Aloe sp. leaf gel and water glass for municipal wastewater
sludge treatment and odour removal
Thameur Jaouadi, Mounir Hajji, Mariam Kasmi, Amjad Kallel,
Abdelwaheb Chatti, Hichem Hamzaoui, Adel Mnif, Chedly Tizaoui
and Ismail Trabelsi
Aloe gel (Alg), which is a natural extract from the Aloe sp. plant, was evaluated in this study for its
potential use as a bioﬂocculant to treat urban wastewater sewage sludge. The gel was used alone
and combined with water glass (WG) under controlled conditions in laboratory experiments. Alg was
found effective to settle the ﬂocculated sludge rapidly and remove distinctive unpleasant odours of
the sludge as highlighted by gas chromatography–mass spectrometry (GC/MS) analysis.
Furthermore, Alg was pH tolerant and had no effect in changing the pH of the wastewater. The
optimum dose of Alg was 3% at which a sludge volume index (SVI) of 45.4 mL/g was obtained within
30 min settling time. To enhance the treatment performances of Alg, WG was also evaluated as an
alkali agent to further reduce the chemical oxygen demand (COD) and ammonia (NH4-N) in the
wastewater. At equal doses of 3% of WG and Alg each, the combined treatment outcomes showed
high turbidity and NH4-N removals of 83 and 89%, respectively, but the overall COD removal was at
best 25%. The settling rate of treated sludge with combined Alg/WG was very rapid giving an SVI of
25.4 mL/g within only 5 min.
Ofﬁce National de l’Assainissement (ONAS),
Mariam Kasmi (corresponding author)
Laboratoire de Traitement et Valorisation des
Rejets Hydriques (LTVRH),
Technopark of Borj-Cedria, 8020, Soliman,
University of Carthage,
Avenue de la Republique, P.O. Box 77, 1054
Amilcar, Tunis, Tunisia
Laboratoire de Valorisation des matériaux utiles,
Centre national des sciences et recherche de
matériaux University of Carthage, Tunis,
Laboratoire Eau-Energie-Environnement (3E), Sfax
National School of Engineering,
University of Sfax,
P.O. Box 1173, 3038, Sfax, Tunisia
College of Engineering, Bay Campus,
Swansea SA1 8EN, UK
Key words |Aloe sp. gel, COD removal, odour removal, sewage sludge, turbidity, water glass
In municipal wastewater treatment, sludge treatment
accounts for about a third of the total wastewater treatment
plant costs (Nowak ), making it one of the important
treatment sections of the process. New cost-effective and
environmentally friendly sludge treatment technologies are
hence needed by wastewater undertakers. In Tunisia,
where this study was carried out, the activated sludge pro-
cess (AS) is the most used biological process for
wastewater treatment and the solid–liquid separation of
the sludge from the treated water remains a challenge due
to overload resulting in poor quality of treated wastewater
(Jemli et al. ). To enhance the performance of the sec-
ondary clariﬁers, addition of organic and/or inorganic
ﬂocculants is widely practiced. The coagulants/ﬂocculants
used in wastewater to aid the separation of sludge can be
either inorganic such as aluminium sulphate, or chemically
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synthetic organic ﬂocculants such as polyacrylamide deriva-
tives (Zahrim et al. ). Although these coagulants/
ﬂocculants have been successfully used for many decades
in the water and wastewater sectors, there have been con-
cerns about their environmental impacts, cost and the
sustainability of over-extraction of raw materials, which
are not renewable resources. The leaching of monomers
from sludges treated with synthetic organic polymeric ﬂoc-
culants has also been highlighted as a signiﬁcant barrier
for the use of wastewater sludge in other areas such as agri-
cultural applications (Abdelaal ). Therefore, demand for
new coagulants/ﬂocculants to improve the characteristics of
the produced sludge to be safely disposed of or utilized in
other sectors (e.g. land spraying) is increasing. In recent
years, bioﬂocculants have emerged as potential substitutes
to traditional inorganic and synthetic polymeric coagu-
lants/ﬂocculants because they are effective in treating
water, generally nontoxic, harmless, less sensitive to pH
changes, and are biodegradable (Giri et al. ;Liu et al.
,). Bioﬂocculants such as Moringa seeds (Menkiti
& Onukwuli ), Lablab purpureus peels (Shilpa et al.
