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两种生物质废物的资源化关键技术研究:剩余污泥与集胞藻
中文摘要

生物质是指一切通过光合作用所形成的有机物质,包括动物、植物、微生物,及其排泄物、垃圾及有机废水等源自生物体的有机质。而生物质废物是生物质利用过程中产生和消费产生的废弃物,它仍然属于生物质的范畴,但能量密度、可利用性等都显著降低。作为两种典型的生物质废物,剩余污泥和微藻受到广泛关注。尤其在中国,由于城镇化水平不断提高而导致的污水处理副产物-剩余污泥产量巨大,微藻爆发会导致严重环境问题,其处理面临巨大挑战,而资源化是降低其处理成本的最佳途径。针对剩余污泥和集胞藻处理及资源化热点问题,本论文系统研究了两种生物质废物的资源化关键技术。主要研究内容和结果如下: (1)运用短时好氧发酵(STAD)技术处理剩余污泥,明确了其对污泥脱水、絮凝和沉降性能及产生物聚合物的影响机制。生物聚合物浓度在STAD过程中先增大后减小,且当生物聚合物浓度较低时,污泥的絮凝性和脱水性均随着生物聚合物浓度的增大而增强。当蛋白质和多糖浓度分别为21.6和12.3㎎/g VSS时,污泥的絮凝性和脱水性最佳。 (2)采用表面活性剂+STAD工艺处理剩余污泥,探索了表面活性剂类型对污泥脱水和产生物聚合物的影响。结果表明:直链十二烷基苯磺酸钠(LAS)、十六烷基三甲基溴化按(CTAB)、椰油酰胺丙基甜菜碱(CAPB)、吐温80 (Tween-80)和鼠李糖脂五种类型表面活性剂均能显著提高溶液中生物聚合物及其组份浓度,CAPB对污泥脱水程度提升效果明显,而其他表面活性剂作用不大。从吸附过程的功能性组份-蛋白质浓度来说,表面活性剂的选择顺序为:CAPB >>CTAB>LAS>鼠李糖脂>Tween-80;从提高剩余污泥脱水程度来说,表面活性剂选择顺序应为:CTAB>CAPB>LAS>鼠李糖脂>Tween-80。 (3)进一步将CaO投加至添加CAPB、CTAB和LAS且发酵8h后污泥中,研究了协同工艺对剩余污泥脱水及产生物聚合物的影响,结果表明:低CaO投加量对生物聚合物浓度影响不大,而增大投加量会显著降低蛋白质和生物聚合物浓度,泥饼含水率均随着CaO投加量的增大而降低,但会显著降低蛋白质的产量,且CAPB+好氧发酵+CaO组合对于强化污泥脱水和产生物聚合物效果最佳;综合考虑蛋白质浓度、泥饼含水率及CaO投加量,后期选用CAPB+好氧发酵+CaO(0.10g/gTSS)协同工艺用以同步提高污泥脱水和产生物聚合物。 (4)系统研究了协同工艺运行时CAPB投加量、pH值、溶解氧(DO)浓度、温度及污泥浓度对污泥脱水及产生物聚合物的影响。协同工艺运行最佳条件为:CAPB投加量0.10g/gTSS,pH值中性,DO 2~3 ㎎/L,温度25~30℃,污泥浓度6~9 g/L。最佳运行条件下CAPB作用机理为:促进生物聚合物水解,增大上清液中易生物降解低分子量组份浓度,提高污泥中功能性微生物活性和比例。继续投加适量的CaO不会影响生物聚合物及其组份浓度,且可与污泥形成多网格状骨架,降低污泥可压缩性,脱水时保持透水性滤层,强化污泥脱水效果。 (5)系统考察了pH值、温度及生物聚合物与重金属离子质量比对生物聚合物吸附Pb²⁺和Zn²⁺的影响,运用响应面设计进行优化,探讨了吸附动力学、热力学及吸附机理。结合响应面优化结果、吸附效果和生物聚合物成本,最佳吸附条件为:pH值6.0,温度35℃,生物聚合物与金属离子质量比2:1;生物吸附为自发过程,符合假二级吸附动力学方程,等温吸附符合Langmuir等温吸附模型。生物聚合物对Pb²⁺和Zn²⁺的最大吸附量分别为793.6和408.4㎎/g,均高于现有报道的最大值。蛋白质在生物聚合物与重金属离子的结合过程中起到重要作用,包括重金属离子与-NH₂之间的络合及与-COOH之间的离子交换。因此,生物聚合物吸附重金属离子的机理包括静电引力、络合和离子交换作用。 (6)探讨了生物聚合物对Zn(Ⅱ)和Pb(Ⅱ)的共吸附性能及机理。Zn(Ⅱ)对Pb(Ⅱ)的生物吸附影响较小,而Pb(Ⅱ)会显著抑制Zn(Ⅱ)的吸附。Zn(Ⅱ)和Pb(Ⅱ)总浓度相同时,生物聚合物对两者的总吸附容量高于相同浓度的单一Pb²⁺离子的溶液,但是低于相同浓度的单一Zn²⁺离子的溶液。生物聚合物对Pb²⁺和Zn²⁺的共吸附过程均可用Langmuir和Freundlich等温吸附模型来描述。共吸附的机理包括生物聚合物中的官能团与金属离子之间的络合作用和离子交换作用。 (7)系统研究了pH值对生物聚合物、重金属离子及吸附过程的影响。添加生物聚合物前后,Pb(Ⅱ)的总去除率均随着系统pH值的增大而增大。pH值的升高使得生物聚合物官能团持续解离和去质子化,这促进生物聚合物通过络合和生物吸附作用去除Pb(Ⅱ)。生物聚合物与Pb(Ⅱ)之间的吸附点位数随着pH值的增大先增大后减小。酸性条件下,生物吸附作用在Pb(Ⅱ)的去除中起主要作用,而在中性或碱性条件下,生物吸附和化学沉淀作用同时存在。碱性条件下,生物吸附和化学沉淀相互影响,且化学沉淀作用优于生物吸附作用。 (8)米用CTAB和LAS协同调理剩余污泥,得到不同组份浓度和比例的生物聚合物,探讨了生物聚合物吸附重金属离子中起主要作用的组份。CTAB调理可制得高蛋白质含量生物聚合物,而LAS调理可以得到高多糖和核酸含量生物聚合物。不同组份比例的生物聚合物对Cu²⁺的吸附容量为258.94~389.92㎎/g,且吸附容量与蛋白质及蛋白质/多糖质量比之间为显著的正线性相关性。结果表明,生物聚合物中蛋白质含量及蛋白质与多糖质量比在其吸附去除Cu²⁺过程中起到了重要的作用。高蛋白质含量的生物质可以直接作为一种性能优异的生物吸附剂来去除废水中重金属离子。 (9)首次运用流式细胞仪与SYTOX染料来评价集胞藻EPS热提取过程中细胞膜的裂解。60℃下热处理20min实现了EPS最大提取率和最少的细胞裂解和EPS变性,为EPS的最佳热提取条件。集胞藻批次生长实验中,P的初始浓度对生物质、SMP和EPS的生长、P的代谢途径和最终分布没有影响,但会影响P不同分布出现的时间。初始阶段,溶液中IP(IPBs)快速且同步吸附到EPS (IP〓)和转移到胞内储磷池(IPP),IP〓耗尽时,IP〓的解吸成为细胞继续生长所需IP的主要来源,且该部分IP不断转化成有机磷(OP),大部分的OP储存在胞内。基于P的质量平衡而提出的P代谢途径模型系统阐述集胞藻生长过程中P的存在形态、分布及动态变化。EPS和IPP在P的吸收、储存及转换过程中起到了重要作用。 (10)首次运用细胞分选(FACS)和显微镜技术实现了集胞藻液中异养菌的定量分析。FACS用来分离混合液中具有自荧光特性的集胞藻和不含叶绿素的颗粒物(异养菌和细胞碎片)。荧光染色法实现了不含叶绿素颗粒物中异养菌和细胞碎片的分离。光学显微镜验证了FACS所确定的划分阈值的精度。运用建立的方法评估磷对集胞藻批次生长中异养菌生长的影响,结果表明集胞藻生长过程中异养菌的比例为3-13%,且低P条件下异养菌的数量较低。 (11)深入探讨了十四烷基三甲基溴化铵(MTAB)对集胞藻回收及色素萃取的影响机理。MTAB投加量低于8%时,凝聚和絮凝作用使得集胞藻形成聚集体,集胞藻回收效率提高;更多的MTAB(>8%)使得EPS释放出来,细胞表面负电荷降低,细胞膜通透性增大,更多萃取剂进入到细胞内部使得色素萃取率增大。投加40%的MTAB可实现集胞藻的最大回收率(62%),而更高投加量使得细胞开始裂解。投加50%的MTAB可实现色素90%以上的萃取率,继续增大投加量不会提高色素萃取效率,表明实现最大程度的色素萃取并不需要完全破碎细胞,这为表面活性剂促进生物质回收和色素萃取奠定了基础。 (12)本课题实现了剩余污泥脱水的同时生产了有利用价值的产物,是一种可持续发展的循环经济模式;对集胞藻的系统研究为其的低能耗快速生长、降低其回收和资源化利用成本打下坚实的基础。 关键词:剩余污泥;好氧发酵;生物聚合物;表面活性剂;重金属吸附;集胞藻;光生物反应器;代谢途径

