| Along with the rapid development of Chinese economy, water environment which our lives depend on tends worse than ever resulting from pollution. It is urgent to make action for water pollution controlling and handling so as to protect our common homeland. At present, technique for municipal wastewater treatment is relatively abundant, simultaneously substantial progress has been made for treatment technique on industrial wastewater treatment. However, in so many industrial parks of cities or towns, a variety of wastewater is discharged and mixed together from different factories with the complexity and diversity of pollutants. Due to mixture with heavy metals and organic compounds and even resulting in multiplex pollution, it is too difficult to find good solution for handling such complicated wastewater by normal process and fit the national stand, otherwise it will cost too much which the customer cannot stand. Therefor, the purpose of this paper is to develop a treatment process for handling successfully such mixing industrial wastewater with heavy metals and organic compounds, and with high treatment efficiency and lower costs for operation. This study has gotten the support by National Special Water Research named Research For Key Technique and Synthesized Pilot Project For Heavy Metal Pollution Controlling in Xiang River. Firstly the detailed investigation for water quality of the final discharged wastewater has been made for a long time in an industrial park with heavy pollution in Hunan Province so that the study object was defined. In this study, Removal efficiency for heavy metals and organic matter and its influencing factors were researched on electro-flocculation as pretreatment, adsorption kinetics, adsorption isotherms, and its influencing factors on diatomite adsorption for heavy metals has also been researched. Comparative study on removal efficiency between particle biofilm-membrane bioreactor and membrane bioreactor for low concentrations of heavy metals, ammonia nitrogen, total nitrogen and organic matter has been done, as well as the difference about the membrane flux, particles of sludge, viscosity of mixture liquid between the two bioators has been studied. Finally, the study on removal efficiency of the electric field-membrane bioreactor has been done. In order to testify the application purpose for this study, a pilot experimental facility has been build up in the integrated wastewater treatment plant, which is still giving figures as a guidance for daily operation and management of this plant.some achievements of the experiment are shown as below.(1) It is effective for electric flocculation to remove heavy metals, and the removal efficiency is more than96.0%of Cu2+, Zn2+, Pb2+, as well as more than82.0%of Cd2+from simulated wastewater. The optimal condition for electric flocculation process to remove heavy metals has been studied, it is shown that the initial pH value is much important for removal efficiency and the optimal pH is6, current density is6A/dm2, plate space is3cm, and the time for electric flocculation is30min. It is proved that electric flocculation process has excellent removal efficiency for COD removal. The efficiency for COD removal can be more than75%under the conditions with pH7, current density10A/dm2, flocculation30min. Therefore, the the process of electric flocculation as a pre-treatment unit is appropriate for mixed industrial wastewater treatment.(2) The feasibility for electric flocculation to remove phosphorus from simulated wastewater was also studied. It is proved that phosphorus removal efficiency can be more than75%under optimized condition.(3)Diatomite can adsorb heavy metals effectively from wastewater, meanwhile the dosage of diatomite has an important impact on the efficiency for heavy metal removal. It is proved that the removal efficiency for heavy metals of Cu2+, Zn2+, Pb2+and Cd2+are85.38%,72.00%,91.70%and65.71%respectively, when thr dosage of diatomite is8g/L. That is, the efficiency is very high. An example can be shown more clearly. When the concentration of Cu2+, Zn2+, Pb2+and Cd2+were5.0mg/L,5.0mg/L,2.0mg/L and0.2mg/L in the influent wastewater, respectively, the corresponding concentration of heavy metals were tested in the effluent as0.73mg/L,1.40mg/L,0.17mg/L and0.07mg/L, respectively, through the electric flocculation process. Thus it gives a good foundation for further removal of heavy metals by subsequent biological treatment.