976 PHYSICAL AND CHEMICAL TREATMENT OF WASTEWATERS
the water. According to Wuhrmann, the dominant reaction
product between the phosphate ion and the ferric ion at pH
above 7 is believed to be FePO 4 , with a solubility product of
about 10^23 at 25°C. The colloidal particle size of the FePO 4
requires a sufficient excess of ferric ion for the formation of
a well-flocculating hydroxide precipitate, which includes the
FePO 4 particles and acts as an efficient adsorbent for other
phosphorous compounds.
It has been reported that for efficient phosphorus
removal (85 to 95%), the stoichiometric amount of 1.8 mg/l
Fe required per mg/l P should be supplemented by at least
10 mg/l of iron for hydroxide formation. Also, the use of
anionic polymer is considered desirable in order to produce
a clear supernatant (Wukash, 1968).Lime Lime reacts with the bicarbonate alkalinity of waste-
water to form calcium carbonate, which precipitates, under
normal conditions:Ca(OH) 2 Ca(HCO 3 ) 2 → 2CaCO 3 ↓ 2H 2 O (8)Normally, 70 to 90% of the phosphorus in domestic sewage
is in the form of orthophosphates or polyphosphates that may
hydrolyze orthophosphates. The remaining phosphorus is
present in the form of organic-bound phosphorus. The removal
of phosphorus can be achieved by direct adsorption on the
surface of calcium carbonate particles. Orthophosphates can
also be precipitated in the alkaline range by reaction with cal-
cium salts to form hydroxyapatite, according to the following
reaction:10Ca(OH) 2 6H 3 PO 4 → 10Ca (PO 4 ) 6 (OH) 2 ↓
18H 20 (9)Schmid and McKinney (1969) observed that hydroxyapatite
was present in soluble form at a pH value above 9.5. They
also found that at pH values of 9.5 or less, phosphorus was
adsorbed onto the growing faces of calcium-carbonate par-
ticles, thereby inhibiting their growth. Buzzell and Sawyer
(1967) have shown that at pH levels of 10 to 11 in the primary
sedimentation tanks, BOD removal of 55 to 70%, nitrogen
removal of 25%, phosphate removal of 80 to 90%, and coli-
form removal of 99% can be expected. Bishop et al. (1972)
have reported that precipitation of domestic wastewater with
lime removed approximately 80% of the TOC, BOD, and
COD; 91% of the SS; 97% of the total phosphorus; and 31%
of the total nitrogen. Phosphates from secondary effluent have
been removed successfully at Lake Tahoe by precipitation
with lime (Slechta and Culp, 1967). Albertson and Sherwood
(1967) found that by recirculating calcium-phosphate solids,
previously formed due to the addition of lime, it was possible
to reduce the lime dosage by about 50%.
Galarneau and Gehr (1997) present experimental results
of their studies on phosphorous using aluminum hydroxide.
de-Bashan and Bashan (2004) present an extensive review of
recent advances in phosphorous removal from wastewaters
and its separation for use as a fertilizer or as an ingredient in
other products.In wastewater-treatment practices, it is detrimental to
form large floc particles immediately in the flocculation step
because it reduces the available floc surface area for adsorp-
tion of phosphorus. Therefore, it is essential to maintain fine
pinpoint flocs in order to get a maximum phosphate removal
by surface adsorption, and this can be achieved by minimizing
the time of their flocculation. This is not the case if the goal is
one of colloidal-solids removal, as is often the case in water
treatment.
The process of coagulation and flocculation in wastewater
treatment can be summarized in the following three steps:- As the coagulant dissolves, positive aluminum
and ferric ions become available to neutralize the
negative charges on the colloidal particles includ-
ing organic matter. These ions may also react with
constituents in solution such as hydroxides, car-
bonates, phosphates, sulfides, or organic matter to
form complex gelatinous precipitates of colloidal
dimensions that are termed “microflocs.” This is
the first stage of coagulation, and for greatest effi-
ciency a rapid and intimate mixing is necessary
before a second reaction takes place. - After the positively charged ions have neutralized
a large part of the colloidal particles and the zeta
potential has been reduced, the resulting flocs are
still too small to be seen or to settle by gravity. The
treatment, therefore, should be flocculation, slow
stirring so that very small flocs may agglomerate
and grow in size until they are in proper condition
for sedimentation. Some evidence suggests that
aggregation of microflocs with dispersed waste con-
stituents is the most important mechanism affecting
coagulation in water treatment (Riddick, 1961). - During the third phase, surface adsorption of parti-
cles takes place on the large surface area provided
by the floc particles. Some of the bacteria present
will also become entangled in the floc and carried
to the bottom of the tank.
Electrocoagulation Electrocoagulation is a process in which
the coagulating ions are produced by electrolytic oxidation of
sacrificial electrodes. This technique has been successfully
used in the removal of metals, suspended particles, colloids,
organic dyes, and oils. An interesting review of this technique
is presented by Mollah et al. (2001). In it the advantages and
disadvantages of electrocoagulation are presented as well as
a description and comparison with chemical coagulation. His
group studied its use in the treatment of a synthetic-dye solu-
tion with a removal of 99% under optimal conditions (Mollah,
Morkovsky, et al., 2004). Another publication (Mollah, Pathak,
et al., 2004) presents the fundamentals of electrocoagulation
and the outlook for the use of this process in wastewater treat-
ment. Lai and Lin (2003) studied the use of electrocoagulation
for the treatment of chemical mechanical polishing wastewater,
obtaining a 99% copper removal and 96.5% turbidity reduction
in less than 100 minutes.C016_005_r03.indd 976 11/22/2005 11:25:16 AM