Mitigation of biofilm in air scrubber system by phosphorus control

Abstract

Air conditioning system is essential to supply the clean air to meet the air quality of various indoor environments such as buildings, subways, and clean rooms. However, the biofilm was formed during long-term operation due to the volatile organic matter and high humidity in the air, causing the various problems such as clogging the water spray device and reduction of the gas-liquid contact surface area. In particular, in the case of cleanroom in semiconductor fabrication facilities using wet scrubber system to remove airborne particles, the application of chemicals such as biocide was restricted. Thus, the development of a new biofilm control strategy is required. In this study, the control of microbial growth was controlled by the control of phosphorus supply. The mechanisms of the biofilm adhesion and microbial growth according to the phosphorus concentration were investigated, and evaluated the microbial activity. The lab-scale air conditioners in cleanroom were built and operated more than 300 days by controlling the concentration of phosphorus in the solution to a normal state (Control, 1P) and 99% removal (P-Limit, 0.01P), and physicochemical properties of the biofilm formed on the eliminator surface were evaluated. The volume and phosphorus accumulation of the formed biofilm decreased as the phosphorus concentration decreased, and the phosphorus limitation contributed change in the structure of the biofilm as well as the amount. The volume of the biofilm based on the same surface area was 26.3 μm$^3$ in the control, but, decreased to 13.2 μm$^3$ in the case of phosphorus limitation. The extracellular polymeric substances (EPS) as one of the main substances forming the biofilm significantly decreased from 6.26 mg to 3.20 mg compared to the same surface area of eliminator. In order to quantitatively evaluate the change in the characteristics of the biofilm formed under the phosphorus limitation, the structural characteristics of the biofilm were evaluated by using the Drip Flow Cell (DFC) and AFM (Atomic Force Microscope). The adhesion force of EPS formed under phosphorus limitation (0.01P) decreased by 83% compared to that of control. Thus, the biofilm formed under phosphorus limitation was easily detached due to low adhesion force, resulting the formation of porous biofilm compared control. As a result of monitoring the signals of soluble microbial by-product-like (SMP) and aromatic protein-like (AP) using FEEM (Fluorescent Excitation-Emission Matrix), despite oligotrophic conditions, microbial activity in control remained high. However, the biofilm formed in the phosphorus limitation condition showed significantly lower activity compared with control. The microbial community of control and P-Limit was evaluated and found that dominant species were significantly altered. The change of dominant species in that was considered to be related to the mitigation of biofilm formation. By phosphorus limitation, Bradyrhizobium and Ralstonia showed 39.81% and 27.11%, respectively in control, while they are decreased to 13.31% and 2.72% in phosphorus limitation. Further research is needed to identify the correlation between the change in the dominant species of microorganisms and the change in the strength of the biofilm such as adhesion by phosphorus limitation.