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Onsite Wastewater Treatment System (OWTS) Manual Summary

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A summary of each of the sections of the revised Onsite Wastewater Treatment System (OWTS) Manual.

Section 1 – General

This section details the purpose of the manual, where the authority to regulate On-site Wastewater Treatment System (OWTS) and the applicability of the manual. This section was not edited in the latest revision.

Section 2 – Sewer Connection Required

If you are close to a sanitary sewer (in a sanitation district) and constructing a new home you are required to connect to the sanitary sewer. This section was not edited in the latest revision.

Section 3 – Definitions

This section lists pertinent terms and their definitions. Several key terms are presented here: 

Code Compliant OWTS means a system that is in conformance with this OWTS Manual. A Code Complaint OWTS can be new or existing.

Code compliant OWTS does not necessarily mean a new system. An existing system can be evaluated and demonstrated to met current design standards. 

A code compliant system does not mean a strict adherence to each and every design standard. If a particular standard cannot be meet, the client can propose an alternative or variance from the standard.  A variance request will include a mitigation measure that ensures at least equal protection of the original standard that cannot be met. Because the intent of original standard is being achieved the OWTS is considered code compliant.

Non-Conforming OWTS means an OWTS that was in compliance with the septic laws, regulations or codes when constructed and has a septic tank and dispersal system.

Cesspool is an excavation in the ground receiving domestic wastewater designed to retain the organic matter and solids while allowing the liquids to seep into the soil. Cesspools differ from seepage pits because cesspool systems do not have septic tanks and are not authorized under this Policy. The term cesspool does not include pit-privies or out-houses which are not regulated under this Policy.

Land Encumbrance means the land area that is eliminated from being utilized for septic dispersal areas. Examples of encumbrances are existing or proposed impervious surfaces such as structures, driveways, paved areas or other hard surfaces, as well as regulatory requirements or easements that eliminate land area for septic dispersal such as setbacks from creeks, rivers, riparian corridors, cut slopes, geological hazards, septic systems, wells, etc.

Section 4 – Criteria for All OWTS

This section contains general provisions and construction standards that apply to all new or replacement OWTS.  This section includes topics such as who can design OWTS, the type of permits required, application submittal requirements, site requirements, and general system requirements.

Section 5 – OWTS Abatements, Abandonments, and Emergency Repairs

This section contains the standards to abate and abandon existing septic systems.

Section 5.3 was added and provides standards regarding emergency repairs. If deemed an emergency, Permit Sonoma will expedite the review and permit process.

Section 6 – Requirements for Approval of Building Permits

When clients apply for building permits, the County also reviews the project and/or the site to ensure an adequate septic system exists. This section details the level of review of existing OWTS relative to the amount of work authorized under building permits.

This section has been significantly revised relative to the April 28, 2018 version presented to the Board of Supervisors in May 2018. The revisions focus on the amount and type of construction that triggers a review of septic systems or triggers the requirement for either a code compliant system or a non-conforming system.

In this version, an increase in wastewater flow – typically as new bedrooms – or an increase in wastewater strength requires a code compliant system. In the prior version, a code compliant system was tied to increases in wastewater but was also required based the amount of construction work on load bearing walls and not necessarily to wastewater.

Building permits not increasing wastewater flow or strength triggers an evaluation to demonstrate that the structure is served by an existing septic system. Cesspools are not considered an OWTS.

Building permits that do not require a set of plans are not evaluated by the Well and Septic section. Several examples include furnaces, water heaters, re-roofs, interior wall coverings, and deck repairs.

The County will also review the site to ensure a reserve system can be constructed in the future should the primary system fail. There are two levels of review depending on how much of the site is encumbered by development and setbacks.

If fifty percent or less land encumbrance, the site will be evaluated to ensure the proposed building does not physically encroach into, onto or adversely affect the existing/primary system and the reserve replacement area. If over fifty percent land encumbrance, a reserve replacement area will be required. This includes site evaluation of soil, percolation rate, separation to groundwater and a designed system.

If the building permit requires an existing OWTS and the client cannot document that one exists or elects to not upgrade or construct a septic system, the building permit will not be issued. The County will not impose any further requirements.

Section 7 – Site Evaluation Methods and Investigation Requirements

The main parameters regarding site evaluations are soil type, soil profile, percolation rate, separation to groundwater and setbacks. Standards and procedures for each of these parameters are presented in this chapter.

While not changing relative to the April 2018 version, we have proposed more options on how to determine the groundwater elevation. In the past, direct observation was the primary method, although we did have exceptions during drought years.

Going forward, we have proposed five methods to determine groundwater elevations:

  1. Direct observations via backhoe pits or auger holes;
  2. Direct observation via existing water wells or monitoring wells;
  3. Indirect observation via soil mottling; or
  4. Compilation of approved readings or observations from any of the first three methods from adjacent or neighboring parcels and/or projects.
  5. Other alternate methods as approved by the Permit Authority.

In this version, a section on when a cumulative impact study is required was added.


Section 8 – Criteria for OWTS Components

This section contains standards for septic tanks, sumps, pumps, leach lines, intercept drains, stream crossings and other basic elements of an OWTS. This section was not edited in the latest revision.

Section 9 – Criteria for Standard OWTS

This section contains the construction standards for standard OWTS or systems consisting of a septic tank and a dispersal system, typically a leach line, but does include other systems such as seepage pits, filled land systems and shallow sloping systems to name a few.

