Environmental Engineering Reference
In-Depth Information
that the volumes of aerobic wastewater treatment tank for nitrogen removal significantly increase.
Recent research pointed out some solutions for this problem. The addition of an aerobic stage to
anaerobic digestion may transform 80% of the ammonia to nitrate (Zupancic and Ros 2008), which
has a beneficial effect of raw wastewater. It provides oxygen in form of nitrate for anoxic treatment
and consequently decreases BOD for the first stage of wastewater treatment.
32.5.2.6 Biosolids composting
Activated sludge contains a lot of nutritive compounds and also many microorganisms. During
the aerobic composting (the consequence of microorganisms), the energy is released in the form
of heat.
The time courses for the important factors in the composting process for activated sludge (the
amount of CO
2
generated, the temperature, weight change and water content of the composting
materials), were simulated by means of regression analysis. Furthermore, we could accurately
estimate the temperature change of the composting materials in the early phase of composting
from the heat balance, including microbial heat generation estimated by regression analysis. At the
optimum aeration regime, we can minimize the time required for the temperature to increase to
65°C, at which most pathogens in compost materials are killed (Kishimoto et al. 1987).
32.5.2.6.1 The Composition
The most important elements for microbe decomposition during the composting are carbon (C)
and nitrogen (N). Carbon represents 50% of cell biomass and is bounded to all energetic processes
in the cell. Nitrogen is a crucial element in proteins, amino acids, nucleic acids, and enzymes,
one of the most important components of cell. The ratio C:N in compost substrate is between
25:1 to 35:1 (Tchobanoglous et al. 2003). When the ratio C:N is smaller than 25:1, larger losses
of nitrogen in shape of free ammonia are occurring, which causes unpleasant smell (Larsen and
McKartney 2000).
32.5.2.6.2 Microorganisms
Composting is a process caused by different microorganisms that decompose organic material to
mineralized form. They need for their own development and activity suitable conditions in the
environment: aeration (ventilation), moisture, and ration of nutrients. We can distribute the course
of composting in three temperature ranges:
1. Mesophilic range, where the temperature is between 20 and 45°C. These fungi and bacteria
prevail in the compost:
Alcaligenaceae, Alteromonadaceae, Bacillaceae, Burkholderiaceae,
Bradyrhizobiaceae, Caryophanaceae, Caulobacteraceae, Cellulomonadaceae,
Flavobacteriaceae, Flexibacteraceae, Hyphomicrobiaceae, Intrasporangiaceae,
Methylobacteriaceae, Microbacteriaceae, Micrococcaceae, Moraxellaceae,
Neisseriaceae, Nitrosomonadaceae, Nocardiaceae, Nocardiopsaceae, Paenibacillaceae,
Phyllobacteriaceae, Propionibacteriaceae, Pseudomonadaceae, Pseudonocardiaceae,
Rhodobacteraceae, Sphingobacteriaceae, Staphylococcaceae, Xanthomonadaceae
(Haruta
et al. 2005).
2. Thermophilic range, where the temperature is between 45°C and 65°C. These fungi and
bacteria prevail in the compost (Schloss et al. 2003):
Micromonasporaceae, Streptomycetaceae,
Thermoactinomycetaceae, Thermomonosporaceae, Streptosporangiaceae, Hydrogenobacte,
Thermus
spp.
3. Final mesophilic range, where the number of thermophilic organisms decreases, and in the
same time increases the number of mesophilic bacteria and fungi. In this phase also higher
organisms appear: protozoa, arthopods, and worms (Schurard 2005).
