These benefits are perfectly matched for this project: construction of a subsurface biofilter cell system for the removal of selenium and nitrate.
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.
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.
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 biofilter cell was created using geosynthetic materials, principally Atlantis tank modules and granular media matrix. The biofilter cell is the primary treatment component of this project.
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.
Design and construction of the biofilter cell were successfully implemented Geosynthetics provided a nonreactive barrier for environmental isolation of the biofilter cell.