micropollutants may decrease due to microbial activity as well as volatilization. The
extent of contaminant depletion depends on the biodegradability and bioavailability
of the compound and of the operating parameters of digestion process (Cea-Barcia
et al. 2013).
Factors enabling the efficient biodegradation of micropollutants include sufficient
abundance of the particular strains possessing appropriate metabolic apparatus. The
low contaminant bioavailability, caused by sorption may inhibit the degradation
process. It was also observed that contaminants characterized by complex chemical
structure, containing the long alkyl chains are slower degraded in comparison with
simpler compounds. In the study of Aemig et al. (2016), which has investigated the
effect of anaerobic digestion on the removal of PAHs, it was concluded that
contaminant degradation during anaerobic digestion was related to distribution
among accessible or non-extractable organic matter pools, the specification of both
pools as well as properties of the contaminant.
During anaerobic digestion, some of the organic micropollutants, e.g., PAHs
present in the feedstock are transferred from low accessible fractions to
non-accessible fractions and thus become nonavailable for microorganisms
(Aemig et al. 2016). Over time pollutants will be slowly remobilized and become
bioavailable again. Moreover, Aemig et al. (2016) showed that contaminant spiked
in a digesting mixture achieved an affinity to both recalcitrant and readily available
fractions. The distribution of a contaminant was governed by physicochemical
characteristics as well as the amount and diversity of each organic fraction in
digestate. Due to the immobilization of contaminants in digestate, the risk of
secondary contamination of soil and water is reduced in comparison with the fresh
feedstock. Considering addition of digestate to polluted soil, to some extent, contaminants may be sorbed by recalcitrant fraction therefore alleviating soil toxicity.
Operating condition of anaerobic digester may influence degradation of some
micropollutants, but it is not a rule. For example, degradation rate of bis
(2-ethylhexyl) phthalate (DEHP) and surfactant nonylphenol ethoxylates (NPE)
was affected by retention time and temperature of the process (El-Hadj et al. 2006;
Patureau et al. 2008), while certain pharmaceuticals and estrogens did not reveal this
dependency (Carballa et al. 2007). As summarized by Stasinakis (2012), most of
phthalates undergo biotransformation due to anaerobic digestion. Some pharmaceuticals such as diclofenac, diazepam, ibuprofen, or naproxen may undergo transformations and be significantly reduced during digestion process, while other, like
carbamazepine, were found to be recalcitrant (Carballa et al. 2006).
During anaerobic digestion, a reduction in water content as well as transformation
of easily bioavailable organic compounds into biogas occur, which consequently
leads to a higher concentration of trace elements in digestate in comparison to the
feedstock. Additionally, vegetal feedstock (e.g., energy crops) may be supplemented
with trace elements, e.g., iron, nickel, cobalt, selenium, molybdenum, and tungsten,
to assure growth and activity of methanogenic bacteria and thus to improve the
biogas production yield and rate (Weiland 2010). Final content of trace elements in
digestate depends on type of feedstock and digestate processing (Kataki et al. 2017).
It was also observed that concentration of trace elements was significantly lower in
5 Potential Use of Waste-to-Bioenergy By-Products in Bioremediation of Total. . .
265
extent of contaminant depletion depends on the biodegradability and bioavailability
of the compound and of the operating parameters of digestion process (Cea-Barcia
et al. 2013).
Factors enabling the efficient biodegradation of micropollutants include sufficient
abundance of the particular strains possessing appropriate metabolic apparatus. The
low contaminant bioavailability, caused by sorption may inhibit the degradation
process. It was also observed that contaminants characterized by complex chemical
structure, containing the long alkyl chains are slower degraded in comparison with
simpler compounds. In the study of Aemig et al. (2016), which has investigated the
effect of anaerobic digestion on the removal of PAHs, it was concluded that
contaminant degradation during anaerobic digestion was related to distribution
among accessible or non-extractable organic matter pools, the specification of both
pools as well as properties of the contaminant.
During anaerobic digestion, some of the organic micropollutants, e.g., PAHs
present in the feedstock are transferred from low accessible fractions to
non-accessible fractions and thus become nonavailable for microorganisms
(Aemig et al. 2016). Over time pollutants will be slowly remobilized and become
bioavailable again. Moreover, Aemig et al. (2016) showed that contaminant spiked
in a digesting mixture achieved an affinity to both recalcitrant and readily available
fractions. The distribution of a contaminant was governed by physicochemical
characteristics as well as the amount and diversity of each organic fraction in
digestate. Due to the immobilization of contaminants in digestate, the risk of
secondary contamination of soil and water is reduced in comparison with the fresh
feedstock. Considering addition of digestate to polluted soil, to some extent, contaminants may be sorbed by recalcitrant fraction therefore alleviating soil toxicity.
Operating condition of anaerobic digester may influence degradation of some
micropollutants, but it is not a rule. For example, degradation rate of bis
(2-ethylhexyl) phthalate (DEHP) and surfactant nonylphenol ethoxylates (NPE)
was affected by retention time and temperature of the process (El-Hadj et al. 2006;
Patureau et al. 2008), while certain pharmaceuticals and estrogens did not reveal this
dependency (Carballa et al. 2007). As summarized by Stasinakis (2012), most of
phthalates undergo biotransformation due to anaerobic digestion. Some pharmaceuticals such as diclofenac, diazepam, ibuprofen, or naproxen may undergo transformations and be significantly reduced during digestion process, while other, like
carbamazepine, were found to be recalcitrant (Carballa et al. 2006).
During anaerobic digestion, a reduction in water content as well as transformation
of easily bioavailable organic compounds into biogas occur, which consequently
leads to a higher concentration of trace elements in digestate in comparison to the
feedstock. Additionally, vegetal feedstock (e.g., energy crops) may be supplemented
with trace elements, e.g., iron, nickel, cobalt, selenium, molybdenum, and tungsten,
to assure growth and activity of methanogenic bacteria and thus to improve the
biogas production yield and rate (Weiland 2010). Final content of trace elements in
digestate depends on type of feedstock and digestate processing (Kataki et al. 2017).
It was also observed that concentration of trace elements was significantly lower in
5 Potential Use of Waste-to-Bioenergy By-Products in Bioremediation of Total. . .
265