Forced-Air Ventilation for Sanitary Sewer Odor Control: What’s My Zone of Influence Going To Be?

When an owner has decided to pursue an odor control solution along a reach of gravity fed sanitary sewer using a forced-air ventilation technology such as a biological system or activated carbon adsorption, one of the most commonly-asked questions is, “What will the zone of influence (ZOI) of the odor control system be?” Here the ZOI is defined as the length of sewer pipe, both upstream and downstream of the air withdrawal point, whose headspace will be maintained under negative differential air pressure by the odor control system fan, thus keeping foul air from escaping to outside atmosphere. Due to a multitude of factors this question can be very difficult to answer accurately simply by performing a desktop evaluation. These factors include the available open headspace in the sewer in question; the number of side-stream sewer connections into the sewer; diurnal wastewater flow patterns; the amount of air dragged in the open headspace not only by the flow carried by sewer in question, but also that which is dragged into it by wastewater flow from the side-stream sewers. Because these factors can change constantly during a given time period, the difficulties in obtaining an accurate it has been shown that full-scale fan testing, whereby the headspace differential air pressure reduction effects of air withdrawal from the sewer are measured directly in the field. The purpose of this paper would be to demonstrate the difficulty in predicting such pressure reduction effects through desktop analysis and to present case studies on odor control studies in Arizona and Minnesota which highlight the different zones of influence, and the reasons for these differences, using field-measured headspace differential air pressure data.