英文摘要

Biomass is an organic substance derived from photosynthesis, including animal, plant, microorganism and its excrete, waste and organic wastewater originating from organisms. Biomass waste is generated during the biomass utilization and it still belongs to biomass but significantly reduced energy density and avaliability. As two kinds of typical biomass waste, waste activated sludge and microalgae are widely concerned. Especially in China, the amount of wastewater treatment by-product, waste activated sludge, was significantly increased due to the improvement f the urbanization level, the eruption of microalgae caused serious environmental problems, and their treatment is facing serious challenges. Resource utilization should be the best way to reduce the treatment cost. In this research, the key technologies of two kinds of biomass waste are systematically investigated and the main contents and results are showed as following: (1)Effects of short-time aerobic digestion (STAD) on sludge properties (dewateability, flocculability and settleability) and microbial extracellular polymeric substances (EPS), and their correlations have been systematically investigated. In the aerobic digestion process, proteins and polysaccharides gradually increased, while nucleic acids increased first and then decreased. The sludge after STAD exhibited better flocculability and dewaterability than that after the prolonged aerobic digestion, but a deteriorating settleability achieved in the process. The linear fitting results showed that proteins and polysaccharides in biopolymers may govern sludge settleability, flocculability and dewaterability. Interestingly, good sludge flocculability and dewaterability were observed when the proteins and polysaccharides in EPS were lower than 21.55 and 12.27 ㎎/gVSS, respectively. (2)Synergistic process of surfactant + STAD + CaO was applied to treat waste activated sludge, and the effect of surfactant and the synergistic process on the sludge dewatering and biopolymers production were investigated. Results should that five different kinds of surfactant, including cetyltrimethyl ammonium bromide (CTAB), linear alkylbenzene sulfonates (LAS), cocoamidopropyl betaine (CAPB), tween-80 and rhamnolipid, could significantly improve the concentration of biopolymers and their compoents. CAPB could improve the sludge dewatering obviously, while other surfactant had little effect. In terms of the functional components of protein in the adsorption process, the selection order of surfactant should be: CAPB >> CTAB > LAS> Rhamnolipid > Tween-80. In terms of improving sludge dewatering, surfactant selection order should be: CTAB > CAPB > LAS > Rhamnolipid > Tween-80. (3)CaO was further added to the activated sludge after aerobic digested for 8 h and adding CAPB, CTAB abd LAS respectively, the effect of synergetic process on the sludge dewatering performance and biopolymers production were investigated. Results showed that low dose of