Preserving Shipboard AFFF Fire Protection System Performance While Preventing Hydrogen Sulfide Formation
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- Sheinson, R.S. & Williams, B.A. Fire Technol (2008) 44: 283. doi:10.1007/s10694-007-0032-6
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There is a very serious problem aboard US Navy ships from generation of toxic hydrogen sulfide (H2S) in Aqueous Film-Forming Foam (AFFF) solutions used for shipboard fire protection. This is the result of the action of sulfate reducing bacteria (SRB) in mixtures of seawater and AFFF, which remain stagnant for significant time periods in shipboard fire protection system piping. Similar to microbial generation of H2S in sewage, over time microbes present in seawater consume organic materials in the AFFF mixture and can deplete the dissolved oxygen. If the reduction-oxidation potential falls low enough, anaerobic action of the SRB on the sulfate present in seawater can then result in H2S generation, reaching dangerous levels. The recommended ceiling for exposure to H2S is only 10 ppm. If the microbes causing oxygen depletion and/or the SRB can be eliminated (or sufficiently minimized), the dangerous generation of H2S would not occur. The Navy Technology Center for Safety and Survivability is participating in a research project for the Naval Sea Systems Command (NAVSEA) to evaluate several treatment modalities for their ability to inhibit H2S formation in AFFF/seawater mixtures and for possible deleterious effects on AFFF performance. Various approaches have been considered employing laboratory evaluations (dynamic surface tension and Ross-Miles foamability), and 28 ft2 (2.6 m2) pool fire extinguishment and burnback protection field tests (Military Standard MIL-F-24385F). The protocol selected for NAVSEA shipboard H2S generation mitigation testing is a combination of a commercial broad-spectrum biocide with a molybdenum compound which is a specific inhibitor of SRB.
KeywordsAFFFAqueous Film-Forming Foambiocidefoamhydrogen sulfideH2Smicrobial growthseawatershipboardsulfate reducing bacteria
AFFF is used in the US military, and in most civilian applications worldwide, as either a “3%” or a “6%” concentrate, alternately referred to as Type 3 or Type 6, respectively. The numbers refer to the percentage of foam concentrate mixed with either fresh water or seawater (e.g., a “6%” AFFF concentrate is nominally used as a mixture of 6% concentrate and 94% water). In the US DoD, the 6% concentrate is used in most shipboard applications, while 3% is used in most land-based applications. A 1% AFFF concentrate is sold by some manufacturers for civilian uses, but is not at present included in the MilSpec. The discharge nozzle can either be handheld, or in cases such as the flight decks of aircraft carriers, built into the ship.
A number of environmental issues have come under more careful consideration in recent years and could impact AFFF usage , including increasing international regulations. One well-publicized environmental issue outside the scope of the present study is the environmental fate and consequences of fluorinated surfactants present in AFFF. The concern that is the subject of this work is the generation of hydrogen sulfide (H2S) when organic rich AFFF remains mixed with natural seawater for long time periods in a non-aerated environment. H2S, causing the familiar odor of rotten eggs, is a toxic gas at higher concentrations. The National Institute of Occupational Safety and Health (NIOSH) Recommended Exposure Limit (REL) for H2S is 10 ppm with the Immediately Dangerous to Life or Health Concentration (IDLH) 100 ppm . H2S is particularly insidious as one acclimates to its odor and might not realize dangerous levels are present.
AFFF for US Navy shipboard fire protection use is generated by mixing the concentrate with seawater as needed. The AFFF-seawater mixture remains in the piping after system use, as the system always remains charged for rapid response. The time interval can be months, and pipe runs on some ship classes can contain hundreds of gallons. Under these conditions, microbes always present in seawater can consume the organic materials in the AFFF mixture and deplete the oxygen originally dissolved in the stagnant AFFF/seawater mixture. If the reduction-oxidation (redox) potential falls low enough, anaerobic sulfate reducing bacteria (SRB) will use the sulfate present in seawater as an oxygen source to metabolize organic components of the AFFF, producing sulfide as a by-product . This can then result in H2S generation reaching dangerous levels. The NRL has been involved in investigating several shipboard incidents of H2S generation from AFFF from 1989 onward. A lesser concern related to microbial activity is ‘scaling,’ i.e., biofouling, of piping (a potentially serious issue for titanium piping, including fire mains, in future ships). Only products that have been certified as satisfying the military specification requirements for AFFF performance are considered in this study. These products all contain fluorosurfactants. Although fluorine-free fire fighting foams have been developed in recent years, their surface tensions are higher. These formulations do not possess the film-forming ability of AFFF, and do not meet the film formation requirement of the MilSpec. More importantly, we know of no fluorine-free fire fighting foam with extinguishment capability approaching the required minimum performance.