), Moroccan cactus extract (Abid et al. ), dates
seeds (Al-Sameraiy ) and Opuntia dillenii (Nougbode
et al. ) have been investigated for turbidity removal pro-
viding removal percentages in the range 78–94%. Moringa
oleifera seed, maize and chitosan were also used in direct
ﬁltration of lake water and were successfully evaluated for
turbidity and microorganism removal as reported by
Mandloi et al. (). A widely reported Indian grown natu-
ral coagulant (Nirmali seeds), Okra seeds pod tips, sap,
plant stalk, and roots have also been studied (Al-Samawi
& Shokralla ). Al-Samawi & Shokralla ()have
used okra extract to treat clay suspensions and have con-
cluded that the okra extract was a powerful polyelectrolyte
coagulant whether it was used as a primary and/or as a
coagulant aid with alum. They also conﬁrmed that the natu-
ral okra extract performed much better than alum at high
turbidity waters. Aloe vera has also been used in water
and wastewater treatment to remove suspended solids, tur-
bidity, chemical oxygen demand, heavy metals and textile
dyes (Lee et al. ;Adugna & Gebresilasie ). Aloe
leaves are well known for their mucilaginous jelly, which
is referred to as Aloe gel (Femenia et al. ;Hamman
). Aloe gel contains mainly monosaccharides such as
glucose and mannose, vitamins, minerals, polysaccharides
and phenolic compounds together with some organic
acids (Hamman ;Mazzulla et al. ). In their study
on textile wastewater, Adugna and Gebresilasie (Adugna
& Gebresilasie ) reported that Aloe steudneri performed
as polyacrylamides for treating the wastewater and
suggested that this natural ﬂocculant can substitute the syn-
thetic ﬂocculant. Nougbode et al. ()have also conﬁrmed
that leaf gels from several Aloe species could be used as
natural ﬂocculants for water treatment. Adsorption was
suggested as the mechanism by which Aloe gel provides
water treatment due to its high ﬁbre content (Adugna &
Gebresilasie ). In addition, other constituents of the
gel such as glyco-aloe-modinanthrone and tannins are postu-
lated to be responsible for the gel’s coagulation property.
Despite being effective to treat water, Aloe gel, similarly to
other natural ﬂocculants, could increase the residual
organic matter in the treated water. According to literature,
the chemical composition of Aloe plants largely depends on
the species analysed but overall the organic matter can
reach up to 81% of the mass of Aloe plants (Eugene et al.
;Radha & Laxmipriya ). In this study, Aloe gel
was evaluated for the ﬁrst time as a bioﬂocculant to enhance
the gravity settling of municipal wastewater sludge. The
study also highlights the increased performance of using
Aloe gel in combination with sodium silicates to obtain
faster settling velocities of the treated wastewater sludge.
Several jar tests were carried out to select the optimal
doses of Aloe gel and sodium silicates and the effects on
pH changes and the volume of sludge produced.
MATERIALS AND METHODS
The sludge samples were collected in two 20 L plastic jerry
cans from the wasted line of a secondary settling reactor of
an activated sludge process of the municipal wastewater
treatment plant (Chotrana II, Tunis, Tunisia). The Chotrana
treatment plant serves a population equivalent to 400,000
with a total wastewater ﬂow of 110,847 m
per year and
serves several industries (textile, slaughterhouse and food
wastewater). The average total solids concentration in the
sludge is TS ¼33 g/L. The sludge samples were immediately
stored after collection in a fridge at 5 C. In certain exper-
iments, the pH of the sludge was adjusted using 1M
NaOH and 1M HCl solutions.
Preparation of Aloe sp. gel
The Aloe sp. leaves were harvested in March 2018 from a
two-year-old Aloe vera plant grown in the garden of the Bio-
technology Center of Borj Cedria, Soliman, Tunisia. The gel
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was prepared as described by Kasmi et al. (). Brieﬂy, the
whole leaves were washed with a tap water and the spikes
which were placed along their margins were removed
before slicing the leaf to separate the skin from the ﬁlet.
