typically needs 12 to 24 h for noticeable improvements in VS destruction and methane production.

An opportunity exists to combine the advantages of HTP and FNA to develop a pretreatment technology with high solubilization with better biodegradability, lower contact time and temperatures to improve the methane potential of sludge while maintaining economic feasibility and circularization in treatment facilities. The effect of combined FNA with HTP pretreatment on the AD process has not been investigated broadly (Wang et al., 2014). Also, HTP + FNA pretreatment has not been compared with other thermochemical pretreatments. As such in this study, in addition to the investigation of combined HTP and alkali and HTP and acid, the impact of combined FNA and HTP pretreatment on the solubilization and biodegradability of TWAS was compared to sole HTP and chemical pretreatments.