| Trichloroethylene (TCE) is one of the industrial widely used chemical solvent, and is also a common organicpollutant in the environment. However, due to inappropriate usage, disposal and treatment of TCE, it seriouslypolluted the air, soil and water, which caused great threat to both environment and human. Extensive researches onchlorinated organic contaminants in groundwater have developed at home and abroad. Bioremediation technology,because of its high processing efficiency, complete degradation, and cause no secondary pollution, has graduallybecome a new technology of organic pollutants treatment in the groundwater. Upflow Anaerobic Sludge Blanket(UASB) reactor is one of the fastest growing high-efficiency anaerobic wastewater treatment process. The UASBreactor has many advantages of lower investment, lower land occupation, lower energy consumption and recyclingresource, which fully embodies the3R (Reduce, Recycle, Reuse) modern environmental thought.In this paper, the study focused on the treatment of TCE wastewater by UASB reactor and optimization of theoperation parameters of UASB reactor. The objectives of this study were (i) to research the influence of TCE on themethane-producing toxicity on the anaerobic granular sludge and the recovery of granular sludge methanogenicactivity;(ii) to study TCE treatment effect in the UASB reactor under continuous operation conditions;(iii) to exploreinfluences of the operation temperature, inffluent pH and HRT on the biodegradation of TCE in the UASB reactor;and (iv) to optimize the operation parameters of UASB reactor by Reesponse Surface Methodology (RSM). The mainconclusions obtained were as follows:The results of anaerobic granular sludge methanogenic toxicity experiment showed that when TCEconcentration were20mg/L and30mg/L, methanogenic activity of anaerobic granular sludge were severely inhibited.Methanogenic active recovery of anaerobic granular sludge experiment results indicated that when the concentrationof TCE were5mg/L,10mg/L and15mg/L, methanogenic activity of anaerobic granular sludge have been restored.However, when TCE concentration were20mg/L and30mg/L, methanogenic activity of anaerobic granular sludgehas not been restored, because TCE caused the death of cells.Adsorption experiment showed that after adding TCE for a period of time, a large amount of TCE has beenadsorbed. After adding TCE approximately0.25h, anaerobic granular sludge adsorption of TCE basically reached theadsorption equilibrium. Subsequently desorption occured, and at about1h time, it reached the desorption equilibrium.Anaerobic granular sludge adsorption and desorption curves of TCE were in line with Freudlich isotherm equation.However, the adsorption and desorption equation were not completely consistent, moreover, adsorption capacity wasgreater than desorption capacity, whicn indicated that part of the adsorption was irreversible. During the operation of UASB reactor, combined the results of adsorption experiment, the removal of TCEunder the anaerobic conditions was only partly due to adsorption of TCE by anaerobic granular sludge, and the restwas removed by biodegradation. This shows that the adsorption and biological removal are intertwined, but mainlybiodegradable.Under the conditions of the temperature of30oC, influent pH value of7.0±0.1, influent alkalinity of (1780±20)mgCaCO3/L, when HRT shortened from25h to5h, the COD removal rate reached up to96.17%in theHRT=25h,while in the HRT=5h, the COD removal rate dropped dowe to83.36%. Under HRT=25h, the TCEremoval rate reached the maximum degradation rate of99.10%, wheras, under HRT=5h, the TCE removal ratedecreased to83.99%. The decrease of HRT would bring a certain impact to the UASB reactor, and every water qualityindicator has declined, and it also had a certain impact on the degradation of TCE in the UASB reactor, especially inthe HRT=5h, the situation was more serious.Under conditions of HRT=15h, influent pH of7.0±0.1, influent alkalinity (1500±100) mgCaCO3/L, whenemperature increased from20oC to30oC, the activity of anaerobic granular sludge was gradually improved. Whenemperature increased from30oC to35oC, the indicators of water quality within the reactor did not change significantly.However, when temperature was40oC, the activity of anaerobic granular sludge was inhibited. COD removalefficiency reached the maximum of91.80%at35oC, while at40oC it dropped to the minimum of78.22%. TCEdegradation rate reached the maximum of97.89%at30oC, whereas at40oC it dropped to the minimum of77.90%.The results indicated that the anaerobic microorganisms within the reactor that adapted to changes in the externaltemperature gradually dominate leading to the improvement of TCE degradation rate in the UASB reactor as thetemperature rises. But when the temperature was too high, TCE degradation rate in the UASB reactor decreased, sothat the microbiological activity decreased, make the efficiency degradation of the TCE.Under the conditions of T=30oC, HRT=15h, when influent pH increased from6.0to7.5, the activity ofanaerobic granular sludge was gradually increased. When influent pH was7.5, COD removal efficiency reached themaximum of90.76%, while influent pH of8.0it dropped to the minimum of85.13%. TCE degradation rate reachedthe maximum of96.76%in influent pH of7.0, whereas n influent pH of6.0, it dropped to the minimum of79.33%.When influent pH was between7.0and7.5, TCE was degraded faster, and the indicators of water quality havereached a good level in the UASB reactor. Moreover, the TCE degradation rate under influent pH less than7.0wassignificantsly less than that of under influent pH greater than7.0, which indicated that acidic environment had agreater impact on the TCE degradation.By applying Central Composite Design (CCD) of Response Surface Methodology (RSM), we chose HRT,temperature and influent pH to optimize the UASB reactor operating conditions. Predicted by the model, the mostsuitable UASB reactor operation conditions were HRT of19.44h, temperature of31.07oC and influent pH of7.13.The predicted COD and TCE removal rate were87.04%and96.52%, respectively. The actual operation conditions ofthe UASB reactor operating conditions wre HRT of20h, temperature of32oC, and the influent pH of7.2, the actualCOD and TCE removal rate were86.87%and97.14%, respectively. |
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