Division 13-09-002
Sewer System Design
Sections:
13-09-002-0001 Guide for Design
13-09-002-0002 Wastewater Design Flows
13-09-002-0002.1 Peak Flows
13-09-002-0003 Sewer Design Capacities
13-09-002-0004 Minimum Pipe
13-09-002-0005 Velocities of Flow
13-09-002-0006 Alignment
13-09-002-0007 Design and Spacing of Manholes
13-09-002-0008 Design of Deep Sewer Lines
13-09-002-0009 Private Sewer Lines
13-09-002-0010 Sewer Services
13-09-002-0011 Sewer Mains
13-09-002-0012 Private Pressure Sewer Mains and Services
13-09-002-0013 On-Site Disposal Systems
13-09-002-0001 Guide for Design
A. Arizona Revised Statutes establishes the control for the design, construction, and operation of sewage systems and waste treatment works to be with the Arizona Department of Environmental Quality (ADEQ). Engineering bulletins produced by ADEQ are to serve as guides for designs of sewage systems. These include, but are not limited to:
1. Bulletin No. 11 - Minimum Requirements for Design, Submission of Plans and Specification of Sewage Works.
2. Bulletin No. 12 - The Septic Tank.
3. Bulletin No. 15 - Package Aeration Plants.
B. More restrictive criteria of design stated in these regulations shall take precedence over ADEQ criteria.
C. Proposed public sewer systems must be gravity flow. No public sewer lift stations will be permitted within the City system. (Ord. 2017-22, Rep&ReEn, 07/05/2017)
13-09-002-0002 Wastewater Design Flows
A. Design flows utilized in the preparation of engineering design reports, plans and specifications shall conform to the criteria set forth in this section as a minimum.
B. Average daily flow estimates based on land use, shall conform to Table 13-09-002-01, Average Daily Flows by Land Use. Where the project land use does not fit within the tabulated categories, an average daily unit flow of one hundred (100) gallons per person per day shall be used.
|
LAND USE |
AVERAGE DAILY FLOW |
|
|---|---|---|
|
Type |
|
Average dry weather flow (ADWF) |
|
Residential Single-Family, Townhouses |
Use 3.5 persons per dwelling unit |
75 gallons per capita per day (gpcd) |
|
Residential Manufactured Homes, Mobile Homes |
Use 3.0 persons per dwelling unit |
75 gallons per capita per day (gpcd) |
|
Residential Condos, Apartments |
Use 2.5 persons per dwelling per unit |
75 gallons per capita per day (gpcd) |
|
Hotel, Motel Tourist |
Use 2.0 persons per hotel/motel room |
75 gallons per capita per day (gpcd) |
|
Commercial |
Use number of acres |
1,000 gallons per acre per day (gad) 3,000 gad PEAK |
|
Industrial |
Domestic flows only |
1,000 gallons per acre per day (gad) 3,000 gad PEAK |
|
Schools, Colleges |
Use enrollment per building |
90 gallons per capita per day (gpcd) |
|
Factors are based on average dry weather flow (ADWF) from City of Flagstaff "Wastewater Management System Facility Plan," prepared by Brown and Caldwell Engineers. |
||
(Ord. 2017-22, Rep&ReEn, 07/05/2017)
13-09-002-0002.1 Peak Flows
All gravity sewer mains shall be designed for peak flow conditions. Peak flow is calculated as the product of the peaking factor and the average daily flow. The peaking factor should be obtained from Figure 13-09-002-01, Ratio of Peak Flow to Dry Weather Flow, based on population.
Figure 13-09-002-01
(Ord. 2017-22, Rep&ReEn, 07/05/2017)
13-09-002-0003 Sewer Design Capacities
Sewer systems, trunk lines, and out-fall lines are to be designed to service the ultimate density of the drainage area. Capacities of lines are to be determined for an entire drainage area, developed or undeveloped, which may be reasonably serviced by the proposed system or by future extensions of the system. Densities will be estimated from the land use plan of the current Regional Land Use and Transportation Plan. Use Table 13-09-002-01, to determine number of persons per unit for different dwelling types. All sewer lines must be designed for peak flow in accordance with Table 13-09-002-01 and Figure 13-09-002-01. The maximum ratio of the depth of flow to the diameter of the pipe, (d/D), shall be seventy percent (70%). (Ord. 2017-22, Rep&ReEn, 07/05/2017)
13-09-002-0004 Minimum Pipe
Gravity sewer lines shall be sized to accommodate the peak design flow subject to the following limitations:
A. The d/D ratio for gravity sewer pipes shall be no greater than seven-tenths (0.7) at the peak flow condition.
B. Minimum pipe size shall be eight (8) inches in diameter.
C. A maximum of one hundred twenty (120) acres of combined commercial and residential development shall drain into an eight (8) inch diameter line. (Ord. 2017-22, Rep&ReEn, 07/05/2017)
13-09-002-0005 Velocities of Flow
A. Velocities in sewer lines shall be determined for design capacities using Mannings formula V = (u/n) (R)2/3 (S)1/2 where V is the average velocity (fps, m/s), u is 1.49 for English units and one (1) for Metric units, n is the Mannings roughness coefficient (as appropriate for the pipe to be used), R is the hydraulic radius (ft), S is slope in (ft/ft).
