Particle size distribution modeling and kinetic study for coagulation treatment of tannery industry wastewater at response surface optimized condition

Abstract

The presence of considerable amounts of Total Suspended Solids (TSS) in wastewater streams can pose a severe threat to aquatic and human life. A high level of TSS will increase water temperature at an alarming rate and decrease dissolved oxygen levels. This study aims to remove TSS from raw tannery wastewater using an active coagulating protein extracted from Moringa oleifera (MO) seeds. The process variables such as, coagulant dosage and pH are varied to test the potential of the coagulant extracted using NaCl and KCl salt solutions, for TSS removal from tannery wastewater. The results show that the NaCl extracted coagulant could effectively remove 89.87% of TSS whereas the KCl extracted coagulant eliminated 76.23% of TSS, at the optimum condition of 40 mL coagulant dosage and 7 pH. The assessment of coagulation rate parameters using differential method of analysis reveals that at optimum condition, the TSS reduction process obeys second order kinetics with a rate constant 3.556 × 10– 5 (mg/L)– 1 min– 1. The volume distribution of suspended solids in raw wastewater is fitted with a log-normal distribution model and Gaussian distribution model. From the magnitude of the coefficient of determination (R2) and Akaike information criterion (AIC) of the Gaussian model (R2 = 0.6864 and AIC = 3.89), it is concluded that the Gaussian model is better than the log-normal distribution model (R2 = 0.4708 and AIC = 6.31) for explaining the volumetric distribution of suspended particles. The numeric distribution of suspended solids in raw wastewater is explained well by the variable β model (R2 = 0.668) rather than the power law model (R2 = 0.196). The 3D response surface plot indicates that maximum TSS removal can be achieved at an optimal condition of 40 mL coagulant dose and 7 pH. The quadratic model developed for the treatment process utilizing the NaCl extracted coagulant shows better response prediction ability (R2 = 0.8118) when compared to the model built for the KCl extracted coagulant (R2 = 0.7911). Both the model equations have a p-value less than 0.05 which indicates that the proposed models are statistically significant. The elliptical contours for TSS removal using NaCl extracted coagulant demonstrate the significant interactions between the process variables. This study exhibited the applicability of MO seed extract for real-time tannery wastewater treatment.