Central Valley's Demo Treatment Process
Water Reuse in California's Central Valley
In the 1960s and ‘70s, the San Luis Unit of the United States Bureau of Reclamation Central Valley Project installed a drainage system in California’s Central Valley with 82 miles of pipelines and ditches that terminated in the Kesterson Refuge. This system was the preferred alternative disposal solution for agricultural drainage rather than discharging brine into the San Francisco Bay. However, in 1986, both the Kesterson Refuge and the drains at Westlands Water District were closed due to high selenium levels, up to 400 parts per billion, that caused genetic malformations in the waterfowl. These high levels in the agricultural drainage were due to the presence of naturally occurring selenium in the Cretaceous and Tertiary marine sedimentary soils along the west side of the Central Valley.
Since that time, irrigation water flows to the SLU have continued, and the only outlet for agricultural drainage is the San Joaquin River via the Grassland Bypass project. These discharges have been limited by increasingly stringent California Regional Water Quality Control Board regulations, prompting the affected parties to sue the Department of Interior for inadequate drainage capability as originally planned in the Central Valley Project. The courts ruled in favor of the plaintiffs, resulting in the 2002 decision to provide an in-valley disposal and drainage water treatment alternative.
USBR investigated selenium removal technologies from 2003 to 2006. Investigations included laboratory bench-testing and field pilot-testing to reduce selenium concentrations in local agricultural drainage water near Firebaugh, California. USBR
issued a Record of Decision in 2007, which selected a drainage alternative based upon a reverse osmosis and biological selenium treatment process (Figure 2). A court order to proceed with the project was issued in December 2009.
Until a viable solution could be found, the five water districts of the SLU developed interim measures to reduce the volume of agricultural drain water. They practiced two-stage irrigation of salt-tolerant crops, such as Jose Tall Wheat Grass that can intermittently tolerate salinity concentrations up to 5,000 milligrams per liter. The drainage collected in the tile drains from these salt-tolerant crops has average salinity concentrations of 13,000 mg/L. This technique greatly reduces the volume of agricultural drain water that needs to be handled.
The 2003-2006 testing performed by USBR was based on treating second-pass drainage water as it offers an economic advantage by allowing for smaller-sized treatment systems.
Designing the Treatment Process
Starting in 2010, our team worked with USBR to design a demonstration treatment process based on the Record of Decision to address the water quality challenges and reduce selenium levels in the drain water. The selected treatment unit processes and their functions included:
Anaerobic biological filter – The GE Advanced Biological Metals (ABMet) System uses a patented advanced anoxic/anaerobic biological treatment process to reduce compounds, such as dissolved metals including selenium and other contaminants, from the feedwater to their insoluble chemical components. The bioreactor consists of a 17-foot-deep granular activated carbon bed seeded with non-pathogenic, facultative anaerobic bacteria that form a biofilm on the inert carbon media. A proprietary molasses-based nutrient is mixed into the influent flow to serve as a carbon source for the bacterial community and maintain the appropriate oxidation reduction potential range for selenium reduction.
Coagulation, flocculation and plate settler sedimentation – This process utilizes the addition of aluminum chlorohydrate along with a static mixer, conventional flocculation and plate settlers to remove solids and organics that carry over from the ABMet process and the recycle clarifiers.
Ultrafiltration membranes and pressure filters – These processes are used for polishing the settled water to reduce the solids introduced into the RO desalination system.
Reverse Osmosis – Two RO test skids were installed, one using brackish water membranes and one using seawater membranes. Both skids will operate at 50 percent recovery due to the high hardness levels. Scale inhibitor is added to control calcium sulfate scale.
The original concept involved feeding RO concentrate to the ABMet reactors to ensure the selenium content of water sent to evaporation ponds would be less than 10 ppb. During initial startup, the system went through multi-day periods when the RO concentrate remained in the ABMet system while support system adjustments and repairs were being made. This ultimately led to formation of calcium sulfate scale in the first stage reactors. To prevent a future recurrence, the process flow was reversed with the ABMet system receiving the second-pass agricultural drain water prior to the RO system. Before making the change, extensive bench-scale testing was performed by GE to show that less than 5 ppb selenium effluent quality could be achieved at the higher hydraulic loading to the reactors. Piping and programming changes were also made to allow for dilution of feed water with permeate if calcium and sulfate levels got too high.
Effluent from the ABMet system is sent to a large effluent holding tank that stores reactor backwashes. Effluent is then pumped from the effluent storage tank to begin the pretreatment process, including coagulation with ACH, flocculation and a plate settler to remove solids that carry over from the ABMet reactors and backwash recycle streams. The effluent from the plate settler is sent to both a media filter and UF system to allow for a comparison of their performance as a polishing step ahead of the RO system.
Two types of RO membranes are being tested (brackish water and seawater) to compare their effectiveness at reducing the high level of boron in the water. Boron concentrations above 1 mg/L limit the crops that can be irrigated with the permeate from the RO system. Boron concentrations higher than 1 mg/L will require various levels of blending with other surface water supplies, depending on the crop’s sensitivity to boron. The selected RO system will need to separate 99 percent of the dissolved salts and remaining selenium from the ABMet effluent into a 50 percent concentrated waste stream. Permeate from the RO system will be reused for irrigation by blending it with other supplies. Removing the selenium to a very low level (less than 5 ppb) in the ABMet reactors is necessary to achieve less than 10 ppb of selenium in the RO concentrate. Realizing less than 10 ppb of selenium concentration will minimize the impacts to wildlife that may come into contact with the open evaporation ponds where the concentrate will be sent for ultimate disposal.
The demonstration treatment plant has a capacity of 200 gallons per minute and a variety of treatment configurations to give water districts the ability to find the optimized treatment scheme to achieve a total selenium concentration of less than 0.010 mg/L in the final RO concentrate. Apart from the treatment process, we are consulting on substantial plant support operations, including a major building structure, drain sump storage tanks and sludge dewatering. Lastly, the demonstration plant design includes additional infrastructure, space and capacity to install alternative treatment equipment for future technology evaluations.
The system is currently operating and collecting cost and performance data. Selenium levels in the ABMet effluent are averaging less than 2 ppb. This data will be a basis for the final design of several full-scale drainage treatment facilities to potentially be constructed across the SLU. To be considered successful, the demonstration treatment plant will need to meet the following objectives:
- Operate 24 hours per day, 7 days per week and year-round for treatment of agricultural drainage water supplied by USBR from seven sumps, via pipeline, and blended within a 500,000-gallon equalization tank supplied by USBR;
- Achieve ABMet process effluent selenium concentration less than 5 ppb;
- Successfully operate the plate settlers, UF system, pressure filters and RO systems to treat 200 gpm of water to consistently achieve:
- Silt density index of 3.0 or lower, using the standard test method from the American Society for Testing and Materials D4189-07;
- Minimum of 50 percent RO product water recovery;
- Prevention of biofouling and scaling of the RO membranes;
- Maximum RO membrane cleaning frequency occurring on a quarterly basis;
- Reject 98 to 99 percent of total dissolved salts, including selenium ions;
- Maximize removal of boron with a target effluent concentration of 5 mg/L.
The treatment of this agricultural drain water will enable effective water reuse, preserve the San Joaquin River as a quality drinking water supply, and advance the cause of sustainable agriculture on the west side of the Central Valley.