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Geosynthetics in the construction of a Southern California subsurface biofilter cell system

August 1st, 2009 / By: / Environmental, Feature, Geogrids, Geotextiles

Introduction

The primary benefits of geosynthetic materials are the flexibility to accommodate a variety of configurations and as a nonreactive barrier for environmental isolation. These benefits are perfectly matched for this project: construction of a subsurface biofilter cell system for the removal of selenium and nitrate using many types of geosynthetic materials.

For the last 25 years, water quality in San Diego Creek in Southern California has been affected by excessive sediments and nutrient levels. The state of California and U.S. Environmental Protection Agency (EPA) regulations required the establishment of limits (i.e., total maximum daily loads—TMDL) on the amount of pollutants that can be discharged into Newport Bay. (San Diego Creek drains approximately 80% of the total area tributary to Newport Bay.)

To meet this EPA requirement, the Irvine Ranch Water District (IRWD) developed a plan to address regional water quality treatment. The plan included the Cienega Filtration Facility adjacent to the Peters Canyon Channel that is specifically for removing selenium from dry weather low flows.

The selenium TMDL is based on meeting the chronic California Toxics Rule (CTR) criterion of five parts per billion (ppb) for protection of aquatic health in dry weather flows. Selenium concentrations in dry weather flows in the Peters Canyon Channel typically range from about 30–50 ppb, but can be much higher at groundwater seeps and weep holes. The current selenium concentration levels are not harmful to humans but have potential to bio-accumulate.

Concept of biofilter cell

The project was designed to pass dry weather flows that are diverted from Peters Canyon Channel through an organically augmented biofilter cell, which was composed of a gravel matrix and constructed using many categories of geosynthetics.

The flows are amended with a carbon source to feed bacteria and create anoxic (oxygen-deficient) conditions in the biofilter cell. Under these conditions, common forms of selenium (i.e., selenate and selenite) are converted into elemental forms of selenium, which have relatively low toxicity and are encapsulated by bacteria growing on the bed materials (e.g., gravel) in the biofilter cell.

The subsurface natural treatment system consists of three major systems:

  • intake and pretreatment system.
  • biofilter cell.
  • finished water system.

The intake and pretreatment system includes an intake wet well and pump, self-cleaning strainer, bag filter, and electron donor tank. The biofilter cell was created using geosynthetic materials and granular media matrix. The finished water system includes an oxygenation system and finished water pump.

The biofilter cell is the primary treatment component of this project. As summarized in the Table 1, the biofilter cell consists of a coarse inert aggregate bed wrapped in an impermeable geosynthetic system. Table 1 Raw water is pumped from Peters Canyon Channel, amended with electron donor, and injected into the biofilter cell through a piped header system.

Within the biofilter cell, an active biofilm is grown, which attaches to the aggregate. The biological activity helps to create anoxic conditions that are favorable for the conversion of soluble selenium compounds to insoluble colloids and precipitates. The insoluble selenium precipitates are adsorbed to the biofilm and sequestered within the biofilter cell. Hydraulic retention time and electron donor feed rate are the variables that can be adjusted to maintain the desired environmental conditions.

Summary of biofilter cell characteristics

Key geosynthetics

  • Construction activities consisted of installing an infiltration gallery in the Peters Canyon Channel, the subsurface biofilter cell, a process area (pump and equipment housing), and a re-oxygenation system. The biofilter cell was constructed using geosynthetics and was covered with 3-4ft of native soil to allow for the development of a future recreational field. A geosynthetic liner system consists of:
  • an HDPE geomembrane, installed to keep the biofilter cell isolated and impermeable.
  • a nonwoven geotextile, used to provide both separation and cushion forprotection of the geomembrane.
  • a geonet/geonet composite–a biplanar HDPE geonet and nonwoven geo-textile–used to collect gas generated by bacteria from the biofilter cell.
  • a biaxial polypropylene geogrid, used to allow for the perimeter 10-ft vertical walls and for the biofilter cell to transition into future treatment biofilter cells.

The construction sequence is summarized as follows:

  1. Subgrade preparation
  2. Placement of 60-mil textured HDPE geomembrane for the bottom of the biofilter cell
  3. Biofilter cell wall construction using geogrid
  4. Compaction of granular media matrix
  5. Geotextile wrapping the biofilter cell as the separation between granular media matrix and geomembrane
  6. Installation of smooth geomembrane to encapsulate the biofilter cell
  7. Placement of geonet on top of the biofilter cell
  8. Placement of geotextile on the side walls of the biofilter cell as the separation between geomembrane and backfill soil
  9. Placement of geonet composite on top of the biofilter cell
  10. Piping
  11. Backfilling and covering

Geosynthetics installation significance

1. A total of 30 PVC piping penetrations through the liner system of the biofilter cell were constructed: 18 monitoring ports on top of the biofilter cell (i.e., vertical pipes) and 12 perforated pipes for inflow, outflow, and gas collection onto the side walls (i.e., horizontal pipes).

Prefabricated HDPE geomembrane boots were fitted over the PVC perforated pipes and extruded to the geomembrane liner. After the extrusion welding, copper wire spark tests were performed for nondestructive seam testing. A probe with a current was passed above the seam with 25mm (1in.) distance between the probe and the seam, and any sparks indicated that a hole was present.

A neoprene gasket and caulking was inserted between the geomembrane boot and PVC pipe annular space and supported in place with two stainless steel band clamps. The caulking was cured for two to three days after which the stainless steel band clamp was secured to the pipe. After the completion of pipe installation, geotextile was wrapped onto the boot as a cushion material during the backfilling of soil.

2. Geomembrane liner on the side walls was protected by an inside and outside geotextile layer from granular media matrix and backfill.

The double-track fusion welding was mainly performed for the side wall seaming. The double-track fusion seams were nondestructively tested using the air pressure test as performed on the top and bottom geomembrane liner systems. At the corners of the biofilter cell, the geomembrane liner was folded and extrusion welded.

3. Two different types of HDPE geomembrane (i.e., 40-mil smooth geomembrane and 60-mil textured geomembrane) were welded together using extrusion welding techniques.

Prior to the welding, asperity of the 60-mil textured geomembrane was grinded and removed. The extrusion welds were tested with the vacuum test method. The test results indicated that these two products could be satisfactorily welded using this technique.

Conclusions

Design and construction of the biofilter cell were successfully implemented using major categories of geosynthetic materials due to the flexibility of geosynthetic materials to accommodate a variety of design configurations. Geosynthetics provided a nonreactive barrier for environmental isolation of the biofilter cell.

Perhaps the greatest challenges with this application of geomembrane were the vertical welding and the abundance of pipe penetrations. To the extent possible, vertical welding should be limited by prefabricating larger portions horizontally and assembling them in a manner that requires little vertical welding. The abundance and variety of specialized geosynthetics provided the designers with a “toolbox” of materials and, thus, the flexibility to develop creative solutions to this challenging environmental issue.

Ronald S. Johnson, P.E., G.E. (rjohnson@geosyntec.com) is a geotechnical engineer, and Sang Yeo, Ph.D. (syeo@geosyntec.com) is a senior staff engineer, both with Geosyntec Consultants. Randy Sundberg, P.E., is a project engineer with the Irvine Ranch Water District.

Acknowledgements

This project was funded by the Irvine Ranch Water District.

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