HomeMy WebLinkAboutSewer Cost of Service Study Draft Report - July 2025
Otay Water District
Draft Sewer Rate Study
July 2025
Otay Water District / Sewer Rate Study
Contents
1. Executive Summary ..................................................................... 1
1.1. Current Sewer Rates .....................................................................................................1
1.2. Proposed Rate Changes ...............................................................................................3
1.2.1. Revised Sewer Return Factors ........................................................................................ 3
1.2.2. Updated Wastewater Strengths ....................................................................................... 3
1.2.3. Fixed Charge Development ............................................................................................. 3
1.3. Cost of Service Sewer Rates ........................................................................................4
1.4. Bill Impacts ....................................................................................................................4
2. Introduction .................................................................................. 6
2.1. Agency Overview ..........................................................................................................6
2.2. Study Overview .............................................................................................................7
2.3. Legal Requirements ......................................................................................................8
2.4. Abbreviations ................................................................................................................8
3. Cost of Service Analysis .............................................................. 9
3.1. Rate Setting Methodology ............................................................................................9
3.2. Units of Service ........................................................................................................... 10
3.2.1. Mass Balance................................................................................................................ 10
3.2.2. Customers ..................................................................................................................... 14
3.2.3. Units of Service Summary ............................................................................................. 15
3.3. Revenue Requirement ................................................................................................ 16
3.4. Cost Functionalization ................................................................................................ 17
3.5. Allocation to Cost Causation Components .............................................................. 18
3.5.1. O&M Allocation ............................................................................................................. 19
3.5.2. Capital Allocation .......................................................................................................... 20
3.5.3. Revenue Offset Allocation ............................................................................................. 20
3.5.4. Cost of Service Allocation Summary ............................................................................. 22
3.6. Unit Cost Development ............................................................................................... 22
3.7. Customer Class Cost of Service ................................................................................ 23
3.7.1. Customer Class Revenue Comparison ......................................................................... 24
4. Cost of Service Sewer Rates ..................................................... 25
Otay Water District / Sewer Rate Study
4.1. Cost of Service System Fees ..................................................................................... 25
4.2. Cost of Service Usage Fees ....................................................................................... 26
4.2.1. Flow Component ........................................................................................................... 26
4.2.2. BOD Component ........................................................................................................... 26
4.2.3. TSS Component ............................................................................................................ 27
4.2.4. Total Cost of Service Usage Fee ................................................................................... 27
4.3. Cost of Service Bill Impacts ....................................................................................... 28
5. Proposed Rate Adjustments ...................................................... 31
5.1. Rate Increase Determination ...................................................................................... 31
5.1.1. Cost Escalation Assumptions ........................................................................................ 31
5.2. Proposed Rates ........................................................................................................... 32
Tables
Table 1: Current Rates ............................................................................................................................. 2
Table 2: Cost of Service Sewer Rates ...................................................................................................... 4
Table 3: Wastewater Volume Balance ................................................................................................... 11
Table 4: Strength Loading Balance ........................................................................................................ 13
Table 5: Customers ................................................................................................................................ 14
Table 6: Units Summary ......................................................................................................................... 15
Table 7: FY 2025 Revenue Requirement ............................................................................................... 16
Table 8: Functionalization Summary ...................................................................................................... 18
Table 9: O&M Allocation ........................................................................................................................ 19
Table 10: Capital Cost Allocation ........................................................................................................... 20
Table 11: Revenue Offset Allocation ...................................................................................................... 21
Table 12: Cost of Service Summary ....................................................................................................... 22
Table 13: Unit Costs .............................................................................................................................. 22
Table 14: Customer Class COS ............................................................................................................. 23
Table 15: Class Revenue Comparison ................................................................................................... 24
Table 16: Cost of Service System Fees ................................................................................................. 25
Table 17: Flow Component .................................................................................................................... 26
Table 18: BOD Component .................................................................................................................... 26
Table 19: TSS Component ..................................................................................................................... 27
Table 20: Total Cost of Service Usage Fee ............................................................................................ 27
Table 21: SFR Bill Impacts ..................................................................................................................... 28
Otay Water District / Sewer Rate Study
Table 22: MFR Bill Impacts .................................................................................................................... 28
Table 23: Commercial Bill Impacts ......................................................................................................... 29
Table 24: Proposed FY 2026 to FY 2030 Rates ..................................................................................... 32
Otay Water District / Sewer Rate Study 1
1. Executive Summary
Public sewer agencies in California typically conduct cost-of-service studies to explain how the proposed rates
(in the draft rate study) and adopted rates (in the final rate study) to be charged to customers reflect the
proportional costs for the agency to provide service to a given parcel, as required by Proposition 218. The
District engaged the consulting firm Raftelis, an expert in sewer rate design and cost of service studies, to
conduct this Sewer Rate Study to examine the costs that Otay Water District is budgeted to incur to provide
sewer service and design a proportional sewer rate structure based on FY 2025 data (test year). The rates and
charges calculated in this report are calculated from the costs to serve each customer class based on their
volume of sewer discharge and the strength (i.e. quality) of the discharge.
1.1. Current Sewer Rates
The District currently has three customer classes based on the land uses for each parcel type: single family
residential (SFR), multi-family residential (MFR), and commercial. Customers in each class pay two monthly
fees: a fixed System Fee and a volumetric Usage Fee. The full schedule of current charges is shown in Table 1
below.
1.1.1. System Fees.
The amount of the System Fee varies by water meter size. All SFR customers pay a System Fee based on a
¾” meter. MFR and commercial customers with larger water meters pay a higher monthly fixed charge than
customers in the same class with smaller water meters.
1.1.2. Usage Fees.
Usage Fees pay for the costs of collection and treatment of sewage that vary based on the volume and/or
strength of the sewage discharged by a parcel.
Because residential sewage strength is generally consistent, the only variable the District uses for SFR and
MFR customers is volume. Their volume of discharge is not measured at the sewer connection (because
sewer meters are cost prohibitive), but instead are based on the parcel’s average monthly water consumption
billed for the months of January through April, commonly referred to as the average winter consumption
(AWC). Water consumption varies throughout the year generally based on irrigation needs, but because
irrigation water is not returned to the sewer, AWC allows the District to use the low-irrigation months to
derive an estimate of the volume of metered water that is returned to the sewer as sewage.
Commercial customers are charged differently than residential customers because both the strength and
volumes are different. Commercial customers have more variability (between customers) in terms of strength
than residential customers do – for example a restaurant that discharges food waste will have a higher-
strength discharge (dirtier wastewater) than an office building that is only discharging sanitary waste.
Commercial customers are therefore subdivided into low strength, medium strength, and high strength
customers.1 The Usage Fees for higher-strength customers are higher than for lower strength customers due to
the increased costs the District incurs to treat higher strength (i.e. dirtier) wastewater. The strength of sewer
1 Strength refers to the quality or “dirtiness” of the wastewater returned to the collection system. This concept is
discussed in more detail in Section Error! Reference source not found.Error! Reference source not found..
Otay Water District / Sewer Rate Study 2
discharge is determined by the biochemical oxygen demand (BOD) and total suspended solids (TSS) in the
discharge.
Each individual commercial customer also has less variability than residential customers on a month-to-
month basis in terms of volume, because, generally, commercial parcels have minimal outdoor irrigation in
comparison to SFR parcels. Volume for commercial customers is based on the monthly average water use for
a full year, referred to as the average annual consumption (AAC).
Additionally, the AWC and AAC are reduced by 15%, known as a “return factor,” to recognize that not all
water used is returned to the wastewater collection system. Measurement of each parcel’s precise sewer use
would be prohibitively expensive to implement. Therefore, the District – consistent with industry standards
throughout the state and country – uses engineering estimates and water consumption data to establish proxy
usage for the purpose of proportionally allocating sewer costs to each parcel AWC is an appropriate measure
of sewer usage for residential customers because it represents the period with the lowest outdoor water use,
providing the best estimate of single-family and multi-family residential indoor water usage. AAC is an
appropriate measure for commercial customers because commercial water use through the primary meter is
generally consistent throughout the year; outdoor use should be through a separate irrigation meter.
The monthly bill calculation is:
(AWC x Return Factor x Usage Fee) + System Fee
For example, a residential customer with an AWC of 9 Ccf currently pays:
(9 x .85 x $3.56) + $19.87 = $47.10
Table 1: Current Rates
(A) (B)
Usage Fee (per Ccf)
1 SFR (AWC) 3.56$ 85.0%
2 MFR (AWC) 3.56 85.0%
Commercial (AAC)
3 Low Strength 3.56$ 85.0%
4 Medium Strength 4.05 85.0%
5 High Strength 5.70 85.0%
System Fee (per Month)
6 5/8" 19.87$
7 3/4" 19.87
8 1" 49.64
9 1 1/2" 99.25
10 2" 158.80
11 3" 297.77
12 4" 496.28
13 6" 992.56
14 8" 1,588.12
15 10" 2,282.91
Current
RatesDescriptionLine Return
Factor
Otay Water District / Sewer Rate Study 3
1.2. Proposed Rate Changes
Overall, Raftelis recommends maintaining the general rate structure, including fixed System Fees that vary by
meter size, the use of return factors to determine wastewater contributions, and volumetric Usage Fees that
vary according to strength of the wastewater. However, Raftelis recommends certain adjustments to the
return factors, wastewater strengths, and the System Fee methodology in order to allocate costs more
proportionately.
