摘要
以餐厨垃圾和果蔬垃圾为代表的城市生物质废物是造成城市生活垃圾传统处理处置过程中出现二次污染的主要原因。本文通过研究水热预处理工艺,强化城市生物质废物的固相有机物水解,使生物质废物能利用高效率的厌氧消化反应器进行消化。综合能耗及厌氧消化性能考虑,水热工艺条件为175℃/60分钟时是较佳的工艺条件。此时,VSS水解率为57.89%,SCOD浓度增加到129.54g/L,粘度降低了46.53%,产甲烷总量提高了5.8%。由于大量有机物转移到液相,单位体积上清液产甲烷量能增加70%,加快了城市生物质废物的甲烷化转化。悬浮固体浓度(SS)不会对水热反应造成明显影响。随着SS浓度增加,餐厨垃圾和果蔬垃圾的屈服应力迅速增加,流动性严重下降。因此水热工艺中对餐厨垃圾和果蔬垃圾的SS浓度应分别控制在10%和5%左右,当原始物料的SS浓度较高时,不宜进一步浓缩。
针对水热后的生物质废物仍然存在大量固相基质的特点,研究固相基质在厌氧消化过程中的特征。在产甲烷潜势(BMP)的实验中发现,以餐厨和果蔬垃圾制成的固相基质具有良好的厌氧消化特性,最终甲烷化转化的部分占固相基质总量的58.9%-74.6%之间。在低于1:5的生物负荷率条件下,消化液中SCOD和VFA均没有出现积累。据此建立了固相基质厌氧消化动力学模型。利用实验室已运行的CSTR反应器对批式实验中求得的固相基质厌氧消化模型参数进行校核,其模型预测结果与实验结果相符。通过模型研究发现,在CSTR反应器中,HRT=20天为VSS的降解曲线的拐点,当HRT小于20天时,反应器内VSS浓度将迅速上升。水热后的生物质废物相对生物固体浓度有所提升,有利于应用ASBR反应器进行消化。
对ASBR进行初步的建模分析表明,反应器内部存在固体积累的可能。利用连续运行的ASBR反应器进行研究发现,固相基质是妨碍ASBR反应器效率进一步提高的关键因素。对比分析表明,当反应器内部生物固体浓度能够维持较高的水平时,即使SRT缩短,也能获得较高的VSS去除率。因此,可以尝试改进ASBR反应器,增加生物固体浓度,以提高转化效率,稳定工况。
The municipal biowaste including kitchen waste and vegetable-fruit waste is the main reason for the secondary pollution in the traditional disposition process of MSW. Through the hydrothermal pretreatment process, the organic materials in solid phase of the biowaste would be hydrolyzed and liquefied, so the biowaste could be converted into methane more efficiently. Considering the energy cost and the anaerobic digestion capacity, the best technological conditions are about 175 degrees of temperature and 60 minutes of reaction time. Under this condition, 57.89% VSS are hydrolyzed, the concentration of SCOD reached to 129.54g/L, the kinetic viscosity reduced about 45.53%, and the total methane production increased about 5.8%. As most of organic matter liquefied, the methane production of per unit volume of supernatant increased 70%. The hydrothermal reaction didn’t influenced by the suspended solid concentration (SS). But with the SS increasing, the yield pressure of biowaste increased sharply caused the fluidity decreased. So in hydrothermal process, the SS of kitchen waste and vegetable waste should below 10% and 5%, while the initial SS of biowaste is pretty high, the biowaste should not be thickened further.
To the characterization of high SS in biowaste being pretreated, the solid substrate behavior in anaerobic digestion is worth studying. In Batch experiment, the solid substrate was pretty easy to be anaerobic digested. Finally about 58.9% to 74.6% solid substrate converted into methane. During experiments, both SCOD concentration and VFA concentration were not accumulated. Based on the experimental data, anaerobic digestion kinetics model of solid substrate established. The value of parameters was calibrated by the data from CSTR reactors. From the model simulation, we found HRT=20d is the flex point for the solid substrate degradation, while HRT is less than 20 days, the solid substrate concentration would increase sharply. The relative concentration of biosolids of the reactor fed on biowaste pretreated would be elevated.
Preliminary analysis of ASBR indicated that the solid substrate could be accumulated in the reactor. The experimental data indicated that the substrate in solid phase is the critical factor to elevate the efficiency of ASBR. Comparative analysis shows that when the biosolid concentration in the reactor could retain high level, most of VSS could be removed with the short SRT. So the ASBR reactor should be further modified to retain biosolid in short SRT.
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