Engineering Case History

UV ENGINEERING CASE HISTORY AND APPLICATIONS 11,1, Engineering Case History 

The City of Fort Benton, Montana, USA, obtains drinking water from the Missouri River. The then current filtration plant (20-30 yr old) was in need of upgrading. Rather than building a new filtration plant, the city built a new 0.088 mVs (2-MGD) treatment plant in 1987. Water is drawn through collectors installed 6-7.5 m (20-25 ft) below the riverbed, a system that allows the riverbed to naturally filter the raw water. Turbidities of water entering the treatment plant average 0.08 NTU. No Giardia cysts have been found in the water (55). 

The water is treated with UV radiation for primary disinfection, then chlorinated for secondary disinfection. An applied chlorine dosage of only about 1 mg/L is necessary. The entire water treatment system is housed in a 2.97 m (32 fU) building. The UV disinfection system consists of six irradiation chambers, two control cabinets with alarms, chart recorders, relays, hour-run meters, lamp and power on-lights, six thermostats, electrical door interlocks, mimic diagrams, and six UV intensity monitors measuring the total UV output. Each irradiation chamber contains one 2.5-kW mercury vapor, medium-pressure arc tube, generating UV radiation at 253.7 nm. 

The system is equipped with a telemetry control system and a fully automated backup system. Each bank of three irradiation chambers has two units on line at all times, with the third unit serving as a backup. In the event that the UV intensity decreases below acceptable limits (20,000 pW-s/cm ) in any of the chambers, the automatic butterfly valve will close, stopping flow through the chamber at that time, the automatic butterfly valve on the standby unit will open. The alarm system is also activated if UV intensity decreases below acceptable limits in any of the chambers. The UV alarm system is interfaced with the automatic dialer and alarm system. In 1987, total equipment costs for the six-unit UV irradiation system with butterfly valves was US 74,587. 

In accordance with the US Environmental Protection Agency, the CT Values for inactivation of viruses by UV radiation is independent of temperature, as shown in Table 4. 

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Damage will be confined to the bubble-collapse region, usually immediately downstream of the low-pressure zone. Components exposed to high velocity or turbulent flow, such as pump impellers and valves, are subject. The suction side of pumps (Case History 12.3) and the discharge side of regulating valves (Fig. 12.6 and Case History 12.4) are frequently affected. Tube ends, tube sheets, and shell outlets in heat exchanger equipment have been affected, as have cylinder liners in diesel engines (Case History 12.1). 

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