Effects of thermal treatment on high solid anaerobic digestion of swine manure: Enhancement assessment and kinetic analysis

Highlights

• Thermal treatment of raw swine manure with solid content >20% at 70 °C was studied.

• It upgraded high solid anaerobic digestion (HSAD)’s methane production rate by 40%.

• Thermal treatment increased biodegradable organics rather than the hydrolysis rate.

• Superimposed first-order kinetics model was firstly used in HSAD and fitted well.

• “Thermal pretreatment of raw manure + HSAD” is of great practical importance.

Abstract

Anaerobic digestion (AD), which is a process for generating biogas, can be applied to the treatment of organic wastes. Owing to its smaller footprint, lower energy consumption, and less digestate, high solid anaerobic digestion (HSAD) has attracted increasing attention. However, its biogas production is poor. In order to improve biogas production and decrease energy consumption, an improved thermal treatment process was proposed. Raw swine manure (>20% solid content) without any dilution was thermally treated at 70 ± 1 °C for different retention times, and then its effect on HSAD was investigated via batch AD experiments at 8.9% solid content. Results showed that the main organic components of swine manure hydrolyzed significantly during the thermal treatment, and HSAD’s methane production rate was improved by up to 39.5%. Analysis using two kinetic models confirmed that the treatment could increase biodegradable organics (especially the readily biodegradable organics) in swine manure rather than upgrading its hydrolysis rate. It is worth noting that the superimposed first-order kinetics model was firstly applied in AD, and was a good tool to reveal the AD kinetics mechanism of substrates with complex components.

Introduction

With rapid expansion of animal farms, the amount of livestock manure is increasing each year (Huang et al., 2011). The National Bureau of Statistics of People’s Republic of China estimated that over 100 million swine were raised in 2014 and produced more than 200 million tons manure. Owing to its high organic content and high sanitation risk (Wang et al., 2014), livestock manure has become one of the most pressing environmental issues in China. At present, land use in cultivated land is the major disposal method of swine manure in China, but has caused severe pollution due to overloading (Qiu et al., 2013). Thus, “reduction” is the most important goal of treating swine manure. Generally, stabilization techniques including composting, aerobic treatment and anaerobic digestion (AD) are recommended to treat manure before land use in Chinese legislation (MEP, 2009). Among them, AD is an environmentally sustainable technique, because it can reduce and stabilize organic matters, recycle energy, reduce pathogens, and decrease unpleasant odor (Appels et al., 2008).

Conventional AD, which is conducted at 2–6% solid content (Liu et al., 2016b), requires large digester volume (Li et al., 2015a). Moreover, it produces large amounts of digestate, and thus poses some severe disposal challenges. HSAD, an emerging AD technique, can overcome the abovementioned disadvantages of conventional AD because its solid content was higher than 8% (Liu et al., 2016b). Many studies have investigated HSAD of various substrates, e.g. sewage sludge (Liu et al., 2016a), manure (Sun et al., 2016), rape straw (Tian et al., 2017), and organic municipal solid waste (Di Maria et al., 2017). However, the methane production rate of HSAD is relatively poor (Zhang et al., 2014). The methane production rate obtained in HSAD of chicken manure at 22.4% solid content was less than 20 mL CH₄/g VS, while that of conventional AD of chicken manure at 5% solid content was around 180 mL CH₄/g VS (Li et al., 2013). Møller et al. (2007) also identified that the methane production rate of swine manure was 300 mL CH₄/g VS in the conventional AD, while it was 200 mL CH₄/g VS in HSAD. Moreover, the methane production rate decreases as solid content increases. For example, it was 410, 385, 361, and 318 mL CH₄/g VS at the VS concentration of 8, 16, 32, and 64gVS/L, respectively (Li et al., 2015b). Obviously, low methane production rate was the bottleneck of the application of HSAD.

Thermal treatment could enhance AD of various substrates (Shao et al., 2013), such as sludge (Dhar et al., 2012, Liao et al., 2016, Liu et al., 2012), food waste (Ariunbaatar et al., 2015), and kitchen waste (Li and Jin, 2015). For example, methane potential of activated sludge increased 34.8% when the sludge was thermally pretreated at 175 °C for 60 min in Liu’s study (Liu et al., 2012). Carrere et al. (2009) also conducted thermal treatment on manure of 5% solid content at 70 °C and 90 °C for 3 h, and it increased the methane production rate by 70% and 89%, respectively. Thermal treatment under 170 °C for 30 min could also enhance the methane production rate of swine manure by 35% at 3.9% solid content (Gonzalez-Fernandez et al., 2008). On the other hand, thermal pretreatment could decrease the hygiene risk significantly as pasteurization, and could meet the requirements of Class A biosolids in US regulation 40 CFR Part 503 (USEPA, 1994). It is worth noting that the solid contents of thermal treatment and AD were usually lower than 17% and 5% in previous studies (Carrere et al., 2009, Ferreira et al., 2014, Gonzalez-Fernandez et al., 2008). So far, the usage of thermal treatment is very limited in practice due to the high energy-consumption.

In order to improve HSAD’s methane production and reduce the digestate, an improved thermal treatment was proposed, i.e. raw swine manure (>20% solid content) without any dilution was treated at 70 ± 1 °C. Then, the thermally treated manure was anaerobically digested at solid content of 8.9% to study the enhancement of the improved thermal treatment on HSAD. Compared to the thermal treatment in previous studies, the improved thermal treatment in this study could save more than 30% of consumed energy and volume of thermal treatment tank. Furthermore, the combined HSAD at 8.9% solid content could also save more than 60% digester volume and heating energy, and decrease 60% digestate as compared to conventional AD. Thus, the process “thermal treatment of raw manure + HSAD” is of great practical importance. To the best of our knowledge, this is the first investigation on this process.

AD is a complex biochemical reaction, and kinetic models can not only facilitate its understanding but also provide key parameters to design reactors, predict methane production, and optimize reaction performance (Zhen et al., 2015). The first-order kinetic model is a very classical model (Zhen et al., 2015) which has been widely used to study AD of various substrates. It assumes the hydrolysis to be the rate-limiting step of AD, and is useful for revealing the AD kinetics mechanism of various substrates (Dennehy et al., 2016, Pellera et al., 2016, Zhao et al., 2016). Pellera et al. (2016) employed this model to identify the effects of alkaline pretreatment on anaerobic digestion of olive mill solid waste, and confirmed a dosage of alkaline of 0–2 mmol/gVS at 25 °C could significantly increase hydrolysis rate. Furthermore, hydrolysis rate could reach 0.28 d⁻¹ in anaerobic co-digestion of food waste and swine manure (Dennehy et al., 2016), while it was 0.02–0.12 d⁻¹ in the AD of fruit residues (Zhao et al., 2016). However, the first-order kinetics model assumes that the substrate is of components with same hydrolysis rate. Obviously, this assumption does not fit with some organic wastes, and therefore a superimposed first-order kinetic model (Gu, 1992) was introduced in this study. This model treats the components of the substrate as three groups according to their hydrolysis rate, i.e. readily biodegradable, poorly biodegradable and non-biodegradable organics. It provides more information about the biodegradation of organic components during AD. To the best of our knowledge, it is the first time that the superimposed first-order kinetic model is applied in AD.