Hydroxyl Systems Inc.

Is Your Moving Bed Biofilm Reactor (Mbbr) Running On All Cylinders?

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Courtesy of Hydroxyl Systems Inc.

Evaluation of a full-scale MBBR process for nitrification is reported. Performance of the MBBR was found to be highly dependent on the hydraulic characteristics of the basin which could be optimized with improved influent flow distribution at the basin inlet. Installing a baffle on the influent pipe or introducing influent at multiple points located laterally across the entire influent end of the basin may provide this improvement. Higher aeration (mixing) rates alone did not improve influent distribution, however, increased mixing intensity focused at the inlet was not considered.
Laboratory bench-scale analyses were conducted to evaluate the maximum substrate utilization rate, r’m, for the nitrifying biomass in the full-scale MBBR process. Assuming a half-saturation constant, Kn, of 1 mg/L, r’m was estimated by statistically fitting the observed data to an attached-growth Monod-type rate expression. Although the data provided an excellent fit, r’m values (20ºC, 8.5 pH) exhibited considerable variation, ranging from 2.024 to 4.418 g/(m2-d) with an average of 3.370 g/(m2-d). Accurate determination of r’m may be affected by conditions in the full-scale MBBR (e.g. NH3-N loading, DO versus NH3-N rate limitation, etc.) and/or the assumption that the quantity of active nitrifying biomass is directly related to the media surface area provided. In order to develop a further understanding of the MBBR process, a standardized technique for the determination of r’m is needed. Results of hydraulic analyses indicated that the full-scale MBBR can modeled as two CFSTRs in series with a bypass flow representing hydraulic deficiencies. Model simulations indicated that hydraulic improvement in the full-scale MBBR could significantly increase the design influent NH3-N concentration and capacity under NH3-N rate limiting conditions. For an MBBR maximized with respect to hydraulic considerations, process performance was found to be highly dependent on r’m. With significant hydraulic deficiencies, process performance is not as sensitive to nitrification kinetics, but rather, is governed by the hydraulic characteristics of the basin.

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