117) In a BOD test, 1.0 ml of raw sewage was diluted to 100 ml and the dissolved oxygen concentration of diluted sample at the beginning was 6 ppm and it was 4 ppm at the end of 5 day incubation at 20°C.The BOD of raw sewage will be

118) The minimum dissolved oxygen which should always be present in water in order to save the aquatic life is

119) The relative stability of a sewage sample, whose dissolved oxygen is same as the total oxygen required to satisfy BOD, is

120) Dissolved oxygen in streams is

121) Facultative bacteria are able to work in