Identify the incorrect statement with respect to a biogas plant.

• A
  It consists of a concrete tank,$10-15$ feet deep,in which bio-wastes are collected and a slurry of dung is fed.
• B
  $A$ floating cover is placed over the slurry,which keeps on rising as the gas is consumed in the tank.
• C
  It has an outlet,which is connected to a pipe to supply biogas to nearby houses.
• D
  The spent slurry is removed and may be used as fertilizer.