Users having a private water supply which is discharged into the city’s sanitary sewer system shall pay sewer service charges as provided in Chapter 6, Article 6 of this code. Provided, however, that where an approved water flow measuring device has been installed, the sewer service charge shall be computed, billed, and paid as provided in § 6-6-19 of this code. Provided further, that where the user is outside the city limits, such user shall pay double the rate provided in § 6-6-19 of this code. An “approved water flow measuring device” within the meaning of this section is a device which is compatible with the city’s reading system and billing software; is plumbed and housed in accordance with the requirements of the city; and is otherwise satisfactory to the city.