Which substances present in water are consumed by microorganisms,including bacteria?

$BOD$ refers to

Primary treatment of sewage is the:

Where are methanogens found in a sewage treatment plant?

Sewage or municipal waste should not be directly passed into rivers,streams and other water bodies because
$I.$ it contains human excreta and other organic waste
$II.$ it contains a number of pathogenic microbes
Which of the statement given above is/are correct?

Secondary sewage treatment is primarily a ......