Sewer ventilation dynamics can be complex, but they are helpful for water/wastewater utilities that wish to control odor. The basic principle of sewer ventilation is that wastewater drags headspace air due to friction between the water surface and the air. Friction between the air and the pipe walls results in a headspace air velocity lower than that of the water. Water level can influence the amount of air being pulled by changing the surface area between the water and air as well as by changing the volume of space available for the air. A pipe’s slope can influence the airflow by increasing or decreasing the water and air velocity.
Sewer systems can be a challenge to model, depending on the complexity of the system layout. Changes in pipe size, water level, and pipe slope within the sewer system can cause sections of the system to become pressurized, pushing foul air out, and other areas to be under vacuum, drawing fresh air into the system. Pressurization is possible whenever an upstream portion pulls more air than a downstream portion. Common causes for pressurization in a sewer system include wet wells, inverted siphons, clogged pipes, and pipe sags.
Hydrogen sulfide, the most common odor in sewers, can be smelled at very low levels such that even slight pressurization in a sewer can allow odors to escape from any openings in the system. Common openings include utility hole lids (pick/bolt holes), wet well hatches, and vents.
Is Your System Pressurized?
If you are receiving odor complaints and want to find out if your system is pressurized, there are two main testing methods: a smoke test and a differential pressure logger. Smoke tests are quicker and less expensive, but they provide limited data. They only determine if the system is pressurized at the time the test is done; they do not measure how pressurized the system is quantifiably. Differential pressure loggers can be rented or bought and can provide days to weeks of data.
How to Control Odor
There are two main types of odor treatment for wastewater: liquid phase and vapor phase. As Justin Angel, PE, discussed in a previous blog, liquid phase treatment can maintain aerobic conditions, shift the pH, or bind the sulfides. These treatment methods aim to either prevent the generation of odorous compounds or their release. The goal of vapor phase treatment is to mechanically draw air from the sewer system into a treatment unit that removes odorous compounds from the air before exhausting it to the atmosphere. Proper sizing of the system is necessary to determine the amount of air that needs to be drawn to create a vacuum within the sewer system that pulls fresh air in rather than letting foul air escape. There are methods for calculating a theoretical air flow rate, but a fan test can determine a more accurate size. A fan test is when headspace air is temporarily exhausted from one location in the collection system to determine the air flow rate required to achieve a vacuum over the desired area of influence within the collection system. A fan test can determine the odor control system’s capacity and how far it should create a vacuum in the sewer system.
Once the capacity is determined, the next step is to evaluate the treatment technologies. Also discussed in Justin’s blog, the standard technologies for vapor phase treatment are chemical scrubbing, biological treatment, adsorption with dry media, and ionization. Each treatment method should be evaluated on a case-by-case basis. They each have their own advantages and disadvantages and depend on the odor levels within the system as well as the potential location of the treatment system.
