The efficacy of Moringa oleifera as a practical application for sustainable water treatment

Abstract

The aim of this research was to determine if the seeds of Moringa oleifera were appropriate for use in potable water treatment. The seeds grow, and are generally used, in regions that lack basic water treatment infrastructure. In these regions, the seeds serve as a primary method of water treatment by removing pathogens and causing flocculation and coagulation of suspended solids. Significant information is available in the literature describing the use of the crushed seeds as an antibacterial agent. The compounds that cause the antibacterial effects have been isolated, identified and tested against a range of pathogenic bacteria. Of these compounds, the M. oleifera cationic proteins were chosen for further research with a focus on the attachment of these proteins to the surface of silica and sand, the inactivation of bacteria using these functionalised surfaces, and the regeneration of the functional surfaces. This thesis outlines the results from studies where the use of sand and silicon dioxide that had been functionalised with MO₂.₁ was investigated for bacterial attachment and inactivation. A method of immobilising MO₂.₁ onto the surface of sand to produce the functional sand (ƒ-sand) was experimentally optimised. The antibacterial effect of the ƒ-sand on Escherichia coli suspended in water was examined by packing ƒ-sand into columns. The columns were found to remove above 90% of the bacteria. Further experiments were conducted to determine whether the inhibited E. coli could be removed from the ƒ-sand without causing the MO₂.₁ to separate from the sand surface. Two regeneration methods were successfully developed leading to the regeneration of the ƒ-sand. In both cases the attachment of the MO₂.₁ was measured quantitatively by determining the amount of MO₂.₁ bound to the ƒ-sand surface, and qualitatively by measuring the amount of E. coli that was removed by the ƒ-sand after it had been regenerated. The first regeneration method used the non-ionic surfactant dodecyl glucoside. It was observed that after the initial exposure of ƒ-sand to dodecyl glucoside, there was an increase in the bacterial removal by the ƒ-sand on subsequent regenerations. The second method involved heating the used ƒ-sand to between 50 ºC and 100 ºC for between 1 minute and 60 minutes. It was found that heating the ƒ-sand to 60ºC allowed for repeated bacterial removal at a constant rate. A laboratory scale-up trial of the ƒ-sand was undertaken to determine the efficiency of the ƒ-sand for removing E. coli from artificially contaminated water. The different experimental parameters that were investigated included varying the concentration of E. coli, operating three columns in series and reversing the direction of flow so that the filter behaved either as a packed bed or a fluidised bed. In this series of experiments a 1.1 log reduction was the highest removal achieved from a 100 CFU/mL E. coli suspension through a fluidised ƒ-sand bed. In the final investigation, the attachment of MO₂.₁ to silicon dioxide (ƒ-SiO₂) and the effect of the ƒ-SiO₂ on E. coli and Micrococcus luteus inactivation were quantified. Both bacteria were shown to attach to the ƒ-SiO₂, and the effect of dodecyl glucoside on these bacteria was investigated. The M. luteus did not separate when exposed to the dodecyl glucoside, but remained attached to the ƒ-SiO₂. The underlying mechanisms for removal were examined by measuring the zeta potential of each system and making physical observations with live cell microscope. The thesis is divided into 8 chapters. These are comprised of the introduction and literature review; where all of the antibacterial components isolated from M. oleifera are discussed. Following the literature review, the methods chapter presents an outline of the general analytical methods used throughout all of the experiments. Chapters 4 to 7 provide the experimental, investigations that focussed on a laboratory-scale investigation using 100g ƒ-sand, and the reusability of ƒ-sand and ƒ-SiO₂. The thesis is concluded in Chapter 8 where the applicability and relevance of ƒ-sand and ƒ-SiO₂ are discussed, followed by the identification of future areas of research.