Fast Track SSO Mitigation and Grit Removal: Protects Environment and Infrastructure

Fast Track SSO Mitigation and Grit Removal - Protects Environment and Infrastructure
The objective of this paper is to present a case study that mitigated a Sanitary Sewer Overflow (SSO) and, at the same time, developed an innovative interceptor grit removal facility designed to protect a downstream siphon crossing of the Mojave River in southern California. The case study covers the time period from identification of the initial problem, identification of the preferred solution and ultimately through final design and construction, and also includes lessons learned along the way. Victor Valley Wastewater Reclamation Authority (VVWRA) is a joint powers regional authority, in the high desert of southern California that collects, treats, and disposes of wastewater from four member agencies. The four member agencies include the Town of Apple Valley, the City of Victorville, the City of Hesperia and the County of San Bernardino Service Areas 42 (Oro Grande) and 64 (Spring Valley Lake). The South Apple Valley Interceptor was constructed in 1981 and crosses the Desert Knolls Wash several times in the vicinity of the Lewis Center for Educational Research (Lewis Center) in Apple Valley, CA. (Figure 1). Over time, VVWRA realized that the original interceptor design within the wash area did not adequately protect the pipe from scour during major storm events. On December 24, 2017, a portion of one crossing (a 15-inch PVC pipeline) was totally exposed and floated/deflected upward during a significant rain event (Figure 2). Following that event, Carollo performed an emergency condition assessment of the existing interceptor resulting in the following recommendations:
- Develop a new alignment that removes the South Apple Valley Interceptor from the Desert Knolls Wash to eliminate the potential for a future SSO.
- Resolve the conflicts with a future San Bernardino County Flood Control District Desert Knolls Wash Phase III Project.
- Explore options to prevent grit (sand and rocks) from entering an existing 16-inch double barrel siphon located downstream of the interceptor and constructed beneath the Mojave River.
- Provide for a relocated flow monitoring and sampling facility.
- Expedite the final design and construction.
We believe this case study can benefit our fellow practitioners specifically by describing how VVWRA implemented an emergency condition assessment, fast track final design with sustainable construction materials, and implemented construction practices to ensure a project with a minimum of a 100-year design life. This approach includes:
- Design of an innovative interceptor grit collection facility constructed with precast polymer manholes, stainless steel slide gates, a PVC pipe bypass pipe system, and composite manhole lids. Figures 3 and 4 illustrate the grit sump and bypass piping. The interceptor grit collection facility was designed to protect a new HDD 16-inch double barrel crossing of the Mojave River in Apple Valley, California from rocks, sand, and debris. Note: The existing Horizontal Directional Drill (HDD) installed 16-inch fusible PVC double barrel crossing was constructed with no means to remove grit and sand.
- Use of corrosion resistant precast polymer manholes and bases. This material provides corrosion free service with an extensive design life. - Use of PVC pipe, which is also corrosion resistant with extensive design life.
- Design of a Fiber Reinforced Polymer (FRP) wastewater sampling facility enclosure. The project was successfully assessed, designed, and constructed on time and under the Contractor's bid price. Total length of time from project onset to project completion was eleven months.
Major lessons learned from the project are:
1. Siphons can be protected from grit and sand deposition by an innovative interceptor grit removal facility. Currently, VVWRA maintenance staff removes debris from the new facility with a VAC truck every 4 months.
2. Long term sustainability or 100-year minimum design life can be achieved for interceptors by specifying corrosion resistant materials.
3. Stake holder involvement during the design helps ensure smooth construction with minimal conflicts.
This paper will focus on the assessment, design approaches, and lessons learned from this project.