Biogas - Digestate As Fertilizer

Treatment of animal manure and slurries in biogas plants results in biodegradation of organic matter to inorganic compounds and methane.
In practice, the anaerobic degradation rate of organic matter from animal manure and slurries is about 40% for cattle slurry and of 65% for pig slurry.

The degradation rate depends at large on feedstock type as showed in table 1.
Due to degradation of organic matter, digestate is easier to pump and easier to apply as fertilizer, with reduced need of stirring, compared to untreated slurry.

	Dry mater [%]	Total N [kg/t]	NH4-N [kg/t]	P [kg/t]	K [kg/t]	Ph
Cattle slurry	6	5	2.8	0.8	3.5	6.5
Pig slurry	4	5	3.8	1	2	7
Digested slury	2.8	5	4	0.9	2.8	7.5

Table 2 shows nutrient distribution in digestate made from household waste.

	Dry matter	Nges [%]	P205 [%]	K2O [%]
Milk products	4 – 8	0.6 – 0.7	0.7 – 1.8
Vegetable waste	5 – 15	3 – 4	0.6 – 0.8	1 – 1.1
Kitchen waste	10 – 20	1.6 – 4.2	0.3 – 1.5	0.4 – 1.1
Vegetable oil	10 – 15	3 – 6	1 – 3	0.1 – 0.2
Fat from waste	25 – 70	0.8 – 3.3	0.1 – 0.5	0.1 – 0.5

References:

Biogas Handbook, Teodorita Al Seadi, Dominik Rutz, Heinz Prassl, Michael Köttner, Tobias Finsterwalder, Silke Volk, Rainer Janssen. ISBN 978-87-992962-0-0