Alum sludge is the sludge discharged from a sedimentation tank in a drinking water treatment plant when polymerized with poly-aluminum chloride (PAC). In this paper, granular alum sludge adsorbent (GASA) was manufactured using powdery alum sludge (PAS) as the raw material and methods such as gluing and pore-forming. The effects of different binders, pore-forming agents, roasting temperatures, and roasting times on the formation of GASA and its dephosphorization performance were investigated. Results showed that the optimum binder was AlCl3 at a mass ratio of 8%, and the best pore-forming agent was starch at a 4% dosage ratio. Meanwhile, the optimum roasting temperature and time were 500 °C and 2 hours, respectively. The specific surface area of GASA was 23.124 m2/g. Scanning electron microscopy suggested that GASA's surface became rough, particles became tight, and average pore size increased, with additional pore channels. P adsorption by GASA reached 0.90 mg/g. The effluent phosphorus concentration of actual tail water decreased to 0.49 mg/L and the removal rate reached 73.5% when the GASA dosage was 20 g/L. The findings of this study are important for the further development of a low-cost adsorbent material for P removal in the future.
The treatment of P-containing sewage has attracted increasing attention because excess P that enters natural water often leads to the eutrophication of water bodies (Alaba et al. 2018). At present, P removal methods in sewage mainly include biological, chemical and physical approaches. Biological methods are difficult to control in practical applications, and the processing effects are unstable. Chemical methods require a large amount of chemicals, and the operating costs are extremely high (Morse et al. 1998). Adsorption is favored because of its low cost, high efficiency and simple operation (Bhatnagar & Sillanpää 2010).
At present, a new type of polymer coagulant, poly-aluminum chloride (PAC) is widely used for coagulation and sedimentation in water supply plants locally and internationally, and produce a large amount of alum sludge every year. Sanitary landfill, marine disposal, and comprehensive utilization of the sludge are the main sludge disposal methods after dewatering (Da et al. 2015). Studies have shown that alum sludge has a large specific surface area and pore volume. At the same time, it has been found that the dominant components in all sludge samples are SiO2, Al2O3, and Fe2O3 (Wu et al. 2004), which possess good selective adsorption capacity for certain anions in water (Ali 2010). Thus, alum sludge can be used as an adsorbent in the field of water treatment. Many local and foreign researchers have studied Al morphology in alum sludge because Al compounds are the key to alum sludge adsorbency. Al in alum sludge exists in an amorphous form (Babatunde et al. 2008). Alum sludge lacks a crystalline structure (Babatunde et al. 2008; Ippolito et al. 2009).
Alum sludge has been used as an adsorbent for P removal because of its good adsorption characteristics relative to P, so the application of alum sludge for P removal in water has become a topic of interest. Experimental trials have demonstrated that alum sludge can efficiently reduce P levels in waste water and P adsorption fits the Langmuir adsorption isotherm equation (Georgantas & Grigoropoulou 2005; Babatunde et al. 2008; Yang et al. 2008; Zhang et al. 2010; Nazirul et al. 2014). The application of Al sludge from water plants to artificial wetland substrates has been studied and it has been found that Al sludge is good for P removal (Razali et al. 2007). Han et al. modified alum sludge by adding Al(OH)3. The results showed that the modified sludge not only had a good removal effect on PO3 but could also adsorb a certain amount of organic P (Han et al. 2011). In addition to P, alum sludge removes turbidity and chemical oxygen demand (COD) (Yang et al. 2009). The development of alum sludge adsorbents has good potential in terms of cost, adsorbent preparation, adsorption process, and removal efficiency. Reusing sludge provides several benefits, such as reducing the amount of solid waste from water plants and producing a low-cost adsorbent for P removal.