The resulting ﬁlets were mixed and homogenized using a
hand electric blender (Moulinex, model genuine). The gel
was used freshly in each experiment immediately after its
Aloe sp. gel analysis
Aloe gel composition in terms of fats, sugars, proteins and
minerals content was determined. Fats assessment was
performed according to the methodology reported by
Hamman (). Sugars were assessed using Dubois proto-
col (Dubois et al. ). Proteins content was determined
using Bradford methodology (Bradford ). As for min-
erals, phosphorus and magnesium concentrations were
determined by means of colorimetric method using vana-
domolybdic complex and atomic absorption. Calcium
concentration was determined using EDTA method; zinc,
copper, and iron assessments were performed using
atomic adsorption method (Rodier et al. ). Sodium
and potassium contents were determined by the ﬂame
photometer method. Lyophilized Aloe sp. leaf gel was
incubated in (1N) H
at 80C for 30 min to extract
cations. Concentrations of Cd, Zn and Fe in the extracts
were determined by atomic absorption spectrophotometry
(SpectrAA 220, Varian, Australia). Protocol adopted from
Zorrig et al. ().
Preparation of water glass
Water glass (i.e. sodium metasilicate (Na
)) was pre-
pared by reacting Tunisian silica sand (SiO
sodium carbonate, Na
(>99%, Honeywell) in a 1:1 M
ratio at temperatures between 1,200 and 1,300C as indi-
cated by Bouaoun et al. ().
Coagulation and ﬂocculation process
The coagulation and ﬂocculation tests were carried out in a
jar apparatus equipped with four steel paddles. The doses
of the Aloe gel varied according to the values 1, 3, 5 and
7% (v/v) of sludge. The coagulation experiments were
performed using a modiﬁed protocol as described by Patil
& Hugar (). The coagulation step was done at high
speed of 200 rpm for 2 min followed by the ﬂocculation
step at 50 rpm for 30 min. The settling time was set to
60 min after ﬂocculation.
Settling experiments were made using a glass column
(46 cm high and 7.8 cm in diameter) in which the height
of the liquid/sludge interface was recorded at regular time
intervals (Zodi et al. ). The effect of pH on sludge settle-
ability was also studied using initial pH values of 7, 11, 12
and 13 adjusted by 1M NaOH solution.
Flocculating rate measurement
To calculate the ﬂocculating rate of Aloe gel, Kaolin clay
was used to make suspensions at 5 g/L in which Alg was
added and stirred for 2 min. After settling for 1 min, the
absorbance at a wavelength of 550 nm of the supernatant
sample was measured by a spectrophotometer UV/VIS
Lambda 25 (PerkinElmer). The ﬂocculating rate was
calculated according to Equation (1) (Liu et al. ),
is the absorbance of the supernatant sample
at 550 nm and A
is the absorbance of the control at
Flocculating rate ¼(A0A1)=A0×100% (1)
The total solids content (TS), suspended solids (SS) and
volatile suspended solids (VSS) were measured according
to the standard methods described by Rodier et al. ().
The total organic carbon (TOC) was measured using a
total carbon analyzer Shimadzu TOC 5050. The total nitro-
gen and phosphorous contents were determined following
the standard methods proposed by Rodier et al. ().
The pH, conductivity (mS/cm) and total dissolved solids
(TDS) (g/L) of each sample were determined using a
multi-parameter instrument (C860, Consort bvba, Belgium).
Chemical oxygen demand (COD) values were measured by
the potassium dichromate colorimetric method using an
open reﬂux system (Rodier et al. ). Ammoniacal nitro-
gen was determined according to the NF T90-15 method
(AFNOR). Sludge settling ability is expressed by means of
the sludge volume index (SVI) which is often recommended
for the characterization of the sludge formation (Tchobano-
glous et al. ;Zodi et al. ;Kallel et al. ). The SVI
is deﬁned by Equation (2).
