Field Testing of Membrane Degasifier

Location of Test Site

The test well was located in the Town of Havana, Florida. It was Well No. 4 and the well configuration was as follows (Report from Mr. Aaron Van Smith, Hatch Mott MacDonald, Florida, July 18, 2008):

Well  ►  Aerator  ►  Chlorination  ►  Ground Storage Tank (GST)  ►  HSP  ►  Elevated Tank

The elevated tank was 450,000 gallons. The floats in GST control the pump, when the well turns on and off. The GST is 100,000 gallons. The well runs at 500-540 gpm. The well depth is 610 ft drilled, 540 ft casing and there are 3 other wells.

Field Testing Procedure

The following water testing procedure was followed:

  1. Take a sample of the inlet and outlet water from the degassing unit; These samples are taken in vials and are completely filled to leave no headspace.
     
  2. Use the LaMotte Single beam Spectrophotometer to measure the inlet and outlet total dissolved sulfide and pH;
     
  3. Using the inlet and outlet pH and temperatures, determine the percentage of un-ionized hydrogen sulfide from Table 1; and
     
  4. Calculate the percentage efficiency of hydrogen sulfide degasification.

Three tests were run (Report from Aaron Van Smith, July 18, 2008). The first and second test determined how much sulfide was being removed from the water via the device. The third test determined how much sulfide was being removed from the water via the treatment plants present treatment, utilizing the aerators and chlorine injection.

Test 1: The inlet water was tested for total dissolved sulfide then the water was ran through the device for approximately 10 minutes and then the device effluent was tested to determine how much sulfide was removed.

Test 2: The inlet water was tested for total dissolved sulfide then the water was ran through the device for approximately 10 minutes and then the device effluent was tested to determine how much sulfide was removed.

Test 3: A sample was taken from right after the aerator. Another sample was taken after the chlorination (sample was taken from a tap at the HSP)

The results of the test were as follows

 

  Test No. 1 Test No. 2 Test No. 3
Initial Dissolved
Sulfide
Concentration
2.2 ppm 2.4 ppm  
Final Dissolved
Sulfide
Concentration
0.38 ppm 0.36 ppm  
Post Aeration     2.35 ppm
Post Chlorination     0.1 ppm
(below detection)

The pH of the water was 7.5 and the temperature was recorded as 25.4 deg C. There was no measurable change in pH between the inlet and degassed water outlet.

Test Data Analysis

Based on the pH and temperature, the % of dissolved hydrogen sulfide is 22.21%. Using this percentage, the following calculations are made:

Havana Test Site Results for Membrane Degassifier 

Supplier: PRD Tech, Inc. Test 1 Test 2 Test 3
Inlet Total Dissolved Sulfide (ppm) 2.2 2.4  
Membrane Degassifier Outlet Sulfide (ppm)  0.38 0.36  
       
Dissolved sulfide after Tray Aerator (ppm)     2.35
Dissolved sulfide after chlorination (ppm)     BDL
      BDL: Below Detection Limit = 0.1 ppm
       
pH of water 7.5 7.5 7.5
Temperature of water (deg C) 25.4 25.4 25.4
       
Percentage of total sulfide that is hydrogen sulfide   22.21%  
(Table 1)      
       
Inlet Dissolved Hydrogen Sulfide in water (22.21%) (ppm) 0.48862
0.53304
 
 
Outlet Dissolved Hydrogen Sulfide (exit of Degassifier) (ppm) 0.084398            0.079956  
       
Dissolved sulfide after Tray Aerator (ppm)     0.521935
       
% Removal Efficiency of Membrane Degassifier 82.73                    85.00  
% Removal Efficiency of Tray Aerator     -2.17
% Treatment Efficiency after chlorination     95.65

 

Notes:BDL: Below Detection Limit is taken to be 0.1 ppm
 

The Membrane Degassifier removal efficiency is 82.73 and 85% for Test 1 and Test 2 respectively, resulting in an exit water hydrogen sulfide concentrations of 0.084 and 0.080, respectively. The chlorination treatment efficiency is 95.7%. This shows that the Membrane Degassifier can perform as well as treatment of hydrogen sulfide using chlorination. Chlorination is expensive requiring more chlorine gas consumption to chemically oxidize the dissolved sulfides in addition to creating disinfection of the water. It is important to emphasize that while Membrane Degassification can only remove the unionized hydrogen sulfide portion of the total dissolved sulfides (percentages given in Table 1), chlorination can remove both the unionized (hydrogen sulfide) and ionized portion (HS-) of the sulfides. Hence, while the chlorination treatment efficiency is higher than the Membrane Degassifier, excess chlorine gas had to be used to achieve this result, since residual chlorine is required in the water. Proper design of the Membrane Degassifier for this water application could have easily resulted in removal efficiencies exceeding 95%.

The tray aerator produced a negative removal efficiency. This was due to the fact that the exit sulfide concentration was higher than the inlet water due to the presence of sulfate reducing bacteria biofilms in the sump and possibly the trays of the tray aerator system. The tray aerator uses a natural draft of ambient air and this allows small amounts of carbon molecules from grass mowers, cars, etc to enter the water, as this air contacts the water falling through the trays. The trays produce a high contact surface area between the water and ambient air, which allows this ambient air carbon compound contamination to enter the drinking water. This contamination, which becomes dissolved organic carbon in the water, then supports the growth of sulfate reducing bacteria (SRBs), and allows this sulfate, which is naturally present in water, to convert partially to sulfide:


SO4 2-     +     CH2O -----------------► HS-     +     HCO3-
Sulfate            Organic                         Sulfide       Bicarbonate


It is important to emphasize that the conversion of sulfate to sulfide by the sulfate reducing bacteria (SRBs) is partial due to limited presence of dissolved organic carbon in the water. Dissolved organic carbon can be present in well waters and is additionally introduced through the ambient air contact in the tray aerator.

Hence, while a clean tray aerator will produce a positive removal efficiency for the dissolve sulfides, growth of sulfate reducing bacteria on the trays and in the sump will result in forming more sulfide from the sulfate in the water and the tray aerator loses removal efficiency, until it becomes a producer of sulfide rather than a removal system. This demonstrates that unless a tray aerator is cleaned periodically, it does not remove hydrogen sulfide from the water.

Another major disadvantage of the tray aerator is that this growth of sulfate reducing bacteria (SRBs) and aerobic bacteria on the trays and in the sump increases the total dissolved organic carbon in the water, resulting in the formation of trihalomethanes when the water is chlorinated. Trihalomethanes (THMs) are a byproduct of the water treatment process. They are formed when natural organic material, such as the decaying bacteria, such as aerobic and sulfate reducing bacteria, reacts with chlorine used to treat the water. This reaction produces “disinfection by-products,” the most common of which are THMs. In 1979, the U.S. Environmental Protection Agency adopted a regulated limit on the amount of THMs allowable in drinking water, of 100 parts per billion.

Finally, tray aerators represent a possible security breach in the drinking water supply, making it possible to easily contaminate the water supply.