Abstract:A study of thermal-alkaline pretreatment and alkaline fermentation of cyanobacteria collected from Taihu Lake was conducted to produce the volatile fatty acids(VFAs). The results showed that thermal-alkaline pretreatment (T=90 ℃, pH=12) promoted the release of cyanobacteria organics into the liquid and provided enough bioavailable substrates for the subsequent acidogenic fermentation. In comparison with the control, the concentrations of soluble chemical oxygen demand(SCOD), soluble polysaccharides and proteins increased by 10.3, 12.3 and 4.8 times, respectively. 3D excitation-emission matrix (3D-EEM) analysis demonstrated that thermal-alkaline pretreatment could improve the bioavailability of cyanobacteria and reduce the content of humic acid-like organics, which is beneficial to the subsequent acidogenic fermentation. In the sequencing batch tests of acidogenic fermentation, the average total volatile fatty acid (TVFAs) concentration reached 18.64 g·L?1, and the VFAs yield was 46%. In the semi-continuous tests, the average TVFAs reached 15.56 g·L-1 and VFAs yield was 26%, the degradation rates of soluble polysaccharides and proteins in cyanobacteria were 50.43% and 47.04%, respectively, and the reduction rates of total suspended solids (TSS) and volatile suspended solids (VSS) reached 24.5% and 43.4%, respectively, which achieved good volume reduction results. However, a large amount of refractory organic materials was still not biodegraded or biotransformed. Key words:cyanobacteria/ thermal-alkali pretreatment/ anaerobic fermentation/ organic release/ volatile fatty acids (VFAs).
图1厌氧产酸发酵罐模式和流程图 Figure1.Anaerobic fermentation tank model and process flow diagram
图2蓝藻热碱预处理后SCOD浓度、溶出率变化(提供给半连续运行反应器) Figure2.Variations of SCOD and solution rate after thermal-alkaline pretreatment (provided for semi-continuous operation)
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Jiangsu Key Laboratory of Anaerobic Biotechnology, School of Environment and Civil Engineering, Jiangnan University, Wuxi 214122, China Received Date: 2019-07-28 Accepted Date: 2019-11-13 Available Online: 2020-05-12 Keywords:cyanobacteria/ thermal-alkali pretreatment/ anaerobic fermentation/ organic release/ volatile fatty acids (VFAs) Abstract:A study of thermal-alkaline pretreatment and alkaline fermentation of cyanobacteria collected from Taihu Lake was conducted to produce the volatile fatty acids(VFAs). The results showed that thermal-alkaline pretreatment (T=90 ℃, pH=12) promoted the release of cyanobacteria organics into the liquid and provided enough bioavailable substrates for the subsequent acidogenic fermentation. In comparison with the control, the concentrations of soluble chemical oxygen demand(SCOD), soluble polysaccharides and proteins increased by 10.3, 12.3 and 4.8 times, respectively. 3D excitation-emission matrix (3D-EEM) analysis demonstrated that thermal-alkaline pretreatment could improve the bioavailability of cyanobacteria and reduce the content of humic acid-like organics, which is beneficial to the subsequent acidogenic fermentation. In the sequencing batch tests of acidogenic fermentation, the average total volatile fatty acid (TVFAs) concentration reached 18.64 g·L?1, and the VFAs yield was 46%. In the semi-continuous tests, the average TVFAs reached 15.56 g·L-1 and VFAs yield was 26%, the degradation rates of soluble polysaccharides and proteins in cyanobacteria were 50.43% and 47.04%, respectively, and the reduction rates of total suspended solids (TSS) and volatile suspended solids (VSS) reached 24.5% and 43.4%, respectively, which achieved good volume reduction results. However, a large amount of refractory organic materials was still not biodegraded or biotransformed.
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--> --> --> 近年来，随着太湖水体富营养化的加剧，太湖蓝藻暴发呈逐年上升趋势。蓝藻不仅会造成水质恶化，破坏水体生态系统，而且多数蓝藻还会释放藻毒素，藻细胞死亡裂解后释放至水体中，威胁城市日常饮用水安全和公众健康[1]。目前，应急打捞是我国减除蓝藻水污染生态灾害和降低再次暴发强度的最直接的有效措施[2]。但打捞的大量蓝藻如不进行及时处理，则会在陆地上发生二次污染。因此，如何实现蓝藻泥的高效处理处置已成为目前亟待解决的环境问题。 蓝藻中富含蛋白质、碳水化合物以及少量的脂质[3]，是理想的厌氧发酵的底物。通过将厌氧消化控制在产酸阶段，可以得到附加值较高的VFAs；并且还能使蓝藻减量，实现蓝藻资源化、减量化的目的。KIM等[4]以提取了脂质的微藻作为底物进行厌氧发酵产酸，产酸效率(1 g VS中VFAs的占比)为36%。SEO等[5]采用提取了脂质的微藻残留物作为底物进行发酵产酸，产酸量达到8.17 g·L?1，产酸效率(1 g COD中VFAs的占比)为40%。然而，目前利用蓝藻发酵生产VFAs的报道不多。 本研究考察了太湖蓝藻泥厌氧发酵产VFAs的效果，并采用热碱预处理对蓝藻进行破壁处理，研究了蓝藻水解及产酸效果，考察了发酵前后底物的降解情况，剖析了预处理前后及发酵后溶解性有机物(DOM)的成分变化，分析预测了工业规模下的蓝藻发酵产酸的经济可行性，为蓝藻厌氧发酵产酸的工业化推广提供了可靠的理论和数据支持。
蓝藻产酸效果如图3所示。蓝藻厌氧发酵过程分为序批式启动阶段和半连续运行阶段。在序批式启动阶段，底物采用TSS=72 g·L?1、VSS/TSS=55%热碱预处理后的蓝藻浆，TVFAs最高可达18.64 g·L?1，产酸效率(1 g VSS中VFAs的占比)最高为46%；在半连续运行阶段，由于更换进料底物，采用TSS=80 g·L?1、VSS/TSS=80%热碱预处理后的蓝藻浆，TVFAs平均浓度为15.56 g·L?1，产酸效率稳定在26%。而EHIMEN等[13]对未预处理的小球藻进行发酵产酸，产酸效率(1 g VS中VFAs的占比)为13%，本研究序批式启动阶段中的产酸效率为其3.54倍，半连续运行阶段的产酸效率为其2倍，这说明预处理能有效提高后续发酵产酸效率。SURESH等[14]以碱-超声预处理蓝藻，以序批式运行得到的产酸效率(1 g VS中VFAs的占比)为27%，本研究序批式启动阶段的产酸效率为其1.7倍。在以蓝藻为底物进行厌氧发酵过程中，所产酸中以乙酸为主，丙酸、丁酸为次，这主要与在碱性条件下，发酵底物中富含碳水化合物、蛋白质有关[15]。
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