Environmental Engineering Reference
In-Depth Information
soluble form, it can be recovered through precipitation of calcium phosphate, Ca
3
(PO
4
)
2
, or
other compounds. Experiments show that calcium phosphate precipitates occur and can
be removed by iltration or centrifugation. Analysis shows crystalline material with the
expected stoichiometry (Ca:P ratio) close to 3:2. Surface area was 188 m
2
/g, indicating ine
particles. Impurities of silicon, iron, and aluminum (present in the original porous matrix)
were low. These tests were done using standard synthetic water; however, possible con-
tamination problems may occur under ield test conditions. For many wastewater applica-
tions, it is anticipated that the recovered phosphate will be able to be used for fertilizer or
other phosphoric acid applications.
Recovery of phosphorus from waste materials has been of high interest in recent years,
and some 22 processes for phosphorus recovery have been identiied (CEEP, 2012). There
are large amounts of phosphorus available in waste streams from agriculture, sewage treat-
ment, and many industrial water streams, especially food and pharmaceuticals. Three
types of phosphorus recovery approaches have been investigated: (i) from liquid streams
(precipitation or sorption/desorption), (ii) from sewage sludge, and (iii) from sludge incin-
eration ash. One problem with sludge is that often it contains aluminum or iron metal ions
from chemicals added to precipitate phosphorous. No matter which approach is used, it
must be economical and many methods are not today. However, in a survey of 417 experts
worldwide (CEEP, 2012), the general opinion was that economically viable processes will
emerge by 2030 and half the experts believe that recovery of phosphorus from liquids is the
most important route. Contaminants in wastewater and sludge do represent a serious issue.
Precipitation methods for recovery of phosphorous compounds and adsorption/desorp-
tion from liquid streams are being pursued by a number of organizations worldwide.
Ty p i c a l ly, C a
3
(PO
4
)
2
or struvite (MgNH
4
PO
4
) compounds are precipitated on ine sand that
acts as a seed for crystallization. Some 330 kg of recovered phosphorus will yield 8000 kg
of struvite, which can be used as a fertilizer supplement or 1584 kg of calcium phosphate.
Calcium phosphate is ideal for additional industrial processing since it is indistinguish-
able, in most respects, from minerals recovered from phosphate rock. Full-scale struvite
recovery processes have been used for >10 years. One problem with that type of process
is that it does not lower the phosphorus in water to <1 mg/L (a commonly accepted dis-
charge limit), so additional polishing reactors are needed.
Meta-PO4 media can remove 99% of phosphorus and reach phosphorous levels <0.1 mg/L.
This is much higher than most chemical precipitation processes, such as struvite crystal-
lization, which only removes 80-90%. Once separated, soluble phosphate ions can be eas-
ily precipitated as calcium phosphate and removed by iltration or centrifugation. This
provides an economic approach to both harvest and recover phosphorous, since the phos-
phorous is really a by-product of the removal process. Calcium phosphate can provide an
economic path to production of usable phosphorus products, such as phosphoric acid. It
can also be used to produce potassium phosphate compounds for use in fertilizer. The
cost to reclaim the phosphorous is the cost to remove/precipitate the PO
4
ions from liquids
remaining after the media is regenerated.
31.6 Economics of Phosphorus Removal
The cost to remove phosphorus will be much lower than competitive sorbents and will
often compete with chemicals (iron and aluminum) used to precipitate phosphorous,
