Firefighting foams: fire or runoff
Published: 01 January, 2006
It was five years ago that 3M announced (May 2000, shortly before InterSchutz 2000) that the company was withdrawing from fluorosurfactant manufacture using perfluorooctanyl sulphonate (PFOS) chemistry.
It was five years ago that 3M announced (May 2000, shortly before InterSchutz 2000) that the company was withdrawing from fluorosurfactant manufacture using perfluorooctanyl sulphonate (PFOS) chemistry.
Further debate about this followed with two Institute of Fire Engineers foam seminars held at the Reebok Stadium in August 2002 and December 2004. Ultimately, this culminated in a UK Department of Environment, Food and Rural Affairs (DEFRA) consultative document at the end of 2004 on the phasing-out and banning of PFOS-based fluorosurfactants.
What is clear is that the environmental impact of firefighting foams has now become of increasing concern both to the fire engineering industry and to end-users.
Commonly-used foams and
international standards
Typically, the firefighting foam may be of three different types: low expansion, medium expansion and high expansion foams, classified according to the ratio of air to liquid in the foam.
The European Standard EN 1568 specifies that high expansion foam shall have an expansion ratio of not less than 500, defined as the volume of a foam sample in litres divided by its mass in kilograms, medium expansion foam of not less than 50, with low expansion foam having a ratio of less than 50.
In practical terms this means that a high expansion foam to EN 1568 will contain a maximum of 2 grams of foam solution per litre of finished foam, i.e., ~2 litres of foam concentrate per cubic metre of finished foam. EN 1568 is also concerned with drainage rates and extinction times.
High expansion foams are particularly useful for filling large voids or building spaces, with foam generators often water-turbine powered. Whether a foam is low, medium or high expansion depends on both the properties of the foam concentrate and the equipment used to proportion or mix the concentrate with water.
Successful production of firefighting foam operationally depends upon (i) using the correct concentrate, whether this be a Class A or Class B product, (ii) mixing the concentrate with water in correct proportion according to the manufacturer’s instructions, e.g., 1%, 3% or 6%, (iii) using properly maintained foam inductors or proportioners and branch-pipes running at recommended feed pressures and flow rates, and finally (iv) having adequate foam concentrate and water supplies available.
Large foam monitors as used, for example, by the petrochemical industry, which may be capable of generating up to 20,000 l/min of finished foam not only require 20 tons of water a minute (obviously!) but also 1200 L/minute (1.2 tonnes) of foam concentrate for a 6% finished product.
Very large incidents, such as at waste disposal sites, chemical process plant, refineries, or even extensive wildland fires, may stretch the local availability of foam concentrate stocks. Many manufacturers maintain large stocks of concentrate centrally that can be called on by local fire brigades when needed.
PFOS-type fluorosurfactants
There seems to be much confusion in the industry as to what constitutes a ‘PFOS’ foam and what it is that is environmentally damaging? Perfluorooctanyl sulphonyl fluoride (C8F17SO2F) - not the chloride as was incorrectly stated in the last issue of F&R - was the chemical feedstock used in the manufacture of 3M PFOS-type fluorosurfactants.
These fluorosurfactants degrade in the environment to give perfluorooctanyl sulphonate (PFOS). The fluorosurfactants themselves can be thought of as containing PFOS in derivative form.
Thus it is quite correct to speak of a PFOS foam concentrate, meaning a foam formulation which both contains derivatised PFOS and degrades in the environment to give PFOS, which has been shown to have an unacceptable PBT profile (PBT = persistence, bio-accumulation and toxicity).
Foam concentrate or finished foam does not, however, contain free PFOS unless it has degraded. PFOS has been shown to be widely dispersed worldwide both in man and other animal species, including in those with no apparent history of exposure.
Human serum samples, which are now generally contaminated, contained no detectable organic fluorine compounds prior to the manufacture of these fluorine-containing compounds in the 1940s and 1950s.
PFOS bio-accumulates in animal tissues with an accumulation ratio between 6,000 and 120,000, as shown by the Etobicoke incident at Toronto International Airport in June 2000. PFOS is toxic to biological systems and the lithium salt of PFOS is an insecticide. PFOS causes disturbances in lipid and steroid metabolism and has also been implicated in human bladder and prostatic carcinoma.
Looking at the chemistry
It is also technically quite correct, although potentially misleading, to say that the fluorotelomer fluorosurfactant foams are PFOS-free and do not degrade to PFOS, as these foams degrade to a chemical entity which is similar to PFOS.