H0SS 1000 (mL=g) (2)
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is the sludge height after 30 min settling (cm),
the initial height of the sludge in the settling column
(cm) and SS is the initial sludge concentration after treat-
The odour from raw and treated sludge were qualitat-
ively assessed by the mean of GC-MS. Four samples
identiﬁed as raw sludge (control), sludge treated with
Alg, sludge treated with water glass and sludge treated
with Aloe gel (3%) plus water glass (3%), were collected
and sealed in 250 mL glass. 100 mL of raw and treated
sludge were used as reactor for 24 h incubation at
room temperature. All samples were subjected to VOCs
Volatile organic compounds (VOCs) were analyzed
using a headspace (TELEDYNE TEKEMAR HT3TM)
coupled with an Agilent GC–MS system (GC with 7890A,
mass detector 5975C with Triple-Axis, insert XL MSD). A
30 mL headspace vial was used for incubation of the
sample performed during 30 min in headspace oven at
50 C, then transferred in a heated line at 85C to avoid
condensation of VOCs and injected in the GC inlet for
2 min. An HP-5 ms (5% phenylmethylsiloxane) column
(Agilent 19091S-433: 2169.66548) was used (30 m ×
250 μm×0.25 μm) at 1.6 mL/min ﬂow with helium N60
(99.9999%) as carrier gas and the run was performed
over 25 min. The temperature programme of the oven
was: 70 C for 2 min, then, 230 C for 20 min and 270 C
for 25 min. The inlet had the following characteristics:
temperature of 250 C, helium gas carrier, split ﬂow of
20 mL/min, splitless split/splitless inlet: splitless injection
mode, 60.688 kPa as inlet pressure, 50 mL/min and 2 min
for purge ﬂow and time, respectively; gas saver on and
20 mL/min for gas saver ﬂow and 2 min for gas saver
time. In this analysis, the used MS had the following speci-
ﬁcations: ms quadrupole at 150C, from 50 to 550 m/z full
scan, 70 eV ion ionized energy, source and transfer line
temperature at 250C. The volatile compounds were ident-
iﬁed by reference to mass spectra and the retention time of
Wiley09 NIST2011 library.
Particle size distribution was determined with a Master-
sizer 2000 laser light scattering instrument (Malvern
Instruments Ltd., UK) as reported by Rahsepar et al.
(). The samples of treated and untreated sludge were
observed qualitatively with a scanning electron microscope
(SEM) (FEI Quanta FEG 650).
RESULTS AND DISCUSSION
The main characteristics of the collected sludge are summar-
ized in Table 1. The results reveal that the mean value of
7.15 for pH, 11,200 mg O
/L for COD and 16 g/L for SS.
VSS content was 2.6 g/L, the TOC was about 473 mg/kg
and the ammonical nitrogen and phosphorus contents
were 400 mg/L and 6.53 mg/kg, respectively. Compared to
a similar sludge reported in Ramirez et al. (),we
observed a low carbon-nitrogen content of Tunisian sludge
and quite high level of COD which might be attributed to
the industrial efﬂuent co-treated at the Chotrana wastewater
Aloe sp. gel characterization
In order to provide meaningful discussion on the effect of
the bio-ﬂocculants on the sludge treatment, the Aloe sp.
leaf gel characteristics were determined (Table 2). The analy-
sis revealed an important amount of soluble sugars of about
22.8 g/100 g of lyophilized gel and high content of proteins
(7.8 g/100 g). Fats minerals and metals were also detected.
Table 2 illustrates other plants composition such as Morin-
ga oleifera seeds and cactus (Opuntia ﬁcus indica) plants,
that various studies pointed out the importance of their
use as ﬂocculant, coagulant or coagulant/ﬂocculants aid
for the removal of turbidity, COD and heavy metal
(Lopez-cervantes et al. ;Leone et al. ;Ben Rebah
& Siddeeg ). As it can be easily noticed, Aloe sp. fat,
protein and total sugars contents are always in the range
of the other plants’values.
Table 1 |Characteristics of sludge generated by Chotrana municipal wastewater
Measured parameters Unit Recorded values
pH –7.15 (at 25 C)
Moisture content % 84 ±1
Total solid content (TS) g·L
Suspended solids (SS) % 16 ±0.5
Total phosphorous mg·Kg
COD mg O
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Treatment of sludge with Aloe gel and water glass
Effect of Alg and water glass addition on sludge’spH
Figure 1 illustrates the effect of the Aloe sp. gel (Alg)
addition on the sludge’s pH. The ﬁgure also shows the
effect of adding water glass (WG) alone or combined with
Alg (Alg-WG) on pH values. Following the addition of
3 mL/100 mL Alg, the pH showed only a modest increase
from 6.95 to 7.9 to remain constant at 7.9 regardless of the
increased amount of Alg added. However, the addition of
3 mL/100 mL water glass exhibited a remarkable increase
in pH from 6.95 to 12.5. A further increase in WG has
increased pH modestly to reach a value of pH 13 after
adding 7 mL/L of water glass. In fact, the alkaline character
of sodium silicate is the main cause of pH increase as shown
by the reaction equation:
Effect on turbidity removal
Figure 2 shows the turbidity changes versus the added
volumes of Alg and WG separately and in combination.