CaO had little effect on the biopolymers production, while high dose of CaO will significantly reduce the concentration of proteins and biopolymers. Water content of the sludge cake gradually increased with increasing CaO dose. The synergetic process of CAPB+ STAD +CaO has the best performance to improve the sludge dewatering and biopolymers production synchronously. Considering the extracted proteins concentration, sludge cake water content and CaO dose, the synergetic process of CAPB + STAD + CaO (0.10 g/gTSS) was used to improve the sludge dewatering and biopolymers production. (4)The effect of CAPB dose, system pH value, dissolved oxygen (DO) concentration, temperature and activated sludge concentration on the sludge dewatering and biopolymers production in the synergetic process were systematically investigated. Results showed that the optimal conditions for the running of synergetic process were as following: CAPB dosage of 0.10 g/gTSS, neutral pH value, DO concentration of 2 ~ 3 ㎎/L, temperature of 25 ~ 30℃, sludge concentration of 6 ~ 9 g/L. With the optimal conditions, influence mechanisms of CAPB on the synergetic process contains: promoting the hydrolysis of biopolymers, increasing the low molecular weight supernatant components that are easily biodegraded, and improving the activity and proportion of functional microorganisms in the activated sludge. Adding sutaible dose of CaO into the digested sludge will not affect the concentration of biopolymer and its components. CaO could form multi grid skeleton with digested sludge, reduce sludge compressibility, maintain porous permeability in the dewatering process, and then enhance sludge dewatering performance. (5)The effects of system pH value, temperature and the mass ratio between biopolymers and heavy metals were systematically investicated; response surface methodology was applied to optimize the biosorption processes; the adsorption kinetics, thermodynamics and mechanisms were discussed. Combined with the response surface optimization results, adsorption efficiency and adsorption cost, the optimal conditions for the adsorption should be controlled as following: pH value of 6.0, temperature of 35℃ and the mass ratio between biopolymers and heavy metals of 2:1. Thermodynamic analyses indicated that the adsorption processes were feasible and spontaneous in nature and the adsorption kinetics was well fit for the pseudo second-order model. Compared with Freundlich and Temkin models, Langmuir model better described the adsorption isotherms. The maximum adsorption capacities of the biosorbent (793.61 mg Pb²⁺/g and 408.38 mg Zn²⁺/g) were markedly higher than those of the reported biosorbents. Metal ions were adsorbed onto the biopolymers surface via electrostatic interaction. Proteins in biopolymers play key roles in the binding of heavy metals. Complexation and ion-exchange between metal ions and the functional groups of -NH₂ and -COOH were existed in the adsorption process. The adsorption mechanisms could be the combined action of electrostatic interaction, complexation and ion-exchange between functional groups and metal ions. (6)The associated adsorption characteristics and mechanisms