Venturi Aeration, Inc.

Enhanced Bioremediation: Contaminated Groundwater and Soil Conditioning via the Venturi Aeration Process

In situ bioremediation has become increasingly successful as a remedial applications technology for the cleanup of hydrocarbon and chlorinated solvent contaminated sites, especially as new products and systems are being configured or as technology is transferred from other environmental or industrial applications to better support the biological contaminant reduction process. This paper describes the transfer of the venturi aeration bioreactor process technology for water and wastewater treatment to field bioremediation applications. The venturi aeration process not only conditions and acclimates microorganisms to their environment but it maintains optimal environmental conditions to eliminate and reduce stress factors. Additionally, it discusses the use of catalytic enzymes to accelerate the biological degradation of hydrocarbon and chlorinated compounds.

Bioremediation is the process of using bacteria, nutrients, enzymes and other biological augmentation products, under controlled conditions, to metabolize organic compounds into carbon dioxide, water and energy for the replication of cells. The principal requirements crucial to the bioremediation process successes are, 1. an active, hydrocarbon or contaminant specific microorganism population, 2. a continuous supply of dissolved oxygen (DO), 3. frequent addition of nutrients (N, P, S, etc.), 4. micronutrients to support biological processes and 5. “conditioning” of their working environment for optimal digestion (pH, temperature, catalytic enzymes, colony concentrations, etc.).

Technology Transfer

While the venturi principle is several hundred years old, the modern venturi aeration process using the Venturi Aeration OxygenatorÔ was developed for municipal, industrial and agricultural wastewater as well as potable water applications to condition the liquids for various desired results. In wastewater applications the device is predominately used to transfer high levels of dissolved oxygen into wastewater for BOD reduction while stoichiometrically oxidizing organic contaminants into benign forms. While in drinking water applications the venturi aeration process has been used to strip weak Henry’s constant substances in order to non-chemically shift pH (viz. CO2), remove soluble radon gas, and strip VOCs (TCE, PCE, etc.) from contaminated aquifers.

pH = 6.3 + Log (alkalinity / carbon dioxide).

Stripping the CO₂ means a logarithmic increase in pH in relation to the nascent (natural) alkalinity. This feature is important to the enhanced bioremediation process.

Since the venturi aeration process is very effective for: adding dissolved oxygen into wastewater, simultaneously stripping partially soluble substances with weak Henry’s constants and developing a biomass– it should be considered for bioremediation.

These three features make it an ideal device for use in environmental bioremediation projects that require high levels of dissolved oxygen to achieve biological reduction (biodegradation) activities and maintain a steady state pH (stripping CO₂) environment in which these biological processes will occur.

Importance of Dissolved Oxygen and pH/ORP Adjustment in Microbial Degradation

As with most bioremediation projects, the rate of contaminant degradation is usually limited by the amount of available dissolved oxygen available to support the microorganisms’ metabolic activity and to a greater extent the pH of the liquid in which degradation occurs. It cannot be emphasized enough that even in the presence of high dissolved oxygen (DO) levels biological activity may slow down if the liquid becomes too acidic (pH < 5.8) from the respired CO₂ of the biological degradation process. ORP (Redox) readings shift from –150 to +200. This large change in ORP is a measure of the increased microbial activity that is occuring.

Note: the principal byproducts of microbial digestion are CO₂ and H₂O. Further, in geographical areas like the Northeast, groundwater is naturally acidic to begin with because of a lack of minerals which can “buffer” pH. Additionally, certain hydrocarbons because of their length, molecular weight, shape and other chemical properties may resist transport into the cell walls of the microorganisms, in these cases the addition of specific enzymes aid in catalyzing the necessary biological reactions.

Unique Design and Operation

The venturi aeration process in wastewater applications routinely will transfer up to 7.5 mg/l of dissolved oxygen into a liquid [20° C at sea level]. However, the Venturi Aerator device can be configured to “saturate” groundwater with dissolved oxygen. It is proprietary knowledge of how the Venturi Aerator can be configured that makes it unique among technologies for enhanced bioremediation. Groundwater extracted from the plume of contamination is pumped into the Venturi Aeration Bioremediation system where the Venturi Aerator adds dissolved oxygen, strips weak Henry’s constant substances and adjusts pH non-chemically, floats free phase product for recovery and supports the growth of a viable biomass. In this manner groundwater is “conditioned” ex situ in the Venturi Aeration Bioremediation Unit, and then is injected into the subsurface vadose zone and groundwater for biological reductions of residual hydrocarbons or chlorinated solvents in the soil and groundwater.

Multiple Functions:

The single Venturi Aerator configured for bioremediation accomplishes all the tasks achieved by an array of different environmental equipment frequently used for bioremediation, i.e.:

1. air sparger with compressor or blower,

2. air stripper tower or column,

3. diffused air floatation (DAF),

4. a fixed film bioreactor,

5. reciculating well, and

6. chemical treatment for pH adjustment

By having only one (1) venturi aeration system accomplish all these combined tasks its makes the capital costs for setting up a bioremediation system very cost effective, and because it is actually moving groundwater to the bioreactor component in a closed loop system it makes the process “dynamic.” Many passive or static bioremediation technologies, e.g. addition of bacteria, nutrients and peroxides, or ORC (oxygen release compounds) require longer periods of time to treat a site just because they are “static.” Additionally, it may eliminate the need for expensive activated carbon adsorbers and their frequent changeouts, transportation and the disposal or reactivation of spent carbon associated with pump and treat systems, depending on clean air emission requirements for the specific project. If there are significant amounts of free phase products present in the groundwater a small oil skimmer or oil/water separator can be added for free phase product recovery directly in the bioremediation unit.

