§ 700-150. OSS DESIGN AND INSTALLATION CRITERIA

(a)

The detailed design and construction of all OSS shall conform to this [Chapter] and technical standards adopted by the Department. All pressure distribution and alternative systems shall be designed by an authorized professional.

(b)

The OSS shall be designed to receive all sewage from the residence or facility served unless otherwise approved by the LA. The design flow shall be established as follows:

(1)

For individual residences, flows of 130 gallons/day/bedroom shall be used for design purposes; and

(2)

For other facilities, the typical values noted in Small and Decentralized Wastewater Management Systems, Crites & Tchobanoglous (1998), EPA Design Manual, the California Plumbing Code, or other document generally accepted as an industry standard shall be used. Any deviations shall be supported by appropriate water usage information and/or the use of low water use fixtures.

(3)

The minimum design flow for an OSS shall be 130 gallons per day.

(c)

Gravity systems and pressure distribution systems shall have the calculation of drainfield area based upon the design flows in Section 700-150-B and loading rates equal to or less than those in Table IV, Maximum Hydraulic Loading Rate for Residential Sewage, and applied only to the bottom of the excavated trench.

Table IV Maximum Hydraulic Loading Rate For Residential Sewage 1,2
Soil Type	Soil Textural Classification	Percolation Rate (mpi)	Loading Rate (gal./ft. /day)
1	Coarse sands, Medium sands	1-3	1.2
2	Fine sands, Loamy sands	4-10	0.8
3	Sandy loams	11-20	0.7
	Loams	21-30	0.6
4	Silt loams, that are porous and have well-developed structure	31-60	0.5
5	Other Silt loams (weak), Sandy clay loams, Clay loams, Silty clay loams	61-120	0.3
6	Sandy clays & Clays of low clay content (<45%) with moderate or strong structure	121-240	0.2

 

Compacted soils, cemented soils, and/or poor soil structure may require a reduction of the loading rate or make the soil unsuitable for the installation of an on-site sewage system.

The maximum hydraulic loading rate for the soil type listed is to be used for calculating the drainfield area required.

Due to the high prevalence of clayey soils present in Sutter County, types 5 and 6 soils shall be subject to the extended site evaluation requirements of Section 700-110.

(1)

If more than one suitable soil horizon is encountered in the soil profile, drainfield trench sizing shall be based on the most restrictive soil within 24 inches beneath the bottom of the trench. Where particle size analysis has been conducted alternative application rates that are between the two soil classes may be considered when the results of soil testing are bordering the determination line between the soil triangle designations.

(2)

The LA may allow the drainfield area calculated from Table IV to be reduced by a maximum of 20 percent to account for trench sidewall infiltration if at least 12 inches of drain rock is used under the distribution pipe and pressure distribution is utilized.

(d)

Effluent Treatment and Distribution.

(1)

The standard of effluent treatment prior to discharge and/or method of distribution in all cases shall meet or exceed the requirements contained in Table V, Effluent Treatment and Distribution for Soil Types and Vertical Separation.

Table V Effluent Treatment and Distribution for Soil Types and Vertical Separation
Soil Type	Vertical Separation 1,4
	≥1 foot to <2 feet	≥2 feet to <3 feet	≥3 feet
1	Treatment Standard 2	Treatment Standard 2	Treatment Standard 2
2	Treatment Standard 2	Pressure Distribution	Pressure Distribution
3—5	Treatment Standard 2	Gravity Distribution or Pressure Distribution	Gravity Distribution or Pressure Distribution
6	Treatment Standard 2	Treatment Standard 2	Pressure Distribution

 

A minimum effective soil depth of 24 inches is required to utilize an OSS.

Depth to groundwater must be ≥5 feet for gravity distribution.

A mound system shall not be used in Type 6 soils.

A minimum of 24 inches is required to groundwater for all initial systems

(2)

A minimum effective soil depth of 24 inches is required to utilize an on-site sewage system for wastewater treatment and disposal.

