Oxygen Transfer in Two-Stage Activated Sludge Wastewater Treatment Plants

Abstract

Aeration is an energy-intensive process of aerobic biological treatment in wastewater treatment plants (WWTP). Two-stage processes enable energy-efficient operation, but oxygen transfer has not been studied in depth before. In this study, α-factors were determined with long-term ex situ steady-state off-gas measurements in pilot-scale test reactors (5.8 m height, 8.3 m3) coupled to full-scale activated sludge basins. A two-stage WWTP with more than 1 Mio population equivalent was studied over 13 months including rain and dry weather conditions. Operating data, surfactant concentrations throughout the two-stage process, and the effect of reverse flexing on pressure loss of diffusers were examined. The values of αmean, αmin, and αmax for design load cases of aeration systems were determined as 0.45, 0.33, and 0.54 in the first high-rate carbon removal stage and as 0.80, 0.69, and 0.91 in the second nitrification stage, respectively. The first stage is characterized by a distinct diurnal variation and decrease in α-factor during stormwater treatment. Surfactants and the majority of the total organic carbon (TOC) load are effectively removed in the first stage; hence, α-factors in the second stage are higher and have a more consistent diurnal pattern. Proposed α-factors enable more accurate aeration system design of two-stage WWTPs. Fouling-induced diffuser pressure loss can be restored effectively with reverse flexing in both treatment stages.

Determination of alpha factors for monitoring of aeration systems with the ex situ off-gas method: experience from practical application and estimation of measurement uncertainty

“…A previous study has shown that the effect of fouling during long-term off-gas measurements could be kept low when applying this maintenance (Schwarz et al 2021). Here, clean water tests repeated over a period of 13 months revealed a decrease of SOTR of 2 to 6% depending on airflow rate and a dynamic wet pressure increase of about 1 kPa.…”

Oxygen Transfer of Fine-Bubble Aeration in Activated Sludge Treating Saline Industrial Wastewater

“…In the present case, the high α values could be explained by the fact that the industrial wastewater is permanently slightly polluted. Surfactants and other possible oxygen-transfer-inhibiting constitutions were not explicitly measured, but elevated concentrations of such substances are usually accompanied by an increased organic concentration in the influent [8,27,30]. An increased concentration of such substances in the influent is, therefore, most unlikely.…”

“…), the given equations for calculating fS (see Section 3.2.) for the corresponding membrane design and a fixed α in the standard oxygen transfer rate in the process water (SOTRPW; kg/h), standardized at DO = 0 mg/L, atmospheric pressure = 101.3 kPa and T = 20 °C: A possible explanation is that the salt shock caused the microorganisms to produce oxygen-transfer-inhibiting substances [6,8,29,30]. Since the influent was stopped during the salt dosage, these substances accumulated in the system (i.e., reactor B1 and B2), thereby enhancing the effect.…”

Development of a Novel Hydrodynamic Sequencing Batch Reactor for Landfill Leachate Treatment by Shortcut Biological Nitrogen Removal

“…Studies on the computational fluid dynamics (CFD) model have revealed that the OTR is directly related to the hydrodynamics inside bioreactors [20,21]. In particular, it has been shown that higher turbulence intensity leads to an increase in the oxygen mass transfer coefficient, and hence oxygen uptake [22][23][24].…”