摘要
目前,我国的大多数城市污水处理厂,特别南方城市的污水处理厂普遍存在生物反硝化脱氮碳源不足的问题,这导致反硝化效率较低,出水总氮不能达标排放。因此,充足的碳源是保证反硝化反应有效进行的必要条件。活性污泥技术作为目前污水处理中运用最普及的一种技术,其产生的大量剩余污泥的处理与处置费用在污水厂运行成本中所占的比例也越来越大。课题针对脱氮达标和剩余污泥处置的问题,利用剩余污泥在碱性条件下厌氧发酵产生的含高碳源的上清液作为反硝化脱氮的补充碳源进行了研究。试验的主要研究结论如下:
首先,通过试验研究了剩余污泥碱解发酵的较优条件以及碱解对污泥絮体的破解作用。确定采用机械搅拌,碱解条件为pH10,并在此基础上,通过一次性投料间歇试验的指标分析、反硝化速率对比以及污泥破解后的扫描电镜照片比较,确定较优的污泥停留时间为SRT=9d。
论文对比了pH10,SRT=9d条件下的剩余污泥碱解发酵上清液、乙酸钠和生活污水三种碳源的反硝化效能和反硝化动力学过程。结果表明,在相似的VFAs/N和MLVSS情况下,上清液碳源的反硝化曲线变化趋势与乙酸钠较为一致,平均反硝化速率略低于乙酸钠,硝酸盐去除率和乙酸钠相当,但远高于生活污水。三种碳源12h的反硝化过程均可分为四个阶段:无限制快速反应阶段、限制性快速反应阶段、慢速反应阶段和内源呼吸阶段。在第一阶段,上清液碳源对应阶段比反硝化速率达到12.169 mgN/(gVSS·h),略低于乙酸钠碳源第一阶段比反硝化速率14.178mgN/(gVSS·h),远高于生活污水该阶段比反硝化速率5.669mgN/(gVSS·h),第二、三阶段上清液碳源比反硝化速率最高,其次是乙酸钠碳源,最低仍是生活污水。结合污泥脱氢酶活性测试结果,认为剩余污泥碱解发酵上清液可以作为反硝化脱氮碳源。
在动力学分析的基础上,以不同VFAs/N比值用上清液单独作碳源进行反硝化批式试验,进一步考察了反硝化各阶段的反应速率,同时进行了上清液回用量的分析,确定了试验初始硝酸盐浓度下的较优VFAs/N比值。结果表明,在反应的6h内,反硝化速率变化也分阶段变化,且随VFAs/N比值的增加VFAs引入量增加,反硝化反应速率迅速得到提高。综合比较反硝化反应的的硝酸盐氮和SCOD去除效果,建议选用VFAs/N比值在2～3之间,该比值范围内脱氮效率高,剩余SCOD浓度对后续处理负荷影响较小。在该比值范围内,反硝化反应主要发生在前3h,持续6h的脱氮效率仅比3h提高10%,从节约投资和运行成本出发,建议运行时间控制在3h内。
将剩余污泥碱解发酵上清液投加到生活污水中进行反硝化,与未投加上清液的生活污水对比,反硝化速率明显提高,在6h内硝酸盐氮降解量可提高到2倍以上,适当VFAs/N下出水硝酸盐氮可低至5mg/L左右,脱氮效率可提高到70%,甚至可接近90%。
根据小试结果,选用合适的VFAs/N比值,将上清液以与生活污水进水体积(1.5L)比例为1/5、1/7.5和1/10用于SBR工艺,使VFAs/TN比值平均值分别为3.12、2.78、2.24,COD/TN比值分别为13.74、9.45、6.10,考察了上述上清液用量条件下,污染物的去除效能和上清液回用对系统存在的影响。结果表明,比例1/10为三个用量条件中的最佳用量,该用量下出水平均COD浓度为47mg/L,平均TN浓度为14.83mg/L,氨氮出水浓度小于2mg/L,出水污染物浓度能达到一级A标的要求。当比例大于1/10时,随着比例增加,TN去除率略有升高,氨氮去除效果基本不受影响,但COD去除率下降,COD出水浓度大幅度增加,不能达标。上清液的回用对SBR系统污泥活性无明显影响。维持上清液适当的用量,氮的引入对系统的影响不大;部分难降解碳源的引入增加了COD处理难度,在较高比例下,出水COD浓度不能达标;上清液引入的磷在短时间内对系统脱氮影响不明显,但长时间的运行,应考虑对上清液中磷元素的处理。
At present, the lack of carbon source for biological denitrification widely exsits in most of sewage treatment plants in China, especially the cities in the south, which leads to low denitrification rates and concentration of total nitrogen in effluent of the plants not meeting the standard. Enough carbon source is an necessary condition for efficient denitrification process. As one of the most popularized technologies for sewage treatment, Activated sludge technology prouduces a lot of excess activated sludge. The percentage of the cost of excess activated sludge dispose becomes bigger in the running costs of sewage treatment plants. This research focus on the two problems. Wasted activated sludge (WAS) was anaerobic digested under alkaline condition. The supernatant generated from the alkaline anaerobic acid-pahse digester, which was full of high carbon source, was recycled to sewage treatment process as supplementary carbon source and investigated. The main conclusions were as following:
Firstly, the preferable conditions for WAS alkaline anaerobic hydrosis and the disruption degree of sludge under alkaline conditon were investigated. The stirring way was determined as mechanical agitation, and the alkaline circumstance was pH10. Then through intermittent test with sludge fed at one time, the parameters of supernatant from alkaline anaerobic acid phase of WAS under different sludge residence time(SRT) were tested and analysised. Considered with denitrification rates and electron micrograph of cracked sludge, the optimum solid residence time(SRT) was determined as 9d.
