Problems with Fat, Oil, and Grease (FOG) in Food Industry Wastewaters and Recovered FOG Recycling Methods Using Anaerobic Co-Digestion: A Short Review

Abstract

Food production industry is the main producer of wastewaters with high fat, oil, and grease (FOG) content. FOG waste can be recovered from the wastewater stream by using physicochemical methods and reused considering their high methane potential. Popular method of FOG reuse is anaerobic co-digestion (AcoD) with wastewater treatment sludge and scum from the primary wastewater sedimentation tanks. This short review focuses on understanding the efficiency of FOG reuse possibilities by AcoD and takes a closer look at problems connected to degradation issues, gives an understanding on microbial changes during FOG degradation, and reviews enzymes involved in the degradation process.

Full text

Problems with Fat, Oil, and Grease (FOG) in Food Industry Wastewaters
and Recovered FOG Recycling Methods Using Anaerobic Co-Digestion:
a Short Review
Elvis Klaucansa*, Karlis Samsb
AURAVIA LATVIA Ltd., Mukusalas 41B, Riga, Latvia
aelvis.klaucans@auravia.lv, bkarlis.sams@auravia.lv
Keywords: fat, oil, grease, FOG, anaerobic codigestion, enzyme usage in biodegradation, food
processing wastewater
Abstract. Food production industry is the main producer of wastewaters with high fat, oil, and grease
(FOG) content. FOG waste can be recovered from the wastewater stream by using physicochemical
methods and reused considering their high methane potential. Popular method of FOG reuse is
anaerobic co-digestion (AcoD) with wastewater treatment sludge and scum from the primary
wastewater sedimentation tanks. This short review focuses on understanding the efficiency of FOG
reuse possibilities by AcoD and takes a closer look at problems connected to degradation issues, gives
an understanding on microbial changes during FOG degradation, and reviews enzymes involved in
the degradation process.
Introduction
Food production facilities are some of the biggest clean water consumers in municipalities due to
their technological demands. Food production wastewaters (WW) usually contain high organic
content together with increased fat, oil, and grease concentrations. Studies showed that FOG
production per capita in developing countries is increasing, and it was 50 kg/year per citizen in 2015;
whereas, in non-developing countries it reached around 20 kg/year on average per citizen [1]. FOG
suppresses WW treatment systems by clogging the pipelines and eliminating further wastewater
treatment processes [2]. There are various options for FOG recovery from WW stream and for use as
renewable materials in order to replace high quality grade raw materials and materials of fossil origin.
The aim of this short review is to highlight the major sources of FOG, to see how their
physicochemical characteristics influence WW treatment, to estimate suitable recovered FOG reuse
techniques with focus on FOG anaerobic co-digestion process, to understand microbial changes, and
to state enzymes and their role in FOG digestion over biogas formation process.
Characteristics of Food Processing Wastewater
Physicochemical characteristics of food processing WW are varying due to food processing
technologies used [3], but they all has similar problems, e.g., high biological oxygen demand (BOD),
increased total suspended solid (TSS) content, they are rich in protein and total nitrogen (Ntot), and
have high FOG concentration. Fat, oil, and grease emission in WW comes from fish filleting, meat
cutting and primary processing, high and low temperature treatment, and product cooking using deep-
frying methods. Table 1 represents the average WW chemical compositions for seafood and meat
processing plants, which use smoking techniques [4], as well as average restaurant WW effluent in
comparison to typical municipal WW parameters [5].

