ADDITION OF UV DISINFECTION FOR KAILUA REGIONAL WASTEWATER TREATMENT PLANT, HAWAII

The Kailua regional wastewater treatment plant near Honolulu, Hawaii, treats flows of 56,818 m3/d (15 mgd) and discharges to Kailua Bay. Concern over the discharge of undisinfected effluent to a bay where humans have direct contact with the water led to design of a disinfection facility.The city and county of Honolulu, Hawaii, chose ultraviolet disinfection, a less commonly used technology for wastewater treatment. Because few operating UV systems for wastewater disinfection exist in Hawaii, a pilot study was conducted to characterize treated wastewater from the Kailua plant and determine if UV disinfection would be effective.The pilot study indicated that UV disinfection would meet the effluent criteria for Enterococci. Several types and configurations of UV systems were evaluated. Based on technical and cost evaluations, the engineers designed a low-pressure system with a vertical lamp configuration.Performance of the UV system indicates that it is effective on a secondary effluent with as much as 20 mg/L suspended solids. However, experience has also shown that oil and grease can interfere with the disinfection process. At Kailua Regional WWTP, therefore, operators have learned that continued vigilance and daily equipment inspections are essential to maintaining performance of the UV disinfection system.