of Pb²⁺ and Zn²⁺ by biopolymers were systematically investigated. Zn²⁺ slightly inhibited Pb²⁺ adsorption, while Pb²⁺ markedly inhibited Zn²⁺adsorption onto the biosorbent. When the total concentrations of Pb²⁺ and Zn²⁺ were the same, the total adsorption capacity of biopolymers for Pb²⁺ and Zn²⁺ in the binary metal solution was more than that for Pb²⁺, but less than that for Zn²⁺ in single metal solution. Moreover, rising proportion of Zn²⁺ concentration in the binary metal solution was found to increase the total adsorption capacity of the biopolymers. Both Langmuir model and Freundlich model commendably described the adsorption isotherms of Pb²⁺ and Zn²⁺ in the associated adsorption system. Complexation and ion exchange between the functional groups and the metal ions played an important role in the associated adsorption process. (7)The influences of pH value on the functional groups of biopolymers, speciation of heavy metals and its removal rate were systemically investigated. With and without adding biopolymers, the removal rate of Pb(II) increased with increasing the system pH value. The continuous dissociation and deprotonation of functional groups in the biopolymer with rising system pH value promoted the biosorption removal of Pb(II) via complexation and ion exchange. The binding site numbers between biopolymers and metal ions increased first and then decreased with rising system pH value. Biosorption removal of Pb(II) was dominant under acid conditions, but biosorption and Pb(OH)₂ precipitation co-existed under alkaline conditions. The biosorption and the Pb(OH)₂ precipitation were interestingly found to interfere with each other under alkaline conditions, and Pb(OH)₂ precipitation excelled the biosorption in the competition of capturing Pb(II). (8)Biopolymers with different proportions of constituents were extracted from activated sludge after the conditioning by cetyl trimethyl ammonium bromide (CTAB) and linear alkylbenzene sulfonate (LAS), and then were used to test their adsorbability for metal ions. CTAB was prominent in increasing proteins (PN) content, and LAS was prominent in increasing polysaccharides (PS) and nucleic acids (NA) content. The biopolymers with different proportions of constituents showed the adsorption capacities of 258.94~ 389.92 ㎎/g for Cu²⁺. The adsorption capacities showed significant positive correlation with proteins (R²=0.9225, P=0.00) and proteins/polysaccharides (R²=0.9797, P=0.00), but negative correlation with polysaccharides and nucleic acids. Proteins and high proteins/polysaccharides were identified to be the key constituents and feature of the biopolymer responsible for binding heavy metals, respectively. The findings denote that all biomass with high protein content could be an excellent adsorbent for metal ions. (9)Our work firstly used flow cytometry (FC) with SYTOX Green (SG) to evaluate EPS solubilization and cell lysis during thermal extraction of EPS from Synechocystis. High EPS-extraction efficiency without cell lysis and EPS denaturalization was achieved with thermal extraction at 60℃ for 20 min for Synechocystis. In the batch growth of Synechocystis, initial P concentration had