Table 1. Wastewater chemical composition in meat and seafood processing in comparison to typical
municipal wastewaters
Parameter
See food processing
wastewaters [mg/L]
Meat processing
wastewaters
[mg/L]
Restaurant
wastewaters
[mg/L]
Typical municipal
wastewaters [mg/L]
Total COD
8 000–18 700
9 600–12 900
1250–4500
210–740
Total BOD
1000–72 000
2 500–8 000
820–3000
150–350
Total suspended solids
(TSS)
500–2000
790–3 350
220–2700
120–450
Total Nitrogen (Ntot)
200 - 300
230–260
–
20–80
Total Phosphorus
(P tot)
–
30–50
–
6–23
FOG [mg/L]
250–5000
100–2000
140–4100
–
Physicochemical Characteristics of FOG and Influence on Wastewater Treatment Process
High FOG concentration in WW is discussed as one of the most growing and hard to resolve
problems due to physicochemical characteristics of waste and problems with its handling during cold
seasons. FOG mainly consists of long chain fatty acids (LCFA) that are bonded to glycerol, esters,
waxes, phospholipids, sterols, and sterol esters. Due to the high concentration of LCFA, it has a
slightly acidic pH value. Fatty acid content in FOG varies but generally wastes are rich in oleic acid
(18:1 cis), elaidic acid (C18:1 trans) and palmitic acid (C16:0) which are hard to degrade and cause
toxic effect to microflora. Material is not classified as biohazardous although its biodegradation is
inhibited by its physicochemical abilities. [6]. Chemical composition of FOG in combination with its
nonpolar nature and specific gravity (less than 1.0) makes them float on top of the water surface. At
room temperature and higher FOG is in semi liquid state and is readily mixable with water. However,
the decrease of temperature results in solidification, and at this state it is hardly movable and
completely water immiscible [7]. FOG tends to stick on nonpolar surfaces, such as fermenter walls
and pipe walls, causing pipe clogging and decreases their lifetime due to corrosive anaerobic
processes and hydrogen sulfide gas formation. FOG’s viscosity correlates with concentration of
unsaturated fatty acids in triglyceride ester composition – the higher the concentration, the lower the
viscosity of FOG. Ineffectively pretreated food processing WW with high FOG content increases the
organic load rate on the further aerobic treatment process and affects the oxygen mass transfer in
aeration tanks [8] as well as sludge dewatering processes by adhesion on biomass surface [9].
FOG Removal from Wastewater Stream
Most of WW pretreatment systems are able to remove up to 90% of FOG from effluent [3]. FOG
removal has to be considered in small food processing facilities and restaurant business scale sites by
gravity or hydro chemical grease traps. The main waste product of this treatment is grease trap waste
[10] with approximate organic matter content ranged from 50 to 80 w/w% [11]. Advanced removal
techniques, e. g., primary sedimentation, dissolved air flotation units, FOG screens, are applied at
industrial food processing facilities and wastewater treatment plants [12]. Depending on technique
used, waste product is classified as waste scum and can contain form 8-50 w/w% of organic matter
[13].
Utilization and Reuse of Recovered FOG
There are numerous possibilities for FOG reuse because of its high energetic value, e.g., agricultural
sector in land farming applications and composting. However, it can have a bad influence on roots of
plants by disturbing the water uptake during regular fertilizing. Deposition of FOG in municipal
landfills is another disposal method; it is considered to be one of the less favorable from

environmental and economical points of view because of methane production during FOG
decomposition and the unused economic potential [4]. Turning FOG into renewable energy is more
reasonable by using transesterification methods and producing biodiesel as alternative to fossil fuel,
and in such way decreasing high quality oil demand [14, 15]. However, many authors point out
problems of recovered FOG: varying chemical composition and differences between industrial brown
sewer grease, WW scum, and grease trap waste from restaurants that affects esterification reaction
[16, 17]. FOG reuse using biological methods is the most effective because of minor effect of varying
material characteristics. Recovered FOG can be used in WW treatment sludge anaerobic co-digestion
(AcoD) processes as extra carbon source in order to increase methane yield. Nevertheless, there are
still many unresolved technical problems with recovered FOG use due to the tendency of fatty clump
formation in system and grease cap formation on top of digestion mass [18]. Use of enzymes and bio-
surfactants has a high potential to enhance FOG digestion efficiency and total methane yield in
anaerobic digestion [19].
Anaerobic Digestion of High Content FOG Waste
There is an open research on FOG waste usage in AcoD aimed at increasing methane yield. Lipids
have 10 times higher biogas potential than carbohydrates and 2 times higher biogas potential than
proteins [20]. Over load of FOG in co–digestion may lead to process inhibition, which is mainly
caused by toxic effect of LCFA accumulation and hydrophobic properties of FOG, such as mass
transfer limitations, sludge flotation and washout, digester foaming, pump and pipe blockages [21].
Many anaerobic digestion plants are not suitable for FOG co-digestion due to technological design.
It is discovered that step feed AcoD methods are more suitable for establishing the optimal organic
loading rate for each individual type of digestion technologies [24]. Technical updates in substrate
dosage combined with bio-surfactant use to improve FOG emulsification can help to achieve even
178 % increase in methane yield for anaerobic digestion plants using grease interceptor waste as co-
digestion substrate for municipal wastewater treatment plant secondary sludge (SS) [22, 23]. Table 2
shows mesophilic semi-continuous AcoD efficiency for FOG containing substrates from different
origin and summarises popular process problems. There is a lack of information about FOG co-
digestion with agro-industrial origin substrates. Agro-industrial biogas production would be able to
benefit from FOG use as alternative source to decrease corn and maize demand, which is manly
subsidized by state governments. Anaerobic reactor techniques, e.g., up-flow anaerobic sludge
blanket reactor and inverse anaerobic sludge blanket reactor process, are promising techniques for
WW treatment with high FOG content [25, 4]. Technology offers to treat WW with high organic
content without separating FOG from WW stream; this would be beneficial for small scale food
factories which are located far from municipal WW treatment plants. Another beneficial aspect of
anaerobic WW treatmet is less activated sludge discharge compared to aerobic treatment technologies
due to methane production. It is possible to turn up to 95% of organic matter into biogas by achieving
up to 600 LCH4/kg VS [26]. The process is sensitive to FOG concentration variance in feed stock
causing sludge flotation. Problem could be solved by use of anaerobic membrane bioreactor
technologies that solve biomass washout problems and increase treatment efficiency [27, 28].

