Soil remediation air and vacuum systems serve different duties in removing, containing, transforming or reducing contaminants in soil and groundwater. The appropriate method depends on contaminant properties, soil structure, groundwater conditions, site use and cleanup objectives.
Air and vacuum systems play an important role in several in-situ remediation technologies. Vacuum can extract contaminated soil gas, while pressurized air can deliver oxygen below ground or transfer volatile contaminants from groundwater into the unsaturated soil zone.
Roots blowers may be used on either the vacuum or pressure side when the required airflow, pressure and gas composition fall within the machine’s approved operating range. However, successful remediation depends on the complete system, not simply on installing a blower with a high nominal capacity.
Site Conditions and Air-Based Remediation Methods
Understanding contaminated soil. Soil is a complex structure containing mineral particles, organic matter, moisture, gases and living organisms. Contaminants can be retained within the soil, dissolved in groundwater, adsorbed onto particles or present as vapor in the spaces between particles.
Common sources of contamination include underground fuel-storage tanks, petroleum and chemical spills, industrial manufacturing sites, refineries and fuel terminals, dry-cleaning facilities, and waste-storage areas, as well as landfills, former military facilities, agricultural chemicals, leaking pipelines, improper disposal practices, and mining and mineral-processing activities.
Contaminants may include gasoline and other petroleum hydrocarbons; benzene, toluene, ethylbenzene and xylenes; chlorinated solvents; volatile organic compounds; semi-volatile organic compounds; and pesticides, as well as heavy metals, cyanides, persistent organic chemicals, and non-aqueous-phase liquids. Air-based remediation methods are particularly relevant to volatile or biodegradable organic contaminants. They are generally less effective for non-volatile contaminants such as heavy metals unless combined with another treatment method.
Soil zones and contaminant location. Subsurface conditions are commonly divided into the unsaturated and saturated zones.
The unsaturated zone, also called the vadose zone, lies above the groundwater table. Its pore spaces contain both air and water. Vacuum can be applied to this zone to induce airflow through the soil and remove contaminant vapor.
The saturated zone lies below the groundwater table. Its pore spaces are substantially filled with water. Pressurized air may be introduced into this zone to promote volatilization or biological activity.
The groundwater table can move seasonally or in response to rainfall and pumping. A remediation system must account for these changes because they affect well-screen exposure, gas-flow resistance, liquid carryover, contaminant distribution, blower operating pressure, and treatment performance. A system designed using only one groundwater measurement may perform differently as site conditions change.
Air permeability of soil. Air does not move equally through every soil type.
Coarse sand and gravel generally allow air to pass relatively easily. Fine sand, silt and clay create greater resistance and may produce highly localized airflow.
Effectiveness depends on soil grain size, porosity, air permeability, moisture, layering and fractures. Groundwater depth, well spacing and applied vacuum or pressure also shape the design. Excessive vacuum can compact soil or draw clean air into preferential pathways, reducing contact with the contaminated zone.
Moist soil normally offers greater resistance to airflow because water occupies part of the available pore space. In tightly packed or clay-rich soil, applying more vacuum does not necessarily create a proportional increase in useful airflow.
Air follows the easiest available route. Preferential pathways can cause one part of the contaminated area to receive strong airflow while another remains untreated.
Soil vapor extraction (SVE) removes volatile contaminants from the unsaturated soil zone.
Extraction wells are connected to a vacuum header. The vacuum system creates a pressure difference that draws air through the soil toward the wells. Volatile contaminants evaporate into the moving air and are transported above ground for treatment.
A typical SVE system includes vertical or horizontal extraction wells, wellhead valves, a vacuum manifold, a moisture separator, a vacuum blower or pump, and pressure and temperature instruments, as well as vapor-treatment equipment, condensate collection, exhaust stack, and a control and safety system. SVE is generally best suited to contaminants that can transfer from the liquid or adsorbed phase into the vapor phase. Performance is influenced by contaminant volatility, soil permeability, temperature, moisture and the degree to which clean air can enter the treatment zone.
