Once each year, the City Manager, based on results of tests conducted during the year, consideration of similar operations and establishments, engineering factors and other relevant matters, shall review or have reviewed for his use, the values of flow, loads and average concentrations of sanitary sewage; and shall review or have reviewed for his use, the limitations as established. Based upon such review, considerations, engineering factors and/or relevant matters, the City Manager may amend, add to and/or delete from Section 933.12 any values, parameters, concentrations, flows and loads which in his judgment merit such amendment, addition and/or deletion.