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 ; Na2 SO4 ; 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.