Vacuum in SVE. The purpose of the vacuum system is to create controlled airflow through the contaminated soil. The objective is not simply to achieve the highest possible vacuum at the blower inlet.
Excessive vacuum can cause unnecessary energy consumption, increased water or condensate carryover, airflow concentrated near a few wells, leakage through poorly sealed surface areas, reduced efficiency in distant soil zones, and blower overheating. Possible consequences include excessive mechanical loading. Insufficient vacuum can result in weak subsurface airflow and limited contaminant removal. The required operating point should therefore be based on the airflow and vacuum needed at the extraction wells under actual soil resistance.
Air sparging introduces pressurized air below the groundwater table. The air travels upward through saturated soil and groundwater.
Depending on the contaminant and site conditions, this can transfer volatile contaminants from groundwater into soil gas, increase dissolved oxygen, support aerobic biodegradation, and encourage movement of contaminant vapor toward the unsaturated zone. Air sparging is frequently combined with SVE. The sparging system introduces air below ground, while the SVE system captures the vapor released above the groundwater table. Without adequate vapor capture, volatilized contaminants may migrate beyond the intended treatment area or reach the surface.
The sparging blower must overcome groundwater hydrostatic pressure, soil resistance, well and pipe losses, injection-device resistance, and required operating margin. A positive-pressure roots blower may be suitable where the required airflow and injection pressure fall within its verified performance range.
Bioventing supplies air to the unsaturated zone to support naturally occurring microorganisms that break down biodegradable contaminants.
Unlike conventional SVE, the primary objective is not maximum contaminant-vapor removal. The system supplies sufficient oxygen to promote biological treatment while limiting unnecessary volatilization.
Bioventing may use air injection, low-rate vacuum extraction, combined injection and extraction, and passive air inlet wells. The required airflow is often lower than in an aggressive SVE system. Oversized equipment can remove contaminants as vapor rather than allowing biological degradation to occur within the soil. Soil oxygen, carbon dioxide, contaminant concentration and biological activity should be monitored to evaluate performance.
Biosparging introduces air into the saturated zone primarily to increase oxygen availability for aerobic microorganisms. It resembles air sparging mechanically, but its main treatment mechanism is biological rather than volatilization.
The required air distribution should provide adequate oxygen without causing uncontrolled vapor migration or excessive disturbance of the contaminant plume.
Biosparging performance depends on contaminant biodegradability, dissolved-oxygen demand, groundwater chemistry, soil permeability, injection pressure, and airflow distribution. Performance also depends on nutrient availability and microbial activity. A high visible airflow is not proof of effective treatment. Monitoring must confirm that oxygen reaches the intended subsurface zones.
Multi-Phase Extraction (MPE) simultaneously removes soil vapor, groundwater and potentially free-phase contaminant liquid.
A high vacuum is applied to specially designed extraction wells. The extracted stream may contain contaminated soil gas, groundwater, fuel or other free product, condensate, soil particles, and foam. This mixed stream requires separation before the gas reaches equipment that is not designed to handle liquid.
A typical MPE system may include extraction wells, liquid lift tubes, a vacuum manifold, a gas-liquid separator, a knockout tank, and a liquid-transfer pump, as well as a vacuum pump, vapor treatment, water-treatment equipment, and automatic level control. A standard roots blower is designed to transport gas, not liquid slugs. Where a roots machine is used on the vapor side, reliable gas-liquid separation and high-level shutdown protection are essential.
Some MPE duties may require a different vacuum technology with greater liquid tolerance. The selection should be based on the actual gas and liquid loads rather than describing every MPE system as a roots blower application.
Extraction and Vapor-Treatment System Design
Extraction wells and airflow distribution. The vacuum blower creates the driving force, but extraction wells determine how that force is applied to the soil.
