Influence of pyrolysis temperature on production of digested sludge biochar and its application for ammonium removal from municipal wastewater

Abstract

Water contamination by ammonium (NH₄⁺) can present considerable risks to natural ecosystems. This work evaluates the potential application of biochar, produced from the pyrolysis of digested sludge, to remove ammonium from water. Anaerobic digester sludge was collected from a municipal wastewater treatment plant in Alberta, Canada, and individual biochars were produced at 50 °C temperature increments between 350 °C and 550 °C. The chemical characteristics of the resulting biochars were determined using elemental analysis, scanning electron microscopy (SEM), BET surface area analysis, and Fourier transform infrared spectroscopy (FTIR). Our findings demonstrate that the biochar yield decreased with increasing pyrolysis temperature, and biochar produced at 450 °C (BC450) had the highest ammonium removal capacity due to its higher surface area and functional group density. The Langmuir isotherm best described the observed ammonium removal capacity by the biochars, indicating that monolayer chemical adsorption may be the dominating mechanism. Biochar ammonium removal capacity was 1.2 mg NH₄N per gram of biochar in municipal wastewater, which is lower than that observed in parallel experiments using a synthetic ammonium solution (1.4 mg NH₄⁺-N/g biochar). This phenomenon is likely due to competition between ammonia and other containments for sorption to biochar surface sites. Our results demonstrate for the first time ever that biochar produced from digester sludge is a promising adsorbent for ammonium removal from municipal wastewater.

Introduction

Ammonium (NH₄⁺) is a soluble pollutant in water systems, influencing potability and causing eutrophication through its role as an oxygen sink during nitrification (Li and Liu, 2009; Yang et al., 2007), while its equilibrium gas phase, ammonia (NH₃), is a potential threat to fish through protein metabolism inhibition (Stuart et al., 2010). Adsorption by various adsorbents (e.g., zeolites, activated carbon and silicates) has been reported to be a promising approach to remove ammonium from water. For instance, the adsorption capability of zeolites was measured as 1.4 mg/g and that of NaOH treated activated carbon to be 2.89 mg/g (Hina et al., 2015; Vu et al., 2018), while the maximum adsorption amount of NH₃ in the micropores of nanostructured silica materials could reach 47.6 mg/g (Roque-Malherbe et al., 2008).

Biochar, as an emerging “cleaner” substitute for activated carbon due to its lower pyrolysis temperature and production cost, is attracting interest with regards to its application in soil remediation (Alam et al., 2018; Kim et al., 2018; Yang et al., 2018). Recent studies have demonstrated that biochar produced from solid wastes can be efficient adsorbents at removing metal and organic contaminants from water (e.g., (Inyang et al., 2012; Mohan et al., 2014; Vithanage et al., 2015)). In particular, organic waste materials, such as wood chips (Veksha et al., 2014), coconut shells (Cazetta et al., 2011), wheat straw (Xu et al., 2016) and bamboo (Fan et al., 2010), as well as inorganic cellular materials, such as coal cinder (Wang et al., 2016), can be converted to biochar through pyrolysis (Bridle and Pritchard, 2004; Hospido et al., 2005; Strezov and Evans, 2009). During pyrolysis, the high organic content of such waste is transformed and fixed at a stable carbon phase. Previous studies on the sludge-derived biochar activation process revealed that the raw material and the pyrolysis temperature are the main factors influencing biochar quality and quantity (Smith et al., 2009; Alam et al., 2018). The thermal decomposition of sludge material under high temperature leads to transport and phase-out of non-carbon elements, such as volatile compounds (Carey et al., 2015), resulting in the bonding of liberated carbon atoms as elementary graphitic crystallites (Smith et al., 2009). The interstices between the crystallites propagate a rudimentary porous structure, increasing the porosity and enhancing the adsorption properties of the resulting biochar (Smith et al., 2009).

Anaerobic digester sludge, which is generated as by-product at municipal wastewater treatment facilities, is often disposed of in landfills or by incineration (Lu et al., 2013), or used for land application after sufficient treatment (Bright and Healey, 2003). However, organic matter and nutrients enriched in these biosolids can be recovered and converted into value-added products, such as biochar, thereby avoiding the environmental and economic costs of disposal.

Digested sludge-derived biochars have been found to effectively adsorb numerous contaminants of environmental concern (Paz-Ferreiro et al., 2018), such as Pb, Cr and As (Jin et al., 2014; Zhou et al., 2015), and numerous types of small and large organics from wastewater systems (Kacan, 2016; Shi et al., 2014; Silva et al., 2016; Yao et al., 2013). Digested sludge can be heated at high temperatures to form biochars that have high porosity (Jindarom et al., 2007), ion exchange capacity, as well as various surface functional groups that can facilitate surface chemical reactions (Chen et al., 2002). Biochar has been studied to remove aqueous ammonium as well (Hou et al., 2016; Shin et al., 2018; Zhang et al., 2017) with the adsorption strength varying amongst biochars produced from different feedstocks and pyrolysis temperatures (e.g., 15.8 mg/g for cotton stalk-biochar and 0.52 mg/g for pine biochar) (Gao et al., 2015; Hina et al., 2015). As far as we are aware, only two studies (Carey et al., 2015; Li et al., 2018) reported the application of digested sludge-derived biochar to remove ammonium from aqueous solution. However, the link between pyrolysis temperatures of biochar and their effects on ammonium sorption is still unclear.

In this study, we hypothesized that digested sludge-derived biochars can be used as an alternative to conventional sorbents in water treatment, particularly considering the competition between ammonium and other contaminants that are frequently found in municipal wastewater. We evaluated and compared for the adsorption capacity of digested sludge-derived biochars at removing ammonium from aqueous solution and from municipal wastewater. Different pyrolysis temperatures were used to convert the sludge into various biochar products. The adsorption processes were then evaluated using isotherm and kinetics models.