Consider the following statements regarding contact stabilization process: 
1) Primary settling tank is not required in some cases. 
2) BOD removal occurs in two stages. 
3) Aeration volume requirements are approximately 50% of those of a conventional — or tapered — aeration plant. 
4) Returned sludge is aerated for 30 min to 90 min in sludge aeration tank. 
Which of the above statements are correct?