(A)   Sewerage quantities.
      (1)   The following guidelines shall be used to compute average dry weather daily flow contributing sewage.
         (a)   The quantity of wastewater that originates from a given area or development shall be calculated from Table 23.31.081(1).
         (b)   Table 23.31.081(1) is based on three people per dwelling unit and 83 gals/day/person.
         (c)   Developments that have a substantially different dwelling unit count per acre may calculate the contributing flow instead of using Table 23.31.081(1).
   ADWF = Dwelling units x gals/day/unit
         (d)   High density zones such as PUD or overlay (M or C) zones require specific calculations for each development. Developments obtaining variances from zoning restrictions and regulations, require calculations for determining wastewater flows.
         (e)   Table 23.31.081(1) is based on gross acreage.
         (f)   The average dry weather flow rate for each zoning area shall be the ADWF from Table 23.31.081(1) times the gross acreage.
   ADWF = Average Dry Weather Flow (from Table A) x gross acreage
         (g)   Computed ADWF for multi-family zoning shall be the number of units per acre times 250 times the gross acreage.
   ADWF = Units/Acre x 250 gallons/day/unit x gross acreage
Table 23.31.081(1)
Land Use
Minimum Lot
Zone Name
Flow Cfs/AC
Table 23.31.081(1)
Land Use
Minimum Lot
Zone Name
Flow Cfs/AC
40 AC
Ag. Rangeland
5 AC
Ag. Productive
5 AC
2.5 AC
Rural Transitional
.5 ac /1 AC
Rural Residential
7,200/1 AC (W)12.5 AC
Single Fam. Res.
125 ac/1 AC (vv)/2.5 AC
Multiple Res.
         (h)   Computed ADWF for a hotel/convention center shall be the number of hotel units times 150 gallons/day times the gross acreage.
      (2)   Infiltration. Infiltration allowance is included in the wastewater flows calculated above.
      (3)   Peaking factor. To compute the peak flow from the average flow, the following peaking factor chart shall be used (Figure 23.31.081(1)).
   (B)   Sewer capacities.
      (1)   General.
         (a)   Sewers shall be designed to carry the peak flow rates (PWWF) from all areas tributary to them. Sewers shall be designed for both size and depth to accommodate developments in upstream tributary areas which would logically be served by them. The peak flow rate at any point shall be the average dry weather flow of all tributary areas times the peaking factor (Figure 1) plus infiltration.
         (b)   Zoning shown in this section is that in effect in the San Benito County Zoning Ordinance. Changes under consideration shall be acknowledged in computing flows. Main sewers shall be designed and constructed to transmit the sewage that will result from the ultimate development of the tributary area in the subdivision. Consideration of the type of development anticipated or existing shall be given in arriving at the design flow but in no case shall any sewer mains be smaller than the size required by the zoning in the areas determined by these standards with the sewer mains flowing half full. In no case shall any sewer by less than six inches in actual inside diameter. Sewers larger than ten inches in diameter may be designed for three-fourths flow. In no case shall a smaller pipe be used in any location than that upstream of the location. Sewer mains shall be designed to withstand the vertical loads that will be imposed on them.
      (2)   Hydraulic design.
         (a)   Minimum velocity for any sanitary sewer shall be two feet per second for pipes flowing at design capacity.
         (b)   Design velocities for sanitary sewers shall be computed using Mannings formula with an “n” value of 0.013 flowing full. The hydraulic elements nomograph shall be used to correct to “n” variable with depth of flow (Figure 23.31.081(2)).
(Cfs x 1.7 = cu m/min)
FIGURE 23.31.081(1)
FIGURE 23.31.081(2)
            1.   Because sanitary sewers are to be designed with no head on the inlet, the maximum design discharge shall not exceed the flow at critical slope and velocity. Due to the unstable flow conditions at critical slope and critical velocity, sanitary sewers should not be designed for these conditions, but maximum design discharge shall not exceed flow under these conditions regardless of greater slope.
            2.   Maximum velocities shall not exceed 16 fps.
            3.   Note. Under normal conditions, a gravity sewer shall not be designed to flow under a head.
         (c)   Minimum size. Minimum street sewer size shall be six inches.
         (d)   Minimum sewer slopes.
