Pump & Treat Remediation Selection and Planning
Pump & Treat Extraction of contaminated water Extraction of free product in the form of a NAPL Removal or destruction of the contaminant from the extracted fluids Optional: Reinjection of treated water; hydraulic control
Initial Steps Thorough site characterization and selection of P&T as the most feasible option. GOAL: Cleanup or Containment? If cleanup, appropriate target level must be set. If containment, must show that contaminant will be successfully kept in the confinement area.
Design Criteria Criteria for well design, pumping system, and treatment depend on: physical site characteristics (soil type, natural flow, etc) contaminant type (dissolved, free product, DNAPL, LNAPL, volatile, nonvolatile, biodegradable
Establishing Effectiveness Monitor the site with wells and piezometers Allows the operator to make iterative adjustments to the system Allows response to changes in subsurface conditions caused by the remediation
Final component: Termination Requirements Termination requirements are based on the cleanup objectives defined in the initial stage of the remedial process. Termination criteria also depend on the specific site aspects revealed during remedial operations (new understanding)
Drawdown Pumping Pumping removes water Lowers the water table near the extraction area to create a cone of depression. Removes dissolved contaminants within the capture zone
Drawdown Pumping
Physical Limitations System designs may fail to contain the contaminant as predicted, allowing the plume to migrate Failure of the pumping equipment may allow the same problem Changes in regional hydrology can affect capture zones adversely Rarely works well in aquifers when the hydraulic conductivity less than 10-5 cm/sec.
Chemical Limitations Residual saturation of the contaminant in the soil pores cannot be removed by ground water pumping. Many contaminants tend to sorb to the soil matrix: Pumping is not applicable to contaminants with high sorption capabilities
Improvements: Direct Removal of NAPL Cone of depression near the extraction well produces a gravity head that pushes flow of NAPL toward the well Increases the thickness of the NAPL layer in the well Generally works well only for LNAPLs (e.g., hydrocarbons)
Improvements: Direct Removal of NAPL One or two pumps. Single-pump configuration: one pump withdraws both water and NAPL Dual-pump configuration: One pump below the water table to remove water and second located in the NAPL layer to recover NAPL
Treatment Technologies Separation of free product from water Air stripping Adsorption onto granular activated carbon Degradation in a bioreactor Constructed wetlands Sprinkler irrigation
Air Stripping Volatile organics are partitioned from ground water by increasing the surface area of the contaminated water exposed to air packed towers, diffused aeration tray aeration spray aeration
Air Stripping: Tower
Air Stripping: Applicability Used to separate VOCs from water Ineffective for inorganic contaminants Henry's law constant (air/water partitioning) determines whether air stripping will be effective K > 0.01 atmospheres - m 3 /mol Used successfully on BTEX, chloroethane, TCE, DCE, and PCE
Air Stripping: Performance For suitable compounds: >99% removal in tower with 15-20 ft of conventional packing Removal efficiencies can be improved: adding a second air stripper in series with the first heating the contaminated water changing the configuration of packing material
Air Stripping: Limitations Inorganic fouling of the equipment (e.g., iron greater than 5 ppm, hardness greater than 800 ppm) Biological fouling (bacteria, slime, insects) is a common problem Requires pretreatment or periodic column cleaning
Air Stripping: Limitations Process energy costs are high Compounds with low volatility at ambient temperature may require preheating of the ground water Off-gases may require treatment based on mass emission rate
Adsorption on Granular Activated Carbon (GAC) Ground water is pumped through one or more vessels containing activated carbon to which dissolved contaminants adsorb When contaminants in the effluent exceeds a certain level, the GAC can be removed and regenerated at an off-site facility removed and disposed
Adsorption on Granular Activated Carbon (GAC)
GAC: Applicability Useful for hydrocarbons, SVOCs and explosives. GAC saturated with explosives cannot be regenerated (!!) Limited effectiveness may be achieved on halogenated VOCs and pesticides Particularly effective for polishing water from other remedial technologies to attain regulatory compliance
GAC: Performance GAC has a long history as a treatment for municipal, industrial, and hazardous wastestreams. Concepts, theory, and engineering aspects of the technology are well developed. A proven technology with documented performance data.
GAC: Limitations The presence of multiple contaminants can hurt process performance. Streams with high suspended solids (> 50 mg/l) and oil and grease (> 10 mg/l) may foul the GAC pretreatment is generally required Water-soluble compounds and small molecules are not adsorbed well Costs are high if used as the primary treatment
Bioreactors Contaminants in extracted water are put into contact with microorganisms Suspended growth biological reactors (e.g., aeration basin) Attached biological reactor: Microbes established on an inert support matrix rotating biological contractors trickling filters
Bioreactors: RBC
Bioreactors: Applicability Primarily to treat SVOCs, fuel hydrocarbons, and any biodegradable organic material May be less effective for some pesticides Pilot-scale field studies conducted on some halogenated compounds, such as PCP and chlorobenzene and dichlorobenzene isomers With co-metabolites are used to treat PCBs, halogenated VOCs
Bioreactors: Performance Well developed technology; used for many decades for municipal and industrial wastewater treatment However, only in the past decade have studies been performed to evaluate the effectiveness of bioreactors in treating ground water and leachate from hazardous waste sites
Bioreactors: Limitations Contaminated ground water is often dilute often will not support an adequate microbial population density especial true for suspended growth reactors. Nutrient addition may be necessary Very high contaminant concentrations may be toxic to microorganisms Low ambient temperatures significantly decrease biodegradation rates
McClellan AFB, Cal. Main COPCs: Chlorinated Solvents Pump and Treat used
McClellan AFB, Cal.
McClellan AFB, Cal.
McClellan AFB, Cal.
Typical Extraction Well
Treatment System Layout
McClellan AFB: Costs Capital: $4,000,000 Annual Operating Costs: $1,240,000
McClellan AFB: Costs Capital: $4,000,000 Annual Operating Costs: $1,240,000
McClellan AFB:Lessons Captures only a small portion of the groundwater contamination present at the site. The system must be significantly expanded to create a zone of capture encompassing other known areas of contamination. Scaling and deposition within the air stipper from precipitation of Ca and Mg salts affected initial operation. Problem was minimized by substituting 2 inch packing for 1 inch packing in the air stripper