ACTIVATED SLUDGE AERATION SYSTEM EFFICIENCY UNDER DIFFERENT PROCESS CONDITIONS IN WASTEWATER TREATMENT PLANTS

Offgas analyses of oxygen transfer efficiency (OTE) at the Terminal Island Treatment Plant (TITP) of Los Angeles document changing performance of fine-pore diffusers in an activated sludge plant over a number of years. Although this plant treats an especially challenging waste stream, the operation of the aeration tanks is little different from many other plants, so these results have wide potential relevance.Recent sessions with larger number of measurements per day and revisiting tanks after a few days or weeks provided improved time resolution, compared to previous work. Samples were more closely spaced, and some samples were taken in the intervals between the aeration grids, at the ends of the tanks, and near the edges of the grids. Tests were also made in which the hood was left in the same position for an hour or two, to determine whether the measurements were subject to significant temporal variations on time scales of a few minutes to an hour or more.Analysis of the full set of measurements made at TITP since 1991 shows detectable effects of changing process conditions, and indicates that the recently measured efficiencies were at the level of systems without diffusers which is a major problem with many aeration systems in activated sludge plants in USA and other countries, in particular plants running under nutrient removal with selector processes. This is probably explained by deterioration of the air distribution system that has now been remedied by a refurbishment program. Also, as in other activated sludge plants in the US, a blower system that lags behind the diffuser technology and that has limited flexibility has made overaeration difficult to avoid in the past. It would have limited the economic benefits of repairs that were limited to the air distribution and diffusion system. Offgas testing can provide evidence of aeration system deterioration when leaks and fouling are large enough to affect local OTEs, but usually the full extent cannot be verified until tanks are dewatered.The recent measurements not only demonstrate variations in tank efficiency, but show the degree to which efficiency losses may develop without being recognized if OTE measurements are not done. Hence, there may be value in similar measurement programs at other plants, for which this study may serve as a prototype.