Our testing included the MilSpec AFFF formulations formerly manufactured by 3M. Production of these products was discontinued several years ago due to environmental concerns about the fluorosurfactants (PFOS) used in these products. Since 3M was previously the major supplier to DoD, these AFFF formulations are still aboard Navy ships. Thus, from a personnel protection standpoint, the prudent course is to include the 3M AFFF in the testing, even though it is no longer manufactured.
H2S Mitigation Approaches
Undesirable H2S formation in AFFF systems results only from a sequence of events involving several necessary factors (written in italics below). At each numbered step of the sequence, various approaches are possible to counteract the sequence step and prevent H2S formation. The approaches for counteracting each of the listed steps and their practicality for shipboard implementation are assessed in the paragraph below each step in the H2S formation sequence:
1. There must be both organic material which can be metabolized by bacteria in sufficient quantity to deplete the available dissolved oxygen, and a source of sulfate.
AFFF contains organic components in addition to surfactants, which are necessary to achieve the necessary foam quantity and performance. About 10% of the dissolved salts in seawater are sulfates (some AFFF formulations contain sulfate also, but their contributions to the total sulfate load in the seawater/AFFF mixture are minor compared to that of the seawater). Reducing the concentrations of organics or sulfate in the AFFF/seawater mixture is not practical.
2. The dissolved oxygen in the seawater must be depleted by aerobic bacteria metabolism, so that anaerobic conditions conducive to SRB functioning exist.
The SRB do not function until anaerobic conditions exist. If the aerobic bacteria population in seawater could be killed or greatly decreased, anaerobic conditions in the piping will be avoided or minimized. Attacking the microbe population can be accomplished via oxidizing and non-oxidizing biocides, ultraviolet (UV) radiation and other modalities. There are practical difficulties getting sufficient UV intensity through seawater, which contains organics and scattering particles. Special windows would also be required. Thus, use of biocides to control aerobic bacteria and prevent the formation of anaerobic conditions is the approach which we chose to pursue.
3. There must be no aeration of the mixture or alternative oxidant sources.
Aeration or adding materials to maintain the redox potential over time is a possible approach. However, it would likely require significant engineering modifications on existing shipboard installations, and was not explored further under this task.
4. SRB must be able to live and function in the anaerobic environment, and sulfide they produce must exceed the solubility limit and undergo no chemical reactions, in order for it to be released as hydrogen sulfide gas.
The SRB could be targeted and attacked by biocides specific to SRB , or sulfide itself might be able to be scavenged chemically . One of these approaches could be a second line of defense after attempting to control the growth of aerobic bacteria.
Complicating the implementation of an approach are the tendencies of bacteria to colonize and form biofilms including on pipe surfaces, crevices and accumulated debris. Bacteria in such protected environments will be more difficult to eliminate. Continued or periodic subsequent treatment with provision for circulation could be necessary, but are beyond the scope of this effort, which was to identify approaches that would not negatively affect AFFF performance.
Selection of Biocides
The commercial fire fighting foam sector has dealt with and handled microbial degradation in other types of foam concentrates. The threat facing the commercial products from microbial growth is much worse for the class of Alcohol Resistant (AR) foams, for which there is no military specification. In order to be able to maintain foam in the presence of alcohols, which can serve as foam breakers, the typical AR formulation is ‘built up’ with organic materials, typically polysaccharides. These materials serve as nutrients for many microorganisms, making the foams much more supportive for microbial life. Therefore AR-AFFFs tend to be very prone to bacterial contamination and degradation. As a result, AR-AFFFs typically contain biocides designed to protect the (unmixed) concentrate from degradation.
