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我国目前农村污水处理基本沿用城市污水处理厂的集中式活性污泥处理方法,然而农村污水存在水量波动大、分布较为分散、维护水平不足等问题,相比城市污水处理率不佳。2021年,全国农村生活污水进行处理的乡村仅占36.94%,远低于城市污水处理率97.89[1]。农村污水处理通常沿用城市污水处理的活性污泥法,活性污泥法虽然能够有效去除污水中的有机物和氮磷,但适合污水合流集中处理模式,然而,大部分农村的管网覆盖率不高,合流集中处理难度大。农村污水COD、TN、TP、氨氮平均值分别为205、62.5、5.5、50 mg·L−1[2],只含有有机物、氮、磷等物质,有毒成分和重金属含量低,可生化性好。而微藻适用于可生化性好的污水净化,利用小球藻和球形红杆菌处理富氮养猪场废水和富碳淀粉废水,TP、COD和TN的回收率分别可达到100%、96%、97%和95%[3]。在利用斜生栅藻处理不同处理阶段城市污水,处沉池污水的TN和TP去除率分别为99.8%和83.1%,二沉池污水的TN和TP去除率分别为98.9%和97.6%[4]。从当地奶业收集的乳品废水作为生长介质(未经任何预处理),用于培养微藻4 d后,COD去除率达到90%以上;培养6 d后氨氮完全消耗[5]。微藻可以利用小型光生物反应器分散式处理可生化性好的农村污水,与活性污泥法相比,无需铺设管网对污水收集集中处理,在适宜的工艺条件下,污水中氮磷的去除率可达90%以上[6]。并且微藻处理污水无需要额外添加碳源用于硝化/反硝化、只需少量曝气、剩余微藻生物质价值高,适合在温度适宜的南方地区处理污水。在微藻处理污水的过程中,通常会有部分有机物难以被微藻高效吸收转化[7]、生物质油脂含量低和藻水分离困难的问题,藻水分离的方法包括絮凝(化学、生物、电)、重力沉降、过滤、浮选、离心等方法,但是这些方法普遍分离成本高或者周期长,限制了微藻处理污水的应用[8]。有研究表明,利用微藻与细菌联合可以大幅提高处理各种污水能力,同时可促进微藻油脂的合成。微藻与细菌之间存在互利共生关系,两者之间的相互作用主要有营养交换、信号传导等[9]。营养交换是藻菌相互作用最基本的关系,微藻通过光合作用产生的O2、释放出有机物可以通过异养细菌的呼吸作用吸收利用。同时细菌呼吸作用产生的CO2又会被微藻光合作用吸收合成生物质。藻菌之间O2、CO2交换可以降低污水处理的曝气需求,减少污水处理造成的CO2排放。共生菌群可以为微藻提供维生素、植物激素、铁载体等。微藻在优化条件下可积累高达50%干生物质占比的油脂,被认为是极具前景的可再生第三代生物能源来源[10]。而且污水中的有机碳为微藻提供了混合营养的环境,混合营养环境相比于普通培养基(BG11)可观察到更高的生物量和脂质生产力[11]。因此,利用微藻处理污水的同时还具有碳减排、生物柴油生产等多重效果[12]。
近年来,微藻生物膜系统受到广泛的重视,因为微藻生物膜生物密度高,可通过机械刮取、挤压等方法收取微藻[13],减少了微藻处理污水后回收生物质的成本。微藻生物膜的形成来源微藻的贴壁生长的特性,当微藻吸附固体基质上时,与其他共生菌群分泌EPS形成稳定的藻菌生物膜,生物膜状态下的比悬浮系统具有更好的光照可用性[14]。微藻生物膜附着基质要求较高,必须考虑到微藻附着效率和形成生物膜后的稳定性,但同时生物膜载体在污水中也会降低反应器透光度的问题。
微藻通过叶绿素a和叶绿素b光合色素吸收光能,具有蓝光(450~480 nm)和红光(605~700 nm)双重吸收波长[15]。特定的红光和蓝光照射微藻可以提高细胞生长速度和脂质生产力[16],有助于提升微藻的光合作用效率。有报道在悬浮微藻体系中加入碳量子点,光合活性显著提高并且微藻的脂质产率提高了34%[17]。但是在悬浮体系中加入碳量子点,碳量子点难以回收,同时新物质引入的可能会给水环境带来潜在的风险。而藻菌共生容易沉淀,沉淀在底部的微藻细胞难以接受到光照,导致光合作用效率低。碳量子点与壳聚糖结合改性的纤维可能有助于改善光照可用性差和藻菌共生易沉淀的问题。微藻在载体表面附着和载体表面性质有关,微藻会在静电力、范德华力等作用下[13],吸附在表面粗糙的载体材料上。由于微藻细胞带负电荷,表面带正电的材料更易于吸附微藻附着,带正电的多孔性材料有效吸附带负电的小球藻快速集和接种,促进了微藻生物膜的形成。脱乙酰基后的壳聚糖含有大量的-NH2基团,带有丰富的正电荷,适合作为生物膜附着载体的改性剂。碳量子点(CQDs)是一种新型的荧光材料,具有光致发光、化学惰性、高生物相容性等优越性能[18],可以通过调节光谱特性和发光强度,实现定向转移未利用光到蓝光和红光。
根据目前微藻系统存在的光利用程度差、附着效率低、生物质油脂含量低的问题。本文设计制备了碳量子点-壳聚糖(CS)交联改性的聚乙烯醇缩甲醛(PVF)纤维作为生物膜附着载体,以该载体为填料构建藻菌生物膜系统,研究该系统净水分散污水协同油脂转化的效能,并解析藻菌生物膜机制。为构建稳定高效的微藻资源化处理[12]污水系统提供理论依据。
碳量子点/壳聚糖@维纶纤维载体强化藻菌生物膜净化农村污水及脂质积累效能
The efficiency of carbon quantum dots/chitosan @ vinylon fiber carrier enhanced algae and bacteria biofilm in purification of rural wastewater and lipid accumulation
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摘要: 本文针对微藻净化污水效率低、藻水分离困难以及微藻含油量低等系列问题,设计制备碳量子点/壳聚糖@维纶纤维(CQDs/CS@PVF)附着载体强化藻菌生物膜系统对污水净化协同产油。结果表明,CQDs/CS@PVF添加形成的藻菌生物膜系统与对照组相比,对污水中的COD、TN和TP的去除速率分别提高了68.2%、120.2%、79.2%,这是由于CQDs/CS@PVF藻菌生物膜系统提高了42.3%的微藻比生长速率,出水达到北京《农村生活污水处理设施水污染物排放标准》一级A标准。污水中的有机物被微藻利用转化为生物质油脂,其含量达到30.1%,脂肪酸C16~C18含量占比为96%,此组份的脂质更适合作为底物合成生物柴油,实现污染物向生物柴油优质原料的转化。CQDs/CS@PVF微藻生物膜快速去除有机物的同时高效生产油脂的机理是藻菌生物膜体系微藻与细菌细胞间距小,从而提高了O2-CO2气体与营养组分交换效率;此外,CQDs/CS@PVF通过拓展藻30%的可吸收光谱可提升20.9%的光合作用效率,非光化学淬灭系数提升了98.0%。以上研究结果可为CQDs/CS@PVF载体强化藻菌生物膜系统高效率处理农村污水同步收获油脂提供参考。