Tenants being billed for trash pick-up through LADWP

I own a duplex in the city of Los Angeles where I pay for sewer and water but the trash pick-up is billed directly to the tenants through LADWP. I was wonder if this is normal for the tenants to be billed directly for trash or should it be included in my LADWP water bill?

thank you for any help/ advice you may be able to provide