carbon to nitrogen ratio (C/N ratio). The optimum ratio fulfilling the metabolic
requirement of microorganisms is about 25–35 parts C to one part N (Gajalakshmi
and Abbasi 2008; Manderson 2009). Higher C/N proportion would result in a
shortage of N causing general slowdown of the process, while the excess of N
gives rise to the production of odorous compounds, mainly ammonia and amines,
with an ensuing negative impact on operators and the environment. Therefore, care
should be taken when the compost biodegradable “ingredients” are being mixed,
considering their characteristic elemental composition. For instance, lignocellulosic
residues generally have high C/N ratio, ranging from values of 400–600:1—in
sawdust, mulch, and wood chips—to 50–150:1 found in cereal straw, hays, silage,
and leaves. Significant exceptions are represented by grass clippings, leguminous
plant residues, and alfalfa hay, having C/N ratios lower than 25:1. Compostable
wastes particularly rich in nitrogen are manures (cattle, horse, sheep), poultry litter,
and activated sludge biosolids arising from sewage treatment plants.
Besides the C/N ratio, the input materials must be carefully screened for their
texture, dimension, and the speed at which they decompose during the process.
These are in fact paramount features of the entire process as they drive the bulk
density, porosity, and the aeration. Compost jargon defines a bulking agent as a
(lignocellulose) material that improves the porosity and prevents the formation of
anaerobic pockets within the composting material. Bulking agents are fundamental
to composting of sludge biosolids since their use entails improved aeration and
adjustment of the C/N ratio. In case of poor degradation of the bulking material,
coarse particles present can be sieved off at the end of the process and recycled.
Moisture is one of the composting variables that affect the microbial activity, as
most of the decomposition occurs in the thin liquid films on the surfaces of the
substrate’s particles. At start, the moisture content is adjusted to about 60–70%,
while lower values (50–60%) are ideal in the later stages of the decomposition. A
low level of moisture results in a significant slowdown of the microbial activity
(especially bacteria, as fungi and actinomycetes are less dependent on water activity—a w ), while the excess of water causes clogging the pores, reduced air exchange,
and subsequent anaerobiosis. Moisture is negatively affected by elevated temperature and an excess of forced aeration; therefore, it is important to moisten the
substrate, especially during the thermophilic phase when the water content can
quickly fall down. The lower extent of moisture in the curing material is one of
the factors prompting the fungal and actinobacterial growth during the maturation
phase.
23.2.4 Composting Systems
Composting is nowadays feasible in various systems, but care should be taken in
defining the various approaches as the terminology used by compost scientists can
be rather confusing (Gajalakshmi and Abbasi 2008; Manderson 2009). Biowaste
processing systems can be categorized on the basis of the system configuration or,
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O. Lhotský et al.
requirement of microorganisms is about 25–35 parts C to one part N (Gajalakshmi
and Abbasi 2008; Manderson 2009). Higher C/N proportion would result in a
shortage of N causing general slowdown of the process, while the excess of N
gives rise to the production of odorous compounds, mainly ammonia and amines,
with an ensuing negative impact on operators and the environment. Therefore, care
should be taken when the compost biodegradable “ingredients” are being mixed,
considering their characteristic elemental composition. For instance, lignocellulosic
residues generally have high C/N ratio, ranging from values of 400–600:1—in
sawdust, mulch, and wood chips—to 50–150:1 found in cereal straw, hays, silage,
and leaves. Significant exceptions are represented by grass clippings, leguminous
plant residues, and alfalfa hay, having C/N ratios lower than 25:1. Compostable
wastes particularly rich in nitrogen are manures (cattle, horse, sheep), poultry litter,
and activated sludge biosolids arising from sewage treatment plants.
Besides the C/N ratio, the input materials must be carefully screened for their
texture, dimension, and the speed at which they decompose during the process.
These are in fact paramount features of the entire process as they drive the bulk
density, porosity, and the aeration. Compost jargon defines a bulking agent as a
(lignocellulose) material that improves the porosity and prevents the formation of
anaerobic pockets within the composting material. Bulking agents are fundamental
to composting of sludge biosolids since their use entails improved aeration and
adjustment of the C/N ratio. In case of poor degradation of the bulking material,
coarse particles present can be sieved off at the end of the process and recycled.
Moisture is one of the composting variables that affect the microbial activity, as
most of the decomposition occurs in the thin liquid films on the surfaces of the
substrate’s particles. At start, the moisture content is adjusted to about 60–70%,
while lower values (50–60%) are ideal in the later stages of the decomposition. A
low level of moisture results in a significant slowdown of the microbial activity
(especially bacteria, as fungi and actinomycetes are less dependent on water activity—a w ), while the excess of water causes clogging the pores, reduced air exchange,
and subsequent anaerobiosis. Moisture is negatively affected by elevated temperature and an excess of forced aeration; therefore, it is important to moisten the
substrate, especially during the thermophilic phase when the water content can
quickly fall down. The lower extent of moisture in the curing material is one of
the factors prompting the fungal and actinobacterial growth during the maturation
phase.
23.2.4 Composting Systems
Composting is nowadays feasible in various systems, but care should be taken in
defining the various approaches as the terminology used by compost scientists can
be rather confusing (Gajalakshmi and Abbasi 2008; Manderson 2009). Biowaste
processing systems can be categorized on the basis of the system configuration or,
474
O. Lhotský et al.