Environmental Engineering Reference
In-Depth Information
Table 12.1 Yearly average values for composition of digestate from the joint co-
digestion plant Ribe Biogas A/S, in Denmark
DM
(%)
N total
(kg/t)
NH
4
-N
(kg/t)
P total
(kg/t)
K total
(kg/t)
Mg total
(kg/t)
Cu total
(g/t)
Ca total
(kg/t)
Year 1
5.6
4.7
3.3
0.9
3.7
0.5
9.7
1.5
Year 2
6.4
4.6
3.1
0.9
3.5
0.5
12.8
1.5
Year 3
6.2
5.2
3.4
1.2
4.1
0.8
2.7
2.5
Year 4
5.8
5.0
3.2
1.1
3.3
0.6
11.6
2.0
Year 5
5.8
4.9
3.2
1.1
2.9
0.5
11.8
1.4
Year 6
5.8
4.8
3.2
1.1
3.2
0.5
10.8
1.5
Source: Adapted from Holm-Nielsen et al. (1997).
water ingress to wastes (preventing water from roofs, downpipes and
farmyards from running into the storage tanks).
When digestate is intended to be used as fertilizer, its composition must be
analyzed and declared in order to facilitate accurate dosage of nutrients and
allow full integration of digestate in the fertilizer plan of the farm, in line
with best farming practices. Declaration is a part of the quality assurance
schemes for digestate in many countries. For example, in Denmark, it is
required that the digestate declaration contains information about the
amount and availability of macro- and micronutrients, the dry matter and
organic dry matter content and the pH value of the produced digestate.
Digestate has a lower dry matter content than the undigested influent: at
least 50% of the dry matter content is converted to methane (CH
4
)and
carbon dioxide (CO
2
). For energy crops, the conversion rate can be as high
as 90% (Murphy et al., 2011). Furthermore, the content of ammonium-N
(NH
4
+
-N) is high in digestate (around 20% higher than undigested cattle
slurry) according to Holm-Nielsen et al. (1997).
12.2.3 The concept of digestate quality management
Quality management and quality control of both feedstock and digestate
aim to provide assurance that the digestate is suitable and safe for use as
fertilizer and that it is perceived as a safe product by crop farmers, food
retailers, politicians, decision makers and the general public (Al Seadi and
Lukehurst, 2012). Quality management and control measures must be
performed throughout the whole closed AD cycle, from production and
supply of the AD feedstock until final utilization of the digestate as fertilizer.
Quality management implies the use of only high-quality AD feedstock
provided through permanent control of feedstock quality, pre-treatment of
specific feedstock types, and a stable and robust AD process with ongoing
monitoring of the process parameters affecting digestate quality (tempera-
Search WWH ::
Custom Search