TEXTILES.ORG
By Felon Wilson
The Olivenhain Municipal Water District’s (OMWD) 4S Water Reclamation Facility (WRF) receives 1 MGD (3785 m3/day) of municipal wastewater, treats it to tertiary standards and discharges none.
The 4S WRF serves a population of 7,500 homes in the 4S Ranch and Rancho Cielo areas, both in San Diego County. Originally a private system, OMWD took over in 1998 and now owns and operates the facility including the collection system consisting of 15 pump stations, 65 miles (108 km) of gravity sewer and 25 miles (42 km) of force main. The WRF has an organic capacity of 2 MGD (7570 m3/day) and treats the incoming wastewater with primary screening and grit removal followed by secondary treatment via extended aeration/clarification. Tertiary treatment is accomplished with shallow-bed sand filtration. The filtrate is disinfected with UV.
All treated effluent is directly recycled through 46 miles (76 km) of purple pipelines and used for non-residential, non-greenspace vegetative applications, including homeowner associations, schools, parks, golf courses and street departments. Recycling the effluent in real time works well in the hot, dry portions of the year, but in cooler, wetter periods (winter), influent volume to the WRF exceeds recycle usage. OMWD has a commitment to zero discharge; in fact, the WRF is designed without a permitted outfall. Excess effluent is stored in a lined impoundment.
The district’s initial impoundment was undersized. In addition, the original reinforced polypropylene liner was failing prematurely due to UV degradation. So, in 2016, the impoundment was enlarged and relined, extending the geomembrane beyond the perimeter access road. By extending the liner beyond the access road to the brow of the reservoir, the district greatly increased storage capacity. This fits the district’s long-term needs:
The Regional Board requires that wastewater treatment plants have 84 days of wet weather (seasonal) storage per MGD based on treatment capacity. The 4S Ranch Pond is a 410 acre-ft pond which equates to capacity of 134 MG (500,000 m3). This provides enough storage for the current design flow maximum of 1.6 MGD (6050 m3/day). The relined pond helps OMWD to meet long-term growth plans.
The impoundment is located west of the WRF and across the busy 4-lane Cove Canyon Road. The plant and the impoundment are both surrounded by residential areas and are very noticeable. The proper operation of both is particularly important.
Average dimensions of the impoundment are 880 ft x 630 ft (268 m x 192 m) with varying width and length as adapted to optimize the available land. To contain the water, OMWD used over 600,000 ft2 (55,000 m2) of 8138 XR-5 Geomembrane to line the basin. The XR-5 was selected after an analysis of basin lining requirements including puncture and UV resistance, strength, longevity, stability, and low thermal expansion contraction properties. The low thermal expansion contraction properties were a must-have as the operation of the storage basin would leave it often empty, and cover was not feasible or cost effective from a capital or O&M standpoint. Further, they wanted the storage facility, like the treatment facility, to be a good neighbor. At their request, Seaman Corporation, the manufacturer of the XR-5, produced a cream-colored version of the product to make the impoundment more aesthetically pleasing, while retaining the material properties.
The completed, lined basin is shown below prior to commissioning.
The winters of 2016 and 2019 were especially wet in southern California and much of the plant effluent required storage, but the design was sufficient to hold all the excess recycled water and the XR-5 contained all the water in the basin until it was ultimately recycled in the community. While other treatment challenges face all WRFs, this one has their recycled water storage under control.
The 4S Ranch Water Reclamation Facility was awarded the 2018 California Water Environment Association and local California Water Environment Association Section Small Plant of the year award.
Information contributed by XR Geomembranes. Learn more about XR Geomembranes here.