Parametric study of coagulant recovery from water treatment sludge towards water circular economy

The domestic water treatment process through the circular economy and resilience values promotes opportunities to tackle water challenges through a transformative approach to provide more sustainable, comprehensive, and structured water and sanitation services. Circular economy attempts are responses to the current unsustainable linear model of ‘take, make, consume, and waste’ to reduce pressure on natural resources and eliminate waste (Gude 2021; Bouziotas et al. 2023). Global demand for drinking water has increased with the increase in the world's population, economic development, rapid urbanization, and changes in consumption patterns.

In conventional water treatment plants, the coagulation–flocculation process effectively removes impurities in water, such as chemical oxygen demand, turbidity, color, suspended solids (SS), heavy metals, oil, and organic matter, where chemicals are introduced and small particles begin to clump and become large flocs; thus, safe and clean drinking water is produced for the public (El-Gaayda et al. 2021; Abujazar et al. 2022). The coagulation process can be considered one of the most typical physicochemical processes in water treatment due to its ease of operation, relatively simple design, and low-energy consumption (Ahmad et al. 2016). This is attributed to the combined cost of coagulant and pH adjustment chemicals as well as the disposal of the resulting sludge or water treatment sludge (WTS) (Keeley et al. 2016).

Coagulation generates large amounts of sludge. In fact, almost 10,000 tonnes of sludge are generated per day (globally). WTS undergoes the dewatering process before being discarded into landfill or is directly discharged into water bodies. WTS has a high concentration of organic matter, inorganic salts, and various oxides (Shahin et al. 2019). Sludge has pathogenic microorganisms and heavy metals, such as zinc, mercury, chromium, and lead, which are difficult to degrade and potentially contaminate soil and water resources (Xu et al. 2019). It can significantly affect water quality and threaten aquatic ecosystems. In addition, the production of WTS is likely to increase as clean drinking water becomes a standard resource. Therefore, coagulant recovery is crucial, so that the metals contained in WTS can be reused and recycled for the subsequent treatment. There are a few methods, including acidification using hydrochloric acid (HCl) and sulfuric acid (H₂SO₄), basification by caustic soda, ion exchange, and electrodialysis processes, that can be employed to recover aluminum and other coagulants from WTS (Ahmad et al. 2021). The acidification process is the most investigated process as it is a cost-effective method to recover coagulants. Yang et al. (2014) studied the potential of recovered coagulant (treated using H₂SO₄) to remove phosphorus in which the phosphorus removal rates reached nearly 100%. Dahasahastra et al. (2022) reported turbidity removal using coagulant recovered from polyaluminum chloride water treatment plants (PAC-WTP) and acidified using HCl.

This study focused on the recovery of alum and PAC coagulants in WTS from two water treatment plants via acidification using H₂SO₄ at different normalities. The performance of recovered alum and recovered PAC was compared with commercial alum in terms of their ability to remove turbidity in surface water treatment under different coagulant dosages and pH values.

WTS was obtained from two water treatment plants (WTPs) in Malaysia. PAC sludge was generated when PAC was used as a coagulant, while alum sludge was generated when alum salt was employed as a coagulant. Each sludge was collected in the WTP sludge lagoon and stored in plastic containers. Raw water was collected at the inlet of the storage tank. The alum and PAC sludges were oven-dried at 105 °C for 3 h to remove the moisture content. The mass/weight of these two sludges before and after heating were measured and recorded. Then, the sludge cake was crushed to produce sludge powder. This work was conducted at the Water and Wastewater Engineering Laboratory, Universiti Malaysia Terengganu.

The alum and PAC sludges were modified using the acidification method, where 97% H₂SO₄ was used. H₂SO₄ with normality values of 0.5, 1.0, 1.5, 2.0, and 2.5 N was prepared by mixing them with 50 mL of deionized water. The alum and PAC sludges were treated with H₂SO₄ and were oven-dried at 105 °C for 3 h to remove the moisture content. The dried sludge cake obtained was ground into powder form and denoted as recovered alum and recovered PAC. The performance of both recovered alum and recovered PAC were evaluated using the jar test experiment.

