How You Landlord Calculate & Mail Tenant Their Water Sewer Bill?

[Lulu Heather]

Hello, I have a rental duplex at Portland, OR. I've just installed the water sub-meter for my tenants to pay for their water/sewer, I want to make their bills by myself without using any Billing Company. I would be grateful if you guys could share your experience how you calculate tenant's sewer volume usage based on your water submeter? (because mine is so weird, I have no idea how City calculate my bill, the sewer usage is always different with the water usage). 

In my main bill, there're other charges as Stormwater, Harbor Superfund, Base Charge - Do you include these charges in tenants' water/sewer bill also? 

Should I bill the tenant monthly or quarterly, as the city send my bill quarterly? Because these charges are different every 3months, so how can I bill the tenant prior I receive the main bill from city? 

Thank you.