1.2.1. Revised Sewer Return Factors
To estimate the sewer discharge from each customer, the District currently applies a uniform 85% return
factor to monthly water use for all customer classes to recognize the fact that not all water served to the
property is returned to the sewer. Raftelis recommends differentiating the return factor for each customer class
to more closely align with the District’s data and industry standard assumptions. The proposed commercial
return factor is 100%, which assumes that all water used by these customers is returned to the sewer system.
The proposed SFR and MFR return factors are 79% and 88%, respectively, to reflect water consumption that
is used for outdoor irrigation needs and therefore does not generate wastewater.
1.2.2. Updated Wastewater Strengths
Raftelis examined the District’s assumptions regarding the strengths of wastewater contributed by each
customer class. Strength refers to the quality, or “dirtiness”, of the wastewater, which the District incurs costs
to treat. As strengths increase, so do the District’s costs to treat (remove the dirtiness) that water to meet
regulatory requirements. The District’s current rates assume that low-strength commercial customers and
residential customers contribute wastewater of similar quality. However, the low strength commercial class
includes offices and retail stores, which generally do not have kitchens and do not discharge food waste or
grease into the wastewater collection system. Therefore, Raftelis recommends assigning residential customers
a higher strength, recovering a slightly higher share of treatment costs from residential customers compared to
low strength customers.
1.2.3. Fixed Charge Development
Raftelis has refined the methodology for the costs recovered in the District’s fixed System Fee in two ways.
The proposed cost of service rates identify customer service costs (a component of the System Fee) to be
recovered from all customers equally, regardless of the amount of wastewater they discharge, since all
customers can benefit equally from customer service.
Additionally, costs associated with the collection system are recovered through a fixed component of the
System Fee that varies by meter size. Larger water meters have the ability to use more water (also known as
“capacity”) than smaller meters. It is a common practice in setting wastewater rates to recover certain costs
associated with maintaining the infrastructure required to provide wastewater service based on capacity, that
is, potential maximum usage. It is a reasonable assumption that all customers with a larger water meter
consume more water, and therefore contribute more wastewater, which drives a demand for more system
capacity (e.g. larger pipes and pumps in the collection system) and therefore should pay a proportionally
higher share of the fixed costs associated with the collection system, which is sized to safely convey
wastewater from all customers.
Otay Water District / Sewer Rate Study 4
1.3. Cost of Service Sewer Rates
Table 2 presents the proposed test year2 sewer rate structure. All rates are based on the cost-of-service analysis
described in Section 3. Note that the rates in this table do not include an annual revenue adjustment (or rate
increase) for FY 2026. Table 24 in Section 5 presents the rates proposed to be effective from FY 2026 through
FY 2030 based on the District’s internal financial planning process.
Table 2: Cost of Service Sewer Rates
1.4. Bill Impacts
Since Raftelis recommends maintaining the same overall rate structure, the process to calculate a bill is the
same as describe above. Under the proposed cost of service rates, the bill for the same SFR customer
consuming 9 Ccf AWC is:
(9 x .79 x $3.77) + $20.75 = $47.55
Figure 1 compares test year SFR monthly bills with current bills at various AWC consumption levels. The
average SFR customer has a monthly AWC consumption of 9 hcf and has a ¾ inch meter; this customer’s bill
would increase 45 cents under the proposed new rate approach, before adjusting for the additional revenue to
2 Test year refers to the year selected to develop the cost of service and design rates. The test year in this study is FY
2025.
(A) (B)
Usage Fee (per Ccf)
1 SFR (AWC) 3.77$ 79%
2 MFR (AWC) 3.77 88%
Commercial (AAC)
3 Low Strength 3.22$ 100%
4 Medium Strength 4.54 100%
5 High Strength 7.17 100%
System Fee (per Month)
6 5/8" 20.75$
7 3/4" 20.75
8 1" 32.00
9 1 1/2" 60.13
10 2" 93.88
11 3" 200.77
12 4" 358.29
13 6" 791.46
14 8" 1,354.03
15 10" 2,141.63
Line Return
FactorDescriptionProposed
Rates
Otay Water District / Sewer Rate Study 5
be collected through the January 1, 2026 proposed rate increase. Additional bill impacts for other customers
can be found in Section 4.3.
Figure 1: SFR Bill Impacts
Otay Water District / Sewer Rate Study 6
2. Introduction
2.1. Agency Overview
Otay Water District (District) is a water, recycled water, and sewer service provider. The State Legislature
authorized the establishment of the Otay Water District in 1956 as a California Special District under the
provisions of the Municipal Water District Law of 1911, Division 20 (commencing with Section 71000) of the
Water Code of the State of California. Otay Water District is a public agency that does not make a profit.
Each customer pays only a fair share of the District’s costs to operate and maintain the public water, recycled
water, or sewer facilities.
Five directors, elected by voters to serve geographically separate divisions, set the District’s ordinances,
policies, taxes, and rates for service. Members of the Board of Directors serve four-year terms of office.
The Board of Directors typically meets in open public session on the first Wednesday of each month at 3:30
p.m. at District headquarters. The public is welcome to attend these meetings.
The District’s wastewater utility provides public sewer service to homes and businesses within the Jamacha
drainage basin, serving approximately 17,000 residents across 4,700 accounts primarily in the Rancho San
Diego area. Figure 2 below provides a map of the District’s sewer service area. Most wastewater is treated at
the Ralph W. Chapman Water Recycling Facility (RWCWRF), which has the ability to treat up to 1.3
million gallons per day. The District also sends sludge byproducts to the Metropolitan Wastewater Joint
Powers Authority (Metro) for processing.
Otay Water District / Sewer Rate Study 7
Figure 2: Otay Water District Sewer Service Area
2.2. Study Overview
Public sewer agencies in California typically conduct cost-of-service and rate studies to ensure there is a strong
nexus between rates charged to customers and costs incurred to provide service, as required by Proposition
218. The District engaged Raftelis to conduct this Sewer Cost of Service and Rate Design Study to establish a
proposed sewer rate structure based on FY 2025 test data. Note that the rates developed in Sections 3 and 4 of
this report do not include an annual revenue adjustment (or rate increase) for FY 2026 approved as part of the
FY 2026 budget adopted by the Board of Directors on June 4, 2025.
Otay Water District / Sewer Rate Study 8
Table 24 in Section 5 presents the rates proposed to be effective from FY 2026 through FY 2030 based on the
District’s internal financial planning process.
2.3. Legal Requirements
California Constitution - Article XIII D, Section 6 (Proposition 218)
Proposition 218, reflected in the California Constitution as Article XIII D, was enacted in 1996 to ensure that
rates and fees are reasonable and proportional to the cost of providing service. The principal requirements, as
they relate to public sewer service are as follows:
1. A property-related charge (such as wastewater rates) imposed by a public agency on a parcel shall not exceed
the costs required to provide the property-related service.
2. Revenues derived by the charge shall not be used for any purpose other than that for which the charge was
imposed.
3. The amount of the charge imposed upon any parcel shall not exceed the proportional cost of service
attributable to the parcel.
4. No charge may be imposed for a service unless that service is actually used or immediately available to the
owner of property.
5. A written notice of the proposed charge shall be mailed to the owner of record of each parcel at least 45 days
prior to the public hearing, when the agency considers all written protests against the charge.
Raftelis follows industry standard rate setting methodologies set forth by the Water Environment Federation
Manual of Practice No. 27: Wastewater Financing and Charges (WEF Manual 27) to meet Proposition 218
requirements and to recommend rates that do not exceed the proportionate cost of providing sewer services
on a parcel basis. The methodology in the WEF Manual 27 is a nationally recognized sewer industry
ratemaking standard.
2.4. Abbreviations
AWC Average Winter Consumption
AAC Average Annual Consumption
BOD Biological Oxygen Demand
FY Fiscal Year
Metro Metropolitan Wastewater Joint Powers
Authority
MFR Multi-Family Residential
O&M Operating and Maintenance
RWCWRF Ralph W. Chapman Water Reclamation Facility
SFR Single Family Residential
TSS Total Suspended Solids
WEF Water Environment Federation
Otay Water District / Sewer Rate Study 9
3. Cost of Service Analysis
Section 3 explains the cost of service (COS) analysis performed for the District’s sewer utility. A COS analysis
distributes a utility’s revenue requirements to each customer class based on their proportionate share of total
burden on the system sewer. The COS analysis completed by Raftelis for the District follows industry
standard cost allocation principles as presented in the Water Environment Federation’s Manual of Practice No.
27, Financing and Charges for Wastewater Systems (WEF Manual No. 27).
3.1. Rate Setting Methodology
The framework and methodology used to develop the COS analysis and to distribute the revenue requirement
to each customer class is informed by the processes outlined in WEF Manual No. 27. COS analyses are
tailored specifically to allocate the District’s costs incurred to collect and treat sewage from each customer
class. However, there are distinct steps in every COS analysis to recover costs from customers in an equitable
and defensible manner:
1. Development of units of service: Each class has unique wastewater characteristics that determine their share
of costs in later steps. These units include annual contributed wastewater, loadings of the BOD and TSS
pollutants, the number of customers, and the number of ¾” equivalent accounts.
