Cement kiln incineration offers an environmentally sustainable solution.
Class B AFFF (aqueous film-forming foams) contain fluorochemicals now generically known as PFAS (perfluoroalkyl substances). Disposal of large volumes of PFAS-contaminated liquid waste remains a major problem, as well as potentially a hugely expensive one, for all Fire Services whether generated as firewater run-off at incidents, or during training, testing and maintenance procedures. Recent development in global PFAS concerns and restrictions mean that there is the need to dispose of legacy AFFF-type firefighting foam concentrate. AFFF formulations originally contained PFOS (perfluorooctane sulphonic acid) and PFHxS (perfluorohexane sulphonic acid), while the later generation Class B AFFF foams contained C8/C6 fluorotelomers without PFOS/PFHxS. More recently so-called ‘C6-pure’ formulations have been produced with C8 fluorotelomer content drastically reduced to <25ppb in order to prevent generation of PFOA (perfluorooctanoic acid). However, with emerging evidence about adverse effects for all PFAS the distinction between the various PFAS has become largely academic as all PFAS are, or breakdown to, highly persistent bio-accumulative, chronically toxic and dispersive pollutants.
Historically uncontained use with run-off soaking into open ground or dilution and discharge into sewers and waterways were cited as appropriate methods of use and disposal for Class B AFFF firefighting foam. However, this has resulted in widespread pollution due to the extreme environmental and biological persistence, long-range environmental transport world-wide, and bioaccumulation with biomagnification up the food chain of toxic PFAS compounds. Fire Services were initially misled by information from manufacturers and suppliers that AFFFs were biodegradable. While this may apply to some of the organic compounds in AFFF, it is not the case for PFAS that are ultimately highly resistant to physical, chemical and biological degradation.
The move to fluorine-free Class B firefighting foams began some twenty years ago with the announcement by the 3M Company in May 20001 that PFOS-based chemistry was being phased-out immediately affecting the availability of certain AFFF products such as 3M LightWater and ATC foams. This was followed by the 2010–2015 PFOA Stewardship Program with part of the industry moving to eliminate C8 chemistry (PFOA and related compounds)2 based partly on the misconception that C6 and shorter-chain PFAS were environmentally and toxicologically benign; a classic example of ‘regrettable substitution’ of one contaminant for another.3
Widespread and growing evidence of a wide diversity of adverse human health, socio-economic and environmental impacts of PFAS pollution prompted major national and international regulatory changes aimed at restricting products containing PFAS as a class rather than the ineffective approach targeting individual components.3 This has resulted in a seismic market revolution towards developing fluorine-free firefighting foams equal to or exceeding the effectiveness or AFFF.
These regulatory restrictions include the UN Stockholm Convention listing of PFOS, PFHxS and PFOA (and their related compounds) in the appropriate Annexes banning or limiting their use,5 legislation passed at EU level ultimately aimed at restricting all PFAS,6 and the very recent US EPA health advisories reducing permissible levels in drinking water for PFOA from 70ppt to 0.004ppt and for PFOS from 70ppt to 0.02ppt, both below the current levels of detection, which are in the region of 2 to 20ppt, so that any detectable level is of concern.4
With the phase-out of AFFF and other PFAS-containing foams has come the problem of waste disposal. PFAS can only be destroyed under very extreme conditions to break the very resistant C-F bond. Extreme chemical methods have not proved viable leaving high-temperature incineration as the main option with few appropriate facilities available. The high cost of PFAS disposal and system decontamination has often provided a financial disincentive to transitioning to fluorine-free alternatives despite the very considerable legal and reputational liability of continued use.
In the first of two articles we outline basic methods for handling PFAS-contaminated liquid waste, and in the second we discuss the emergence of cement kiln co-processing as a safe, accessible and cost-effective option for PFAS destruction that can be implemented under strict licensing and regulatory control.
Many examples exist worldwide in which groundwater contamination has resulted from PFAS foam use including impacts on drinking water supplies, especially near military airfields where PFAS foams were released during training, testing or system faults. One of the most egregious and well documented examples occurred at the Australian Defence Force’s Oakey (Queensland) Army Helicopter base in which 1.43 million litres of AFFF concentrate was discharged during training to groundwater over a 25-year period, contaminating the drinking-water supply for the township of Oakey; this amounted to about one IBC (1,000L) of foam concentrate being released regularly to the environment every week for a quarter of a century! A similar pattern of contamination occurred at Tindal RAAF airbase. In both instances the limit of reporting for PFOS/PFHxS in groundwater was the accepted USEPA value of 0.07 microgram/L (70ppt) PFOS for lifetime exposure, more recently drastically reduced to 0.02ppt (0.02ng/L) in June this year. With the exception of a few major incidents such as Coode Island or Buncefield, relatively little PFAS pollution has been the result of use in emergency situations.
