Journal of Science and Technology
1 (2), 2007, 234-244
©BEYKENT UNIVERSITY
REMOVAL OF SUSPENDED SOLID
MATERIALS FROM THE WASTEWATER OF
NATURAL DIMENSION STONE CUTTING
PLANTS BY FLOCCULATION
Savaş ŞENER
Department of Environmental Engineering, University of Mersin, 33343
Mersin, Turkey, +90-324-361-00-01/7089; e-mail: [email protected]
ABSTRACT
During the processing of natural dimension stones (NDS), the wastewater
produced is either re-used in the process or discharged into the aquatic
environment. The suspended solids must be removed from the effluent prior to
re-using or discharging into environment. In order to remove the fine particles,
large basins are needed to provide sufficient retention time due to the extreme
low settling velocities, therefore, organic polymers (flocculants) have to be
added to increase the settling velocity and the efficiency of settling basins.
In this study, flocculation characteristics of NDS namely marble, granite and
basalt were investigated using three commercial flocculants such as anionic
(A-110), cationic (C-592) and non-ionic (N-100 plus). The primary particle
properties such as particle size, size distribution as well as their dispersion
properties like solid concentration, suspension pH and zeta potential on
settling rate and supernatant concentration in solids have also been considered.
Results showed that among the three different flocculants tested, anionic
flocculant for marble sample; cationic flocculant for granite and basalt samples
gave the best result. The flocculant dosage affected significantly the settling
rate of the suspension as well as the clarity of the supernatant. The anionic
A110, flocculant with 3 mg/L dosage showed the optimum performance for
marble suspension while the cationic C592 flocculant with 4 and 3 mg/L
dosage found enough for granite and basalt suspensions, respectively.
Keywords: Dimension stone, marble, basalt, granite, removal of suspended
solids, flocculation
234
S. Şener
ÖZET
Doğal yapı malzemelerinin işlenmesi sırasında çıkan atıksu ya proses
içerisinde tekrar kullanılmakta ya da sucul ortama deşarj edilmektedir. Deşarj
öncesi atıksudaki askıda katı maddeler giderilmelidir. Düşük çökme
hızlarından dolayı çok ince boyuttaki katıların çöktürülebilmesi için çok geniş
alanlı havuzlara ihtiyaç duyulduğu için çökme hızlarını artırmak için organik
polimerler (flokülant) kullanılır.
Bu çalışmada, mermer, granit ve bazalt tanelerinin çöktürülmesinde üç tip
(anyonik A-110, katyonik C-592 ve iyonik olmayan N-100 plus) ticari
flokülantların çöktürebilme karakteristikleri incelenmiştir. Çalışmalarda tane
boyutu ve dağılımı, bunun yanında tanelerin süspansiyon içindeki davranışını
belirleyen katı derişimi, pH ve tanelerin zeta potansiyellerinin çökme hızı ve
duru fazdaki katı derişimine etkisi incelenmiştir.
Elde edilen sonuçlar denenen üç flokülantın arasında mermer taneleri için
anyonik, bazalt ve granit için ise katyonik flokülantlar en iyi sonucu vermiştir.
Flokülant dozu tanelerin çökme hızını dolayısıyla atıksuyun duruluğunu
belirlemektedir. Mermer tozu içeren atıksuda 3 mg/L dozda anyonik A-110;
granit ve bazalt atıksuyunda ise katyonik C592 flokülantının sırasıyla 4 ve 3
mg/L dozları yeterli bulunmuştur.
Anahtar Kelimeler: Doğal yapı malzemeleri, mermer, bazalt, granit, askıda
katı madde giderimi, flokülasyon
1. INTRODUCTION
Natural dimension stones (marble, basalt and granite) are cut into slabs or
plates with rotating blades and are finished by polishing processes. Water is
mainly used as a cooling media in the cutting processes and takes away all the
particles produced during cutting and polishing. Suspended solids in the
wastewater produced can lead to the development of sludge deposits when
untreated wastewater is discharged in the aquatic environment. The wastewater
containing fines is pre-treated through sedimentation tanks prior to discharge
or is recycled to reuse the treated wastewater instead of fresh water. In order to
remove the fine particles, large basins are needed to provide sufficient
retention time to settle the slow settling fines completely [1]. Otherwise, the
re-circulated water containing high solid concentration when reused reduces
the usable life of the cutting blades.