Table 2. FOG co-digestion methods
FOG source
FOG
content
FOG
Loaded
from
total
VS*
HRTa
[d]
OLRb
[gVS/L/day]
Methane yield
[LCH4/kgVSadd]
Biogas
increase
[%]
Methane
conc.
[%]
Stated problems
Ref.
Grease interceptor
waste from
municipal waste
water treatment
plant.
10 v/v%
65.5
v/v%
20
133
752
317
66,9
Process inhibition after 84%
addition of FOG from total
VS.
Unstable substrate dosage.
Unstable process
performance.
[23]
FOG collecting
facility waste
2.97 w/w%
64%
15
2.34
598
137
66,8
Process inhibition after 74%
addition of FOG from total
VS.
Long microbial acclimation
time and recovery time.
[29]
Fat waste from veal
cutting
98 w/v%
2.5%
40
1.3
520
219
61
pH drop during incubation
time.
Low organic loading rates
due to LCFA inhibition.
[30]
Extracted FOG from
restaurant food
waste
71.8 w/v%
42%
20
5.2
n. d.c
45.4
n. d.
Process inhibition after 55%
addition of FOG from total
VS.
Unstable digestion process.
LCFA accumulation.
[31]
Grease trap sludge
from meat
processing plant
57%
30%
20
1.15
456
52
67.5
Negative impact of Oleic
acid deposition.
[32]
Grease sludge from
primary clarifier
skimmings in
municipal WW
treatment plant
64%
40%
15
3.1
660d
46
70
Process inhibition and
instability from 60-90% of
FOG addition from total VS.
Foaming effect.
[33]
aHRT – hydrolic retention time; bORL – organic loading rate; cn. d. – not determined; d– value stated as total biogas production

Microbial Cultures Involved in FOG Digestion
Understanding the changes in microbiome induced by FOG digestion using AcoD is one of the key
factors for successful FOG conversion to biogas. Syntrophic β-oxidizing bacteria work as primary
triglyceride digesters, and their relative abundance increases together with LCFA concentration.
During the changes of microbial consortium, an increase of Proteobacteria - Syntropohomonas sp.
species is detected and it inhibits Methanogenic arhea concentration [34]. LCFA deposition inhibits
acetoclastic and methanogenic arhea growth disrupting their metabolism [35], which slow down
hydrolysate conversion into methane [36, 37]. Lately researchers have found that Gram-negative
facultative anaerobic Proteobacteria Rheinheimera sp. and Bacillus sp. can digest FOG under
anaerobic conditions and decrease LCFA deposition [38]. Well-known Pseudomonas putida strains
can use LCFA as sole carbon sources [39] by help of self-synthesized ramnolipids; therefore, it is
important to enhance abundance of these cultures in anaerobic digestion [40] to decrease toxic effect
of LCFA. There is open discussion about substrate pre-treatment with fungi from Penicillium sp.,
Aspergillus sp. and Rhizopus sp. species [41], but one of the main disadvantages is that FOG substrate
has to be dewatered and made to undergo solid state pre-treatment, which slows down the mass flow
and has unfavourable maintenance [42]. Microbial enzyme dosage could cover the gaps in the biogas
formation process by helping to prepare substrates for the next fermentation steps.
Enzymes Involved in FOG Degradation
Complex triglycerides are broken down into glycerol and LCFA with help of extracellular enzymes
of bacteria and fungi. Degradation process involves complex enzyme content that consists of lipases,
esterases, proteases, and amylases [43]. While lipases are mainly involved in insoluble long chain
triglyceride decomposition, esterase converts partly soluble molecules [44]. There are studies on
recombinant lipase use from E. coli that is capable to keep the hydrolytic ability over broad
temperature and pH ranges and that shows better performance than using wild-type lipase ferments
[45]. Research on immobilized lipase derived from Candida rugosa showed a 65% recovery potential
with only 30% enzyme activity decrease after digestion. These data provide new evidence of
economic potential of immobilized enzymes in FOG co-digestion technologies [46].
Conclusions
Research shows that FOG is one of the top problems in WW treatment both from industrial and
restaurant scale origins due to their physicochemical characteristics and degradation bi-products.
Reuse of recovered FOG by AcoD with SS is one of the most popular methods, but it is still facing
unresolved problems with FOG dosage and toxic effect from LCFA accumulation. Promising tools
for problem solving is use of bio-surfactants and lipolytic enzyme producing bacteria bio-
augmentation. There is also luck of information for FOG usage in Agro-industrial biogas production
which could decrease green mass usage. Further investigation is needed to more understand influence
of LCFA on the process as well as to find the optimum balance between the abundance of bacteria
and methanogenic arhea.
Acknowledgements
This study was supported by the European Regional Development Fund
(ERDF) within the framework of “Latvian Food Competence centre”
Ltd. project Nr. 1.2.1.1/16/A/004 “LATVIAN FOOD INDUSTRY
COMPETENCE CENTER” research project Nr.19, conducted by
AURAVIA LATVIA Ltd.

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