Improving aeration systems in saline water (part II): effect of different salts and diffuser type on oxygen transfer of fine-bubble aeration systems.

The objective of the present study is to investigate the different effects on the oxygen transfer of fine-bubble aeration systems in saline water. Compared to tap water, oxygen transfer increases due to the inhibition of bubble coalescence. In Part I of the present study, we investigated in laboratory-scale experiments the effect of design of diffuser membrane. The objective of Part II is the assessment of effects of different salts, diffuser type and diffuser density. We measured the concentration of various salts (MgCl2; CaCl2; Na2SO4; NaCl; KCl) above which coalescence is fully inhibited and oxygen transfer reaches its maximum (referred to as the critical coalescence concentration; CCC). For this purpose, we developed a new analytical approach, which enables investigation of the coalescence behaviour of any aeration system and (mixed) salt solution quickly and easily by evaluating the results of oxygen transfer tests. To investigate the transferability to large scale and the effect of diffuser type and density, we repeated lab-scale experiments in a 17,100 L pilot-scale test tank and carried out additional tests with tube and plate diffusers at different diffuser densities. The results show that despite the higher pressure drop, diffusers with dense slit density and smaller slits are to be recommended in order to improve efficiency of aeration systems in saline water.

Improving aeration systems in saline water: measurement of local bubble size and volumetric mass transfer coefficient of conventional membrane diffusers.

In this study, for the first time, the influence of the design of conventional membrane diffusers on the volumetric mass transfer coefficient (kLa) 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, kLa 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, kLa 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.

Fouling mitigation in anaerobic membrane bioreactors using fluidized resin beads.

This study focuses on the use of fluidized resin beads to mitigate fouling during ultrafiltration (UF) of the effluent of an anaerobic bioreactor. Two different module configurations were tested: A fluidized bed of resin beads was generated in a tubular UF membrane, and a hollow fiber (HF) UF membrane was submerged into a fluidized bed, respectively. During filtration of anaerobically treated synthetic wastewater using the tubular module, fluidized resin beads with a diameter of 0.5-0.71 mm did not show any beneficial effect. In contrast, the presence of fluidized resin beads (diameter of 0.5-0.71 and 1.00-1.25 mm) in the HF module reduced the fouling rate significantly. Furthermore, particle diameter and the bed voidage affected the cleaning efficiency of a pre-fouled membrane in the HF module. Interestingly, short-term filtration tests (<2 h) of a dextran solution showed that fluidized resin beads are able to minimize concentration polarization of a macromolecule, even in the tubular module. Therefore, it is supposed that fouling of the anaerobically treated synthetic wastewater was mainly attributed to the deposition of colloidal and particulate matter.

Design of fine-bubble aeration systems for municipal WWTPs with high sea salt concentrations.

The impact of sea salt on the aeration coefficient kLa of fine-bubble aeration systems was investigated in clean water and in a model substance for simulating activated sludge. The investigations were carried out at laboratory and pilot scale. Sea salt was dosed in the form of artificial seawater, real seawater or simplified, by adding NaCl. The investigated media showed an increase of kLa with rising sea salt concentrations up to 10 g/L. The ratio between kLa and sea salt concentration is impaired by the air flow rate and the type of applied diffusers. Literature data show that the favourable effect of sea salt on kLa can also be found in the activated sludge of conventional municipal wastewater treatment plants. The results of this study allow the derivation of salt correction equations to be used in the design of fine-bubble aeration systems for taking into account the favourable impact of sea salt on kLa. By that means, a more precise design of respective systems will be enabled.

Energy, cost and design aspects of coarse- and fine-bubble aeration systems in the MBBR IFAS process.

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.