1. Introduction
With the development of social economy and the improvement of people’s living standards, the annual output of livestock and poultry manure in China has reached 3.8 billion tons in recent years [
1]. The number of Chinese pig stock reached 442 million by 2016 [
2]. The large scale of the pig breeding industry has brought a series of environmental problems in terms of water pollution and air pollution [
3], especially the environmental pollution caused by pig manure [
4]. Untreated pig manure can produce large amounts of noxious gases, and pig manure can easily penetrate into the ground due to rain erosion, polluting soil and groundwater and entering water bodies with underground runoff, thus polluting water resources [
5]. In order to improve the efficiency of farming, a large number of heavy metals are added to the animal feed in the modern husbandry industry, which exceeds the animal’s absorption capacity and is excreted in the form of feces and urine. In 2017, the total heavy metals sourced from manures was 2.86 × 10
5 t with the predominant contribution originating from pig manure (71.52%) [
6] posing a very high pollution risk. As, Cr, Hg and Cd in livestock feces are of low content but of great toxicity [
7]. Heavy metals gather in animals and plants through the food chain. The high concentration of Cu and Zn in the untreated manure will be accumulated in plants by their entrance to the environment. The efficiency of photosynthesis decreases with the excessive accumulation of Cu and Zn and plant growth is inhibited [
8]. The original edible and medicinal value of plants is also reduced; for example, the antinociceptive activity of guava is limited by the excessive accumulation of Zn and Cu [
9]. Excessive intake of Cu and Zn will damage the stomach, cardiovascular and nervous system [
10]. In addition, excessive intake of Zn inhibits the absorption of Cu and Fe and excessive Cu damages protein, lipids and DNA [
11]. The harm to the environment caused by pig manure is an environmental problem that needs to be solved urgently, and the rational and full utilization of pig manure resources is an emerging industry with great development prospects.
The anaerobic composting of pig manure can not only effectively solve the problem of pig manure accumulation but also obtain biogas and other substances, which is a common treatment method. However, traditional composting and aluminum sulfate treatment brings many hidden dangers of waste and secondary pollution [
12]. Heavy metals can inhibit the activity of microorganisms during anaerobic digestion and reduce gas production [
13]. Many anaerobic composting additives also have a certain negative effect on methane production. Zhao et al. found that the addition of biochar and manganese sulfate to the anaerobic composting of pig manure could inhibit the activity of methanogens and lead to the decline of methane production [
14]. The study of Ren et al. also proved that the addition of clay had a similar effect [
15]. Hu et al. found that adding a filamentous microalgae to the dry anaerobic digestion of pig manure could improve the methane production of the fermentation system [
16], but microalgae additives had the problems of high cost and low energy conversion efficiency [
17]. The advantages of the dry co-digestion of food waste and pig manure are the low cost of kitchen waste and high energy conversion rate, but there is also the problem of high fatty acids inhibiting methane production [
18].
As an agriculturally developed country, many crops, such as rice, bamboo, wheat and corn, are planted in China, which produces much surplus straw. Due to the relatively high cost of traditional straw treatment and the large amount of harmful gases produced by open-air incineration (NO
2, CO, SO
2) [
6], a new technology needs to be developed. Anaerobic digestion is a simple, reliable and environmentally friendly technique which is not only treats solid waste reasonably well but also increase methane production to truly realize a circular economy by combining livestock manure, crop straw and inorganic minerals. Straw itself can produce biogas under anaerobic digestion [
19]. Many scholars have confirmed that anaerobic digestion can effectively promote the gas production of the system by controlling the dry matter of the crop straw and livestock manure to a certain level. For example, Zhao et al. conducted the anaerobic digestion of pig manure and oat straw in different proportions. They found that mixed anaerobic digestion could improve the gas production when the C/N ratio was 27 [
20]. Xiang et al. used rice straw and pig manure as raw materials for anaerobic digestion and found that the methane production of the test group was improved with the addition of rice straws which were treated by alkaline and ultra-sonication [
21]. Li et al. found that corn stalks pretreated with NaOH had higher methanogenic performance under the condition of high organic loading rate and 40 °C [
22].