The dentification effiency and denitrification dynamics of the three carbon sources, such as supernatant of sludge alkaline digestion, acetic acid sodium and sewage, were investigated and compared. The results shows, under the similar conditions of VFAs/N ratio and MLVSS, the denitrification curve of supernatant carbon source was similar to acetic acid sodium, the average denitrification rate was little lower than to acetic acid sodium, and nitriate removal rate was approximate to acetic acid sodium, and much higher than sewage. The twelve-hour denitrification process of three carbon sources all could be divided to four phases:the non-restrict fast reaction phase, the restrict fast reaction phase, the slow reaction phase, and the endogenous respiration phase. In the first phase, the specific denitrificaiton rate of supernatant carbon source achived 12.169 mgN/(gVSS·h), which was lower than acetic acid sodium of 14.178mgN/(gVSS·h), but much higher than sewage of 5.669 mgN/(gVSS·h). In the second and third phases, the supernatant carbon source had the biggest specific denitrificaiton rate, followed by acetic acid sodium, and the last still was sewage watewater. Combining the results of TTC-DHA test and the above, it is considered that the supernatant form WAS alkalin anaerobic acid phase digester could be used as the carbon source for denitrificaiton.
On the basis of dynamics analysis, denitrification tests with supernant used as single carbon source under different VFAs/N ratios were proceeded. Reaction rates of every phase of denitrifiction was investigated. The recycling dosage of supernatant was analysed, and the optimum ratios under the initial nitrate concentration was determined.The results shows that in the running phase of 6h, denitrification rates also changed in different phases, and with the increase of VFAs/N, the intake of VFAs increased, and the denitrification rates was quickly improved. Comparing the nitrate and SCOD removal results, VFAs/N ratio between 2～3 was advised. Under the ratio extent, the denitification reaction mainly occurred in the first 3h phase, the nitrate removal rate of 6h was 10% higher than 3h. In order to reduce the investment and running cost, running time was advised to control in 3h.
The supernatant from WAS alkaline anaerobic acid phase digester was invested to denitrificaiton process of sewage wastewater. Compared to denitrfication process of only sewage wastewater, the denitrificaiton rate with supernatant dosage was improved obviously. The degradated amount of nitrate nitrogen in six hours could be improved more than twice. Under proper VFAs/N ratio, the nitrate nitrogen concentration of effluent could be decreased to about 5mg/L, and the denitrification effiency could increase to 70%, even almost to 90%.
Recording the results of tests on pilot scale, and the advised VFAs/N ratio, supernatant from WAS alkaline digestion was recycled to SBR process, with the volume being 1/5, 1/7.5 and 1/10 of sewage influent. The corrsponding average VFAs/TN were seperately 3.12、2.78、2.24, and average VFAs/TN were seperately 13.74、9.45、6.10. The efficiency of pollutants removal and the effect of supernatant recycle to system under the dosage of supernatant were investigated. The optimum dosage among the three ratios is 1/10, under which the average effluent COD concentrition was 47mg/L, average effluent TN concentrition was 14.83mg/L, and the effluent NH4+-N was less than 2mg/L, all meeting the discharge standard. When the recycle ratio was higher than 1/10, the removal rate of total nitrogen increased a bit with the ratio getting bigger, and the ammonia nitrogen removal was not affected. While the rate of COD removal decreased, and the effluent COD concentrition increasing by a wide margin couldn’t meet the standard. Supernatant recycle had no remarkable effect to the SBR activated sludge system. The effect of intake of nitrogen to the system was litte when the recycle dosage was appropriate. Under higher recycle ratio, the effluent COD concentrition couldn’t meet the discharge standard, because the intake of some nonbiodegradable carbon source increase the difficulty of COD removal. The phosphrous carried into the system by supernatant recycle had no remarkable effect to total nitrogen removal in a short time. But dispose of phosphrous should be considered for long time running.
引文
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