(3)

On-site sewage systems requiring more than 500 lineal feet of drainfield trench shall utilize pressure distribution.

(e)

Holding Tanks.

(1)

Persons shall not install or use holding tank sewage systems for residential development whether seasonal or year-round. This prohibition may be waived by the LA:

a.

For interim uses limited to handling emergency situations; and

b.

For repairs as permitted under Section 700-170-A (4).

(2)

A person proposing to use a holding tank sewage system shall:

a.

Secure an approved operational permit from the LA; and

b.

Use a holding tank on the current approved wastewater tank list.

(f)

Septic Tanks:

(1)

Must be on the approved list of watertight septic tanks;

(2)

Shall have the following minimum liquid capacities for a single-family residence:

Number of Bedrooms	Required minimum liquid tank volume (gallons)
≤ 2	1,000
3—4	1,500
5—6	2,000
Each additional bedroom	250 per bedroom

 

(3)

Shall have three times the daily design flow with a minimum of 1,000 gallons for non-residential facilities;

(4)

Shall have clean-out and inspection accesses at or above grade;

(5)

Shall have access risers and covers that are watertight, constructed of a durable material, and secured with a lockable lid or otherwise secured to prevent unauthorized entry;

(6)

Must be designed with protection against floatation and groundwater intrusion in high groundwater areas;

(7)

Must be equipped with a Department approved effluent filter;

(8)

Must be tested in place and demonstrated to be watertight by the commercial installer and/or authorized professional prior to use; and

(9)

In multi-compartment tanks or when two or more tanks are used in series, the primary compartment or tank shall not have a liquid capacity of less than 500 gallons or less than two-thirds of total liquid capacity, whichever is greater.

(g)

Pump Tanks:

(1)

Must be included on the approved list of watertight pump tanks;

(2)

Shall have a liquid capacity of at least two times the daily design flow with a minimum capacity of 500 gallons;

(3)

Shall have cleanout and inspection accesses at or above finished grade;

(4)

Shall have access risers and covers which are watertight, constructed of a durable material, and secured with a lockable lid or otherwise secured to prevent unauthorized entry;

(5)

Must be designed with protection against floatation, ground water intrusion, and surface water inflow; and

(6)

Must be tested in place and demonstrated to be watertight by the commercial installer and/or authorized professional prior to use.

(h)

Location of Septic Tanks and Pump Tanks.

(1)

Septic tanks and pump tanks shall be located in an accessible location for pumping and maintenance. Septic tanks and pump tanks located under paving or in areas subject to vehicular traffic must be reinforced to withstand the additional loading caused by potential vehicular traffic. A California State Professional Engineer shall determine the appropriate specifications for the reinforced tank.

(i)

Building Sewer and Gravity Effluent Pipe.

(1)

Pipe used for the construction of a building sewer line beyond the building plumbing shall be a minimum of three inches inside diameter and of plastic that shall be PVC ASTM D3034, ABS Schedule 40, or the equivalent. Effluent gravity sewer pipe shall be of the same material and size as the building sewer pipe.

(2)

Construction of the building sewer line shall be such as to secure watertight joints and it shall be on a grade of not less than / ₄ inch per foot. Any such pipe or piping four inches or larger in diameter shall have a slope of not less than / ₈ inch per foot.

(3)

Construction of the effluent sewer line shall be such as to secure watertight joints and it shall be on a grade of not less than / ₁₆ inch per foot.

(4)

No tees or ells exceeding 45 degrees shall be permitted in the building sewer line except for plastic long bend 90 elbows or sanitary tees. All aggregate horizontal changes in direction exceeding 135 degrees shall have accessible cleanouts.

(5)

Building sewers of three inch diameter shall have cleanouts installed at intervals of not more than 50 feet and sewers of four inch diameter and larger shall have cleanouts installed at intervals of not more than 100 feet. Where the effluent sewer line exceeds 100 feet in length, a cleanout shall be placed every 100 feet.

