Using light emitting diodes at 450 nm for in vitro treatment of water intended for human consumption

This study aimed to evaluate the in vitro antimicrobial effect of exposing raw water intended for human consumption to light (λ= 450 nm) and to investigate the correlation between the results obtained and physical and chemical parameters. Fifteen (15) samples of raw water were collected from households in a rural area of ​​Santo Antônio de Jesus – Bahia (Brazil), from November to December 2016. A 100 mL aliquot of each sample was exposed to a lighting system consisting of two high intensity light emitting diodes, with a wavelength of 450 nm and luminous flux of 200 lumens per 10 h. Quantifications of heterotrophic bacteria, total coliforms and temperature started at time zero and were done every two hours until the end of exposure to light. Bacteriological analysis was repeated after 72 h of being exposed to light. pH, dissolved oxygen and salinity analyses were performed before each experiment. After a 10h illumination at 450 nm light emitting diodes (LEDs), the dosage of light received by the water samples was 581.8 J/cm2. There was a significant reduction in the two bacteriological parameters analyzed after treatment (p = 0.000). There was an average decrease in heterotrophic bacteria counts from 3.44 to 1.86 log CFU/mL and total coliforms from 2.45 to 1.02 log CFU/mL. Mean reductions of heterotrophic bacteria were 97.01% and total coliforms were 95.61%. After 72 h, both counts increased; there was significant growth between heterotrophic bacteria (p = 0.000), but there was no significant growth for total coliforms (p = 0.058). pH (p = -0.981, p = 0.000), dissolved oxygen (ρ = -0.529, p = 0.043) and temperature (ρ = 0.521, p = 0.047) were related to the percentage reduction of heterotrophic bacteria. The method is shown to be effective in disinfecting raw water in vitro under different physical and chemical conditions.