A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science, School of Environment, Resources and Development

Thesis (M.Sc.) - Asian Institute of Technology, 1985

The laboratory experiment was investigated to study the role of silt and sea water in purification process in Chao Phraya River Estuary. The effect of silt on biochemical oxygen consumption was investigated by serial BOD bottles method. The presence of silt increased ultimate oxygen consumption (L) while kaolin had little or no effect. The effect of mixing increased ultimate oxygen consumption. silt and kaolin had no effect to deoxygenation coefficient (k). The effect of mixing on deoxygenation coefficient was not revealed. The adsorption of organic matter on silt was investigated by COD determination. The easily decomposiable organic matter in polluted canal (klong) was adsorbed on silt easier than the more stable organic matter. The effect of cations and sea water on the coagulation was investigated by Jar Test. Silt of Chao Phaya River was coagulated by artificial sea water (more than 10% of sea water contents). The Mg++ ion played the most effective role in coagulation process among the cations (Mg++, Ca++ and Na+) in sea water. Kaolin was not coagulated by artificial sea water, The canal water inhibited coagulation of silt by sea water due to the organic matter present in canal water. The settling velocities of silt and flocs were determined by using setting column. The settling velocity of silt-flocs that were formed by artificial sea water (2.7 x 10-3cm/s). The calculation by House's model (the suspended-load-distribution equation) showed that the increase in settling velocity by coagulation did not change the distribution of suspended sediment in the estuary.