Wastewater Collection System Odor Control and Corrosion Abatement
Relief Interceptor Ventilation Pilot and Odor Control
Perkins Engineering Consultants, Inc. (PECI), conducted pressure and odor monitoring on two 36-inch, one 30-inch and upstream and downstream 42-inch interceptors to evaluate the impacts of ventilating the collection system on pressure and hydrogen sulfide levels. The testing demonstrated that many of the sewer headspaces in the project area are normally pressurized and that mechanical ventilation can maintain a vacuum on areas subject to odor complaints. Floodplain, tree canopy, existing easements, critical water designations and other surrounding land use issues were considered in evaluating pilot and full scale foul air withdrawal locations. Selecting the capacity of the odor control system involved balancing construction cost and vessel size against incremental reductions in collection system pressures. Due to relatively low measured hydrogen sulfide levels and the prevalence of other reduced sulfide compounds, dry media adsorption was recommended for treatment.
Regional Wastewater Collection System Ventilation Dynamics Investigation
PECI personnel, serving as a subconsultant to a partnered firm, performed a month-long field investigation of gravity sewer ventilation dynamics in parallel 39 and 54-inch interceptors serving multiple customer cities. Headspace H2S, pressure/vacuum levels, and other parameters were measured both with and without the influence of mechanical ventilation. It was shown that mechanical ventilation can produce a vacuum in a sewer headspace over a mile upstream from the pull point, and for a significant distance downstream as well. Headspace H2S Concentrations were reduced 50 to 75% under continuous ventilation, contributing to corrosion management.
Regional Collection System Odor Evaluation
PECI personnel assessed a host City with source and potential solutions for collection system odors in a nearby wholesale customer’s collection system. Odor complaints were being attributed to the host's treatment plant, but dispersion modeling showed that plant odors were unlikely to be perceived in complainant's neighborhoods. An investigation revealed pressurized headspace situations, particularly upstream of a major inverted siphon. Chemical treatment and evacuation of the headspace were considered. Improvements are under consideration by both Cities. Our personnel recently updated the comprehensive odor dispersion model for the host city's 24 MGD treatment plant.
Force Main Discharge Ventilation Pilot and Odor Control
PECI conducted fan testing and odor sampling on a 21-inch gravity line downstream of a force main discharge to gravity to evaluate the impacts of ventilating the collection system on hydrogen sulfide and pressure levels. Before initiating the pilot study, a potential property for the odor control system was identified east of the selected ventilation manhole. Data were collected at the six ventilation rates over two separate sampling days. Increasing ventilation rates consistently increased the vacuum imparted on upstream and downstream manholes. Measured hydrogen sulfide levels exceeded recommended values for activated carbon adsorption and biofiltration. Bioscrubbing was recommended with carbon polishing, due to the proximity of sensitive neighbors.
Regional Wastewater System - Corrosion Management Plan
PECI personnel, along with several other firms, completed a system-wide corrosion management plan for a regional wastewater system serving nine cities in northwest Tarrant and Denton counties. Work involved pole camera inspections of approximately 100 underground structures followed by manned entries and detailed inspection of over a dozen structures warranting closer inspection. The plan prioritized numerous projects for both capital and O&M funding, and will develop recommendations for systematic future inspection of pipelines, HVAC equipment, metering facilities, manholes, and other facilities.
Collection System Odor Abatement Evaluation
PECI personnel have performed testing of collection system headspace air quality and pressures to resolve odor complaints in neighborhoods along two major regional system interceptors. Smoke and pressure testing revealed that headspace pressurization is occurring in gravity interceptors due to surcharging over a mile downstream from the points where complaints are occurring. Fan testing has been performed to identify mechanical ventilation rates for developing a headspace vacuum upstream in the affected neighborhood using a fan located downstream near the client’s treatment plant.