Integrated treatment process of pulp and paper mill effluent using coagulation and heterogeneous photocatalysis

Abstract

The pulp and paper industry generates 10-250 m³ of wastewater per ton of pulp and paper products. Accordingly, it is the third largest wastewater-generating industry with more than 700 types of discharged toxic contaminants. These substances are difficult to degrade by natural means and present hazardous risks to the human health (e.g. carcinogenicity, mutagenicity and endocrine disruptors) and ecosystem (e.g. scum formation, thermal impact, and eutrophication). However, conventional biological treatment methods are incapable of degrading these complex and bio-recalcitrant substances. Therefore, the main objective of this research was to develop a more sustainable integrated treatment system (consisting of coagulation process and heterogeneous photocatalysis) to effectively remove suspended solids and persistent organic contaminants in the raw pulp and paper mill effluent (PPME). Firstly, a detailed preliminary study of utilizing Cassia obtusifolia (C. obtusifolia) seed gum as a natural coagulant to pre-treat raw and undiluted PPME was investigated. At recommended coagulation process conditions (initial effluent pH = 5, C. obtusifolia seed gum dosage = 0.75 g/L, slow-mixing velocity and time = 10 rpm and 10 min, respectively, and settling time = 1 min), C. obtusifolia seed gum removed high loadings of total suspended solids (TSS) and chemical oxygen demand (COD) of up to 86.9 and 36.2%, respectively. Following that, the interactive effects of coagulation process conditions were evaluated. Under optimal pre-treatment conditions (C. obtusifolia seed gum dosage = 0.17 g/L, alum dosage = 0.09 g/L, and slow-mixing time = 3.40 min), the combined use of C. obtusifolia seed gum and alum at natural pH of raw PPME (pH 7.2) yielded 89.6 and 55.4% of TSS and COD removals, respectively. Pre-treatment of raw and undiluted PPME using coagulation in the present work established that C. obtusifolia seed gum could be used as an alternative plant-based coagulant or as a potential coagulant aid in pre-treatment of complex industrial effluent. Photoactive Fe₂O₃-TiO₂ was synthesized in the absence of solvent and at ambient conditions using mechanochemical process (via ball milling) for the subsequent photocatalysis of pre-treated PPME. Detailed characterization studies revealed enhanced intrinsic properties of the fabricated Fe₂O₃-TiO₂. The highest photodegradation efficiency of pre-treated PPME (62.3% of COD removal) was achieved using Fe₂O₃-TiO₂ synthesized at 20 min of milling time, 250 rpm of milling speed, and 1 mol% of Fe₂O₃ loading. Then, the interactive influences of photocatalytic operating conditions and reusability of Fe₂O₃-TiO₂ were investigated. Under optimal photocatalytic conditions (initial effluent pH = 3.88, Fe₂O₃-TiO₂ dosage = 1.3 g/L, and air flow-rate = 2.28 L/min), 80.6% of COD was removed from pre-treated PPME using Fe₂O₃-TiO₂. The present study established that the mechanochemical process enabled a greener and facile synthesis of a highly photoactive, recyclable and retrievable Fe₂O₃-TiO₂ that could effectively degrade industrial PPME. Lastly, this research concluded that the studied integrated treatment system of raw PPME (using coagulation process and heterogeneous photocatalysis) achieved an overall treatment efficiency of 89.6 and 91.6% TSS and COD removals, respectively. This study has successfully demonstrated an effective and environmentally more sustainable integrated treatment system to degrade raw PPME.