The liquid side fouling is attributed to inorganic precipitates, dry biomass, biofilm, and a combination of the three.
The inorganic fouling is classified as Type A fouling. It increases head loss, but it can increase OTE by decreasing the effective pore size of the diffuser (Kim, 1993). Biofouling by dried or wet biomass is classified as Type B fouling, which increases headloss, but decreases OTE since the biofilm forms bigger secondary pores on the diffuser surface. Type C foulingis caused by the mixture of inorganic precipitates and biofilm. The mixture may increase OTE by decreasing effective pore size, but it certainly decrease energy efficiency by increasing head loss.
Type B diffuser fouling appears most common in biological wastewater treatment. As can be seen in Fig. 1, the average -factor of new diffuser is 0.5, but it decreases to 0.4 when the diffusers age 2-24 months and it further decreases to 0.35 afterwards. By definition, fouling factor, , of 2-24 month old diffusers is 0.8 (=0.4/0.5) and that for more than 24 month old diffusers is 0.7 (=0.35/0.5).
Fig. 2 shows bubbles from a diffuser cleaned only the right half of the surface. It is apparent that the bubbles from the cleaned surface are much smaller than the bubbles from the fouled surface. Regarding the factors affecting the OTE of the two different groups of bubbles, following are relevant.
- Total available surface area is inversely proportional to the bubble size. If bubble size decreases to a half, total surface area doubles. As a result, OTE can be doubled with a half sized bubbles.
- Large bubbles produce stronger turbulence when they rise. As a result, specific mass transfer coefficient on coarse bubble surface is larger than that on fine bubble surfaces.
- Overall, OTE increases as bubble size decreases, but not as much as the bubble surface area suggests.
Fig. 1. Effect of diffuser fouling on – and -factor in CAS (Rosso, 2007a).
Fig. 2 shows how the Type B fouling affects bubble size that in turn affects OTE.
Fig. 2. Bubbles produced from fouling area is much larger than the bubbles from cleaned area in a same diffuser (After Rosso and Stenstrom, 2007)
© Seong Hoon Yoon