To make use of AFFF manufacturers’ experience and ideas regarding inhibition of bacterial growth, we contacted all the manufacturers with AFFF products on the Qualified Procurement List (QPL, signifying certification of the product’s compliance with Military Standard MIL-F-24385F). In the course of these contacts, the manufacturers gave information on controlling bacterial growth in AR-AFFF concentrates, which contain high levels of nutrients. Thus the adaptation of the methods used to control bacteria in AR-AFFF concentrates to controlling bacteria in AFFF/seawater mixtures was identified as a promising approach.
For AR-AFFFs, the biocide is intended to prevent bacterial degradation of the undiluted concentrate. After dilution, there is always an issue of bacterial attack of the foam mixture. The implication, mentioned by some of the manufacturers, is that higher antimicrobial concentrations would likely be needed to provide long-term protection for seawater/AFFF mixtures in shipboard piping.
All the manufacturers felt that bleach (hypochlorite), used as a disinfectant to control bacterial growth (including as an approach to prevent hydrogen sulfide formation), would adversely interact with their products and degrade performance. It was felt hypochlorite might react with both glycol ethers such as butyl carbitol (used as foam extenders in AFFF) and the surfactants in AFFF, particularly during an extended period of storage. Instead, antimicrobial chemicals were suggested and are employed by the industry to control bacterial growth in their products.
A biocide used to control aerobic bacteria for this application must have long-duration effectiveness, have broad spectrum effectiveness against many different organisms, be effective at neutral and slightly alkaline pH, and have minimal environmental persistence, reasonable toxicity for personnel safety, and approval for use in non-potable water systems.
After consideration of a variety of biocides, the class which appeared most promising, and was selected for further testing, was that of polycyclic amines. Examples of this class include Dowicil 75 (manufactured by Dow Chemical) and Busan 1024 (manufactured by Buckman Labs, Memphis). Both of these compounds were suggested by foam manufacturers, and are used in AR-AFFF formulations to prevent bacterial growth. This prior use in foam formulations suggests that these biocides are unlikely to interfere with AFFF performance.
Biocides in this class are slowly hydrolyzed by water, eventually producing formaldehyde, which is the main anti-bacterial active ingredient. As the active material is only produced after hydrolysis, handling of the parent compound poses less risk to personnel. Also, formaldehyde has a fairly short lifetime in the environment, avoiding the problems of using an environmentally-persistent toxin.
A number of other types of biocides also work by degradation to produce formaldehyde. In some cases the rate of hydrolysis is very high, meaning the biocide is intended to be used as a short-term “shock treatment” to kill bacteria, rather than protection over an extended time period. In other cases, the rate of hydrolysis in dependent on pH, and the biocides will not give a consistent behavior over the pH range (roughly 6–9) likely to be encountered in shipboard seawater/AFFF mixtures.
We expect that while the same concentrations of antimicrobial in the diluted water-AFFF mixture that is used in the commercial foam concentrate would achieve effectiveness for controlling bacteria in the liquid mix, further increased factors (at least by 10× if not 100×) would be required to combat the higher bacteria concentrations from seawater especially those microorganisms in biofilms. If a continued effective presence of antimicrobial can be maintained, it might not be necessary to dose for destroying biofilms protected microbes.
Development of resistance is a potential concern. It might be unlikely that any one biocide will provide a long-term solution. Not all bacteria will be equally susceptible to the compounds and cells could develop resistance over time. A strategy for long-term control might need to include switching biocides periodically.
Whatever means is employed to address H2S generation must not cause a significant decrease in AFFF fire protection capability. Thus, this task focused on literature research to identify mitigation approaches, evaluation of their impact on AFFF properties via laboratory screening evaluations (surface tension and foaming properties), and fire performance field evaluations. Foam film capability to float on less dense flammable liquids depends on adequate reduction of surface tension. Foam must maintain itself without draining (breaking down) too quickly. And most importantly, the modalities performing the most satisfactorily in the laboratory screening evaluations must not show unsatisfactory results in field fire tests.
