November 21, 2017

Division Three - Standards for Sanitary Sewers - Chapter II. Design Standards

Chapter II. Design Standards

Sec. 302-1 Sanitary Sewers Design Criteria

The following guidelines shall be used to compute contributing sewage:

302-1.01 The average family unit shall be two and eight tenths (2.8) persons per residential dwelling unit or its equivalent, and two and twenty four hundredths (2.24) persons per multi-family residential unit.

302-1.02 The average discharge of domestic sewage shall be one hundred (100) gallons per day per person. In the absence of any specific information to the contrary, the design peak discharge shall be estimated as follows:

Peak Factor = 1.8 x (Average Discharge)

302-1.03 Infiltration allowances will be in addition to the above.

302-1.04 In the absence of any specific information to the contrary, the peak design discharge for commercial and industrial sewage shall be:

Land Use Peak Discharge Rate Peak Factor

Commercial 0.010 CFS/GA (6,460 GPD/GA) 1.8

Industrial 0.015 CFS/GA (9,695 GPD/GA) 2.0

Flow rates may be obtained from approved references or other acceptable source.

302-1.05 Depth of flow in main and trunk sewers: Main sewers up to fifteen (15) inches inside diameter shall be designed to flow not more than seven-tenths (0.7) full; Trunk or main sewers over fifteen (15) inches inside diameter may be designed to flow full.

Note: Under no condition is a gravity sewer ever to be designed to flow under a head.133

Sec. 302-2 Sewer Velocity and Quantities

302-2.01 Minimum design velocity for any sanitary sewer shall be two (2) feet per second, for pipes flowing full or half full, or with the approval of the Director, one and six tenths (1.6) feet per second. Acceptable criteria for allowing 1.6 feet per second would be as follows: Pipes designed for 1.6 feet per second velocity at peak shall flow at least 40% full for computed average flows. Without considering the above, the following pipe slopes may be used:

Land Use Peak Discharge Rate Peak Factor
Commercial 0.010 CFS/GA (6,460 GPD/GA) 1.8
Industrial 0.015 CFS/GA (9,695 GPD/GA) 2.0

302-2.02 Unless special provisions approved by the Director have been provided, design velocities for sanitary sewers shall not exceed 10 feet per second. Velocities are to be computed using Manning's formula with a constant "n" value for pipes half full or full as follows:

PVC Pipe “n” = 0.011

Other Pipe “n” = 0.013

Sec. 302-3 Maximum Discharge

The maximum design discharge shall not exceed the flow at critical slope and velocity.

Sanitary sewers shall not be designed for flow conditions at critical slope and velocity.

Sec. 302-4 Manholes

302-4.01 Manholes shall be placed at the intersections of all six (6) inch and larger sewer lines.

302-4.02 Manholes shall be placed at all changes of alignment, either vertical or horizontal.

302-4.03 The maximum distance between manholes shall be 400 feet, except for mains 15 inches in diameter or larger, the maximum distance may be increased to 600 feet. Minor exceptions may be approved by the Director. In areas where the slope of the sewer line exceeds 1%, the maximum distance between manholes shall be reduced to 350 feet.

302-4.04 A one tenth (0.1) foot energy drop shall be allowed for flow through manholes with a deflection of 45° or more.

302-4.05 Invert elevations of varying size pipes leading into and out of manholes shall be set by matching elevations of the pipe soffits.

302-4.06 Building drains or house laterals which terminate in a manhole shall be elevated a minimum of six (6) inches and a maximum of 12 inches above the flowline of the manhole.

302-4.07 Manholes shall be placed only in locations that provide ready access to the sewer for maintenance and emergency service.

302-4.08 All manholes shall be as shown on standard drawing STANDARD PRECAST MANHOLE, Plate S-5. Manholes shall be constructed to grade. Precast reinforced concrete manhole risers and tops shall conform to ASTM Designations C-478 with openings over outgoing sewer. Precast tops shall be eccentric cone type. All cement used in the construction of concrete manholes shall be Type II and conform to ASTM Designation C-150. Manholes outside of paved areas shall be constructed to prevent drainage water from entering manhole. Approved water stops are required for all PVC sewer lines entering any manholes.

302-4.09 Frame and cover shall be as shown on standard drawing CAST IRON MANHOLE FRAME AND COVER, Plate S-6. They shall be interchangeable with Alhambra Foundry Model A-1270 and conform to ASTM A 48-83 Class 35B gray cast iron with minimum strength 35,000 psi for H20-44 highway loading.

Sec. 302-5 Joints

All joints in the precast concrete sections, except adjusting rings, shall be set in an elastomeric sealant.

Sec. 302-6 Drop Manholes

Whenever the vertical distance between the inverts of sewer line connection and a manhole exceeds 18 inches, a Standard Drop Manhole, Plate S-3 or S-4 shall be constructed. Drop Manholes shall be permitted only for compelling reasons.

Sec. 302-7 Rodding and Flushing Inlets

Standard rodding and flushing inlets may be allowed at the terminal end of sewer segments constructed for later extension, such as at the edge of a subdivision, except where the sewer segment exceeds 200 feet in length, in which case a standard manhole shall be constructed. See Plate S-7 and S-8.

