The water quality of sewage-fed Varthur Lake, Bangalore, India has been measured at five different locations. A BOD removal of 70% (filterable) was achieved when the lake functioned as an anaerobic-aerobic lagoon for 6 months at an estimated residence time of 5 days. During this period, the biota of the lake, especially primary producers (phytoplankton, algae), treated the water to nearly standard water quality levels. The growth and spread of macrophytes (water hyacinth) renders the lake anaerobic and reduces its capacity to treat the water. Keeping an open surface, and permitting microalgal growth provides a high level of water treatment, and may be used in a larger number of small towns to enable local reuse of water.
We are grateful to the Ministry of Environment and Forests, Government of India and the Indian Institute of Science for financial and infrastructural support. We thank Mr. Khan M A, Principal, KK English School, Varthur and Ms. Ali Rani, Biology Teacher, KK English School for useful suggestions during discussions, and Ms. Hema, Ms. Aafia, Mr. Umraaz, Ms. Nagendra, Mr. Ismail, Mr. Harshit – students of K K High School, Mr. Arun D T, Mr. Varun, Ms. Indu Mathi, Ms. Sanyo Sabu, Ms. Mohanambal for active participation during the field experiments.