Nature preserves, vineyard covered foothills, and the Napa River virtually surrounds the northern California city of American Canyon in a permanent greenbelt of spectacular scenery. Regarded as the gateway to the Napa Valley, this small community of 13,000 perches on the southern tip of this world-famous wine producing region located just 35 miles northeast of San Francisco.
For many years the population grew at a relatively modest rate, adding only a few hundred residents annually. As a result, relatively simple, conventional water and wastewater treatment systems provided adequate service. In 1975, American Canyon stopped using its outdated oxidation ponds, and began pumping its municipal wastewater north, to Napa County’s Soscol Advanced Waste Treatment Facility, which could discharge a higher quality effluent to the environment.
After more than two decades of outsourcing its wastewater treatment, city officials in American Canyon began to consider constructing plant of their own. In 1997, the city decided it could economically treat and recycle municipal wastewater in an environmentally responsible manner and began planning and designing a new wastewater treatment plant (WWTP).
MBR enables water reuse program
Three wastewater treatment processes capable of meeting Title 22 requirements were evaluated by the city—oxidation ditch, extended aeration system and membrane bioreactor. Upon concluding the evaluations, ZENON ZeeWeed® MBR was selected as the best available technology for the city’s wastewater treatment needs. Pilot tests revealed that ZeeWeed MBR provided the best value for the city, produced the highest quality effluent and offered the most robust and reliable operation.
In September 2002, the new 2.5 MGD WWTP released its first treated effluent into an environmentally sensitive wetlands that was specifically constructed to demonstrate the benefits of using high quality recycled water as a year-round water source for freshwater wetlands.
As the city continues to develop its water reuse program, the majority of the WWTP effluent will be diverted to urban and agricultural applications, while the balance of the effluent will continue to supply the wetlands.
To produce such high quality reuse water for the wetlands, the ZeeWeed MBR first passes the city’s wastewater—a mixture from 85% municipal and 15% industrial sources—through a 3mm fine screen for trash removal. BOD for the raw wastewater is typically around 225 mg/l, and TSS is around 250 mg/l.
After pretreatment, the wastewater flows into the bioreactor’s anoxic zone for denitrification. From here, the mixed liquor enters the aerobic zone. The plant is equipped with four process trains, three of which consistently treat municipal sewage, while the fourth treats either municipal or industrial wastewater. Expansion space is provided within each train, and if necessary, membrane cassettes can be added to increase the plant’s capacity by 1.25 MGD, for a total of 3.75 MGD.
ZeeWeed reinforced, hollow-fiber membranes are immersed directly into the aerobic zone of the bioreactor, and draw treated effluent into the fiber using a gentle suction. Cyclic aeration is used to scour the surface of the membranes and prevent solids from accumulating. The membrane acts as a physical barrier, preventing suspended solids and pathogens from entering into the final effluent. Because sludge settling is not required in this process, clarifiers can be eliminated and the MBR plant is significantly smaller than a comparable conventional facility.
The ZeeWeed MBR consistently produces high quality effluent with BOD less than 2 mg/l, TSS less than 1 mg/l, turbidity less than 0.5 NTU, phosphorus less than 6 mg/l, nitrogen less than 2 mg/l and coliforms of less than 2 mpn index/100 ml.
Following the successful implementation of membrane filtration at the WWTP, water managers began considering similar technology to expand the city’s drinking water treatment plant (DWTP). American Canyon’s population had grown by nearly 1?3 from 2001 to 2004 and demand for potable water from residents and businesses was far exceeding the conventional plant’s 2.5 MGD capacity. During peak seasons as much as 2.5 MGD of additional water was being purchased from Napa County.
City councilors and water managers once again initiated an evaluation process to determine the best available membrane technology to increase drinking water production. ZENON ZeeWeed membranes were again selected for their performance, compact plant size and technological advantages. Moreover, the city acknowledged that operational efficiencies in training, labor and maintenance could be gained by incorporating a ZeeWeed system in both of its municipal plants.
Scheduled for commissioning in the fall of 2004, the 3 MGD ZeeWeed plant will draw water from the North Bay Aqueduct, which has an average turbidity of 40 NTU, although can experience spikes that exceed 200 NTU. Average TOC is 5 mg/l, manganese 0.70 mg/l, iron 1.6 mg/l, and color is 130 PCU.
A 1 mm screen will provide pretreatment before alum is added and the water enters the flocculation tanks of the two process trains. From these tanks, the water will flow by gravity into the membrane tanks. A slight suction applied to the ZeeWeed membranes will draw permeate into the membrane where it will be pumped to a clearwell. The plant has been designed to produce treated water with a turbidity of less than 0.1 NTU, greater than 4 log removal of Giardia and Cryptosporidium, and greater than 2 log removal of viruses.
Two additional process trains are available to expand the plant to a total capacity of 6 MGD. American Canyon’s existing conventional plant will continue to operate along with the new DWTP, bringing the city’s total potable water production to 5.5 MGD, with future capacity for 8.0 MGD.
Recycled water preserves potable water supplies
But, expansion of the DWTP may be a long way off. Once American Canyon’s water recycling program is fully operational, the city predicts that it will save about 1.85 MGD of potable water by using recycled water for non-potable applications such as irrigation and industrial process water. With new, sustainable sources of water, the city can positively address the increased demand that growing populations and industry will place on the city’s water resources.
Since the WWTP was commissioned over two years ago, American Canyon has become a destination for many wastewater engineers and municipal planners that are seeking solutions to their own current and future water management strategies. The facility currently hosts four to five tour groups per month that come from throughout the U.S. and the world. Many more will no doubt come to learn how ZENON membrane technology is not only treating wastewater and drinking water, but also how membranes will enable American Canyon to achieve sustainable and environmentally responsible water supplies well into the future.
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