Environmental Science & Engineering - www.esemag.com - November 2004
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Tanks provide innovative filtration system to protect downstream habitats


The Napa River, one of California’s largest Central Coast Range rivers, drains 426 square miles on its 50-mile journey from Mt. St. Helens to the San Pablo Bay. An estuary system comprises the last 17 miles of its journey.

Over the years, stream bank erosion and plant life removal have created significant river sedimentation. In fact, 70,000 to 200,000 tons of sediments enter the Napa Valley watershed every year due to development, roads and hillside vineyards. Unfortunately, the sediment deposits into stream channels and ponds which impair fish spawning and reduce habitat diversity and the food supply for fish. In an effort to protect the river’s ecosystem, the state has identified the Napa River as a 303(d) body of water in the Clean Water Act. Section 303(d) identifies bodies of water whose effluent limitations are not sufficient to meet state and federal water quality standards.

In April 2003, when work began on the US$31.5 million Maxwell Bridge replacement project over the Napa River, maintaining the river water quality was key. The state’s Regional Water Quality Control Board requires that water discharge not exceed 10 percent in areas where natural turbidity is greater than 50 NTU.

“By August, we had six, 48-foot deep cofferdams in the middle of the Napa River that we had to dewater,” said Shane Dees of C.C. Meyer, the project contractor.

Using four 4-foot wide, 65-footlong interlocking steel plates, C.C. Meyer created a series of temporary watertight enclosures that could be pumped dry to expose the river bottom. Only then could the contractor build footings, columns and drive piles 30 feet under the river to support the bridge. Once the footings and columns were completed, the cofferdams were backfilled up to the riverbed and pulled out of the river. “During excavation, we were stirring up a lot of mud in the cofferdam,” said Mr. Dees. “It was very difficult to pump 1,000 gallons of water a minute and try to keep it clean.”

At various times during the monthslong excavation and installation project, the cofferdams became so muddy that the water could not be cleaned without operating up to three filtration systems. “We didn’t have the money or the space for such a large set up,” said Mr. Dees. “With a lot of work, we were able to convince the state environmental agencies to allow us to pump from one cofferdam to the other, and wait for the sediment to settle.”

Creating a successful filtration solution
Two filtration systems were put in place – one on the east side of the river and one on the west side. The systems consisted of 15 Baker Style (US) 20,000-gallon settlement tanks, three 4-pod sand filters with Flygt submersible pumps, two Baker Roll-off boxes, carbon filters and berms to provide secondary containment.

“It was a pretty tight space. We needed to design the system to fit within the confined space and allow room to drive 130 foot piles into the river,” said John Coon of Baker Tanks. “In order to meet space requirements, we had to build platforms and lift the tanks in by crane rather than drive them onto the jobsite. As a result, the decreasing elevation of each tank allowed the water to cascade from tank to tank to maintain the necessary water flow.”

Baker T-style tanks were chosen due to their smaller footprint and ease of modification. “We were able to make different size valve alterations to the tanks to accommodate high and medium discharges allowing for settling and skimming of cleaner water,” said Mr. Coon. “We customized each tank with two 10-inch pipelines to maintain the (US) 1,000-gallon-aminute flow, and manifolded the tanks so we could add equipment if needed.”

Testing the waters
To comply with the State of California’s Regional Water Quality Control Board’s water quality regulations, upstream and downstream readings had to be taken at various intervals. A turbidity meter was used to conduct the readings. The turbidity levels had to be brought down from 1,000 NTUs to the natural turbity of the river.

Given the high turbity levels, Napabased ProTech General Contracting Service was brought in to institute a multi-step suspended material clarification process. A chemical coagulant was added to the influent water and mixed to form a dense flocculant that settled out by gravity. The flocculant was then collected in a series of settlement tanks. This is a particularly efficient method to remove suspended material as it effectively removes more than 90% of particles greater than 25 microns.

To meet the specific needs of the bridge replacement project, ProTech also used an aluminum chlorhydrate coagulant, which is a high-molecularweight polymer specifically designed for water treatment systems. The coagulant was injected into the influent water at 50 ppm, using a metering pump that can maintain a system flow rate of 500 gpm running 24 hours per day. Because proper mixing of the coagulant and raw water was necessary, the coagulant was injected into the line at least 20 feet upstream from the filtration system. This method helped prevent violent agitation or high-speed mixing which can decrease coagulant activity. Gravity settled the Napa River sediment in the clarifier tanks. The water was routed through a sand filter for final polishing.

Careful planning ensures efficient filtering
To avoid delay and extra expense involved in filtering at high tide, Mr. Coon took it upon himself to consult the tide charts. “With careful planning, we were able to work on other areas of the job during high tide which allowed us to churn out close to one million gallons of water a day – a couple of times we were running 24/7,” he said.

For further information contact Baker Tanks at
Tel: (562) 430-6262 or email: AReimers@BAKERTANKS.com.

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