Design, Construction and Operation of Innovative Selenium Treatment System

During the course of remedial activities conducted by WPI on a major drinking water aquifer restoration project, it was determined that groundwater contains naturally occurring selenium at concentrations ranging from 1 to 70 µg/L. In 2004, based on the California Toxics Rule and the State Implementation Plan, the  Los Angeles Regional Water Quality Control Board implemented a new regulatory limit for selenium discharge under the National Pollution Discharge Elimination System (NPDES) of 4.5 µg/L.

 

In response to the new NPDES requirements, WPI designed and implemented a pilot program in 2005 for removal of selenium from groundwater using biological reduction. At that time, there was no economically-feasible commercial process available to reduce dissolved phase selenium concentrations below 5 µg/L. Experiments with the pilot system were completed in November 2007. Results indicated that biological reduction using a packed bed process had the potential to meet the 4.5 µg/L NPDES discharge requirement.

 

In March 2008, WPI began construction of a full-scale biological selenium removal system . The system included two pressurized bioreactors, three computer-controlled pumps, and instrumentation for flow/level monitoring. In September 2008, the original single inlet groundwater manifold was replaced with two; one for wells containing high concentrations of selenium and the other for wells with low selenium concentrations. The manifold connected to the high selenium wells was routed to the biological system, while the low selenium manifold was routed directly to an air stripping system. From August 2008 to February 2009, the biological system operated on an experimental basis, achieving the target selenium removal concentration at a flow rate of 20 gpm.

 

During 2009, WPI continued to operate the biological system while implementing numerous process improvements to increase the process flow rate and the selenium removal capability.

 

Process refinements included:

  • Improving the stability and reliability of the organic feed system by construction of a temperature-controlled housing, the installation of a more 
  • reliable metering pump, and installation of a mass flow meter
  • Installation of a third bioreactor to increase contact time
  • Specification of a new primary groundwater pump to allow operation of the bioreactors at elevated pressure
  • Installation of pressure transducers and a PLC-based pressure monitoring system
  • Extensive modifications to the selenium system PLC program to permit data-logging of bio-system flow rates and pressures, allow automatic startup/shutdown of the system, allow remote data access, and automatically shut off high selenium wells when the bio-system is not operating
  • Optimizing media backwash techniques including development of compressed air/water jetting system to minimize clumping of media
  • Design and installation of a PLC-controlled motorized media leveling system to prevent channeling of water through the media bed

 

The bio-treatment system has been in full compliance with the 4.5 µg/L NPDES selenium discharge limit since July 2009. Since September 2010, the bio-system has been operational 95 to 99% of the time at a flow rate of 40 gpm. The selenium concentration in the bio-system effluent has generally been less than 1 µg/L.