Since conducting a large number of field tests of fire extinguishment is impractical, we first conducted laboratory measurements to see if additives had a deleterious effect on AFFF properties before proceeding to field testing. Two laboratory tests were employed to give an initial evaluation of the effect of various antimicrobial additives on AFFF properties. One was an evaluation of foaming using the Ross-Miles foaming protocol. This protocol, which is widely used in evaluation of foaming properties but differs from the MIL-F-24385F test, measures the amount of foam generated, its water content, and persistence, by a gravity-feed of liquid into a receptacle. None of the additives tested caused a noticeable difference in the foaming properties of any of the AFFF formulations according to the Ross-Miles test.
The effect of the additives on surfactant performance was studied by recording the dynamic surface tension (DST) of the additive/AFFF combination . DST measures not only the amount by which the surfactant reduces surface tension, but the speed at which it does so by forming a surface monolayer.
Glutaraldehyde is widely used as an effective non-oxidizing sterilizing agent . However, it also proved to be rapidly consumed, leaving no residual protection, and generating a precipitate. For these reasons, and other handling concerns, it was not considered further.
Anaerobic SRB in the AFFF system can be attacked directly with molybdate . Molybdate is not a general antimicrobial but as it is chemically similar to sulfate, it is taken up by the SRB. But the SRB are not able to process it, and it interferes with their viability. If addition of molybdate is employed, it might be necessary to provide increments over time.
Dowacil polycyclic amine antimicrobial formulations were further evaluated in the laboratory, as were molybdate additives. Neither of these additives had a deleterious effect on DST, and both were judged worthy of further investigation, including fire protection performance testing.
Fire Protection Performance
Mixtures used for accelerated aging tests
AFFF concentration (%)
Aging at 65°C (days)
Before oven aging, while none of the AFFF solutions had a noticeable color, the solutions varied considerably in turbidity. The National and especially the 3M AFFF solutions were the cloudiest. After oven aging for 10 days at 65°C all of the solutions containing Dowicil 75 had a yellowish color. National solutions were still cloudy, along with 3M solutions.
Surface Tension Results
Dynamic surface tension was measured with a Kruss BP-2 maximum bubble pressure tensiometer. This instrument allows determination of the short-time rate at which the AFFF components lower the surface tension of water. The instrument gives a set of data of surface tension versus surface age, with the surface tension gradually decreasing as the surface age increases. At large values of surface age (10 s or more) the value of the DST approaches the value of static surface tension determined by a ring tensiometer. Tensiometry was performed on all samples used in the field fire performance tests, as well as mixtures of the four AFFF formulations at 3% and 6% (half-strength and full-strength) in artificial (as opposed to natural) seawater. Unlike the natural seawater samples used in the fire testing, the artificial seawater samples were not aged at an elevated temperature.
Dynamic Surface Tensions (DST) of AFFF mixtures
All Type 6 AFFF mixed at concentrations indicated (6% = full strength; 3% = half strength)
DST@10 s Age (mN/m)
3M @6% artificial seawater
3M @3% artificial seawater
3M @3% natural seawater aged (sample #5)
3M @3% natural seawater + molybdate (.5 g/L)/Dowicil aged (#6)
Chemguard @6% artificial seawater
Chemguard @3% artificial seawater
Chemguard @3% natural seawater aged (#9)
Chemguard @3% natural seawater + molybdate (.5 g/L)/Dowicil aged (#10)
Ansul @6% artificial seawater
Ansul @3% artificial seawater
Ansul @3% natural seawater aged (#7)
Ansul @3% natural seawater + molybdate (.5 g/L)/Dowicil aged (#8)
National @6% artificial seawater
National @3% artificial seawater
National @3% natural seawater aged (#1)
National @3% natural seawater + Dowicil aged (#2)
National @3% natural seawater + molybdate (5 g/L) aged (#3)
National @3% natural seawater + molybdate (.5 g/L)/Dowicil aged (#4)
As noted in the table, all the MilSpec QPL Type 6 AFFF mixtures evaluated demonstrated some degradation in surface tension reduction going from full strength @6% to half strength @3% (artificial seawater, not higher temperature aged). There is only half as much surfactant and it must diffuse from the bulk liquid to the surface and then align. The decrease in surface tension was least for 3M, increasing slightly through Chemguard and Ansul, and greatest with National AFFF.
