Turbidity removal from low turbidity synthetic water using crude aqueous extract from Moringa oleifera seeds

Abstract

Turbid river water extracted for drinking water usage is conventionally treated with inorganic coagulants such as aluminium sulphate or ferric chloride. Natural polymers such as cationic proteins from seeds of Moringa oleifera (drumstick tree) have received increased attention over the past three decades and shown to be effective alternatives over conventional coagulants. Past researchers reported various efficiencies and dosages in reducing turbidity of highly turbid water (efficiency of >90%) prepared synthetically. However, they are relatively inefficient in aqueous solutions with low initial turbidity value (<50 NTU). The cause of this inefficiency of turbidity reduction varies since different types of clay were used, mostly uncharacterised variations of kaolin and bentonite. It is hypothesized that the size of colloids and their stability in the turbid water affects the efficiency. The aim of this study was to assess the coagulation activity of a representative natural coagulant, M. oleifera to reduce turbidity of a stable colloidal solution prepared from bentonite having defined size. The optimum dosage of M. oleifera and turbidity reduction efficiency (TRE) were determined using jar test. The bentonite stock solution (BSS) was prepared by mixing bentonite powder comprising mainly of Na-Ca montmorillonite [(Na,Ca)0.3 (Al,Mg)2 Si4 O10 (OH)2. H2O] with either deionised or tap water. Two separation techniques used, sedimentation and centrifugation, yielding BSS with a colloidal size range which conforms with the stable size range of 0.1 to 0.7 μm as observed in natural water. The BSS was further diluted to obtain the synthetic turbid water (TW) of desired turbidity. The TWs were tested for colloidal stability with the most stable TW used for further coagulation activity testing. Centrifugation of BSS prepared with deionised water was found to produce the most stable TW. Likewise, TW prepared by centrifugation also resulted in a well defined and stable colloid with size range between 0.05 to 0.5 μm simulating the colloids found in low turbidity natural water. MO efficiently removed turbidity of TW up to 98% at a dosage of 2.28 gmo/kgTW.