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Sewer Service Charges
Calculating Service Charges
Sewer Service Charges are charged annually, either on property tax assessments or directly billed to the commercial customer. The sewer service charge for commercial businesses is based on the following formula:
- Amount of potable water (gallons) used in prior year ÷ 54,750 x strength factor x current rate per EDU
To understand this formula, 54,750 is the annual water use in gallons for an Equivalent Dwelling Unit (EDU). The Strength Factor is based on the State Water Resources Control Board Revenue Program Guide, which calculates the typical strength for wastewater generated by various business types. View the list of strength factors by business category in Section 5.01.030 of the NapaSan Code.
Unmetered Commercial, Public Utilities, & Public Agency Facilities Service Charges
Unmetered commercial, public utilities, public agency facilities, and schools (both public and private) are charged as:
- Elementary Schools: 1 service unit (EDU) per 48 students
- Other: 1 service unit (EDU) per 10 employees/occupants
- Secondary Schools: 1 service unit (EDU) per 17 students
Calculating Service Charges
Industrial businesses, including winery-related operations, require a permit to discharge process waste to the sewers. Industrial users are charged monthly for sewer service based on a formula. The formula is designed to provide a multiplier for high-strength flows. There is a minimum of 1.0 service unit. Biochemical Oxygen Demand in mg/L (BOD) and Total Suspended Solids in mg/L (TSS) are determined through the use of samplers:
- Step 1: Flow Factor
- Flow Factor = Average daily flow (in gallons) ÷ 150 gallons/day
- Step 2: Strength Factor
- Strength Factor = 0.58 + (BOD ÷ 265 mg/L × 0.15) + (TSS ÷ 303 mg/L× 0.27)
- Step 3: Calculate Equvalent Dwelling Units (EDU)
- EDU = Flow Factor × Strength Factor
- Step 4: Calculate Monthly Sewer Use Fee
- Monthly Sewer Use Fee = EDU × Current Rate per EDU ÷ 12
Calculating Service Charges
The Sewer Service Charge (SSC) is the fee that customers are charged each year (billed annually or monthly) for collection and treatment of sanitary waste from their home or business. The rate is based on the typical volume and strength of the wastewater generated by a single family home, which is called an Equivalent Dwelling Unit (EDU).
2023/24 Sewer Service Charges
| Type of Unit | Number of EDU | Fiscal Year 2022 to 2023* |
|---|---|---|
| Condominiums and Townhouses | 0.91 | $672.13 |
| Duplex, each unit | 0.88 | $649.97 |
| Mobile Home | 0.75 | $553.95 |
| Overnight Trailer Park, per space | 0.40 | $295.44 |
| Single Family Dwelling | 1.00 | $738.60 |
| Triplex, Fourplex, and Apartments | 0.72 | $531.79 |