PHOSPHATE REMOVAL FROM WATER BY RED MUD
USING CROSSFLOW MICROFILTRATION
G. AKAY
1
*, B. KESKINLER
2
,A.C°AKICI
2
and U. DANIS
2
1
Department of Chemical Engineering, University of Nottingham, Nottingham NG7 2RD, U.K. and
2
Faculty of Engineering, Atatu
È
rk University, 25240 Erzurum, Turkey
(First received October 1996; accepted in revised form June 1997)
AbstractÐRed mud (which is a waste material formed during the production of alumina and primarily
contains ferric and aluminium oxides) was used as an adsorbent for the removal of phosphate ions from
water using cross¯ow micro®ltration. It is shown that phosphate ions act as a coagulant for red mud par-
ticles, forming a compressible cake with a compressibility index of unity. As shown also by dead-end ®l-
tration experiments, the cake resistance decreases with increasing phosphate concentration. The phosphate
rejection is a strong function of feed dispersion pH, phosphate and red mud concentration ratio as well as
the concentration of co-ions such as sulphate ions used to adjust the dispersion pH. Under certain con-
ditions, especially when pH = 5.2, steady state permeate ¯ux and phosphate rejection reach a maximum
with 100% rejection achievable. The permeate also contain metal ion impurities originating from red mud
and their concentrations increase with increasing red mud concentration and decreasing pH. The eects of
other process variables, cross¯ow velocity and membrane pore size on phosphate rejection and permeate
¯ux are also studied. # 1998 Elsevier Science Ltd. All rights reserved
NOMENCLATURE
C(t), C*= Transient and steady state phosphate concentrations in
permeate (mM)
C
PF
= Phosphate concentration in the feed (mM)
C
RF
= Red mud concentration in the feed (g/l)
D
43
= Volume average particle size (mm)
D
32
= Surface average particle size (mm)
d= Membrane pore size (mm)
J(t), J*= Transient and steady state permeate ¯uxes (m s
À1
)
n= Compressibility index (-)
R(t), R*= Transient and steady state phosphate rejections (-)
R
C
, R
M
= Cake and membrane resistances (m
À1
)
S= Particle size span (-)
t= Time (s)
U= Cross¯ow velocity (m s
À1
)
V= Permeate volume per unit ®ltration area (m)
a= Speci®c resistance (m kg
À1
)
a
0
= Proportionality constant (equation 4)
DP= Transmembrane pressure drop (Pa)
m= Viscosity of permeate (Pa s)
1. INTRODUCTION
Red mud (RM) is a waste material formed during
the production of alumina when the bauxite ore is
subjected to caustic leaching. It is composed (see
Table 1) mainly of non-toxic ®ne particles of silica,
calcium, iron and possible toxic metal, aluminium.
The toxicity and colloidal nature of the RM par-
ticles as well as the relatively large quantities gener-
ated (1±2 tons RM per ton of alumina produced)
create a serious pollution hazard.
The utilisation of RM, as a cheap adsorbent for
phosphate has been considered by Shannon and
Verghese (1976) and Siao and Akashi (1977). It was
demonstrated that RM, at exceedingly high
dosages, was a good phosphorus removal medium
for domestic wastewater. However, some RM par-
ticles remained in the water thus causing turbidity
and toxicity due to the presence of aluminium, and
sludge production was prohibitively large. As a
result, the direct use of RM as an industrial phos-
phorus precipitant was not possible. The conven-
tional biological treatment process such as trickle
bed ®lters and activated sludge operations have lim-
ited phosphate removal capacity dictated by the
phosphate uptake ability of the micro-organisms
during the growth phase. Consequently, some of
the soluble phosphates are still present in the eu-
ent in the biological treatment processes. When
phosphate removal is a priority, chemical treatment
may need to supplement or even replace conven-
tional biological treatment. Typically, raw domestic
wastewater has a total phosphorus concentration of
approximately 10 mg P/l, the principal forms of
phosphate being orthophosphate (5 mg P/l), pyro-
phosphate (1 mg P/l) and tripolyphosphate (3 mg
P/l) together with smaller amounts of organic phos-
phates (Eilbeck and Mattock, 1987). However,
industrial waste waters (such as in detergent manu-
facturing and metal coating processes) may contain
phosphate levels well in excess of 10 mg P/l. The
removal of phosphate from aqueous streams,
whether of domestic or industrial origin, consists of
Wat. Res. Vol. 32, No. 3, pp. 717±726, 1998
# 1998 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
0043-1354/98 $19.00 + 0.00
PII: S0043-1354(97)00236-4
*Present address: Department of Chemical and Process
Engineering, University of Newcastle, Newcastle upon
Tyne NE1 7RU, U.K.
717