Enhancing biomethane production and phosphorus recovery from aerobic granular sludge through thermal alkali pretreatment

Despite the increasing implementation of aerobic granular sludge (AGS) systems in wastewater treatment, the efficient recovery of energy and phosphorus from AGS-derived excess sludge remains a critical challenge, limiting the overall sustainability of the process. This study investigated the digestibility of low-temperature thermal-alkali pretreated excess sludge from the AGS process and phosphorus recovery as struvite from digested sludge to enhance resource recovery potential, including energy and phosphorus from excess sludge. The primary objectives of this study were to improve the anaerobic biodegradability of excess sludge through low-temperature thermal-alkali pretreatment, to assess phosphorus recovery via struvite precipitation, and to perform a comprehensive economic evaluation of the integrated recovery process. The pretreated sludge samples exhibited an increase in soluble chemical oxygen demand (sCOD) concentrations, which can be attributed to enhanced solubilization induced by the pretreatment. The maximum methane production of 216 ± 22 mL CH₄/g VS was achieved for the pretreated sludge sample at 100 °C and pH 10 (S3). The low-temperature thermal-alkali pretreatment before the anaerobic digestion can be recommended as an effective approach for the enhancement of the BMP, with an enhancement of 27 % for S3 concerning the sludge sample without any pretreatment (S0). On the other hand, the pretreated sludge samples of S4 (at 100 °C and pH 11) and S5 (at 100 °C and pH 12) had lower improvement in methane production compared to S3, which might be explained by the inhibition effect of sodium hydroxide (NaOH) concentration. The phosphorus recovery rate increased with the increment of pH values for each Mg/P ratio. The optimum phosphorus recovery rate (91.9 ± 3.7 %) was obtained at a magnesium/phosphorus (Mg/P) molar ratio of 1.5:1.0 and pH 10. Besides, the unit total cost and profit for struvite production were calculated as 3.40 $/ton sludge and 12.83 $/ton sludge, respectively. Overall, low-temperature thermal-alkali pretreated with struvite crystallization demonstrated the viability of an economically and environmentally sustainable way to enhance the recovery of resources from excess sludge. These findings highlighted the technical effectiveness and economic viability of integrating thermal alkali pretreatment with struvite crystallization to improve resource recovery from AGS excess sludge, offering a promising pathway toward circular wastewater management.