Aloe sp. leaf gel and water glass for municipal wastewater sludge treatment and odour removal

In municipal wastewater treatment, sludge treatment accounts for about a third of the total wastewater treatment plant costs (Nowak 2006), 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 process (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. 2015). To enhance the performance of the secondary 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 synthetic organic flocculants such as polyacrylamide derivatives (Zahrim et al. 2010). Although these coagulants/flocculants have been successfully used for many decades in the water and wastewater sectors, there have been concerns 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 flocculants has also been highlighted as a significant barrier for the use of wastewater sludge in other areas such as agricultural applications (Abdelaal 2004). 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 coagulants/flocculants because they are effective in treating water, generally nontoxic, harmless, less sensitive to pH changes, and are biodegradable (Giri et al. 2015; Liu et al. 2010, 2015). Bioflocculants such as Moringa seeds (Menkiti & Onukwuli 2010), Lablab purpureus peels (Shilpa et al. 2012), Moroccan cactus extract (Abid et al. 2009), dates seeds (Al-Sameraiy 2012) and Opuntia dillenii (Nougbode et al. 2013) have been investigated for turbidity removal providing 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. (2004). A widely reported Indian grown natural coagulant (Nirmali seeds), Okra seeds pod tips, sap, plant stalk, and roots have also been studied (Al-Samawi & Shokralla 1996). Al-Samawi & Shokralla (1996) have used okra extract to treat clay suspensions and have concluded 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 natural 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, turbidity, chemical oxygen demand, heavy metals and textile dyes (Lee et al. 2015; Adugna & Gebresilasie 2018). Aloe leaves are well known for their mucilaginous jelly, which is referred to as Aloe gel (Femenia et al. 1999; Hamman 2008). Aloe gel contains mainly monosaccharides such as glucose and mannose, vitamins, minerals, polysaccharides and phenolic compounds together with some organic acids (Hamman 2008; Mazzulla et al. 2012). In their study on textile wastewater, Adugna and Gebresilasie (Adugna & Gebresilasie 2018) reported that Aloe steudneri performed as polyacrylamides for treating the wastewater and suggested that this natural flocculant can substitute the synthetic flocculant. Nougbode et al. (2016) 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 2018). In addition, other constituents of the gel such as glyco-aloe-modinanthrone and tannins are postulated 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. 2011; Radha & Laxmipriya 2015). 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.

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 m³ per year and serves several industries (textile, slaughterhouse and food wastewater). The average total solids concentration in the sludge is TS = 33 g/L. The sludge samples were immediately stored after collection in a fridge at 5 °C. In certain experiments, the pH of the sludge was adjusted using 1 M NaOH and 1 M HCl solutions.

The Aloe sp. leaves were harvested in March 2018 from a 2-year-old Aloe vera plant grown in the garden of the Biotechnology Center of Borj Cedria, Soliman, Tunisia. The gel was prepared as described by Kasmi et al. (2018). 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 preparation.

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 (2008). Sugars were assessed using Dubois protocol (Dubois et al. 1956). Proteins content was determined using Bradford methodology (Bradford 1976). As for minerals, phosphorus and magnesium concentrations were determined by means of colorimetric method using vanadomolybdic 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. 2009). Sodium and potassium contents were determined by the flame photometer method. Lyophilized Aloe sp. leaf gel was incubated in (1N) H₂SO₄ at 80°C 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. (2010).

Water glass (i.e. sodium metasilicate (Na₂SiO₃)) was prepared by reacting Tunisian silica sand (SiO₂ > 99%) with sodium carbonate, Na₂CO₃ (>99%, Honeywell) in a 1:1 M ratio at temperatures between 1,200 and 1,300°C as indicated by Bouaoun et al. (2017).

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 (2015). 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. 2009). The effect of pH on sludge settleability was also studied using initial pH values of 7, 11, 12 and 13 adjusted by 1 M NaOH solution.

The odour from raw and treated sludge were qualitatively 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 analysis.

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 85°C 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 specifications: ms quadrupole at 150°C, from 50 to 550 m/z full scan, 70 eV ion ionized energy, source and transfer line temperature at 250°C. The volatile compounds were identified by reference to mass spectra and the retention time of Wiley09 NIST2011 library.

Particle size distribution was determined with a Mastersizer 2000 laser light scattering instrument (Malvern Instruments Ltd, UK) as reported by Rahsepar et al. (2017). The samples of treated and untreated sludge were observed qualitatively with a scanning electron microscope (SEM) (FEI Quanta FEG 650).

The main characteristics of the collected sludge are summarized in Table 1. The results reveal that the mean value of 7.15 for pH, 11,200 mg O₂/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 ammoniacal nitrogen and phosphorus contents were 400 mg/L and 6.53 mg/kg, respectively. Compared to a similar sludge reported in Ramirez et al. (2018), 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 treatment plant.

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 analysis 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 Moringa 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. 2011; Leone et al. 2016; Ben Rebah & Siddeeg 2017). As it can be easily noticed, Aloe sp. fat, protein and total sugars contents are always in the range of the other plants' values.

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 calcium 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 particles.

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. However, 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 content 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 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 provoked 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 Ca²⁺ and K⁺, is necessary for the gelatinous properties of mucilage (gel) (Hamman 2008). 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 structure that is composed of various carbohydrates, such as L-arabinose, D-galactose, L-rhamnose, D-xylose and galacturonic acid (Vijayaraghavan et al. 2011). In this context, it was reported that galacturonic acid is significantly implicated 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 (–NH₂) groups, as well as hydrogen bonds. These functional groups are considered as preferred groups for the flocculation process (Sepúlveda et al. 2007).

Figure 4 illustrates the changes of COD and ammonia nitrogen 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 observations were also reported by Ramavandi & Farjadfard (2014). 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 NH₄⁺-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 NH₄⁺-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 NH₄⁺-N removal 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 constituents (e.g. proteins) may aggregate and fold at high doses (Gupta et al. 1998), which reduces the number of active functional groups available for NH₄⁺ accumulation and adhesion. The action of water glass was much more pronounced since its alkali character changes ammonium to ammonia gas (pKa = 9.25 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 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 noteworthy that both Alg and combined treatments recorded better results compared to the settling of raw samples without 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 (Supplementary Material).

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 (i.e. WG) and sludge with possible ion binding with inorganics in the wastewater (e.g. Ca²⁺) (Okano et al. 2015; Zuo et al. 2015).

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 formation of large aggregates showing pillars of 10–20 μm of 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. 2015).

The outcome of this novel combined treatment of wastewater sludge by addition of Aloe gel and water glass highlighted the clear macroscopic aspect of coarse agglomerates of sludge with raw sludge having suspended fine particles.

The activated sludge flocs are known to have a complex 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 distribution and the presence of small particles. Next to bacteria, the flocs contain extracellular polymeric substances (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. 2001). Consequently, considering the reported Aloe gel compositions and the involvement of its ingredients in the flocculation/coagulation process, the preeminent role of Aloe gel in the flocs formation mechanism could be considered. Moreover, 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.

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 disappearance 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. 2009). A human olfactory sensing experiment was carried out and showed that the twenty people did not detect any unpleasant 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 conditions. 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 turbidity 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 particles. Furthermore, the Aloe leaf gel odour removal efficiency was revealed through VOCs analysis of the treated 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-friendly sludge.

The Supplementary Material for this paper is available online at https://dx.doi.org/10.2166/wst.2020.123.