摘要
针对传统污泥厌氧消化反应速率慢、消化时间长、池体体积庞大、生物气中甲烷含量低等问题,采用强化预处理的技术研究其对污泥厌氧消化的促进作用。本文分析比较了单独超声波、单独碱解以及碱和超声联合应用对污泥的破解效果;考察了污泥经超声波和碱联合作用后对厌氧消化性能的影响,初步探讨了污泥预处理对厌氧消化反应的促进机理,并在莫诺方程的基础上,建立了预处理污泥厌氧消化反应过程的动力学方程。主要研究成果如下:
碱和超声波的组合方式对污泥的破解效果明显优于超声波和碱的单独处理,其不但可以提高污泥的水解速率,而且可以减少碱的投加量,缩短超声破解的时间。由SCOD随破解时间的变化曲线可知,各种预处理方式对污泥的水解过程可以分为快速水解和慢速水解两个阶段。对快速水解阶段进行动力学分析可知,碱和超声波同时作用的预处理方式获得的水解速率最高,为130.3 mg SCOD /(L·min)。
污泥经碱和超声波同时作用后进行厌氧消化,其生物气产量、有机物去除率、固体削减率均较控制组有显著提高,并且在维持同样的污泥产气率和有机物去除率的条件下,碱和超声波的预处理作用可使厌氧消化反应的污泥停留时间由一般的20d缩短到8d。预处理污泥在停留时间为8d时,生物气产量为控制组(污泥停留时间为20d)的2.6倍,VS和TCOD的去除率也分别较控制组(20d)提高了22.87%和14.38%。
对预处理污泥在厌氧消化中的水解率、产酸率和产甲烷率进行研究得到:与控制组相比,碱和超声预处理可以提高污泥厌氧消化的水解率、产酸率和产甲烷率。预处理污泥在停留时间为8d时,其水解率、产酸率和产甲烷率分别较控制组(20d)提高了82.38%、76.47%和86.54%。对于原污泥的消化反应,水解阶段始终是整个厌氧消化过程的限速步骤;对于预处理污泥的消化反应,当污泥停留时间大于等于8d时,水解反应是整个厌氧消化过程的限速步骤;当污泥停留时间小于5d时,有机物的水解率和产酸率远大于其产甲烷率,因此,产甲烷反应是整个厌氧消化的限速步骤。
通过对污泥上清液中有机物的成分组成分析和消化系统中产甲烷活性的测定得到预处理促进污泥厌氧消化性能的机理为:碱和超声的共同作用使污泥中的有机物能快速的由固相进入液相,污泥在破解预处理过程中的溶解速率远远高于其自溶速率,有机物的利用率较控制组得到显著提高。与污泥中溶解性有机物的量相对应,预处理污泥在消化反应中生成的有机酸量较控制组也有显著提高,并且当污泥停留时间大于等于8d时,这些生成的有机酸能够迅速被降解转化。当反应器经过预处理的污泥培养驯化后,其消化污泥的产甲烷活性较控制组有显著提高,并且在较短的污泥停留时间下还可以保持较高的产甲烷活性。采用一级动力学方程对消化反应中有机物的降解进行动力学分析可知,碱和超声波预处理可使反应的K值提高89.58%(以TCOD计算),从而加快了有机物的降解速度,使污泥厌氧消化的性能得到提高。
在莫诺方程的基础上,结合微生物增殖和底物降解的规律,根据微生物和底物的物料衡算方程,得出反应器厌氧消化动力学的四个基本参数为:YG=0.0726, b=0.0413d~(-1),K=-9.65和k0=0.025,在这四个参数的基础上得到的预处理污泥厌氧消化系统的动力学方程式。
Anaerobic digestion is a common process for waste activated sludge stabilization. However, in anaerobic digestion, the biological hydrolysis is so difficult, which results in the slow degradation, long residence time and large volume. In order to reduce the impact of rate-limiting step, several pretreatments for sludge hydrolysis are presented. In this thesis, the effects of ultrasonic pretreatment, alkaline pretreatment and combination of these two methods are compared. The efficiency and mechanism of anaerobic digestion of sludge by alkaline and ultrasonic pretreatment are studied. In addition, basing on the Monod equation, the kinetics equation of anaerobic digestion of pretreated sludge is deduced. The main conclusions of this study are as follows:
It has been found that the combined ultrasonic and alkaline treatment is more effective than alkaline or ultrasonic treatment alone in releasing SCOD and VSS solubilization, which can not only improve the hydrolysis rate, but also reduce the pretreatment time in ultrasonic pretreatment and alkaline dose in alkaline treatment. There are two distinct phases in sludge hydrolysis with pretreatment. The first phase is a very rapid increase in solubilization, followed by a much slower second phase. According to kinetic analysis for first rapid phase, the initial hydrolysis rate of the simultaneous alkaline and ultrasonic treatment is the highest among these methods being 130.3mgSCOD / (L·min).
Comparing with the non-pretreated sludge, the reduction of organic matters and biogas production in the anaerobic digestion with simultaneous alkaline and ultrasonic disintegrated sludge can be improved significantly. After pretreatment, the sludge retention time (SRT) of anaerobic digestion can be reduced from 20d to 8d, and the digestion system performs well. When the retention time is 8d, the anaerobic digestion of pretreated sludge shows 2.6 times higher gas production than that of untreated sludge (SRT=20d), and removal efficiency of volatile solid (VS) and total chemical oxygen demand (TCOD) increased by 22.87% and 14.38%, respectively.
Comparing with the non-pretreated sludge, the percentage hydrolysis, acidification and methanogenesis of TCOD in the anaerobic digestion with pretreated sludge can be improved significantly. When the sludge retention time is 8d, the hydrolysis, acidification and methanogenesis of TCOD with pretreated sludge increased by 82.38%, 76.47% and 86.54%, respectively, compared to the control group. For the non-pretreated sludge, the hydrolysis is found to be the rate-limiting step of the whole digestion process. For the pretreated sludge, when the SRT≥8d, hydrolysis was the rate-limiting step. However, when the SRT<5d, the hydrolysis and acidification of TCOD are higher than the methanogenesis, and the methanogenesis become the rate-limiting step in the anaerobic digestion.
By analyzing the component of released liquid in the anaerobic digestion, the mechanism of enhancing the anaerobic digestion of sludge by alkaline and ultrasonic pretreatment is explored. The results show that when the sludge is pretreated by the simultaneous alkaline and ultrasound, the substances inside and outside the cell can be released into the liquid rapidly, and the utilization ratio of organic matter is increased. The solubilization rate of sludge in pretreatment process is higher than the decay rate of the sludge. Corresponding to concentration of the soluble substance in the sludge, the higher quantity of VFA is production in the anaerobic digestion with pretreated sludge, and when the SRT≥8d, the produced VFA in the anaerobic digestion can be converted to the biogas quickly. It also found that the activity of methane forming bacteria in the anaerobic digestion fed with pretreated sludge is higher than the control group and at the reduced retention time it can be stable. In order to determine the improvement of the process kinetics a first-order degradation model of organic matter is used over the anaerobic digestion system. Comparing with the control, the K of digestion process with pretreated sludge is increased by 89.58%.
Basing on the Monod equation, the kinetic equation of anaerobic digestion of pretreated sludge is deduced. Four basic parameters for reactor are YG=0.0726, b=0.0413d~(-1),K=-9.65 andk0=0.025, respectively.
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