2. Revenue requirement determination: The rate-making process begins by determining the revenue
requirement for the test year, also known as the rate-setting year. The revenue requirement represents the
full cost that the District must recover from ratepayers in order to reliably provide sewer service. The
revenue requirement includes the District’s operating costs, annual debt service, capital expenditures, and
reserve funding as projected based on the annual budget, including any miscellaneous revenues that offset
costs by reducing the total revenue that must be collected from rates. The test year for this study is FY
2025, using the adopted FY 2025 budget; these rates are then adjusted by District staff to recover the FY
2026 revenue requirement before adoption at a Proposition 218 Hearing to be held on October 1, 2025,
including a proposed effective date of January 1, 2026 as described in Section 5. The revenue requirement
in this study was previously determined through the District’s internal financial planning process and
provided to Raftelis. Later years (FY 2027, 2028, 2029, and 2030) are also presented in this report, which
provides the maximum potential rates for each fiscal year, except that those rates may be further adjusted
based on pass-through wholesale costs or inflation costs not anticipated in this study.
3. Cost functionalization: O&M expenses and capital assets are categorized by their function in the system.
Functions identified in this study include wastewater collection, lift stations, screening and grit removal,
aeration, secondary settling, residual processing and disposal, customer service, and general administrative
overhead.
4. Cost causation component allocation: Functionalized costs are then allocated to cost causation
components based on their burden on the system. Cost causation components include flow (the volume
of wastewater discharges), strength loading characteristics such as BOD and TSS (the “dirtiness” of
wastewater discharges), billing services, and capacity.
5. Unit cost development: The revenue requirement for each cost causation component is divided by the
appropriate units of service to determine the unit cost for each component.
6. Revenue requirement distribution: The unit cost is utilized to distribute the revenue requirement for each
cost causation component to customer classes based on each customer class’s service units.
Otay Water District / Sewer Rate Study 10
7. Rate design: After allocating the revenue requirement to each customer class, we design and calculate rates
to recover costs from the customers that cause the District to incur them. This process also includes a rate
impact analysis and sample customer bill impacts.
8. Administrative record preparation and rate adoption: The final step in a rate study is to develop the
administrative record in conjunction with the rate adoption process. This report serves as the
administrative record for this study. The administrative record documents the study results and presents
the methodologies, rationale, justifications, and calculations used to determine the proposed rates. A
thorough and methodological administrative record serves two important functions: maintaining
defensibility in a stringent legal environment and communicating the rationale for revenue adjustments
and proposed rates to customers and key stakeholders.
3.2. Units of Service
The first step of the cost-of-service process is to determine the units of service that will be used to assign costs
to each customer class. Each customer class has unique wastewater characteristics that are quantified and used
to assign costs, ensuring that each class pays their share of costs in proportion to the impacts their wastewater
system use characteristics place on the system.
3.2.1. Mass Balance
A key feature of a sewer COS is the mass balance, which is used to estimate contributed wastewater flows and
pollutant strengths using District data and assumptions based on industry standards. The goal is to balance
the assumed wastewater flow and strengths directly contributed by customers with known information about
wastewater from samples taken at the RWCWRF and Metro influent points to ensure that they are
reasonable. The resulting volumes and pollutant loadings are used to distribute costs to the customer classes
in Section 3.7.
3.2.1.1. Volume and Return Factors
The first component of the mass balance concerns balancing the total volume of wastewater treated by the
District with wastewater contributed directly by customers. Table 3 presents this process.
Otay Water District / Sewer Rate Study 11
Table 3: Wastewater Volume Balance
Column C, Line 1 shows the total amount of wastewater metered at the RWCWRF and Metro. Consistent
with industry standards, this study assumes that 5% of that total is from infiltration and inflow, which is
wastewater that enters the collection system via breaks and gaps in the pipes rather than through a service
connection to an actual customer. The Net Plant Influent on Line 3 is the total estimated wastewater
contributed by the District’s customers.
After determining the net influent, we must estimate the amounts contributed by each customer class. This
begins with billed volumes, as obtained from the District’s records. Commercial customers are billed based on
their average use per month over a 12-month period (average annual consumption, or AAC). SFR and MFR
customers are billed based on their average use per month in the January through April period3 (average
winter consumption, or AWC). The amounts in Column A are the total volumes billed for the test year.
The next step requires us to estimate the portion of water used that is returned to the sewer collection system.
Most water is used indoors and is returned to the sewer. However, a portion of water use is for irrigation
outdoors, which is not returned to the sewer. The mass balance recognizes this by applying return factors
(shown in Column B) to estimate wastewater contributions (Column C, the result of multiplying the values in
Column A by the values in Column B for each customer class).
3 The winter period spans 4 months to allow the use of 2 bi-monthly bills from Helix Water customers in the calculation
of the winter average. Raftelis compared the AWC calculated using the District’s winter period definition of January
through April to average water use from alternative periods and found the current definition to be reasonable and
substantially similar to other options.
(A) (B) (C)
1 Total at RWCWRF and METRO 594,097
2 Less: I&I 5.0%29,705 -
3 Net Plant Influent 564,392
Commercial
4 Low Strength 57,762 100.0%57,762
5 Medium Strength 13,683 100.0%13,683
6 High Strength 3,996 100.0%3,996 ---
7 Subtotal 75,441 75,441
8 Net Residential Flow 488,952
9 Less: MFR 88,079 88.0% 77,510 -
10 Net SFR 521,631 78.9% 411,442
Line Description
Billed
Wastewater
(Ccf)
Return
Factor
Contributed
Wastewater
(Ccf)
Otay Water District / Sewer Rate Study 12
This study assumes that 100% of water billed to commercial customers is used indoors. Many commercial
customers have separate irrigation meters for their outdoor water use needs; water delivered through
irrigation meters are not billed for sewer use and are not included in Column A. Other customers that do not
currently have an irrigation meter have a financial incentive to acquire one if their outdoor needs are
significant. Therefore, it is a reasonable assumption that all water used through potable water meters by
commercial customers is returned to the sewer.
However, many SFR and MFR customers do not have an irrigation meter and potentially use a significant
portion of their water for outdoor needs. Raftelis developed unique sewer return factors for each of these
customer classes. For MFR customers, this factor was developed using the ratio of water use in the lowest use
month compared to the average monthly use for the entire year for this customer class over a three-year
period. The lowest use months are March and April, which is consistent with the fact that this period is
generally the rainiest in the region and customers would have very little outdoor water use. It is a reasonable
assumption that all water used in in the lowest month by MFR customers is for indoor needs.
For the SFR class, the return factor is calculated from all of the preceding assumptions by subtracting
commercial wastewater contributions (Column C, Line 7) and MFR wastewater contributions (Column C,
Line 9) from the net influent (Column C, Line 3); the remainder must be from the SFR class. The SFR return
factor is the result of dividing the estimated SFR wastewater contribution (Column C, Line 10) by the SFR
billed wastewater (Column A, Line 10). Raftelis confirmed that this is a reasonable result by deriving the
average water use per capita per day implied by this data. The District serves 4,611 SFR accounts (cited in
Table 5 below) with an average household density of 3.07 persons4 and population of approximately 14,142,
resulting in a 59.6 gallons used indoors per person per day. This value is consistent with those seen elsewhere
in California and the United States.
3.2.1.2. BOD and TSS Pollutants
The mass balance also estimates the total strength loadings for each class, as presented in Table 4. Strengths
are expressed in milligrams of pollutant per liter of wastewater. The strengths in Columns B and D are then
applied to the contributed wastewater in Column A to derive the pounds of each pollutant in Columns C and
E5.
4 Per the Census Bureau ACS Survey Table B25124 for the Rancho San Diego census defined place.
5 The calculation of pounds also includes the conversions of milligrams to pounds and liters to Ccf.
Otay Water District / Sewer Rate Study 13
Table 4: Strength Loading Balance
The process and use of data to estimate strength loadings is conceptually similar to that used to develop
contributed volumes. The average BOD strength of wastewater influent at RWCWRF is known from the
District’s records (Line 1). Assumptions must be made regarding commercial strengths (Lines 4 to 6) that
result in reasonable implied SFR strengths (Line 10). This study assumes BOD loadings of 200 mg/L and
TSS loadings of 175 mg/L for low strength commercial customers and that loadings for medium and high
strength commercial customers are 2 and 4 times higher, respectively. Overall, this results in an implied SFR
BOD strength of 296 mg/L and TSS strength of 234 mg/L, which is comparable to the average influent
concentrations. Since SFR customers are the majority of customers, this is reasonable.
Raftelis believes these assumptions regarding commercial class strengths are valid for two reasons. First, they
are within the range of values identified by studies completed by other utilities around California and the
United States. In other words, the strengths assumed for the District’s customers are consistent with those
assumed for similar customers in other areas. Second, the assumed strengths are consistent with the District’s
data regarding wastewater characteristics of RWCWRF influent as well as residential flow. An assumption of
25 mg/L or 500 mg/L for low strength customers, for example, would result in significantly different and
unreasonable implied residential strengths. Instead, the residential strengths are consistent with data from
sampling studies conducted by other communities around California and the United States.
