Biogas digestate quality and utilization - The Biogas: Science, Production and Applications - page 286

Table 12.10 Example data for the performance of evaporation

DM
(%)

Organic
DM (%)

TN
(g/kg)

PO 4 -P
(g/kg)

COD
(g/kg)

Inflow 3.1 1.7 3.1 0.3 45

Concentrate 11.0 8.3 9.0 1.0 108

Condensate 0.05 0.05 0.04 0.0 < 1

Source: Adapted from Heidler (2005).

The Anammox process

For the removal of ammonia, the Anammox (anaerobic ammonium
oxidation) process can be applied. This is a microbial process that takes
place in many natural environments and is also the trademarked name of the
ammonium removal technology subsequently developed. The Anammox
process is performed by bacteria, with Planctomyces and Pirellula being the
most well-known genera, and consists of two steps that take place in the
same reactor: (I) partial nitrification performed by ammonia-oxidizing
bacteria; (II) conversion of the resulting ammonium and nitrite to
dinitrogen gas and about 15% nitrate (not illustrated), performed by
Anammox bacteria, according to:

2NO 2 þ

4NH 4 þ þ 2NH 4 þ þ 4H þ þ

ð I Þ 3O 2 ! 2H 2 O

NO 2 !

4NH 4 þ þ

ð II Þ N 2 þ 2H 2 O

Evaporation

The evaporation of digestate can be an interesting solution if sufficient waste
heat is available at the biogas plant. A considerable amount of the solids
should be removed beforehand. In technical-scale applications (such as
energy crop digestion) this is often done by combining screw press
separators and vibration sieves to remove fibers and thus reduce clogging
of the heat exchangers. However, a considerable amount of cleaning will still
be needed. By evaporation, a nutrient-rich concentrate is produced and a
condensate is recovered. The ambition is to have low nitrogen concentra-
tions in the condensate and high nitrogen concentrations in the concentrate.
Considerable amounts of sulfuric acid are added to reduce the evaporation
of ammonia, which implies high costs. Apart from that, the produced
condensate does not meet direct discharge levels (see Table 12.10). If direct
discharge is required, the condensate can be purified via a reverse osmosis
step. In conclusion, digestate evaporation requires the use of considerable
chemicals and the availability of waste heat, direct discharge levels of the
condensate cannot normally be obtained.

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