Environmental Engineering Reference
In-Depth Information
12.1
Examples of total nitrogen (TN) concentration in different
feedstock types (in kg per ton of fresh matter).
used. The list of feedstock materials must be regularly revised and updated
according to the latest knowledge. For more details about AD feedstock, see
Chapter 2 of this topic.
Digestate quality is positively influenced by well-controlled and stable AD
processes and process parameters such as temperature, pH, conductivity,
concentrations of volatile fatty acids (VFA), retention time of feedstock,
process temperature, etc. More details about these process parameters can
be found in Chapter 10 of this topic. The quality of digestate can be
compromised by inadequate handling and unsuitable storage and applica-
tion (Al Seadi and Lukehurst, 2012), allowing chemical, physical or
biological re-contamination (from un-sanitized materials) or facilitating
losses of valuable nutrients by evaporation or leakage.
12.2.2 Digestate characteristics and declaration
The nutrients that are supplied by the feedstock are also present in the
digestate (Fig. 12.1). Therefore, the content of plant nutrients in digestate
varies between AD plants according to the composition of the feedstock
digested and will also vary over time for the same AD plant. This is
illustrated in Table 12.1, where the variation of the yearly average
composition of digestate produced at Ribe Biogas Plant in Denmark is
used as an example. The slight increase in dry matter content during the five
years in this particular example was due to measures employed to reduce
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