Santa Cruz Meets THM/DBP Challenge with Unique Air Stripping Process - Case Study
Santa Cruz has successfully demonstrated an innovative 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 EZ Tray air stripper was installed and its performance has greatly surpassed the City’s goals, including up to 90% THM removal.
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 raw 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 parts per billion (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. During pilot testing, it became apparent that the packing was vulnerable to gumming up and clogging, which would necessitate 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 early on.' (Note: if the problem is scale deposits, often the tower is flooded with an acid solution, which adds another whole category of complication and risk.)
McKinney and other Water Department personnel had an opportunity to observe an E-Z Tray® Air Stripper (U.S. Patent Number 5,518,668) from QED Environmental Systems in use at Ford Ord near Monterey. McKinney says, 'Fort Ord is a decommissioned army base where they are doing groundwater remediation. 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 after looking at several brands, decided on the E-Z Tray® stripper because of its stainless steel construction and simple, straightforward tray removal. 'I don’t remember their names off the top of my head,' McKinney says. 'Some of them were made out of fiberglass. We liked the QED air stripper because of the stainless steel construction, the removable trays, really simple, very straightforward. The other units, you had to open a giant door unfastening 20 bolts or 30 bolts. It just seemed very cumbersome.
He adds, “The entire QED unit is NSF approved, where all the other units hadn’t received NSF approval but they were going to try to get a waiver for it based on materials, and that just is a longer process.'
Performance Factors Make The Difference
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, 'Budget-wise, I had to get approval for it. We have two clarifiers, and originally the idea was bring in one air stripper and see if it was going to be effective or not. We can run 700 gallons per minute through both clarifiers, that’s about 350 gallons per minute each, and originally we were going to go with two 500 gallon per minute (air stripper) units so we had room for future expansion.
“That went over our formal bid limit and that would have made us go into a full-fledged RFP process, which we didn’t want. So we downsized it to a 350 gallon per minute unit for our test unit, and we are running that, and it’s actually meeting our needs, with the idea that if we needed a second one, we could just stick it right next to the first one and then use that.'
The small footprint of the various E-Z Tray® Air Stripper models played a role in the buying decision. According to McKinney, 'Because (QED) had multiple sizes and shapes that we could pick from, it made it easier to fit it specifically right next to the clarifiers, so it was a very significant factor.'
'It was a matter of trying to get the right size unit, the flow rate that we wanted, because we have three different pumps that feed the clarifier… small, medium and large. The medium puts out 400 gallons
per minute, just a bit larger than the stripper’s rated flow, so the flow rate I think is 350 or 360.
“We sometimes run our medium pump hydraulically over – we go above the hydraulic limit on it, and that was part of the decision-making process. The large pump puts out 500 gallons per minute, and we made sure that the unit can actually handle 500 gallons per minute in terms of the hydraulic loading, and then we just understood that if we ran it at that rate, the percent removal would be effectively lowered.
'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 with this. When we cranked it up to 500 gallons per minute, it went down to about 80% removal. That’s where you have the flexibility of looking at what size you have 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.'
Low Maintenance Is Another Key
In addition to ease of cleaning, visual monitoring through glass doors was an attraction, although in practice darkness at the back of the trays makes it hard to see whether the air holes are being clogged. However, McKinney says, 'You can certainly see an equal air flow through the unit.'
The one time they pulled the trays, after 3 months, McKinney reports, 'There was very little sign of organics or anything on them. There was no scaling whatsoever. There was a little slime; it’s typical biofilm that 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.'
Because the air stripper is sited outdoors in the California sunshine, algae growth was somewhat of an issue, but this is being eliminated by putting a tarp over the front of the unit to keep out the light, and lifting it to observe the stripper operation when required.
When asked about the importance of operation and maintenance (O&M) costs in the decision, McKinney replies, 'Well, there is very little O&M cost on the unit, really just electricity to run the blower, and since we have solar energy at our treatment plant it’s hard to measure what impact that blower system has. So I would say it’s pretty minimal.'
QED Online Modeler
The QED online process modeler was useful in predicting performance for THM removal. Santa Cruz actually got better removal than predicted, especially for brominated species, which were totally eliminated. According to McKinney, 'We were expecting all the chloroform to be evaporated off and we would be left with the brominated species, and it actually is performing the exact opposite. All the brominated species are completely gone and what’s left over is just the tiny amount of the chloroforms.
'So we have 100% removal on all the brominated species, and probably 85% removal on the chloroform, and it averages out to be about 90% removal… I think we got better removal than what the model predicted.'
Installation & Customer Service
Asked about QED customer support, from the factory and local representatives, McKinney replies, 'It was great. We put the purchase order through and then worked with one of the design engineers. I had to get all the designs approved by our engineering department and that was pretty straightforward.
When the unit arrived, equipment setup was simple and straightforward. '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.'
Where does it go from here? McKinney answers, 'I am thinking that down the road one of the things we are looking at with the air stripper, we are very comfortable putting it on the raw water side of our system because if there was anything that was sucked into the blower and got into the water supply, or if we stripped out too much chlorine or anything like that, it would all be caught in the rest of the treatment process.
'But there is definitely an opportunity to potentially use this on a potable water tank. There would need to be a couple design features with the air vent, to prevent contamination.