Environmental Engineering FOURTH EDITION

198 ENVIRONMENTAL ENGINEERING

Phosphate may be removed chemically or biologically. The most popular chem-
ical methods use lime, Ca(OH)2, and alum, A12(S04)3. Under alkaline conditions,
the calcium ion will combine with phosphate to form calcium hydroxyapatite, a
white, insoluble precipitate that is settled out and removed from the wastewater.
Insoluble CaC03 is also formed and removed and may be recycled by burning in a
furnace:

CaCO3 -+ C02 + CaO.

Quicklime, CaO, is then slaked by adding water and forming lime, that may be reused:

CaO + H20 -+ Ca(OH)2

The aluminum ion fi-om alum precipitates as very slightly soluble aluminum phosphate,

M3+ + Poi- + m044,

and also forms aluminum hydroxide

AI3+ + 30H- ?, M(oH)3$,

which forms sticky flocs that help to settle out phosphates. Alum is usually added in
the final clarifier. The amount of alum needed to achieve a given level of phosphorus
removal depends on the amount of phosphorus in the wastewater. The amount of sludge
produced may be calculated by using stoichiometric relationships.

EXAMPLE 9.8. A wastewater contains 6.3 mg/L P, and it is found that an alum dosage

of 13 mg/L as A13+ achieves an effluent P concentration of 0.9mgL:

A3+ -I- Po$ -+ Alp04

Molecular weights: 27 95 122 (or31 asP)

or, from stoichiometry, 3 1 mg of Premoval results in the formation of 122 mg aluminum

phosphate sludge:

(5.4 Premoved ) (g ) = 2 1.3 mg ALP04 sludge produced.

The amount of M3+ used to produce the APO4 is

( 5.4 y - P removed ) (27mgA13+ ) = 4.6 mg M3+

(31 mg P)

The excess aluminum, 13 - 4.7 = 8.3 ma, probably formed aluminum hydroxide:

AI3+ + 3(0H-) + Al(OH),J.

Molecular weights: 27 (3) (17) 78