Modeling odor dispersion from a wastewater treatment plant based on olfactometry measurements vs. theoretical H2S emissions

The new odor regulations in Israel include the use of AERMOD dispersion model to assess the odor impact of emitting sources. Odor emission inputs should base on air samples collected at the source and analyzed by dynamic olfactometry according to the international standard EN13725. Before the new regulations, environmental authorities and consultants often evaluated odor emissions from wastewater treatment plants (WWTPs) from theoretical H2S emissions calculated by the WATER9 model. Considering H2S as the primary odorant of wastewater treatment odor, these values were used as the input in AERMOD. The present study aims to compare the two inputs (theoretical H2S vs. measured odor emissions) and to examine how they affect odor impact assessments from WWTPs. We focus on the activated sludge WWTP of Afula city in Jezre'el Valley, northern Israel (41,000 inhabitants; 7500 m3 day−1) which is located about 3 km west of the city.The input of odor emissions (odor units (OU)/s) was obtained from a series of sampling missions that included the main wastewater collection, storage and treatment units. Air samples were collected into 60-L Tedlar bags by the standard lung method and an EPA-type flux chamber. Olfactometry was conducted on Odile 2510 (Odotech Inc., Canada). The input of H2S emissions was obtained from WATER9, based on the concentration of H2S at the inlet stream and other physico-chemical parameters. AERMOD was used to assess the impact of each unit and that of the whole plant. The model was used to assess the worst-case impact for 98% of the time, as required by the Israeli regulations for existing facilities.The total H2S emission from the whole plant was calculated as 160,900 μg/s whereas the total measured odor emission was 110,800 OU/s. Considering odor threshold of ∼1 ppb H2S (1.42 μg/m3), the odor impact from the whole plant is similar based on the two inputs. Yet, the relative contribution of each treatment unit was different by using the two approaches. The main difference is the relative contribution of the sewage inlet and the aeration basin: The sewage inlet trench contributed 47% out of the total H2S emission vs. only 9% out of the total odor emissions. On the other hand, the aeration basin contributed 36% out of the total H2S emissions vs. 69% out of the total odor emissions. The advantages and disadvantages of each approach are discussed.