If the tenant is "filthy"? due to a mental disability, then you need to proceed with caution to avoid any violation of the Fair Housing Act. Maybe Garry can give you the correct procedure required under law if this is the case with your tenant.
Some States do require a reason to be given for ending a rental agreement. This might be a requirement of your State?
You can look up Portland Oregon relocation fee law, new for 2017 which requires the landlord to pay between 2900 to 4500 dollars to ask a tenant to leave a rental house without cause in the case of a month to month. This does not include at the end of a fix term lease, a house shared with the landlord or the landlords own home which has been rented for less than 3 years.