Digestate as Fertilizer

Digestate upgrading techniques — Nutrient extraction
Membrane filtration
Liquid
digestate
Ultrafiltration (UF)
UF-concentrate
UF-permeate
RO-concentrate
Reverse osmosis (RO)
Water
(RO-permeate)
Modules of reverse osmosis
In membrane filtration, different membranes
with decreasing pore sizes are usually used
(micro, ultra, nanofiltration, and finally reverse
osmosis). Ultrafiltration (UF) with a pore size of
0.01 – 0.05 micrometres (µm) and reverse osmosis
(RO) with a pore size of 0.0005 – 0.005 µm are usually
used for treating liquid digestate. Since membranes
are very sensitive to solids, using flocculants in separation
and/or flotation is usually necessary to produce a
liquid fraction that is as particle-free as possible. With
increasing clogging, the energy requirement rises
sharply due to frequent cleaning intervals and the
throughput decreases. The power consumption is between
10 – 30 kWh el
/m³ input.
6.6 Nutrient extraction
The goal of nutrient extraction is to produce fractionated single nutrients that can be used as mineral fertilizers or
in the chemical industry. In addition, the digestate is removed to the point where it can be discharged into water.
At present, these processes are not very common when upgrading digestate. The upgrading steps for complete
nutrient extraction include filtration, phosphate precipitation, and ammonium stripping.
In precipitation, magnesium ammonium phosphate (MAP) or calcium phosphate salt is obtained, which is used
directly as a readily available mineral fertilizer or for further fertilizer production. In stripping, ammonium sulphate
solution can be produced from the gas scrubber and, if necessary, a nitrogenous lime fertilizer as well. In addition,
a nitrogen-reduced digestate is obtained, which can be recirculated to the digestion process. This nitrogen sink
makes it possible to use higher quantities of nitrogen-containing feedstocks, such as dry poultry manure without
biological inhibition. The wastewater with a high proportion of organic matter and nutrients can be spread like a
liquid digestate or undergo further treatment.
Precipitation
Permeate
from fi ltration
pH increase
Precipitation of
phosphate salts
Technical
filtration
Ammonium
residual water
End product:
e.g. struvite
The precipitation of the phosphate occurs from
the liquid fraction, from which all solids must
be separated. This is done through several filtration
stages up to microfiltration with a pore size of <
0.1µm. Increasing the pH value shifts the solubility equilibrium
of the phosphates. Phosphate salts precipitate as
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