4.4 Nutrient Removal/Recovery from Sludge
117
4.4.1.2 Precipitation Processes
Phosphorous in sewage sludge is bound to several organic and inorganic moieties.
The addition of alkalinity and changing the pH to around 8.5 could precipitate phosphorous by some processes like the Phostrip technology, as introduced previously
(Fig. 4.3). Although it is not applied to the sludge directly, it recovers phosphorous
in the form of calcium phosphate from a phosphorous-enriched sludge processing
side-stream by precipitation. In this process, acetic acid could be added to the stripper
to increase the amount of released phosphorous [13, 43].
Phosphorous can also be precipitated from sewage sludge in the form of struvite,
a crystalline magnesium ammonium phosphate hexahydrate (NH 4 MgPO 4 .6(H 2 O))
containing P 2 O 5 at approximately 30% by weight. Although formation of struvite
during wastewater treatment operation is undesired as it leads to pipes blockage, it
can be utilized as a fertilizer [13]. Struvite can be produced from digested sewage
sludge by adding sulfuric acid as a leaching agent, and a divalent salt of Mg such as
MgCl 2.
4.4.1.3 Wet Oxidation Process
In the wet oxidation process, organic fraction of sludge is oxidized and destructed
using pure oxygen and supercritical water (T > 374 °C and P > 221 bar). The
phosphorus can then be extracted through precipitation using calcium hydroxide
[43, 46].
4.4.1.4 Physical, Chemical and Thermal Processes
The physical, chemical and thermal processes for phosphorous recovery involve
dissolving phosphorous using physical, chemical and thermal operations followed
by precipitation. Examples of these processes are KREPO, Kemicond, Seaborne,
BioCon and SEPHOS methods. The KREPO process is a thermal process that heats
the sludge up to 140 °C under pressure and adjust its pH with sulfuric acid to 1–
3. Phosphate, metals and a large fraction of organic compounds are dissolved and
separated from the solids. The pH is then increased and the iron phosphate is separated
from the liquid phase by precipitation [13, 43].
The Kemicond technology can be applied to raw sludge treatment using sulfuric
acid and hydrogen peroxide (H 2 O 2 ) followed by a two-stage dewatering process.
Sulfuric acid dissolves compounds such as iron phosphate and hydroxides, and
hydrogen peroxide oxidizes the dissolved iron (II) to iron (III). The dissolved phosphates are re-precipitated as ferric phosphate and are recovered by dewatering of
solids. To enhance the dry solids recovery, polymer is commonly added at the
dewatering stage [13].
117
4.4.1.2 Precipitation Processes
Phosphorous in sewage sludge is bound to several organic and inorganic moieties.
The addition of alkalinity and changing the pH to around 8.5 could precipitate phosphorous by some processes like the Phostrip technology, as introduced previously
(Fig. 4.3). Although it is not applied to the sludge directly, it recovers phosphorous
in the form of calcium phosphate from a phosphorous-enriched sludge processing
side-stream by precipitation. In this process, acetic acid could be added to the stripper
to increase the amount of released phosphorous [13, 43].
Phosphorous can also be precipitated from sewage sludge in the form of struvite,
a crystalline magnesium ammonium phosphate hexahydrate (NH 4 MgPO 4 .6(H 2 O))
containing P 2 O 5 at approximately 30% by weight. Although formation of struvite
during wastewater treatment operation is undesired as it leads to pipes blockage, it
can be utilized as a fertilizer [13]. Struvite can be produced from digested sewage
sludge by adding sulfuric acid as a leaching agent, and a divalent salt of Mg such as
MgCl 2.
4.4.1.3 Wet Oxidation Process
In the wet oxidation process, organic fraction of sludge is oxidized and destructed
using pure oxygen and supercritical water (T > 374 °C and P > 221 bar). The
phosphorus can then be extracted through precipitation using calcium hydroxide
[43, 46].
4.4.1.4 Physical, Chemical and Thermal Processes
The physical, chemical and thermal processes for phosphorous recovery involve
dissolving phosphorous using physical, chemical and thermal operations followed
by precipitation. Examples of these processes are KREPO, Kemicond, Seaborne,
BioCon and SEPHOS methods. The KREPO process is a thermal process that heats
the sludge up to 140 °C under pressure and adjust its pH with sulfuric acid to 1–
3. Phosphate, metals and a large fraction of organic compounds are dissolved and
separated from the solids. The pH is then increased and the iron phosphate is separated
from the liquid phase by precipitation [13, 43].
The Kemicond technology can be applied to raw sludge treatment using sulfuric
acid and hydrogen peroxide (H 2 O 2 ) followed by a two-stage dewatering process.
Sulfuric acid dissolves compounds such as iron phosphate and hydroxides, and
hydrogen peroxide oxidizes the dissolved iron (II) to iron (III). The dissolved phosphates are re-precipitated as ferric phosphate and are recovered by dewatering of
solids. To enhance the dry solids recovery, polymer is commonly added at the
dewatering stage [13].