Unravelling the reason for seasonality of foaming in sewage-fed urban lakes

Abstract

Foaming surface waters are a global phenomenon but are understudied. Bellandur lake in India has gained international attention due to its foaming events, which occur seasonally after rainfall. This study investigates the seasonality of foaming and the sorption/desorption of surfactants onto sediment and suspended solids (SS). Results show that foaming lake sediment can contain as much as 3.4 g of anionic surfactant/ kg of dry sediment, and its concentration was proportional to the organic matter (OM) content and surface area of the sediment sample. This is the first study to demonstrate the sorption capacity of SS in wastewater, which was found to be 53.5 ± 4 mg surfactant/g SS. In contrast, only a maximum of 5.3 mg surfactant was sorbed/g of sediment. The lake model analysis revealed that sorption is a first-order process and that surfactant sorption on SS and sediment is reversible. SS was found to desorb ∼73 % of sorbed surfactant back to the bulk water, while sediment desorbed 33–61 % of sorbed surfactants proportional to their OM. Contrary to the common assumption, rain does not dilute the surfactant concentration but instead increases the foaming potential of lake water through desorption from SS.