(A) (B) (C) (D) (E)
1 Total at RWCWRF and METRO 594,097 288 1,066,786 232 861,158
2 Less: I&I 29,705 200 37,092 200 37,092 -----
3 Net Plant Influent 564,392 292 1,029,694 234 824,066
Commercial
4 Low Strength 57,762 200 72,126 175 63,110
5 Medium Strength 13,683 400 34,171 350 29,900
6 High Strength 3,996 800 19,958 700 17,463 -----
7 Subtotal 75,441 268 126,256 235 110,474
8 Net Residential Flow 488,952 296 903,438 234 713,592
9 Less: MFR 77,510 296 143,215 234 113,120 -----
10 Net SFR 411,442 296 760,223 234 600,472
Line BOD
(lbs)
TSS
(mg/L)
TSS
(lbs)Description
Contributed
Wastewater
(Ccf)
BOD
(mg/L)
Otay Water District / Sewer Rate Study 14
3.2.2. Customers
Table 5, Columns A through F present the number of customers served by the District, arranged by their
customer class and meter size as reported by the District’s billing records.
Columns G and H show the meter equivalent ratio calculation, which is later used to allocate Capacity-
related costs appropriately and equitably. Larger meters impose larger demand and require greater capacity
within the water system. Equivalent meter units in this study are based on AWWA-rated hydraulic capacities
and are calculated to represent the potential burden on the sewer system relative to a base water meter size.
Capacity ratios are calculated by dividing larger meter capacities by the base meter capacity. The base meter
in this study is a 3/4-inch meter. AWWA capacity ratios (Column G) are calculated by dividing the capacity
of each meter size (Column H) by the capacity of a 3/4-inch meter (Column H, Line 2).
This information is later used to develop unit costs and allocate costs in Section 3.7.
Table 5: Customers
(A) (B) (C) (D) (E) (F) (G) (H)
Meter Size
1 5/8" 105 - - - - 105 1.00 30
2 3/4" 4,349 - 20 3 1 4,373 1.00 30
3 1" 157 - 5 - - 162 1.67 50
4 1 1/2" 2 - 12 5 4 23 3.33 100
5 2" - 39 16 5 2 62 5.33 160
6 3" - 5 1 - - 6 11.67 350
7 4" - 6 - - - 6 21.00 630
8 6" - - 1 - - 1 46.67 1,400
9 8" - - - - - - 80.00 2,400
10 10" - - 1 - - 1 126.67 3,800 ------
11 Total Accounts 4,613 50 56 13 7 4,739
12 Total Eq. Meters 4,722 392 339 46 25 5,525
Comm.
Low
Comm.
Medium
Comm.
High Total Eq. Meter
Ratio
Safe Flow
(gpm)Line Description SFR MFR
Otay Water District / Sewer Rate Study 15
3.2.3. Units of Service Summary
Table 6 summarizes the units of service presented above. Flow, BOD, and TSS units were developed in Table
4, columns A6, C, and E. The number of customers were presented in Table 5, Line 11; the account and ¾”
equivalent meter totals in this table are annualized by multiplying the number of customers by 12 in order to
show the total number of bills the District will issue. These units are later used to proportionally distribute
costs to customer classes in Section 3.7.
Table 6: Units Summary
6 Due to practical billing constraints, SFR units are developed using a return factor of 79% (rounded from 78.9% shown
in Table 3).
(A) (B) (C) (D) (F)
Class
SFR 412,088 761,417 601,415 55,356 56,668
MFR 77,510 143,215 113,120 600 4,708
Commercial - - -
Low Strength 57,762 72,126 63,110 672 4,064
Medium Strength 13,683 34,171 29,900 156 556
High Strength 3,996 19,958 17,463 84 300 -----
Total 565,039 1,030,888 825,009 56,868 66,296
Units: hcf lbs lbs Number of
Accounts,
Annualized
Number of
Eq. Meters,
Annualized
Eq. MetersDescription Flow BOD TSS Accounts
Otay Water District / Sewer Rate Study 16
3.3. Revenue Requirement
Table 7 shows the rate revenue requirement for FY 2025 (also referred to as the test year or rate-setting year).
The revenue requirement is split into operating and capital categories (Columns A and B), which are later
allocated based on O&M expenses and capital assets respectively. The expenses (Lines 1-3) are equal to FY
2025 budgeted expenses. The non-rate revenue (Line 5) includes interest earnings, late fees, cost sharing
revenue, and other miscellaneous revenues that are applied as offsets to the final rate revenue requirement.
The contribution to reserves adjustment (Line 6) is equal to FY 2025 projected net cash change and represents
the decrease in the rate revenue requirement resulting from a projected use to reserves of FY 2025. All
aforementioned values are from the District’s internal financial planning forecast for FY 2025 in absence of
any additional rate increase. The final rate revenue requirement (Line 9) is calculated as follows:
Total revenue required from rates (Line 7) = Revenue requirements (Line 4) + Non-Rate Revenue (Line 5) + Reserves (Line
6)
Table 7: FY 2025 Revenue Requirement
(A) (B) (C)
Expense
1 O&M 2,903,700$ 2,903,700$
2 Debt Service 164,200 164,200
3 Rate Funded Capital 1,286,000 1,286,000 ---
4 Subtotal 2,903,700$ 1,450,200$ 4,353,900$
5 Non-Rate Revenue (110,000)$ (358,192)$ (468,192)$
6 Contribution to / (Use of) Reserves -$ (427,708)$ (427,708)$ ---
7 Total Revenue Required 2,793,700$ 664,300$ 3,458,000$
Line Description Operating Capital Total
Otay Water District / Sewer Rate Study 17
3.4. Cost Functionalization
Assigning costs to functional categories is a critical step in the COS process. Functionalization identifies
various activities (functions) and their costs that the District must carry out in order to provide general
wastewater service. Put simply, the District must collect dirty wastewater, treat it to allowable standards of
cleanliness, and manage the administration of the utility. The functions for the sewer utility are:
1. Collection: related to the system of pipes that collect and convey wastewater discharged by customers to
the RWCRWF or Metro facilities.
2. Lift Stations and Pumping: related to operating lift stations that pump wastewater in parts of the
collection system that cannot be gravity fed.
3. Screening and Grit Removal: related to the first step of wastewater treatment, the removal of large
physical materials such as trash or other debris that has entered the collection system.
4. Aeration: a step in the wastewater treatment process that introduces oxygen to promote bacterial growth
in the wastewater and increase aerobic biodegradation of organic materials.
5. Secondary Settling: related to the removal of organic solids that remain in the sewage following the
aeration process.
6. Residual Processing and Disposal: related to disposal of sludge which is extracted from the wastewater
treated at RWCRWF. The District sends sludge to Metro for disposal and is billed for this service.
7. Customer Service: related to billing and operation of customer call centers.
8. General: related to overhead such as utility management, finance, or other activities not directly related to
one of the above functions.
This step is completed for both the operations and maintenance (O&M) budget and the book value of the
District’s plant assets in service. O&M was functionalized by District staff during the budgeting process based
on a review of contractual, maintenance, and labor related costs expected in FY 2025. Assets were
functionalized according to their description in the District’s records.
The O&M functionalization also includes an additional Metro Contingency function. The District’s FY 2025
budget includes $900,000 for payment for wastewater treatment and sludge disposal services from Metro.
However, this payment is not representative of an average year. The District typically treats most of it’s
wastewater and sends only sludge (which has a high BOD and TSS concentration) to Metro for disposal.
Approximately once every three years, including in FY 2025, the RWCRWF is temporarily shut down for
maintenance. When this happens, all of the District’s untreated sewage is sent to Metro. Metro’s charge to the
District is higher in FY 2025 due to the additional volume of sewage. In the remaining two out of three years,
the District sends a lower volume of sewage at a higher strength loading. Therefore, the FY 2025 budgeted
payment to Metro is separated into a baseline amount, included in the Residual Processing function, and a
Metro Contingency amount to allow for more precise allocation in Section 3.5.1.
The result of the functionalization process is shown in Table 8.
Otay Water District / Sewer Rate Study 18
Table 8: Functionalization Summary
3.5. Allocation to Cost Causation Components
The cost allocation process identifies the types of customer demands that are met by different functions of the
system. Cost causation components are used to allocate costs to customers based on their use of the system.
Cost causation components identified in this study include:
1. Flow: costs that are related to the quantity of wastewater generated.
2. BOD: costs that vary based on the biological oxygen demand (BOD) strength parameter of wastewater;
Biochemical oxygen demand is a measure of wastewater strength based on the amount of organic matter
contained in wastewater prior to treatment and measured in milligrams per liter (mg/L) or parts per
million (ppm). The more organic matter in the wastewater, the more oxygen the microorganisms need to
utilize the matter in the wastewater.
3. TSS: costs that vary based on the total suspended solids (TSS) strength parameter of wastewater; TSS is a
measure of wastewater strength based on the amount of solid particles in suspension in wastewater prior to
treatment, also generally expressed in milligrams per liter (mg/L) or parts per million (ppm).