Full Scale System Tested

In the first full-scale demonstration project of this system, the Venturi Aeration Bioremediation unit, was at a site with contaminated groundwater (diesel fuel and gasoline). It was positioned on the site so the various hoses could be moved around the plume of contamination as required to influence the entire plume. The system was connected to the existing wells that were part of on-going “passive” remedial activities. The environmental contractor had been adding hydrogen peroxide to all the monitoring wells and observation wells to provide as much DO as possible approximately every two weeks. The process of adding diluted 35% hydrogen peroxide, microorganisms and nutrients took two man days two times a month.

With the Venturi Aeration Bioremediation unit the influent line into the unit was connected to an existing 0.5 hp Grundfoss pump in a four (4″) recovery well on the down gradient side of the plume. Groundwater was extracted at a rate of 30 gpm, and the pump was put on a timer to allow the well to periodically recharge. The groundwater was pumped directly into the Venturi Aeration Unit that immediately stripped the carbon dioxide from the groundwater changing the pH non-chemically. The pH of the extracted groundwater was consistently in an acidic range of 5.6 to 5.8 and was shifted to a neutral range 7.2 to 7.4 without addition of any chemicals exclusively by stripping the carbon dioxide. This is extremely beneficial to the microorganisms.

Conditioned Groundwater

The Venturi Aerator unit added dissolved oxygen (DO) by discharging the aspirated liquid into a stilling well. The continuous flow forced the highly aerated liquids to migrate to the bottom of the stilling well where they were attached with fine micro bubbles. These micro bubbles allow residual amounts of free phase product to float where they could be removed. DO stratified in the Venturi Aerator Bioremediation unit, at the top of the tank it measured 16.0-18.0 mg/l, in the middle 10.0-12.0 mg/l, and at the bottom 3.5-4.0 mg/l. Conditioned groundwater with a pH of 7.4 and DO levels of 10.0-12.0 mg/l were then injected into the plume using a diaphragm pump. Because of the pulling action of the Grundfoss extraction pump and the pushing action of the diaphragm injection pump, groundwater moved through the plume every 1:20 minutes from the injection well to the extraction well. This time was measured by adding a non-toxic biodegradable food grade coloring to the injected groundwater and timing its movement through the groundwater by observing its extraction into the Venturi Aeration unit. At a flow rate of 30 gpm, 1,800 gph can be extracted, conditioned and reinjected (43,000 gpd). For this pilot test site it is estimated that a 100% turn of the groundwater in the plume was achieved every five (5) days. By causing a movement of the groundwater to the extraction well, extracted groundwater could be “conditioned” and treated ex-situ prior to reinjection in the upgradient injection wells with microorganisms and nutrients already added to the neutral pH liquids. Groundwater temperatures at this site are 14-17° C and depth to groundwater averages 22’5″.

Hydraulic Flow

At the full-scale pilot system after several days of operation, groundwater had migrated into the plume from the surrounding areas and the surrounding outlying observation wells were dry while the system was operating. This indicates that the groundwater had migrated to the extraction points aiding in “soil washing” and reducing chances for smear in the vadose zone.

Preventing Iron Fouling

This site also has free iron (Fe++) at 35 mg/l. Iron can foul well screens and other system components. By oxidizing the iron in the Venturi Aeration unit, iron is immobilized and does not reenter the groundwater to plug the slotted well screens. However, for those sites with higher levels of iron in groundwater, a Hydro-Cat Catalytic Ionizer can be added to the system to keep iron colloidal in order to prevent it from fouling system components. Iron scaling also is reversible using the Hydro-Cat Ionizer.

Enzyme Catalyzed Biodegradation

While no catalytic enzymes have been added to this pilot test site, they have been added to another hydrocarbon contaminated groundwater site in California where there were significant increases in metabolic reactions were directly attributable to the enzymatic activity. The field microbiologist calculates enzyme-catalyzed reduction rates have increased by almost 1000% based on SPC (standard plate count). The enzymes (QuellzÔ ) are protein-based substances that help catalyze these reactions. By attaching themselves to hydrocarbons, enzymes allow certain hydrocarbons that are ordinarily hydrophobic in nature to cross the cell walls (membranes) into the microorganism for degradation. Regardless of the amounts of DO used to enhance degradation, certain hydrophobic molecules, by nature, are unable to cross the cell membrane thereby leaving the molecule undegradable by biological processes. In these cases, enzymes help catalyze cellular transport of these hydrophobic molecules for degradation.

Summary

The Venturi Aeration treatment process for used for treating wastewater and potable water is directly transferable into the environmental field for enhanced bioremediation. One Venturi Aerator can take the place of several more pieces of expensive environmental cleanup equipment making the cost of remediation economical while more importantly accelerating the degradation of hydrocarbon and solvent contamination in both the saturated vadose zone and in groundwater. For hydrophobic hydrocarbons, adding catalytic enzymes helps catalyze beta-oxidative reductions that would not occur biologically, or would otherwise occur very slowly.