The results show that as the added volume increases, the
turbidity decreases. According to Figure 3, Alg was less
effective than water glass to remove turbidity (Alg: 45%;
WG: 89%). This could be attributed to the formation of cal-
cium silicate as main precipitate following the reaction of
water glass (sodium silicate) and calcium content in the
sludge (Table 1). The obtained precipitate had a potential
to adsorb ﬁne particles, making the supernatant free of
Effect of Alg and WG addition on SS and VSS
Figure 3 illustrates the effect of the different doses of Aloe
gel and water glass added separately or in combination on
both SS and VSS removal. The obtained results show that
the SS content increased instead of being reduced. How-
ever, no signiﬁcant effect was noticed for VSS removal
with both treating agents used separately or combined.
The progressive addition of WG in the sludge at contents
higher than 1% induced a reestablishment of the SS con-
tent to its original value (∼20.3 g/L) after a drop to
14.5 g/L at 1% WG (Figure 3(b)). In order to investigate
the combined effect of Aloe gel and WG addition on the
sludge treatability, further experiments were carried out
at ﬁxed Alg content of 3% while WG dosages were
Figure 1 |pH values variation following the Aloe sp. leaf gel (Alg), water glass and aloe þ
water glass addition to sludge at different volume.
Figure 2 |Turbidity variation with different volume of Aloe sp. leaf gel (Alg), water glass
and Aloe gel þwater glass, removal rate.
Table 2 |Aloe leaf gel biochemical characterization compared to Moringa oleifera seeds
and Cactus (Opuntia ﬁcus indica)ﬂours
sp. leaf gel
et al. 2016)
et al. 2011)
Fats 4.8 36.7 2.38
Soluble sugars 22.78 18.4 67.6
Proteins 7.82 31.4 7.24
Ca 3.02 ––
Mg 0.98 ––
Na 3.03 ––
K 3.89 ––
P 0.01 ––
Fe 0.87 ––
Cu 0.04 ––
Zn 0.01 ––
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Figure 3 |Variation of suspend solid (SS) and volatile suspended solid (VSS) of sludge following the application of (a) Aloe sp. leaf gel (Alg), (b) water glass and (c) Aloe þwater glass.
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varied from 1 to 7%. The more WG was added, the higher
the SS content was recorded (ranging from 20.4 g/L at 1%
WG dosage to reach 27.5 g/L at 7% WG). The VSS values
recorded a moderate increase from 9 g/L at 1% to 10 g/L
at 7% (Figure 3(c)).
The mechanistic of ﬂocculation/coagulation as pro-
voked by Aloe gel was reported by many studies. In fact,
gels from Aloe species are composed of low protein and
lipid contents, and polysaccharide is considered as the
main ingredient. Interestingly, Aloe species are known by
the polysaccharide mucilage production. Furthermore, the
presence of minerals, such as Ca
, is necessary
for the gelatinous properties of mucilage (gel) (Hamman
). The Aloe sp. coagulation/ﬂocculation mechanism
could be interpreted as hypothesis by analogy to the
cactus, where the high ﬂocculation/coagulation capacity
of cactus was mainly attributed to its polysaccharide struc-
ture that is composed of various carbohydrates, such as
L-arainose, D-galactose, L-rhamnose, D-xylose and galac-
turonic acid (Vijayaraghavan et al. ). In this context,
it was reported that galacturonic acid is signiﬁcantly impli-
cated as the main active coagulant agent, based on its
polymeric structure. This polymeric structure provides a
bridge for particles to adsorb. Moreover, the functional
groups of cactus polysacharides included carboxyl
(-COOH), hydroxyl (–OH) and amino or amine (–NH
groups, as well as hydrogen bonds. These functional
groups are considered as preferred groups for the ﬂoccula-
tion process (Sepúlveda et al. ).