            1.   Purpose. Minimum slope requirements are necessary to assure self-cleansing and self-oxidizing velocities in order to avoid significant generation of hazardous, odorous and corrosive sulfur compounds.
            2.   Minimum slopes. Slopes of sewers shall equal those needed to assure the required minimum velocity.
            3.   Substandard slopes. Slopes below the minimum slopes may be used to avoid pumping only upon specific approval of the Engineer. The approval should be solicited well in advance of completion of design.
            4.   Pipe for substandard slopes. Pipe in substandard slope areas, and pipe in all areas downstream from substandard slope areas to the point where the peak flow rate is four times that in the section with substandard slope, shall be corrosion-resistant sewer pipe.
   (C)   Sewer location.
      (1)   Roads.
         (a)   The centerline of sewers constructed in county roads shall not be closer than four feet to the centerline of the road or closer than two feet to the curb. In existing roads, the preferred sewer locations shall be off the pavement edge at least two feet. Exceptions to these location requirements may be made only on approval of the Engineer.
         (b)   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 inches.
      (2)   Water-sewer separation. The state’s Department of Health requirements for separation shall be met in locating sewers. A minimum of ten feet horizontal separation between water and sewer lines shall be maintained.
      (3)   Easements. Location of sewer lines in easements shall be kept to a minimum. Whenever possible, sewers shall be placed in the public roadway. Where sewer lines are located within easements, the easements shall be as follows:
         (a)   Granted with final map;
         (b)   Be granted to the entity accepting and maintaining the sewers;
         (c)   Be dedicated to and accepted by San Benito County;
         (d)   The minimum width of any easement for sanitary sewer purposes shall be ten feet. In special cases of terrain, depth of sewer line and the like, the required width may be modified with the approval of the County Engineer; and
         (e)   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 that are subject of any overlapping easement.
      (4)   Future extensions. When an area outside the proposed project can be logically served by future extension of a proposed sewer, the proposed sewer shall extend to the proposed project boundary or to the end of a paved street as determined by the County Engineer in a manner to facilitate the future extension.
      (5)   Alignment. Sewers shall be laid on a straight alignment and grade between manholes except that curved sewers may be used subject to the following requirements:
         (a)   All curve data shall be shown on plans.
         (b)   Minimum radius of curvature and joint deflections shall be as recommended by the pipe manufacturer and approved by the Public Works Department.
         (c)   All deflections shall be at the pipe joints or by specially metered pipe sections. Actual bending of the pipe itself will not be allowed.
         (d)   A manhole shall be constructed at either the B.C. or E.C. of all curves.
         (e)   Vertical curves may be used in combination with horizontal curves. Where vertical curves are used, the sewer shall be sized for the flattest slope within the curve.
         (f)   Maximum combined horizontal and vertical deflection at any joint shall be as recommended by the manufacturers but in no case more than three feet.
         (g)   The arithmetic sum of all horizontal and vertical deflection in curved sewers between adjacent manholes shall not exceed 60 degrees.
   (D)   Depth of sewers.
      (1)   Basic requirements. Sewers shall be installed at a depth that will provide suitable service to the properties connected and will allow subsequent installation of water lines in accordance with the water/sewer separation rules so as to require a minimum of special construction of the water lines other than joint spacing.
      (2)   Standard depths. Minimum cover for any sewer main shall be four feet within the street section, and three feet in sewer easements. Any deviation from this cover shall require special design and approval by the County Engineer.
      (3)   Exceptions.
         (a)   Special pipe cover shall be used when total cover over public and house connection sewers is less than four feet, and may be required when total cover will not be placed immediately after pipe installations, or when other special conditions exist.
            1.   Two to four feet of cover shall require the use of ductile iron pipe or other approved bedding or encasement.
            2.   Less than two feet of cover shall require encasement. In no case will cover less than one foot be permitted.
         (b)   All pipelines shall be designed and constructed to a safety factor of one and one-half. The Engineer, during design, shall consider impact and dead loads imposed upon the pipe both during construction and after construction. The construction plans shall show any required special pipe-zone considerations.
   (E)   Structures.
      (1)   Manholes.
         (a)   Locations.
            1.   Manholes shall be located at all abrupt changes in alignment or grade and at all junctions.
            2.   Manholes shall be located at least every 400 feet along lines smaller than 24 inches in diameter and larger will usually be at 400 feet but may be extended subject to the approval of the Engineer.
