What is the purpose of electrostatic precipitator in ecology unit?

return and fresh air filtration i think 

Due to electrostatic charge dust is removed and air is cleaned

Electrostatic precipitation is a method of dust, and small particulates collection that uses electrostatic forces, and consists of discharge wires and collecting plates. A high voltage is applied to the discharge wires to form an electrical field between the wires and the collecting plates, and also ionizes the gas around the discharge wires to supply ions. When gas that contains an aerosol (dust, mist) flows between the collecting plates and the discharge wires, the aerosol particles in the gas are charged by the ions. The Coulomb force caused by the electric field causes the charged particles to be collected on the collecting plates, and the gas is purified.

Energy Efficient,

In electrostatic precipitator lot of energy is required to

Electrically charge the grease,

where as Ecology unit works on simple4 stages

Filtration principle which doesn’t require any energy to clean the air.

Easy Monitoring,

In Denair standard ecology unit, Filter status can be easily

Monitored in the status panel,

In case of Electrostatic precipitator grease is released into the ducts or

environment when the plates get covered in grease.

Low operating cost,

Cost of cleaning or changing filter is low in case Ecology

unit whereas in electrostatic precipitator the degreaser used to clean the plates

are very expensive; hot water and waste disposal are required; daily cleaning is a major operating cost.

Additional Cost involved in Electrostatic Precipitator: Heated water supply and its drainage will add additional cost to the actual cost.

Dear colleagues,

Please note that Samar wants to know purpose of electrostatic precipitator in ecology unit. We know what is elec. precipitator, but the point is that: What is its benefit ecologically?

Respect to all other answers I think the Basharat' answer is the most related one.

Thanks Basharat.

The Electrostatic precipitator is charge the heavy particles which harm the enviroment much more then the tiny one and ESP also reduce th SOx emission.

electrostatic precipitator (ESP) is a filtration device that removes fine particles, like dust and smoke,

Gents,

for the same airflow and grease content, what is the comparison in filter change cycle (on average) for a multi stage filter Ecology unit (panel-bag-hepa arrangement) vs the cleaning cycle for an electro static precipitator. 

it consist of electric charges which in turn attracts ducts and smoke due to charge difference they are attracted and electrons in there field decides the charge they carry accordingly the opposite charges are  attracted  to  each other and they collected and filtered.