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The nature, distribution and cycling of dissolved hydride-reducible and 'hidden' arsenic in the surface and interstitial waters of estuarine systems has been studied by cryogenic trap hydride-generation atomic absorption spectroscopy (HG-AAS) methods. With 'hidden' arsenic compounds no volatile hydride is formed upon treatment with sodium borohydride and pretreatment methods are necessary. Batch ultra-violet irradiation pretreatment processes for the detection of 'hidden' arsenic were further improved by the addition of persulphate resulting in the effective breakdown of non-hydride reducible arsenic compounds to 'inorganic' arsenic forms in 4-8 hours. Photolysis with persulphate using a microwave driven lamp reduced the time required to 2.5 minutes for freshwater samples, but concurrent heating effects made it unsuitable for estuarine water samples. A novel inline photooxidation HG-AAS technique was also developed which converted both hydride-reducible organoarsenic and non-hydride reducible arsenic species to 'inorganic' arsenic with ca. 92±4% efficiency. A seasonal study of the surface waters of Southampton Water and the estuary of the River Itchen revealed that during the summer months dissolved hydride-reducible 'inorganic' arsenic concentrations decreased as the hydride-reducible methylated arsenic concentrations increased. Dissolved 'hidden' arsenic concentrations were consistent with the release of complex organoarsenicals from planktonic organisms by active excretion, during senescence and grazing. These complex organoarsenic compounds are then broken down to simpler forms by chemical and biological processes. Deposition of these organoarsenic compounds into the sediments is accompanied by further breakdown and release into the overlying water column. Only once the decomposition has reduced the chemical complexity of the organoarsenicals can they be detected as hydride-reducible arsenic species.

The focus of this study has been on the ability of crushed seed suspensions of the pan-tropical tree Moringa oleifera Lam. to clarify turbid waters. Information regarding the coagulant effect of the seeds was first published in 1979. Since then a number of studies have been carried out to establish the effectiveness and nature of the coagulant effect. These are reviewed. The coagulation effect has been attributed to water soluble, low molecular weight cationic proteins contained within the seed kernel. The mode of activity has been proposed to be charge neutralisation via the patch mechanism. In an attempt to further the understanding of coagulation behaviour, four water sources in Malawi and Zimbabwe have been examined. A modified theory of protein coagulation that involves a combined charge neutralisation and protein precipitation mechanism is presented. Further work required to fully determine the mechanism of activity is discussed. In order to establish whether seed solutions could be considered for use within continuous flow water treatment systems a pilot plant was designed and constructed in Malawi. Details of the design are presented. Studies undertaken over a three-year period demonstrated that, as a coagulant, seed solutions were effective. Subsequent to these trials, an evaluation of the efficacy of using seed solutions on an operational treatment works was undertaken. Factors that might be potential constraints to utilisation at this scale, for example seed processing and dosing solution preparation, were investigated. Although effective as a coagulant, it has been concluded that the use of powdered seed suspensions is not a viable option for large-scale applications. Amongst those constraints identified, problems with the preparation of dosing solutions, the potential for increasing Total Organic Carbon levels in treated water and regulatory issues are considered.