Sec. 302-8 Polypropelene Coated Manhole Steps

Manhole Steps shall be M.A. Industries Model PS-2PF or approved equal Polypropelene Coated Manhole Step.

Sec. 302-9 Sewer Main Location, Alignment, Cover and Size

302-9.01 Location

All sanitary sewers and appurtenant structures shall be a minimum of five (5) feet from the roadway center line. In no case shall a sanitary sewer line be located closer than five (5) feet from an existing or proposed gutter lip. Sanitary sewers shall be centered on travel lanes to the greatest extent possible. In unstable soil, the minimum distances may be increased by the Engineer. Minor exceptions may be approved by the Director.

The entire sanitary system shall be located as mentioned above and shall be designed to clear all other existing or proposed utilities by a minimum of six (6) inches. Clearances with water lines shall conform to Chapter V of this Division.

Location of sewer mains in easements shall be subject to the approval of the Director. Sewer mains in easements shall be kept to a minimum.

Where sewer mains are located within easements, the easements shall be:

    1. Granted with final map, or 
    2. Be granted to the entity accepting and maintaining the sewers, or
    3. Be dedicated to and accepted by Kern County.
    4. The minimum width of any easement for sanitary sewer purpose shall be ten (10) feet. In special cases of terrain, depth of sewer line, access, etc. the required easement width shall be increased based on 1:1 cut slope plus three (3) feet for pipe zone. Width of easement to be approved by Director.
    5. All easements shall include right of ingress and egress over adjoining property for maintenance, replacement and operation. No permanent structures shall be constructed in such easements, except fences, and except utilities which are the subject of an overlapping easement. Vehicular access shall be provided to all manholes except as specifically exempted by the Director.

302-9.02 Alignment

Whenever possible, sewer lines shall be laid out in a straight line between structures. Curved sewer lines will be allowed under the following conditions:

    1. All curve data shall be shown on the plans.
    2. Minimum radius of curvature and joint deflections shall be as recommended by the pipe manufacturer and approved by the Director. Alignment shall be concentric to street or easement centerline.
    3. All deflections shall be at the pipe joints or by specially mitered pipe sections. Actual bending of the pipe itself will not be allowed.
    4. Manholes shall be constructed at the B.C. and E.C. of all curves.
    5. Any non-metallic sewer line constructed on a horizontal curve shall have a No. 10 bare soft copper detector wire placed over the pipe line prior to backfilling. The detector wire shall be brought up to the manhole frames at each end of the curved pipeline.
    6. Identification tape, designed for use on sewers, shall be placed one foot above on top of the sewer main along its entire length. Identification tape shall also be placed on the top of sewer laterals extending from one foot above the sewer main to the right of way line.

302-9.03 Cover

Minimum cover for any sewer main shall be four (4) feet within the street section, and three (3) feet in sewer easements, and over sewer laterals within the street section. Any deviation from this cover shall require special design and approval by the Director.

302-9.04 Size

Minimum line size for any sewer main shall be eight (8) inches in diameter. For sewer mains of 150 feet or less which cannot be extended, (e.g. cul-de-sacs) sewers shall not be less than six (6) inches in diameter or as approved by Director.

Sec. 302-10 Building Sewers

302-10.01 Minimum size of any sanitary building sewer within road right of way and/or dedicated easement to serve individual residences, commercial structures, etc. shall be four (4) inches. Actual size of building sewers larger than four (4) inches shall be determined by fixture unit requirements as per the current edition of the California Plumbing Code.

302-10.02 Quantity: A separate building sewer lateral shall be provided to each lot, parcel or building. Subdivision of land into large parcels subject to potential future re-subdivision or multiple building sites may require additional laterals or increased lateral size.

Sec. 302-11 Special Equipment and Structures

302-11.01 All special equipment such as package treatment plants, their appurtenances, and their effluent disposal areas and methods shall be designed, located, and constructed so as to preclude contamination, pollution, nuisance, and structural and mechanical instability. Proposals and plans for package treatment systems and disposal facilities shall be subject to the review and approval of: (1) the Regional Water Quality Control Board (RWQCB) for design, contamination elements of pollution and nuisance aspects. Upon approval, waste discharge requirements will be issued by said RWQCB. (2) the Engineering, Surveying & Permit Services Department for structural and mechanical integrity. Special structures, such as pump stations, pressure lines and sags, etc. shall be subject to the approval of the Engineering, Surveying & Permit Services Department and the maintaining District.

302-11.02 Plans for individual septic tank systems shall be subject to the review and approval of the County Environmental Health Services Department and by the County Building Inspection Division. Reclaimed water usage shall be subject to the review and approval of the State Department of Health Services.

302-11.03 Sewer Pumping Stations: Sewer lift stations shall be designed to lift the design peak flow as determined using the criteria in section 302-1 of this manual. The lift station shall be equipped with two pumps, each capable of discharging the peak flow. Multiple pump stations may be considered where dual pump stations are infeasible, subject to the approval of the Director and/or the maintaining District. Said facilities shall be capable of passing the peak flow with any one pump out of service. Final design is subject to the approval of the Director and/or maintaining District.


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Bakersfield, CA 93301-2370

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