Abstract: In this study, carbon quantum dots/chitosan @ poly (vinyl alcohol) fibers (CQDs/CS@PVF) were designed and prepared to address a series of issues such as low efficiency in sewage purification by microalgae, difficulty in algae water separation, and low oil content in microalgae. Synergistic oil production by attaching carriers to enhance algal biofilm systems for wastewater purification. The results show that CQDs/ CS@PVF compared with the control group, the removal efficiencies of COD, TN, and TP in wastewater by the added algal biofilm system increased by 68.2%, 120.2% and79.2 %, respectively, which was due to CQDs/ CS@PVF-loaded algal biofilm system increased the growth rate of microalgae by 42.3%, and the effluent could meet the Class I A standard of Beijing Water Pollutant Discharge Standard for Rural Domestic Sewage Treatment Facilities. The organic matter in sewage was utilized by microalgae and converted into biomass oil with a content of 30.1%, and the content of fatty acid C16-C18 accounted for 96%. The lipid of this component was more suitable as a substrate for the synthesis of biodiesel, realizing the conversion of pollutants into high-quality raw materials for biodiesel. The mechanism of CQDs/CS@PVF-loaded microalgae biofilm for rapid removal of organic matter and efficient production of oil was small distance between microalgae and bacterial cells in the algal bacterial biofilm system, thereby improving the exchange efficiency of O2-CO2 gas and nutrient components. In addition, CQDs/ CS@PVF could expand the absorbable spectra of algae by 30%, which could lead to the increase of the photosynthetic efficiency by 21%, and the increase of non photochemical quenching coefficient by 98%. This study provides a reference for CQDs/CS@PVF carrier-enhanced algal biofilm system to efficiently treat rural sewage and simultaneously harvest oil.
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Key words:
- microalgae /
- rural sewage /
- biodiesel
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表 1 一级反应动力学模型拟合参数
Table 1. First order reaction kinetics model fitting parameters
系统 COD TN TP R2 k R2 k R2 k 悬浮 0.981 1.043 0.986 0.462 0.981 0.629 PVF 0.998 1.319 0.994 0.543 0.998 0.902 CS@PVF 0.998 1.385 0.983 0.582 0.998 0.95 CQDs/CS@PVF 0.999 7 1.754 0.985 0.938 0.990 51 1.127 表 2 logistic模型拟合参数
Table 2. Logistic model fitting parameters
系统类别 R2 k CQDs/CS@PVF 0.990 1 0.846 71 CS@PVF 0.999 08 0.676 43 PVF 0.996 69 0.648 27 悬浮 0.999 17 0.595 04 表 3 不同系统下微藻脂肪酸占比
Table 3. Proportion of microalgal fatty acids in different systems %
脂肪酸 悬浮 PVF CS@PVF CQDs/CS@PVF C14:0 0.63 0.8 0.92 0.43 C16:0 40.58 36.49 37.11 37.03 C16:1 7.30 7.18 6.42 6.22 C18:0 2.89 2.26 2.05 2.98 C18:1 25.81 29.94 29.34 29.15 C18:2 11.33 16.81 15.53 15.53 C18:3 8.58 5.18 5.66 5.65 C20:1 0.41 0.55 0.36 0.36 C20:1 0.36 0.37 0.54 0.58 C22:1 2.11 1.42 2.07 2.07 饱和脂肪酸 44.10 43.55 43.08 43.44 不饱和脂肪酸 55.90 56.45 56.92 56.56 C16-C18 96.49 96.86 96.11 96.56 -
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