In the first large-scale application of its kind, the City of Fillmore, in Ventura County, California, expanded its treated wastewater disposal capacity by installing subsurface percolation, a simple yet innovative approach that has been shown to be cost-effective and environmentally sound. The innovative disposal system has restored operational reliability and provided secondary benefits to the community.
In concept, the subsurface percolation system (SPS) is very similar to a leach field, which is commonly used for disposal of primary effluent in small private septic systems. The primary difference between the two types of systems is that the SPS disposes of much cleaner secondary effluent on a larger municipal scale. Construction of the first phase of the Fillmore system was completed during the summer of 1994. It has a design capacity of over 2.2 mgd.
Initial testing of SPS has exceeded design expectations, and plans for a second construction phase may not be required to serve anticipated development. The City's system consists of perforated polyethylene pipe arches buried in a network of shallow trenches beneath a five-acre site. The network is divided into four operational zones. Treated effluent flows through the pipe arches and percolates into highly permeable subsurface soils.
The Disposal Problem
The City of Fillmore has historically relied on a series of five percolation/evaporation ponds for disposal of treated effluent. Although the disposal ponds performed well most of the year, they periodically experienced a significant loss in percolation capacity during very rainy periods. High water levels in the adjacent Santa Clara River caused groundwater levels to rise up to the pond bottoms. This condition essentially stopped percolation and converted the disposal ponds into holding ponds. In some years, these high water events extended over a period of weeks. The storage capacity of the ponds was exceeded and resulted in direct discharges of secondary effluent to the river.
Although these discharges were disinfected, diluted by high river flows, and allowable under the City's NPDES permit, they were undesirable. City leaders were interested in finding an environmentally-sound disposal option that would reduce or eliminate surface discharge events. Furthermore, the Regional Water Quality Control Board could change the permitted status of these discharges in the future when the City's NPDES permit requires renewal.
Finding Solutions-A Pilot Test
In 1989, the City embarked on a wastewater planning effort that identified the need for additional disposal capacity. As the City explored the feasibility of alternative disposal methods, the concept of SPS emerged. The Ventura Regional Sanitation District (VRSD), the City's contract operator, conceived the idea while researching literature on perforated pipe arch products commonly used in stormwater applications. The high quality secondary effluent and highly permeable subsurface conditions along the Santa Clara River appeared to be ideal conditions for this kind of system. Moreover, the concept appeared to have a number of operational advantages and secondary benefits. A pilot test of subsurface percolation was performed to confirm the technical feasibility of the concept.
Two types of systems were started up over a period of about 18 months. The primary objective of the testing was to observe how the acceptance rate of the subsurface soils varied with time, loading, and fluctuating groundwater conditions. Unfortunately, much of the west coast was suffering through a drought during testing, so the effects of extremely wet weather could not be observed. Good data was obtained, however, on the relationship between acceptance rate and time.
One pilot set-up used a gravel-filled trench with a perforated PVC distribution pipe (Fig 1), while the other set-up (Fig 2) used a pipe arch product manufactured by Infiltrator Systems, Inc., Old Saybrook, Connecticut. The pilot work was conducted using secondary effluent at the treatment facility site which was adjacent to the site planned for the new disposal system. Effluent dosing rate was controlled using a float system that maintained a one-ft dosing depth as measured from the trench bottom. Collected data included metered flow, effluent BOD and TSS content, and depth to groundwater.
At the end of the testing program, both systems proved to be effective. Test data showed that continuous operation of both systems resulted in a decline of percolation capacity that eventually leveled off at a relatively uniform rate. This data was consistent with other research that suggests that effluent solids, filtered by the trench bottom and sidewalls, create a bio-mat that eventually masks the infiltrative surface and reduces percolation capacity.
In terms of unit capacity, the initial volume of both systems was in the range of 40 gpd/sq ft of trench bottom. This rate dropped within about six months and leveled off in the 5 to 10 gpd/sq ft range. During testing, operational problems were experienced that interrupted flow to the system for periods of a week or more. After the pilot systems had rested and dried out, percolation capacity rebounded to 80p;85 percent of the initial capacity when placed back in service. This observation led to the conclusion that the system should be divided into zones and operated with run/rest cycles to optimize disposal capacity.
Concept Feasibility to Design
With pilot testing completed, the next task was to compare the feasibility and costs of SPS and ponds, and confirm geotechnical suitability of the proposed site. In a final report on Alternative Wastewater Disposal Using Subsurface Percolation, SPS was shown to have advantages over the pond system, at comparable life-cycle costs. These economic and non-economic advantages included
The City subsequently purchased the site and proceeded with design work for eligibility under the State's Revolving Fund loan program. Final design work addressed several key issues that were identified during the feasibility study:
Initial testing of the system has been encouraging. During dry weather conditions, the initial capacity of the system exceeded 40 gpd/sq ft of gross site area. Considering the expected decline in percolation rate with time, the system is projected to have a long-term capacity of approximately 10 gpd/sq ft. With frequent resting and rotation of operating zones, a 2.2 mgd system capacity is expected to be achieved. The final test of the system-operation during high groundwater conditions-may occur this winter.
About the Author:
Craig Lichty was project manager for Kennedy Jenks Consultants, San Francisco, California, the design engineering firm on the Fillmore project.