Response surface optimization with central composite design of a coagulation process using Opuntia microdasys coagulant for turbid water treatment

Abstract

Water treatment using chemical coagulants is still a challenging issue in developing countries due to the high cost of purchase and the residuals of coagulants in sludge. Bio-coagulants are considered suitable substitutes for chemical coagulants. These materials are environmentally benign, non-toxic, and generate reduced sludge quantities in comparison to traditional chemical coagulants. This research aimed to evaluate the performance of different parts (cladodes and seeds) of locally available cactus species (Opuntia microdasys) as novel natural coagulants. Turbidity removal from turbid water was monitored under varying pH and doses of cladodes and seeds. Additionally, the effect of coagulation by cladodes and seeds on water treatment was investigated and optimized. Fourier transform infrared (FTIR) spectroscopy was conducted to elucidate the coagulation mechanism and experiment of coagulation was designed using response surface methodology (RSM). Seeds resulted in the optimum turbidity removal efficiency of 98% at pH 7 and 60 mg/L coagulant dose, while the cladodes resulted in a maximum turbidity removal of 85% at pH 7 and 40 mg/L coagulant dose. Analyses of variance (ANOVA) revealed that coagulant dose and pH both play a very important role in turbidity removal. Protein-specific functional groups (Amide I (C=O), medium 1° amines, and medium aliphatic amines) were identified by FTIR analysis to confirm the presence of protein content in cladodes and seeds likely responsible for the mechanism of coagulation. Floc sizes of cladodes and seeds, analysed
through micro-photographic analysis, were less than 10 µm and up to 50 µm, respectively.