Flocculation and antibacterial performance of dual coagulant system of modified cassava peel starch and alum

Abstract

The present study examined the synergistic effect of alum-modified cassava peel starch as dual coagulants in removing total suspended solids and Escherichia coli in dam water. The optimisation study was carried out to evaluate the effects of pH, coagulant dosage, and settling time on the flocculation performance of the coagulants. Alum as sole coagulant and combination of alum and native cassava peel starch were also studied for comparison purpose. Characterisations of the coagulants using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, and proton nuclear magnetic resonance spectroscopy studies confirmed the successful modification of cassava peel starch. Characterization studies revealed changes in surface morphology, elemental distribution after the modification. Pores, rough surfaces and several new element including Cl and Ca were observed on the surface of the modified starch. The presence of important active groups such as citral that responsible for antimicrobial and flocculation function was also reflected in the modified starch coagulant. The optimisation study indicated distinct performance of alum-modified cassava peel starch compared to alum alone and alum-cassava peel starch in terms of both flocculation and antibacterial functionality. High removal of both TSS and E. coli was achieved with the treatment using the combination of alum-modified cassava peel starch with superior removal of 92.75% and 100%, respectively, within 10 min of settling time using the recommended working conditions of alum-modified cassava peel starch dosage of 7.5–50 mg/L at pH 7. Additionally, the characterisation of flocs indicated that the employment of alum as sole coagulant and alum combined with the modified starch possessed distinctive features. Scanning electron micrograph showed that the flocs formed using alum-modified cassava peel starch was highly compacted clusters with appearance of E.coli cells embedded in the floc matrix, whereas for alum as the sole coagulant, the flocs were irregular-shaped and aggregated. Besides, the difference in peak occurrence from analysis reflected different mechanisms of floc formation using alum alone and alum-modified cassava peel starch. From the floc analysis study, charge neutralisation, bridging and sweep flocculation were the mechanisms that are expected to govern the coagulation and flocculation process.