Rate Modification

NapaSan is legally required to ensure that all the costs associated with collecting and treating wastewater are aligned with the rates being charged and that these costs are distributed fairly across all customer classes.  To be sure these requirements are met, NapaSan conducted an in-depth rate study that began in January of 2020.   

The results of the rate study show that wastewater flow and strength assumptions used to calculate bills need to change.  These changes will impact customer classes differently but, in general, costs for commercial and industrial customers that put greater demand on the sewer system will increase, single-family homes will stay the same, and apartments, condos and other residential properties will vary based on their typical demand on the system.  

Below, you will find information about the rate structure changes and what the changes mean for different customer classes.  

Residential Customers

How are sewer service charges calculated and paid?

For residential customers, the sewer service charge (SSC) appears as an assessment on your property tax bill. For a single-family residence, the SSC is currently $738.60 per year.  The sewer service charges for other types of dwellings are calculated in relation to the wastewater generation associated with a single-family residence, or equivalent dwelling unit (“EDU”).  

What is changing?

NapaSan currently assumes that a single-family home discharges approximately 210 gallons of wastewater per day.  As part of the rate study, residential wastewater discharge was analyzed.  Results showed that a single-family residence, or EDU, now discharges an average of 117 gallons per day. The average wastewater discharge for other types of dwellings was still lower than a single-family residence, but some flow assumptions need to change.  The table below shows the current assumed fraction of an EDU associated with each dwelling type and the recommended EDU based on the rate study results. 

Dwelling Type

Current EDU Ratio

Proposed New EDU Ratio

Single-family Residence












Mobile Home



Single-family Residence with Accessory Dwelling Unit



These changes would result in the following rate changes for different dwelling types:

ClassCurrent YearYear 1Year 2Year 3Year 4Year 5
Single-family Home$738.60$738.60$738.60$738.60$738.60$738.60
Mobile Home$443.16$480.09$517.02$553.95$590.88$627.81
Accessory Dwelling Unit$738.60$369.30$369.30$369.30$369.30$369.30

Commercial and Industrial Customers

How are sewer service charges calculated and paid?

Commercial and industrial customers are billed according to the number of equivalent dwelling units (EDUs) utilized each year. An EDU is the measure of the strength and volume of wastewater discharge associated with a typical single family home.  The EDU calculation is based on measured potable water usage (which reflects wastewater generation) and the strength of the wastewater. 

Commercial and industrial customers typically generate more wastewater than a single family home, and the wastewater discharged is typically stronger and more intensive to treat than the wastewater from a single-family home.  For this reason, NapaSan calculates bills for commercial customers by calculating the water usage for the year, dividing by the gallons associated with an EDU, and then adjusting it by a fixed strength factor specific to the type of business. Strength factors are a measure of the concentration of biochemical oxygen demand (BOD) and total suspended solids (TSS) in wastewater.  Industrial customers’ bills are calculated in the same manner, but are based on an assumed strength factor or an actual strength factor calculated using measured BOD and TSS values. 

What is changing?

The rate study found that assumptions made in the current rate structure regarding non-residential wastewater discharge need to be changed.  Currently, an EDU is based on 210 gallons of wastewater discharged per day. The study showed that 117 gallons per day is more accurate for an EDU.  While there has been a decrease in wastewater flow, there has not been a decrease in wastewater strength, so the assumed BOD and TSS values associated with an EDU should be increased to reflect the true strength of the wastewater NapaSan receives.   The table below shows the current EDU flow assumptions and the proposed assumptions based on the rate study results.

EDU Flow Assumptions






210 gallons per day

175 mg/L

200 mg/L


117 gallons per day

314 mg/L

359 mg/L

Commercial/Industrial 3-Year Water Use Average:

NapaSan will base future billing on a three-year average of water usage (compared to only one year), providing more predictability for ratepayers and NapaSan.

Wastewater “Strength” Charges

The formula for calculating and charging for wastewater strength, or the concentration of TSS and BOD, is changing. Depending on the type of business and how much water is used, this change will have varying impacts on commercial and industrial customers. These changes were made to properly allocate the costs of treatment.  

                Current                                                   Proposed

                FLOW – 50% of cost                               FLOW -58% of cost

                BOD – 25% of cost                                  BOD – 15% of cost

                TSS – 25% of cost                                   TSS – 27% of cost 

These changes to the strength factor calculation formula will result in a change in the proposed strength factor for many business types. Click the link below to view the new strength factors associated with different business categories.

Current and Proposed Commercial Strength Factors

Commercial Rate Calculation Formula

The formula used to calculate charges for commercial customers is:

Gallons of potable water used in prior year ÷ gallons associated with 1 EDU x Strength factor (SF) x cost per EDU  

Currently, the formula is:

Gallons of potable water used in prior year ÷ 76,650 x SF x $738.60

The proposed new formula would be:

Gallons of potable water used in prior year ÷ 42,705 x SF x $738.60

Industrial Rate Calculation Formula

The formula used to calculate charges for industrial customers involves multiple steps.  

  • Step 1: Calculate Flow Factor
  • Step 2: Calculate the Strength Factor
  • Step 3: Calculate EDU
  • Step 4: Calculate Monthly Sewer Use Fee

The table below shows the current and proposed changes to the formulas used to calculate industrial user charges.    

Current Formulas

Proposed Formulas

Flow factor = Average daily flow (gallons) ÷ 210 gallons/day

Flow factor = Average daily flow (gallons) ÷ 117 gallons/day


Strength factor = 0.50 + (BOD ÷ 175 mg/L × 0.25) + (TSS ÷ 200 mg/L× 0.25)


Strength factor = 0.58 + (BOD ÷ 314 mg/L × 0.15) + (TSS ÷ 359 mg/L× 0.27)


EDU= Flow factor x Strength Factor

No change

Monthly charges = EDU x $738.60 ÷ 12


No change


Bill Impacts

Changes in the flow assumptions and the associated concentration changes will result in a sewer service charge increase for many non-residential customers.  To minimize the impacts to customers, the flow and concentration changes will be phased in over a 5-year period.   

Want to talk with NapaSan staff about how your bill will change?

NapaSan staff will be available for phone or online one-on-one appointments to review your charges and answer your questions about specific rate impacts.  To schedule an appointment, please call (707) 258-6000.  Our staff will take down your questions and contact information. Then our team will gather all the data and information regarding your bill before scheduling a convenient time to discuss your specific bill impacts. 

Public Hearing and Protest Process

The Public Hearing was held on Wednesday, March 31, 2021 at 6pm.  

Public Information Sessions

NapaSan held two public information sessions in February and March of 2021.  A recording of the March 10th information session can be viewed by clicking the link in the "Recorded Meetings" section in the top right column of this page. 


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