The effect of flow equalization and low-rate prefermentation on the activated sludge process and biological nutrient removal

    Research output: ThesisDoctoral ThesisCollection of Articles


    The flow and load variations in the wastewater plant influent complicate the operation of the biological treatment process and harm the process performance. Moreover, when the biological nutrient removal (BNR) process is implemented in the plant, the plant influent is often lacking readily biodegradable organic matter. Readily biodegradable organic matter can be produced by prefermentation. In this research project, the primary clarifiers at the Pihlajaniemi WWTP in Savonlinna were modified in order to tackle the problems caused by flow variation and the lack of suitable organic matter. This study demonstrated that diurnal flow variations were efficiently levelled out in the existing primary clarifier basin volume. Surprisingly, a significant amount of organic matter was transformed into a more accessible form for the BNR bacteria when only flow equalization was in operation, but the attempt at enhancing the VFA production by adding an internal sludge recycle was not successful. The raw sludge removal in the equalization/prefermentation basin was not compromised in the modified operation. There is no commonly accepted method for assessing the magnitude of flow variations or the efficiency of the flow equalization. This thesis introduces a coefficient of flow variation that can be used for this purpose. The modifications in the pre-treatment were beneficial for the process performance. The main improvement in the biological process performance was observed with nitrification. The improvement could be mainly attributed to the diurnal flow equalization. Moreover, it could be demonstrated that the increased heterotrophic assimilation and also prefermentation influenced nitrification. The results on nitrification are of high importance because the effects of dynamic influent and feed water characteristics on nitrification have not been widely studied, especially not in full-scale. Moreover, the sludge settling characteristics were improved in the equalization/prefermentation process train compared with the reference process train. This, together with the more constant flow rate, enabled better hydraulic control of the secondary clarifiers. It can be concluded that the modification of the existing primary clarifier to a multifunctional pre-treatment basin is a feasible solution for the improvement of the BNR process performance. This process modification could be widely implemented in Finland because the majority of middle-sized and large WWTPs have primary clarifiers. Nitrification is usually the limiting part of the biological wastewater treatment. Therefore, implementation of equalization/prefermentation would enable a reduction in the aerated process volume. The economic balance of the modifications is clearly positive.
    Translated title of the contributionVirtaaman tasauksen ja esifermennoinnin vaikutus aktiivilieteprosessiin ja biologiseen ravinteiden poistoon
    Original languageEnglish
    QualificationDoctor's degree
    Awarding Institution
    • Aalto University
    • Vahala, Riku, Supervising Professor
    • Kiuru, Heikki, Thesis Advisor, External person
    Print ISBNs978-952-60-5152-9
    Electronic ISBNs978-952-60-5153-6
    Publication statusPublished - 2013
    MoE publication typeG5 Doctoral dissertation (article)


    • flow equalization
    • prefermentation
    • biological nutrient removal


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