A table illustrating the typical length of leach line was created and inserted into this section as part of this revision. The design length of a leach line is determined by the soil type and/or percolation rate (hydraulic loading rate), the soil profile (absorption area) and the amount of wastewater. Given that assumptions were made on these parameters, the presented table is for illustration purposes and cannot be used directly for design purposes.

Section 10 – Criteria for Water Reuse

This section includes standards for gray water systems. There are three levels of gray water systems:  laundry to landscape, simple systems and complex systems. All three systems are required to comply with the California Plumbing Code, but only the latter two require an application and permit from Permit Sonoma. This section was not edited in the latest revision.

Section 11 – Criteria for Commercial, Industrial & Institutional OWTS

This section contains the standards for non-residential OWTS and components such as wineries, package treatment plants, flow equalization and grease interceptors. This section was not edited in the latest revision.

Section 12 – Non-Standard Experimental and Alternative OWTS Process

This section contains the process for experimental and/or new technology to be introduced and placed in the field on a trial basis. The main criteria include the equipment being certified by the National Sanitation Federation and the system being able to separate solids from liquids, nutrient reduction and pathogen reduction comparable to a standard OWTS.

The process is for a proponent to submit an application detailing the unit or equipment. A total of twenty units may be installed – ten in each Regional Water Board jurisdiction. The proponent monitors the systems and collects performance data. This data may be used to move the experimental systems into the alternative OWTS program. This section was revised to clarify the criteria and approval process.

This section also contains the process for the Alternative OWTS program. The main criteria include either being in the Sonoma County Experimental OWTS program with acceptable results or having a comparable number of units installed in another jurisdiction with acceptable performance data.

The process is for a proponent to submit an application detailing the unit, the number of installations, the performance data and other details. Once approved, there is no limit to the number of Alternative OWTS that can be installed within the County.

Section 13 – Non-Standard Experimental and Alternative OWTS Construction Standards

This section contains the construction standards for the approved experimental and alternative OWTS.

Section 14 – Non Standard and/or Commercial OWTS Operational Permit and Monitoring

All non-standard OWTS are required to be placed into the operational monitoring program, commonly referred to as the OPR program. Section 14 contains the standards and reporting requirements for this monitoring program. A few of the requirements include the installation of monitoring wells surrounding the system, submittal of performance data such as flow rate and operational status, an access easement in favor of the County and an annual fee. This section was not edited in the latest revision.

Section 15 – Vesting Certificates

This section contains details regarding vesting certificates. Vestings come in the two varieties – one for an approved design and one for an installed OWTS. Vestings are intended to protect property owners from changes in regulations. The approved design vesting is valid for three years and the vesting for an installed OWTS is valid for two years. This section was not edited in the latest revision.

Section 16 – Subdivisions and Lot Line Adjustments Requirements

When creating new subdivisions of land one guiding principle is that each new parcel will be able to properly dispose of the wastewater generated on that parcel. This applies to major subdivisions, minor subdivisions and lot line adjustments.

Easements for sewage disposal may be considered for major subdivisions if the standards contained in Section 16 are met. This section was not edited in the latest revision.

Section 17 – Variance Requirements

If a particular standard cannot be met, the client can propose an alternative or variance from the standard. A variance request will include a mitigation measure that ensures at least equal protection of the original standard that cannot be met. The request needs to include why a variance is needed or why the original standard cannot be achieved. The request also needs to include how the proposed alternative meets or achieves the original standard.


This section presents the requirements for variances and includes a list of common code requirements that are subject to variance requests. Also presented are associated standards that provide equal treatment for the original standard. This section was revised since April 2018.

Section 18 – Variance Prohibition and Special Standards Areas

There are specific areas in Sonoma County where the Regional Water Board and/or the Board of Supervisors have prohibited variances. The Regional Water Board’s Total Maximum Daily Loads (TMDL) will make many of these prohibition areas a moot point. The County plans to seek the revocation of many of these areas after the Russian River pathogen TMDL is adopted. This section was not edited in the latest revision.

Section 19 – Dispute Resolution

Occasional there are disputes regarding project approvals that do not get resolved at the staff level. The process to resolve these disputes is to use the chain of command starting with staff, supervisor and then manager. If not resolved at this level, the client may request to convene the Dispute Resolution Panel.

The Land Use Advisory Panel appoints the members of the Dispute Resolution Panel. The Dispute Resolution Panel is a six-member panel and the membership is detailed in this chapter. The Dispute Resolution Panel hears the case and then provides a recommendation to the Director of Permit Sonoma. The Director reviews the recommendation and makes a final determination. This section was not edited in the latest revision.

Section 20 – Tier 3 Treatment, Monitoring, Inspection and Sampling for Supplemental Treatment Units

Section 20 presents standards for OWTS that are subject to the OWTS Policy Tier 3. The OWTS Policy Tier 3 includes OWTS that are within a specified geographical area and there is not an approved Total Maximum Daily Load Implementation Plan or an Advanced Protection Management Program covering the same geographical area. The geographical area for Tier 3 is specified in the State’s OWTS Policy.   This section was not edited in the latest revision.

Appendix A – Approved Experimental and Alternative Systems

Appendix A was added to this version and presents a listing of the currently approved experimental OWTS and currently approved alternative OWTS.