Fat, oil and grease reduction in commercial kitchen ductwork: A novel biological approach
Introduction
FOG build up in commercial kitchen ventilation ductwork has been an area of concern within the sector for many decades (Farrell et al., 2011), yet this is not reflected in the academic discourse. An extensive literature review yielded little academic work resulting in effective FOG reduction solutions within these hot and humid environments.
The increasing attention towards energy reduction, particularly in the profligate energy using environment of commercial kitchens, calls for closer consideration of the energy impacts of technologies installed in these buildings (Mudie et al., 2013). In the same vein, when considering heat recovery in commercial kitchens, there is no greater barrier to the utilisation of waste heat from kitchen ductwork than FOG deposits (CIBSE, 2009, Fisher et al., 2013). This paper introduces a novel grease reduction system against the background of what is currently known about the composition, measurement and control of FOG in commercial kitchen ducts and pays particular attention to its sustainability.
The removal of FOG from air flows also contributes to increased safety and reduction in insurance premiums. In 2005, the Building Research Establishment (BRE, UK) examined data from the London Fire Brigade and found 700 fires involving the HVAC system in London that year. Of these, the ignition of FOG within the duct represented the second largest principle cause of ventilation fires after the ventilation components. In the USA, each year grease fires result in over $100 million dollars in direct property damage within commercial kitchens (BRE, 2005). The National Fire Protection Agency (NFPA, USA) reports that restaurants are the most at risk from fires spreading into the ductwork (23.72% of all duct fires), and that this is due to grease build up (BRE, 2005). Improperly cleaned air may also result in unpleasant odours and greasy film deposits on neighbouring properties, and this may incur legal issues (DEFRA, 2005). Recent research has attempted to characterise emissions from cooking a variety of foods in a commercial catering environment in terms of volume, particle size and composition (Schauer et al., 1996, Gerstler et al., 1999a, Gerstler et al., 1999b, Kuehn et al., 2008, Kuehn et al., 2009a, Kuehn et al., 2009b, Fisher and Swierczyna, 2014). In general, cooking particulates are made up of hundreds of different long and short chain fatty acids, esters and carboxylic acids, several types of poly-aromatic hydrocarbons (PAHs) and lactones, among many other volatile organic compounds (VOCs) (McDonald et al., 2003, Kuehn et al., 2008, Kuehn et al., 2009a, Kuehn et al., 2009b, Abdullahi et al., 2013).
Source apportionment is particularly challenging as species and classes of a compound are not exclusive to a particular source (Schauer et al., 1996, Schauer et al., 1999, McDonald et al., 2003, Kuehn et al., 2008). A recent review of the field concluded that while further knowledge of source-related chemical composition would be beneficial, quantitative source apportionment remains imprecise and potentially inaccurate in realistic situations with mixed cooking source types contributing to atmospheric concentrations (Abdullahi et al., 2013).
Fig. 1, Fig. 2 provide examples of source-apportioned quantities and particle sizes both in the extract plume and the duct (Kuehn et al., 2008, Kuehn et al., 2009a, Kuehn et al., 2009b). In the plume, the majority of grease mass corresponded to particles >10 µm, primarily originating from a solid fuel broiler and a wok (Kuehn et al., 2008, Kuehn et al., 2009a, Kuehn et al., 2009b). However, the majority of the grease mass emission in the ductwork was found to be in the vapour phase or associated with particles smaller than 1 µm in size (Kuehn et al., 2008, Kuehn et al., 2009a, Kuehn et al., 2009b).
Gerstler et al., 1999a, Gerstler et al., 1999b, Gerstler, 2002 and Sippola and Nazaroff, 2005 performed extensive work concerning deposition of liquid borne particles in exhaust and ventilation ducts. Gerstler, 2002 found that decreasing the exhaust velocity dramatically decreases the rate at which particles deposit on the wall, due to a decrease in flow turbulence. Sippola and Nazaroff, 2005 concluded that deposition to duct walls and ceilings was greatly enhanced in ducts which included bends; developing turbulence was associated with greater deposition than straight ducts with fully developed turbulence. Conversely, it has also been suggested that decreasing the velocity of air flow is likely to adversely impact the indoor air quality and human exposure to pollutants (Rim and Novoselac, 2010).
Sippola and Nazaroff, 2005 also found that deposition levels were greater in insulated ducts compared to uninsulated ducts. It was hypothesised that this was due to the roughness of the insulation, though this was not quantitatively explored. Nevertheless, decreasing insulation levels to reduce FOG deposition is likely to adversely impact any heat recovery potential.
Section snippets
Current approaches to FOG removal
The following sections outline current methods and processes for FOG reduction used in commercial kitchens, and their viability. The UK guidance on cleanliness of ductwork states; “Total grease removal is not normally feasible”, and “Due to (high levels)…the majority of the ductwork cleaning will be by manual rather than mechanical methods.” (BESA, 2013). In practice, manual cleaning may occur every 6 weeks for the heaviest loads (BESA, 2013). Anecdotal evidence suggests that sufficient care
Measurement of deposit thickness and associated regulation
As this study focuses upon examining solid deposition over time, the important work of ASHRAE in developing a method to characterise airborne cooking effluent (ASTM Standard F2519 (ASTM, 2015, Schrock and Knappmiller, 2015)), as well as technologies such as Tapered Element Oscillating Microbalance (TEOM) and piezocrystals were disregarded.
Ramesh (2010) investigated methods of film thickness measurement for the application of grease surface deposit assessment and stated that very little work has
Materials and methods
The objective of the experimental program was to determine if biological agents could reduce the FOG deposits on an already soiled duct, and prevent the further build-up of FOG during operation.
FOG deposit thickness
The level of FOG deposit at each of the four measurement locations was found to decrease over the duration of the experimental period. The initial average thickness of 1239 µm was reduced by 47% to an average of 668 µm in 51 days. The average deposit thickness was determined from the 20 measurements at each location per day to provide an average for that location per day. The averages of each of the four locations were again averaged to provide the total FOG deposit thickness for the duct on each
Conclusions and further work
In response to the issues presented by current FOG reduction technologies, a novel system utilising a reagent containing Bacillus subtilis was trialled in situ, during normal kitchen operation. Experimental data and implications of this study are particularly industrially relevant.
Direct FOG deposit thickness measurement and microflora activity tests confirm that the novel system reduces FOG deposits in commercial kitchen ductwork by 47% in seven weeks. The system mitigates build-up of FOG,
Funding
This work was supported by the Engineering and Physical Sciences Research Council [grant no EP/G037787/1].
Acknowledgement
Special thanks must be made to Mitchells & Butlers plc, James Sharman, Richard Felgate, David Fielding, Dr. Neil Smith, Sue Wyeth-Price and Quintex Ltd (particularly Peter Evans, James Drake and Simon Jarman), without whom this study would not have been possible.
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