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4 Nitrogen and Phosphorous Recovery from Municipal Wastewater …
4.4 Nutrient Removal/Recovery from Sludge
Wastewater sludge is a source of different nutrients, organic compounds and inorganic
materials. The nutrients in sludge mainly originate from human excreta but also from
detergents and effluents of food, pulp and paper and chemical industries [43]. The
nutrients such as phosphorous and nitrogen can be recovered from sludge to be used
as fertilizers, and recovering phosphorous from sludge is considered as a promising
approach of resource recovery from waste [13]. Different types of physical and
chemical methods can be used to extract the nutrient from sludge, as discussed
below.
4.4.1 Phosphorous Removal/Recovery from Sludge
Phosphorous can be recovered from the ash generated in sludge incineration plants.
During incineration, phosphorous and heavy metals are transferred to the ash, from
which phosphorous could be then recovered from ash using leachants [43]. However,
the phosphorous recovered from ash is limited due to the impurities present in the
recovered products. Other methods for recovering phosphorous with low impurity
levels from sludge include seeding, precipitation, wet oxidation, physical/chemical
and thermal processes [13, 43]. Depending on the type of separation method,
phosphorous can be recovered in the forms of iron phosphate, calcium phosphate,
phosphoric acid and struvite [13].
4.4.1.1 Seeded Processes
Seeded processes use a seed material to develop calcium phosphate crystals. As an
example, in the Crystalactor process, a portion of settled activated sludge from the
secondary clarifier of a biological nutrient removal plant is fed into reactors filled
with sand as the seeding material. A lime solution ((Ca(OH) 2 ) is then added to the
reactor to increase the pH to around 8 and to increase the concentration of calcium
ions for effective precipitation of calcium phosphate on the sand seeds. The settled
calcium pellets are then removed from the bottom of the reactor, containing around
40–50% calcium phosphate [13].
As another seeding process, the P-RoC method recovers phosphate from sludge
using tobermorite-rich waste material from the construction industry as the seed. The
process is similar to the above Crystalactor technology; however, no lime addition
is required as tobermorite is composed of calcium silicate hydrates, which could
stimulate the precipitation of calcium phosphate [13].
4 Nitrogen and Phosphorous Recovery from Municipal Wastewater …
4.4 Nutrient Removal/Recovery from Sludge
Wastewater sludge is a source of different nutrients, organic compounds and inorganic
materials. The nutrients in sludge mainly originate from human excreta but also from
detergents and effluents of food, pulp and paper and chemical industries [43]. The
nutrients such as phosphorous and nitrogen can be recovered from sludge to be used
as fertilizers, and recovering phosphorous from sludge is considered as a promising
approach of resource recovery from waste [13]. Different types of physical and
chemical methods can be used to extract the nutrient from sludge, as discussed
below.
4.4.1 Phosphorous Removal/Recovery from Sludge
Phosphorous can be recovered from the ash generated in sludge incineration plants.
During incineration, phosphorous and heavy metals are transferred to the ash, from
which phosphorous could be then recovered from ash using leachants [43]. However,
the phosphorous recovered from ash is limited due to the impurities present in the
recovered products. Other methods for recovering phosphorous with low impurity
levels from sludge include seeding, precipitation, wet oxidation, physical/chemical
and thermal processes [13, 43]. Depending on the type of separation method,
phosphorous can be recovered in the forms of iron phosphate, calcium phosphate,
phosphoric acid and struvite [13].
4.4.1.1 Seeded Processes
Seeded processes use a seed material to develop calcium phosphate crystals. As an
example, in the Crystalactor process, a portion of settled activated sludge from the
secondary clarifier of a biological nutrient removal plant is fed into reactors filled
with sand as the seeding material. A lime solution ((Ca(OH) 2 ) is then added to the
reactor to increase the pH to around 8 and to increase the concentration of calcium
ions for effective precipitation of calcium phosphate on the sand seeds. The settled
calcium pellets are then removed from the bottom of the reactor, containing around
40–50% calcium phosphate [13].
As another seeding process, the P-RoC method recovers phosphate from sludge
using tobermorite-rich waste material from the construction industry as the seed. The
process is similar to the above Crystalactor technology; however, no lime addition
is required as tobermorite is composed of calcium silicate hydrates, which could
stimulate the precipitation of calcium phosphate [13].