Fluorotelomer foams contain, typically, a partially fluorinated tail attached to a sulphur atom, such as C6F13CH2CH2-S-, as part of a sulphonamide. Under appropriate environmental conditions, fluorotelomer foams can degrade to either perfluorocarboxylic acids, CnF2n+1COOH, or the fluorotelomer sulphonates, such as C6F13CH2CH2SO3-, also known as 6:2 FtS or H-PFOS and which contains a total of eight (8) carbon atoms, NOT six (6) as often claimed. Results in the scientific literature strongly indicate that 6:2 FtS should degrade to 6-carbon perfluorohexanoic acid (PFHxA : C5H11COOH) in activated sewage sludge through a bacterial partial dehydrofluorination process involving the -CF2-CH2- linkage. That this does not appear to happen in groundwater (see below) is probably due to the sampled groundwater being substantially sterile, i.e., bacteria-free, and oxygen-free. It is, however, known that perfluorochemicals, including perfluoro-carboxylates and perfluoroalkyl sulphonates, become concentrated in wastewater treatment sludge (Higgins et al., 2005).
Contrary to claims in the trade press and even in the scientific literature, possibly intended to deflect environmental scrutiny from the fluorotelomers, the fluorotelomer sulphonate (6:2 FtS or H-PFOS) is chemically very similar to PFOS and is considered a structural analogue of PFOS by practising chemists - both have a chain length of eight (8) carbon atoms only being distinguished by four fewer fluorine atoms and the -CH2-CH2- unit in 6:2 FtS. (see molecule illustration)
The correct IUPAC name for 6:2 FtS (H-PFOS) is 1H,1’H,2H,2’H-perfluorooctane sulphonic acid. Structural analogues of the same chain length often have similar physical and chemical properties, showing a progression of these properties based on chain length.
The 6:2 FtS fluorotelomer sulphonate has been found at very high levels in groundwater and to be extremely persistent at old military firefighting training sites. Indeed 6:2 FtS (H-PFOS) may be even more stable than PFOS itself based on the measured groundwater concentrations (Schultz, Barofsky and Field (2004)).
Currently we do not know for certain whether fluorotelomer sulphonates also bio-accumulate and are biologically toxic, although their structural similarities with PFOS should produce a high index of suspicion. More independent research is urgently needed to answer these environmental questions.
Another source of confusion concerns the difference in potential environmental impact between foam concentrate or finished foam and their degradation products. There would appear to be a very low risk of toxicity from using foam concentrate or finished foam either to firefighters or the environment, based on the manufacturers’ Material Safety Datasheets (MSDS).
Groundwater issues
Fluorotelomer surfactants, like all fluorosurfactants irrespective of their means of manufacture, are detergents and as such have acute biological toxicity to aquatic systems.
The long-term problem is, however, that fluorotelomer environmental degradation products are extremely persistent, surviving in groundwater for at least decade, and are, as yet, of unknown bio-accumulative capacity and toxicity.
There is no ‘safe’ level for discharge of a substance which will accumulate indefinitely within an environmental compartment and which is of unknown biological activity. It is well nigh impossible to remove a contaminant from groundwater once it is there!
All fluorosurfactant foams, whether PFOS or fluorotelomer based, break down to chemically stable, long-lived organohalogen degradation products. Dispersive use where containment and disposal may not possible, such as in some firefighting operations, is likely to result in discharge of contaminated run-off to groundwater or controlled waters.
Under List 1 of the UK Groundwater Regulations 1998, discharge to groundwater of organohalogen compounds or materials than can degrade to organohalogen compounds, a category which includes all fluorosurfactants, is explicitly prohibited.
Materials can only be downgraded to List 2 if the Enforcing Authority is prepared to issue a derogation in writing based on a lack of persistence, bio-accumulation and toxicity. Clearly not the case for fluorosurfactants.
Concentrate in sewage
sludge
The UK regulations are based on the implementation of the EU Groundwater Directive 1979 and all EU member states have the obligation to subsume this directive within their national legislation under the concept of subsidiarity. Thus uncontrolled discharge of fluorosurfactants and their degradation products to groundwater is not only environmentally irresponsible but actually illegal in the United Kingdom with the possibility of prosecution, and probably also in all other European countries which have incorporated the EU Groundwater Directive.
Even discharge of fluorosurfactants to the foul sewer may be fraught with difficulties, although technically legal so long as the water authorities are informed.
A recent paper from Jennifer Field’s group has shown that perfluorinated degradation products appear to be concentrated in sewage sludge from water treatment plants. In the UK at least, most of this sludge goes to land-fill although some is also put directly onto agricultural land. This represents another fairly effective way of further contaminating groundwater!
Fire extinguishers with PFOS
The manufacture of PFOS-based fluorosurfactants has now been phased out worldwide. There remain, however, large stocks of PFOS foam concentrate held by the petrochemical and process industries, as well as smaller quantities held by municipal fire services.