High-temperature incineration (>1,100°C for PFOS) with residence times greater than 2 seconds is recommended for destruction of PFAS waste.5 This requires specialist incinerators, is expensive and not particularly efficient; for example, destruction of PFOS requires 485kJ/mol, equivalent to 30 litres of gasoline to destroy one kilogram of PFOS. Incinerator conditions must be very strictly controlled otherwise hydrofluoric acid and fugitive PFAS can be released in the flue gas. Moreover, very high temperature hazardous waste incinerators are not commonly available and normal domestic waste incinerators are completely unsuitable for PFAS destruction.
The Queensland Department of Environment and Science facilitated PFAS destruction trials using a cement rotary kiln in 2016–17 as proof-of-concept for an efficient and environmentally acceptable method of destroying both liquid and solid PFAS-containing waste in large quantities under strictly regulated conditions achieving destruction efficiencies of 99.999% (PFOS, PFOA and PFHxS). Cement Australia (Geocycle) successfully commenced the safe and sustainable destruction of AFFF (PFAS) materials under licence in 2018 at Cement Australia’s Gladstone Queensland cement kiln. Geocycle also began destruction of contaminated soils and solid materials in 2016 through the Railton (Tasmania) cement kiln. Cement kiln destruction of hazardous waste such as poly chloro-biphenyls (PCBs) has been recommended by the Basel Convention5 and has also been used for decades by the aluminium smelting industry for incineration of carbon spent pot-linings (SPL) with very high fluorine content (~10%), which is ultimately captured in the cement as inert calcium fluoride minerals.
In a recent US EPA workshop held in Cincinnati, Ohio, Patterson and Dastgheib (2020) have identified cement kiln destruction of PFAS-loaded ion-exchange resin as simple and cost effective with the resin acting as a useful fuel supplement.6 Moreover, these authors state that ‘… Ion-exchange and activated carbon absorption are identified as the most mature and feasible technologies for PFAS removal …’ [from waste liquids], for example, to extract PFAS from large volumes of firewater run-off at major incidents or from legacy foam concentrate. However, to be effective the ion exchange resin used needs to be compatible with the ionic form of the PFAS in the waste (cationic, anionic and/or zwitterionic).
A technical briefing note from the US EPA (February 2020) points out that ‘… Few experiments have been conducted under oxidative and temperature conditions representative of field-scale incineration …’.7 The Queensland 2016–17 trials and subsequent licensing of cement kiln technology for routine PFAS destruction predates this comment by some considerable time.
Complete destruction or mineralization (to CO2, H2O, HF and fluorine minerals) of perfluoroalkyl substances is made difficult because of the strength of the C-F bond. This normally requires very high temperatures and long residence times. Moreover, the need to scrub toxic and acidic hydrogen fluoride, HF, from the flue gas adds significantly to the cost.
Thermal destruction of PFAS and the mechanisms involved have been recently reviewed extensively by Longendyke et al. (2022), although these authors singularly failed to acknowledge the availability of very high temperature calcium-catalysed incineration in rotary cement kilns in spite of noting that the presence of calcium compounds could trigger defluorination at lower temperatures.8 Or that this technology is routinely used for the destruction of hazardous waste as recommended by the Basel Convention,5 including cement kiln co-processing of fluorine-contaminated carbon spent pot linings from the aluminium smelting industry.
Various methods for PFAS-waste destruction or removal have been investigated, including plasma arc incineration, sonolysis, ozonolysis, catalysed photolysis, catalysed oxidative or reductive cleavage, activated carbon (GAC), ion-exchange resins, electrocoagulation and reverse osmosis.9 These methods are generally expensive, specialised, not commonly available, only concentrate PFAS, suffer from incomplete mineralization, or are only small-scale and not commercialised.
Cement kiln incineration, however, uses existing facilities, does not require modification of the plant or process, is relatively cheap, uses no extra energy, produces no extra CO2, is environmentally sustainable and, most importantly, is able to handle substantial amounts of both liquids and solids. Safety margins for temperatures and residence times are also far higher than standard commercial incinerators. The presence of calcium from limestone and calciferous clay used in clinker production has the dual benefit of catalysing the destruction of PFAS at lower temperatures10 plus it captures the fluorine as inert calcium fluoride, thus avoiding fluorine releases in the kiln flue gas. Rotary cement kiln incineration for PFAS destruction has already been briefly discussed in a previous publication.11
The operation and licensing conditions necessary for efficient and environmentally protective PFAS-containing waste destruction in rotary cement kilns will be discussed in Part 2 of this article.