12.7.3.1 Preliminary Design of Soil Vapor Extraction Treatment Program
(1) Description of Treatment Program
1) Principle of Technology. Soil gas extraction technology is an in-situ remediation technology that requires contaminants in the soil to be transferred out and treated through a medium, such as air or steam. Soil gas extraction technology is suitable for removing a variety of contaminants with high vapor pressure or higher boiling point than water, such as chloride solvents and volatile gases. Soil gas extraction can treat high concentrations of contaminants including contaminants in non-aqueous liquids. Soil gas extraction is a fast and effective remediation technique. According to the U.S. Environmental Protection Agency, soil gas extraction is the most widely used in-situ remediation technology.
Soil gas extraction technology is through the arrangement of the unsaturated soil layer in the extraction wells to the soil into the air flow, the air flow through the soil, volatile/semi-volatile organic pollutants with the air into the vacuum wells, so that the soil can be remediated, the gas is ultimately disposed of at ground level, the technical process is shown in Figure 12.71. To increase the pressure gradient and air flow rate, in many cases a number of air injection wells are also installed in the contaminated soil. Usually, the minimum depth of the extraction wells is 1.5m, and there have been successful examples of the deepest reached 91m.
Figure 12.71 Schematic diagram of the soil gas extraction system
Advantages of the soil gas extraction technology:Operationally available, there can be standardized equipment to operate; wide range of pollutants to deal with the treatment of the scale of the treatment; does not damage the soil structure as well as the potential value for recycling of wastes; does not cause secondary pollution.
Limitations of soil gas extraction technology: the conductivity of the lower soil will cause uneven distribution of gas flow; for low permeability of the soil treatment is not effective; the water table is too high will affect the treatment effect; discharge gas needs further treatment.
Soil gas phase extraction technology can also be combined with other technologies, such as groundwater air injection technology and groundwater extraction technology, can be used simultaneously to deal with soil and groundwater contamination, often called multi-phase extraction technology. China has realized complete sets of equipment, serialization and automation in this technology. Usually the equipment is divided into three major systems, extraction system, separation system, tail gas purification system. First of all, through the extraction system for the separation of pollutants in the soil pumping, and then gasified pollutants containing water gas sent to the separation system to remove particles and water, and then through the tail gas purification system, to achieve the exhaust emission standards, and ultimately can effectively reduce the concentration of pollutants in the soil, does not produce secondary pollution.
2)Case introduction. Soil gas extraction technology was first successfully developed by the U.S. TerraVac Company in 1984, and gradually developed into the most commonly used remediation technology for soil and groundwater pollutants in the 1980s.
a. In an industrial site in southern Ontario, Canada, VEI developed and applied soil gas extraction technology to treat site contamination. The main treatment of volatile organic pollutants in the soil, including gasoline, diesel, chlorinated ethylene and ethane. The project effectively stopped the migration of VOCs to groundwater.
b.DoDHF is a former Hamilton Air Force Base located in Novato, California, USA, about 20 miles north of San Francisco. Gasoline contamination of the local soil and shallow groundwater aquifer has left the soil and groundwater with oil and methyl tertiary butyl ether. The remediation was achieved by utilizing soil vapor extraction to effectively reduce organic contaminants in the soil and Methyl Tertiary Butyl Ether (MTBE) in the groundwater.
(2) Implementation scheme
A typical soil vapor extraction system is shown schematically in Figure 12.72.
Figure 12.72 soil gas extraction system design structure schematic
Soil pollution control construction program should have the project before the implementation of the relevant experiments (pilot) to determine the parameters of the specific determination. Its system design should include the following aspects:
1) the structural design of the well.
2)Design of piping system.
3)Design of the vapor handling system.
(3) Implementation process
1) Pilot test. Before proceeding to finalize the final restoration plan, the technology generally needs to undergo a pilot test to obtain the basic data for the final plan, so that the economic and technical aspects of the designed plan will be more appropriate.
It is recommended that one primary monitoring well and two observation wells be installed 3-5m around the site. The purpose of this system installation includes: testing the possibility of pumping on the site; determining the design parameters including vacuum pressure, air flow, etc.; and determining the specifications of the necessary equipment and ancillary facilities.