Alum sludge discharged from sedimentation tanks in drinking water treatment plants can effectively remove P from sewage. However, powdery alum sludge (PAS) typically forms colloidal suspensions during the water treatment process, which leads to high turbidity and influences the water treatment effect. PAS is difficult to settle and separate after use. Thus, recycling alum sludge is relatively difficult (Nazirul et al. 2014). In order to solve the problems above, this experiment utilizes the clay-like characteristics of PAS, molding it by gluing, making it porous by adding a porogen, and improving its mechanical strength by roasting at a high temperature, so that porous plastic granular alum sludge adsorbent (GASA) with good properties can be produced. (Xu et al. 2015). Taking the lost ratio and the removal rate of the simulated P waste water as performance indices, we analyzed the effects of different binders and pore-forming agents in GASA formation and determined the optimum dosages of additives. The effects of various roasting temperatures and times on the properties of GASA were investigated, and the best conditions were selected. Finally, GASA was applied to actual tail water, and the removal efficiency of total phosphorus by GASA was investigated.
MATERIALS AND METHODS
Experimental material preparation
Alum sludge was obtained from a sedimentation tank of a water supply plant in Nanjing with the PAC dose in the range of 20–25 mg/L. After the alum sludge has been air dried to a paste, dried in a constant temperature-drying oven at 60 °C, pulverized and filtered through a 100-mesh sieve, then dried at 102 °C to a constant weight, it was sealed and stored at room temperature. The preparation process of the GASA is shown in Figure 1(a) and 1(b). Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) analysis on alum sludge was performed and it was found that the mass percentages of the main chemical elements of the alum sludge were as follows: O: 52.62%, C: 12.26%, Si: 14.36%, Al: 9.17%, Fe: 6.47%, Ca: 1.10%, Mg: 0.77%, and other elements: 3.25%.
Pure KH2PO4 was dried at 110 °C for 2 hours. A certain amount of dried KH2PO4 was dissolved in distilled water to prepare a certain concentration of P-containing waste water. After sealing, the sample was kept in a cold storage (4 °C).
The actual tail water was taken from the biochemical effluent of a printing and dyeing plant waste water treatment facility. The actual tail water contains a certain concentration of PVA (polyvinyl alcohol) and dye. The tail water quality is shown in Table 1.
|Water quality index .||COD (mg/L) .||Total nitrogen (mg/L) .||Total phosphorus (mg/L) .||pH .|
|Water quality index .||COD (mg/L) .||Total nitrogen (mg/L) .||Total phosphorus (mg/L) .||pH .|
Performance indicators of GASA
Adsorption performance was examined by measuring the P removal rate using the following steps. A total of 0.5 g adsorbents were added to 100 ml of simulated P-containing waste water. The pH was adjusted to 7.0 and the mixture was placed in a 20 °C constant temperature shaker for 24 hours (SHA-C, Guohua Electric Co., Ltd, China). The pH of the liquid was monitored using a precision pH meter (PHS-3, Shanghai Precision Scientific Instrument Co., Ltd, China). The residual P concentration was measured by ammonium molybdate spectrophotometry to calculate the P removal rate.
The specific surface area and pore volume of adsorbents have a considerable influence on the adsorption performance of the obtained adsorbent material (Makris et al. 2004). The specific surface area of the GASA after being formed was measured and compared with that before being formed.
Actual tail water treatment
100 ml of actual tail water was placed in conical flask and 0.50 g, 1.00 g, 1.50 g, 2.00 g, 2.50 g, 3.00 g, or 3.50 g GASA were added to the conical flask, which was sealed. The total phosphorus and COD values in the solution were measured after shaking in a constant temperature oscillator (20 °C, 150 r/min) for 48 hours.
RESULTS AND DISCUSSION
Choice of binders
The binder makes the PAS granular and ensures that the adsorbent is shredded as small as possible during use to avoid affecting water turbidity. The binder also overcomes the issue of PAS being difficult to separate and recycle. The addition of some inorganic binders also could enhance the adsorption performance of the adsorbent. In this test, two inorganic binders, namely Na2SiO3 and AlCl3, were selected, and polyvinyl alcohol (PVA) and carboxymethyl cellulose (CMC) were selected as the two organic binders (Baklouti et al. 1996; Hua et al. 2015).