Effect of Alg and WG addition on COD and NH
Figure 4 illustrates the changes of COD and ammonia nitro-
gen concentrations with the added volume of Aloe gel and
water glass separately and combined. At low doses, the
recorded data reveal a slight increase of COD value when
Aloe gel was added to the sludge (Figure 4(a)). Similar obser-
vations were also reported by Ramavandi & Farjadfard
(). This increase in COD following the addition of Alg
was expected due to the organic matter content of Aloe
gel which is rich in organic substances (e.g. carbohydrates
and proteins). Yet, above 1 mL/100 mL of added Alg, the
COD values of the solution showed a decreasing trend but
the overall COD of the solution remains higher than the
initial COD of the sludge until the applied dose of Alg
exceeded 5 mL/100 mL Alg. A further increase in Alg
doses above 5 mL/100 mL results in COD values slightly
lower than the original COD value of the sludge. In contrast,
the addition of WG alone or combined with Alg resulted in a
much more effective COD removal than Alg reaching a
removal percentage of about 25% at a dose of 7 mL/
100 mL water glass.
Figure 4(b) shows the removal of NH
-N at different
doses of Alg and WG. According to Figure 5(b), the addition
of Alg and WG alone or combined at low doses resulted in a
signiﬁcant reduction in ammonia. At a dose of 1 mL/
100 mL, the removal of NH
-N was 66% for all agents to
reach 89% at 7 mL/100 mL WG. A further increase in Alg
dose above 1 mL/100 mL did not increase NH
but it made it worse (only 40% removal at 7 mL/L), possibly
due to a competition with COD removal as illustrated in
Figure 4(a) where COD removal with aloe gel becomes
relevant only at high doses. In addition, Alg active constitu-
ents (e.g. proteins) may aggregate and fold at high doses
(Gupta et al. ), which reduces the number of active
functional groups available for NH
adhesion. The action of water glass was much more pro-
nounced since its alkali character changes ammonium to
ammonia gas (pKa ¼9.25 at 25 C) that can be easily
removed from solution via the increase of pH to 12.9
(Figure 1), which justiﬁes the high ammonia removal of
89% obtained following the addition of WG.
Figure 4 |Variation of COD (a) and ammoniacal nitrogen (b) with volume of Aloe sp. leaf
gel (Alg), water glass and Aloe þwater glass removal rates.
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Performances of Aloe gel and water glass on sludge
Figure 5 depicts the sludge settling performance for Alg and
for combined Alg-WG treatments as well as the raw sludge
without any coagulant being added. The SVI values are
also indicated in Figure 5. The combined Alg-WG treated
sample exhibited a net drop from 100 mL to reach 15 mL
within only 5 min. The settled volume stabilized at 10 mL
after 10 min until the end of the experiment (90 min). The
Alg treated sample settled volume was limited to 28 mL
after 40 min or higher settling times. However, it is note-
worthy that both Alg and combined treatments recorded
better results compared to the settling of raw samples with-
out addition of any of the coagulants; where, at best, a ﬁnal
settling sludge volume of 34 mL was recorded after 80 min
settling. The effect of the optimal dosages of the combined
treatment of sludge after settling using 3% of Alg and 3%
of WG dosages showed a net difference of settling rates
Figure 6 gives the particle size distribution of the
untreated sludge, sludge with Aloe gel and sludge with
both Aloe gel and water glass. Particle size shifts are clearly
observed for the Alg-WG treated sludge. The sludge particles
shifted from ﬁne sizes (∼100 μm) to larger particles and the
agglomerates were found to have a bimodal distribution
with a mean size value of 300 μm. This might explain the
accelerated settling velocity observed in the presence of
WG.The increased particle size as a result of WG addition
could be explained by the formation of large aggregates
resulting from the interaction between silicate polymers
Figure 5 |Effect of Aloe gel (•) and combined Aloe gel-Water glass (▪) addition on the
treated sludge compared to the raw sample (▴) during 90 min.
Figure 6 |Raw and treated sludge particle master size distribution.
Figure 7 |SEM image of raw sludge (a), and aloe gel (b) combined Aloe gel and water
glass (c) treated sludge.
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Figure 8 |Mass spectra of VOCs from the untreated sludge (a) and the treated sludge using Aloe sp leaf gel (b) and water glass (c).
9T. Jaouadi et al. |Municipal wastewater sludge treatment and odour removal Water Science & Technology |in press |2020
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(i.e. WG) and sludge with possible ion binding with inorgan-
ics in the wastewater (e.g. Ca
)(Okano et al. ;Zuo
et al. ).