Inorganic passivators are also used to improve the gas production performance and passivate the heavy metals of the anaerobic digestion of livestock manure. Clay minerals, such as attapulgite (AT) and sepiolite (SE), have high specific surface areas, developed pore structure and unique crystal structure [
23]. Van der Waals force (physical adsorption), chemical bond force (chemical adsorption) and electrical adsorption force (ion exchange adsorption) appear easily between heavy metal ions and inorganic passivators. Heavy metals in feces inhibit microbial activity and hinder anaerobic digestion [
24] The content of heavy metals decreases by these forces, which is conducive to the production of biogas and the application of agricultural irrigation [
25]. The addition of fly ash also has a positive effect on the on the production of methane [
26]. Yang et al. found that methane yield was significantly improved by 25% under the optimal biochar dosage of 5–10% [
27]. At present, many scholars have also proved that adding passivator to pig manure could achieve the effective solidification of heavy metals and reduce their pollution to the environment. For example, Kong et al. found that adding calcium magnesium phosphate fertilizer, waste mushrooms and other substrates promoted the humification process of compost and improved the passivation performance of Cu, Zn, Cd, Cr and Pb [
28]. Zheng et al. found that the bioavailability of Zn and Cu in pig manure composting process could be reduced by adding a certain proportion of SE [
29]. At the same time, scholars also found that modification of passivator [
30] and change of particle size [
31] could further improve its passivation effect and reduce its bioavailability.
However, at present, many related studies lack the comparison with the single material performance of gas production and the passivation of heavy metals. The research and experimental data in this area are very important for the assessment of heavy metal pollution risk and improvement of gas production. Therefore, this experiment takes fresh pig manure and corn straw as digestive raw materials, and the purpose of this study is as follows: (1) evaluate the effects of different compound passivators on the total amount of biogas produced by the anaerobic digestion of pig manure, the average methane concentration and the passivation effect of Zn and Cu; (2) measure the oxidation-reduction potential and pH of mixed materials and carry out the preliminary study on the changes of physical and chemical properties of mixed anaerobic digestion of pig manure straw; and (3) study the effects of different compound passivators on organic structure changes before and after the anaerobic digestion of pig manure by Fourier transform infrared spectroscopy (FTIR).
4. Conclusions
The gas production efficiency of anaerobic digestion with addition of corn straw and passivator was better than single materials. The problem of insufficient gas production can be dealt with through the promotion of microbial activity using mixed materials. Heavy metals in pig manure can also be better passivated during anaerobic digestion with the addition of mixed material. When the ratio of pig manure to straw is 8:2, adding an inorganic passivator could effectively increase gas production and the passivation of Cu and Zn performance during anaerobic digestion of pig manure. The passivation performance of the composite passivator for Cu and Zn was better than a single passivator, the optimal ratio of passivators for AT, FeSO4 and SE is 7.5 g/L, 5 g/L and 7.5 g/L. The vibration peak intensity of hydroxyl and carbon-hydrogen bonds in the pig manure residue decreased after adding passivation, which were shown in the FTIR spectrum, the decrease indicated that macromolecular organic matter was decomposed. The vibration peak intensity of carboxylic acid and carbonyl increased. The content of humus increased, and heavy metals in pig manure were easily combined with humus. The phenomenon improved the passivation efficiency of heavy metals in the anaerobic digestion process.
Energy stress was relieved by the improvement of gas production performance, and biogas residue was used as safer fertilizer in farmland irrigation by the efficient passivation of Cu and Zn. This study can provide a theoretical basis for the safe application of biogas fertilizers. This technology deserves the large-scale promotion with the above advantages and cheap material sources. However, the actual environment is more complex than the laboratory, which leads to uncertainty regarding the actual benefits of composite passivators on the anerobic digestion. More experiments on the application in industry and agriculture need to be carried out to prove the actual impact of composite passivators on the environment. In addition, for industrial production, the preparation conditions of composite passivator need to be further optimized according to different environmental conditions and requirements.