Indoor and outdoor air quality assessment of four wastewater treatment plants.

The study assessed the air quality of four wastewater treatment plants (WWTPs) by monitoring levels of hydrogen sulfide (H2S) and endotoxin. Samples were taken over a 1-year period (2001-2002). The unit operations at each WWTP were categorized as: (a) grit removal, (b) primary clarification, (c) biological treatment, (d) secondary clarification, (e) sludge dewatering, and (f) digestion. Temperature and humidity were monitored simultaneously, whereas airborne H2S and endotoxin were monitored at each of the six unit operations in each plant. Carbonaceous biochemical oxygen demand (CBOD) and total incoming flow of the day of visit were also recorded. The geometric means of H2S concentration were less than 1 ppm and endotoxin ranged from 6-1247 EU/m3. A mixed model analysis of covariance (ANCOVA) was used for the statistical analysis. While temperature was not associated with the levels of both contaminants, humidity was influential on the level of H2S (p < 0.01) but not of endotoxin. CBOD did not affect the levels of either contaminant; however, incoming flows showed an association with the levels of H2S (p < 0.05). The concentrations of H2S in the six unit operations were statistically different, whereas endotoxin did not show any differences in concentrations between units. Individual comparisons proved that concentrations of H2S in the grit removal and sludge dewatering unit operations were statistically higher than the other operations. Overall, the concentrations of H2S varied depending on total incoming flow, humidity, and different unit operations. This trend was not observed for endotoxin. The results showed that the factors analyzed affected concentrations of H2S and endotoxin differently. Therefore, different control methods for endotoxin and H2S need to be considered to effectively reduce their concentrations at WWTPs.