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

Abstract

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 lab-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 to investigate 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.

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

“…Our previous studies showed that in SW, f S,max as well as the maximum k L a were higher with a small slit membrane design than one with large slits [1]. For disc diffusers, the k L a increases by 10% when using the small slit diffuser membrane design compared with diffusers with the large slit design [2]. However, the improved k L a is accompanied by an increased diffuser pressure drop (due to the smaller slits) and, therefore, by a higher energy requirement for air supply.…”

“…The normalised k-value (k N ; -) describes the slope of the linear increase in f S until c Salt /CCC Salt = 1.0 and f S reaches f S,max . Since only the diffuser membrane design and q A,Disc affects f S,max or bubble diameter, but not the salt type [2,[11][12][13], it is reasonable that solutions of different salts and salt mixtures with the same c Salt /CCC Salt ratio and aerated with the same diffuser would present similar f S values. Other studies already showed the advantages of using a dimensionless concentration as independent parameter for analyses of hydrodynamics and oxygen transfer with different salt solutions in bubble columns [13][14][15].…”

“…However, the improved k L a is accompanied by an increased diffuser pressure drop (due to the smaller slits) and, therefore, by a higher energy requirement for air supply. Nevertheless, model calculations based on SW test results show that the aeration efficiency improves due to the higher oxygen transfer [2].…”

“…The oxygen transfer of fine-bubble aeration systems is enhanced by a high salt concentration (c Salt ; g/L) due to the inhibition of bubble coalescence. In former studies, we assessed the different factors influencing the oxygen transfer in saline water (SW) [1][2][3].…”

“…Hence, f S takes values greater than 1.0 and increases linearly with an increasing c salt . The increase in f S depends on the airflow rate, type of diffuser and design of diffuser membrane [2]. When the c Salt reaches a certain concentration, which is specific for each salt and mixed salt solution, the coalescence is fully inhibited and f S reaches its maximum (f S,max ) [1].…”