Abstract
In this study, freezing–thawing (FT) pretreatment of different freezing time and freezing temperatures was investigated to find the effect on anaerobic digestion (AD) of wheat straw (WS). The freezing temperature gradient is − 10 °C, − 20 °C, − 40 °C and − 80 °C, and the freezing time gradient is 12 h, 24 h, 48 h and 96 h. Total methane production exhibited a mere distance among all samples. Morphology change sculptured by SEM showed this method broke the structure of WS leaving fragments and pores in varying degrees. Three kinetic models were performed on WS to represent the behavior of experimental data. Kinetic model parameters of total methane production and lag phase time showed that logistic function model had the best fit, followed by modified Gompertz model, yet transfer function model lost efficacy in this experiment. Logistic function model was then used to reveal the influence on lag phase caused by freezing time and freezing temperature, and the results implied that FT pretreatment can shorten the lag phase time of AD, providing a 21.39% improvement under the optimal conditions of − 20 °C 96 h. The analysis of response surface regression shows that the freezing temperature has more effect on the lag phase time of anaerobic digestion than freezing time. Warmer freezing temperature of − 20 °C do better than − 80 °C on lag time, which can be achieved in most cold regions, so this treatment can occur naturally in such area without additional energy input.Graphical abstract
Highlights
With the increasing energy demand of developing countries like China, the problem of lacking fossil fuels is getting more and more serious
Logistic function model was used to reveal the influence on lag phase caused by freezing time and freezing temperature, the results implied that FT pretreatment can shorten the lag phase time of anaerobic digestion (AD), providing a 21.39% improvement under the optimal conditions of -20°C 96 h
Increasing surface area means the acceleration of hydrolysis, which leads to a sever accumulation of volatile fatty acids (VFA), and the pH value should be lower than untreated assay, or even acidification (Dumas et al 2015;Veluchamy et al 2018)
Summary
With the increasing energy demand of developing countries like China, the problem of lacking fossil fuels is getting more and more serious. Burning fossil fuels causes air pollution, which directly worsens the environment and can result in the formation of secondary pollutants (Fan et al 2019). Wheat straw (WS) mainly consisting of cellulose, hemicellulose and lignin, is an available source at low cost, that can be used to produce acids convert to biogas (Shen et al 2019). These components of WS are closely combined with each other to form a complex three-dimensional structure on the cellular scale, making WS free from microbial attack (Kumari et al 2018). Hydrolysis is a rate limiting step in the AD with straw as substrate (Cornejo et al 2019)
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