(j)

Distribution boxes:

(1)

Shall be required on all conventional gravity systems;

(2)

Shall be constructed and installed to provide equal flow of effluent to all outlets;

(3)

Shall be set on stable soil or otherwise supported to prevent misalignment;

(4)

Shall be durable, watertight, and equipped with an adequate removable cover;

(5)

Shall not be constructed or installed where the invert of the inlet pipe is less than one inch above the level of the invert of the outlet pipes, nor shall the invert of the outlet pipes be less than two inches above the floor of the distribution box; and

(6)

Shall not be installed within three feet of the drainfield trenches.

(k)

Drainfield.

(1)

All drainfields shall be installed or located to comply with the following design criteria:

Trench Design Criteria
Maximum length of each trench	100 feet
Maximum width of trench	36 inches
Minimum width of trench	12 inches
Minimum depth	6 inches
Maximum depth of trench	24 inches
Minimum distance of undisturbed earth between disposal trenches	7 feet

 

Without pressure distribution.

The minimum trench depth shall be 12 inches from original grade if 12 inches of drain rock is used below the distribution pipe.

The bottom of the drainfield shall not be deeper than 24 inches below the finished grade, unless written approval is given by the Health Officer.

The Health Officer may allow a reduction in separation of individual trenches to not less than four feet on repairs or if a 24-inch wide or narrower trench is used.

a.

The length of all drainfield trenches in conventional gravity systems shall be the same length with a maximum variance of 15%;

b.

The grade of the bottom of drainfield trenches and drainfield lines shall be level with a maximum grade of two inches per 100 feet;

c.

The minimum depth of drain rock under drainfield lines shall not be less than six inches;

d.

The amount of drain rock over drainfield lines shall not be less than two inches; and

e.

The drain rock in the drainfield shall terminate at the intersection of the drainfield trench sidewall and the effluent sewer line, and such intersection shall be at least three feet from the distribution box and five feet from the septic tank or pump tank.

(2)

Drainfield trenches shall not be excavated during wet soil conditions to prevent smearing and/or compaction of the soil interface. All smeared or compacted soil surfaces in the trench shall be scarified and the loose material removed.

(3)

All distribution piping for gravity drainfields shall be a minimum three-inch diameter Polyethylene (PE), ABS, or PVC perforated sewer pipe.

(4)

Drain rock:

a.

Shall be one-half to two and one-half inches in diameter, with no less than 100% passing a two-inch sieve by weight and no more than five percent passing a one-half inch sieve by weight; and

b.

Must be durable, clean, washed, non-deteriorating gravel, free of organic materials and fines, and with the percent by weight passing the U.S. No. 200 sieve no greater than 0.5%.

(5)

Drainfield trenches shall have an approved barrier material consisting of untreated building paper (40—60 lbs.), straw (two inches compacted minimum), or a geotextile filter fabric placed between the gravel or gravel substitute and soil cover. This requirement may be waived by the LA when gravelless chambers are used.

(6)

All drainfield trenches shall have a minimum of one observation port for each lateral located at the distal end of the trench.

(l)

Cover of the Drainfield.

(1)

The cover material shall be a loamy material with less than 30% clay content;

(2)

The minimum depth of cover over the drainfield shall not be less than nine inches of cover material;

(3)

The maximum depth of cover over the drainfield shall not exceed 18 inches except by special permission of the LA;

(4)

The soil cover shall extend at least five feet beyond the limits of the drainfield trenches and graded at a maximum slope of 3:1. On sloping sites, a downslope correction factor shall be used to maintain the required maximum slope of 3:1;

(5)

The required grade of the drainfield trenches must be maintained while backfilling; and

(6)

The soil cover shall be graded to prevent ponding, seeded, and covered with an approved erosion control material if necessary.

(Ord. No. 1632, § 1, 3-28-2017)