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King County Wastewater Treatment Division has operated its South Treatment Plant since the 1960s. The plant was originally located in a rural setting with a large buffer. However, over the years the buffer has been shrinking as commercial, industrial, and some residential neighbors have continued to move closer to the plant as the area has developed. For unknown reasons, the odor produced by the plant and the offsite impacts have increased over the past couple of years. While some channels are covered and treated for odor control and the solids processes are enclosed and treated, the majority of the liquid processes are still uncovered. King County began a study in 2001 to evaluate the current odor control system and look at ways to upgrade the odor control system and be a good neighbor to the surrounding community.

The odor study began with a review of the current odor control system and baseline odor dispersion modeling using data from a handheld hydrogen sulfide (H2S) meter. The odor control system includes the following units:

Primary odor reduction towers (PORTs): Two single-stage primary process chemical scrubbers that treat influent division channel, aerated grit basins, primary influent distribution channels, primary effluent collection channels

Secondary odor reduction tower (SORT): One single-stage secondary process chemical scrubber that treats settled sewage channels, return activated sludge (RAS) mixing box and channel extension, and primary effluent distribution channel

DAFT ORT: One single-stage chemical scrubber followed by two carbon towers that treat six dissolved air flotation thickeners (DAFTs)

Dewatering ORT: One single-stage chemical scrubber followed by two carbon towers that treat the dewatering building (belt filter press room, biosolids truck loading conveyor, biosolids loading bays)

One organic biofilter that treats the influent sewer

The baseline odor dispersion modeling identified the main odor sources to be the headworks (screenings and grit), primary clarifiers, and aeration basins. Additional odor sources that did not rank highly after the baseline modeling, but were also suspected of potentially causing offsite impacts were: mixed liquor and RAS channels, thickened sludge blending tank open hatch, digester vents or leaks, and biosolids cake truck loading area (when doors were open).

In the summer of 2002 odor source sampling using a flux chamber was performed. Samples were analyzed for hydrogen sulfide, odor, reduced sulfur compounds, ammonia, and amines. The results were surprising. The main odor source was the four aeration basins, which had by far the highest mass emissions of any other source at the plant. Other sources were scrubbers (including the biofilter), followed by the primary clarifiers as shown in Figure 1. Notable present odorous compounds were hydrogen sulfide, trimethyl amine, and dimethyl disulfide.

In each aeration basin, the odors were strongest at the end of the first pass, but continue throughout the basin. Each basin has an anaerobic selector, which is approximately the first half of the first pass. Once the wastewater passes the selector, the odorous compounds are effectively stripped by the aeration process. An interesting finding from the sampling is that the odors were sampled twice, in August and October. In August the selector was on, and in October the selector was off. While the odors at the plant were perceived to be less at the plant during the October sampling event, the data show that the results for both hydrogen sulfide and odor were identical during both events.

At present the main task is to determine what is causing the aeration basin odors. A process study of the current treatment plant process is under way that includes analysis of the raw influent and treatment processes together with the results from the odor sampling. Once the source of the odor is identified, process changes will be implemented to mitigate the odors. Odor sampling may be repeated if process changes appear to reduce the odors significantly. Odor dispersion modeling was performed, with and without controls, using the new sampling data.

Odor controls were recommended based on the results of the odor dispersion modeling. The odor control approaches include a combination of permanent process changes, basin covering, optimization of scrubbers, and addition of new scrubbers.

Document Type: Research Article


Publication date: January 1, 2003

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