Table 2.

Summary on current control techniques for VOCs removal.

Techniques	Principle	By-Product	Advantage	Disadvantage	Ref.
Botanical purification	Air is passed through a planted soil or directly on the plants. The contaminants are then degraded by microorganisms and/or plants, the precise mechanisms being unclear.	CO2, organic and amino acids	Low cost, no secondary pollution, beautifying the indoor environment	The purification effect is bad for high concentration pollutants	[28,62]
Catalytic combustion	Combustion of VOCs at low temperature with the help of a catalyst.	CO2, H2O	Wide range of application coverage, high efficiency, no secondary pollution	Not suitable for gas containing dust particles and droplets	[63,64]
Bio-filtration	Bio-filtration is a process in which contaminated airs passed through a biological stuffing medium that supports many kinds microorganism that biodegrade the VOCs.	Biomass	Little or no energy needs to be added in the form of heat or radiation to support this process	The equipment is big, long residence time, easy to jam	[65,66]
Absorption	Absorption is used to remove VOCs from gas streams by contacting the contaminated air with a liquid solvent.	Wastewater	Product recovery can offset annual operating costs	High demands on absorbent, complex process, high cost	[24]
Zeolite based adsorption	Air pollutants are adsorbed onto zeolites, often as filtration post-treatment.	Spent zeolite and collected organics	Effective in more than 90% RH as the adsorbent might be too specific	Pollutant reemission	[67]
Activated carbon based adsorption	VOCs are removed from the inlet air by physical adsorption onto the surface of the carbon.	Spent carbon and collected organics	Recovery of compounds, which may offset annual operating costs	They are flammable, difficult to regenerate for high boiling solvents, promote polymerization or oxidation of some solvents to toxic or insoluble compounds, and require humidity control.	[68]
Membrane Separation	Pollutants are passed through a membrane into another fluid by affinity separation.	Exhausted membrane	No further treatment, simple process, small energy consumption, no secondary pollution	The stability of the membrane was poor	[69]