Abstract
Previous studies showed that adding zero valent iron (ZVI) can increase the methane production and degradation rate of organic waste by improving the performance of anaerobic digester. However, our study firstly found that ZVI (37 μm, 10 g/L) inhibited the anaerobic digestion (AD) of cow manure and lignocellulose. ZVI significantly increased the methanogenic rate of cow manure in the first 6 days, but decreased the accumulative methane yield and volatile fatty acids yield by 10.3 and 12%, respectively. The effect of ZVI on AD of liquid biomass separated from cow manure was positive, but the effect on solid biomass was negative. These results indicated that ZVI enhanced the AD of easily biodegradable organics but inhibited the biodegradation of refractory organics (lignocellulose). By analyzing the varying effects of ZVI in diverse anaerobic systems, it was found that the effects were influenced by the characteristics of substrate and inoculum-substrate ratio. This study suggested that only proper ZVI addition can improve the AD process depending on the feeding materials.
Highlights
Anaerobic digestion (AD), with the merits of reducing organic pollution, energy recovery, and low operation cost, has become a widespread pathway in converting organic waste to energy (Romero-Güiza et al, 2016)
Cow manure was not used as a substrate in previous studies, which might be the reason for the difference
The final total soluble iron of C-CM1 was 3.80 ± 0.13 mg/L, higher than the original total soluble iron at the first day. These results showed that the soluble iron was enough in this system for microorganisms and enzymes, as ingested iron from animal feed was excreted in cow manure (Yang et al, 2018), which again confirmed that the effect of zero valent iron (ZVI) in AD system was affected by the characteristics of the substrate
Summary
Anaerobic digestion (AD), with the merits of reducing organic pollution, energy recovery, and low operation cost, has become a widespread pathway in converting organic waste to energy (Romero-Güiza et al, 2016). Agricultural solid waste, including livestock manure and crop straw, is regarded as a reused biomass resource due to its high content of biodegradable organic matters and nutrients (Lu et al, 2016). As a major carbon source of agricultural solid waste in AD system, lignocellulose is difficult to be biodegraded because of its highly resistant and recalcitrant biomass structure (Sawatdeenarunat et al, 2015). To improve the biodegradability of substrate and the efficiency of AD, several approaches have been suggested and evaluated in many studies, such as mechanical or chemical pretreatment (Hendriks and Zeeman, 2009), co-digestion with several mixed substrates (Wang et al, 2018), and introduction of additives (Romero-Güiza et al, 2016).
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