Annual In-Situ Monitoring of Diffuser Material Properties for Fine-Pore Aeration Systems

Aeration is the costliest process in wastewater treatment plants, accounting for at least 45% of wastewater treatment energy costs (Reardon, 1995). Fine pore diffusers were designed to reduce the operational costs of aeration. These diffusers may be composed of sintered ceramics or polymeric membranes with mechanically punched pores on their surfaces; both types are designed to shear the volumetric air flow from the blowers into fine bubbles (ASCE, 1989). Compared to the bubbles created by coarse bubble diffusers, fine bubbles have a higher surface-to-volume ratio and hence a longer travel time in the water column, in creasing their oxygen transfer efficiency (IWA, 2008). While newly installed fine pore diffusers are initially more efficient in terms of oxygen transfer than coarse bubble diffuser s, fine bubble diffusers are prone to fouling and the degradation of the polymeric material properties (for membrane diffusers), as confirmed in previous research (e.g., Wagner and von Hoessle, 2004; Kaliman et al, 2008; Rosso et al, 2008). Fouling can be observed on diffusers as a visible layer of surface biofilm. The rate of material degradation depends upon diffuser and wastewater characteristics, as well as process operational parameters; however, to date there is no way to predict the degradation rate accurately. Degradation of the diffuser surface inevitably reduces the efficiency of the diffusers, which results in an increase in the operational costs for wastewater treatment plants.The goal of this project was to determine the quantitative link between time in operation, diffuser material changes, fouling biofilm accumulation, and changes diffuser performance for fine-pore diffusers installed in municipal treatment processes. In order to quantify the degradation of these diffusers, the thickness and hardness of the diffuser surface were studied for three diffuser compositions. Because material properties vary with membrane composition, three different polymeric diffusers were selected for this investigation: silicone diffusers; ethylene propylene diene monomer (EPDM); polyurethane diffusers. Ceramic diffusers were not included in this paper because ceramic is biologically inert. In order to determine the rate of change of diffuser thickness and hardness, we installed in-situ diffuser tubes and periodically harvested samples of diffuser membranes for testing, while maintaining their functionality (Fig. 1).