(4)The main influence factors on adsorption efficiency for heavy metal by diatomite were pH value and concentration of heavy metals in the influent wastewater, and especially for pH value. Normally the dsorption effect is poor under acidic condition, whereas ideal result can be gotten under neutral or weakly alkaline condition. An example can be shown. When pH value is given6±0.2, the removal efficiency for Cu2+, Zn2+, Pb2+and Cd2+will be90.00%,77.54%,94.07%and72.14%, respectively. However, the removal efficiency is almost no change along with the pH value further increasing. When the concentration of heavy metal ion in solution is low, the adsorption amount for a diatomite unit is also low reasonably. Along with increasing of the concentration of heavy metal, more space of diatomite starts to make action with heavy metal ions until tending adsorption saturation point. This means that the adsorption amount will keep increasing along with the concentration ascending of heavy metal. However, once the concentration of heavy metal ion is over1.0mmol/L, the increasing extent of adsorption amount is slowdown.(5)The adsorption trend is similar for diatomite to adsorb Cu2+, Zn2+and Cd2+, that is, the adsorption efficiency for the three ions are85.38%,78.77%and68.57%, respectively, in the first30minutes. Subsequently, the increasing extend starts to decrease. However, the adsorption trend for diatomite to adsorb Pb2+is so different compared with the above three ions. The action is very rapid so that the adsorption efficiency is more than81.48%in the first5minutes and88.15%in10minutes, which is tending the utmost for removal.(6)When the stable equilibrium is reached for diatomite adsorbing the four heavy metal ions of Cu2+, Zn2+、Cd2+、Pb2+, it is shown that the adsorbed amount of Pb2+is much more than the others. For example, when the adsorption amount for the four heavy metal ions of Cu2+、Zn2+、Cd2+、Pb2+is tested in the first5minutes, it will be4.21,3.46,6.80and11.0mg/L, respectively, that is, the sequence of adsorption amount for the four heavy metals is Pb2+> Cd2+>Cu2+>Zn2+.This is also means that diatomite as an adsorbent has an obvious selectivity for different heavy metal ions.(7)Experimental study on adsorption isotherms for diatomite adsorbing the above4heavy metals was done, and regression analysis was also made with Langmuir and Freundlich Equation in the same time. It is shown that Langmuir adsorption isotherm equation can fit the result better than Freundlich adsorption isotherm, with the fitting regression coefficient R2value0.94, and among0.78to0.81, respectively.(8)The comparative study has been done on the removal effect on particulate biofilm-membrane bioreactor and ordinary membrane bioreactor. Experimental result is shown that both of them have an outstanding effect for pollutants removal including heavy metals, COD and ammonia, and can fit the demand of the national standard--Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB18918-2002). However, both of bioreactors have a low efficiency for TN removal, resulting from the continuous aeration and prohibiting the growth of denitrifying bacteria.(9)In this study, compared with ordinary membrane bioreactor, particulate biofilm-membrane bioreactor has gotten a more efficient effect on heavy metal, COD, ammonia and TN removal, due to the enhancement of adding diatomite.(10)The rate of decline of membrane flux can be delayed by adding diatomite in the MBR reactor. It is proved that adding diatomite can improve the living environment for microorganism, so that the sludge concentration can increase faster with average particle size of flocs increasing up to77.37%, sludge viscosity reducing with41.18%. Thus it will decrease the formation velocity for sludge cake layer. In the meantime, adding diatomite can make sludge cake layer more loosen, permeable, and filterable. In a word, it is important for adding diatomite to decrease the growth rate of enduring pressure for membrane, slow down pollution of membrane, and extend the life cycle for membrane module.(11)Electricity-membrane bioreactor has a higher efficiency for COD removal and the removal rate can keep more than90.0%. There is no much influence for removal effect by changing current density.(12)Even with impact load, there is little influence on the efficiency for electricity-membrane bioreactor removing ammonia. It is shown such system has a very strong capacity of self regulation.(13)It has a great influence on the efficiency for electricity-membrane bioreactor to remove TN by changing current density. For example, current density is changed from110mA to150mA, the remove rate will be improved until the maximum83.8%. If keeping increasing current density, the remove rate for TN starts to decline. Impact load will have some influence for the removal rate of TN. It is shown that the system can keep ideal operation when the concentration of TN of influent wastewater is changed from46mg/L to88mg/L. but when the value of TN is up to90mg/L, efficiency for the system removing TN starts to decline. The experiment result is shown that the assembled unit within electric flocculation and electric membrane bioreactor can treat effectively complicated wastewater discharged from industrial zone. |
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