Zero Valent Iron Significantly Enhances Methane Production from Waste Activated Sludge by Improving Biochemical Methane Potential Rather Than Hydrolysis Rate

Abstract

Anaerobic digestion has been widely applied for waste activated sludge (WAS) treatment. However, methane production from anaerobic digestion of WAS is usually limited by the slow hydrolysis rate and/or poor biochemical methane potential of WAS. This work systematically studied the effects of three different types of zero valent iron (i.e., iron powder, clean scrap and rusty scrap) on methane production from WAS in anaerobic digestion, by using both experimental and mathematical approaches. The results demonstrated that both the clean and the rusty iron scrap were more effective than the iron powder for improving methane production from WAS. Model-based analysis showed that ZVI addition significantly enhanced methane production from WAS through improving the biochemical methane potential of WAS rather than its hydrolysis rate. Economic analysis indicated that the ZVI-based technology for enhancing methane production from WAS is economically attractive, particularly considering that iron scrap can be freely acquired from industrial waste. Based on these results, the ZVI-based anaerobic digestion process of this work could be easily integrated with the conventional chemical phosphorus removal process in wastewater treatment plant to form a cost-effective and environment-friendly approach, enabling maximum resource recovery/reuse while achieving enhanced methane production in wastewater treatment system. L arge amounts of organic matter from wastewater are converted into waste activated sludge (WAS) during biological treatment processes in wastewater treatment plants (WWTPs)1 . Anaerobic digestion of WAS has been widely applied to stabilize and reduce the volume of WAS as well as produce a renewable bioenergy resource in the form of methane 2-5 . The anaerobic digestion process generally consists four stages, i.e. hydrolysis, fermentation, acetogenesis and methanogenesis for methane production 6 . However, the application of anaerobic digestion is often limited by the slow hydrolysis rate and/or poor biochemical methane potential (or degradation extent) of the WAS 7-11 . In order to effectively enhance methane production from WAS in anaerobic digestion, a number of strategies have been developed, such as thermal, chemical, and mechanical methods [12][13][14][15][16][17][18][19][20][21] . However, most of these methods are cost intensive owing to high energy input or large chemical requirements 7,22,23 . Thus, alternative cost-effective approach to improve methane production from WAS in anaerobic digestion process is highly desired.Zero valent iron (ZVI), a cheap reducing agent, has been widely used in wastewater pretreatment, groundwater purification and soil remediation 24,25 . Recent studies have found that ZVI addition in anaerobic reactors for biological wastewater treatment could significantly improve chemical oxygen demand (COD) removal by ca. 25% [26][27][28][29] . Indeed ZVI can lower oxidation-reduction potential and serve as an acid buffer, thus helping maintain a stable and favourable condition for methan...

Exploring the effects of ZVI addition on resource recovery in the anaerobic digestion process

“…It is also noteworthy that the ZVI did not modify the kinetics of the process, as the k H values did not vary significantly in the presence of ZVI. These observations are in agreement with previous work, which postulates that ZVI mainly enhances the methane potential introducing electrons into the system but it does not improve the hydrolysis kinetic of the anaerobic digestion process [27]. Moreover, a recent work hypothesizes that ZVI can enhance the hydrolysis of refractory compounds [10].…”

“…Although this was previously explained by the improvement of the environmental redox conditions, since ZVI can considerably decrease the ORP [6], this explanation has been recently discarded [10]. As such, the increase of the process rate has been questioned, as the ZVI seems to affect to the BMP only [27]. Additionally, there are no convincing evidences indicating that ZVI increases universally hydrolysis rates, as has been depicted in Figure 1d, and therefore ZVI addition should not be viewed as a method for improvement of the methane production rate (since the hydrolysis is the limiting step of the AD process).…”

Effective Pb2+ removal from water using nanozerovalent iron stored 10 months

“…In the past few years, different methods were developed to synthesize iron nanoparticles (Brumfiel 2003;Hassan 2005;Zhang et al 2015;Su et al 2016), modify their surface properties (Mahfuz and Ahmed 2005;Karlsson et al 2004;Rajajayavel and Ghoshal 2015), and improve their efficiency for field delivery and reactions (Liu et al 2015;Glavee et al 1995;Khalil et al 2004;Xu et al 2005;Zhang et al 1998;Schrick et al 2002;Elliott and Zhang 2001). Extensive investigations reported that nanoscale iron particles are effective for the removal of a wide environmental contaminants such as heavy metal ions like As(III), Pb(II), Cu(II), Ni(II), and Cr(VI), chlorinated organic solvents, organochlorine pesticides, some inorganic compounds (Elliott and Zhang 2001;Liu et al 2005;Alowitz and Scherer 2002;Cao et al 2005;Kanel et al 2005), and organic dyes (Alowitz and Scherer 2002;Cao et al 2005;Kanel et al 2005).…”