Well design should define screen depth and length, casing diameter, filter-pack material, surface seal and spacing. The design also needs connection-pipe dimensions, an individual control valve and sampling points. Poorly sealed wellheads or surface cracks can admit atmospheric air near the extraction point, allowing the blower to move high airflow without drawing effectively from deeper contaminated zones.
Each well should be monitored and balanced independently. Relevant measurements may include wellhead vacuum, branch airflow, vapor concentration, temperature, moisture carryover, and oxygen. Performance also depends on carbon dioxide and explosive-gas concentration. Adjusting valves only according to the total header vacuum can conceal poorly performing wells.
Vacuum manifold design. The manifold connects multiple extraction wells to the treatment system. It should be designed to minimize unnecessary pressure loss and prevent condensate from collecting in uncontrolled locations.
Important considerations include total simultaneous airflow, distance from wells to the blower, pipe diameter, number of bends and fittings, individual well resistance, and pipe slope. The design should also consider condensate drainage, chemical compatibility, underground settlement, freeze protection, and inspection and cleaning access. The main header must carry the combined flow from all active wells. It should not automatically be the same diameter as an individual extraction branch.
Undersized manifolds can produce excessive pressure loss, weak airflow from distant wells, excessive vacuum at nearby wells, difficult balancing, increased blower power, and longer remediation time.
Moisture and condensate management. Soil gas frequently contains substantial moisture. Water may condense as the extracted gas cools in above-ground pipes or treatment equipment.
Liquid can also enter because of high groundwater level, rainfall infiltration, incorrect well-screen position, excessive applied vacuum, low points in the manifold, and water collected during shutdown. Possible consequences include faulty separator controls.
The system may require an inlet knockout tank, demister, a moisture separator, automatic drain, high-liquid-level alarm, and a high-level blower shutdown. The system should also account for a condensate-transfer pump and secondary containment. Collected liquid must be characterized and managed as potentially contaminated material. The separator should be sized for both routine condensate and credible liquid surges. A small drain pot alone may not protect the blower from a sudden groundwater slug.
Extracted-vapor treatment. Contaminated vapor should not normally be discharged directly to the atmosphere.
Treatment options can include activated-carbon adsorption, thermal oxidation, catalytic oxidation, condensation, biological treatment, and chemical scrubbing, as well as combined treatment stages.
The appropriate method depends on contaminant composition, vapor concentration, total airflow, moisture, temperature, and required removal efficiency. Performance also depends on local discharge requirements and expected concentration changes over time. Vapor-treatment equipment creates pressure loss. This resistance must be included when selecting the vacuum system, particularly when the treatment device is placed on the blower discharge. Carbon beds can become increasingly restrictive as they collect moisture or particulate material. Their condition should be monitored rather than treated as a fixed pressure loss.
Contaminant concentration changes. An SVE system often removes high contaminant concentrations during early operation. Concentrations may then decline as easily accessible contamination is removed.
The decrease may not remain continuous. Vapor concentration can rebound after the system is stopped because contaminants redistribute from less permeable soil regions.
Operational strategies may include continuous extraction, pulsed operation, periodic shutdown and rebound testing, seasonal adjustment, individual well cycling, and reduced blower speed at lower demand. A variable-frequency drive can help match blower capacity to changing site conditions. The permitted minimum blower speed and cooling requirements must still be respected.
Blower Selection and System Protection
Roots blower selection should be based on the required operating point at the blower inlet.
The enquiry should define required airflow, inlet vacuum or discharge pressure, minimum and maximum duty points, soil-gas temperature, moisture and contaminant composition. It should also state vapor concentration, particle load, active-well count, manifold losses, treatment resistance, site altitude, control range and operating hours. Give airflow reference conditions explicitly: actual inlet volume, standard flow and free-air flow are not interchangeable. Check motor power and temperature across the full operating range, including operation with wells closed or treatment equipment restricted.
