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餐厨垃圾具有有机质含量高、易生物降解的特性[1]。厌氧消化是实现餐厨垃圾减量化、稳定化和资源化的关键技术之一[2]。区别于传统厌氧消化,干式厌氧消化(TS>15%)具有沼液产量少、容积产气率高等优点[3]。因此,干式厌氧消化已在国外得到大量应用,但国内该技术的推广应用仍属起步阶段[4]。随着我国生活垃圾分类的实施,一方面,大量的餐厨垃圾得以分离并亟需资源化处置;另一方面,分类后得到的餐厨垃圾不同于以往未分类的生活垃圾,其杂质含量减少、有机质含量进一步提高;这对于干式厌氧消化工艺意味着更高的有机负荷和潜在的底物有机组成失衡,最终造成抑制影响。
传统湿式厌氧消化常用碳氮比来衡量底物的有机组成,合适的碳氮比为20~30[5],过高或过低都会对产甲烷微生物的代谢造成抑制。而对于干式厌氧消化,由于其含固率高,有机质含量以及挥发性脂肪酸、氨氮等中间产物浓度远高于湿式厌氧消化工艺,其底物有机组成失衡往往会导致更严重的酸化或氨氮抑制影响。因此,较为平衡的底物有机组成更为重要。餐厨垃圾中易被厌氧微生物用以产甲烷的有机物以多糖、蛋白质和脂质为主。不同有机物由于在组成结构、微生物代谢途径等方面的差异,导致他们具有不同的产甲烷速率和甲烷产量。例如,多糖和蛋白质水解产生的小分子单糖以及氨基酸将通过糖酵解途径产生有机酸,蛋白质水解同时释放一定量的氨氮;脂质水解的主要产物为中长链脂肪酸,中长链脂肪酸将通过β氧化每次脱除2个碳生成1分子乙酸和1分子H2实现逐步降解。多糖具有较快的降解速率;脂质主要产物(中长链脂肪酸)的降解却被认为是厌氧消化的限速步骤。此外,多糖、蛋白质和脂质的理论产甲烷量也有较大的差异[6],根据各有机物的元素组成通式估算,多糖、蛋白质和脂质的理论产甲烷量分别为416、496和1 014 mL·g−1。但是,由于厌氧消化微生物代谢的能量消耗以及抑制性影响,各类有机物往往难以被完全降解,实际甲烷产量往往低于理论甲烷产量。综合以上信息可知,导致餐厨垃圾干式厌氧消化过程中产甲烷速率和产甲烷量变化的成因是较为复杂的。即使在相近的碳氮比下,有机物组成也会存在差异;因此,碳氮比并不能充分地指示餐厨垃圾干式厌氧消化工艺是否稳定运行。
对于餐厨垃圾干式厌氧消化,有研究[7-8]报道了产甲烷量、产甲烷速率和厌氧消化稳定性受碳氮比和有机物组成比例的影响;然而,对于各有机组分比例(多糖、蛋白质、脂质)的变化对有机组分的降解以及底物甲烷转化率的影响仍不明确。为此,本研究针对餐厨垃圾干式厌氧消化工艺甲烷转化率的限制性因素进行了分析。以馒头、豆腐和食用油分别代表餐厨垃圾中多糖、蛋白质和脂质3类主要厌氧消化代谢底物,通过批量厌氧消化实验,分析基于有机组成差异的餐厨垃圾干式厌氧消化中甲烷转化率及其限制性因素,以期为餐厨垃圾干式厌氧消化过程中高效稳定产甲烷提供参考。
餐厨垃圾干式厌氧消化工艺中甲烷转化率及其限制性因素
Analysis of restrictive factors of methane conversion based on organic composition in dry anaerobic digestion of food waste
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摘要: 餐厨垃圾的有机组成是影响其干式厌氧消化甲烷转化率的重要因素。通过批量实验并结合混料模型设计,以馒头、豆腐和食用油分别代表餐厨垃圾中多糖类、蛋白质类和脂质类为主的底物,研究餐厨垃圾有机组成对干式厌氧消化各有机组分降解率、甲烷产生量的影响;并探讨了餐厨垃圾有机组分降解对甲烷转化率的限制性因素。结果表明,多糖、蛋白质和脂质的平均降解率分别为62.87%、41.96%和29.62%;餐厨垃圾中多糖和蛋白质比例的增加会显著降低甲烷转化率,而脂质比例的增加显著提升了甲烷转化率。此外,高多糖比例的餐厨垃圾干式厌氧消化可导致pH降到7以下,产甲烷菌可利用的碳源以CO2的形式散失;高蛋白质比例下蛋白质结构发生转化,难以被微生物进一步利用;甲烷转化率与脂质降解率具有正相关性,高脂质比例下产甲烷速率在初期受限,因而导致较长的迟滞期。本研究结果有助于优化有机组成以提高餐厨垃圾干式厌氧消化甲烷转化率。Abstract: Organic composition is an important factor which influences the methane conversion in dry anaerobic digestion of food waste. The effects of organic composition on organic degradation and methane conversion were investigated in dry anaerobic digestion of food waste based on the batch mode experiments, in which steamed bread, tofu, and oil were represented polysaccharide-rich, protein-rich, and lipid-rich food waste, respectively. And