Effectiveness of ultra-violet radiation units in the bactericidal treatment of seawater

Abstract

Four commercially available ultra-violet (u.v.) radiation units (A, B, C, D) and a specially designed u.v. radiation unit (Kelly-Purdy) were tested for their effectiveness in the bactericidal treatment of seawater to be used in controlled shellfish purification systems. The bactericidal effectiveness of a unit was based on its absolute ability to produce a seawater equivalent to coliform bacterial standards for potable water and its relative ability to reduce the bacterial content of the seawater, as measured by reduction in fecal coliform bacteria and bacteria enumerated by the 35°C and 20°C plate count methods. Units A, B, C and D were the immersed lamp type in which the water to be treated was in direct contact with u.v. radiation lamps; A and C were equipped with “hot” cathode, high intensity lamps (135 μW cm −2 at 1 m) and B and D with “cold” cathode, low intensity lamps (73 μW cm −2 at 1 m). The Kelly-Purdy unit, built specifically for seawater treatment, was equipped with u.v. lamps of intermediate intensity (85 μW cm −2 at 1 m) suspended above the water. Results of the study demonstrated that although units A, B, C and D could be used to treat seawater, all four were affected adversely by turbidity. Units A and C were least affected by turbidity and thus most readily adaptable for seawater treatment. Under most turbidity conditions units B and D would require an efficient filtration system. Tests indicated that the four commercial units would require installation of flow regulation devices, and changes in the lamp compartment to permit rapid disassembly for lamp cleansing and metering of u.v. radiation intensity. The Kelly-Purdy unit produced a bacterially acceptable seawater under most turbidity conditions, but was affected adversely by the higher turbidities (> 80 Jackson Turbidity Units), indicating the need for a filtration system or the establishment of precise flow rate curves for turbidities above the critical level.