Facing a noncompliant ventilation system during its preliminary treatment building (PTB) upgrade, the Arlington County Water Pollution Control Plant innovatively rerouted foul air.
NFPA 820 insights
- By moving foul air to the gravity thickeners as make-up air, resolving odor control capacity issues and fugitive emissions, Arlington County Water Pollution Control Plant ventilation issues were resolved.
- Engineers also upgraded the internal ventilation system with new fans, ductwork, monitoring and physical separations to meet NFPA 820 standards.
The Arlington County Water Pollution Control Plant recently upgraded its preliminary treatment building (PTB) to replace the 30-year-old influent screenings handling equipment. During design, the facility discovered that the building’s ventilation system, installed in the early 1990s, was not compliant with NFPA 820: Standard for Fire Protection in Wastewater Treatment and Collection Facilities. The building was designed before NFPA 820 being issued as a standard.
To upgrade the ventilation system, the design team needed to identify how the additional air would be treated. Foul air fans on the roof of the PTB transferred air across the site through an abandoned sewer to a centralized odor control system on the south side of the plant that also services other areas of the plant, including the influent pump station (Four Mile Run lift station), two gravity thickeners, the dissolved air flotation treatment building and two sludge storage tanks.
The odor control system was already beyond capacity, so continuing to direct the air to this system was not a good option. Additionally, the sewer system was over-pressurized, causing fugitive odors at the plant due to foul air flowing out through existing manholes in the system. The design team considered several options to provide additional treatment at the PTB without adding to the overwhelming centralized system.
The plant intends to add a new odor control system on the north side in the future, so providing a dedicated odor control system for the PTB was not desirable. Tying the PTB into the existing north odor control system was considered but was determined to be cost-prohibitive.
An innovative solution was developed to transfer the air from the PTB to the existing gravity thickeners. By using the odor control exhaust from the PTB as make-up air for the gravity thickeners, the total airflow to the centralized odor control system was reduced to within the system’s capacity. This also reduced the over-pressurization of the sewer system, thus mitigating the fugitive odors at the existing manholes.
Within the building, the existing odor control system provided the 12 air changes per hour (ACH) required to lower the electrical hazard classification from Class 1 Division 1 to Class 1 Division 2. However, the ventilation system was deficient in several areas:
- Most of the exhaust air was drawn from the below-grade flow channels, with minimal air drawn from the building. This was mitigated by adding new odor control fans with ductwork drawing air from the upper and lower levels of the screen room.
- The makeup air was introduced on the upper level only through two roof-mounted supply fans with hot-water duct heaters. No distribution ductwork was provided to distribute the air to the lower level. This was mitigated by adding a new hot-water makeup air unit with ductwork introducing air into the upper and lower levels of the screen room.
- The odor control system fans and the supply fans did not include flow monitoring with local alarm stations and remote monitoring. This was mitigated by adding flow monitoring to the odor control exhaust fans and the make-up air unit with local alarm stations and remote monitoring through supervisory control and data acquisition (SCADA).
- The basement level of the PTB was not physically separated from the upper levels of the building. This level is connected to other areas of the plant through below-grade tunnels. This was mitigated by creating physical separation between the basement and the upper levels of the building.
- The administrative area of the PTB and the chemical storage area were not physically separated from the screen room. It was not possible to create physical separation between these areas due to the existing configuration of the building. A combustible gas detection system (with local and remote alarms through SCADA) was added in the corridor in the administration area that is adjacent to the screen room. The owner was instructed to evacuate the building if these alarms are activated and not to re-enter until the alarm has been cleared. The existing chemical area is positively pressurized with respect to the screen room. Because no upgrades were provided for the chemical storage area, no further mitigation measures were implemented.
