(fluoroquinolone) in swine manure was found in the resulting compost [112]. Similarly, composting of turkey litter spiked with some antibiotics led to the full removal
of chlortetracycline, whereas reduction of monensin and tylosin varied between
54 and 76% of initial concentration and sulfamethazine was not removed at all
[113]. It is postulated that sorption processes seem to be the most feasible elimination pathway for many APIs during composting [110, 111], thus hampering the
mineralisation of these chemicals. However, most studies on composting-induced
degradation of APIs do not consider the mass evolution of feedstock (e.g. formation
of humic substances) during composting and the mechanisms underpinning the API
degradation, so leading to inaccurate conclusions on the composting efficiency in the
removal of APIs [114]. In addition, the impact of composting on API degradation
has been a research topic mainly investigated at lab scale using API-spiked manures,
so the aging effect has not been considered. Aging of hydrophobic organic pollutants
in soil is a well-known phenomenon whereby pollutant availability and biodegradation decrease as the time that pollutants remain in soil increases [115]. A similar
assumption has not been considered in composting studies of API-contaminated
feedstocks where organic matter content is higher than that in agricultural soils.
Likewise, complementary strategies such as vermicomposting (use of earthworms in
composting of solid organic residues) have not been deeply investigated. Indeed,
some benefits could be obtained with vermicomposting technology compared to
aerobic composting. For example, the quality of compost, in terms of physicochemical properties, produced from green waste (trimmings and litter) was higher with
vermicomposting than with composting [116]. Additionally, enzymes such as phosphatase and β-glucosidase showed a higher activity in the vermicompost than in
compost, both produced from cattle manure [117]. The impact of vermicompost on
soil physicochemical and biological properties was reviewed by Lim et al. [118],
who concluded that vermicompost has a higher beneficial impact on plant growth
and soil fertility than compost, because the former contains a larger amount of
available nutrients and plant growth-stimulating substances (phytohormones),
which probably degrade during the thermophilic phase of aerobic composting.
Vermicomposting is an oxidative process mainly driven by earthworms and
microorganisms, whereby organic wastes are broken down and transformed into a
fine and porous peat-like material named vermicompost [119]. This bio-oxidative
process occurs in a mesophilic environment (<30
C) created by the continue activity
of epigeic earthworms (e.g. Eisenia spp.), which aerate and facilitate heat dissipation
during organic matter decomposition. Vermicomposting of organic waste has been
described by Domínguez [106] in two actions: the earthworm gut-associated processes (GAPs) and the cast-associated processes (CAPs) (Fig. 3). The GAPs involve
the physical break down (e.g. grinding in gizzard) and biochemical transformations
of organic matter ingested by earthworms. Secretion of enzymes from the earthworm
gut epithelium and exoenzymes secreted by gut symbionts provide a biochemical
cocktail to decompose the organic matter [120, 121]. Nutrients are absorbed at the
gut epithelium, and secretion of substances such as mucus, urea and ammonia will
form the chemical composition of the egested material (casts). It is interesting to
highlight that during GAPs, the initial microbial composition and activity of the
352
J. C. Sanchez-Hernandez
of chlortetracycline, whereas reduction of monensin and tylosin varied between
54 and 76% of initial concentration and sulfamethazine was not removed at all
[113]. It is postulated that sorption processes seem to be the most feasible elimination pathway for many APIs during composting [110, 111], thus hampering the
mineralisation of these chemicals. However, most studies on composting-induced
degradation of APIs do not consider the mass evolution of feedstock (e.g. formation
of humic substances) during composting and the mechanisms underpinning the API
degradation, so leading to inaccurate conclusions on the composting efficiency in the
removal of APIs [114]. In addition, the impact of composting on API degradation
has been a research topic mainly investigated at lab scale using API-spiked manures,
so the aging effect has not been considered. Aging of hydrophobic organic pollutants
in soil is a well-known phenomenon whereby pollutant availability and biodegradation decrease as the time that pollutants remain in soil increases [115]. A similar
assumption has not been considered in composting studies of API-contaminated
feedstocks where organic matter content is higher than that in agricultural soils.
Likewise, complementary strategies such as vermicomposting (use of earthworms in
composting of solid organic residues) have not been deeply investigated. Indeed,
some benefits could be obtained with vermicomposting technology compared to
aerobic composting. For example, the quality of compost, in terms of physicochemical properties, produced from green waste (trimmings and litter) was higher with
vermicomposting than with composting [116]. Additionally, enzymes such as phosphatase and β-glucosidase showed a higher activity in the vermicompost than in
compost, both produced from cattle manure [117]. The impact of vermicompost on
soil physicochemical and biological properties was reviewed by Lim et al. [118],
who concluded that vermicompost has a higher beneficial impact on plant growth
and soil fertility than compost, because the former contains a larger amount of
available nutrients and plant growth-stimulating substances (phytohormones),
which probably degrade during the thermophilic phase of aerobic composting.
Vermicomposting is an oxidative process mainly driven by earthworms and
microorganisms, whereby organic wastes are broken down and transformed into a
fine and porous peat-like material named vermicompost [119]. This bio-oxidative
process occurs in a mesophilic environment (<30
C) created by the continue activity
of epigeic earthworms (e.g. Eisenia spp.), which aerate and facilitate heat dissipation
during organic matter decomposition. Vermicomposting of organic waste has been
described by Domínguez [106] in two actions: the earthworm gut-associated processes (GAPs) and the cast-associated processes (CAPs) (Fig. 3). The GAPs involve
the physical break down (e.g. grinding in gizzard) and biochemical transformations
of organic matter ingested by earthworms. Secretion of enzymes from the earthworm
gut epithelium and exoenzymes secreted by gut symbionts provide a biochemical
cocktail to decompose the organic matter [120, 121]. Nutrients are absorbed at the
gut epithelium, and secretion of substances such as mucus, urea and ammonia will
form the chemical composition of the egested material (casts). It is interesting to
highlight that during GAPs, the initial microbial composition and activity of the
352
J. C. Sanchez-Hernandez