4. Customer: component that serves to directly allocate customer service function costs.
5. Capacity: collection system costs that are related to the quantity of wastewater discharged but that the
District has chosen to recover in the fixed charge.
6. General: costs not directly related to any of the causation components above. General costs are reallocated
proportionally to all other causation components.
Each function is allocated differently to the cost causation component that is best related to the way the
District incurs costs.
1. Collection: allocated fully to the Capacity component. The size and cost of the collection system is directly
related to amount of wastewater it is designed to convey. Once the pipes are designed and installed in the
ground, the cost is fixed and does not vary from year to year.
2. Lift Stations and Pumping: allocated to the flow component since lift stations are designed and built based
on the amount of wastewater they are expected to move. Costs are directly related to the size of the lift
station.
3. Screening and Grit Removal: allocated fully to TSS since this operation is related to the removal of solids.
(A) (B)
Function
1 Collection Pipes 431,858$ 21,446,143$
2 Lift Stations and Pumping 416,091 3,719,468
3 Screening & Grit Removal 126,151 271,694
4 Aeration Basins 244,303 122,919
5 Secondary Settling Basins 115,068 41,444
6 Residual Processing 500,825 10,551
7 Customer Service 168,654 -
8 Metro Contingency 456,370 -
9 General 444,380 294,374 --
10 Total 2,903,700$ 25,906,592$
AssetsLine Description O&M
Otay Water District / Sewer Rate Study 19
4. Aeration: allocated fully to BOD since this process is related to the removal of organic materials that affect
the amount of BOD.
5. Secondary Settling: allocated fully to BOD since this process is related to the removal of organic materials
that affect BOD.
6. Residual Processing and Disposal: Metro’s billings to the District include cost components for volume
and pollutants that directly correspond with the flow, BOD, and TSS cost causation components used in
this study to develop the District’s retail rates. Therefore, this cost is allocated using factors derived directly
from a five-year average of Metro’s charges to the District.
7. Customer Service: allocated fully to the Customer Service cost causation component to allow recovery on
an equal, per customer basis. Regardless of the amount of wastewater they contribute, all customers receive
one bill each month. The costs to send this bill is the same for all customers.
8. Metro Contingency: allocated 33% to flow since in 1 of out 3 years this amount is used to pay Metro for
accepting increased wastewater flows. The remaining 67% is allocated to the General component to
recognize the fact that this amount may be used on a variety of other general activities in other years.
9. General Overhead: allocated to the General component for proportional reallocation to all components.
3.5.1. O&M Allocation
Table 9 shows the allocation of functionalized FY 2025 O&M expense forecast to each cost causation
component according to the criteria described above. Total O&M expenses associated with each function
(Column H) were determined in Table 8.
Table 9: O&M Allocation
(A) (B) (C) (D) (F) (G) (H)
O&M Function
1 Collection Pipes 0.0%100.0%431,858$
2 Lift Stations and Pumping 100.0%416,091
3 Screening & Grit Removal 100.0%126,151
4 Aeration Basins 100.0%244,303
5 Secondary Settling Basins 100.0%115,068
6 Residual Processing 38.7% 17.2% 44.1%500,825
7 Customer Service 100.0%168,654
8 Metro Contingency 33.0%67.0%456,370
9 General 100.0%444,380 -------
10 Subtotal 760,665$ 445,403$ 346,972$ 168,654$ 431,858$ 750,148$ 2,903,700$
11 Reallocation of General 264,963 155,147 120,861 58,747 150,429 (750,148) - -------
12 Total 1,025,628$ 600,550$ 467,833$ 227,402$ 582,287$ -$ 2,903,700$
13 Percent 35.3% 20.7% 16.1% 7.8% 20.1% 0.0%
Line Description TotalGeneralFlow BOD TSS
Customer
Service Capacity
Otay Water District / Sewer Rate Study 20
3.5.2. Capital Allocation
Capital assets are utilized in COS analyses to allocate the capital revenue requirement to the cost causation
components. The District’s assets are representative of long-term capital investment in the District’s sewer
system. Therefore, functionalized capital assets, as presented in Table 8, are used to allocate capital costs to
the cost causation components. Functionalized assets are allocated to the cost causation components using
the same allocation factors as were used to allocate O&M costs in Table 9.
Table 10 shows the allocation of capital assets by function to each cost causation component. The resulting
overall asset allocation, expressed as a percent on Line 12, is then used to allocate the test year capital costs,
including rate fund capital, debt service, and the use of reserves on Lines 13 to 15. The total test year capital
needs are shown on Line 16.
Table 10: Capital Cost Allocation
3.5.3. Revenue Offset Allocation
Non-rate revenues collected by the District serve to offset the rate revenue requirement by providing funding
that reduces the amount that must be recovered from rates to fund each cost causation component. Table 11
presents the allocation of these items. Non-rate revenue items in the operating fund are allocated in the same
proportion as O&M expenses, expressed as a percent on Line 12 of Table 9. The exception to this is payments
received from the Spring Valley Sewer District for the District’s operation of a shared lift station facility,
which is fully allocated to the Flow cost causation component because the cost to the District is based on the
(A) (B) (C) (D) (F) (G) (H)
Asset Function Asset Value
1 Collection Pipes 0.0% 0.0% 0.0% 0.0% 100.0% 0.0%21,446,143$
2 Lift Stations and Pumping 100.0% 0.0% 0.0% 0.0% 0.0% 0.0%3,719,468
3 Screening & Grit Removal 0.0% 0.0% 100.0% 0.0% 0.0% 0.0%271,694
4 Aeration Basins 0.0% 100.0% 0.0% 0.0% 0.0% 0.0%122,919
5 Secondary Settling Basins 0.0% 100.0% 0.0% 0.0% 0.0% 0.0%41,444
6 Residual Processing 38.7% 17.2% 44.1% 0.0% 0.0% 0.0%10,551
7 Customer Service 0.0% 0.0% 0.0% 100.0% 0.0% 0.0%-
8 General 0.0% 0.0% 0.0% 0.0% 0.0% 100.0%294,374 -------
9 Subtotal 3,723,554$ 166,175$ 276,346$ -$ 21,446,143$ 294,374$ 25,906,592$
10 Reallocation of General 42,797 1,910 3,176 - 246,491 (294,374) - -------
11 Total 3,766,351$ 168,085$ 279,522$ -$ 21,692,634$ -$ 25,906,592$
12 Percent 14.5% 0.6% 1.1% 0.0% 83.7% 0.0%
Capital Expense
13 Rate Funded Capital 14.5% 0.6% 1.1% 0.0% 83.7% 0.0% 1,286,000$
14 Debt Service 14.5% 0.6% 1.1% 0.0% 83.7% 0.0% 164,200
15 Use of Reserves 14.5% 0.6% 1.1% 0.0% 83.7% 0.0% (427,708) -------
16 Total 148,652$ 6,634$ 11,032$ -$ 856,174$ -$ 1,022,492$
17 Percent 14.5% 0.6% 1.1% 0.0% 83.7% 0.0%
TotalLine Description GeneralCapacityFlow BOD TSS
Customer
Service
Otay Water District / Sewer Rate Study 21
volume of flows through the shared facility. Revenues in the capital funds are allocated according to capital
assets, as shown in Line 12 of Table 10.
Table 11: Revenue Offset Allocation
(A) (B) (C) (D) (F) (G)
Operating Fund
1 Late Fees 35.3% 20.7% 16.1% 7.8% 20.1% 24,000$
2 SVSD Shared Facility 100.0% 12,000
3 Availability Fees 35.3% 20.7% 16.1% 7.8% 20.1% 53,000
4 Interest 35.3% 20.7% 16.1% 7.8% 20.1% 21,000 ------
5 Subtotal 46,615$ 20,269$ 15,789$ 7,675$ 19,652$ 110,000$
Expansion Fund
6 Interest Income 14.5% 0.6% 1.1% 0.0% 83.7% 500$ ------
7 Subtotal 73$ 3$ 5$ -$ 419$ 500$
Betterment Fund
8 Availability Fees 14.5% 0.6% 1.1% 0.0% 83.7% 51,000$
9 Interest Income 14.5% 0.6% 1.1% 0.0% 83.7% 1,700 ------
10 Subtotal 7,662$ 342$ 569$ -$ 44,128$ 52,700$
Replacement Fund
11 Annexation Fees 14.5% 0.6% 1.1% 0.0% 83.7% 116,292$
12 Interest Income 14.5% 0.6% 1.1% 0.0% 83.7% 188,700 ------
13 Subtotal 44,340$ 1,979$ 3,291$ -$ 255,382$ 304,992$ ------
14 Total 98,690$ 22,593$ 19,654$ 7,675$ 319,581$ 468,192$
Line Description TotalFlow BOD TSS
Customer
Service Capacity
Otay Water District / Sewer Rate Study 22
3.5.4. Cost of Service Allocation Summary
Table 12 summarizes the net cost of service allocation from the values developed in Table 9, Table 10, and
Table 11.
