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To: Dave Heckman - Construction Engineer
Thru: Rich Barrows - QA Engineer
From: Alan Woodmansey - Highway Engineer (PDP)
Subject: Chitosan Treatment of Sediment Laden Water - Washington State I-90 Issaquah Project
This report presents the results of a project site product review made by WFLHD members on February 26, 2002. The purpose of the site visit was to determine if Chitosan turbidity reduction methods currently in use are adaptable and adequate to meet WFLHD's current and/or future project needs for improving construction site runoff water.
The I-90 Sunset Interchange project is being constructed 30 km east of Seattle at Issaquah. The project has heavy sediment load in the runoff/drainage water due to construction impacts on the valley hillside. Typical runoff water has a turbidity of 150 Nuephelometric Turbidity Units (NTU), but treated water has turbidity levels around 1-NTU.
This picture is looking west with I-90 on the left. The sediment-laden water is shown here just before it is piped to the settling ponds. The settling ponds are about 1km downstream at the bend in the road straight ahead. The treated water is returned to the ditch about 300m ahead near the tall evergreen tree.
Project Water Processing
Sediment laden water flowing from the construction site is directed to one of two settling/holding ponds. This project has a 300,000-gallon pond and a 700,000-gallon pond. The ponds have concrete bottoms with bentonite sides. These ponds are part of the project design and in the future will be used to treat roadway storm water runoff. From the ponds, water is pumped using a submerged 35-hp pump. As soon as the pumped water enters the pipe, it has Chitosan (Liqui-Floc) added. The Chitosan causes the fine sediment particles to bind together and is subsequently removed with the sediment during sand filtration. Chitosan also removes phosphorous, heavy minerals, and oils from the water. Chitosan is a non-toxic, biodegradable substance made from shellfish; it is commonly used by aquariums to clean water and is safe for fish. The water is pumped 16 m from the pond into four 54" diameter silica sand filtration pods under 35 psi pressure. The pods can filter an average of 475 gpm, which includes a 2-minute backwash cleaning cycle every 10 minutes. The backwash water and sediment is sent back to the settling pond. The filtered water is pumped uphill to near where it was taken from the ditch and then allowed to infiltrate into the ground.
The 300,000-gallon settling pond. The intake pump is below the two floating barrels in the middle. The filtration pods are up the embankment to the left.
WFLHD observers were Rich Barrows, Janice Halvorsen, Gary Wrightman, Steve Zaske, and Alan Woodmansey.
WSDOT Engineer was Savina Uzunow (email@example.com).
Baker Tanks Representatives were Bill Benner (Sales Rep), Randy Preston (Operations Manager), & Tim Ferris (Primary Contact and Regional Manager), phone 1-800-225-3712.
Natural Site Solutions Company- John MacPherson (Senior Chemist) Chitosan technical expert (206)730-5370, http://www.naturalsitesolutions.com/
Other Information & Concerns
This project is not using any Gel-floc because Liqui-floc is better suited for this filtration process. We did not see any passive contact use of Gel-floc, although this is another viable flocculation technique when placed in-stream. One Gel-floc fabric sock can treat up to 500,000 gallons of storm water. Dose rates for Liqui-floc are 0.5 mg/L or about one gallon of Chitosan per 20,000 gallons of water.
No pH adjustments are necessary for removing sediment on this project since the water pH of 6-8 is within the range needed for the Liqui-floc to work. Liqui-floc becomes very ineffective outside of this pH range. Neither the Chitosan nor the filtration process changes the pH level. However, if influent pH levels were adjusted to allow treatment, effluent pH levels are a concern due to fish sensitivity. The Baker Tank representatives said that if needed, they could easily and inexpensively increase the influent pH using a mixing vault with sodium bicarbonate. The effluent could then be decreased with an in-line additive prior to returning to surface waters.
Chitosan is an important additive in the filtration process. Sand filtration apparently can remove up to 50% of the turbidity alone while the Chitosan with sand filtration removes up to 99% turbidity.
Sediment removed from the settling pond can have the effect of a "green burst" when spread out on the ground. High concentrations of nitrogen in the Chitosan act as fertilizer and promote significant vegetative growth in places where it is spread.
This project is limited on the amount of water it can filter because Washington State requires infiltrating all of the project water and not discharging it to any surface waters. The reason given by a state engineer for this was because the water permit was already done for infiltration and getting it changed could be a long and difficult process. The entire system is not operated during rain showers because the ground is not capable of infiltrating additional water during those times.
Chitosan in a filtration process is a process that Baker Tanks has been improving over the past two years. It could be considered experimental because of its relative newness for this application, and it is not yet considered a best management practice. However, Baker Tanks has used it on about 24 projects. They continue to improve their processes and equipment.
Baker Tanks has expanded its filtration line to include sand filters (ranging from 74 gpm to 1,000 gpm) and portable cartridge trailers capable of accepting wound string particulate filters and carbon canisters. The cartridge filter trailers are useful in low gpm applications. Baker is currently involved in another project utilizing real time data with turbidity, temperature, and pH monitoring sensors. This technology provides vital statistics on a 24-hour basis.
An empty sand filtration pod weighs 1750 kg (8000 lbs), with sand 4630 kg (21000 lbs), and with water 6400 kg (29000 lbs). Four pods in the shipping container are a permitted hauling load on highways.
The following project filtering costs were provided by the WSDOT engineer:
Initial set up and mobilization - one time cost: $4,950
Monthly rates for renting:
Sand Filter system: $3,465
Approximate highest renting monthly cost = $12,031