Match List – I with List – II

List – I (Chemicals)	List – II (Purpose)
(A) FeCl33​	(1) Added to wastewater during activated sludge treatment to promote microbial growth
(B) Ca(OH)22​	(2) Added to lower pH after chemical precipitation in soda-lime process
(C) CO22​	(3) Used in the soda-lime process for removal of hardness
(D) O22​	(4) Added to aid in coagulation of particles before sedimentation

Choose the correct answer from the options given below:

Correct option is D

Introduction:
· (A) FeCl₃ → (IV)
· Ferric chloride is a common coagulant used to aggregate suspended particles before sedimentation in water and wastewater treatment.
· (B) Ca(OH)₂ → (III)
· Calcium hydroxide (slaked lime) is used in the soda-lime process to remove water hardness by precipitating calcium and magnesium ions.
· (C) CO₂ → (II)
· Carbon dioxide is added after softening to lower the pH and neutralize excess lime, especially after lime-soda treatment.
· (D) O₂ → (I)
· Oxygen is supplied in the activated sludge process to support aerobic microbial growth during wastewater treatment.

Information Booster:

Coagulants like FeCl₃ are essential for turbidity removal.
· Ca(OH)₂ reacts with CO₂ to form calcium carbonate in hardness removal.
· Supplying oxygen improves biodegradation efficiency in biological treatment units.