Wastewater Disposal Goes Underground
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
- Providing Operational Flexibility and Performance-Based on investigation of subsurface soils and historic groundwater conditions, SPS was projected to outperform the pond system during wet periods. Its shallower configuration made it less vulnerable to rising groundwater. The plan was to use the existing ponds for disposal during dry weather, for disposal/storage during the onset of wet weather, and then switch to SPS during very wet weather. This approach addressed the City's biggest concern-undesired surface water discharges.
- Reducing Operations and Maintenance Costs-The SPS eliminates the need to perform many routine and expensive O&M tasks such as rodent control, weed and mosquito abatement, and algae and weed removal from pond bottoms.
- Eliminating Public Concerns about Odors and Mosquitoes-Because of its subsurface nature, the SPS eliminates odors, mosquito breeding habitats, and the need to spray undesirable pesticides, which benefits the environment and reduces public health concerns.
- Achieving Multiple Land Use-An interesting benefit of the SPS is that it allowed the city to purchase one site that serves the community in two ways: as a wastewater treatment plant and as a community park. Final planning is underway for construction of soccer fields at the five-acre site.
- Enhancing Adjacent Land Values-The City's Community Development Department is actively promoting commercial and residential development in the area surrounding the treatment facility. Instead of having to mitigate developer concerns over the presence of un-sightly ponds, the City is able to use the new community park to promote itself. The SPS/park concept has provided a buffer zone around the existing plant, enhancing adjacent land values that would have been adversely affected by an expanded pond-type disposal system.
- Eliminating Flood Protection and Site Security Requirements- Because the Santa Clara River is prone to flooding, the ponds are surrounded by a flood protection dike. In addition, human contact with the ponds must be restricted, so they are also enclosed by a security fence. The costs and visual impacts of the dikes and security fencing are eliminated because the system's below-grade configuration restricts discharge to surface waters during flooding and eliminates human contact.
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:
- Four operational zones for periodic resting and drying
- Parallel trench separation to optimize capacity while minimizing the total system length
- Shallow depth for natural aeration and best use during high groundwater conditions
- Access ports for observing effluent dosing depth along zone perimeters, and
- Monitoring wells, up- and down-gradient from the site, to observe groundwater quality changes.
Construction and Initial Operation The system was constructed at a rate of about 500 linear feet per day. Because the trenches were shallow and the pipe arch sections lightweight, only two men and a backhoe were required. About 43,000 ft of trenches, distribution piping and appurtenances was constructed on a five-acre site at a final construction cost of approximately $430,000. The cost also includes clearing an existing orchard and removing/ replacing localized areas of unsuitable soils.
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.