Adding granular activated carbon into anaerobic sludge digestion to promote methane production and sludge decomposition

Highlights

• Granular activated carbon was used for the first time to enhance sludge digestion.

• Granular activated carbon accelerated direct interspecies electron transfer.

• Granular activated carbon accelerated interspecies hydrogen transfer.

• Granular activated carbon improved methane production and sludge decomposition.

Abstract

Anaerobic digestion of waste activated sludge is frequently restricted with the slow fermentation rate due to its complex components. Carbon materials have been reported to enhance syntrophic metabolism of anaerobic wastewater treatment, but its function in anaerobic digestion of sludge has yet to be investigated. In this study granular activated carbon (GAC) was added into a batch-mode anaerobic sludge digestion reactor with an attempt to improve the sludge digestion. The results showed that with adding GAC from 0 to 5.0 g, the methane production increased by 17.4%, and the sludge reduction rate increased by 6.1 percent points (from 39.1% to 45.2%). In the 20 days’ digestion, GAC obviously enriched hydrogen-utilizing methanogens, Geobacter, and other methanogens capable of direct interspecies electron transfer, which could enhance the electron exchange between syntrophs and methanogens to accelerate substrate consumption and methane production.

Introduction

Anaerobic digestion (AD) has been deemed to be an energy-efficient and promising method to dispose wasted activated sludge (WAS) produced from municipal wastewater treatment plant for its high value-added products and low operating cost (Feng et al., 2015, Garrido-Baserba et al., 2015, Larsson et al., 2015). Anaerobic sludge digestion is generally categorized into three stages (Lv et al., 2010), i.e. (1) hydrolytic acidogenesis of complex organic compounds, (2) acetogenesis, and (3) methanogenesis. Unlike AD of simple substrates whose hydrolytic acidification goes fast, waste activated sludge is composed of high-strength macromolecule organics, such as polysaccharide and protein, and then their decomposition is quite slow (Bougrier et al., 2006), which results in the low fermentation efficiency and long retention time required for the anaerobic sludge digestion. To improve the AD of waste sludge, chemical, thermal, mechanical, ultrasound and ozone pretreatment processes (Carrère et al., 2010, Serrano et al., 2015, Xu et al., 2010) have been developed to accelerate the decomposition of the sludge. However, the current pretreatment usually involves considerable energy consumption and high expenditures, making them less acceptable for practical application.

On the other hand, H⁺ as a main electron acceptor of AD is of low oxidative-reductive potential (E_H+/H2 = −414 mV, pH = 7.0), indicating that it is difficult to oxidize most substrates including important intermediates (E_NaD+/NaDH = −320 mV, E_FADH/FADH2 = -220 mV, E_Fd-ox/Fd-red = −398 mV, pH = 7.0) of AD at the standard temperature and pressure (STP), which may lead to a halt of the anaerobic respiration accompanied with accumulation of H⁺ and intermediates, like volatile fatty acids (VFAs). This is an important reason for the slow rate of AD of substrates such as WAS. Thermodynamically, lowering H₂ partial pressure to a quite low level (<10⁻⁴-10⁻⁵ atm) (Feng et al., 2014) is a solution to make anaerobic respiration forward according to Nernst Equation. Hydrogenotrophic methanobacterium, capable of converting H₂/CO₂ to methane, is responsible to reduce the H₂ partial pressure during AD (Stams et al., 2006). The working mode of electron transfer between syntrophs and methanogens via hydrogen as electron carrier or so-called interspecies hydrogen transfer (IHT) has been recognized for more than half century (Shen et al., 2016). However, the exchange of H₂ between syntrophs and methanogens is fragile, which is not only because hydrogen-consuming methanogen is not the dominant methanogens in most digesters, but also because H₂ diffusion between H₂ producers and H₂-consuming methanogens is slow (Stams et al., 2006). Slight interruption of H₂ consumption may break the balance of syntrophic metabolism. For the AD of sludge, the high-strength complex components involved require a smoother electron exchange among anaerobes to accelerate the sludge digestion.

Recently, it was reported that direct interspecies electron transfer (DIET) was an alternative to exchange electron between syntrophs and methanogens during AD (Reguera et al., 2005). In DIET interactional microbial species forge biological electrical connections, which was confirmed in co-cultures of Geobacter sulfurreducens and Geobacter metallireducens (Summers et al., 2010), co-cultures of Geobacter metallireducens and Methanosaeta (Rotaru et al., 2014a) or Methanosarcina (Rotaru et al., 2014b), and mixed cultures of anaerobic digesters (Cruz Viggi et al., 2014). The cells established biological electrical connection through their conductive nanowires pili. More recently it was confirmed that the conductive materials like carbon cloth, biochar, graphite and magnetite could serve as the similar function as pili to assist the electrical connection of syntrophic metabolism. As results, adding conductive materials was reported to enhance DIET and then increase the methane production from anaerobic treatment of wastewaters (Chen et al., 2014, Liu et al., 2012).

As suggested above, the slow electron exchange rate among anaerobes is an important reason for the low efficiency of AD of WAS. Together with the findings that conductive materials could enhance DIET to supplement the insufficient electron exchange between syntrophs and methanogens, we considered that conductive materials likely accelerated the electron transfer in anaerobic sludge digestion as to enhance the decomposition of sludge. Currently, the studies on DIET for enhancing AD were mostly focused on anaerobic treatment of wastewaters. Until now, the effect of DIET via conductive materials on digestion of waste activated sludge has not been clarified. Granule activated carbon (GAC), an inexpensive material frequently serving as a biocarrier or adsorbent in wastewater treatment process, has been demonstrated the applicability in facilitating DIET in methanogenesis due to its conductivity. In this study, GAC was added into anaerobic digester of WAS for the first time with an attempt to improve the sludge decomposition and methane production. The aim of this study is to investigate the effects of GAC on the sludge digestion. We expect to provide a simple but effective method to improve the AD of sludge.