B. Design velocities are to be within the range of two (2) fps to ten (10) fps flowing full.
1. A minimum velocity of two and one-half (2.5) fps is recommended in order to prevent deposition. The following chart is a guideline for the limits of slope for smaller diameters based on velocity and the n value for the pipe flowing full condition. Capacity must also be considered.
|
Pipe Size (inches) |
Min. Slope (%) 2 fps** |
Max. Slope (%) 10 fps* |
||
|---|---|---|---|---|
|
|
n = 0.013 |
n = 0.010 |
n = 0.013 |
|
|
8 |
0.34 |
4.91 |
8.29 |
|
|
10 |
0.26 |
3.65 |
6.16 |
|
|
12 |
0.20 |
2.86 |
4.83 |
|
|
15 |
0.15 |
2.12 |
3.59 |
|
* PVC (n = 0.010) DIP (n= 0.013)
** Minimum slopes for PVC shall also be calculated using a coefficient of roughness of 0.013.
(Ord. 2017-22, Rep&ReEn, 07/05/2017); Ord. 2024-13, Amended, 4/16/2024 (Res. 2024-15)
13-09-002-0006 Alignment
Sewers shall be laid with straight alignments between manholes. Curvilinear sewers are not permitted. Straight alignment shall be checked by using a laser beam. Sewers shall be located in street right-of-way (ROW) and shall generally run parallel to property lines or street centerline and located out of the wheel path (preferably in the center of the street). Gravity sewer alignments shall be located as set forth in the latest edition of standard utility locations for the City of Flagstaff Engineering Details. (Ord. 2017-22, Rep&ReEn, 07/05/2017)
13-09-002-0007 Design and Spacing of Manholes
A. Manholes are to be installed at the end of each line; at all changes in grade, size, horizontal or vertical alignment; at all intersections of mains and service connections greater than six (6) inches in diameter; and at distances not greater than four hundred (400) feet for sewers twelve (12) inches or less, and five hundred (500) feet for sewers greater than twelve (12) inches.
|
PIPE SIZE (INCHES) |
MAXIMUM MANHOLE SPACING (FEET) |
|---|---|
|
Less than 12" |
400 |
|
Greater than 12" |
500 |
B. Five (5) foot diameter manholes are required wherever the sewer main diameter is twelve (12) inches or greater, whenever there are two (2) or more inlets, whenever the manhole depth is twelve (12) feet or greater or whenever the manhole is designed with a drop sewer connection. Clean-outs are not allowed.
|
PIPE SIZE (INCHES) |
MANHOLE DEPTH (FT) |
MANHOLE DIAMETER (INCHES) |
FRAME AND COVER DIAMETER (INCHES) |
|---|---|---|---|
|
Less than 12" |
12 and less |
48 |
24 |
|
Greater than 12" |
Greater than 12 |
60 |
30 |
|
15" and larger |
Any |
60 |
30 |
|
Drop Manholes |
Any |
60 |
30 |
C. A drop manhole is to be used when a sewer enters a manhole two and one-half (2.5) feet or more above the manhole invert in accordance with MAG Detail No. 426.
1. If there is less than two and one-half (2.5) feet of fall, redesign of sewer grades is required to result in a maximum of one-half (0.5) foot above the flow line of the outlet.
2. Sewer grades shall be normally designed to provide one-tenth (0.1) foot fall from the flowline inlet to the flowline outlet within the manhole.
3. When a sewer main joins a ten (10) inch or greater main, the top of each pipe shall match at their intersection of the manhole.
a. The maximum horizontal deflection angle (inlet to outlet) for an eight (8) inch main shall be ninety (90) degrees.
b. For mains ten (10) inches and larger the maximum defection angle shall be sixty (60) degrees.
c. The minimum flow line radius shall be two (2) feet.
D. Concrete caps on manholes located outside roadways or parking lots shall have a continuous No. 3 rebar centered in the cap.
E. One (1) adjustment ring or one (1) row of bricks is required on all manholes. The ring and cover shall not be set directly on the cone.
F. Manhole covers shall have a pickhole and watertight manhole covers shall have a concealed type pickhole for removal of the cover. Bolts on watertight manhole lids shall be stainless steel.
G. Where corrosive conditions due to septicity or other causes are anticipated, consideration shall be given to providing corrosion protection on the interior of the manholes.
H. Manholes shall be pre-cast concrete or poured-in-place concrete type. Manhole lift holes and grade adjustment rings shall be sealed with non-shrinking mortar.
1. Inlet and outlet pipes shall be joined to the manhole with a gasketed, flexible water-tight connection or any water-tight connection arrangement that allows differential settlement of the pipe and manhole wall to occur.
I. Watertight manhole covers shall be used whenever the manhole is located in a floodplain, wash, or other areas known to be subject to stormwater runoff.
