Kinetic studies on organic degradation and its impacts on improving methane production during anaerobic digestion of food waste
Introduction
Anaerobic digestion of food waste is attracting more and more attention worldwide for recovering energy and reducing greenhouse gas emissions [1], [2]. There have been many studies focused on the effect of operating parameters on methane yields, such as operation mode (batch or continuous), temperature (mesophilic or thermophilic), moisture content (wet or dry), organic loading rate, presence or absence of co-substrates (co- or mono-digestion) and hydraulic retention time [3], [4], [5], [6]. Furthermore, in order to increase digestion efficiency and improve the methane yield, anaerobic biodegradability of food waste in two-stage [7] and three-stage [8] anaerobic digesters have also been studied, and various pretreatment methods [6], [9], [10] have been proposed. The energy ratio and economic feasibility were also conducted [9].
Meanwhile, to allow the prediction of kinetic parameters and to help elucidate the digestion process, some kinetic models have been proposed to describe the process of substrate degradation and biogas production. Simplified generalized models based on first order models have predominantly been employed for parameter estimation, improving the understanding of the biological process and aiding in predicting the behaviour of biological system when designing anaerobic system [11]. It was concluded that kinetic parameters, such as those of biogas production and methane yield, may vary when different models and substrates were used [12]. Li et al. [13] compared three kinetics models, including first-order kinetics, the transfer function model and the cone model for different livestock manures as feedstocks and with different substrate concentrations. The results showed that the cone model had better performance than the first-order and the transfer function models. Kafle and Kim [14] compared the modified Gompertz and first-order kinetics models and showed that, better fitting result was found for the modified Gompertz model. El-Mashad [15] observed that the Cone model best described the cumulative biogas production data, whereas the exponential model was the worst predictor of the experimental data. Moreover, due to the structural and numerical complexity, many models cannot be applied for automatic monitoring or robust simulation of different substrates and process conditions [16]. It is important to highlight that previous studies devoted to the kinetic parameters during digestion of food waste were simplified to focus on fitting the experimental data of biogas/methane production [10], and very few studies centred on the detailed kinetic degradation properties of organics in food waste (i.e. volatile solids, total solids, lipids and proteins) and their correlations during the anaerobic digestion of food waste. Additionally, food waste can present important differences as the composition can vary with factors such as food availability, seasonal variation and consumption patterns. For food waste, lipids are one of the main organic components and may have a bi-directional effect on digestion [17]. However, calculation of the hydrolysis constant using kinetic models from the previous study was only made for a combined fraction of carbohydrates and proteins, omitting the lipid fraction [18]. Moreover, Miron et al. [18] suggested that the hydrolysis constant value might not be a universal constant, as it is no more than a specific calculation for a given substrate under certain conditions. However, it could be noted that previous studies devoted to the kinetic parameters during digestion of food waste (including mono- and co-digestion) were confined to using collected food waste with limited composition ranges [19] and co-digestion with other organic waste (such as dairy manure [20] and sewage sludge [21]).
Therefore, there is a need to extend kinetic models to organics reduction and verify whether kinetic parameters meet this important assumption. Thus, it is necessary to make comparisons of these kinetics models (i.e. the exponential, Fitzhugh, Cone, transference function and modified Gompertz models), which were used to determine the methane production potential, maximum methane production rate and lag time for anaerobic digestion by fitting the measured methane yields [13], [16], [22], [23], [24], [25], [26], [27], and find the appropriate one by model validation for parameter estimation. To further increase the digestion efficiency and improve the biomethane production, making the overall process more energy sustainable, the interaction of organics degradation and their impact on methane production should be studied.
The objectives of this paper are to investigate the degradation performance of organics (i.e. total solids, volatile solids, lipids, and proteins) and maximize the methane yield of food waste by optimizing organics degradation during food waste digestion. This work contributes to improvement the understanding of: (a) the applicability and validation of five simplified and widely applied models for predicting biomethane production performance; and (b) the correlation between the organics reduction in terms of volatile solids, proteins and lipids in food waste. Finally, the optimizations for enhancing biomethane production through the improvement of organic degradation were suggested.
Section snippets
Food waste
Food waste was collected from three different canteens. Impurities, such as big bones, plastics, and metals were manually removed from the food waste. Samples collected from the same canteen were mixed with a kitchen blender to ensure uniform and representative experimental materials. The mixed samples were then macerated to an average size of 1–2 mm. All samples were stored at 4 °C in a refrigerator for the subsequent experiments. The basic compositions and characteristics of the three kinds
Food waste characteristics and process parameters
The performance of anaerobic biodegradability for 12 substrates was evaluated using batch tests. The total solid and volatile solid content in the 12 food waste samples ranged from 16.7% to 24.7% (wet basis) and 95.2% to 98.2% (dry basis), respectively (Table S2). Additionally, the organic composition in the feedstock on a volatile solid basis was: lipids from 6.1% to 45.5%, proteins from 18.6% to 46.3%, and carbohydrates from 18.0% to74.7% – indicating significant variations in the volatile
Conclusions
This paper investigated the degradation performance of organics (i.e. total solids, volatile solids, lipids, and proteins) and the maximum methane yield of food waste by optimizing organics degradation during food waste digestion. This work showed that the methane yield (385–627 mL/g volatile solid) increased exponentially with the organic reduction, while the volatile solid reduction increased exponentially with the lipid degradation and linearly with protein degradation. The reduction of
Acknowledgements
This work was supported financially by the Major Science and Technology Program for Water Pollution Control and Treatment (2017ZX07202005) and the China Scholarship Council (CSC).
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