increased moisture content required by the biodigester
(Alves 2016).
An important factor in the use of food waste for the
production of biogas and biomethane is the presence of toxic
compounds. In addition to the conditions of temperature,
pH, availability of nutrients, agitation, feeding rate and the
presence of a high-performance microbial community, it is
necessary to ensure that possible toxic compounds are present in concentrations safe for the system. These inhibitors
can be produced during the process or added unreasonably
during feeding. Each inhibitor is different in terms of its
effects, which can occur continuously or intermittently
depending on the diet and the consistency of its composition.
Some inhibitory compounds that can be generated during
the process are ammonia, volatile organic acids and hydrogen sulfide (Chen et al. 2016; Kwietniewska and Tys 2014).
According to Poggi-Varaldo et al. (1997), ammonia is considered an inhibitory compound in mesophilic systems at
concentrations of 2.8–8 g/kg and in thermophilic processes
from 2.5 to 4 g/kg. Ammonium results in the loss of
potassium from methanogenic microorganisms and can have
effects similar to Ca
2+ and Na
+ (Deublein and Steinhauser
2008). The pH has a great influence on the degree of toxicity
of ammonia, which is only toxic in its non-ionized state, just
as it occurs for hydrogen sulfide (H 2 S). The production of
H 2 S is directly proportional to the concentration of sulfates
and proteins in the substrate. According to Mendonça
(2009), the organic load and sulfate ratio greater than 10
does not cause the inhibition of bacteria. When this proportion is exceeded, the sulfate-reducing bacteria start to
compete with the methanogenic bacteria for H 2 and acetate,
reducing the efficiency of the system (Callado et al. 2017).
Feeding with high protein contents can also result in
foaming, which is common when adding large loads of lipid
materials. Foaming can last up to 3 weeks and reduce biogas
formation by 50% (Kougias et al. 2015). To control foaming
ideally, the feed rate should be regulated, avoiding overload.
Alternatively, especially when foaming is unavoidable, the
use of defoamers is used. There are several defoamers
available on the market, but the choice must be carefully
made in order to guarantee non-toxicity to the microbial
community, biodegradability and a minor impact on the
plant’s operating costs.
Heavy metals, such as cadmium, mercury, arsenic and
lead, can act in the enzymatic inhibition, harming the biotransformation of organic matter in terms of yield and speed.
Some metals that have a positive effect at low concentrations, such as zinc and copper, can become toxic if the limit
is exceeded (Abdel-Shafy and Mansour 2014). In addition to
the concentration of metals, toxicity is also influenced by pH
in the biodigester and oxidation state. The effect of pesticides
on biodigestion is still poorly explored, but agricultural
waste is often contaminated; the presence of lindane and
DDT, for example, has already been described as an inhibitor of methane production (approx. 34% at a concentration
of 5 mg/L and 45% at 10 mg/L) (El-Gohary et al. 1986).
6 Conclusion
Anaerobic digestion of food wastes is a complex process that
requires adequate processing of the substrate prior to fermentation. Knowing the chemical composition of this substrate, regarding the content of macro, microelements,
contaminants and potential inhibitors is essential to obtain
stable and active microbial communities that result in controlled and productive processes. For this reason, the biochemical potential of methane should not be considered as
the only parameter for substrate selection. The control of the
process also plays an important role to avoid turbulence in
the system and the formation of inhibitory compounds that
may harm the biogas productivity. The use of food wastes
for the production of biogas is an important tool in reducing
socioeconomic and environmental losses caused by the
considerable losses in the food chain, contributing to the
construction of a society that makes more rational use of the
available natural resources. The Itaipu demonstration unit is
the main reference in Brazil for large-scale biodigestion of
food wastes. Visited by researchers and institutions interested in the theme throughout the year, it contributes significantly to scientific and technological advancement in a
country in which this technology is undergoing
consolidation.
References
Abdel-Shafy, H., & Mansour, M. S. M. (2014). Biogas production as
affected by heavy metals in the anaerobic digestion of sludge.
Egyptian Journal of Petroleum, 23, 409–417. https://doi.org/10.
1016/j.ejpe.2014.09.009.
ABRELPE. (2019). Panorama of Solid Waste in Brazil 2018/2019.
Brazilian Association of Public Cleaning and Special Waste
Companies. http://abrelpe.org.br/panorama. Acessed May 2020.
Al Seadi, T., Rutz, D., et al. (2008). Biogas handbook. Esbjerg:
University of Southern Denmark.
Alves, I. R. F. S. (2016). Evaluation of codigestion in biogas
production. Doctoral Tesis, Federal University of Rio de Janeiro.
Andrade, V. T. (2007). Construction and evaluation of hydrocyclone
models for wastewater treatment. Dissertation, Campinas State
University.
Angenent, L. T., Karim, K., Al-Dahhan, M. H., Wrenn, B. A., &
Domiguez-Espinosa, R. (2004). Production of bioenergy and
biochemicals from industrial and agricultural wastewater. Trends
Biotechnology, 22, 477–485.
Araujo, A. P. C. (2017). Biogas production from organic waste using
Anaerobic Biodigester. Monography: Federal University of
Uberlândia.
Banks, C. J., Zhang, Y., et al. (2012). Trace element requirements for
stable food waste digestion at elevated ammonia concentrations.
