How Much Does Mobile High Vacuum Dual Phase Extraction Cost in the Bay Area?

 

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I was on  the phone with a property owner and her attorney talking about possible cleanup options for her property. They explained that the property had undergone cleanup using soil vapor and groundwater extraction, but the problem still remained. While the initial cleanup work definitely moved the property toward case closure, regulators were looking to bring contaminant levels down even further because a nearby domestic water supply well was threatened.

As we discussed options, the property owner spoke up and asked about mobile high vacuum dual phase extraction (HVDPE). She had heard that it was very effective, but expensive. I explained that although it is expensive up front when compared to other cleanup technologies, the shortened cleanup time and effectiveness of HVDPE can make it cost competitive. I told her that in many cases, the amount of cleanup accomplished after two years of soil vapor and groundwater extraction could be attained by two months of mobile HVDPE. 

“Well,'' she said, “why aren’t we looking at it for my site? I am definitely interested in something that’s going to end this problem and give me back my property sooner rather than later.” Then she asked, “How much does  HVDPE cost?”

I explained that it's hard to tell because each site and its circumstances are different. For example, it’s hard to gauge how much contamination is available for extraction, so it’s hard to estimate how long HVDPE will have to operate. Often, a pilot test is done to determine HVDPE performance. Of course, that didn't answer her question.

After a brief pause in the conversation I said, “Okay, lets go over costs for a job I recently finished using mobile HVDPE to get an idea of what it could cost at your site. Let’s begin with a short description of mobile HVDPE and then we’ll get into the costs.”

What is Mobile HVDPE?

HVDPE uses a heavy duty vacuum pump to generate high air flow rates and vacuum pressures. For example, at a recent job flow rates ranged between 136 cubic feet per minute (cfm) to 199 cfm and vacuum pressures ranged from 16 inches of mercury (inHg) to 19 inHg. For reference, a typical household vacuum operates at flow rates between 50 cfm and 100 cfm and vacuum pressures around 6 inHg.

For use in cleanup, the vacuum pump is connected to a sealed well targeted for cleanup. The well seal allows for an extraction pipe to be installed through it and into the well. One end of the extraction pipe is placed below the water and the other end extends through the well seal. The top of the extraction pipe is connected to an air/water separator, which in turn is connected to the heavy duty vacuum pump. A valve installed in the side of the well allows a controlled flow of air to enter the well, as necessary. Once vacuum is applied to the extraction pipe, the depth of the pipe and the well control valve are adjusted to remove as much subsurface soil gas and contaminated groundwater as possible. Put the HVDPE system on a truck or trailer with a generator and it’s a mobile system.

RRM DPE unit

The advantages of mobile HVDPE are:

  • Both soil gas and groundwater are removed together. As the process unfolds, groundwater is removed from the soil which opens air flow pathways previously blocked by water. This results in more air flow through the soil over a larger volume of soil which in turn increases contaminant removal and decreases cleanup time.
  • The transfer of volatile contaminants from soil and groundwater to soil gas is significantly increased under a high vacuum which means contaminants are removed a lot faster.
  • Dual phase extraction causes mixing of air (oxygen) and groundwater, creating conditions that support and stimulate naturally occurring bacteria that consume contamination - known as biodegradation.
  • Faster removal rates and enhanced biodegradation mean much faster cleanup times.
  • A mobile system avoids the high cost of building a system on site and breaking it down once cleanup is complete.

Compared to groundwater and soil vapor extraction, mobile HVDPE reaches a larger volume of contaminated soil and groundwater per well, it removes more contaminants, it removes contaminants faster, and there is no infrastructure to remove after cleanup. Mobile HVDPE works best for cleanup of volatile petroleum hydrocarbons (from service stations) and solvents (from dry cleaners).

What Does Implementation Look Like?

Typically, a site has been investigated prior to use of mobile HVDPE. There are usually three or more  groundwater monitoring wells at the site, and the general location of contaminant “hot spots” or areas of high contaminant levels have been identified. It is possible that an additional well, or wells, is needed to clean up either a larger area or deeper zone of contamination. 

Cal Clean at Greenbaker WTF 1Scheduling mobile HVDPE with a contractor is easy enough, but usually there is a moderate lead time before the contractor can show up on-site. Typically, a 30-day term is contracted, but daily rates and extended rates are possible. In our case, we initially contracted for 30 days and then extended operation. Contractors use either thermal oxidation or granulated activated carbon (GAC) to treat contaminated soil gas, and GAC to treat groundwater prior to discharge.

Implementation at our site was straight forward. The contractor showed up in the morning, met with one of our staff for a briefing, and began installing extraction pipes in the wells and running hoses between the wells and the HVDPE unit. The contractor also set up a groundwater treatment system that discharged to the sanitary sewer. We were up and running on the first day the contractor arrived at the site. Daily operation was conducted by the mobile HVDPE contractor.

At the close of operation, the contractor disassembled the extraction piping, hoses, and groundwater treatment system, and demobilized from the site. Our staff did a final inspection of the work area, disconnected the sanitary sewer access, and conducted confirmation groundwater sampling of the site’s wells.

