Abstract
Effects of salt on anaerobic digestion are dosage-dependent. As salt is a widely used condiment in food processing, effects of salt are bound to be considered when food waste is digested. In this study, salt addition effects (0, 2, 4, 6, 9, 12 g∙L−1) on biogas and methane yields and kinetics of biogas production were researched. Meanwhile, component characteristics (food waste featured in carbohydrate, protein and fat, respectively) and fermentation concentrations (5 and 8 gVS∙L−1) were also taken into consideration. Results showed that 2–4 g∙L−1 salt addition was the optimal addition dosage for AD systems as they not only have the maximum biogas and methane yields, but also the maximum vs. removal in most cases. Also, according to the results of a modified Gompertz model, which is used to predict biogas and methane production rates, suitable salt addition can accelerate biogas production, improving the maximum biogas production rate (Rmax). Factorial design (2 × 2) proved that interaction of salt and fermentation concentrations was significant for food waste featured with carbohydrate and with protein (p < 0.05). High salt addition and fermentation concentration can break the AD system when the feeding material was food waste featured with carbohydrate, but for food waste featured with protein, interaction of fermentation concentrations and salt addition can alleviate inhibition degrees.
Highlights
Considerable amounts of food waste are discharged annually; it is estimated that 1.3 billion tons of food waste were produced in 2018 globally and the amount of food waste could keep increasing in the 20 years [1,2]
The only two exceptions were those for high carbohydrate food waste (HCFW) digestion with 8 gVS·L−1 fermentation concentration (FC), g·L−1 salt addition had the maximum total specific biogas production (SBP) (764 mL·g−1 Volatile solid (VS), Table 2) and for high protein food waste (HPFW) digestion with gVS·L−1 FC, 4 g·L−1 salt addition had the maximum total SBP (628 mL·g−1 VS, Table 2)
The only exception was when HCFW was digested with 8 gVS·L−1 FC; 4 g·L−1 salt addition had the maximum total specific methane production (SMP) (269 mL·g−1 VS, Table 2)
Summary
Considerable amounts of food waste are discharged annually; it is estimated that 1.3 billion tons of food waste were produced in 2018 globally and the amount of food waste could keep increasing in the 20 years [1,2]. Food waste may cause many problems, such as bad odors and the emission of greenhouse gases. Is considered as a good option for organic waste treatment, as the products of AD are biogas and digestate. Biogas can be used to generate electricity and heat and digestate can be used as fertilizer [3]. Salt mentioned in this article refers to table salt, which is a widely used food flavoring agent and its content in food is between 2% and 5% in mass fraction [4]. The main composition of salt in China is NaCl, taking up more than 97%, and the corresponding sodium content is 39% approximately [5]
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