Due to the extreme low settling velocities, organic polymers (flocculants) have
to be added to increase the settling velocity and the efficiency of settling
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Removal Of Suspended Solid Materials From The Wastewater Of Natural Dimension
Stone Cutting Plants By Flocculation
basins. Therefore, flocculation enhances subsequent sedimentation by
increasing particle size, resulting in increased settling rate [2].
Flocculation is the physical process of bringing the destabilized very fine
particles (e.g., <5^m) with polymolecules (flocculant) in contact to form larger
flocs that can be more easily removed from suspension [3]. Flocculants are
mostly synthetic polymers based on repeating units of acrylamide and its
derivatives, which may contain either cationic or anionic charge. For nonionic
polymers such as polyacrylamides, hydrogen bonding and/or hydrophobic
forces are believed to be the main driving forces for adsorption. Polymeric
flocculants provide enhanced formation of larger aggregates (flocs) of particles
by three general mechanisms called charge neutralisation, electrostaticpatching and bridging. Although various flocculation mechanisms are possible,
the most important is flocculation by bridging. Bridging is considered to be a
consequence of the adsorption of the segments of the flocculant
macromolecules onto the surfaces of more than one particle. It is generally
believed that low molecular weight polymers tend to adsorb and neutralise the
opposite charges on the particles. Consequently, flocculation is caused by the
reduction in the electric double layer repulsion between particles. Charge patch
attraction occurs when the particle surface is negatively charged and the
polymer is positively charged. Bridging is considered to be a consequence of
the adsorption of the segments of flocculant macromolecules onto the surfaces
of more than one particle [4].
While adsorption of the polymer is necessary for bridging flocculation to
occur, it is important to realize that adsorption and flocculation are not
separate, sequential processes, but occur simultaneously [5]. Optimum
flocculation occurs at flocculant dosages corresponding to a particle coverage
that is significantly less than complete. Incomplete surface coverage ensures
that there is sufficient unoccupied surface available on each particle for
adsorption during collisions of segments of the flocculant chains attached to
the particles [6].
There are only a few studies on flocculation of marble wastewater [7-10]. In
the studies, zeta potential of the marble particles exhibited negative charge
within the all pH employed and both cationic and anionic flocculants were
found to flocculate the marble particles at different levels. In the study
performed on flocculation of travertine suspension [11], the effects of
suspension pH and charge density of anionic polyacrylamide (PAA) on
flocculation behavior of two different natural stone suspensions (NSS) marble
and travertine were investigated by settling rate and turbidity as indicators. At
pH of 11, the settling rate of both natural stones increased significantly with
anionic polymer.
236
S. Şener
In this study, flocculation characteristics of fines of different natural dimension
stones namely marble, basalt and granite were investigated using three
commercial flocculants such as anionic, cationic and non-ionic. The primary
particle properties such as particle size, size distribution as well as their
dispersion properties like solid concentration, suspension pH and zeta potential
on settling rate and supernatant concentration in solids have also been
considered.
2. MATERIALS AND METHOD
In the experiments three different wastewater of natural dimension stones,
marble, granite and basalt, were used. The pH and solid ratio of the marble,
granite and basalt slurries were 9.1, 8.5 and 7.6, and 21.52%, 29.64% and
18.91% (w/w), respectively. The particle size analysis determined by sub-sieve
technique using the Andreasen Pipette.
Anionic Superfloc (A110), non-ionic (N-100 Plus) Superfloc flocculants and
cationic Magnafloc (C592), obtained from the American Cyanamid Company
and Allied Colloids Inc., respectively, were used for the flocculation
experiments.
