| Presently, the high concentration of grit in waste water treatment plants’ inflow is widespread. This phenomenon mainly owned to the imperfect drainage system and the highly strength of urban construction. The waste water treatment plant(WWTP) in our country mostly adopts the oxidation ditch or sequencing batch reactor activated sludge process(SBR), these two processes do not contain the primary settling tank(PST). Process contains PST, such as Anaerobic-Anoxic-Oxic(A2/O), often cancelling the operation model of the PST, to insure sufficient carbon source for biological nitrogen and phosphorus removal. This operation model make fine grit(particle size <200μm), which should be removed in PST, inflow into the biochemical disposal unit. Fine grit deposit at the bottom of bioreactors, or suspend in the activated sludge. The suspend part often directly result in a decrease of the MLVSS/MLSS ratio. Currently, a lot of WWTPs’ MLVSS/MLSS ratio is only about 0.3~0.5, some even be as low as 0.2. Practice has proved that the high concentration of grit in WWTPs’ bioreactors will not only accelerated the wear of equipment, but also reduces the efficiency of anaerobic digestion of sludge. The treatment of excess activated sludge will be restrained. Therefore, how to remove the fine grit in activated sludge and improve the ratio of MLVSS/MLSS, has become a difficult problem must be resolved. Our research group developed a grit-sludge separation module(patent: ZL201110159013.X) to remove grit from the activated sludge. The separation of grit in activated sludge make MLVSS/MLSS ration increased. A serious of experiment was made to further improve the separation efficiency. This paper studied on the suspension properties of grit with different particle size in mixed liquid. And use the ultrasonic pretreatment to release fine grit from activated sludge. Using computation fluid dynamics(CFD) method to simulated the influence of structure parameters on separation efficiency, to find the optimal parameters for the grit-sludge separation module. The main conclusions are as follows:① The suspension properties of grit in mixed liquid were distinguished by the particle size. Grit suspended with a free state or covered by activated sludge floc, others maybe swept by sludge. Result showed that the critical size of grit is 33μm, which can be swept by activated sludge floc. As the extension of running time, 33μm grit was gradually swept by the sludge. And it required 15 days for those grit been swept.② The 50μm and 66μm grit keep always suspended with a free state. The average removal efficiencies of them(65.7% and 72.2% in mixed liquid) are closed to 66.6% and 73.1% in water. While more, both suspension properties and removal efficiencies of them do not change with the extension of running time. Grit with 14μm particle size can be swept by activated sludge in a short running time, average removal efficiency is 31.9%.③ Ultrasonic pretreatment mainly degradation the organic components in activated sludge, does not affect the content of fine grit. There is a positive correlation between degree of degradation and ultrasonic time. Conclusion shows that a longer ultrasonic time pretreatment makes the MLSS reduce more, and higher cracking rate of sludge. After ultrasonic pretreatment for 60 min, the MLSS reduction for 1#~4# are 1545mg/L、1374mg/L、1545mg/L、1517mg/L respectively. MLISS to MLSS ratio is within the range from 0.157 to 0.159(close to 0.159 in control group). With the increase of ultrasonic time, the specific oxygen uptake rate increase first and then decrease, it reaches the maximum value in about 10min~15min.④ Ultrasonic pretreatment can release 33μm grit in 10min~15min. After released by pretreatment method, the suspension of swept grit changed to a free state. Result showed that 50μm grit keep always suspended with a free state. The ultrasonic process cannot affect the suspension property and removal efficiency. It also does not affect the 14μm grit’s suspension property and removal efficiency.⑤ Among all the structure parameters, Lc, Di, Du and Do showed to be the main factors affect the separation efficiency. When the parameters set to be Dc=75mm, Li=300mm, Di=15mm, Lo=50mm, Lc=100mm, Du=10mm, Do=20mm, the grit-sludge separation module get a highly separation efficiency. The following parameter, Dc=150mm, Li=600mm, Di=30mm, Lo=100mm, Lc=200mm, Du=20mm, Do=40mm, is suitable for production. And the capacity for this module is 19.98m3/h.⑥ Through the invention about fine grit’s suspension property, ultrasonic pretreatment for releasing grit from activated sludge, structure parameters’ influence on separation efficiency, draw the conclusions as following:(1) Grit with particle size >33μm showed the suspension property of free state, can be easily removed.(2) The critical size grit will be gradually swept by activated sludge. After 15 days running, the swept completed. Grit can be easily removed from the mixed liquid in these 15 days. And there is no need for ultrasonic pretreatment if grit removed before swept by the activated sludge.(3) Grit with particle size <33μm can be swept by activated sludge in a short running time. Ultrasonic pretreatment cannot release this grit from activated sludge floc, and it also does not improve the separation efficiency. So, this fine grit will not taken into consider for its strengthen removing. All those conclusions will give the operation a good guiding significance. |
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