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Technoeconomic assessment of removing the residual chlorine dioxide from drinking water

Gkika Vasileia

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Summary

Water disinfection is an essential stage in its treatment process, aiming to eliminate pathogenic microorganisms and protect public health. Chlorine dioxide is a very powerful disinfectant, capable of effectively neutralizing microbes. Its importance is particularly prominent in drinking water treatment, as it mitigates the formation of harmful by-products associated with conventional chlorine use. The aim of this thesis is to evaluate the most suitable chemical dechlorination method among selected chemicals for the removal of residual ClO2 from water, as well as to assess the implementation cost of these methods in water treatment facilities. The chemicals used were sodium sulfite, sodium metabisulfite, ammonium chloride and sodium thiosulfate, all in powder form, while sodium thiosulfate was also tested in tablet form. The results of the experimental investigation showed that Na2SO3 achieved exceptionally high dechlorination efficiency (98.7 %), making it the most efficient solution in terms of performance. Similarly high effectiveness was observed with Na2S2O5, with dechlorination efficiency of 97.4 %. Na2S2O3, exhibited significant variation, as in powder form, its efficiency ranged from 12-87 %, while in table form it achieved dechlorination greater than 98.5 %, highlighting the effect of the material’s form on its effectiveness. In contrast, NH4Cl had insufficient efficiency (approximately 40 %) and was deemed inadequate for use as a dechlorinating agent. The techno-economic evaluation included the investigation of the procurement cost of chlorine dioxide, as well as the estimation of the cost of procuring the chemicals for the dechlorination of ClO₂ in a water treatment plant with a maximum capacity of 10,000 m³/d. The analysis revealed that Na2S2O5 is the most economical option among the tested dechlorinants, with an annual cost of €9,260 and consumption of 10,290 kg/y, combining high efficiency with relatively low usage. Na2SO3 follows, with an annual cost of €10,230 and lower consumption of 6,820 kg/y, offering a balanced option. Finally, Na2S2O3·H2O, despite its effectiveness, had the highest annual consumption (16,790 kg/y), which increases procurement costs (18,470 €/y) and imposes greater storage and handling requirements. Overall, Na2SO3 stands out as the optimal choice for dechlorination applications, combining high efficiency and low cost and is recommended for applications where consistently maximum effectiveness is required. Na2S2O5 is suggested for cases requiring maximum efficiency, despite slightly higher consumption, whereas NH4Cl is not recommended due to its low performance. In conclusion, the technoeconomic comparison confirms that the choice of dechlorinating agent should be based on both effectiveness and application suitability, in order to ensure environmental protection without excessive costs.

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