The Effect Of Fine Grit Particle Size On Activated Sludge MLVSS/MLSS And Prediction Research

With the popularization and application of oxidation ditch and SBR process without the primary settling tank, and in order to ensure sufficient carbon source for phosphorus and nitrogen removal, some WWTPs usually by-passed the primary settling tank during operation, lead to the fine grit(<200?m) which supposed to be removed in the primary settling tank entered into the biochemical pool freely, or deposited in biochemical bottom, or suspended in the sludge mixture. The activated sludge MLVSS/MLSS is unstable, stay in low status in the rainy season. When activated sludge MLVSS/MLSS is too low, in order to maintain the wastewater treatment efficiency, WWTPs have to elevate the activated sludge MLSS concentration passively. Previous studies found that particle size is the main factor affecting the fate of fine grit in biochemical pool, how to predict activated sludge MLVSS/MLSS according to the characteristics of influent fine grit and realize active control of sludge concentration is of great significance to the accurate operation and control in WWTPs. Based on the operation situation research of the 13 WWTPs in the key basin, this paper mainly focused on the effect of different size fine grit on the activated sludge MLVSS/MLSS during the input process, the activated sludge MLVSS/MLSS prediction model based on the effect of fine grit particle size and the affected active sludge recovery prediction model is established. The research results show that:153.8% of the researched WWTPs influent SS concentration were higher than 500mg/L, and mainly were fine grit with particle size lower than 200?m(cumulative distribution frequency were higher than 96%). 77% WWTPs cancelled or by-passed the primary settling tank, the grit removal amount of the grit chamber is 1/6 ~ 1/3 to the design value, the desanding effect is unstable. The biochemical pool all have the problem of grit sedimentation, the activated sludge MLVSS/MLSS is commonly between 0.3~0.5, even as low as 0.2. The influent SS/BOD5 in rainy season is significantly higher than the dry season, activated sludge MLVSS/MLSS changes with the season significantly.2In order to characterize the suspension characteristics of different size fine grit in the biochemical pool, the suspension ratio ? which is defined as the proportion of fine grit suspended in the activated sludge to the influent total fine grit, is introduced in this paper. The research results show that the fine grit suspension ratio was mainly affected by particle size under a certain hydraulic condition, the smaller the particle size is corresponding to the larger suspension ratio ?, the suspension ratio and fine grit particle size has significant linear relationship:a= -0.0084 d +0.931(R2=0.9798).3When the fine grit is scouring in the biochemical pool by the rainwater runoff during the rainy season, it will accumulate in the activated sludge, and the activated sludge MLVSS/MLSS ratio reduces with the increase of operation time, the prediction model is:The model shows that the activated sludge MLVSS/MLSS during the fine grit input process is not only related to influent fine grit concentration, particle size, but also related to the influent COD concentration and sludge age.4When there is no exogenous fine grit input into biochemical pool during the dry season, with the discharge of excess sludge, the fine grit in the activated sludge was freed, the activated sludge MLVSS/MLSS ratio f increases gradually during the release process, the prediction model is:This suggests that the activated sludge MLVSS/MLSS of the release process is not only related to the fine grit accumulation concentration during the rainy season, but also related to the influent COD concentration, sludge age and hydraulic retention time.5Use the activated sludge MLVSS/MLSS prediction model based on the fine grit particle size effect, the WWTPs can accurately predict the activated sludge MLVSS/MLSS according to the characteristics of influent fine grit, thus realize active control of sludge concentration and maintain stable MLVSS, which is of great significance to the accurate control and stable operation of WWTPs.