Dual Phase Collection System Odor Investigations: The Liquid Phase Might Not Be Enough!

It is a well known fact that investigations of odor production in gravity fed sanitary sewers often involve the need for a full understanding of the liquid phase causes of dissolved sulfide production. Related parameters that are typically investigated include wastewater temperature, pH, existing dissolved sulfide concentration and BOD5.The question that is rarely answered in these investigations and studies is, “What is causing odorous air to be forced from the gravity collection system?” Without a full understanding of the headspace ventilation and air pressure characteristics of the sewer system, a permanent odor control solution might not be possible. This presentation will focus on a case study from a recent project in which both liquid phase odor production parameters and headspace ventilation and air pressure characteristics were quantified. Analytical techniques and methods will be presented along with the data from these projects that resulted in the implementation of successful odor control solutions.Figure 1 presents an example of typical air pressure data that was gathered on a recent sanitary sewer odor control project in New Mexico.The air pressure data shown in Figure 1 was taken in a manhole that was a known source of odor complaints and positive sewer headspace air pressure. This had been determined by desktop analysis and subsequently confirmed by air pressure testing. An in-ground biofilter was designed and built near the location of the manhole to remove odorous air from the gravity sewer and treat it through wood-chip media. Two blowers were installed for the biofilter: one duty and one standby. As shown in Figure 1, the natural air pressure in the manhole (with no blowers running) was upwards of 0.15 inches of water column, which resulted in odorous air being forced from the sewer to atmosphere on a daily basis. When Blower 1 is turned on, the air pressure immediately is reduced to −0.5 inches of water column. Blowers 1 and 2 working simultaneously result in an air pressure value of −0.8 inches of water column. When both blowers are turned off the air pressure immediately returns to approximately 0.15 inches of water column.The paper will show that this type of analysis is a valuable tool in identifying the ultimate source of odor release in a gravity fed collection system and designing the proper odor control ventilation systems to ensure that odors are no longer released from the sewer pipes.