THEY'RE AT THE FENCE LINE – AN ODOR CONTROL MASTER PLAN

The City of Orlando, FL faces the same problems that many wastewater treatment plants have as houses get closer and closer every year. In 1997 the City implemented an odor control plan at the 25 MGD Water Conserv II Reclamation Facility (WRF) to address the highest priority odor sources. As a result, the screen and grit facility at the plant headworks was covered and a biofilter was installed to treat 2,000 cfm of air. The biofilter successfully reduced hydrogen sulfide (H2S) from 5.6 ppm (inlet) to 0.006 ppm (outlet) achieving a 99.9% removal efficiency.As new homes and apartments were constructed adjacent to the plant, the City embarked on the second phase of the Odor Control Master Plan using dispersion modeling to predict both odor levels in terms of dilutions to threshold (D/T) and the frequency of occurrence of odors above a detection level D/T of 7. The ISCST model was used to generate isopleths of peak D/T concentrations around the plant on aerial photographs. Peak D/Ts were used to correct the model from hourly average levels to less than a 30 second average because odors can be a nuisance even if detected for a short time. Local meteorological data was used to predict how many times events would occur each year. Events were considered odor detection for a 30 second to 1 hour duration, which are called threshold exceedance frequencies (TEFs).Odor testing and odor control system performance data are presented in the paper. Modeling results, including isopleth contours of D/T levels and TEF contours on aerial photos are also included. Model results predict that the peak D/Ts can be reduced from a range of 37–118 at the fenceline around the plant to about 8–15 at the fenceline with the recommended odor control. An overall average reduction of 84% of peak D/Ts could be achieved. In addition, the TEFs could be reduced from 145–830 events (odors above detection level) per year to 4–47 events at the fenceline with odor control. In the most sensitive areas, where houses are right up to the fenceline, peak D/Ts are predicted to be 10 and the events are predicted to be about 10 times per year.In an effort to achieve its goal of rare (if any) barely detectable odors, the City will replace a fine mist scrubber that achieves only 47% odor reduction on a sludge storage tank, cover anoxic basin effluent channels and install surface mixers in the anoxic basins to replace the existing low rate diffused air system. The source of diffused air for the activated sludge process is covered primary clarifiers. Therefore, the H2S concentration of the diffused air entering the aeration basins is about 5 ppm and it has a D/T of 660. At the anoxic tank surface the outlet D/T of the denitrification tank had an H2S concentration of about 2 ppm and a D/T of 250. For the aeration basins, running in an aerobic mode, the D/T was 16 and H2S was 0.1 ppm, providing effective treatment of the odorous air in the activated sludge diffusion process. Using low horsepower surface mixers, will reduce odor emissions but will also be a net energy savings from the diffused air process. A biofilter with synthetic media will be used to treat air from the anoxic basin channels and the fine mist scrubber at the sludge thickening building will be modified to a bioscrubber.The City of Orlando, Conserv II WRF presently has a variety of odor control systems but has implemented an Odor Control Master Plan in a proactive approach as residences move right up to the fenceline. Dispersion modeling has been an effective tool to clearly define the remaining areas of the plant that require odor control.