The SEM images (Figure 7) show that the structures of
the raw sludge and the treated sludge with Alg-WG have
an abundance of micro pores. The SEM images suggest
that signiﬁcant changes in the ﬂoc’s morphology occurred
following the addition of Alg-WG as evidenced by the for-
mation of large aggregates showing pillars of 10–20 μmof
length between aggregates in the presence of Alg-WG. The
observed building structure might be attributed to calcium
silicate due to the reaction of silicates in the water glass
with calcium in the sludge (Okano et al. ).
The outcome of this novel combined treatment of waste-
water sludge by addition of Aloe gel and water glass
highlighted the clear macroscopic aspect of coarse agglom-
erates of sludge with raw sludge having suspended ﬁne
The activated sludge ﬂocs are known to have a com-
plex and heterogeneous composition. Furthermore, the
ﬂocs characteristics (size, structure, etc.) vary widely
according to the surrounding environment variation
which could affect their dewaterability mainly the size dis-
tribution and the presence of small particles. Next to
bacteria, the ﬂocs contain extracellular polymeric sub-
stances (EPS) and various inorganic and organic
molecules. Water represents the main component of the
microbial aggregates, followed by the EPS and biomass.
The EPS embody a highly hydrated matrix reaching 98%
of water content (Keiding et al. ). Consequently, con-
sidering the reported Aloe gel compositions and the
involvement of its ingredients in the ﬂocculation/coagu-
lation process, the preeminent role of Aloe gel in the
ﬂocs formation mechanism could be considered. More-
over, it has been reported that Aloe gel can inﬂuence
sludge ﬂoc characteristics, including: morphological
aspects; physical properties; and chemical constituents of
polymeric substances and metallic ions.
Performances of Aloe gel on sludge odour removal
The mass spectrum of VOCs emitted by the untreated
sludge is illustrated in Figure 8(a). The analysis allowed
the detection of four compounds identiﬁed as Toluene
(peak 1), Benzenamine, N-ethyl- (peak 2), 4-ﬂuoro-1,2-
xylene (peak 3) and phenol, 2,6-bis(1,1-dimethylethyl)
(peak 4) with retention times (min) of 5.087, 12.394,
13.415 and 17.981 and relative areas (%) of 41.03, 38.34,
12.76 and 7.86, respectively. Figure 8(b) and 8(c) illustrate
the mass spectra of VOCs emitted by the treated sludge
using Alg and WG, respectively. The spectra show that
the addition of either Alg or WG results in the disappear-
ance of peaks 2, 3 and 4 initially detected among the
untreated sludge VOCs. Nevertheless, a new compound
identiﬁed as D-limonene with retention time (min) of
9.867 and relative area of 48.64% was detected after Alg
addition. Limonene is one of the most widespread terpenes
in the ﬂavour and fragrance industry (Zodi et al. ). A
human olfactory sensing experiment was carried out and
showed that the twenty people did not detect any unplea-
sant smell from the treated sludge, which provides a
further evidence of the removal of odour following the
application of Alg.
This study presented a new insight of the potential use of
Aloe gel for the treatment of municipal wastewater sludge
as a novel bioﬂocculant. Aloe sp. leaf gel, at different
doses (1, 3, 5 and 7%), was tested as bioﬂocculant via
sequential treatments that used coagulation, ﬂocculation
and sedimentation processes under deﬁned operating con-
ditions. The Aloe gel showed good performance for
sludge solid–liquid separation. To enhance the Aloe gel
action, a further step was necessary by the addition of
water glass. The combined treatment showed that the use
of aloe gel and water glass at 3% yielded a removal of tur-
bidity up to 83%, and 89% of ammonia nitrogen. Moreover,
the resulting ﬂocs from the treatment using aloe gel and
water glass generate much coarser particles and readily
settling sludge, typically with only 5 min of settling time.
SEM investigation enabled the observation of multi pillars
of 10 to 20 μm length located within the sludge intra par-
ticles. Furthermore, the Aloe leaf gel odour removal
efﬁciency was revealed through VOCs analysis of the trea-
ted sludge which indicated the disappearance of odour-
causing substances following application of Aloe gel or
water glass. The proposed process would be an attractive
approach not only to reduce the treatment cost but also
to minimize environmental concerns and generate eco-
The Supplementary Material for this paper is available
online at https://dx.doi.org/10.2166/wst.2020.123.
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