CASE STUDY: Santa Cruz meets THM/DBP challenge with unique air stripping process

Santa Cruz, California has successfully found a way for the City to meet its water quality goals while using process recycled water as part of its supply source. Using the recycled water had caused concern about reliably meeting the U.S. EPA THM/DBP potable water quality standards in its finished water. The City evaluated various aeration treatment processes and chose an E-Z Tray® Air Stripper from QED as the best fit for meeting their water quality and O&M goals. The E-Z Tray air stripper was installed and its performance has greatly surpassed the City's goals, including up to 90% THM removal.

Background
The City of Santa Cruz, California depends on surface waters for 95% of its municipal water supply; only 5% comes from groundwater. About 10% of the water stream going into the treatment process comes from recycled water. An engineering study revealed increasing levels of trihalomethanes (THMs) in the reclaimed water, sometimes up to 150-200 ppb. The presence of trihalomethanes is primarily a byproduct of drinking water disinfection. The increased THM levels in the recycled water were putting Santa Cruz in danger of exceeding the U.S. EPA's maximum contaminant level of 80 ppb in potable water, so the city began evaluating methods to reduce THMs before adding recycled water to the treatment stream.

Search for a Solution
Their first approach was a packed tower air stripper. Pilot tests showed that the packing was vulnerable to gumming up and clogging, necessitating laborious cleaning. Terry McKinney, Production Superintendent, City of Santa Cruz, says, "We didn't know how we would do maintenance on a tower other than taking all the packing out, which we did not want to do. So that's why the tower was eliminated."(Note: for scale deposits, often the tower is flooded with an acid solution, adding more complication and risk.) McKinney had an opportunity to observe an E-Z Tray® Air Stripper (U.S. Patent Number 5,518,668) from QED in use at Ford Ord near Monterey. They gave us a quick rundown on how to pop the trays out. We went with the smaller tray design, the split trays, which is wonderful. They are very easy to clean, and we really haven't had to clean them that much. The air agitation in the aerator keeps the particles in the recycled water from settling."

Comparison Of Alternatives
Santa Cruz considered other tray-type strippers, but decided on the E-Z Tray® stripper. "We liked the QED air stripper because of the stainless steel construction, the removable trays, really simple, very straightforward“ McKinney says. “The entire QED unit is NSF approved, where all the other units hadn't received NSF approval."

Plant data for THM Removal in the City of Santa Cruz (CA) drinking water treatment system. Results show consistent THM reductions of 80-96%.

Performance And Cost Factors
Capital cost was also a factor in the choice, but one which proved to be no problem, given the wide range of E-Z Tray models and price levels. McKinney says, “We normally get 90% removal. We haven't done a lot of testing on it, but one flow rate was about 300 gallons per minute, 350-ish, and we got 90% removal removal with this. When we cranked it up to 500 gallons per minute, it went down to about 80% removal. You have the flexibility of looking at stripper size in relation to flow rates, and you can go over the flow rate if you can accept the lower percent removal, and 80% is still fine."

Installation & Customer Service
Asked about customer support from the QED representatives, McKinney replies, "It was great. We put the purchase order through and then worked with one of the design engineers. When the unit arrived, setup was simple. "It was very easy. I mean it's all pretty much self-contained. We did the piping on our end so there were modifications to our clarifier system. Essentially we had inlet valves and outlet valves on it and everything was hooked up. Dialing in the air and getting the air to water ratio where we wanted it was easy. Customer service was exceptional."

Low Maintenance Is Another Key
In addition to ease of cleaning, visual monitoring glass doors was an attraction. McKinney says, "You can certainly see an equal air flow through the unit.” The one time they pulled the trays, after3 months, he reports: "There was very little sign of organics or anything on them. There was no scaling whatsoever. There was a little slime, typical biofilm you would get in a pipe and it was not detrimental at all. It was just on there; you could feel it, it was slippery, but all the holes were perfectly fine and open."