Covered Processes, Ventilation and Negative Pressure

Covers applied to open processes at wastewater facilities have evolved from floating domes that were used to minimize product volatilization on petroleum storage tanks to tight fitting panels with snug fitting male/female panel interfaces. In the first wastewater cover applications, domes were fixed to the outer walls of process tanks, worked as process caps and, in most cases, were not ventilated. The covers were either sealed to try to trap odors or open to the atmosphere to allow passive ventilation. Both of these options reduced overall odor impacts but allowed for small volumes of highly concentrated odor releases. The next step in cover evolution was to include ventilation and odor control for covered processes to eliminate the periodic high concentration release.Once ventilation and odor controlled were also considered, the headspace volume became an important cost consideration. Although more expensive initially, flat covers were more cost effective in many large volume applications when ventilation and odor control costs were also considered.Original cover ventilation theory considered that covers would work as intended as long as infiltration was less than the ventilation rate since make-up air is necessary for ventilation. As more covers were installed, it became clear that tighter covers were still necessary and achievable. Tighter covers were needed to decrease the wind impact across a cover and to minimize short-circuiting.Maintaining a negative pressure under a cover with dedicated intakes and uptakes operating will assure maximum odor capture and proper ventilation. Proper smoke and ventilation testing is needed during the design, specification and installation to qualify each cover. Smoke testing is recommended to pinpoint areas of concern prior to infiltration testing.