An often overlooked problem is that of hand-held foam fire extinguishers which, for maintenance purposes, must be discharged regularly and refilled. These extinguishers are most frequently discharged by maintenance personnel down car park drains, onto the grass verge or down toilets, without checking first where the surface water is going to or whether they need to inform their local waste water treatment plant.
Indeed, water treatment undertakings may not even be aware of the problem of fluorosurfactants being organohalogens under the Groundwater Regulations! There are a very large number of extinguishers in use representing a very considerable and potentially unknown reservoir of PFOS-based foam.
Phasing out PFOS
In a sense PFOS foams are history. What remains is to be decided is the way in which the exit strategy will be managed at regulatory level.
DEFRA in the UK is taking a European lead in this respect, having formulated a strategy which, if accepted by other European member states, will result in there being a five-year derogation period for the phasing out of all use of PFOS-based fluorosurfactants with the ultimate high temperature incineration of remaining PFOS-containing material.
High-temperature incineration of the fluorosurfactant residues is environmentally friendly as scrubbing of the flues gases containing hydrogen fluoride (HF) yields calcium fluoride (CaF2), the naturally occurring mineral fluorite or fluorspar originally used as the source of the fluorine atoms.
Whether this regulatory strategy is subsequently applied to all fluorosurfactants, not just those derived from PFOS but also including the fluorotelomers, remains to be seen and will be determined by environmental and toxicological studies in progress.
What risk reduction strategies are available to fire brigades for reducing the environmental impact of firefighting foams?
The responsible approach for Procurement Officers is to consider replacing foam stocks, as use permits, with either AFFF and AFFF-AR low-fluorine foam concentrates or even with fluorine-free formulations depending on operational imperatives.
For many municipal fire brigades fluorine-free foam may be a realistic option as a number of these preparations have now achieved full compliance with the test standards and do show performance equivalent to classical fluorosurfactant AFFF, such as Arctic Foam’s RF-6(tm) series or Bio-For(tm) and Ecopol(tm) from Bio-Ex.
As pointed out recently by Ted Schaefer from 3M Australia at the Reebok Seminar in December 2004, it is not necessary to have film-forming in order to get AFFF-like extinguishing capability.
Moreover, municipal fire brigades often use firefighting foam under true dispersive conditions under which containment is all but impossible and contamination of the water table all but certain. In this sort of situation the use of a long-term environmentally ‘clean’ foam - for example, a fluorine-free Class B or a Class A foam - is a natural solution.
Real problems remain, however, for the petrochemical and chemical process industry. Here Class B foams are required with extreme performance capabilities in terms of spreading, burn-back, fuel pickup or foam blanket stability, for large hydrocarbon tank fires, currently only satisfied by fluoro-protein formulations such as Angus Fire’s FP70 Plus(tm).
Even then containment of foam-contaminated run-off is necessary in order to protect the environment. For municipal fire brigades, however, there may be little logic in using a high-performance Class B AFFF-AR foam if the local area risk inventory does not warrant it.
How many local authority fire brigades actually need such a high specification and relatively expensive product? Is it not perhaps part unnecessary overkill or a macho attitude associated with failure to conduct a suitable and sufficient assessment of risk?
A high-risk strategy
Fire services often try to avoid having a multiplicity of extinguishing media available on appliances, in order to simplify operational procedures, equipment and training. To a harassed Procurement and Training Officer a so-called ‘one stop’ foam concentrate would have its attractions, if it could be realised.
A recent suggestion in the trade literature that a Class B AFFF-AR fluorosurfactant foam can be used - not only for deep-seated hydrocarbon and alcohol fires at 3% - but also for hydrocarbon spills at 1%, for training at 1% and as a Class A or wetting agent at 0.8%, is environmentally irresponsible.
This represents a high-risk strategy for any operational officers as they would be liable to prosecution under the UK Groundwater Regulations, or its equivalent, and would not have the protection of the Memorandum of Understanding between the Local Authorities and the Environment Agency unless human life and health could be shown to have been at risk - thus excluding training and most Class A wildland fire or structural building incidents.
Moreover, Class B fluorosurfactants, being both hydrophobic and oleophobic, do not penetrate carbonaceous fuels such as wood nearly as well as made to measure Class A hydrocarbon surfactants and are thus not so efficient.
In addition, most fire service appliances with round-pump proportioning only use one set induction rate, so that a ‘one stop’ foam would be used in practice at only 3%. Simplification of operational procedures is all very well but the ‘one size fits all’ philosophy cannot be applied to extinguishing agents!
So if you can minimise or reduce your use of fluorosurfactant-containing foam for fighting fires, without hazarding operational capability, do so by using either low-fluorine or fluorine-free formulations, or even a different extinguishing technology. Do not disregard the environmental issues of using fluorosurfactants!
