Phosphorus recovery from sewage sludge through integrated thermochemical treatment and sequential wet extraction

• Main Author: Tiwari, Satya Brat
• Other Authors: Lim Teik Thye
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering; Sewage sludge; Thermochemical treatment; Phosphorus; Resource recovery; Biochar; Pyrolysis; Hydrothermal treatment; Heavy metal; NMR; Leaching; Hydrochar
• Online Access: https://hdl.handle.net/10356/180994
• Institution: Nanyang Technological University

Phosphorus (P) is extracted from its natural mineral phosphate rock, which is finite and has a skewed geographical distribution. It is critical for the survival of life on earth and is mainly used for fertilizer production. P recovery from sewage sludge (SS) is critical for addressing the dual challenges of unsustainable fertilizer production and increasing eutrophication. However, the most prominent approach to P recovery from SS, i.e., acidic extraction of P from thermally treated SS residues (such as ash and char) is complicated by toxic trace element (TE) contamination. This PhD thesis investigates a novel approach combining integrated thermochemical treatment and sequential wet extraction to enhance P recovery from SS without TE contamination. The research is structured around three core studies, each addressing specific aspects of the process, collectively offering a comprehensive understanding of this novel method for P recovery. The first study assessed the feasibility of the novel process and explored the underlying mechanisms for high P recovery efficiency. In this process, dry powdered SS was mixed with dry powdered alum sludge (AS) and subjected to acidic extraction in the pH range of 3 – 4. The amendment with AS under acidic conditions aimed to convert apatite phosphorus (AP) (mainly calcium phosphate (Ca-P)) to non-apatite inorganic phosphorus (NAIP) (mainly aluminum phosphate (Al-P)). Subsequently, the solid residue was extracted with an alkali at high pH (>12). Amending SS with AS reduced orthophosphate loss during acidic pretreatment by 59% compared to unamended SS. The highest orthophosphate alkaline recovery achieved was 74% for the amended SS. Various analytical techniques, including, solid-state NMR, and XPS; and modelling tools such as Visual MINTEQ and central composite design were employed to elucidate the mechanisms underlying the improved recovery. The findings demonstrated that the conversion of Ca-P to Al-P during acid pretreatment through Ca-P dissociation and adsorption of released orthophosphate onto Al(OH)₃, significantly boosted alkaline P recovery, making this method applicable to diverse sludge compositions globally. Building on these findings, the second study explored an integrated approach combining sequential wet extraction and pyrolysis for P recovery from SS. SS+AS mixture underwent acid pretreatment, pyrolysis at varying temperatures, and subsequent alkaline extraction from char. The highest alkaline P recovery efficiency, 88%, was achieved from the amended SS char prepared at 400°C. Pyrolysis at high temperatures (> 600°C) led to the conversion of NAIP to AP and immobilization of P in the char matrix, resulting in lower alkaline P recovery (< 60%). Most TEs were encapsulated in the char matrix, except for Zn, which partially volatilized above 600°C. This study highlighted that AP to NAIP conversion during acidic pretreatment and optimal pyrolysis conditions were crucial for maximizing P recovery. Advanced characterization techniques, including solid and liquid-state NMR, XPS, XRD, and FTIR, were applied to investigate the underlying mechanism, indicating that this integrated approach effectively isolated P from TEs, enhancing the viability of the method for practical applications. Addressing the challenges associated with processing dry sludge and pyrolysis, the third study investigated the effects of hydrothermal conditions in the acidic pretreatment of SS with AS. This process involves acidic hydrothermal carbonization (HTC) followed by alkaline extraction from the residual solids (called hydrochar). A central composite design experiment optimized HTC conditions, achieving a maximum alkaline P recovery of 82% under optimal conditions. Solid-state NMR analysis revealed that P associated with aluminum in the hydrochar occurred via surface complexation rather than AlPO₄ formation. Detailed analysis using ICP-MS indicated that most TEs were retained in the hydrochar, resulting in a P-rich and TE-deficient extract. This approach potentially offered a cost-effective and energy-efficient pathway for P recovery, with the scope for further refinements to meet regulatory standards for TE concentrations in fertilizers. In conclusion, this thesis presents a robust framework for P recovery from SS through integrated thermochemical treatment and sequential wet extraction. The studies collectively demonstrate that appropriate amendments and extraction conditions can significantly enhance P recovery while addressing TE contamination issues. This integrated methodology offers a sustainable solution for P recovery, aligning with global efforts towards resource recovery and environmental sustainability, and presents a promising avenue for large-scale implementation.