         (b)   Drop manholes. Drop manholes shall be constructed in accordance with standard drawings and shall be used wherever sewers enter manholes at more than 24 inches above the outlet elevation of the manhole. Vertical curves may be used to eliminate drop manholes with approval of the Engineer.
         (c)   Design.
            1.   Manholes shall be constructed in accordance with standard drawings. Manhole designs, which in the opinion of the Engineer, provide access to the mains, a stable working platform, and freedom from splash and turbulence, equivalent to or better than the design shown herein, will be approved.
            2.   Grades of the channels within manholes shall be as follows (except where the sewerage agency has different requirements):
               a.   When the main channel flows straight through the manholes, carry the upstream and downstream pipe grades to the center of the manhole.
               b.   When the main channel curves through the manhole, in addition to the pipe grades, provide additional fall in the invert equal to the loss in energy head to be expected.
            3.   For side inlets to manholes where the pipe size is less than the main line, align the pipe soffit of the side inlet with that of the main inlet.
            4.   For side inlets to manholes when the pipe size is the same as the main, provide a drop of 0.1 feet between the inlet invert and the invert of the outlet of the main line.
         (d)   Protection. Where new proposed sewers are to be connected into a manhole in active use, the designer shall call for such protection as is necessary to prevent construction debris from being washed into the active sewers. Plugged inlets or other suitable protection shall be called for in the active manhole before beginning manhole modifications or proposed sewer cleaning.
         (e)   Elevations. Finished elevations of frames and covers shall be set in relation to finished grades of the completed road surface or finished surrounding grade when outside of roads.
      (2)   Cleanouts.
         (a)   Requirements. Dead end sewers not over 200 feet in length shall terminate in standard manholes or cleanouts. Dead ends over 200 feet long shall terminate in standard manholes unless future extension of the dead end will include a manhole within 400 feet of the upper-most manhole, in which case a temporary cleanout is permitted. Where dead ends are on a slope of 0.01 or greater, the length for use of a cleanout may be extended to 300 feet.
         (b)   Location. Unless greater length is necessary to serve the property, end structures shall be placed ten feet beyond the down grade property line on the last lot served.
         (c)   Design. Cleanouts shall be constructed generally in accordance with standard drawings.
      (3)   Service laterals.
         (a)   Requirements. Wherever it is known or can be reasonably assumed that a building sewer connection is required, a service lateral shall be shown on the plans and installed to the property line as a part of the street sewer construction, prior to paving. Service laterals shall be installed whenever possible during construction of the sewer main using prefabricated fittings.
         (b)   Size. Service laterals for single dwellings and small single stores or offices shall be four inches or larger providing the Plumbing Code does not require the building sewer to be larger than four inches. All other service laterals shall be six inches or larger and at least equal to the size of the building sewer.
         (c)   Depth. Service laterals shall be at the minimum depths herein provided and in addition such depth shall be sufficient to provide a connection to any point on the lot within the established building setback lines (eluding any area steeper than five horizontally to one vertically) with a cover of one foot and a slope of not less than 0.02. Any exception to this requirement must have approval by the County Engineer.
         (d)   Design. Service laterals shall be constructed in conformity with standard drawings.
         (e)   Future connections. Unused openings shall be tightly sealed and supported in a manner to facilitate their future location and use.
         (f)   Backflow prevention. Sewers shall be designed to preclude the backflow of sewage into laterals. If it is infeasible to install the waste receiving inlets from any building at least one foot higher than the rim elevation of the next upstream manhole, or other structure providing hydraulic relief, then backflow prevention devices shall be provided.
         (g)   Connection to cesspools or septic tanks. Cesspool seepage pits shall not be connected to any public sewer. Septic tanks shall not be connected to any public sewer except where part of an approved septic tank effluent pumping (STEP) design.
         (h)   Curb markings. The location of all sewer service laterals shall be marked by an “S” cast into the concrete of the curb at completion of construction.
      (4)   Structures and pipes.
         (a)   Roads. All structures and pipe placed under public roads shall be of sufficient strength to support with an adequate factor of safety the backfill, road surfacing and HS20 and alternative truck loading with impact.
