The use of an open channel, low pressure UV reactor for water treatment in low head recirculating aquaculture systems (LH-RAS)

Abstract

This study examined the effectiveness of an open channel, low pressure (LP), ultraviolet (UV) reactor for water treatment in a low head (LH) recirculating aquaculture system (RAS). Currently available UV reactors use high pressure pumps and submerged bulbs. The reactor in this study makes use of the head gained by the water recirculation of the LH-RAS without the need for additional pumps, while the bulbs are located above water surface to ease maintenance and lower costs. The UV reactor's output was characterized and evaluated by measuring the fluence rate distribution at various X, Y locations and average UV fluence rate delivered to the water surface using spherical chemical actinometry. The average UV fluence rate measured in dry conditions at the surface of the test water was 2.3 mW cm −2. Measurements of the exponential decay of irradiance in the water with the increase in water depth corresponded well with transmittance results, and were used to estimate the reactor's UV dose at different water flow rates. Using collimated beam apparatus (CBA), the inactivation of heterotrophic microorganisms in the RAS as a function of UV fluence showed that survival of bacteria dropped significantly as the UV dose increased and then stabilized at higher UV doses. Log survival at the UV reactor installed in the RAS was used to back calculate the average UV dose, which resulted to be of ∼2 mJ cm −2 based of the CBA data, and ∼4 mJ cm −2 based on dose equations at water flow rates of 100 m 3 h −1. Only a certain amount of sensitive bacteria can be inactivated by fluence rates delivered at UV doses of 2–4 mJ cm −2, thus to increase the delivered UV dose, it is suggested to increase the power of the UV bulbs, and to use this system in water with low transmittance and gravitational flow as in LH-RAS, for the benefit of controlling pathogenic bacteria proliferation.