Estimation of gas transfer coefficient with micro bubble technology on green roof runoff water

Abstract

Green open spaces reduction serving as crucial water recharge areas, is a consequence of urban development. Green roofs, incorporating vegetation atop buildings, present a viable solution to mitigate this loss by replicating some functions of green spaces. However, the retention capacity of green roofs is limited, leading to runoff. Reusing this runoff can enhance rainwater capture efficiency, yet the water typically fails to meet clean water standards due to low dissolved oxygen (DO) levels. Micro bubble technology offers a potential remedy by injecting gas into the water to elevate DO levels. To optimize micro bubble usage, it is essential to determine the gas transfer coefficient (kLa), which is influenced by various parameters such as the type of gas and the duration of treatment. This study utilized 60L samples of green roof runoff water, subjected to micro bubble treatments of 30-min and 60-min. Two types of green roof media were tested: vegetated (Portulaca grandiflora) and unvegetated. The kLa values were derived by modeling DO levels during the treatments. The results indicated that the highest kLa values for both 30-min and 60-min exposure were observed in runoff water from green roof with Portulaca grandiflora, 0.2533/min and 0.3781/min of kLa values, respectively.