         (b)   Other pipes and structures. Sewers under other pipes and structures shall be protected from damage and shall be constructed so as not to endanger the other pipe or structure. The concrete blanket method is not required where the distance between the outside of crossing pipes exceeds 12 inches unless required by water-sewer separation rule.
         (c)   Flexible joints. Flexible joints that allow for differential settlements or other movement of sewer pipe, sewer structures, adjacent pipe and adjacent structures shall be provided where sewer lines enter encasements, manholes or other structures. Flexible joints shall be within one foot of the structures and shall be indicated on the plans.
      (5)   Force mains and lift stations.
         (a)   Requirements. All sewage shall reach the system by gravity flow, in a fresh condition susceptible to conventional sewage treatment processes. Where extreme hardship conditions prevail and a substantial area cannot be sewered by gravity sewers in accordance with these requirements, a sewage pumping station may be installed. No pumping facilities shall be incorporated in sewer plans without approval of the Engineer.
         (b)   Lift stations design. Lift stations, where allowed, shall incorporate the following features:
            1.   Pumps or other devices shall be duplex. Minimum desirable size is four inches. Maximum velocity in suction is five feet per second;
            2.   Capacity shall be provided to handle ultimate peak flow from the tributary area with the largest pump out of service. Stage installation of pumps is allowed providing space is provided for future installations;
            3.   Access shall be provided to site for removal and repair of equipment;
            4.   A means of dewatering force mains shall be provided;
            5.   The lift station shall not be in county road right-of-way except with permission from the Engineer;
            6.   Paint all machinery and piping;
            7.   Provide an approved high-water and power failure alarm; and
            8.   County shall be provided with complete manufacturer’s brochures, technical data, O&M manuals and the like for all equipment and controls.
         (c)   Submersible lift station design. Lift stations shall meet the minimum requirements of “SWPA Handbook, Submersible Sewage Pumping Systems” by Submersible Wastewater Pump Association. Pumps shall be rail mounted with automatic disconnect.
         (d)   Structural. Structures shall be reinforced concrete, fiberglass and epoxy coated steel. In residential areas, structures shall be below ground. Provide a hatch suitable for the removal and replacement of major equipment components. Hatches shall be spring loaded (Bilco type) with metal provided in brass, stainless steel or aluminum materials. Where applicable, provide equipment-lifting eyes in roof of station. All structures shall have ample working room around machinery. Minimum clearance to wall shall be 24 inches or greater as needed for adequate maintenance.
         (e)   Mechanical and piping. The design shall satisfy the minimum requirements of the state’s Health Code. A reduced pressure principle device or air gap is required on all domestic water connections. A one-inch metered connection is to be provided adjacent to the station. Provide an automatic air blower system to exchange air every six minutes minimum. Provide an automatic sump pump (slope floor to sump) with check valve in discharge pipe to wet well. Provide standby capacity equal to the largest single unit. Make provisions for standby emergency power connection for use during power outages. Sump pumps and air blowers shall be easily removable for maintenance.
         (f)   Actuation of pumps. Provide switch to alternate lead pump in system. The control system shall be an approved electronic type.
         (g)   Electrical. All electrical installations shall comply with the National Electrical Code and Division of Industrial Safety requirements and County of San Benito Electrical Code. Use enclosed prefabricated electrical panels mounted above ground level outside the pump station. Provide running time meters for all motors (use reset type). Provide explosion-proof electrical appurtenances below ground or an approved type disconnect and time delay. Provide separate blower system within all electrical panels located below ground. Provide an approved type alternator and float switches. A manual switch adjacent to the motor that will override the control panel shall be provided. Switch shall be mounted at motor height. Adequate lighting and electrical outlets shall be provided. Outlets shall be mounted at motor height. Color code all wiring and piping (National Standard Coding).
         (h)   Force main design. Force mains need not comply with the requirements for gravity sewers. Force mains shall be designed in accordance with good engineering practice. Velocity shall be between two and ten feet per second.
      (6)   Treatment and disposal.
         (a)   General.
            1.   Treatment facilities shall include all necessary components and amenities as required by the county to ensure a complete, automated, operating facility that will lead itself to minimum maintenance and operational costs.
            2.   Storage or auxiliary pumping capacity shall be provided as required to prevent overflow during power outages, the duration of which shall be based on power company records and projections for the area in which the facilities will be constructed.