The lost ratio and P removal rate for the two inorganic binders of Na2SiO3 and AlCl3 are shown in Figure 2(a). As shown in Figure 2(a), at any addition ratio and taking AlCl3 as the additive, the lost ratio of the obtained adsorbents was lower than that of Na2SiO3, and the P removal rate was higher than that of Na2SiO3. With the increase in the additive dosage, the P removal rate, using the adsorbent prepared with AlCl3 as a binder, gradually increased. The lost ratio also decreased from 3.6% to 1.2%. However, the P removal rate using Na2SiO3 as an adhesive gradually decreased, and the lost ratio remained essentially unchanged at 4.7%. Therefore, AlCl3 as a binder can not only reduce the lost ratios of the obtained adsorbent particles but can also improve the P removal rate. When 8% Al was added, the P removal rate reached 87.71%, and the lost ratio also decreased to 1.32%. Although the lost ratio continued to decrease when the dosage of AlCl3 continued to increase, then the effect was insignificant. Therefore, for economic reasons, the optimum AlCl3 dosage is 8%.
PVA, CMC, and other organic adhesives are suitable for use at low temperature. If the adsorbent with an inorganic binder was unroasted at high temperature after drying, then the GASA disintegrated directly in water or even brokeup completely. Hence, the adsorbent with an inorganic binder needed to be roasted at high temperature. However, GASA added to an organic binder was difficult to form after roasting at high temperature. The obtained adsorbent was scattered (its tightness or solidity was not high) and it exhibited a high lost ratio. Under normal temperature conditions, after PVA and CMC were added to the dry pellets of PAS, the lost ratio of the GASA in water was extremely low. Therefore, GASA with organic binder does not require roasting at a high temperature. Instead, the process should be carried out under normal temperature conditions.
Figure 2(b) shows the effect of CMC and PVA addition on the lost ratio and P removal rate of the GASA. As shown in Figure 2(b), the lost ratio of the granular adsorbent decreased with the increase in the amounts of CMC and PVA at room temperature and remained stable at approximately 4% to 2%. The P removal rate in the solution from adsorbent particles of both binders decreased with the increase in dosage due to the excessive addition of the organic binder, which clogs the pores of the adsorbent and reduces its adsorption performance (Mikutta et al. 2004). The addition of excessive organic additives also decreased the amount of GASA per unit weight, thereby decreasing the P removal efficiency.
The contrast between Figure 2(a) and 2(b) shows that the removal rate of P from the GASA prepared with the organic binder at room temperature was much lower than that prepared with AlCl3 at the same lost ratio. Therefore, AlCl3 was selected as the binder for further tests due to its positive effects.
Choice of pore-forming agents
After granulating and forming, PAS aggregated, so the pores may have been blocked, thereby decreasing the adsorption performance. Pore-forming agents were decomposed at high temperature. Therefore, both the space occupied by pore-forming agents and the gas escaping after decomposition can increase the porosity of the obtained adsorbent (Xu et al. 2015). In this test, NaHCO3 and starch were compared with each other.
The lost ratios and P removal rates of the granular adsorbent were measured. As shown in Figure 3, when NaHCO3 was used as a pore-forming agent, the lost ratio increased from 4% to 10% with the increase in the mass ratio of NaHCO3 from 2% to 10% because NaHCO3 will decompose to produce CO2 gas at high temperature. The addition of excessive NaHCO3 is inconducive to adsorbent formation (Co et al. 2015). Conversely, the P removal rate increased with the increase in NaHCO3 addition because the CO2 gas formation at high temperature increased the number of pores in the granular adsorbent, which was beneficial to adsorption. The lost ratio increased with the increase in NaHCO3. Several adsorbent particles broke down into powder, which strengthened the mass transfer effect and increased the P removal rates.
When starch was used as a pore-forming agent, its dosage exhibited minimal effect on the lost ratio of the resulting adsorbent. The lost ratio was essentially <1%, which was much lower than when NaHCO3 was used as a porogen. This result was because starch possesses a certain degree of cohesiveness. The P removal rate first increased, then decreased because the incorporation of excess starch at a certain temperature led to its incomplete decomposition and excess residue. This process reduced the amount of PAS per unit mass of GASA. All of these led to a reduction in the amount of P adsorbed. Therefore, starch is the best pore-forming agent, and the optimum dosage was 4%.