Electrokinetic studies were performed, to determine zeta potential of the
particles in the slurry, in the Rank Brothers Electrophoresis Apparatus. Effect
of pH on the sign and values of zeta potential were examined. In the apparatus,
platinum electrodes were used and all measurements were conducted at 80V
and 298 K. The suspension was ultrasonicated in ultrasonic bath (35 kHz) for
5 min in order to break up aggregates in the suspension prior to measurement.
The resultant suspension (100 mL) was conditioned for 10 minutes during
which pH was adjusted to predetermined pH by addition of a NaOH or HCl
(0.1 N) solution under continuous stirring. A WTW 340i model pH meter was
used for measurement of pH of solutions. The zeta potential values were
calculated from electrophoretic mobility using the Smoluchowski equation
[12].
The flocculation experiments were carried out at room temperature in Velp
FC45 model Jar Test apparatus. For each test a 250 mL sample of the
suspension was placed into a 500 mL glass beaker. Prior to the flocculation
tests 0.1% (weight/volume, w/v) stock solution of the flocculant was prepared
with distilled water and diluted to 0.01% (w/v) before use. The dispersed
suspension was first conditioned at 500 rpm for 5 minutes and the solution of
flocculant was added to the suspension under continuous mixing condition.
After 2 minutes of time the stirring speed was reduced to 100 rpm for 2
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Removal Of Suspended Solid Materials From The Wastewater Of Natural Dimension
Stone Cutting Plants By Flocculation
minutes, to allow floc growth. After various settling time, 20 mL of
supernatant was taken out at a fixed distance of 2 cm below the air-liquid
interface. In all of the flocculation tests, the effect of various operating
parameters, such as pH, type and amount of flocculant, on the performance of
flocculation were tested. Concentration of total suspended solid (TSS) in the
supernatant was used as an indicator for the flocculation tests.
3. RESULTS AND DISCUSSION
3.1. CHARACTERIZATION OF THE SUSPENSIONS
3.1.1. PARTICLE SIZE ANALYSIS OF THE FINES
Particle size determination by the sub-sieve sizing technique is based on the
Stokes law and employs the relationship between time and travel distance of
particles and particle settling in a viscous liquid. The particle size distribution
of the marble, granite and basalt particles in their suspensions are given in
Figure 1. According to the results obtained from the figure, 80% of the total
marble, granite and basalt particles (d80) are finer than 40 |im, 30 |im and 19
|im, respectively.
£
cu
5:
100 -T
80 --
? 60w
w
TO
I
40-
JO
! 20 -E
°
0-1
Figure 1. Particle size distribution of solids in the wastewater.
238
S. Şener
3.1.2. ZETA POTENTIAL OF THE SOLIDS
The zeta potentials of marble, granite and basalt l were determined as a
function of pH and are given in Figure 2. The electrokinetic measurements
could not be done due to the dissolution of marble particles below pH 6. As
seen in the figure, an isoelectrical point for marble was obtained at about pH
11 below which zeta potential is positive. The isoelectrical point for neither
granite nor basalt was obtained within pH 6-12 where zeta potential values
were all negative.
PH
granite
—•— basalt
—A— marble
Figure 2. Zeta potential of solids as a function of pH.
It is known that the magnitude of zeta potential exerts an important influence
on the state of aggregation of the particles in water. When the magnitude of
zeta potential is high, the electrostatic repulsion between particles causes the
minerals to disperse readily and form stable suspensions. On the other hand,
for higher magnitude of zeta potential, the attraction between the particles and
oppositely charged polymer surface causes formation of bridging forming
agglomerates resulting in faster settling characteristics.
3.2. STABILITY OF SOLID SUSPENSIONS IN THE ABSENCE OF
POLYMER
The samples of the suspensions are allowed to settling in the absence of
polymer. Total suspended solids (TSS) in the supernatant as a function of
settling time in the absence of any polymer are given in Figure 3. As seen in
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Removal Of Suspended Solid Materials From The Wastewater Of Natural Dimension
Stone Cutting Plants By Flocculation
the figure, without any flocculant addition, settling rate is extremely low and
macroscopic observation revealed that the suspended solids were very fine to
colloidal and their settling was not possible even after their settling time is
increased.
settling time, min
Figure 3. Total suspended solid (TSS) in the supernatant as a function of settling time
in the absence of any polymer.
