5.1.3 Soil Composting with Digestate as a Bioremediation
Strategy and Organic Matter Post-treatment
Energy production from renewable sources and reduction of greenhouse gases
(GHGs) emission are currently strongly encouraged in many countries all over the
world. Anaerobic digestion is a process aimed at biogas, mainly methane, production
from organic residues (Weiland 2010). Nevertheless, the production of biogas from
organic matter generates large amount of organic by-products, which are digestates
in solid and liquid forms. The physicochemical characteristics of the final digestate
depend on properties of applied feedstock and operating parameters of the process
(Appels et al. 2008; Kataki et al. 2017). Digestate production is estimated to range
between 5 and 80% of input feedstock (Möller and Müller 2012).
Reasonable practice of organic waste management includes the reduction of the
amount of wastes deposited on landfills and the facilitation of organic carbon return
into the environment. It can be achieved by field application of the digestate solid
fraction, e.g., obtained through soil–liquid separation, as a fertilizer or an amendment during agricultural activities or bioremediation treatments (Mata-Alvarez and
Macé 2004).
Solid digestate has at least three main advantages over fresh feedstocks, e.g.,
animal manures, food waste, and sewage sludge: (1) During the digestion process
organic matter becomes stabilized (Sect. 5.3.2) and nutrients such as nitrogen and
phosphorus are accumulated in digestate in bioavailable forms. However, nutrient
status depends on the initial nutrient amount in the feedstock and process parameters
and may differ significantly between digestates (Tampio et al. 2016). (2) The amount
of organic contaminants in digestate is lower in comparison with feedstock due to
contaminant degradation during anaerobic digestion (Sect. 5.3.5). (3) The digestion
process has a positive effect on the decrease of pathogen content (Sect. 5.3.4).
Although digestate has lower pathogen content in comparison with fresh feedstock, anaerobic digestion does not ensure complete sanitation of the by-product
(Ottoson et al. 2008; Trémier et al. 2013). The European Union considers the solid
fraction of digestate as a waste (EEC 2008) and requires a posttreatment step before
land applications (Tambone et al. 2015). To improve digestate quality and meet
legislation requirements implemented by many countries, posttreatment of digestate
is required (Ottoson et al. 2008; Trémier et al. 2013).
Composting of digestate mixed with contaminated soils results in double benefit:
efficient soil bioremediation and improved characteristics of digestate (Fig. 5.1).
Composting in biopiles is an ex situ bioremediation technique, efficient for the
cleanup of soil contaminated by TPH. Biopile is composed of contaminated soil
mixed with organic matter and bulking agent if needed, organized into wide piles,
which usually range from 2 to 4 m height (Jørgensen et al. 2000). Air is supplied in
biopiles, aeration may be passive (e.g., pipeline system) or active (e.g., pile mixing).
The course of composting may be divided into three main intervals in accordance to
intensification of various processes, which depend on temperature. Temperatures
above 50
C provide sanitization, for temperatures between 45 and 50
C intensive
250
A. Gielnik et al.
Strategy and Organic Matter Post-treatment
Energy production from renewable sources and reduction of greenhouse gases
(GHGs) emission are currently strongly encouraged in many countries all over the
world. Anaerobic digestion is a process aimed at biogas, mainly methane, production
from organic residues (Weiland 2010). Nevertheless, the production of biogas from
organic matter generates large amount of organic by-products, which are digestates
in solid and liquid forms. The physicochemical characteristics of the final digestate
depend on properties of applied feedstock and operating parameters of the process
(Appels et al. 2008; Kataki et al. 2017). Digestate production is estimated to range
between 5 and 80% of input feedstock (Möller and Müller 2012).
Reasonable practice of organic waste management includes the reduction of the
amount of wastes deposited on landfills and the facilitation of organic carbon return
into the environment. It can be achieved by field application of the digestate solid
fraction, e.g., obtained through soil–liquid separation, as a fertilizer or an amendment during agricultural activities or bioremediation treatments (Mata-Alvarez and
Macé 2004).
Solid digestate has at least three main advantages over fresh feedstocks, e.g.,
animal manures, food waste, and sewage sludge: (1) During the digestion process
organic matter becomes stabilized (Sect. 5.3.2) and nutrients such as nitrogen and
phosphorus are accumulated in digestate in bioavailable forms. However, nutrient
status depends on the initial nutrient amount in the feedstock and process parameters
and may differ significantly between digestates (Tampio et al. 2016). (2) The amount
of organic contaminants in digestate is lower in comparison with feedstock due to
contaminant degradation during anaerobic digestion (Sect. 5.3.5). (3) The digestion
process has a positive effect on the decrease of pathogen content (Sect. 5.3.4).
Although digestate has lower pathogen content in comparison with fresh feedstock, anaerobic digestion does not ensure complete sanitation of the by-product
(Ottoson et al. 2008; Trémier et al. 2013). The European Union considers the solid
fraction of digestate as a waste (EEC 2008) and requires a posttreatment step before
land applications (Tambone et al. 2015). To improve digestate quality and meet
legislation requirements implemented by many countries, posttreatment of digestate
is required (Ottoson et al. 2008; Trémier et al. 2013).
Composting of digestate mixed with contaminated soils results in double benefit:
efficient soil bioremediation and improved characteristics of digestate (Fig. 5.1).
Composting in biopiles is an ex situ bioremediation technique, efficient for the
cleanup of soil contaminated by TPH. Biopile is composed of contaminated soil
mixed with organic matter and bulking agent if needed, organized into wide piles,
which usually range from 2 to 4 m height (Jørgensen et al. 2000). Air is supplied in
biopiles, aeration may be passive (e.g., pipeline system) or active (e.g., pile mixing).
The course of composting may be divided into three main intervals in accordance to
intensification of various processes, which depend on temperature. Temperatures
above 50
C provide sanitization, for temperatures between 45 and 50
C intensive
250
A. Gielnik et al.