Biofiltration of asphalt emissions: Full-scale operation treating off-gases from polymer-modified asphalt production

In response to complaints from nearby residents, a biofilter was designed i nstalled, and tested for treating odors in one of three odorous emission st reams from an asphalt plant pl producing polymer-modified asphalt. Hydrogen sulfide (H2S) was determined to be the most prevalent gaseous reduced sulf ur compound and was detected in the emission stream only when polymer mater ial war being added to raw asphalt. Emission scream H2S concentrations were quite variable, ranging from 16 to approximately 30, 000 ppm (v/v) and con sidered the likely compound contributing most to the plant's odor complaint s. The biofilter was effective in controlling odor from the production process and removed an overall average of 65% of the H2S during polymer addition, and for H2S concentrations less than 400 ppmv, removal averaged 98%. These removal efficiencies reflect data from the biofilter operating at 2.5-minut e empty bed residence time in 1996 and a 6.1-minute empty bed residence tim e in 1997. The biofilters bed became increasingly acidified during the plant's 1997 op erating season producing a pH gradient through the bed ranging from a high of 6.6 to a low of 3.1. The bed medium moisture content remained constant a t about 60% (wet weight basis), but changes were observed in the water pote ntial: no correlation to performance was determined Changes in the microbia l community reflected the bed acidification trend with acidophiles becoming generally more numerous in the bed's deeper portions and in the mid to lat e season when the bed was most acidified. Bed acidification did not impart the biofilter's H2S removal efficiency. Nearby residents were surveyed and roughly half of the respondents indicate d that the odor conditions had improved, one-third felt odor conditions wer e unchanged ana the remaining 15% felt odor conditions were worse despite t he fact that only one of three of the plant's odorous emission streams were treated by the biofilter. Plans are to implement biofiltration for odor co ntrol at all of the facility's emission points.