Study Of Natural Attenuation And Enhanced Natural Attenuation Of Leachate Pollutants In Aquifer

A majority of landfill is a simple disposal of solid waste in our country. The leachate pollution in the subsurface environment is serious as a result of no maturity leachate collection system and no measure of preventing leakage. The traditional technique is pumping and treating of leachate, but these processing is expensive.Some research indicated that the natural undergruoud environment has the potential to remedy the contaminants. So natural attenuation of contaminants in the subsurface environment has been recognized as a treatment because of no fixing and maintenance of equipment in the remediation process and lower cost.Natural attenuation rarely can be used as a sole treatment process since non-engineered biodegradation processes are very slow. Prior to 1994, natural attenuation was not an acceptable remediation technique. However, by 1996, most U.S. states had adapted policies concerning intrinsic bioremediation and by 1998, the Air Force evaluated it as an option for remediation at most of their petroleum-contaminated sites. The US EPA recognizes natural attenuation for remediation of Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), Resource Conservationand Recovery Act (RCRA) and underground storage tank (UST) regulated sites.Although many protocols exist, agreement on the requirements for evaluating and monitoring natural attenuation has not been achieved. Which parameters that must be monitored and the frequency and location of data collection are aspects of the protocols that need to be standardized. The current study of natural attenuation is mainly about pollution of petroleum hydrocarbon.According to the USEPA (1999), natural attenuation is the''use of natural processes to contain the spread of the contamination from chemical spills and reduce the concentration and amount of pollutants at contaminated sites''. It can also be termed as intrinsic remediation, bioattenuation and intrinsic bioremediation. It is mainly used for remediation of the aquifer when the contamination source has been removed but can also be used when the source is still present or if some hot spots are removed.Natural attenuation is the reaction of physics, chemistry and biology to promote the intrinsic remediation. The natural processes including convection, dispersion, dilution, adsorption, volatilization, chemistry and biological degradation can be divided into the non- destructive process and the destructive processes as a whole.The objective of this experiment is to study the natural attenuation of leachate through the transportation and transformation of contaminants in aquifer. At the same time the bioattenuation is distinguish form non- bioattenuation by inhibiting bacteria growth. And then the efficiency of different chemistry reagent that enhance natural attenuation is compared in this study.Some conclusion is found through the simulation research of natural attenuation and enhancing natural attenuation in laboratory:(1)The adsorption of COD_Cr, NH₄⁺ and total phosphorus (TP) by aquifer media of grit is preferably simulated by Langmuir adsorption isothermal. The maximum theory adsorbance of COD_Cr, NH₄⁺ and total phosphorus(TP) basing on Langmuir adsorption isothermal is 46.73mg/kg,32.36mg/kg,6.03mg/kg, respectively. As the large adsorption of COD_Cr, the grit's adsorption is different from its character, so the method of ascertaining the adsorption of grit needs promoting. Firstly, the error of biology reaction can be removed through inhibitor which can inhibit organism degradation by bacteria. Secondly the most theory adsorbance of COD_Cr also can be ascertained by mensurating the change of organism on the grit.(2) The retardation coefficient(R) of COD_Cr and NH₄⁺ in this simulated experiment is 1.217 and 1.13, respectively. The result shows that the retardance of grit to COD_Cr's transportation is bigger than to NH₄⁺'s.(3)The biological decay coefficient(λ) is 0.0008d-1 through kinetics equation of biodegradation( COD_Cr = 64785 e^-0.0008t )which is simulated by the monitor data. The organic constituents of leachate is degraded by bacteria which can use the electron accepter such asO2, NO3-, Fe(â…¢), SO₄^2-, CO2 to sustain the microorganism living while the organism is the electron supplier in aquifer.(4) The COD_Cr value of leachate is continually reduced as natural attenuation existing in aquifer,but the efficiency of COD_Cr degradation is only 12.64%.In simulated experiment the pH value is increased in the total trend from6.25 in former leachate to about 8.4 in the end. While the change current of Eh value opposing to the pH declines from 125mV in the groundwater to -220mV that is close to the Eh of leacthe. The curve of NH₄⁺,NO3-andNO₂- shows that the degradation of variety of nitrogen by microbe is not prominence.(5) The addition of Fe₃⁺, SO₄^2- as electron accepter and PO43-as nutrition in the aquifer to enhance natural attenuation can well increase the degradation rate of COD_Cr, viz 44.76%, 38.26%and 27.11%. Three kinetics equation of biodegrad–ation by injection of Fe³⁺, SO42- and PO43- is respectively COD_Cr= 64785 e^-0.0142t , COD_Cr = 64785 e^-0.0097tand tCOD_Cr = 64785 e^-0.0053. The coefficient of them is 0.0142 d-1, 0.0097 d-1 and 0.0053d-1 which is declined in turn .According to the above experiment data, the addition of Fe³⁺ as a method of enhancing natural attenuation is better than any others in the simulated experiment.The quantity usage of various chemistry reagent that enhance natural attenuation just bases on the need of theory. In experiment process, the quantity of Fe³⁺ is too much to enhance the degradation of leachate. The reason is that injection of Fe³⁺ cause the pH value lower so that the acid environment represses the growth of the microorganism. The content of SO₄²⁺ is also too high which can influence the quality of the groundwater. But the quantity of PO₄^3- according to the proportion of C, N and P do not influence the distant site so much. So it is necessary to make sure the quantity of the chemistry reagent for safe and efficient application in later work.Because of the restrict of time and the experiment condition, this text of study on natural attenuation and enhancing natural attenuation of leachate in aquifer is only limited by the laboratory stage. But the natural attenuation as a kind of economical and convenient remediation can be applied on pretreatment or deepness treatment. Its appliance needs further research.