3.3. FLOCCULATION STUDIES
3.3.1. EFFECT OF FLOCCULANT TYPE
Three different flocculants with 6 mg/L addition dosage were tested for three
samples of suspensions at inherent pH value separately. The performance of
the flocculants was evaluated with respect to concentration of TSS in the
supernatant. The results are given in Figures 4-6.
As seen in the figures, anionic flocculant for marble sample; cationic
flocculant for granite and basalt samples gave the best result.
Among the three different flocculants tested for the marble suspension, the
most effective proved to be the anionic one for probably because of the
positive surface charge of the surface of the marble particles. Similarly, the
positive charged surface of cationic flocculant for the negatively charged
granite and basalt particles were more effective to form bridging. The results
are consistent with the results of zeta potential measurements. The polymer
bridging mechanism is responsible for flocculation of the suspensions [11].
240
S. Şener
Coulombic interactions between flocculant chains and surface of the particles
play a decisive role on polymer adsorption onto minerals surface.
On the other hand, hydrogen bonding between hydrogen of the polymeric
chains and oxygen of the various minerals surface seems to be the driving
mechanism of adsorption of the non-ionic flocculant onto particles. In all case,
the non-ionic flocculant was found to be considerably effective.
Figure 4. Total suspended solid (TSS) in the supernatant of marble wastewater as a
function of settling time (at pH 9.1 and 6 mg/L flocculant addition).
Figure 5. Total suspended solid (TSS) in the supernatant of granite wastewater as a
function of settling time (at pH 8.5 and 6 mg/L flocculant addition).
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Removal Of Suspended Solid Materials From The Wastewater Of Natural Dimension
Stone Cutting Plants By Flocculation
settling time, min
Figure 6. Total suspended solid (TSS) in the supernatant of basalt wastewater as a
function of settling time (at pH 7.6 and 6 mg/L flocculant addition).
3.3.2. EFFECT OF FLOCCULANT AMOUNT
The settling curves for marble, granite and basalt suspensions as a function of
flocculant dosage were obtained for each polymer and the results are given in
Figure 7.
flocculant dose, mg/L
Figure 7. Total suspended solid (TSS) in the supernatant as a function of different
flocculant dosage (Flocculants used: anionic A110 for marble, cationic C592 for granite
and basalt).
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S. Şener
It is clear that flocculation performance increased with increasing addition
dosage. As regards the effect of dosage, it is shown that the increase of
polymer addition results in an increasing settling rate, possibly due to coverage
of the particles surface with polymer chains and a neutralization of the surface
charge. The experimental results revealed that there is an optimum as regards
concentration of the polymer added into the particles suspension. The anionic
A110 flocculant with 3 mg/L dosage showed the optimum performance for
marble suspension, while the cationic C592 flocculant with 4 and 3 mg/L
dosage was found to be enough for granite and basalt suspensions,
respectively.
4. CONCLUSION
The following results can be inferred from this study:
• An isoelectrical point for marble was obtained at about pH 11 below which
zeta potential is positive. The isoelectrical point for neither granite nor basalt
was obtained within pH 6-12 where zeta potential values were all negative.
• The electrostatic forces (Coulombic interactions) play an important role on
polymer-particle attachment and thus on the flocculation.
• The negative charged anionic flocculant adsorbed to the positively charged
surface of marble particles; the positive charged cationic flocculant adsorbed
to the negatively charged surfaces of granite and basalt particles. The nonionic flocculant was found to be considerably effective for all samples.
• The increase of polymer addition results in an increasing settling rate. As
regards concentration of the polymer, 3 mg/L anionic polymer dosage in
marble suspension, 4 and 3 mg/L cationic polymer dosages in granite and
basalt suspensions gave the optimum flocculation performance.
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