| 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.
|
PDF Full Text Request