the restrictive factors of methane conversion were further discussed. Results showed that, the average degradation rate of polysaccharide, protein and lipid was 62.87%, 41.96%, and 29.62%, respectively. Methane conversion rate decreased with increasing proportion of polysaccharide and protein, while that increased with increasing proportion of lipid. Dry anaerobic digestion of food waste with high proportion of polysaccharides resulted in pH below 7, and the available carbon source of methanogens was lost in the form of CO2. Protein structure was transformed under high protein ratio, which was difficult to be further utilized by microorganisms. There was a positive correlation between the rate of methane conversion and lipid degradation. The rate of methane production was limited at the initial stage of anaerobic digestion under high lipid ratio, resulting in a longer lag period. The results of this study are helpful to optimize the organic composition and improve the methane conversion rate of dry anaerobic digestion of food waste.
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Key words:
- organic composition /
- methane conversion rate /
- dry anaerobic digestion /
- food waste
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表 1 接种污泥及底物基本特征
Table 1. Basic characteristics of inoculum sludge and substrates
供试样品 质量分数/% C/N 质量浓度/(mg·L−1) 质量分数/% TS VS SCOD 氨氮 溶解性蛋白 溶解性多糖 总蛋白 总多糖 总脂质 接种污泥 18.68±1.74 6.03±0.32 6.70±0.02 6430.00±818.05 1 696.25±219.06 1 463.98±75.18 1 287.86±65.28 0.08±0.01 0.04±0.00 0.05±0.00 馒头 26.65±0.19 26.56±0.19 20.12±0.01 − − − − 0.06±0.02 0.79±0.06 0.03±0.00 豆腐 17.33±0.04 16.70±0.06 4.19±0.00 − − − − 0.56±0.07 0.15±0.00 0.10±0.01 食用油 99.93±0.01 99.90±0.01 902.42±1.04 − − − − 0.11±0.08 0.09±0.02 0.79±0.05 表 2 实验设计
Table 2. Design of experiments
% 实验组 馒头 豆腐 食用油 A 100 0 0 B 0 100 0 C 0 0 100 D 66 17 17 E 17 66 17 F 17 17 66 G 33 33 33 注:表中数据为挥发性固体质量分数。 表 3 不同有机组成底物的甲烷转化率
Table 3. Methane conversion rate of substrates with different organic compositions
实验组 最终累积产
甲烷量/(mL·g−1)理论产甲
烷量/(mL·g−1)甲烷
转化率/%A 88.22 479.36 18.40 B 162.73 541.94 30.03 C 861.07 898.22 95.86 D 276.67 568.93 48.63 E 430.37 601.90 71.50 F 711.25 780.17 91.17 G 500.48 653.75 76.56 -
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