The Sterling Water Treatment Plant team made a quick and capable adjustment to reverse osmosis for uranium removal.
Operators at the Sterling Water Treatment Plant went from on-site well water chlorination to a $30 million, 9.5 mgd reverse osmosis (RO) plant with deep injection wells for waste disposal. It has been an interesting experience.
“We were tasked with operating a very different facility than any of us were familiar with,” recalls David Beck, lead operator. “We had to learn new equipment and understand new processes well enough to not only operate, but to solve issues.”
The city began planning for the new plant in 2008, when the Colorado Department of Public Health and Environment issued an enforcement order to address naturally occurring high uranium in the drinking water.
The city hired Hatch Mott MacDonald (HMM) as project manager and design engineer for the facility, which became operational in November 2013. Deep injection Well No. 1 went online in October 2013, and Well No. 2 in March 2014. HMM received the 2015 Engineering Excellence Award for the project from the American Council of Engineering Companies (ACEC) of Colorado.
Today, the plant’s operators produce water that meets all standards and also tastes good — sweet success after many months of construction, training, commissioning and the ongoing challenges of running and maintaining a complex system. “To me, our greatest success is success,” says Beck. “Navigating through challenges while producing a product that exceeds regulatory standards is our greatest achievement.”
Before the new plant was built, the city’s water treatment system consisted of 12 wells and five chlorine injection sites. In the distribution system, pressure was maintained by two elevated storage tanks and two ground storage tanks. A few operators from the distribution and collection crew handled the water treatment.
In September 2008, the Department of Public Health issued an enforcement order to the city to ensure long-term compliance with maximum contaminant levels for uranium (0.030 mg/L) and total trihalomethanes (0.080 mg/L).
“At this time, Sterling’s water system had an annual running average of 0.044 mg/L uranium and 0.083 mg/L total trihalomethanes,” says Beck. The raw water also contained high levels of sulfate (438 mg/L), total dissolved solids (1,177 mg/L) and hardness (376 mg/L). Many local homes and businesses used ion-exchange softeners and under-the-sink reverse osmosis systems to treat their water.
The city hired HMM to do a cost-benefit study on a treatment plant. The consultants recommended RO technology and two EPA Class 1 deep injection wells to dispose of the uranium-contaminated brine reject water. The 7,000-foot wells send the brine below an impermeable rock layer.
Normally used in the oil and gas industry, deep injection wells allow the brine to seep into natural rock formations. “In Sterling’s case, there is a water reservoir within these formations that had to be proven of worse water quality than the brine being injected before permitting could take place,” says Beck. Injection well permitting took a year, and plant construction took 20 months. It took an additional 10 months to build a pumping house for Well No. 2.
After entering the plant, 83 percent of the raw water is filtered in three filter chambers (Parker Hannifin). The water is treated with antiscalant and sent to the RO process. The remaining 17 percent is filtered and sent to the blend process (Graver Technologies filtration system).
“The blend process is credited with 3.5-log removal and is used to mix with permeate water from our three RO trains (H2O Innovation),” says Beck. “Since the RO process filters out essential minerals, blending filtered water with RO-treated water ensures a balanced, noncorrosive product.”
The trains operate at 82 percent recovery. The permeate is sent to a 17,000-gallon tank that feeds a mixing chamber, where operators add the blend water, sodium hypochlorite and sodium hydroxide. From there, the water enters a 300,000-gallon baffled clearwell, where it achieves the required chemical contact time. The RO concentrate (0.2 to 1.2 mgd) is sent to a concentrate storage tank before being pumped to the injection wells.
The new plant took a year to reach compliance. “We were out of compliance until we had a full year of analysis below all MCLs,” says Beck. “So, although the plant was fully operational in November 2013, we had to operate for a full year under all MCLs before the enforcement order was lifted in December 2014. Today, the finished water quality is excellent, with 0.0049 mg/L uranium, 70 mg/L hardness, 77 mg/L sulfate, 280 mg/L TDS and 0.00243 mg/L TTHM.
System startup happened in stages. “Dave and I were the only ones involved during construction and initial startup, and we didn’t know much about the process,” says Jeff Reeves, utilities superintendent. “We had to learn everything very quickly.”
Says Beck, “We got to see each piece of equipment being installed, and we learned about it as it went online.” Reeves and Beck came on board just before construction began, and they represented the city for all decisions after that. Two more operators were hired later in the commissioning stage; neither had water treatment experience.
H20 Innovation trained the team on the RO and clean-in-place systems. Manufacturers’ representatives trained them on everything else: chemical pumps, RO feed systems and pumps, finished water motors and pumps, filter chambers, inline analyzers, process water feed system, SCADA and HVAC systems.
“As each piece of equipment came online, the FactoryTalk SCADA program (Rockwell Automation) was updated to incorporate the new equipment,” says Beck. “I sat with AmWest Controls so I could learn the new controls and validate the alarms.”
A few weeks after RO system startup, a calcium carbonate scaling problem in the concentrate removal system halted operations. Chemical dispersants used in the RO process are designed to keep minerals in solution.
“We quickly found out that these dispersants were not designed to overcome the time the brine was in the storage tank,” says Beck. “No one foresaw this issue because deep injection is not common for RO facilities. The only other RO facility in Colorado with a deep injection well does not have a brine storage tank.”