106
E. B. Sydney et al.
(Alves 2016).
An important factor in the use of food waste for the
production of biogas and biomethane is the presence of toxic
compounds. In addition to the conditions of temperature,
pH, availability of nutrients, agitation, feeding rate and the
presence of a high-performance microbial community, it is
necessary to ensure that possible toxic compounds are present in concentrations safe for the system. These inhibitors
can be produced during the process or added unreasonably
during feeding. Each inhibitor is different in terms of its
effects, which can occur continuously or intermittently
depending on the diet and the consistency of its composition.
Some inhibitory compounds that can be generated during
the process are ammonia, volatile organic acids and hydrogen sulfide (Chen et al. 2016; Kwietniewska and Tys 2014).
According to Poggi-Varaldo et al. (1997), ammonia is considered an inhibitory compound in mesophilic systems at
concentrations of 2.8–8 g/kg and in thermophilic processes
from 2.5 to 4 g/kg. Ammonium results in the loss of
potassium from methanogenic microorganisms and can have
effects similar to Ca
2+ and Na
+ (Deublein and Steinhauser
2008). The pH has a great influence on the degree of toxicity
of ammonia, which is only toxic in its non-ionized state, just
as it occurs for hydrogen sulfide (H 2 S). The production of
H 2 S is directly proportional to the concentration of sulfates
and proteins in the substrate. According to Mendonça
(2009), the organic load and sulfate ratio greater than 10
does not cause the inhibition of bacteria. When this proportion is exceeded, the sulfate-reducing bacteria start to
compete with the methanogenic bacteria for H 2 and acetate,
reducing the efficiency of the system (Callado et al. 2017).
Feeding with high protein contents can also result in
foaming, which is common when adding large loads of lipid
materials. Foaming can last up to 3 weeks and reduce biogas
formation by 50% (Kougias et al. 2015). To control foaming
ideally, the feed rate should be regulated, avoiding overload.
Alternatively, especially when foaming is unavoidable, the
use of defoamers is used. There are several defoamers
available on the market, but the choice must be carefully
made in order to guarantee non-toxicity to the microbial
community, biodegradability and a minor impact on the
plant’s operating costs.
Heavy metals, such as cadmium, mercury, arsenic and
lead, can act in the enzymatic inhibition, harming the biotransformation of organic matter in terms of yield and speed.
Some metals that have a positive effect at low concentrations, such as zinc and copper, can become toxic if the limit
is exceeded (Abdel-Shafy and Mansour 2014). In addition to
the concentration of metals, toxicity is also influenced by pH
in the biodigester and oxidation state. The effect of pesticides
on biodigestion is still poorly explored, but agricultural
waste is often contaminated; the presence of lindane and
DDT, for example, has already been described as an inhibitor of methane production (approx. 34% at a concentration
of 5 mg/L and 45% at 10 mg/L) (El-Gohary et al. 1986).
6 Conclusion
Anaerobic digestion of food wastes is a complex process that
requires adequate processing of the substrate prior to fermentation. Knowing the chemical composition of this substrate, regarding the content of macro, microelements,
contaminants and potential inhibitors is essential to obtain
stable and active microbial communities that result in controlled and productive processes. For this reason, the biochemical potential of methane should not be considered as
the only parameter for substrate selection. The control of the
process also plays an important role to avoid turbulence in
the system and the formation of inhibitory compounds that
may harm the biogas productivity. The use of food wastes
for the production of biogas is an important tool in reducing
socioeconomic and environmental losses caused by the
considerable losses in the food chain, contributing to the
construction of a society that makes more rational use of the
available natural resources. The Itaipu demonstration unit is
the main reference in Brazil for large-scale biodigestion of
food wastes. Visited by researchers and institutions interested in the theme throughout the year, it contributes significantly to scientific and technological advancement in a
country in which this technology is undergoing
consolidation.
References
Abdel-Shafy, H., & Mansour, M. S. M. (2014). Biogas production as
affected by heavy metals in the anaerobic digestion of sludge.
Egyptian Journal of Petroleum, 23, 409–417. https://doi.org/10.
1016/j.ejpe.2014.09.009.
ABRELPE. (2019). Panorama of Solid Waste in Brazil 2018/2019.
Brazilian Association of Public Cleaning and Special Waste
Companies. http://abrelpe.org.br/panorama. Acessed May 2020.
Al Seadi, T., Rutz, D., et al. (2008). Biogas handbook. Esbjerg:
University of Southern Denmark.
Alves, I. R. F. S. (2016). Evaluation of codigestion in biogas
production. Doctoral Tesis, Federal University of Rio de Janeiro.
Andrade, V. T. (2007). Construction and evaluation of hydrocyclone
models for wastewater treatment. Dissertation, Campinas State
University.
Angenent, L. T., Karim, K., Al-Dahhan, M. H., Wrenn, B. A., &
Domiguez-Espinosa, R. (2004). Production of bioenergy and
biochemicals from industrial and agricultural wastewater. Trends
Biotechnology, 22, 477–485.
Araujo, A. P. C. (2017). Biogas production from organic waste using
Anaerobic Biodigester. Monography: Federal University of
Uberlândia.
Banks, C. J., Zhang, Y., et al. (2012). Trace element requirements for
stable food waste digestion at elevated ammonia concentrations.
106
E. B. Sydney et al.