How Much Does Mobile HVDPE Cost?

Cleanup at the site continued over five consecutive HVDPE periods, each punctuated with review of the data and a decision to continue operation. Costs are provided in the following table.

Screen Shot 2022 04 06 at 4.39.15 PMFirst Event 30-day mobile HVDPE

The total cost for the first 30-day event including rental and operating expenses was $67,898. In addition to the contractor costs, consulting services for health and safety, setup and startup of the HVDPE unit, data reduction and review, regulatory liaison, and sanitary sewer compliance cost about $8,000.

Contaminant concentrations went from 11,000 parts per million by volume (ppmv) to 3,700 ppmv with 8,586 pounds of contaminant removed. The contaminant removal rate averaged 313 pounds per day. The cost to remove contamination over the first 30-days was about $8 per pound of contaminant.

With the first 30 days of operation complete, stakeholders discussed further operation. The data indicated that more contaminants were available for removal and the regulatory authorities wanted the operation to continue until the contaminant removal rate dropped to 8 pounds per day. It was decided to continue operation.

Second Event - 40-day mobile HVDPE

The second HVDPE event lasted 40 days and cost  $90,236. Consulting services were about $6,400. During operation, contaminant concentrations went from 3,700 ppmv to about 1,200 ppmv with 4,746 pounds of contaminant removed. The contaminant removal rate averaged 93 pounds per day. The cost to remove contamination was about $28 per pound of contaminant. Again, stakeholders decided that continued operation was warranted.

Third Event 30-day mobile HVDPE

The third HVDPE event lasted 30 days and cost $66,621. Consulting services cost about $8,640. During operation, contaminant concentrations went from 4,200 ppmv to about 2,100 ppmv with 2,607 pounds of contaminant removed. The contaminant removal rate averaged 117 pounds per day, and the cost to remove contamination was about $26 per pound. Stakeholders remained committed to further treatment based on the data.

Fourth Event - 30-day mobile HVDPE

soil gas sampling 1 1The fourth HVDPE event lasted 30 days and cost  $66,887. Consulting services cost about $2,718. Contaminant concentrations went from 1,500 ppmv to about 1,100 ppmv with 2,143 pounds of contaminant removed. The contaminant removal rate averaged 76 pounds per day, and the cost to remove contamination was about $31 per pound of contaminant. While there was resistance to continued operation because of the mounting cost, stakeholders agreed to continue operation to bring the contaminant removal rate down further.

Fifth Event 19-day mobile HVDPE

The fifth HVDPE event lasted 19 days and cost $42,061. Consulting services cost about $2,000. Contaminant concentrations stayed flat and ranged from 760 ppmv to  890 ppmv with 1,142 pounds of contaminant removed. The contaminant removal rate averaged 52 pounds per day, and the cost to remove contamination was about $52 per pound of contaminant. Although the contaminant removal rate remained significantly above 8 pounds per day, the mounting cost pushed stakeholders to end HVDPE.

Factors That Drive Cost (Why Does it Cost so Much?)

Some of the factors that drive mobile HVDPE cost include:

  • High vacuum pressure - Heavy duty equipment and plumbing are necessary to generate and withstand high vacuum pressures, and heavy duty transport is necessary to carry the weight. Heavy duty equipment, plumbing and transport drive up costs.
  • Type of contaminant - Petroleum hydrocarbons (like those found at gas stations) in soil vapor can be burned as part of treatment prior to discharge. Halogenated compounds (typically found at dry cleaners) cannot be burned without treating the combustion products prior to discharge. As a result, GAC is typically used to treat halogenated compounds, which costs more.
  • Groundwater treatment - The amount of groundwater generated varies based on subsurface conditions and how much dewatering (lowering the water table) is required for cleanup. The treatment cost is directly linked to the amount of groundwater generated, so scenarios with larger amounts of groundwater will cost more. 
  • Treated groundwater discharge - There are limited discharge options for treated groundwater. Not all sanitary sewer districts will take the treated water, use of the storm drain may not be possible, and areas for land discharge are limited. Any option will require regulatory permitting and have varying monitoring and sampling requirements. 
  • Soil type - Air flow through soil varies by soil type, and can affect cleanup time. The tighter the soil, the harder it is for the air to flow. This limits the amount and rate at which contaminants can be removed and can drive up cost.
  • Generator fuel - the mobile HVDPE uses a generator for electric power and the cost of fuel can change quickly. Fuel usage drives cost. 

In Conclusion

We described mobile high vacuum dual phase extraction (HVDPE), and what implementation looks like. We noted the advantages of HVDPE and provided a summary of costs for a recent project. We took a look at factors that can drive HVDPE cost.  

We found mobile HVDPE to be so effective, we purchased our own unit. It is available for cleanup or pilot testing. Take a look at our services tab for more information. If you are looking for a mobile HVDPE unit, or are just wondering if HVDPE is the right choice for your site, we can help. Call us at (831) 475-8141 or click the button below to schedule a free consultation.