3M: Some degradation going from full strength to half strength, but still better or equal to full strength of other formulations. No significant effect at 10 s of natural seawater, aging, or adduct treatment.
Chemguard: Some degradation going from full strength to half strength. Control in natural seawater not degraded with aging. Some degradation of natural seawater formulation aged with adducts, but still lower surface tension than all Ansul and National half strength mixtures.
Ansul: Some degradation going from full strength to half strength. Ansul aged half strength natural seawater close to artificial seawater (bit better, probably not significant difference); significant surface tension increase with adducts.
National: Degraded going from full strength to half strength. Very significant degradation of control and all adduct mixtures aged in natural seawater.
Accelerated aging tests were previously performed with 0.5 g/L glutaraldehyde adduct on the four Type 6 QPL certified AFFFs at full strength in artificial seawater (prepared with salt composition and concentration as specified in ASTM D-1141-52 using Lake Products Co. Inc. Sea Salt), using the same aging protocol. These tests were focused primarily on the stability of glutaraldehyde in AFFF/seawater mixtures, and thus do not provide a direct comparison of the effect of artificial versus natural seawater on AFFF stability and surface tension. No significant changes were observed in DST for any of the AFFFs in the artificial seawater/glutaraldehyde between the unheated and aged samples. This indicates that either the increase observed for National Foam in the aging tests in natural seawater only happens in natural seawater, or (less likely) the glutaraldehyde affected the results.
Fire Test Performance Evaluations
Fire extinguishment and burnback times for aged formulations (protocol of MilSpec MIL-F-24385F)
Extinguishment time (MIL Spec: ≤45 s)
Burnback Time (MIL Spec: ≥300 s)
3M control (sample #5)
3M w/adducts (#6)
Chemguard control (#9)
Chemguard w/adducts (#10)
Ansul control (#7)
Ansul w/adducts (#8)
National control (#1)
National w/adducts (#4)
In the table, the time results are compared to the performance times required by the AFFF specification. This is done for illustration only. The conditions used are not the same as those used for the specification protocols (MIL-F-24385F does not include a half strength test of aged AFFF in salt water). As noted above, test conditions for this task were selected to be more stringent to better allow differentiation of any performance decrement. The significant comparison is the degree of performance decrement relative to the control tests.
Fire Test Performance and DST Correlations
Ansul and National foams showed decreased fire extinguishment performance compared with 3M and Chemguard products. Ansul and National AFFF performance decreased further with adducts. The performance of all National tested mixtures decreased markedly when aged in natural seawater. These fire test results correspond very well with DST results. DST is a valid predictor for fire extinguishment capability. Those mixtures whose DST exceeded 22 mN/m at a surface age of 10 s had significant increases in extinguishment times. Mixtures with 10 s DST values lower than 22 mN/m all had extinguishment times clustered in a narrow range from 32 to 35 s. Burnback properties (more dependent on foam resilience and resistance to radiant energy, not on film formation or spreading) do not correlate with DST, with all mixtures exceeding the minimum required burnback times.
Several approaches to mitigating H2S formation from stagnant seawater/AFFF have been considered and eliminated. Antimicrobials offer an option without decreasing fire protection performance for at least one currently available Qualified Procurement List AFFF commercial product. Thus, if the techniques explored in this work prove to be effective in mitigating H2S production, there is an acceptable option available. NAVSEA is conducting antimicrobial effectiveness evaluations under realistic conditions using firemain and AFFF piping aboard inactive US Navy ships, with preliminary results indicating their validity.
Naval Sea Systems Command (NAVSEA O5P4), the entity responsible for the AFFF military specification, has sponsored this effort. This project benefited from contributions by Douglas Barylski, NAVSEA; MPR Associates personnel, especially Lynessa Ehrler and John Hilliert, as contractors for NAVSEA; and a number of NRL people, including John Farley, Clarence Whitehurst, Will Bricker, Leila Hamdan and Brenda Little.