HMM suggested an acid injection system to lower the concentrate pH and help keep the dissolved minerals in solution. Hydro Construction helped the operators pull pumps and investigate the issue. What they saw was alarming. “There were basically rock formations covering the pump screens, pipes and pump impellers, and there was no way to scrape it off,” Beck says.
Plant operators came up with the idea to soak the pumps in CLR (Jelmar). “It’s an off-the-shelf product for removing calcium, lime and rust,” says Beck. “We went around and bought all the product the local stores carried.” They capped the end of a 6-foot-long 10-inch pipe and inserted the entire pump and pipe assembly into that piece of pipe, then filled it with CLR.
Within a few hours, the pump was good as new, so they did the same thing with the other three pumps.
Although they were back in business, the operators needed a long-term solution. “The acid injection system proposed by HMM was an option that would certainly work, but it had a couple of drawbacks,” says Beck. “At the feed rates required for hydrochloric acid, the operational costs were extremely high.
“The other acid available was sulfuric acid, which could cause potential issues with the deep wells. The brine is already near its saturation limit for sulfate, so using sulfuric acid could cause sulfate to come out of solution.” This was an issue, since sulfate fouling of a deep injection well is not reversible. Antiscalant provider King Lee Technologies supplied a product that worked immediately.
According to Beck, the operators’ greatest challenge is dealing with new issues that arise: “Our facility is still relatively new, so when an issue does come up, it is likely something we haven’t faced before. And there aren’t any references to an established solution. Each issue has to be approached objectively.”
For example, the operators noticed inline pH values slightly different from expected levels based on dosages and known water quality. “The answer isn’t necessarily that the dose needs to be changed or the probe is bad,” says Beck. “It would have to be approached with an open mind to many possibilities. One question to ask would be: Is this value correct? If so, you would approach it from a quality control position. If the value is wrong you would start looking at the probe.”
It turned out the problem wasn’t the probe. The team discovered that the probe yielded different values when testing the same water in a beaker versus inline. In the beaker, it read the correct value. “In our case, there was stray current in the water, which threw off the probe by a couple of tenths,” says Beck. “Operation is pretty straightforward when everything is running as you expect. But with some issues, you really have to be creative to find the cause.”
Plant operators face ever-changing raw water quality. “We have to stay on our toes and continuously monitor membrane performance,” says Beck. “Since the treatment technique before we built the new plant was limited to chlorine injection, these raw water changes were never monitored, and we had very little previous data.”
Although the wells are relatively close to each other, their water quality is different. Operators see subtle changes in each well year-round in conductivity, hardness, alkalinity and organics.
Another concern is deep injection well failure. “No one will give me a definite answer on how long they will last, since every injection well is different and subject to unknown conditions in the reservoir they inject into,” says Reeves.
Besides the analysis and reporting on the wells, the plant must perform a yearly pressure falloff test, giving insight on how the reservoir is responding to injection. Every five years, the plant performs a mechanical integrity test to prove that the wells are mechanically sound. They also perform a radioactive tracer test to see where the injection fluid flows, making sure none is passing the rock barrier above the reservoir.
“If the injection wells fail because they reached their capacity, we have four other sites where we could drill more,” Reeves says. “If a well fails because of scaling or biological fouling, we will hire a company to rehab it.”
Beck adds, “We monitor and record data on the wells as we run them. We can’t monitor everything going on in the injection zones, but by monitoring pressure and flow changes, we should be able to catch any fouling in time to schedule a remedy.”
Despite these unknowns, Beck is confident that the team will do its best to meet current and future challenges while producing safe, high-quality water.
Operators at the Sterling Water Treatment Plant do it all. They clean and calibrate instruments, change RO pretreatment/blend system filters, and perform routine pump and motor maintenance. They spend substantial time collecting and analyzing data so they can monitor the health of the RO membranes, plan ahead for maintenance and investigate issues.
“Most Class A facilities have different departments to handle operation, equipment and instrument maintenance, laboratory testing and groundswork,” says David Beck, lead operator. “We don’t have separate departments for these, so our operators need to be well rounded. We try to make the maintenance routine, and based on manufacturers’ recommendations, so we can stay on top of things before they become an issue.”
The team works well together. “We all play our part,” Beck says. “We’re a small community, and it’s hard to find employees with RO experience. Our operators had to learn on the job, and that took a lot of dedication.”
Jeff Reeves, utilities superintendent, adds, “Since we couldn’t find people with RO experience, we looked for those who were interested in water treatment and who had taken relevant courses like biology, and we trained them from the ground up.”
Reeves has been with the city for 10 years and holds Class B Water Treatment, Class C Wastewater Treatment, Class 4 Distribution, Class 4 Collections Systems, and Class D Industrial Waste certifications. Beck reports to Reeves and has been with the city for eight years. He holds Class A Water Treatment, Class 4 Distribution and Class 3 Collections Systems certifications. Reporting to him are operators:
- Nada Baker, Class A Water Treatment, Class 1 Distribution, Class 1 Collections Systems, Class D Industrial Waste
- Theresa Haydel, Class B Water Treatment, Class 1 Distribution
- Eli Krueger
- Justin Bartlett
New hires receive on-site training and offsite classroom instruction in water treatment. “We focus their training on what will help us out the most at the plant,” says Reeves.