Effect of roasting temperature and roasting time
Roasting temperature exhibited considerable influence on the adsorption performance of the GASA. Figure 4(a) shows that when the roasting temperatures were between 400 °C and 700 °C, the lost ratios of the resulting adsorbent were low, that is, <1.2%. When the roasting temperature was <300 °C, the lost ratio was high. This is because at the lower temperatures, the rate of solid-state reaction was excessively low, and GASA could be effectively formed. Except for a small amount of substances that were soluble in water, GASA was mainly broken into fine particles. The P removal capacity of the resulting adsorbents first increased and then decreased with the increase in temperature. At 500 °C, the P removal effect was optimal. As the temperature continued to increase, the P removal effect of the resulting adsorbents on P started to deteriorate.
The reason for the results above is that the combustion of the pore-forming agents was incomplete at a low temperature, and the resulting adsorbent possessed few pores, a small surface area, and low adsorption capacity. With the increase in temperature, the porous agent combustion tended to be complete, thereby improving the pore volume and specific surface area of the resulting adsorbent and increasing its adsorption performance. Extremely high temperature destroyed the GASA structure, thereby resulting in the collapse and blockage of the adsorption pores as well as the poor adsorption performance of the resulting adsorbents (Hua et al. 2015).
In summary, at a roasting temperature of 500 °C, the resulting adsorbents displayed the highest P removal rate, with relatively low lost ratios. Thus, 500 °C is the optimal roasting temperature.
Roasting time influenced on the formation of GASA. Figure 4(b) clearly shows that the adsorption performance of the adsorbents obtained after a roasting time of 1–2 hours was better than after >2 hours. Appropriate roasting time can remove impurities from the GASA. When the roasting time was excessively long, the internal structure of GASA was damaged, and the adsorption performance was reduced. The lost ratios of the adsorbent that were obtained under a roasting time in the range of 2–3 hours were the lowest. When the roasting time exceeded 4 hours, the lost ratios of the resulting adsorbent increased, which was unfavorable for the formation of the GASA because excessively long sintering times lead to the deterioration of the clay material's properties, (Metzger & Yaron 1987). Excessively long roasting time also consumes considerable energy.
In addition to the factors above, selecting a calcination time of approximately 2 hours at a roasting temperature of 500 °C was economical and reasonable.
Specific surface area determination
The N adsorption–desorption isotherms of PAS and GASA obtained under optimum conditions are shown in Figure 5(a) and 5(b), respectively. The specific surface area and pore volume were calculated according to the BET specific surface formula. The performance of the adsorbent after molding was further examined, and the results are shown in Table 2.
|Sample .||SBET (m2/g) .||Pore volume (cm3/g) .||Average pore size (nm) .|
|Sample .||SBET (m2/g) .||Pore volume (cm3/g) .||Average pore size (nm) .|
Micropores play a major role in alum sludge adsorption, and their specific surface area reaches 104.9 m2/g (Makris et al. 2004). But the specific surface area of the GASA was only 23.124 m2/g. This confirms that the specific surface areas of alum sludge in various water plants differ (Wang et al. 2016). And alum sludge characteristics are influenced by the quality of the drinking water source, coagulant type, and treatment plant system (Siswoyo et al. 2014). As shown in Table 2, compared with PAS, the GASA only showed a slight decrease both in specific surface area and pore volume, and maintained the good characteristics of the original powder. And the average pore size increased to 13.206 nm after molding.
The pore diameter distribution curves of the two samples are shown in Figure 6(a) and 6(b). A pore size of 3.29 nm in the original PAS was the most widely distributed. After molding, the proportion of pores with a pore diameter of 10 nm increased significantly. These pores were the ones formed by the decomposition of the pore-forming agent.
