Studies On Influence Factors For Sludge Ozonation And Sludge Reduction Performance Of Combined Processes

Municipal wastewater is commonly treated by activated sludge process. With the increase of urban scale and wastewater flowrate, this process will generate an increasing amount of excess activated sludge, which need for expensive treatment and final deposal within a variable legal and regulatory framework. To resolve this problem, a novel sludge reduction process by ozonation, in which ozone lysis and cryptic growth take place. Some basical researches, such as the reaction pattern of sludge ozonation, the influence factors, kenitic analysis and optimal methods, and hybrid application researches, such as that is combined with CAS process (conventional activated sludge) and with MBR (membrane bioreactor) one, were investigated to disclose the design methods and operational performance.The effects of some important parameters on the sludge reduction performance of the activated sludge systems in three sludge samples with different MLSS concentration were studied at different ozone concentration. The results showed that during ozonation, the variations of both MLSS and rCOD (released COD) showed the quasi-liner trends in relationship, and rCOD released by unit mass sludge ozonation was in the range of 0.96 to 1.04. The active bacteria amount showed a index decrease pattern and the sterilization was completed in just few minutes. The soluble N and TP in mixed liquor showed increase trends, while pH value being dropped from 8 to 5. It has been shown from the experimental results that both the control of the reaction time and the increase of the cell lysing ratio are favourable to keep the stable performance of activated sludge systems.The influence factors during ozonation and mass transfer was studied respectively. The results showed that of the influence factors including the input ozone concentration, ozone dose, initial MLSS concentration, content of extracellular polymer substance, reduced matters concentration, pH value and operating mode of sludge reduction unit, etc., the input ozone concentration, initial MLSS concentration and operating mode of sludge reduction unit are the key factors. With the increase of ozone concentration, the cell lysing rate increased, while ozone dosage and low initial MLSS concentration were favorable for sludge to be liquified. For operating mode, at the same energy cosumption, the lysing effect of continous operating mode was better than intermittent operating one.The kinetic studies showed that at certain ozone dose the MLSS concentration and active bacteria amount could be depicted by linear equation and exponential equation respectively, and the rCOD could also be depicted by linear equation. The sterilization ratio showed that the residues in ozonized sludge mixed liquid were not suitable to recycle to aeration basin, which might be the main cause of the increase of dead cells.As to the optimization of sludge reduction process, the results showed that low sludge yield processes are valuble to decrease energy consumption and treatment costs. Low MLSS concentration is favorable for celles to lyse. The determination of ozonation time should be considered based on both the cell lysing ratio and cost-effectiveness, and a better sludge reduction effect was achieved at the cell lysing ratio within the rang of 0.72-0.8. The optimal height of the bulk mixed liquid was determined according to the ozone utilization rate higher than 90%, and the reactor height should be designed up to 1.5 to 2.0 times that of optimal height.The sludge flowrate for SO (sludge ozonation) unit could be determined by the value of q/Qmax, and among the main design factors were wastewater flowrate Q, the SCOD decrease between influent and effluent (S0-Se ), observed sludge yield Yobs, cell lysing ratioξ, released COD per unit MLSSηresulted from ozonation, and yield coefficient Y2. All above factors should be determined before the design of CAS-SO process or MBR-SO process. In this study, the q for CAS-SO process and MBR-SO process is 9 L·d-1 and 2 L·d-1, repectively.Under the control of the given parameters, a better operational performance for each process with of 80 days duration was obtained. As compared with control stage, some parameters did not varied significantly, such as MLSS, COD and ammonia nitrogen, except for nitrate, TN and TP, because there were different nitrogen transfer route without EAS wasting, which could cause the increase of TP concentration in effluent. From analysing the sludge characteristic of CAS-SO process, it was found that the introduce of SO unit could hardly deteriorate sludge characteristic which included the EPS concentration, settling performance, active bacteria amount, DHA activity, specific oxygen uptake rate, and so on.In comparison of the CAS-SO process with MBR-SO process, it was found that MBR-SO process was better than CAS-SO process whether in technical or economic index. However, in terms of operational performance, both the two processes have their own respecvtive merits. Moreover, it was also found that the costs of MBR-SO process for treating unit volume wastewater was lower, only accounted for half that of CAS-SO process. The cost of both the processes for treating unit volume of sludge was accounted for 9.5% and 5.83% of unit wastewater treatment expenditure, respectively.