There is hereby adopted the following exhibit A requiring engineering design reports for each project, which shall describe existing conditions and problems, and describe the proposed solution, which shall be followed by all persons seeking to connect to the sanitary sewer system of the city:
Exhibit A, "Sewer Facilities Construction Standards", prepared for the city by Molzen-Corbin Consulting Engineers.
SECTION 109
SANITARY SEWER DESIGN CRITERIA
This section presents the criteria, standards, and regulations related to the design of sanitary sewer systems for general development service. It does not cover the criteria for design of major interceptor sewers or treatment facilities. The material is directed to the competent design professional and is not intended to be a detailed design handbook. Criteria and standards presented are those determined to be the minimum acceptable values necessary to result in system designs having satisfactory function characteristics, durability, and operational suitability. It is expected that the designer will strive for the best design to suit the circumstances involved, and that designs will reflect sound professional judgment at all times.
109.1 Governing Regulations
Ordinances and policies related to the design and operation of sanitary sewer systems include the following:
1. Subdivision Ordinance
2. Sewer Use Ordinance (Ordinance 85-14)
3. Sewer and Utility Expansion Charge Rates
109.2 Areawide Wastewater Collection and Treatment Facilities
109.2.1 Plan
109.2.1.1
Engineering Design Criteria
Design Capacity Criteria
Off-site flows will be considered by the designer and shown in
this design calculation.
109.2.1.2 Sanitary sewer design shall conform in size, location, and arrangement to adopted plans. Governing plans may include, but are not limited to, the approved area plan and City Master Plan. The City should be consulted for information regarding applicable plans for areas under design consideration.
109.2.1.3 In residential areas, the following design flows will be used per house, apartment, townhouse, or mobile home:
Average Peak
Houses 380 gpd 855 gpd
Apartments, mobile 290 gpd 650 gpd homes, townhouses
109.2.1.4 Commercial sewage flows will be considered on a case-bycase basis by the designer. The designer will present the basis for the commercial sewage flow used in his design calculations.
109.2.1.5 Design flow shall be 1.2 x peak flow.
109.2.1.6 Design is for full pipe flow at the design flow.
109.2.1.7 Manning's Formula is to be used for determination of pipe flow velocities and capacities using a value for Manning's "n" = 0.013.
a. Peak velocity - Velocity at peak flow conditions.
b. Average velocity - Velocity at average flow conditions.
109.2.1.8 Minimum velocity in sewer shall be 2.0 fps. Maximum velocity shall be 10 fps.
109.2.1.9 Minimum collector size allowable is 8 inches.
109.2.2 Manhole Criteria. (See Standard Details 5200-109.22SS-A, B, and C)
109.2.2.1 Manholes must generally be located on the center line of street right of way or of street width if the street is not concentric with the right of way. Manholes for straight lines in curved streets may be located as much as 5 feet off from center line of street or right of way; however, required clearances from other utilities must be maintained. The offset of such manholes is to be dimensioned from center of manhole barrel to the center line of the street or right of way.
109.2.2.2 Standard minimum manhole depth is 6.0 feet, measured from rim to invert, unless otherwise approved by City.
109.2.2.3 The required inside diameter for a manhole is determined as follows:
a. Minimum inside diameter is 4.0 feet.
b. 6.0 feet minimum inside diameter manholes to be used where indicated on the improvement plans.
109.2.2.4 Flow will not be permitted to change in direction horizontally by more than 90° in a manhole.
109.2.2.5 The minimum drop through a manhole will be 0.10 feet except in the case of a single line straight through manhole in which case the normal slope of the line shall be maintained. Where flows converge at a manhole, the inverts should be designed to produce a smooth water surface at design flow with no backwater conditions in any of the incoming lines.
109.2.2.6 The maximum distance between manholes allowed is 350 feet.
109.2.2.7 Cleanouts will not be allowed in lieu of manholes on the sewer system unless approved by the City.
109.2.3 Line Criteria
109.2.3.1 Sanitary sewer materials and installation methods must comply with the requirements set forth in the New Mexico Standard Specifications for Public Works Construction (as modified herein) and Standard Details.
109.2.3.2 Minimum line size allowed: 8 inch inside diameter.
109.2.3.3 Curvilinear sewer lines will not be acceptable.
109.2.3.4 Sections of line that are flat relative to the upstream line are to be avoided. As much as possible, continuous flow velocity and capacity will be provided. The energy gradient should slope generally parallel to the slope of the invert with no abrupt changes or slopes opposite to the direction of flow.
109.2.3.5 Line depth should be sufficient to provide gravity service to property contiguous to the line. Additional depth may be required to provide service. Generally, house services shall be a minimum of 2 feet below finished floor elevation, measured at a point on the finished floor edge farthest from the sewer main.
109.2.3.6 The main lines are to be located within public right of way except as noted in Section 109.2.3.7 following and are to be aligned in accordance with the Primary Utility locations, Exhibit 109-1. Where the Primary Utility locations do not apply, the following criteria apply:
EXHIBIT 109-1
a. The New Mexico Department of Public Health policy on the proximity of water and sewer lines: "whenever possible, it is desirable to lay parallel water and sewer lines at least 10 feet apart horizontally, and the water line should be a higher elevation than the sewer. If this is not possible, separate trenches will be required in all cases (this shall be effective even though one line has been installed prior to the other), and the water line shall be at least 2 feet above the sewer. When water and sewer lines cross each other, the water line shall be at least 3 feet above the sewer; otherwise, the sewer shall be of ductile iron pipe". A preferred alternative is encasement in concrete for 10 feet on each side of the water line. If ductile iron pipe is used, it must be run from manhole to manhole. See Standard Details S-200-109.236SS and S-200-109.323SS.
b. Main lines must be located so they can be maintained without disturbing any sidewalk, curb, gutter, or any other utility. The required trench must be totally within the paved roadway.
c. Written approval of the City must be obtained for any deviations from the Primary Utility locations.
