Feasibility of applications of waste driven composite for the treatment of Paper and Pulp industry wastewater

Abstract

The agro-based pulp and paper (P&P) industry produces wastewater with persistent color due to lignin derivatives and other organic compounds, which conventional treatment methods have failed to address effectively. This study explores Advanced Oxidation Processes (AOPs) for decolorizing real industrial effluents. Following circular economy principles, industrial discards such as furnace blast sand (FBS) and foundry sand (FS) were repurposed as substitutes for iron in the synthesis of Fe-TiO₂ composites used in the treatment process. The results highlight AOPs and material reuse as promising strategies for enhancing environmental sustainability in wastewater management. The present study aimed to reduce the residual color in real wastewater streams from an agro-based P&P industry. Wastewater samples were collected from three distinct stages: the first, known as the UASB stream, which has highest amount of color was sourced from the initial stage involving straw washing and pulping; the second, the I/L or alkali stream, was obtained from the paper-making, chemical recovery, and bleaching stages; and the third, the O/L or outlet stream, was collected after secondary treatment, though it still exhibited significant color. Due to the varying properties and color intensities of these streams, different treatment methods were employed. A two-step process combining coagulation/flocculation, followed by a simultaneous dual process of simultaneous photocatalysis and photo-Fenton, was employed to treat both the UASB and I/L streams effectively. For O/L stream direct application of dual process was successful. Coagulation-flocculation was applied as a pretreatment to reduce the color load that hindered light penetration, making direct application of the dual process (photocatalysis and photo-Fenton) impractical for the UASB and I/L streams. This pretreatment step was necessary to address persistent organics like phenols and lignin derivatives, which were not effectively removed in the initial stage, allowing for enhanced color removal. For implementation of dual process, a visibly active and low-cost composite was prepared using industrial waste materials, such as FS, FBS and clay mixed in the ratio 1:1:2 respectively. A thin film of TiO₂ was coated on spherical composite beads, enabling photocatalysis, while iron leaching from the composite triggered the photo-Fenton process, effectively combining both effects within a single system. vii The study utilized three different reactors Batch, Recirculation, and Once-through to treat three distinct wastewater streams (UASB, I/L, and O/L). Initially, batch-scale experiments were conducted for all streams, optimizing variables such as pH, coagulant dose, bead surface area coverage, and oxidant dose. Under optimized conditions, the dual process achieved 90.62% color removal for the UASB stream in 90 min. 87.17% for the I/L stream in 120 min. and 91.6% for the O/L stream in 60 min. Further adjustments were made in the recirculation reactor by varying factors such as bead size and flow rate, alongside the previously mentioned parameters. Treating 5L of effluent per stream yielded 89.74% color removal for the UASB stream in 90 min. 84.5% for the I/L stream in 90 min. and 75% for the O/L stream in just 45 min. To optimize the process more efficiently, Design Expert software with Box-Behnken Design (BBD) was used for the I/L and O/L streams. The results showed a close match between predicted and experimental values, confirming the software's effectiveness. Both streams achieved high R² values (>0.8), validating the reliability of the model. The color removal study was also conducted using a once-through reactor approach for all three streams, employing similar operating parameters. Optimized conditions resulted in color removal efficiencies of 94.4% for the O/L stream, 88% for the I/L stream, and 84% for the UASB stream within just 45 minutes. To identify the primary reactive species responsible for color removal, different scavengers were used. The significant reduction of ~40-50% in color removal upon the addition of TBA quencher indicated that hydroxyl radicals (OH• ) played a major role in the process. Various characterizations, including SEM, EDS, UV-DRS, and FT-IR, HRTEM, XPS etc. were conducted to verify the catalyst's integrity, confirm the presence of elements, and identify the complexes formed. A key challenge in using composite beads is maintaining the hybrid effect over multiple recycles, which requires continuous iron extraction from the support material while preserving the surface activity of TiO2. In this study, the composite beads were successfully recycled for over 100 recycles with minimal reduction in color removal efficiency. The hybrid effect, encompassing both iron leaching and TiO2 activity, remained consistently effective throughout these recycles. Mineralization study was also performed in terms of % COD reduction. Cost analysis was also performed in scale-up trials employing once through reactor. This study is the first to report the successful implementation of a fixed-bed in-situ dual process using industrial waste materials, along with scale-up trials, for the treatment of real agro-based P&P industry wastewater