Table 12: Cost of Service Summary
3.6. Unit Cost Development
Having established the units of service for each customer class, the next step in the COS process is to calculate
a unit cost of service for each cost causation component. Table 13 presents this calculation. The costs in
Column A were previously summarized in Table 12 and the units in column B were summarized in Table 6.
The unit cost in column C is the result of dividing the cost by the units.
Table 13: Unit Costs
(A) (B) (C) (D) (F) (G)
Expenses
1 O&M 1,025,628$ 600,550$ 467,833$ 227,402$ 582,287$ 2,903,700$
2 Capital 148,652 6,634 11,032 - 856,174 1,022,492
3 Non-Rate Revenue (98,690) (22,593) (19,654) (7,675) (319,581) (468,192) ------
4 Total 1,075,590$ 584,592$ 459,211$ 219,727$ 1,118,880$ 3,458,000$
TotalCapacityCustomer
ServiceLine Description Flow BOD TSS
(A) (C)
Cost Component
1 Flow 1,075,590$ 565,039 hcf 1.90$
2 BOD 584,592 1,030,888 lbs 0.57
3 TSS 459,211 825,009 lbs 0.56
4 Customer Service 219,727 56,868 Number of Accounts, Annualized 3.86
6 Capacity 1,118,880 66,296 Number of Eq. Meters, Annualized 16.88 -
7 Total 3,458,000$
Units of Service
(B)
Unit CostLine Description Cost
Otay Water District / Sewer Rate Study 23
3.7. Customer Class Cost of Service
Finally, the cost to serve each customer class is calculated by multiplying the customer class units of service
for each cost causation component by the unit cost for each cost causation component, as performed in Table
14. For example, the SFR Flow amount of $784,439 shown in Line 1, Column A is determined by
multiplying 412,088 Ccf (Table 6, Line 1, Column A) by $1.90 (Table 13, Line 1, Column C)7. Table 14
shows, that of the $3,458,000 required to operate the system, the SFR class is responsible for $2,721,249.
Table 14: Customer Class COS
7 The unit costs in Table 13 are rounded to the nearest penny for display purposes. For this reason, manually calculating
the distributed class cost of service as described does not exactly equal the results shown in Table 14.
(A) (B) (C) (D) (E) (F)
Class
1 SFR 784,439$ 431,782$ 334,756$ 213,885$ 956,388$ 2,721,249$
2 MFR 147,545 81,214 62,964 2,318 79,457 373,498
Commercial
3 Low Strength 109,954 40,901 35,128 2,596 68,588 257,167
4 Medium Strength 26,047 19,378 16,643 603 9,384 72,053
5 High Strength 7,606 11,318 9,720 325 5,063 34,032 ------
6 Total 1,075,590$ 584,592$ 459,211$ 219,727$ 1,118,880$ 3,458,000$
Line TotalCapacityDescription Flow BOD TSS
Customer
Service
Otay Water District / Sewer Rate Study 24
3.7.1. Customer Class Revenue Comparison
Table 15 compares to the total test year revenue requirements distributed to each class (Table 14, Column G)
to the total revenue collected from each class under current rates. Note that the total revenue under either rate
structure corresponds to the revenue requirement from Table 7, indicating that the test year rates and rate
structure changes are revenue neutral. Distributional impacts to the various customer classes are a result of
the District’s changing cost structure and wastewater characteristics relative to the prior study that established
the current rates, and due to refinement of the methodology to maintain adherence to cost of service
principles.
Table 15: Class Revenue Comparison
(A) (B) (C) (D)
Class
1 SFR 2,696,710$ 2,721,249$ 24,539$ 0.9%
2 MFR 397,144 373,498 (23,646) -6.0%
Commercial
3 Low Strength 272,044 257,167 (14,877) -5.5%
4 Medium Strength 63,735 72,053 8,318 13.1%
5 High Strength 28,367 34,032 5,666 20.0%----
6 Total 3,458,000$ 3,458,000$ -$ 0.0%
Line Description
Revenue at
Current
Rates
Revenue at
Proposed
Rates
$ Change % Change
Otay Water District / Sewer Rate Study 25
4. Cost of Service Sewer Rates
4.1. Cost of Service System Fees
Table 16 presents the detailed calculation of the cost of service fixed System Fees, which are based on the
Customer Service and Capacity unit costs. Customer costs do not vary to by connection size or type.
Therefore, the Customer unit cost, previously derived in Column C, Line 4 of Table 13, is applied uniformly
to all System Fees (Column A).
Capacity costs vary with the potential to contribute wastewater. The collection system is sized to safely
convey the wastewater generated in a given area, based on the number and type of customers. The size of a
customer’s water meter is used as a proxy for potential wastewater contributions. As part of the permitting
process for new property uses, the District requires a count of water fixtures, such as sinks, toilets, and
faucets, that will be installed at a new location. The required meter size is directly based on these factors.
Therefore, it is reasonable to assume that customers with a larger water meter generate more wastewater.
To derive the rate component that recovers Capacity costs, the unit cost (Table 13, Line 6, Column C) is
multiplied by the AWWA capacity ratio for each meter size (Table 5, Column H). For example, the 1” meter
Capacity cost (Column B, Line 3) is derived by multiplying the 5/8” equivalent cost (Line 1) by 1.67 (Table 4,
Column H, Line 3).
The fixed charge components in Columns A and B are added and rounded to the next penny in Column C to
derive the total System Fee.
Table 16: Cost of Service System Fees
(A) (B) (C)
Meter Size
1 5/8" 3.86$ 16.88$ 20.75$
2 3/4" 3.86 16.88 20.75
3 1" 3.86 28.13 32.00
4 1 1/2" 3.86 56.26 60.13
5 2" 3.86 90.01 93.88
6 3" 3.86 196.90 200.77
7 4" 3.86 354.42 358.29
8 6" 3.86 787.60 791.46
9 8" 3.86 1,350.16 1,354.03
10 10" 3.86 2,137.76 2,141.63
Line Description
Customer
Service
Capacity
Related Total
Otay Water District / Sewer Rate Study 26
4.2. Cost of Service Usage Fees
Usage Fees are designed to recover the portion of the rate revenue requirement allocated to the Flow, BOD,
and TSS cost causation components.
4.2.1. Flow Component
Table 17 shows the calculation of the Flow rate component. Each class’s share of costs was previously
calculated in Table 14, Column A and is restated below in Column A. Billable wastewater volumes from
Table 6, Column A are shown in Column B. The Flow rate component in Column C is the result of dividing
Column A by Column B. All customers pay the same amount per Ccf since Flow costs are those related to the
volume of wastewater the District handles.
Table 17: Flow Component
4.2.2. BOD Component
Table 18 shows the calculation of the BOD rate component. Each class’s share of costs was previously
calculated in Table 14, Column B and is restated below in Column A. Billable wastewater volumes from
Table 6, Column A are shown in Column B. The BOD rate component in Column C is the result of dividing
Column A by Column B. This rate component varies between customer class in proportion to the BOD
strength of their contributed wastewater.
Table 18: BOD Component
(A) (B) (C)
Class
SFR 784,439$ 412,088 1.90$
MFR 147,545 77,510 1.90
Commercial
Low Strength 109,954$ 57,762$ 1.90$
Medium Strength 26,047 13,683 1.90
High Strength 7,606 3,996 1.90
Description Cost Billed Units Rate ($/Ccf)
(A) (B) (C)
Class
SFR 431,782$ 412,088 1.05$
MFR 81,214 77,510 1.05
Commercial
Low Strength 40,901$ 57,762$ 0.71$
Medium Strength 19,378 13,683 1.42
High Strength 11,318 3,996 2.83
Description Cost Billed Units Rate ($/Ccf)
Otay Water District / Sewer Rate Study 27
4.2.3. TSS Component
Table 19 shows the calculation of the TSS rate component. Each class’s share of costs was previously
calculated in Table 14, Column C and is restated below in Column A. Billable wastewater volumes from
Table 6, Column A are shown in Column B. The TSS rate component in Column C is the result of dividing
Column A by Column B. This rate component varies between customer class in proportion to the TSS
strength of their contributed wastewater.
Table 19: TSS Component
4.2.4. Total Cost of Service Usage Fee
Table 20 presents the total cost of service Usage Fee. The components in Columns A, B, and C (derived in
Table 17, Table 18, and Table 19) are added and rounded to the next penny in Column D to derive the total
Usage Fee.
Table 20: Total Cost of Service Usage Fee
(A) (B) (C)
Class
SFR 334,756$ 412,088 0.81$
MFR 62,964 77,510 0.81
Commercial
Low Strength 35,128$ 57,762$ 0.61$
Medium Strength 16,643 13,683 1.22
High Strength 9,720 3,996 2.43
Cost Billed Units Rate ($/Ccf)Description
(A) (B) (C) (D)
Class
SFR 1.90$ 1.05$ 0.81$ 3.77$
MFR 1.90 1.05 0.81 3.77
Commercial
Low Strength 1.90$ 0.71$ 0.61$ 3.22$
Medium Strength 1.90 1.42 1.22 4.54
High Strength 1.90 2.83 2.43 7.17
Description Flow BOD TSS Total
Otay Water District / Sewer Rate Study 28
4.3. Cost of Service Bill Impacts
The following Tables present sample monthly bills for each customer class. Bill impacts reflect the complete
effect of the cost of service Usage Fee, cost of service System Fee, and updated return factors in the test year
but do not yet include the impact of the District’s revenue increase that will be applied before rate adoption in
January 2026. SFR bills utilize the most common meter size and a range of average winter consumptions to
provide a representative sample of impacts across many customers. MFR bill impacts show data for the
District’s actual customers. Bills for individual apartment buildings have been consolidated into a total for
each of the five complexes in the District. Commercial bills impacts include all bills for each of the District’s
actual commercial customers.