The powdered sludge samples were characterized using several methods. The chemical composition and surface morphology of the sludge were analyzed using scanning electron microscopy (SEM), JEOL (JSM 6360LA). The sludge samples were also subjected to Fourier-transform infrared (FTIR) spectroscopy in the range of 4,000–600 cm⁻¹ to determine the surface functionalities of the sludge. The pH value and solid content of the sludge were measured according to the Standard Methods for Examination Water and Wastewater (APHA 2017).

Turbidity removal was tested using the jar test experiment, where 500 ml of raw water sample was poured into six beakers. Water quality parameters, including turbidity, pH, temperature, and total dissolved solids, were recorded before the experiment was conducted. The prepared coagulants, namely alum, PAC, recovered alum, and recovered PAC, were added into a beaker containing 500 mL of water. The mixture was then stirred at 120 rpm for 2 min, followed by slow stirring at 80 rpm for 20 min. The mixture was kept stagnant for 45 min to allow the flocs to settle down. The sediments formed were removed using the filtration system equipped with a vacuum pump to collect the water.

The performance of both recovered PAC and recovered alum prepared with 0.5, 1.0, 1.5, 2.0, and 2.5 N H₂SO₄ was tested using the method described above. The test was repeated by adding different dosages of recovered PAC and recovered alum (10–60 mg/L). Experimental work was also conducted at different pH values (3, 5, 7, 9, and 11). Water quality was measured and recorded. The best acid concentration was chosen due to the higher percentage of turbidity removal.

In the double bond region (1,500–2,000 cm⁻¹), a medium bond peak was detected around 1,635.64 cm⁻¹. This indicated the presence of a carbonyl double bond. There is a specific and unique region called the fingerprint region (600–1,500 cm⁻¹). Moreover, a weak bond between 912.33 and 1,002.98 cm⁻¹ is represented by the multiple absorption band. The number of peaks for recovered alum in Figure 2(b) showed a single bond region (2,500–4,000 cm⁻¹), where a medium narrow bond was found, which defined that there were hydrogen bonds in the material. A medium absorbance peak was detected around 1,629.85 cm⁻¹ in the presence of a double bond region. This signified that several carbonyl double bonds existed. The absorption bands from 910.40 to 1,002.98 cm⁻¹ indicated multi-band absorption. The IR spectra for both recovered coagulants were almost similar.

Both PAC and alum sludges underwent acidification treatment using 0.5–2.5 N H₂SO₄. As the normality of the acid increased, the percentage of turbidity removal increased to a certain degree and then decreased due to the Al-flocs dissolution process, affecting the coagulation process (Zand & Hoveidi 2015). As shown in Figure 3, the highest turbidity removal percentage was recorded at 1.5 N H₂SO₄ (50.51%) for recovered alum and 1.0 N H₂SO₄ (66.07%) for recovered PAC. Meanwhile, the lowest turbidity removal percentage was observed at 2.0 N H₂SO₄ for recovered alum and 1.5 N H₂SO₄ for recovered PAC, which was suitable for sludge acidification. Turbidity removal increases monotonically for different rates of acid as the coagulant dose also increases (Ahmad et al. 2016). The optimum condition of Al-flocs, which is comparatively predominant and adsorbs more colloidal particles, may be due to the presence of more adsorptive sites on aluminum hydroxide flocs (Zand & Hoveidi 2015). Therefore, sludge acidification using 1.5 and 1.0 N H₂SO₄ for each sludge was found to be less efficient in removing suspended colloids from river water.

The result also showed that the highest turbidity removal of 86% was obtained by recovered alum with 50 mg/L dosage. This study compared the performance of both commercial and recovered alum, where recovered alum exhibited higher turbidity removal of up to 6% compared to commercial alum. The turbidity removal decreased slightly with the addition of more recovered alum from 50 to 60 mg/L due to the destabilization of colloidal particles. Moreover, the turbidity removal percentage of recovered PAC and commercial PAC with 40 mg/L dosage was almost similar at 80%. The percentage of turbidity removal recorded was almost similar for both commercial and recovered PAC, i.e., 97% at 30 mg/L dosage. This proved that that recovered coagulant has the potential to be used in the water treatment process, thus reducing the dependency on commercial coagulant and reducing WTP-operating costs.

