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RULE §317.2Sewage Collection System

  (9) Active geologic faults. For systems to be located in areas of known active geologic faults, the design engineer shall locate any faults within the area of the collection system and the system shall be laid out to minimize the number of sewers crossing faults. Where crossings are unavoidable, the engineering report shall specify design features to protect the integrity of the sewer. Consideration should be given to joints providing maximum deflection and to providing manholes on each side of the fault so that a portable pump may be used in the event of sewer failures. Service connections within 50 feet of an active fault should be avoided.

  (10) Erosion control. Erosion or sedimentation control that minimizes the effects of runoff shall be provided during the construction phase of a project. This requirement will be reviewed on a case-by-case basis.

(b) Capacities.

  (1) Sources. The peak flow of domestic sewage, peak flow of waste from industrial plants, and maximum infiltration rates shall be considered in determining the hydraulic capacity of sanitary sewers.

  (2) Existing systems. The design of extensions to sanitary sewers should be based on the data from the existing system. If this is not possible, the design shall be based on data from similar systems or paragraph (3) of this subsection, new systems.

  (3) New systems. New sewers shall be sized using an appropriate engineering analysis of existing and future flow data. The executive director shall have the authority to determine the reliability and appropriateness of the data utilized for sizing the system. In the absence of local reliable flow data and engineering analysis, new sewer systems shall be designed on the basis of an estimated daily sewage flow contribution as shown in the table in §317.4(a) of this title (relating to Wastewater Treatment Facilities). Minor sewers shall be designed such that when flowing full they will transport wastewater at a rate approximately four times the system design daily average flow. Main trunk, interceptor, and outfall sewers shall be designed to convey the contributed minor sewer flows.

(c) Design details.

  (1) Minimum size. No sewer other than service laterals and force mains shall be less than six inches in diameter.

  (2) Slope. All sewers shall be designed and constructed with slopes sufficient to give a velocity when flowing full of not less than 2.0 feet per second. The grades shown in the following table are based on Manning's formula with an assumed "n factor" of 0.013 and constitute minimum acceptable slopes. The minimum acceptable "n" for design and construction shall be 0.013. The "n" used takes into consideration the slime, grit, and grease layers that will affect hydraulics or hinder flow as the pipe matures.

Attached Graphic

  (3) High velocity protection. Where velocities greater than ten feet per second will occur when the pipe is flowing full, at slopes greater than those listed in paragraph (2) of this subsection, special provisions shall be made to protect against pipe displacement by erosion of the bedding and/or shock.

  (4) Alignment. Sewers shall be laid in straight alignment with uniform grade between manholes unless slight deviations from straight alignment and uniform grade are justified to the satisfaction of the executive director.

  (5) Manhole use. Manholes shall be placed at all points of change in alignment, grade, or size of sewer, at the intersection of all sewers and the end of all sewer lines that will be extended at a future date. Any proposal which deviates from this requirement shall be justified to the satisfaction of the executive director. Clean-outs with watertight plugs may be installed in lieu of manholes at the end of sewers which are not anticipated to be extended. Such installations must pass a leakage test and a deflection test for all flexible lines.

    (A) Type. Manholes shall be monolithic, cast-in-place concrete, fiberglass, precast concrete, high-density polyethylene (HDPE), or of equivalent construction. Brick manholes shall not be used, nor shall brick be used to adjust manhole covers to grade.

    (B) Spacing. The maximum required manhole spacing for sewers with straight alignment and uniform grades are in the following table. Reduced manhole spacing may be necessary depending on the utility's ability to maintain its sewer lines. Areas subject to flooding require special consideration to minimize inflow.

Attached Graphic

    (C) Inflow and infiltration control. Watertight, size-on-size resilient connectors allowing for differential settlement shall be used to connect pipe to manholes. Pipe to manhole connectors shall conform to ASTM C-923. Other types of connectors may be used when approved by the commission. Manholes should not allow surface water to drain into them. If manholes are located within the 100-year flood plain, the manhole covers shall have gaskets and be bolted or have another means of preventing inflow. Where gasketed manhole covers are required for more than three manholes in sequence, an alternate means of venting shall be provided at less than 1,500 foot intervals. Vents should be designed to minimize inflow. Impervious material should be utilized for manhole construction in these areas in order to minimize infiltration.

    (D) Manhole diameter. Manholes shall be of sufficient inside diameters to allow personnel to work within them and to allow proper joining of the sewer pipes in the manhole wall. The inside diameter of manholes shall be not less than 48 inches.