Table 21: SFR Bill Impacts
Table 22: MFR Bill Impacts
Meter Size
Average
Winter
Consumption
Current
Billed WW
Proposed
Billed WW Fixed $ Volume $ Total $ Fixed $ Volume $ Total $ $ %
1 3/4" 3.00 2.55 2.37 19.87$ 9.08$ 28.95$ 20.75$ 8.93$ 29.68$ 0.74$ 2.5%
2 3/4" 5.00 4.25 3.95 19.87 15.13 35.00 20.75 14.89 35.64 0.64 1.8%
3 3/4" 9.00 7.65 7.11 19.87 27.23 47.10 20.75 26.80 47.55 0.45 1.0%
4 3/4" 11.00 9.35 8.69 19.87 33.29 53.16 20.75 32.76 53.51 0.36 0.7%
5 3/4" 14.00 11.90 11.06 19.87 42.36 62.23 20.75 41.70 62.45 0.21 0.3%
6 3/4" 28.00 23.80 22.12 19.87 84.73 104.60 20.75 83.39 104.14 (0.46) -0.4%
Line
ImpactCustomer Demand Current Cost of Service
Meter Size
Average
Winter
Consumption
Current
Billed WW
Proposed
Billed WW Fixed $ Volume $ Total $ Fixed $ Volume $ Total $ $ %
1 Multiple 1,791 1,522.28 1,576.01 3,732.02$ 5,419.32$ 9,151.34$ 2,645.16$ 5,941.56$ 8,586.72$ (565)$ -6.2%
2 Multiple 3,663 3,113.48 3,223.37 5,875.60 11,084.00 16,959.60 3,473.56 12,152.10 15,625.66 (1,333.93) -7.9%
3 Multiple 433 368.33 381.33 297.77 1,311.26 1,609.03 200.77 1,437.62 1,638.39 29.36 1.8%
4 Multiple 947 805.31 833.73 595.54 2,866.89 3,462.43 401.54 3,143.16 3,544.70 82.27 2.4%
5 Multiple 505 429.53 444.69 158.80 1,529.13 1,687.93 93.88 1,676.48 1,770.36 82.43 4.9%
Line
ImpactCustomer Demand Current Cost of Service
Otay Water District / Sewer Rate Study 29
Table 23: Commercial Bill Impacts
Meter Size
Average
Winter
Consumption
Current
Billed WW
Proposed
Billed WW Fixed $ Volume $ Total $ Fixed $ Volume $ Total $ $ %
1 3/4" 3.58 3.04 3.58 19.87 12.32 32.19 20.75 16.25 37.00 4.81 14.9%
2 3/4" 6.92 5.88 6.92 19.87 20.94 40.81 20.75 22.28 43.03 2.22 5.4%
3 3/4" 5.00 4.25 5.00 19.87 24.23 44.10 20.75 35.85 56.60 12.51 28.4%
4 3/4" 5.00 4.25 5.00 19.87 15.13 35.00 20.75 16.10 36.85 1.85 5.3%
5 3/4" 1.17 0.99 1.17 19.87 4.03 23.90 20.75 5.31 26.06 2.16 9.1%
6 3/4" 24.83 21.11 24.83 19.87 75.14 95.01 20.75 79.95 100.70 5.70 6.0%
7 3/4" 14.92 12.68 14.92 19.87 45.15 65.02 20.75 48.04 68.79 3.77 5.8%
8 3/4" 5.67 4.82 5.67 19.87 17.16 37.03 20.75 18.26 39.01 1.98 5.3%
9 2" 86.58 73.59 86.58 158.80 261.99 420.79 93.88 278.79 372.67 (48.12) -11.4%
10 3/4" 23.42 19.91 23.42 19.87 70.87 90.74 20.75 75.41 96.16 5.42 6.0%
11 3/4" 5.83 4.96 5.83 19.87 17.64 37.51 20.75 18.77 39.52 2.01 5.4%
12 1 1/2" 501.00 425.85 501.00 99.25 1,724.69 1,823.94 60.13 2,274.54 2,334.67 510.73 28.0%
13 3/4" 1.00 0.85 1.00 19.87 3.03 22.90 20.75 3.22 23.97 1.07 4.7%
14 3/4" 1.00 0.85 1.00 19.87 3.03 22.90 20.75 3.22 23.97 1.07 4.7%
15 1 1/2" 32.50 27.63 32.50 99.25 157.46 256.71 60.13 233.03 293.16 36.44 14.2%
16 2" 140.75 119.64 140.75 158.80 425.91 584.71 93.88 453.22 547.10 (37.61) -6.4%
17 3/4" 4.17 3.54 4.17 19.87 12.62 32.49 20.75 13.43 34.18 1.69 5.2%
18 3/4" 0.08 0.07 0.08 19.87 0.24 20.11 20.75 0.26 21.01 0.90 4.5%
19 2" 41.33 35.13 41.33 158.80 200.24 359.04 93.88 296.34 390.22 31.17 8.7%
20 1 1/2" 148.17 125.94 148.17 99.25 448.36 547.61 60.13 477.11 537.24 (10.38) -1.9%
21 2" 7.67 6.52 7.67 158.80 23.21 182.01 93.88 24.70 118.58 (63.43) -34.9%
22 2" 55.42 47.11 55.42 158.80 167.70 326.50 93.88 178.45 272.33 (54.17) -16.6%
23 1" 25.83 21.96 25.83 49.64 78.16 127.80 32.00 83.17 115.17 (12.63) -9.9%
24 2" 43.25 36.76 43.25 158.80 130.87 289.67 93.88 139.27 233.15 (56.53) -19.5%
25 3/4" 32.58 27.69 32.58 19.87 98.59 118.46 20.75 104.91 125.66 7.20 6.1%
26 1" 38.08 32.37 38.08 49.64 115.23 164.87 32.00 122.62 154.62 (10.25) -6.2%
27 2" 80.50 68.43 80.50 158.80 277.12 435.92 93.88 365.47 459.35 23.43 5.4%
28 2" 12.50 10.63 12.50 158.80 37.83 196.63 93.88 40.25 134.13 (62.50) -31.8%
29 2" 36.00 30.60 36.00 158.80 108.94 267.74 93.88 115.92 209.80 (57.94) -21.6%
30 2" 157.75 134.09 157.75 158.80 764.30 923.10 93.88 1,131.07 1,224.95 301.85 32.7%
31 2" 73.58 62.54 73.58 158.80 253.30 412.10 93.88 334.05 427.93 15.83 3.8%
32 10" 2,241.75 1,905.49 2,241.75 2,282.91 6,783.54 9,066.45 2,141.63 7,218.44 9,360.07 293.62 3.2%
33 3/4" 2.92 2.48 2.92 19.87 10.05 29.92 20.75 13.26 34.01 4.08 13.7%
34 1 1/2" 74.75 63.54 74.75 99.25 257.33 356.58 60.13 339.37 399.50 42.92 12.0%
35 1 1/2" 1.00 0.85 1.00 99.25 4.85 104.10 60.13 7.17 67.30 (36.80) -35.3%
36 1 1/2" 11.50 9.78 11.50 99.25 34.80 134.05 60.13 37.03 97.16 (36.89) -27.5%
37 1 1/2" 79.25 67.36 79.25 99.25 239.81 339.06 60.13 255.19 315.32 (23.75) -7.0%
38 1 1/2" 74.58 63.39 74.58 99.25 256.74 355.99 60.13 338.59 398.72 42.73 12.0%
39 1 1/2" 58.42 49.66 58.42 99.25 176.78 276.03 60.13 188.11 248.24 (27.79) -10.1%
40 3/4" 40.17 34.14 40.17 19.87 121.55 141.42 20.75 129.35 150.10 8.67 6.1%
41 1" 341.00 289.85 341.00 49.64 1,031.87 1,081.51 32.00 1,098.02 1,130.02 48.51 4.5%
42 3/4" 7.25 6.16 7.25 19.87 21.94 41.81 20.75 23.35 44.10 2.29 5.5%
43 1" 10.58 8.99 10.58 49.64 32.02 81.66 32.00 34.07 66.07 (15.59) -19.1%
44 2" 146.58 124.59 146.58 158.80 443.55 602.35 93.88 471.99 565.87 (36.48) -6.1%
45 3/4" 1.00 0.85 1.00 19.87 3.03 22.90 20.75 3.22 23.97 1.07 4.7%
Line
ImpactCustomer Demand Current Cost of Service
Otay Water District / Sewer Rate Study 30
Meter Size
Average
Winter
Consumption
Current
Billed WW
Proposed
Billed WW Fixed $ Volume $ Total $ Fixed $ Volume $ Total $ $ %
46 3/4" 1.00 0.85 1.00 19.87 3.03 22.90 20.75 3.22 23.97 1.07 4.7%
47 2" 37.92 32.23 37.92 158.80 114.75 273.55 93.88 122.10 215.98 (57.56) -21.0%
48 1 1/2" 51.00 43.35 51.00 99.25 154.33 253.58 60.13 164.22 224.35 (29.23) -11.5%
49 2" 22.58 19.19 22.58 158.80 68.33 227.13 93.88 72.71 166.59 (60.54) -26.7%
50 1" 115.92 98.53 115.92 49.64 350.77 400.41 32.00 373.26 405.26 4.85 1.2%
51 2" 137.75 117.09 137.75 158.80 416.83 575.63 93.88 443.56 537.44 (38.20) -6.6%
52 2" 22.25 18.91 22.25 158.80 67.33 226.13 93.88 71.65 165.53 (60.60) -26.8%
53 2" 56.25 47.81 56.25 158.80 193.64 352.44 93.88 255.38 349.26 (3.19) -0.9%
54 2" 10.58 8.99 10.58 158.80 32.02 190.82 93.88 34.07 127.95 (62.87) -32.9%
55 2" 46.08 39.17 46.08 158.80 139.44 298.24 93.88 148.38 242.26 (55.98) -18.8%
56 1 1/2" 57.25 48.66 57.25 99.25 173.24 272.49 60.13 184.35 244.48 (28.01) -10.3%
57 1 1/2" 52.75 44.84 52.75 99.25 181.59 280.84 60.13 239.49 299.62 18.77 6.7%
58 1 1/2" 28.50 24.23 28.50 99.25 86.24 185.49 60.13 91.77 151.90 (33.59) -18.1%
59 1 1/2" 31.75 26.99 31.75 99.25 96.08 195.33 60.13 102.24 162.37 (32.96) -16.9%
