Ecology Unit in Mechanical Design

If you mean that the requirement is to clean exhaust air (i.e. that is removed from internal spaces and discharged to outside) even from the BOH (If you mean kitchens of restaurants, etc.) - there is no such requirement in LEED.

BUT I know about local practiсe that require to provide grease traps in local exhaust traps of such premises (BOH, or other that has similar contaminants). It can be part of your local "EPA" (if such exist) requirements or just standard practice for convenience (to reduce grease contamination in ducts and exhaust fan). 

Sounds like a more appropriate question for the IEQ Prequisite for Minimum Indoor Air Quality Performance.

Your question is not appropriate for this particular forum. From an energy perspective such an air cleaning device would be considered a process load and simply modeled identically in both models. This is not a LEED required element so if you want to know why the mechanical designer included an air cleaner you need to ask them. Also please keep in mind that these forums do not exist solely for you to extract information. They also serve as a source of information for others. So if someone else has a question about air cleaning devices they would first search in the appropriate LEED prerequisite/credit forum. Posting questions in unrelated forums makes it more difficult for others to find the answers they are seeking. 

Keep in mind that the experts who monitor these forums are giving their time and expertise for free. I have been doing so for 14 years and made the suggestion I did dozens of times before without anyone giving me a hard time about it. My original response was to try and help you find an answer. As the expert who monitors this forum I am not an expert on indoor air quality issues. Another reason to post it in the appropriate forum is that the experts who monitor individual forums do not subscribe to all of them. So posting in the appropriate forum sends a notification to an expert who will be more likely to be able to answer your question. It also sends a notification to others who monitor particular forums who again may have the same question as you do or have the expertise themselves to answer your question. Searching a key word is not the only way that people with such questions find their answers. So my original response was not intended to prove you wrong but to help you find a better answer than I can provide.