Two-step granular activated carbon filtration in drinking water treatment

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Doctoral thesis (article-based)
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81, [81]
Helsinki University of Technology, Laboratory of Water and Wastewater Engineering, Teknillinen korkeakoulu, vesihuoltotekniikan laboratorio, 33
In the treatment of Finnish surface waters, the main objective is to reduce natural organic matter (NOM) before the water is disinfected. In many treatment plants, granular activated carbon (GAC) filtration is applied for the enhanced removal of NOM after conventional treatment. The aim of this study was to assess the performance of two-step GAC filtration in NOM removal and to determine its effects on the water quality in the distribution system. Furthermore, the optimal operation conditions of the existing process scheme at Helsinki Water's treatment plant after installing two-step GAC filtration and UV disinfection was evaluated. The study revealed that the major factors influencing the effective NOM removal in GAC filtration are the choice of GAC type and the frequency of regeneration. Other factors, such as empty bed contact time and biological degradation rate, were of minor importance. However, even the fully exhausted "old" GAC effectively decreased the assimilable organic carbon (AOC) and the regrowth of heterotrophic bacteria in the distribution system. The most interesting finding of the study was that introducing GAC filtration and UV disinfection increased the nitrification in the chloraminated distribution system. Introducing GAC filtration is a challenge for the more efficient use of the existing treatment process. Although optimal ozonation probably provided the highest biological activity in the following exhausted GAC filters, alone it does not justify the use of ozone. After installing GAC filtration and UV disinfection, the role of ozone is rather to provide an additional barrier against unexpected changes in the source water quality.
adsorption, natural organic matter, optimization, biological processes
Other note
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