109.2.3.7 Sanitary sewer main lines may be located outside public right of way only under the following conditions:
a. Prior written approval is given by the City.
b. The main line must be located as follows:
(1) In a paved, permanent access easement, or
(2) In a planned green space with access suitable for sewer line maintenance equipment.
(3) If (1) and/or (2) above are impossible due to prior platting, the situation will be handled as a special case.
c. A permanent easement will be granted for exclusive use of water and sanitary sewer. It must be possible to excavate any buried water or sanitary sewer with 1:1 side slopes from the bottom of the pipe, without disturbing any sidewalk, curb and gutter, or any other utility. The required trench must be totally within the paved roadway and the exclusive underground easement. A minimum width easement of 20 feet is required for a single utility and 25 feet for water and sewer.
d. Compliance with the New Mexico Department of Public Health policy on the proximity of water and sewer lines must be achieved.
e. In private streets, Primary Utility locations apply where possible.
109.2.4 Trenching and Backfillinq. Trenching shall be performed in accordance with improvement plans and specifications. Trenching shall be of sufficient depth and width to properly install the pipe. Padding or bedding material to be placed around pipe shall be a finely graded material free of rocks in excess of one inch in diameter and shall be free of any sharp objects and deleterious material. Bedding material shall be compacted around the pipe and at least 6 inches above the pipe. The remainder of the backfill material shall be select material compacted to a density of at least 90% of the maximum density as determined by ASTM D1557. When located under roads, sidewalks, etc., the top 6 inches of ditch backfill shall be compacted to a minimum density of 95% of maximum density as determined by ASTM D1157. The above trenching and backfilling methods shall apply to all utility lines (sewer, water, gas, etc.) installed in the subdivision. See Standard Drawing S200-109.323SS.
109.2.5 Service Connections (private collection systems and individual service connections).
109.2.5.1 Four inch (4") minimum size service connections must be made to the main line (Standard Detail S200-109.251SS) except at the end of cul-de-sacs where connection to a manhole is permitted in the manner shown in Standard Detail S200-109.25SS.
109.2.5.2 Six inch (6") service connections are permitted where a 6-inch tee in the main exists.
109.2.5.3 Four inch (4") mechanical taps are permitted to tappable main lines 8 inches and larger. Mechanical taps must conform to the requirements of the New Mexico Plumbing Code, latest edition.
109.2.5.4 All service connections shall have a minimum slope of 1/4 inch per foot toward the main within the public right of way.
109.2.6 Sewage Lift Stations
109.2.6.1 Sewage lift stations shall be set pit type, with submersible, easily-removable sewage pumps.
109.2.6.2 The lift station shall have 100% standby capability with one pump out of service at peak flow.
109.2.6.3 Sized for a maximum of four (4) pump starts per hour.
109.2.6.4 In order to provide standardization for maintenance and repair parts, pumps shall be Flygt brand.
109.2.6.5 Lift station controls shall be supplied by pump manufacturer.
109.3 Construction Criteria
109.3.1 General. Any special crossing permits or easements required for installation of sewer lines or appurtenances are to be obtained by the developer in the name of the City at the developer's expense.
109.3.2 Materials
109.3.2.1 All construction shall conform to the Standard Details and the Subdivision Regulations unless otherwise approved by the City.
109.3.2.2 All construction will conform to the New Mexico Standard Specifications for Public Works Construction as modified herein, unless otherwise approved by the City.
109.3.2.3 Sewer Lines
a. Materials
(1) DIP - ASTM/ANSI A746, push on joints.
(2) PVC - Gravity sewer lines 18 inches and larger, ASTM F697 and/or UNI-89, 15 inches and smaller, ASTM D3034, SDR 35.
(3) Force Mains
(a) PVC - AWWA C900 pressure Class 150 minimum.
(b) DIP - AWWA C151
b. Installation
(1) Trenching and backfilling per pipe manufacturer's instruction/recommendations, or Standard Detail S-200-109.323SS.
(2) Testing - Compaction testing required every 500 feet. Backfill in road will require compaction testing every 200 feet.
109.3.2.4 Manholes
a. Materials - See Standard Details.
b. Installation - See Standard Details.
109.3.2.5 Lift Stations
a. Type - Wet pit manhole type with hinged cover and submersible pumps on slide rail for easy removal. Sewage piping in lift station to be ductile iron.
b. Installation - Shutoff valves and check valves to be external to the lift station.
109.2.3.6 Service Connections
a. Types - In-line service tees or wyes to be provided on new sewer lines.
b. Installation - See Standard Details.
109.2.3.7 Testing of Service Lines. Refer to New Mexico Standard Specifications for Construction of Public Works.
109.2.3.8 In case of conflict or discrepancy between these regulations, Standard Details, and NMSSPWC, these documents will govern in the following order of priority:
1. Subdivision Regulations
2. Standard Details
3. NMSSPWC
(Ord. 1987-05, 5-19-1987; amd. 2000 Code)