no impact on the production of biomass, SMP, and EPS. While the initial P concentration affected the rate and the timing of how P was transformed among internal and external forms of inorganic P (IP) and organic P (OP), the trends were the same no matter the starting P concentration. Initially, IP in the bulk solution was rapidly and simultaneously adsorbed by EPS (IP〓) and taken up as internal IP (IP〓)- As the bulk-solution’s IP was depleted, desorption of IP〓 became the predominant source for IP that was taken up by the growing cells and converted into OP〓. Based on the P mass balance, the proposed a set of transformation pathways for P during the growth of Synechocystis. Key is that EPS and intracellular P pool play important and distinct roles in the uptake and storage of P. (10)Our work firstly combined flow cytometry (FC) and fluorescence activated cell sorting (FACS) for the quantification of heterotrophic bacteria in cultures of Synechocystis. Particles not containing chlorophyll--heterotrophic bacteria and cell debris -- were separated from mixed cultures using FACS based on autofluorescence of Synechocystis. Heterotrophic bacteria were differentiated from cell debris using FC with SYTOX green fluorescence. Using microscopy, we verified that FACS was able to quantify heterotrophic bacteria in Synechocystis cultures effectively. Applying these methods to batch cultures of Synechocystis showed that the count proportions of heterotrophic bacteria were significant (3 ~ 13%) and low P produced low heterotrophic bacteria. (11)Systematically explored the mechanisms by which MTAB enhances biomass harvesting and pigments extraction from Synechocystis. At a low dose (≤ 8%), MTAB mainly brought about coagulation and flocculation, which led to aggregation that improved harvesting. MTAB dose from 8% to 40% released and solubilized EPS, which lowered the biomass’s negative charge, but also led to the breakup of the large aggregates. The increase of cell permeability in this stage allowed EA to pass into the cells and achieve better pigment extraction. With a MTAB/Biomass % ratio of 40%, complete EPS removal was achieved with minimal cell lysis, yielding a maximum harvest efficiency of 62%, even more MTAB (> 40%) led to cell lysis. Pigment extraction plateaued at > 90% recovery for MTAB/Biomass % ratios of greater than 50%, but which almost remained stable with more MTAB, indicating that complete cell lysis was not required to achieve the maximum extraction efficiency. This work lays the foundation for the practical application of surfactant for biomass harvesting and pigment extraction. (12)This project synchronously achieved the waste activated sludge dewatering and valuable biopolymers production, which is a sustainable development mode for the circular economy; Systematic study on Synechocystis laid the solid foundation for its low energy consumption and rapid growth, reducing the cost of biomass harvesting and resource utilization. Key Words: Activated sludge; Aerobic digestion; Biopolymers; Surfactant; Heavy metal adsorption; Synechocystis sp. PCC 6803; Photobioreactor; Metabolic pathway

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