         (b)   Regulatory standards. Pertinent requirements of the following agency standards, including all changes thereto, shall be considered and complied with, except that in the event of conflict, the stricter design criteria shall govern.
            1.   Laws and standards of State of California Department of Public Health;
            2.   The Porter-Cologne Water Quality Control Act and the Regional Water Quality Control Board;
            3.   Ordinances of the County of San Benito; and
            4.   Others as appropriate
         (c)   Design reports. A design for the proposed project to be constructed shall be submitted to and approved by the county. In addition to general requirements of this standard, the following requirements apply:
            1.   An engineering report on the development of the site including but not limited to soils survey, geology, ground and surface water hydrology, water supply, liquid waste disposal, probable population densities, effects of construction and other development activities on the existing environment and conformance with the master regional or county plans.
            2.   A master plan relating to the disposal of wastes anticipated from the ultimate development; the plan shall conform to the regional or county master plan for sewage disposal. Stage development may be acceptable for some components of the system if the stage development can be proved financially feasible and if approved by the county.
            3.   The filing of a report on the proposed waste discharge in the county’s name by the developer, accompanied by the appropriate filing fee and pursuant to Cal. Water Code § 13260 (unless the waste is to be discharged into an existing community sewerage system), to the Central Coast Regional Water Quality Control Board.
         (d)   Access. All weather access shall be provided to all major functional units. A minimum width of 12 feet is required. Turnaround area for the largest service vehicle shall be provided per CALTRANS standards.
         (e)   Structures.
            1.   Shall provide protection against weather and vandalism, shall be designed to blend architecturally with the character of the development, and shall provide maximum efficiency and minimum operational and maintenance costs to the district.
            2.   Laboratory storage and/or warehouse facilities may be required where the new development cannot practically utilize existing county facilities. An “in-lieu” fee will be required where it is practical to expand existing county facilities to accommodate new development.
         (f)   Treatment process. Removal requirements for treatment processes will be as required by the waste discharge requirements issued by the Regional Water Quality Control Board, but in no event shall the five-day biological oxygen demand reduction be less than 85% using sampling procedures reflecting plant flow through time. Process design shall be based on the larger of the design flow rate requirements or the average five-day BOD requirements. Comminution equipment will be required.
         (g)   Outside lighting. All mechanical areas shall be adequately lighted for night operation and maintenance.
         (h)   Metering, recording and monitoring equipment.
            1.   Treatment plants.
               a.   Metering, recording and monitoring equipment shall be provided at all treatment plants. All such equipment shall be compatible with existing county equipment.
               b.   Generally, but not necessarily limited to, requirements are as follows:
                  (i)   Flow meter and recorder;
                  (ii)   Recording, continuous reading chlorine analyzer to detect chlorine residual and equipped with adjustable low and high alarm contacts; and
                  (iii)   Radio monitoring and alarm equipment.
            2.   Pumping stations.
               a.   Flow meter and recorder (when required); and
               b.   Radio monitoring and alarm equipment.
         (i)   Chlorination facilities. Chlorination facilities consist of gas chlorinator(s) and their associated equipment. Chlorinators shall be capable of pacing sewerage flows either by a flow proportional chlorinator or a series of individual chlorinators activated from the pump control panel as approved by the District. Installations shall be provided with a continuous chlorine monitor to detect chlorine residual and shall have adjustable high and low alarm contacts. Final effluent standards shall meet RWQCB standards. Chlorination contact facilities shall be designed to provide not less than 30 minutes contact time at peak discharge rate and shall be arranged to permit bacteriological and chemical sampling of the effluent.
         (j)   Design flow rate.
            1.   Waste stabilization lagoons, aerated lagoons and similar process with long detention times - average dry weather flow (ADWF).
            2.   Extended aeration plants - 50% of peak flow (PF) but not less than the average dry weather flow (ADWF).
         (k)   Effluent disposal.
            1.   Unless a development can be connected to an existing sewage system, all effluent must be disposed of in accordance with the Discharge requirements of the Regional Water Quality Control Board. On-site disposal must be substantiated by soils, geological, hydrological and related studies, and this resultant report submitted to the county.
            2.   Effluent must be contained within the disposal site and metering and recording devices must be installed to measure any runoff from the disposal area.
            3.   Drainage structures must be capable of passing a 100-year event.