With the MBBR IFAS (moving bed biofilm reactor integrated fixed-film activated sludge) process, the biomass required for biological wastewater treatment is either suspended or fixed on free-moving plastic carriers in the reactor. Coarse- or fine-bubble aeration systems are used in the MBBR IFAS process. In this study, the oxygen transfer efficiency (OTE) of a coarse-bubble aeration system was improved significantly by the addition of the investigated carriers, even in-process (∼1% per vol-% of added carrier material). In a fine-bubble aeration system, the carriers had little or no effect on OTE. The effect of carriers on OTE strongly depends on the properties of the aeration system, the volumetric filling rate of the carriers, the properties of the carrier media, and the reactor geometry. This study shows that the effect of carriers on OTE is less pronounced in-process compared to clean water conditions. When designing new carriers in order to improve their effect on OTE further, suppliers should take this into account. Although the energy efficiency and cost effectiveness of coarse-bubble aeration systems can be improved significantly by the addition of carriers, fine-bubble aeration systems remain the more efficient and cost-effective alternative for aeration when applying the investigated MBBR IFAS process.

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 α_mean, α_min, and α_max 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.

We summarized the experience from three decades of oxygen transfer testing and aeration research at Technical University of Darmstadt to validate the oxygen transfer efficiency of modern fine-bubble diffusers. 306 oxygen transfer tests in clean water of 65 different fine-bubble diffusers, carried out in the same test tank under identical test conditions, were analysed and compared with previous results. As a result, we could show that the performance of fine-bubble aeration systems has increased by 17% over the last three decades. Therefore, modern well designed and operated aeration systems can achieve SSOTE values between 8.5 and 9.8% · m⁻¹. Additionally, a comparison of various diffuser types and diffuser densities was done. Based on the new results an exemplary cost/benefit analyses for a 100,000 PE WWTP shows the calculation of an optimized diffuser density with respect to investment and operating costs.

In this study, for the first time, the influence of the design of conventional membrane diffusers on the volumetric mass transfer coefficient (k_La) and bubble size in tap water (TW) and saline water (SW) was investigated (up to 15 g/L NaCl). By using a new analytical approach, k_La and the bubble size along the ascent of the bubble swarm were measured simultaneously and in real time. The results show that in TW, after collision bubbles merge into larger bubbles by coalescence. In SW, coalescence is inhibited by salt. Due to the smaller bubble size, k_La increases to more than double compared to TW. The results show that in SW, membrane diffusers with dense slit patterns and smaller slit lengths are to be recommended in order to enable improved utilization of oxygen in saline water.

With regard to the water shortage in several regions of South East Asia, the paper focuses on the development of a sustainable Industrial WasteWater Management Concept with the focus on Reuse (brand name: IW²MC → R) to reduce water consumption from natural resources. The IW²MC → R includes the sustainable treatment of wastewater in Industrial Parks (IP) to provide reuse water for different purposes. The main objective is to reach the highest possible Industrial Park Reuse Factor (IPRF). The IPRF describes the relation between wastewater inflow to the Central Wastewater Treatment Plant and the outflow of reuse water for different applications. The Infrastructure Reuse Factor (IRF), one component of the IPRF, relates to infrastructural reuse applications (e.g. irrigation, street cleaning, toilet flushing). To determine the IRF, a Model Industrial Park is applied. A first calculation resulting in an IRF of ∼25% includes reuse applications for irrigating green spaces, street cleaning, and toilet flushing. In case, other applications for reuse water are considered (e.g. cooling or firefighting water), the IRF can be higher than 25%. Thus, the IW²MC → R provides a sustainable solution strategy, especially for water-stressed regions, to drive new IP developments by reducing water extraction from natural resources.