Ozone Water Treatment, Color Removal and Ozone Purification
In 1991, the Hot Springs Pool and Lodge Resort installed a DIN-Standard ozonation system for treating the world’s largest hot springs-fed swimming pool. The following main reasons were responsible for this decision:
- The suppliers of gaseous chlorine had dwindled down to a single source, as it had become increasingly difficult to find a supplier willing to transport gaseous chlorine through the treacherous mountain passes and tunnels to reach this remote facility.
- There was uncertainty whether the license for continued chlorine usage would be renewed.
- There was deep concern on the part of lodge management 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 on the site.
- There was uncertainty as to the price structure for future chlorine supplies.
- There was great incentive to abandon chlorine treatment because of reduced insurance rates associated with ozone treatment. Indeed, liability insurance costs have decreased dramatically since the ozonation process was installed.
- 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.
- There were also reports of significant algae growths which forced unscheduled pool closings, and added additional operating expenses to the pool.
Pertinent Water Quality Parameters
This mountain resort is unique in that the water fed to the two open-air pools is from a mountain hot spring. Because of this, the pools 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:
- Bacterial disinfection
- Viral inactivation
- Algae control
- Compliance with Colorado State health codes, which require a minimum ORP of 650 mV in the pool
To guarantee bacterial disinfection and viral inactivation, the ozonation system is designed to meet a minimum CT value of 1.6 mg/l-min. which 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 25°C.
Pool Dimensions and Hydraulics
The total volume of this outdoor pool is 1,072,300 gallons, occupying a total area of 30,195 ft². The 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 90°F (32°C) in winter and 86°-89°F in summer. Temperature of the springs water is 124°-130°F. 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 during holidays.
Pool turnover is designed for four hours with a flow rate of 4,500 gal/min. Makeup water per day is circa 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).
The Water Treatment Process
This is shown schematically in Figure 1. From the balance tank, water is treated with circa 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.
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).