Pick up the incorrect statement from the following :

• Septic tanks are horizontal continuous flow type of sedimentation tanks
• Septic tanks are generally provided a detention period of 12 to 36 hours
• Septic tanks are completely covered and high vent shafts are provided for the escape of foul gases
• Septic tanks are generally classified as tanks which work on the principle of anaerobic decomposition
• None of these.