Which process is used to remove physical particulate matter from sewage?

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?

In anaerobic sludge digesters,what do anaerobic bacteria digest?

What gases are produced in anaerobic sludge digesters?

The measurement of $BOD$ (Biochemical Oxygen Demand) in wastewater provides information about the amount of which of the following?

$BOD$ is concerned with