Advantages and disadvantages of available phenol removal technologies^65,66.

Methods	Advantages	Disadvantages	References
Biological treatments
Biological degradation	• Phenol is consumed by microorganisms such as bacteria, algae, yeast, and fungi and convert them to harmless compound	• Can lead to toxic by-products	67–69
		• Growth control problem
		• Not suitable for high concentrations of phenol
		• Sludge production
		• Requires use of co-solvent when the concentration of phenol is low
Enzyme degradation	• Enzymatic reactions are specific in nature and happened under moderate pH and temperature	• Non-reusability of the enzymes	69–71
	• Higher catalytic efficiency and lower cost than the traditional chemical methods	• Enzyme instability in the harsh environment of the wastewater
Chemical treatments
Oxidative process	• In gaseous oxidation, there is no increase in the volume of wastewater and sludge	• Use of expensive chemicals
	• Simplicity of application	• Incomplete oxidation of phenol	72 and 73
		• Safety issue due to the use of hazardous chemicals
		• H2O2 needs to be activated by some other means
		• Wet oxidation of phenol is not economical due to the need for high pressure and temperature
Electrochemical destruction	• No need for expensive chemicals	• Requires expensive equipment	74
	• Sludge is not produced	• High energy consumption
		• Safety issues in handling toxic chemicals
Photochemical	• Phenols are greatly degraded, and sludge is not produced	• By-products are formed	75 and 76
		• Expensive equipment is needed
Fenton reagents (H2O2 + Fe(ii) salts)	• Fenton reagents are environmentally safe and therefore can be easily handled	Sludge production	77 and 78
	• No need for expensive and complicated apparatus
Irradiation	• Effective oxidation at lab scale	The requirement of a high amount of dissolved O2
Physical treatments
Membrane filtration	• Removes all types of dyes	• Concentrated sludge is produced	79
Electrocoagulation	• Economically feasible	• Production of large amounts of sludge	80
Distillation	• Phenols are separated from aqueous media based on relative volatility	• Needs high energy consumption	37
		• Used for low concentration of phenol removal
Adsorption	• Good for removal of phenol	• Regeneration is difficult	4, 81–83
	• Need mild temperature and pressure	• Regeneration need calcination or the use of solvent
	• Economical	• Many adsorbents have low adsorption efficiency
	• Easy to operate and no expensive equipment's required	• Not suitable for adsorption low level of phenol
		• Sometimes chemicals are used for adsorbent modification which are expensive and toxic
		• Sometimes need a high amount of adsorbent required
Liquid–liquid extraction	• Easy to operate	• Sometimes has low selectivity	66, 83–85
	• Performed at mild conditions	• Use of toxic, flammable, and volatile solvents in the extraction process
	• The extract can be recycled as a raw material	• Regeneration of solvent might be expensive and challenging
Ion exchange	• Regeneration of adsorbent	Not effective for all dyes	66