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The topic of this cumulative thesis is to investigate the influence of diffuser design on the oxygen transfer of fine-bubble aeration systems in wastewater treatment at increased salt concentrations. In the activated sludge process, as the common aerobic biological treatment process, fine-bubble aeration systems are preferred to satisfy the oxygen demand of microorganisms. Thereby, aeration is usually the most energy-intensive part of the activated sludge process, accounting for 50 % to 80 % of total requirement. To ensure high energy efficiency of aeration and thus of the entire treatment process, designers of WWTP must therefore consider all influencing factors including salinity. When salinity rises, coalescence is increasingly inhibited. The resulting decrease in bubble size leads to an increase in the gas-liquid interface, followed by a tremendous increase in oxygen transfer. Within this thesis, the oxygen transfer of various conventional fine-bubble aeration diffusers in tap- and saline water as well as in saline activated sludge was measured. The scope was to assess the effect of diffuser membrane design, diffuser type, diffuser density and salt type on oxygen transfer at different salt concentrations. Simultaneously, bubble size along the ascent of the bubble swarm were measured via image analyses at different levels of ascending bubble swarm in tap water and in saline water. The measurements in tap- and saline water took place in a 250 L glazed bubble column and in a 17,100 L glass-steel-frame tank. Oxygen transfer tests in saline activated sludge were conducted in a pilot scale activated sludge tank with an aerated water volume of 2.250 L. The plant was operated for 269 days with saline industrial wastewater influent. The oxygen transfer into the activated sludge was measured continuously by off-gas method. The results show, that in tap water the oxygen transfer depends predominantly on diffuser density and type of diffuser as well as on the depth of submergence. In contrast, the diffuser membrane design has no effect on oxygen transfer in tap water, although bubble size measurements showed that the slit length of diffuser membrane affect the size of bubbles close to the diffuser (primary bubbles). The reason is, that bubbles coalescence rapidly during their ascent. Therefore, using fine-slitted diffusers in tap water does not lead to an improved oxygen transfer compared to large-slitted diffusers of the same type. Rather, the aeration efficiency decreases due to the higher pressure drop of fine-slitted diffusers. However, when the salt concentration increases, the increasing inhibition of coalescence leads to a decreasing bubble size and a tremendously rising oxygen transfer. When the salt concentration exceeds the critical coalescence concentration (CCC), coalescence is completely inhibited and the oxygen transfer reaches its maximum. The CCC is specific for each salt or salt mixture. Using a new self-developed analytical approach, for the first time CCC was determined for various single salt solutions for conventional fine-bubble diffusers. If coalescence is completely inhibited, the bubble size equals the size of primary bubbles, which results in an improved oxygen transfer using fine-slitted diffusers. Oxygen transfer measurements in saline water as well as in saline activated sludge show, that oxygen transfer increases up to 23 % compared to large-slitted diffusers of the same type. Despite higher pressure drop of fine-slitted diffusers, aeration efficiency increases up to 17 %. Thus, at elevated salt concentrations, the efficiency of fine-bubble aeration systems can be significantly improved by adjusting the diffuser membrane design, a fact, that must also be considered in the future in the design of aeration systems. Therefore, an optimized design approach is proposed, which enables an appropriate design of fine-bubble aeration systems at increased salt concentrations. |