From these results, the turbidity removal efficiency was relatively high for both recovered coagulants compared to commercial coagulants. As the coagulant dosage increases, positive charges become more dominant leading to electrostatic repulsion between particles (Wang et al. 2021; Khazaie et al. 2022). Table 1 shows the analysis of variance (ANOVA) performed at a significant level (α) of 0.05 with a p-value of 0.685973, which was higher than 0.05. It indicated that the rejection of H_o was weak, and there was no statistically significant difference in the mean of the four types of coagulants due to increased dosage.

The best performance of alum was seen at pH 7, and its performance decreased to some extent at pH values of 5 and 8. The highest performance of PAC was observed at pH 5 and 6 (Wei et al. 2015). Based on Figure 5, it demonstrated that when the dosage increased, the final pH of PAC and recovered PAC increased, but the final pH of alum and recovered alum increased to some extent and then decreased. The pH disturbed the balance of reactions between organic functional groups and hydrogen ions and Al hydrolysis products. At low pH, hydrogen ions compete more vigorously with metal hydrolysis products (Demissie et al. 2021; Guo et al. 2023). The results showed that turbidity removal was dependent on pH and coagulant dosage. The highest turbidity removal was achieved at pH 5–6 for PAC and pH 6–7 for alum (Wei et al. 2015). From Table 2, the significance level (α) of 0.05 and the p-value of 0.054202, which was greater than 0.05, implied that there was no statistically significant difference. This result also proved that the rejection of H_o was weak. There was no statistically significant difference in the mean of the final pH with increasing dosage with the four types of coagulants.

The effect of pH on turbidity removal using PAC may be related to the formation of high polymeric positive hydrolyzates and Al(OH)₃ in the solution with a pH between 6 and 8 (Gong et al. 2022). Therefore, the turbidity removal percentage was lower at pH 3, 9, and 11 for both recovered coagulants.

Table 3 presents the ANOVA results at a significant level (α) of 0.05. The p-value of 0.11080052 indicated that there was no statistically significant difference. Since the p-value was greater than 0.05, the evidence to reject was weak. Thus, there was no statistically significant difference in the mean of the turbidity removal percentage on pH.

Coagulant recovery from WTS is crucial and contributes to the circular economy in the water sector. The WTS collected from the WTP can be recycled and reused in the subsequent cycle of water treatment. The acid digestion method proposed in this study was able to promote high recovery efficiency with good quality recovered coagulant and low treatment cost. This study also promotes the reduction of WTS disposal in the landfills and dependency on commercial coagulants.

The alum and PAC present in WTS sludge could be recovered with the addition of H₂SO₄ with various normalities. The optimum H₂SO₄ normality for recovered alum and recovered PAC were 1.5 and 1.0 N with 66 and 50.5% turbidity removal, respectively. More than 80% turbidity removal was achieved with a 40 mg/L dosage of recovered alum and recovered PAC. The quality of water treated with recovered alum and recovered PAC were almost similar to the commercial alum and PAC (80%). Increasing the dosage of recovered alum to 50 mg/L exhibited an increase in turbidity removal to 85%. This study compared the performance of commercial and recovered coagulants and found that the recovered alum showed higher turbidity removal of up to 6% compared to the commercial alum. The potential of recovered PAC and recovered alum as coagulants can be further explored to obtain water quality comparable to that treated with commercial coagulants and also beneficial for the reduction of WTP-operating costs. In addition, this study can be the baseline for the future development of more sustainable waste management, particularly in Malaysia. Nevertheless, the performance of recovered alum and recovered PAC must also be tested for the removal of total suspended solids, volatile suspended solids, biochemical oxygen demand and chemical oxygen demand before commercialization and scale-up for the pilot study.

The authors gratefully acknowledge financial support from Universiti Malaysia Terengganu through Interdisciplinary Impact Driven Research Grant (UMT/ID²RG/55444/2023).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.