J. Locked manhole covers may be required in isolated easement locations or where vandalism is anticipated.
K. When connecting to an existing manhole, coring will not be accepted. The connection shall be made with a new cast-in-place manhole.
L. Manholes should be located outside of sidewalks, bikeways, bike lanes, and FUTS trails when feasible. (Ord. 2017-22, Rep&ReEn, 07/05/2017; Ord. 2024-13, Amended, 4/16/2024 (Res. 2024-15))
13-09-002-0008 Design of Deep Sewer Lines
When the depth of a sewer line exceeds fifteen (15) feet, then: services will not be allowed; the pipe must be class 350 ductile iron or designed to withstand the trench and traffic loads; and, an easement wider than twenty (20) feet may be required. Sewer lines deeper than twenty-five (25) feet will not be allowed. (Ord. 2017-22, Rep&ReEn, 07/05/2017)
13-09-002-0009 Private Sewer Lines
Private sewer mains shall not be allowed. Existing private sewer mains shall be maintained as required and regulated by the Arizona Department of Environmental Quality. (Ord. 2017-22, Rep&ReEn, 07/05/2017)
13-09-002-0010 Sewer Services
A. Sewer services shall be installed perpendicular (not parallel) to the right-of-way or easement, within the right-of-way or easement, and shall not be installed across another parcel or lot except where service is perpendicular to, and entirely within, a public utility or sewer easement. Private easements across separate parcels will not satisfy the requirements of this section. Sewer services are prohibited on sewer transmission mains that are eighteen (18) inches or larger.
B. Sewer service locations shall be located by branding an "S" on the top or face of the curb.
C. Sewer services shall be located a minimum of five (5) feet from the outside of a manhole wall.
D. An approved backwater valve shall be installed when the finished floor elevation of a building is one (1) foot or less above the nearest upstream manhole or clean-out rim elevation. A self-explanatory tabulated numerical listing of the lots requiring backwater valves shall appear on the plans. The backwater valves shall not be installed within a public right-of-way or easement.
E. If sewer services are installed via saddles, the minimum spacing between services shall be five (5) feet, and there shall be one (1) connection per length of VCP and two (2) per length of PVC main.
F. When a sewer service is required to be abandoned, it shall be abandoned at the property line and capped using the appropriate material (PVC, clay, or concrete). (Ord. 2017-22, Rep&ReEn, 07/05/2017; Ord. 2024-13, Amended, 4/16/2024 (Res. 2024-15))
13-09-002-0011 Sewer Mains
A. Where reasonable, sewer mains shall be located under a paved surface. Where this is not possible, the engineer should give consideration to access and maintenance issues. The engineer should examine the possibility of redesigning and the project layout, e.g., roads, building envelopes, and drainage patterns, in order to facilitate access and maintenance to all sewer pipes and appurtenances with the appropriate equipment and vehicles for repairs and/or preventive maintenance operations.
B. Dead end easements shall be avoided unless they are for a sewer which has no potential for future extension.
C. Manholes shall be located in the right-of-way or an easement which provides access for emergency response, repairs, and/or preventative maintenance operations.
D. Sewer lines constructed in washes and floodways shall have their crowns at least two (2) feet below the one hundred (100) year storm scour depth and shall be constructed with DIP. The DIP shall extend a minimum of ten (10) feet each side of the one hundred (100) year storm scouring. (Ord. 2017-22, Rep&ReEn, 07/05/2017)
13-09-002-0012 Private Pressure Sewer Mains and Services
A. All proposed public sewer systems shall be gravity flow. Public pressure sewer systems including piping, lifts, and appurtenances are prohibited. No public sewer lift stations will be permitted within the City system.
B. Private pressure sewer systems, including individual pressure sewer services are not allowed unless approved by the Utilities Division and the City Engineer. Off-site extensions of the public system in order to provide gravity service may be required. Should a private system be allowed, the following criteria shall be addressed prior to plan approval:
1. A provision for continued operation by the appropriate Class or Grade Operator as required in AAC R18-05-114.
2. A provision for scheduled routine operation and maintenance by qualified personnel and an operation and maintenance manual approved by ADEQ.
3. An emergency spill prevention and response plan shall be kept at the site and include provisions for twenty-four (24) hour response and mitigation by qualified personnel.
4. In accordance with AAC R18-9-E301, sewer collection, force mains, and lift stations having the design flow of ten thousand (10,000) gpd or more shall maintain and revise, when needed, an operation and maintenance plan at the operator’s control center (office) and the appropriate field person’s vehicle.
5. When a lift station is installed as an interim condition until the future extension of a gravity main, the developer shall pay to the City Utilities Division the estimated cost of decommissioning and removing the lift station and connecting to the gravity main. (Ord. 2017-22, Rep&ReEn, 07/05/2017)
13-09-002-0013 On-Site Disposal Systems
On-site disposal systems will be permitted only as outlined in AAC R18-9-A309. On-site disposal systems shall be designed to allow immediate future ties to sewer lines when available. Percolation testing and design shall meet County Health Department and ADEQ requirements. (Ord. 2017-22, Rep&ReEn, 07/05/2017)