Rainwater is a potentially useful source of water for urban households. However, it may contain significant bacterial contamination that needs to be treated to make the water safe to drink.
In particular, the bacteria Pseudomonas aeruginosa (P. aeruginosa) has been implicated in waterborne and food-borne diseases, and is now considered a primary infectious agent.
P. aeruginosa has been detected in stored rainwater by many researchers. Finding a way to inactivate P. aeruginosa in rainwater could be an important step in improving water availability.
One potential way to disinfect rainwater is solar disinfection (SODIS). By exposing rainwater to sunlight, the amount of P. aeruginosa can be greatly reduced. The portion of solar radiation which plays a major role in generating the antimicrobial activity is believed to be UV-A, 315 – 400 nm, and to a lesser extent visible violet and blue light in the range of 400 – 490 nm.
In addition, the increase in water temperature through solar heating is known to work with solar radiation to further enhance the removal of P. aeruginosa. The disinfection efficiency of three different designs was studied in a variety of conditions.
In the first study, the survival rate of P. aeruginosa was evaluated in 4 different plastic water bottles. Water Bottle 1 was left totally transparent with no modification. Bottle 2 had a reflective metal applied to the back half of the bottle, while Bottle 3 had a black absorptive coating applied to the back half. A fourth bottle was kept in the dark to function as a control.
In the second study, the water temperature of three bottles was measured during the course of a sunny day. As in Study 1, there were different backings on the plastic bottles.
In a third study, the effect of water turbidity on the survival rate of P. aeruginosa was measured. Turbidity is a measure of the cloudiness of the water, which is usually due to dust and other small particles. Samples in bottles were placed in the sunlight with 3 different levels of turbidity.
Tables and figures adapted from Amin MT, Nawaz M, Amin MN, Han M (2014) Solar Disinfection of Pseudomonas aeruginosa in Harvested Rainwater: A Step towards Potability of Rainwater. PLoS ONE 9(3): e90743. doi:10.1371/journal.pone.0090743
Which of the following would be the BEST explanation for the results in Study 3?