Rising Sludge in Secondary Settlers Due to Denitrification

Mogens Henze, Rene Dupont, Peter Grau, Alejandro de la Sota

    Research output: Contribution to journalJournal articleResearchpeer-review

    Abstract

    High suspended solids concentrations in settler effluents can be caused by rising sludge, which is the effect of flotation of solids by nitrogen gas resulting from biological denitrification. Many factors influence the nitrogen gas bubble evolution. The most important factor is the rate of biological denitrification. Factors like nitrogen gas solubility and oxygen concentration in settler influent only play a minor role. The hydraulic retention time in the bottom part of the settler is, for all practical purposes, so high that sufficient nitrogen gas will be generated at temperatures above 20°C, if the nitrate content in the influent to the settler is above the critical one. For temperatures around 20°C the critical nitrate-nitrogen concentration is 6–8 g NO3−-N/m3. The best measure in order to avoid rising sludge is to denitrify the wastewater in the treatment process ahead of the settler.
    Original languageEnglish
    JournalWater Research
    Volume27
    Issue number2
    Pages (from-to)231-236
    ISSN0043-1354
    DOIs
    Publication statusPublished - 1993

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