During this period, technicians were required to pump air through fans to the main monitoring wells. Volatile organic compounds, liquids and temperature changes are observed by adjusting airflow and vacuum pressure. From this, optimized operating parameters are designed. The following procedure, Figure 12.73, describes the work steps:
Figure 12.73 Pilot workflow diagram of the soil gas extraction system
2) Soil pumping system design and implementation. A soil gas extraction system is an in-situ remediation technique that removes volatile and some semi-volatile contaminants from the soil by drilling/pitting holes in the soil and using a vacuum to direct the flow of air through the soil in unsaturated (seepage) areas. The extracted contaminant-laden air can be treated and recycled for discharge.
Soil vapor extraction remediation systems are often used in conjunction with other remediation techniques, such as air aeration and dual (multiple) layer extraction. A soil extraction system design plan needs to be prepared after pilot testing to address the following:
a. Operating airflow, vacuum pressure, temperature control, VOC control.
b.Specifications, depth, and location of on-site wells, associated piping layout.
c.Final equipment, cost and list of appurtenances.
d.Test plan.
e.Operation manual.
3) Remedial operation of the soil extraction system. It is anticipated that the soil pumping system will take 1 month to install and 1 to 2 weeks to test. The system should operate for 12 to 18 months. Operation of the system will need to be primarily automated, and during that time, the remediation engineer's technicians will need to perform monthly commissioning for the following (but not limited to):
a. Check the condition of each well.
b.Monitor VOCs and temperatures in each well.
c.Check the operation of the soil pumping system.
d. Regulate airflow and pressure in each well.
e.Observe if any shortcuts form.
f. Record operating data for evaluation and future commissioning.
g. Perform necessary maintenance.
Each run takes 1 to 3d of time to debug.
4) Soil verification sampling and site closure. The site requires three rounds of verification soil sampling, analyzing for benzene, total petroleum hydrocarbons, and polycyclic aromatic hydrocarbons (PAHs), to assess remediation operational performance. The first round is required six months after the system begins operation, the second round is required one year after operation, and the third round is required during the 12 to 18 month period prior to cessation. The remediation contractor will need to prepare a site closure report to demonstrate remediation results, soil conditions, risk issues, and more.
(4)Timing
The entire remediation period for the preliminary design is planned to be 2 ? years, with the following timing for the various treatment phases:
Pilot testing:2 to 4 weeks;
Soil pumping system design and installation:1 to 2 months;
Operation of the soil pumping system as well as maintenance:12 to 18 months.
12.7.3.2 Preliminary Design of Groundwater Pollution Treatment Program
(1)Introduction of Treatment Program
The extraction treatment technology refers to the technology of pumping out the polluted groundwater for treatment. Advantages of the technology:Long-lasting, easy to operate, low cost, little damage to the ecology, permanent removal of pollutants, suitable for petroleum-based pollution treatment. However, the implementation of this technology has a longer period, the need to treat the pumped groundwater increases the corresponding cost, the effect of the treatment of low permeability strata is poor, and the treatment time is longer for this site (relatively low permeability coefficient). The extraction treatment combined with artificial intervention natural attenuation technology is a method to accelerate the natural attenuation rate of pollutants by using artificial gas injection to improve the self-purification ability of the aquifer itself, taking into account the shortcomings of the extraction treatment technology.
For the gas station groundwater pollution treatment project, the preliminary design of this study adopts the QS-3 monitoring wells in the project site for pumping, and in addition arranges several injection wells and installs the groundwater pollution treatment device--WaterlooEmitter transmitter to inject oxygen into the groundwater (Figure 12.74 ) to increase the oxidation of the aquifer itself to promote the oxidative degradation of pollutants in the aquifer itself, and at the same time, the pumped water will be treated by activated carbon adsorption, and discharged after meeting the standards.
Figure 12.74 Photograph of Waterloo Emitter prototype
The Waterloo Emitter is a device that specializes in groundwater remediation, and its main function is that, after air is injected into the device through external pressure, the device is able to deliver the oxygen in the air to the groundwater as molecules rather than bubbles in order to increase the groundwater's oxidizing itself, oxidizing some specific organic pollutants into carbon dioxide and water, and improving the self-purification ability of groundwater itself.
