My internship was at the Water Treatment Plant (WTP) with the objective of studying the operational and technological conditions required to reduce the concentration of chlorites in the treated water.
When chloride dioxide (ClO2) is used at the Water Treatment Plant as a pre-oxidant (for the oxidation of Iron and Manganese, to prevent undesired taste and odours and also to avoid the excessive formation of halogenated sub products from disinfection), chlorites (ClO2-) and chlorates (ClO3-), sub products which are potentially harmful to human health, are noted as sub products of the reaction to obtain chlorine dioxide (ClO2).

My internship included various studies, such as, the study of the water treatment process, the study of certain parameters of the water along the treatment process, as well as the operational conditions of the system and the influences these had in chlorite formation. The study of the chlorine dioxide generator’s efficiency. And also, the study of a method and application, compatible with the current installations, to remove or reduce chlorites within the provisory limit of 0,7 mg/L set by the WHO.

For this purpose, I try the addition of a specific Powder Activated Carbon – PAC . All the experimental trials were done on a lab bench scale (Jar-test), where all necessary precautions were taken into account to guarantee simulated hydraulic behaviour of the Fontaínhas Water Treatment Plant (agitation times and unit retentions) and maintain the similarity of coagulation, flocculation and adsorption processes (type and dosage of constituents). The doses and conditions fixed for the “Jar-test” essays were chosen to best represent the hydraulic and operational behaviour of the Water Treatment Plant. For both types of carbon studied and in both throughput conditions, there were positive results in the reduction of chlorites, in which the dosage of added carbon was the main factor. The highest values of removal were 64% with one type and 57% with another, inferior or very close to the limit set by the World Health Organization.

Now, the Water Treatment Plant adopted this method and it works just fine!