Sewer Siphon Assessment and Air Jumper Design

In response to customer complaints about sewer odor, Orange County Sanitation District (OCSD), California launched a project to investigate the ventilation requirements of existing sewer siphons, to verify adequacy of existing air jumper operation, and determine the need for new air jumpers or additional air jumper capacity. The challenge was assessing sewer siphon air jumper adequacy without clear industry standards or guidance for comparison and to then develop air jumper sizing guidelines and recommendations for retrofit replacement or addition of air jumpers at 17 of the existing 87 OCSD sewer siphon locations.

A literature search for sewer siphon air jumper sizing guidance was conducted. For each of the 17 sewer siphons studied: siphon inlet and outlet structures were inspected; sewer flow was measured; headspace air pressure or vacuum was measured; air flow into or out of the upstream and downstream siphon structures was measured; and dissolved and headspace hydrogen sulfide levels were measured. Relationships were theorized and developed between: depth of wastewater flow in the sewers; velocity of wastewater flow; sewer headspace airflow rates; and sewer headspace pressure or vacuum with manhole covers in place. General design guidelines for sewer siphon air jumpers were developed and recommended and these guidelines were applied to the field data for each of the studied 17 siphons. Sewer siphon air jumper retrofit replacement or additions are currently in the final design phase with construction scheduled to start in February 2007.

The Orange County Sanitation District consists of a service area of 471 square miles, including 21 city and three special district member agencies. The regional transmission system owned and maintained by OSD includes 475 miles of interceptor and trunk sewer lines. The local collection sewers are owned and maintained by the member agencies.

The OCSD system also includes 17 pump stations and two regional wastewater treatment plants treating a total average daily flow of 243 million gallons per day (mgd).

The regional OCSD sewer transmission system includes 87 sewer siphons. These inverted sewer siphons range in size from 15 to 60 inches in diameter and typically cross under large storm drain box culverts within heavily congested, multi-lane boulevard intersections in Orange County, California.

OCSD is very sensitive to odor complaints from the public. It is OCSD’s goal to eliminate all sources of OCSD sewer system related odor complaints. OCSD has maintained a Geographical Information System (GIS) tracking of the location of all odor complaints. It was determined that there are a number of odor complaint “hotspots” in the vicinity of many of the siphons, despite a very pro-active siphon cleaning and maintenance program. OCSD prioritized the odor complaint hotspots in the vicinity of siphons and then moved forward with this project to investigate the adequacy of air jumpers for the siphons with the most odor complaints. The need for air jumpers and how to adequately size the air jumpers for each of these odor hotspot locations is the subject of this paper.

In the past, the OCSD’s primary method of sewer siphon odor control has been to provide a “siphon vent” or “air jumper” and to seal manholes in the vicinity of sewer siphons. Of the 17 siphons reviewed in this project, six siphons do not have an air jumper, due possibly to oversight or to physical limitations accepted in the original project designs. It was suspected, and confirmed, that for most of the siphons possessing air jumpers, the existing air jumpers were not adequately sized to avoid upstream sewer headspace pressurization and release of malodorous air. This is partially evidenced by the chronic odor complaints for these siphon areas. The escaping airflow rate, intensity of odor, and location of escape are transitory and very difficult to determine or control. This often results in a public nuisance in the form of unpleasant odors.

Currently, OCSD utilizes odor control mitigation through sealing upstream and downstream manholes near the siphons, which drives the release of malodorous air to some other typically unknown location.

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