60 2" 31.42 26.71 31.42 158.80 108.16 266.96 93.88 142.65 236.53 (30.44) -11.4%
61 2" 152.33 129.48 152.33 158.80 524.40 683.20 93.88 691.58 785.46 102.26 15.0%
62 1 1/2" 56.58 48.09 56.58 99.25 274.13 373.38 60.13 405.68 465.81 92.43 24.8%
63 1 1/2" 35.42 30.11 35.42 99.25 121.93 221.18 60.13 160.81 220.94 (0.25) -0.1%
64 3/4" 9.00 7.65 9.00 19.87 27.23 47.10 20.75 28.98 49.73 2.63 5.6%
65 2" 5.00 4.25 5.00 158.80 15.13 173.93 93.88 16.10 109.98 (63.95) -36.8%
66 2" 98.83 84.01 98.83 158.80 299.06 457.86 93.88 318.23 412.11 (45.75) -10.0%
67 3/4" 1.33 1.13 1.33 19.87 4.02 23.89 20.75 4.28 25.03 1.14 4.8%
68 3" 281.67 239.42 281.67 297.77 852.33 1,150.10 200.77 906.98 1,107.75 (42.36) -3.7%
69 1 1/2" 19.83 16.86 19.83 99.25 60.01 159.26 60.13 63.85 123.98 (35.27) -22.1%
70 1 1/2" 16.08 13.67 16.08 99.25 48.66 147.91 60.13 51.78 111.91 (36.00) -24.3%
71 3/4" 16.42 13.96 16.42 19.87 49.69 69.56 20.75 52.87 73.62 4.07 5.8%
72 3/4" 1.25 1.06 1.25 19.87 3.78 23.65 20.75 4.03 24.78 1.12 4.7%
73 1 1/2" 38.83 33.01 38.83 99.25 188.13 287.38 60.13 278.41 338.54 51.16 17.8%
74 6" 17.83 15.16 17.83 992.56 53.95 1,046.51 791.46 57.41 848.87 (197.64) -18.9%
75 1 1/2" 54.25 46.11 54.25 99.25 164.16 263.41 60.13 174.69 234.82 (28.60) -10.9%
76 1 1/2" 72.25 61.41 72.25 99.25 218.63 317.88 60.13 232.65 292.78 (25.10) -7.9%
Line
ImpactCustomer Demand Current Cost of Service
Otay Water District / Sewer Rate Study 31
5. Proposed Rates
5.1. Rate Increase Determination
The District’s projected overall costs of sewer service for the next 5 years were determined by the Board of
Directors as part of the annual budget process. The District commissioned a Cost-of-Service Study to
determine the most equitable way to proportionally allocate among all customers the District’s projected costs
of sewer service, in compliance with the California Constitution, Article XIII D, Section 6 (“Proposition
218”).
These proposed rates and charges, if adopted, would implement the Cost-of-Service Study’s recommended
sewer charges for a period of five years, and pass-through to customers unanticipated increased costs due to
inflation or City/County charges.
5.1.1. Cost Escalation Assumptions
As part of the rate-setting process, the District projects the revenue needed in future years based on
assumptions and projections of cost increases including, among other things, general and construction cost
inflation (CPI and CCI, respectively), and increases in wholesale wastewater treatment or collection services
(provided by the City of San Diego and the County of San Diego). The District projects such cost increases
based on historical data and trends. The proposed annual increases to sewer rates and charges each year from
2027 to 2030 assume an average annual rate of cost inflation of 6.3% for power, administrative, materials, and
payroll costs (CPI), an annual rate of 6% wholesale conveyance and treatment services (except for FY 2028,
in which year a rate of 8.88% is assumed) (City/County rates), and a rate of 4% for construction costs for
capital improvement projects (CCI).
If overall inflation or construction inflation increases in amounts that exceed those assumptions, then,
pursuant to Government Code section 53756, the District is proposing to pass-through such unforeseen
inflationary increases directly to customers in future years as follows:
» If the annual change in the Consumer Price Index (“CPI”), San Diego-Carlsbad Area as of January 31 of
the preceding year, exceeds the assumed rate for the applicable year, the District will pass through the
difference.
» If the City or County rates exceed 6% in any given year, or 8.88% in FY 2028, the District will pass
through the difference.
» If the annual change in the Engineering News Record Construction Cost Index (“CCI”) measured from
February to February of the following year, exceeds the assumed rate for the applicable year, the District
will pass through the difference.
Collectively, the proposed adjustments described above are referred to as the “Pass-Throughs.” The District
may implement the Pass-Throughs at any time commencing January 1, 2026, through and including
December 31, 2030, upon notice of such adjustment to each impacted property owner and customer of record
at least 30 days prior to implementation. The Pass-Throughs may be applied to sewer rates and fixed charges
as applicable, and in no event shall the Pass-Throughs cause the proposed rates to exceed the cost of
providing sewer service.
Otay Water District / Sewer Rate Study 32
5.2. Proposed Rates
Table 24 presents the proposed rates for FY 2026 to FY 2030. To determine the FY 2026 rates, the proposed
rate increase of 4.1% is applied to the cost-of-service rates developed in this report. For example, the cost-of-
service rate of $3.77 x the proposed increase factor of 1.041 = $3.92. Future year rates are derived by applying
the proposed rate increases to the previous year’s adopted rates.
Table 24: Proposed FY 2026 to FY 2030 Rates
1 Rate Increase 4.1% 5.2% 9.8% 9.8% 9.8%
Usage Fee (per Ccf)
2 SFR (AWC) 3.56$ 3.77$ 3.92$ 4.12$ 4.53$ 4.97$ 5.46$
3 MFR (AWC) 3.56 3.77 3.92 4.12 4.53 4.97 5.46
Commercial (AAC)
4 Low Strength 3.56$ 3.22$ 3.35$ 3.52$ 3.87$ 4.25$ 4.67$
5 Medium Strength 4.05 4.54 4.73 4.98 5.46 6.00 6.59
6 High Strength 5.70 7.17 7.46 7.85 8.62 9.46 10.39
System Fee (per Month)*
7 5/8" 19.87$ 20.75$ 21.60$ 22.72$ 24.95$ 27.40$ 30.08$
8 3/4" 19.87 20.75 21.60 22.72 24.95 27.40 30.08
9 1" 49.64 32.00 33.32 35.05 38.49 42.26 46.40
10 1 1/2" 99.25 60.13 62.60 65.86 72.31 79.40 87.18
11 2" 158.80 93.88 97.74 102.82 112.90 123.96 136.11
12 3" 297.77 200.77 209.02 219.89 241.44 265.10 291.08
13 4" 496.28 358.29 373.02 392.42 430.87 473.10 519.46
14 6" 992.56 791.46 823.99 866.84 951.79 1,045.06 1,147.48
15 8" 1,588.12 1,354.03 1,409.68 1,482.98 1,628.32 1,787.89 1,963.10
16 10" 2,282.91 2,141.63 2,229.65 2,345.59 2,575.46 2,827.85 3,104.98
*The current System Fees shown in this table apply to commerical and multiresidential customers.
Single-family residential customers currently pay a System Fee of $19.87 per month, the charge
for a 3/4" meter. Under the rates proposed for 2026 and after, with limited exceptions all customers
will pay the System Fees shown here for their actual installed meter size.
Line Description Current 2025 COS 2026 2027 2028 2029 2030