ASSESSMENT OF ODOR FORMATION MECHANISMS IN AN ACTIVATED SLUDGE BASIN AT THE NORTHEAST WASTEWATER TREATMENT PLANT

The City of Philadelphia Water Department (PWD) operates a 210 MGD secondary wastewater treatment facility, the Northeast Water Pollution Control Plant (NE WPCP) located in the Bridesburg section of Philadelphia. PWD has been continuously working on odor control strategies at the NE WPCP. Improvements made include improved ventilation from process areas, process changes in the aeration basins, and the use of chemical treatment in the Return Activated Sludge (RAS). However, even with these modifications, some off-site odors still exist. The predominate odor detected both onsite and offsite is described as “canned corn”, which has been associated with dimethyl sulfide (DMS).Under previous work, CH2M HILL and PWD worked together to conduct an initial modeling assessment and ranking of the relative off-site impact of various sources at the NE WPCP. From this study, the aeration basins and the set 2 primary sedimentation tanks (PST's) were identified as the major sources to be focused on in future work and the odor contributions from various reduced sulfur compounds were defined.The key mission of this project is to define the causes of aeration basin odor emissions from the NE WPCP, recommend ways to mitigate emissions, and determine a method to monitor odor emissions, with a focus on the major sources of the off-site odors. The sampling effort was guided by two primary theories as to how odors are forming at the NE WPCP:The interaction of dissolved sulfide (DS) with methyl groups in a series of reactions to form DMS:The breakdown of amino acid compounds in the treatment process forms methyl mercaptan (MM) and DMS:Since it is easier to measure organic sulfides than free methyl groups, the focus of the sampling effort was on organic sulfides. Volatile solids may be a surrogate for methyl groups, but direct measurement of methyl groups in the liquid phase is not well established. By tracking the formation of reduced sulfide compounds across the treatment processes, the primary formation mechanism can be identified.Collection of wastewater samples for analysis by VA Tech was complemented by field measurements at each sampling location by PWD staff using hand-held instruments. Process information collected by the SCADA system was recorded to document process flows and other operating parameters. The sampling approach included several rounds of sampling, with the results of one round establishing the objectives of the next.The Water Environment Research Foundation (WERF) odor bottle standard is based on the assumption of reproducible headspace gas concentrations, if the headspace is in thermodynamic equilibrium with the sample in the bottle. The following analytical method was proposed for the evaluation of odor samples from the aeration basin influent, effluent, and side streams. The method was based on the collection of liquid samples in a PET bottle and analysis of the headspace concentrations for organic sulfide compounds, including MM, DMS, and dimethyl disulfide (DMDS). The PET bottle approach isolates and concentrates the odor forming potential of these compounds at concentrations that can be measured analytically. The procedure is therefor particularly valuable for determining odor generation potential of samples and analysis of plant operating conditions that create tendencies toward higher odor generation. This lab approach was critical to studying the formation pathways and trends for odor generation.The following sampling events were conducted.Round 1 sampling was intended to examine the odor formation potential across the wastewater treatment plant.Round 2 sampling involved sample collection for use in bench-scale testing at VA Tech, additional testing of aeration basins and field testing of primary sludge at the Southeast (SE) and Southwest (SW) Water Pollution Control Plants (WPCP).During the round 3 testing, the PWD and CH2M HILL project team targeted a day when an odor event was occurring.In the round 4 sampling event, wastewater samples were collected immediately upstream of the treatment plant to see if the strong odor formation potential reaction observed in round 3 could be associated with a particular influent stream.The analyses performed by VA Tech do not quantify actual emission rates for the process units, but define the odor formation potential of a sample or mixture once it has become anoxic or anaerobic. Analyzing the concentrations in the headspace of the sample containers allows the analyst to quantify compounds that would not be detectable using a flux chamber or field sampling techniques. Comparing the headspace results of samples collected across the plant or during different sampling rounds allows the odor formation mechanisms to be understood. Additional tests that included oxygen addition or increase dissolved sulfide concentrations as well as mixtures with RAS from the Blacksburg Wastewater Treatment Plant provided additional information about the odor formation mechanism and possible mitigation measures.