Ozonation of The Hot Springs Swimming Pool in Glenwood Springs, CO

by Pool Builders on 01-31-2006 in Articles

In 1991, the Hot Springs Swimming Pool and Lodge Resort installed a DIN-Standard ozonation system for treating the world's largest hot springs-fed swimming pool. Their were many reasons for this decision:

1. There was only one remaining supplier of gaseous chlorine available to the pool's management, as it had become increasingly difficult to find a supplier willing to transport gaseous chlorine through the dangerous passes of the Rocky Mountains to reach the mountain-town of Glenwood Springs.

2. There was uncertainty whether or not the license for chlorine usage would be renewed.

3. There was concern with the storage of chlorine gas in close proximity to the swimming pools. On any given day, there could be as many as four one-ton chlorine gas cylinders near the pool.

4. There was uncertainty about the price of future chlorine supplies.

5. Despite meeting Colorado Health Department regulations of maintaining a 0.4 mg/l chlorine residual in the water, water quality parameters of turbidity, color (green), and overall water quality at times left much to be desired. There were frequent reports of eye and skin irritations, and isolated reports of infections.

6. There were also reports of significant algae growths which forced unscheduled pool closings, and added additional operating expenses.

7. There was incentive to abandon chlorine treatment because of reduced insurance rates associated with ozone treatment. The Lodge's liability insurance costs have decreased dramatically since the ozonation process was installed.

Water Quality Parameters

The mountain resort is unique in that the water fed to the two open-air pools is from a fresh, natural mountain hot spring. These are outdoor pools, open the year round. Because the water source is from underground hot springs, considerable iron, manganese, algae spores, dissolved organics, and hydrogen sulfide contaminate the raw waters. All of these contaminants are oxidized by ozone treatment installed only in the larger of the two pools. The smaller (therapy) pool employs untreated spring water.

From a hygienic standpoint, the ozone treatment system on the large pool is designed to meet the following criteria:

o Bacterial disinfection

o Algae control

o Viral inactivation

o Compliance with Colorado State health codes

The ozonation system is designed to meet a minimum CT value of 1.6 mg/l-min to guarantee bacterial disinfection and viral inactivation. This will guarantee inactivation many more than 3-logs of Giardia cyst inactivation (if any are present) and many more than 4-logs inactivation of enteric viruses, according to the U.S. EPA requirements for ozone primary disinfection of drinking waters at 25oC.

Pool Dimensions and Hydraulics

The total volume of the swimming pool is 1,072,300 gallons, occupying a total area of 30,195 ft2. This outdoor pool is 405 ft in length and 100 ft at its widest point. The hot springs water flows naturally at the rate of 3.5 mgd. The average pool water temperature is regulated (by dilution with City tap water) at 90oF (32oC) in winter and 86-89oF in summer. Temperature of the springs water is 124-130oF. The Hot Springs Lodge & Pool at Glenwood Springs is situated at 6,000 ft above sea level. The average number of people visiting the pool throughout the year is between 1,000 and 1,300 per day, which sometimes rises as high as 3,000 people each day during the holiday seasons.

Pool turnover is designed for four hours with a flow rate of 4,500 gal/min. Makeup water per day is ca 500,000 gal, supplied by the natural hot spring. Most of this water is not consumed or discharge to waste, however, but is believed to drain through cracks in the piping system beneath the pool (the ozonation system was retrofit and was not part of a new pool design).

Water Treatment Process

This is shown schematically in Figure 1. From the balance tank, water is treated with ca 1 mg/l of polyaluminum chloride (PACl) flocculant just prior to ozonation. Ozone dosage at maximum loading is 1,200 g/h, reacted over a minimum two minutes retention time in a 9,000-gallon reactor. Ozone is injected under vacuum using a bypass turbine pump, a venturi injector, and a static mixer contacting device.

Application of ozone is monitored and controlled by ORP readings taken at the reactor effluent. ORP also controls the pH (adjusted by manual addition of HCl). Output of the ozone generator can be adjusted between 10 and 100 percent capacity. The majority of the time, the reactor tank outlet has an ORP level of ca 850 mV, with ORP levels in the pool waters proper of ca 750 mV. The pH varies between 6.9 and 7.5.

After ozonation, the water is passed through a gravity filter having a total surface area of 864 ft2. The bottom part of the filter consists of sand and gravel; the upper part of granular activated carbon. The filter has four cells, one cell of which is backwashed manually each day.

Sodium hypochlorite solution is added to adjust the residual chlorine level to 0.4 mg/l.

System Performance

This system has now been in operation for 15 years. The quality of water produced has shown a striking change in color from green to dark blue; and even though the pool is situated outdoors, algae control has been excellent. This is attributed to the combination of ozone destruction of algae spores followed by maintenance of 0.4 mg/l of chlorine residual (ORP levels between 850 and 750 mV).

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