Consider the following statements as regards a Septic Tank:
1) The size required is large and uneconomical when serving more than roughly 100 persons.
2) It can remove around 90% of BOD and 80% of suspended solids.
3) As compared to the sludge holdings of a plain sedimentation tank, a septic tank can hold about 90% less of sludge volume.
4) Frequent removal of sludge is not required
Which of the above statements are correct?