| Currently, how to remove fine grit(paticle size of < 200 ?m) has become an urgent problem for wastewater treatment systems which do not set the primary settling tanks(PSTs), such as the oxidation ditch, or omit the PSTs in operation. The existing grit chamber technologies, which have been designed to enhance the grit removal, can remove the fine grit with a particle size of > 100 ?m effectively. However, for the fine grit of < 100 ?m in bioreactors, the suspension properties and removal techniques remains unknown. A previous study by the author's team has demonstrated that particle size of fine grit has significant impacts on its fate characteristics in biological wastewater treatment systems. Fine grit with a particle size of ? 73 ?m becomes suspended in activated sludge and directly leads to a decrease in MLVSS/MLSS of activated sludge, which also is the main reason why MLVSS/MLSS of activated sludge is commonly low in China's wastewater treatment plants(WWTPs). This paper investigated the operation status of grit removal system and the accumulation situation of grit in bioreactos in China's WWTPs. With fine grit of ? 73 ?m as the research object, this paper studied the suspension properties of fine grit with different particle sizes in bioreactors. Based on the grit-sludge separator developed by the author's team independently(China Patent number: ZL201110159013. X), this paper examined the effect of suspension mode of fine grit on the performance of grit-sludge separator, explored the release methods of fine grit in activated sludge aggregates, and optimized the structure of grit-sludge separator. Finally, this paper developed the integrated grit-sludge separation technology that matcthed the suspension properties of fine grit with different particle sizes and its application research was conducted. The main conclusions of this paper are as follows:(1) This paper investigated the operation status of grit removal system and the accumulation situation of grit in bioreactos in 13 WWTPs of China, such as Hangzhou QG WWTP and Wuzhou DY WWTP. The survey results shows that the grit removal efficiency of grit chamber is undesirable. 69% of survey WWTPs do not set the primary settling tanks(PSTs) or omit the PSTs in operation. MLVSS/MLSS of activated sludge in bioreactors is low. Fine grit in WWTPs is mainly brought in by storm runoff and influent SS/BOD5 in rainy reason is significantly higher than that in dry season, which results in the variation of activated sludge MLVSS/MLSS with season. At present, WWTPs adjust MLSS according to experience, which leads to the significant fluctuations of MLVSS concentration. The survey results indicates that it has significant value and good prospects to improve the grit removal efficiency of grit chamber, develop the separation technology of fine grit from activated sludge, and establish the precise adjustment mode of activated sludge MLSS based on the prediction of MLVSS/MLSS.(2) A smaller particle size corresponds to a larger suspension ratio, which is defined by as the proportion of fine grit brought in by influent that is suspended in the activated sludge, and a smaller MLVSS/MLSS of activated sludge. The suspension ratio is linearly related to the particle size of fine grit(y=0.996-0.010x), and the MLVSS/MLSS of activated sludge and the suspension ratio of fine grit is significantly correlated(y=2.84/(12.27x+3.06)). Based on the material balance equation of fine grit suspended in bioreactors, this paper developed the prediction model of the effect of fine grit on MLVSS/MLSS of activated sludge. According to the prediction model, WWTPs can adjust the MLSS accurately, which is conducive to the stable operation of WWTPs.(3) The suspension modes of fine grit in activated sludge is influenced by the particle size and suspension time. Fine grit with a particle size > 33 ?m is freely suspended in activated sludge and the suspension time has no impact on its suspension modes. Fine grit of 14~33?m is freely suspended in activated sludge during the early stages, but it is picked up and bound by activated sludge flocs after 15 d. Once the fine grit of ? 14 ?m is suspended in activated sludge, it is quickly picked up and bound by activated sludge flocs.(4) The suspension modes of fine grit affect the performance of grit-sludge separator directly. Fine grit is freely suspended in activated sludge, which is conducive to the separation of fine grit from activated sludge. Fine grit is picked up and bound by activated sludge flocs, which results in the decrease of the separator's removal efficiency. After fine grit with a particle size of 33 ?m is bound, the grit removal efficiency and separation degree of the separator are reduced by 43.7% and 43.3%, respectively. As fine grit in WWTPs is mainly brought in by storm runoff, grit-sludge separators should be operated during and after storms in order to remove the fine grit of > 14?m. On the one hand, it can prevent the negative effects caused by the fine grit accumulated in activated sludge. On the other hand, it would avoid the binding of freely suspended fine grit of 14~33?m by biomass aggregates and the decreased efficiency of grit-sludge separator.(5) ultrasonic pretreatment can hardly release the fine grit of ? 14 ?m in activated sludge aggregates, due to re-flocculation of the disintegrated sludge particles which is promoted by the release of intracellular polymers and which results that fine grit of ? 14 ?m maintains being picked up and bound by activated sludge. Thus the ultrasonic pretreatment can't improve the separator's removal efficiency for fine grit of ? 14 ?m in activated sludge. Moreover, other sludge pretreatment methods are still difficult to enhance the separation performance of the separator for fine grit of ? 14 ?m. Therefore, grit-sludge separators should focus on the removal of fine grit of >14 ?m in activated sludge, and the optimization of the separator's structure should achieve the enhanced removal of fine grit of >14 ?m.(6) The structure of grit-sludge separator was optimized using computational fluid dynamics. The structure of separator after optimization is that cylindrical body diameter, inlet size, vortex finder length, cylindrical part length, overflow diameter, underflow diameter, and conical part angle are 75 mm, 15 mm, 50 mm, 100 mm, 20 mm,10mm and 12°,respectively. Compared with the separator before optimization, the feed pressure of separator after optimization is decreased 48.0%, which indicates that the operation energy of the separator reduces significantly, and the grit removal efficiency is improved. The removal efficiency of fine grit of 15?m is increased to 83%, which indicats that optimized separator can remove the fine grit of > 14 ?m efficiently.(7) Based on the optimized grit-sludge separator, the integrated grit-sludge separation equipment was developed, which was conducive to the promotion and application of grit-sludge separation technology. Its main parts include the coarse sieve, sludge storage tank, liquid level controller, sludge lift pump, grit-sludge separator, fine sieve, overflow sludge storage tank and sieve shaker. The application research of the integrated equipment in Wuzhou DY WWTP shows that during the separation of fine grit in activated sludge, the total removal efficiency for ISS is 24.3%, and the water content and organic content of coarse rejection-fine grit mixture discharged are 49.1% and 0.187, respectively. The results indicated that the integrated grit-sludge separation equipment can remove fine grit in activated sludge efficiently and have a good dewaterability for the coarse rejection and fine grit discharged by the equipment. |
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