| In recent years, there are some problems in urban wastewater treatmentplants(WWTP), such as low organic components in sewage sludge,mixtureMLVSS/MLSS is between0.25~0.5, much lower than the normal value0.75, anddeposited sludge height in oxidation ditch which cancelled primary settling tank is up to1~2m. These phenomenon have become a great problem for the normal operation ofWWTP. This phenomenon in the mountainous urban WWTP of the three gorgesreservoir area is especially prominent. This maybe have relations with the increase ofexogenous inorganic suspended solid(ISS), caused by the cancellation of primarysettling tank, chemical phosphorus removal and extension of sludge rententiontime.Based on existing research, few relative studies have been done, and there are alsolack of studies about the fate of ISS in wastewater treatment system and the mechanismof MLVSS/MLSS decline. Based on the analysis of ISS distribution characteristic,particle size is considered as an important factor affecting the fate of particles. Based onthis, the effect of fine sediment and chemical phosphorus removal on inorganic solidaccumulation in the activated sludge system was studied first; second, typical particlesize distribution characteristic of fine sediment in sewage treatment system was studied;finally, the comprehensive effect of fine sediment accumulation on WWTP’s operationwas also researched.The main conclusions are as follows:1) Influent fine sediment concentration had important influence on ISSaccumulation and recoverycapability of activated sludge.When influent fine sedimentcontent(ISS/COD) with particle size less than43um was not more than0.6,MLVSS/MLSS can remain above0.6;when ISS/COD was more than1.2,MLVSS/MLSS dropped to below0.44; when ISS/COD was more than2.4,MLVSS/MLSS dropped to below0.25. Further study found that if ISS/COD was morethan1.8, it would continue to affect therecoverycapability of activated sludge system,and MLVSS/MLSS and MLSS is difficult to return to the initial value after60d. Finesediment can improve the settleability of activated sludge and dewatering performance,but also increase the density of the activated sludge at the same time, making sludgeeasier to settling in the biochemical treatment tank, increasing the difficulty of agitationand aeration, even resulting in a decrease of working volume of biochemical treatmenttank. Fine sediment will not inhibit microbial activity, but will reduce the oxygen uptake rate of unit mass sludge.2)According to the dosage stipulated in theCode for design of outdoor wastewaterengineeringand operation manner in WWTP,use ferric sulfate and aluminum sulfate aschemical phosphorus removal agent. The results showed that MLVSS/MLSS can stillmaintain at0.6after operation for50d. The chemical sludge’s contribution to the ISSwas equivalent to the ISS/COD of0.1, but the effect on MLSS and MLVSS/MLSS wasgreater than that of fine sediment. Chemical phosphorus removal can also improve thesludge settleability and dewatering performance, but will inhibit microbial activity,reducing the oxygen uptake rate.3)The influent fine sediment in an activated sludge system may be discharged fromthe reactors via effluent, or suspend in the activated sludge, or deposite at the bottom ofthe reactors, or discharged from the reactors via excess sludge. The results showed thefine sediment with particle size less than73um had a uniform proportion in mixture,excess sludge and deposited sludge, and the value was＞21%、＞31%、＜47%respectively. The fine sediment with particle size more than106um mainly deposited inthe bottom of reactor, and the proportion was＜4%、＜20%、＞75%respectively.Studies further found that the sludge MLVSS/MLSS ratio decreased easily. Under thecontinuous influence offine sediment with particle size less than73um, MLVSS/MLSSdecreased from0.84to0.26after40d. In a word, large solid particles were more likelyto deposit in the bottom of the reactor, causing sediment deposition, but fine particlessuspended in mixture more easily, causing sludge MLVSS/MLSS ratio decrease.4)Through1year on-site monitor of A WWTP in Chongqing, it found that theannual average concentration of fine sediment was809mg/L, and ISS/COD was1.27,much higher than the ordinary sewage of0.1. During rainfall process, fine sedimentcontent increased to1243mg/L, and the maximum of ISS/COD was1.98, increased by113%than the background period.After a rainfall, MLVSS/MLSS only decreased from0.37to0.32, but MLSS rised from5380to6700mg/L.Rainfall caused a lot of sedimentinto the activated sludge, resulting in MLVSS/MLSS decreased. In order to ensure thewastewater treatment efficiency, WWTP had to improve MLSS, leading to MLSSincreased from3000mg/L in the dry season to8000mg/L in the rainy season, andMLVSS decreased from0.57to0.25. This operation manner of WWTP will hinder theoxygen transfer, increase the oxygen supply, eventually make WWTP’s operating costsincrease more than44%. Study further found that high sediment concentration ininfluent and sediment particle size less than73um were the main reason for MLVSS/ MLSS reducing.The study of B WWTP showed that the sediment deposition in the bottom of biochemical treatment tank was very serious. For oxidation ditch, fine sediment deposited mainly in the anoxic period. Its deposition height was from0.36m to1.68m, and deposition volume was1061m3, leading to hydraulic retention time of anoxic period decreased by57%, and TN removal rate reduced by20%. The accumulation of fine sediment increased40000～50000yuan cost for WWTP regularly, stopped WWTP running at the same time, affectting the normal operation of the WWTP.5) Based on the distribution characteristics of fine sediment in sewage treatment system, the accumulation model of ISS is established, the formula was Chemical sludge mixed with mixture evenly and did not deposited in the bottom, thus, its distribution coefficient was1. It verified that the model can predict the effect of fine sediment content, chemical sludge and fine sediment with different particle size to MLVSS/MLSS and MLISS concentration. |
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