(2)Case Introduction
In 2007, an oil spill at a gas station in Guelph, Ontario, Canada, caused groundwater contamination, and the groundwater contaminants at the project site were mainly volatile petroleum hydrocarbons (VPHs), benzene and polycyclic aromatic hydrocarbons (PAHs). The initial contamination range was 30m long, 15m wide and 30m deep, and the lithology of the aquifer was mainly chalk. The Waterloo Emitter, a groundwater contamination controller, was selected for this example project to treat the treated aquifer with oxygen delivery, thus enhancing the natural biodegradation of the contaminants in the aquifer.
The project*** involved the placement of 14 sets of Waterloo Emitter emitters so that they were installed within 4 m of the water table, and the Waterloo Emitters were placed on a "fence" set up perpendicular to the direction of groundwater flow (Figure 12.75) to cut off the contamination plume. the contamination plume. Dry air (containing 21% oxygen) was released into the groundwater contaminated area through 14 vapor injection wells installed at the site via piped Waterloo Emitter emitters to directly provide oxygen molecules necessary for degradation of the contaminants.
Figure 12.75 Photograph of the Waterloo Emitter site layout at a gas station in Ontario, Canada
Monitoring during the remediation process showed a one-degree decrease in the levels of the characteristic contaminants in the groundwater at the site after the start of the remediation work. After one month of operation, the average Eh value of the monitoring wells increased by 880% and the contaminant concentration decreased by an average of 9.6mg/L, with a maximum decrease of 27mg/L. Within six months, the contaminant concentration level in the groundwater was reduced to the analytical detection limit, which meets the standard limits for contaminant concentrations in soil, groundwater and sediment under the Ontario Environmental Protection Act. The treatment was decommissioned after one year of implementation, an excellent completion of the groundwater remediation and treatment project.
(3) Preliminary Engineering Layout
The application of extraction treatment combined with artificial intervention natural attenuation technology for the construction work of remediation and treatment of shallow groundwater caused by refined oil leakage from Shell gasoline stations*** entails the following steps:
1) Site leveling. Mainly refers to the existing site for leveling, water, electricity has reached the construction requirements.
2)Construction of the gas injection well. Preliminary design to around the gas station station area arranged 12 gas injection wells. Monitoring wells into the depth of 40m or so, the water section of 20 to 40m, down into the diameter of 108mm steel pipe.
3) equipment installation. Steam injection wells into the well after washing and pumping, and down into the groundwater pollution treatment device - Waterloo Emitter a number of groups, and with 1 set of gas supply system. The depth of treatment was determined to be 25-35 m below the surface. the existing monitoring wells of the gas station were utilized as pumping wells and pumping equipment was installed for pumping treatment. The pumped groundwater is treated by activated carbon adsorption using homemade activated carbon adsorption device, and is discharged after the treatment meets the standard.
4) Governance operation. Governance operation period is mainly for equipment maintenance and daily monitoring work, mainly including: pumping system stability maintenance, sewage treatment system stability maintenance and adsorption material replacement, daily monitoring of water quality and other work.
12.7.3.3 Site soil, groundwater joint management program
After the above analysis, it can be seen that the gas station site pollution management can be used to soil vapor extraction technology and extraction treatment combined with artificial interventions natural attenuation technology for joint use in the management of soil and groundwater, respectively, the program can be called multi-phase extraction technology that is, due to the extraction wells of groundwater and soil management extraction system, Pipeline and management system has some similarity, the soil and groundwater management system design can be considered in an integrated manner (Figure 12.76).
During the construction of extraction wells, the pumping/injection requirements of soil and groundwater are considered comprehensively for design, upholding the principle of multi-purpose use of one hole and reducing the construction investment. At the same time, it is recommended to adopt the soil gas phase extraction series device, and select the multi-phase extraction treatment set for the groundwater and soil of this project, so as to improve the treatment efficiency.
At the same time, in the site layout design for the later reconstruction of the gas station, the arrangement and rational arrangement of the relevant facilities and equipment, such as gas injection wells, pumping wells, pipelines and ditches, gas (water) treatment equipment and discharge of water after treatment, are considered in an integrated way in the treatment process, so as to reserve appropriate space for the treatment work. The later reconstruction and daily operation of the gas station shall not affect the treatment of underground water and soil pollution of this project.
Figure 12.76 Schematic diagram of the structure of the joint soil-groundwater treatment system for gas stations