Microstructure of adsorption material
The PAS and GASA prepared under the optimal conditions were subjected to SEM, as shown in Figure 7. The SEM images of the PAS and GASA showed that the surface morphology of the two samples were essentially the same, and the alum sludge surfaces before and after molding were relatively rough. The GASA after molding was more compact than before molding. At the same time, GASA still had many pores and these were not blocked due to the molding process. Many pore channels were still visible on the surface, which was consistent with the results of the specific surface area determination.
Actual tail water treatment
The removal effects of the GASA dosage on the total phosphorus and COD of actual tail water is shown in Figure 8(a) and 8(b), respectively. As shown in Figure 8(a), the effluent phosphorus concentration dropped to 0.49 mg/L and the removal rate reached 73.5% when the GASA dosage was 20 g/L. If GASA continued to be added, the effluent phosphorus concentration decreased slightly, and the effect was not obvious.
In the simulated phosphorus-containing waste water, the adsorption amount of phosphorus by GASA reached a maximum of 0.90 mg/g when the pH was 4.0. When the pH was higher than 4.0, the adsorption capacity gradually decreased as the pH increased, and when the pH was 10.0, the adsorption amount decreased to 0.65 mg/g. In actual tail water, the adsorption capacity of GASA for phosphorus was only 0.1–0.23 mg/g. It was lower than the adsorption amount in simulated water. This is because the organic matter in the actual tail water affects the adsorption of phosphorus by GASA.
GASA also had a certain removal effect on COD in actual tail water, as is shown in Figure 8(b). With the increased dosage, the removal rate of COD increased gradually. However, when the dosage was 20 g/L, the COD removal rate reached 35% and the effluent COD concentration decreased to 74 mg/L, but did not change much after this point. This was because all the organic matter that could be adsorbed by GASA in the tail water had been completely adsorbed, so the COD removal rate could no longer be reduced by increasing the GASA dosage.
Activated C is a universal absorbent for removing various contaminants from water. However, the widespread use of activated C is often limited because of its high cost (Bhatnagar & Sillanpää 2010). Researchers have attempted to develop inexpensive adsorbents that utilize large amounts of agricultural, industrial, and municipal wastes. Red mud, fly ash, slag and waste Fe(III)/Cr(III) hydroxide have been investigated for phosphate removal from water. But alum sludge has a larger specific surface area than those of red mud and fly ash (Bhatnagar & Sillanpää 2010). Furthermore, alum sludge is available almost free of charge and causes major disposal problems. The use of alum sludge as a low-cost absorbent could not only partly reduce the volume of alum sludge and the cost of waste disposal, but could also reduce the pollutants in waste water at a reasonable cost, including in terms of electricity and chemical additives.
PAS was used as a raw material to study the optimum preparation conditions of GASA. The effects of different binders and their dosages; pore-forming agents and their dosages; and roasting temperatures and times on the adsorption properties of the GASA were studied by single-factor tests. The results showed that alum sludge acting as a P adsorbent can secure P removal. When the binder used was AlCl3, the effect was optimum when the mass ratio was 8%. The best P removal rate was as high as 87.71%, with a lost ratio of 1.32%. Starch was the best pore-forming agent, and the optimum dosage was 4%. The optimal roasting temperature and roasting time were 500 °C and 2 hours, respectively. The GASA after molding were more compact than before molding and the surface remained rough with additional pores. Thus a porous plastic alum sludge adsorbent with good properties can be produced, which can also solve the problems of PAS. The adsorption capacity of phosphorus by GASA reached 0.90 mg/g in the simulated waste water, while it was only 0.23 mg/g in the actual tail water. When the adsorbent dosage reached 20 g/L, the effluent phosphorus concentration of the actual tail water decreased to 0.49 mg/L, the removal rate reached 73.5%, and the COD removal rate was 35%. The findings of this study are important for the further development of low-cost adsorbent materials for P removal in the future.
This work was supported by the Primary Research & Development Plan of Jiangsu Province (No. BE2016703), the Natural Science Youth Fund of Jiangsu Province (No. BK20171017), the National Natural Science Youth Fund of China (No. 51707093) and the Science and Technology Program of the Ministry of Housing and Urban-Rural Development of China (2014-K7-010).