Pressure and vacuum duties: Roots blowers can support different soil-remediation duties:
Typical roots blower duties by remediation process
| Remediation process | Typical blower duty |
|---|---|
| Soil Vapor Extraction | Vacuum extraction |
| Air sparging | Positive-pressure air injection |
| Bioventing | Low-pressure injection or extraction |
| Biosparging | Positive-pressure air injection |
| Combined SVE and sparging | Separate vacuum and pressure duties |
| Multi-Phase Extraction | Vacuum duty after suitable gas-liquid separation |
The pressure-side and vacuum-side duties should be calculated separately. One blower should not automatically be assumed capable of performing both functions at the same time.
Blower protection. A soil-remediation blower may be exposed to moisture, particles and chemically contaminated gas.
The installation may require inlet moisture separator, high-liquid-level shutdown, particle filtration, vacuum-relief valve, pressure-relief protection, and non-return valve. The system should also account for discharge-temperature monitoring, motor-current monitoring, flexible connector, suitable piping supports, gas detection, and emergency isolation. The protection arrangement depends on whether the machine operates under vacuum or positive pressure. A vacuum-relief valve can admit controlled air if inlet vacuum becomes excessive. A pressure-relief valve protects a positive-pressure blower from excessive backpressure. These devices serve different functions and should not be interchanged.
Safety, Monitoring, and Applicability
Flammable and toxic vapor safety. Extracted soil gas may contain flammable or toxic compounds.
The system design should evaluate lower explosive limit, maximum expected contaminant concentration, oxygen concentration, ignition sources, electrical classification, and static electricity. The system should also account for gas leakage, equipment ventilation, exhaust-treatment temperature, emergency shutdown, and safe sampling procedures. A standard blower package must not be installed in a potentially explosive atmosphere unless the complete equipment configuration is suitable for the specified hazardous area. Dilution air should not be introduced without understanding its effect on total airflow, treatment capacity and flammability.
Remediation performance monitoring. Blower operating data alone cannot demonstrate that the soil has been remediated.
Performance monitoring may include airflow from each well, wellhead vacuum, subsurface pressure response, vapor concentration, contaminant mass-removal rate, and groundwater level, as well as soil oxygen and carbon dioxide, treatment-unit efficiency, condensate quantity, blower power and temperature, and rebound after shutdown. A decline in contaminant concentration can mean that remediation is progressing, but it can also indicate poor airflow through the contaminated zone. Concentration must therefore be interpreted together with airflow and subsurface pressure data.
Limits of air-based remediation. Air and vacuum systems are not suitable for every contaminated site.
Possible limitations include very low-permeability soil, high soil moisture, deep contamination, non-volatile contaminants, strong contaminant adsorption, and complex geological layering, as well as shallow buildings or utility corridors, uncontrolled vapor-migration risk, high groundwater fluctuations, aggressive or incompatible gases, and difficult exhaust-treatment requirements. Pilot testing is often valuable before full-scale equipment selection. It can reveal the relationship between applied vacuum, airflow, well spacing and the actual radius of influence.
Glossary
Air Sparging: Injection of air below the water table to promote volatilization and, in some cases, aerobic biodegradation.
Biosparging: Controlled air or oxygen injection in the saturated zone primarily to enhance aerobic biodegradation.
Bioventing: Controlled delivery or extraction of air in unsaturated soil primarily to support aerobic biodegradation.
Multi-Phase Extraction: Simultaneous or coordinated recovery of soil gas, groundwater and potentially separate-phase product under vacuum.
Saturated Zone: The subsurface zone in which interconnected pores are filled with groundwater.
Soil Remediation: The controlled treatment, containment or removal of contaminants from soil, soil gas or groundwater.
Soil Vapor Extraction: A remediation process that applies vacuum to remove volatile contaminants as soil gas from the unsaturated zone.
Vadose Zone: The unsaturated subsurface zone between ground surface and the water table.