STUDY OF HYDROGEN SULFIDE EMISSIONS FROM WASTEWATER TREATMENT PLANTS

Orange County Sanitation District (OCSD) operates two large Wastewater Treatment Plants (WWTPs) in Southern California providing services to more than 2.2 million residents and numerous businesses. Plant No. 1 treats approximately 3.9 m3/sec (90 million gallons per day – mgd) of wastewater and Plant No. 2 treats about 7.0 m3/sec (160 mgd). Both plants are located in highly populated residential areas. Therefore, reduction of toxic and odorous emissions at both plants has been a high priority for OCSD.Most odor-generating substances in wastewater treatment facilities can be classified as either inorganic gases or organic vapors with two principal odorous gases being ammonia and hydrogen sulfide. Since the odor threshold for ammonia is about three orders of magnitude higher than for hydrogen sulfide it is the latter which is usually mostly responsible for odor problems at WWTPs. In addition, H2S is a highly toxic compound, which affects cardiovascular and respiratory human organs. As a result, emissions of hydrogen sulfide from facilities are usually highly restricted and regulated.Hydrogen sulfide is produced and used in numerous industrial applications, as well as in agriculture and scientific research. But the main source of hydrogen sulfide at WWTPs is its natural formation by the decomposition of human waste and other organic products. As a result, high concentrations of H2S are routinely detected in wastewater and biosolids as well as in air emissions from WWTPs.The presented paper contains information about hydrogen sulfide emissions from OCSD WWTPs. While some emissions sources such as chemical scrubbers are studied relatively well, there is almost no information related to emissions from digesters, junction boxes, sludge drying. The emissions data is based upon engineering calculation, modeling and source testing. The paper contains an estimate of H2S emission for different processes and comparison with other major compounds in WWTPs emissions data.