The Effect of Liquid Handwashes on Freshwater Alga; Chlorella vulgaris

Abstract

There is much concern about the hazardous effects of hand washing products on the environment due to their increased use during the Covid-19 pandemic. Even though these products have been released into streams for several years, their fate and behaviour are not well known yet. This study aimed to evaluate the impact of four regularly used handwashing liquids in the Sri Lankan market on freshwater algae. Chlorella vulgaris, the test organism, was grown in the OECD medium amended with four selected concentrations (0.50, 1.00, 1.50, and 2.00 mL/L) of four liquid handwashes, and a control sample was maintained without adding handwash. An inoculum of Chlorella vulgaris was added to each test concentration to obtain 105 CFU/mL initial algal count. The growth of Chlorella vulgaris was detected using the optical density measurements at 430 nm in quintuplicate. The procedure was repeated at 24-hour intervals for one week and at 72-hour intervals until 21 days. The relative growth of test samples based on optical density was calculated as a percentage in comparison to the untreated control, and the effect of each hand wash on algae was determined in terms of percentage survival. According to the graphical interpretation of the results, survival percentages drop as handwash concentration rises, but surprisingly, they rise at 1.50 ml/L in three products. The repeated measures ANOVA technique is used to analyse the effect of handwash concentration and the interactive effect of concentration and time on algal growth. According to the statistics, there are no significant differences in the growth when increasing the concentration of three products (p < 0.05). However, growth has increased when increasing the concentration only in one product, and the growth has increased with time (p < 0.05) in all products. It can be concluded that liquid handwashes affect the growth of freshwater algae in either a positive or negative manner. This may be due to the collective effect of constituents of the handwashes tested.