What maintenance costs can I charge tenants?

[Rylanda Wilson]

I have a 5 unit apartment building complete with a separate laundry room, utility room and individualize large storage rooms on site for each tenant.  I am having a problem with the main drain backing up frequently even though I pay a professional drain service to clear it out.   I am having to pay every 4 months for a drain clean out at a cost of $600+ each time.  And every 2 months I use an approved enzyme in each apartment sink and the laundry room sinks.  I have not had this frequent problem until this year.  Is this a cost that I can pass on to the tenants in a special assessment?  I am willing to pay 1/6th.  Please advise.