    (E) Manhole inverts. The bottom of the manhole shall be provided with a "U" shaped channel that is as much as possible a smooth continuation of the inlet and outlet pipes. For manholes connected to pipes less than 15 inches in diameter the channel depth shall be at least half the largest pipe diameter. For manholes connected to pipes 15 to 24 inches in diameter the channel depth shall be at least three-fourths the largest pipe diameter. For manholes connected to pipes greater than 24 inches in diameter the channel depth shall be at least equal to the largest pipe diameter. In manholes with pipes of different sizes, the tops of the pipes shall be placed at the same elevation and flow channels in the invert sloped on an even slope from pipe to pipe. The bench provided above the channel shall be sloped at a minimum of 0.5 inch per foot. Where sewer lines enter the manhole higher than 24 inches above the manhole invert, the invert shall be filleted to prevent solids deposition. A drop pipe should be provided for a sewer entering a manhole more than 30 inches above the invert.

    (F) Manhole covers. Manhole covers of nominal 24-inch or larger diameter are to be used for all sewer manholes.

    (G) Manhole access. Design of features for entering manholes shall be guided by the following criteria.

      (i) It is suggested that entrance into manholes in excess of four feet deep be accomplished by means of a portable ladder. Other designs for ingress and egress should be given careful evaluation considering the safety hazards associated with the use of manhole steps under certain conditions.

      (ii) Where steps are used, they shall be made of a noncorrosive material and be in accordance with applicable OSHA specifications as published by the United States Department of Labor.

    (H) Testing. Manholes shall be tested for leakage separately and independently of the wastewater lines by hydrostatic exfiltration testing, vacuum testing, or other methods acceptable to the commission. If a manhole fails a leakage test, the manhole must be made watertight and retested. The maximum leakage for hydrostatic testing shall be 0.025 gallons per foot diameter per foot of manhole depth per hour. Alternative test methods must ensure compliance with the above allowable leakage. Hydrostatic exfiltration testing shall be performed as follows: all wastewater lines coming into the manhole shall be sealed with an internal pipe plug, then the manhole shall be filled with water and maintained full for at least one hour. For concrete manholes a wetting period of 24 hours may be used prior to testing in order to allow saturation of the concrete.

  (6) Sag pipes (inverted siphons). Sag pipes shall have two or more barrels, a minimum pipe diameter of six inches and shall be provided with necessary appurtenances for convenient flushing and maintenance. The manholes shall have adequate clearances for rodding, and in general, sufficient head shall be provided and pipe sizes selected to assure velocities of at least three feet per second at design flows. The inlet and outlet details shall be arranged so that the normal flow is diverted to one barrel. Provisions shall be made such that either barrel may be taken out of service for cleaning.

(d) Alternative wastewater collection systems. Use of alternative wastewater collection systems may be considered when justified by unusual terrain or geological formations, low population density, difficult construction, or other circumstances where an alternative wastewater collection system would offer an advantage over a conventional gravity system. An alternative wastewater collection system will be considered for approval only when conditions make a conventional gravity collection system impractical. Alternative wastewater collection system types include pressure sewers (septic tank effluent pumping or grinder pump systems), small diameter gravity sewers (minimum grade effluent sewers or variable grade effluent sewers), vacuum sewers, and combinations thereof. Alternative wastewater collection systems are comprised of both on-site (interceptor tanks, pumps, pump tanks, valves, service laterals) and off-site components (collector mains, force mains, vacuum stations, clean-outs, manholes, vents, and lift stations). Pressure sewer systems, small diameter gravity sewers, and vacuum sewers will be approved on a case-by-case basis. The engineering report must justify the design of alternative wastewater collection systems to the satisfaction of the executive director. The EPA's Manual of Alternative Wastewater Collection Systems (EPA/625/1-91/024), the WEF's Alternative Sewer Systems (MOP FD-12), or other appropriate engineering literature should be used as the basis for design.

  (1) Management. A responsible management structure under the regulatory jurisdiction of the Texas Commission on Environmental Quality shall be established, to the satisfaction of the executive director, to be in charge of the operation and maintenance of an alternative wastewater collection system. A legally binding service agreement shall be required to insure the alternative wastewater collection system is properly constructed and maintained. The required elements of the service agreement are as follows.

    (A) The document must be legally binding.

    (B) Existing septic and pump tanks that are to be used as interceptor tanks for primary treatment, wastewater storage, or pump tanks prior to the discharge into an alternative sewer system must be cleaned, inspected, repaired, modified, or replaced if necessary, to minimize inflow and infiltration into the collection system prior to connection.

    (C) The utility shall have approval authority for the design of the system including all materials and equipment prior to the installation of an interceptor tank, pressure sewer pump tank, or vacuum system appurtenances. The materials shall comply with standard specifications submitted to and approved by the executive director.

    (D) The utility must be able to approve the installation of the interceptor tank, pressure sewer pump tank, or vacuum system appurtenances after construction to ensure the installation was as specified.

    (E) The utility must be responsible for the operation and maintenance of the system including any interceptor tank, pressure sewer pump tank, or vacuum system appurtenances incorporated.

    (F) The utility must be able to stop any discharges from any collection system appurtenances in order to prevent contamination of state waters.

    (G) The utility shall submit a maintenance schedule to the executive director which outlines routine service inspections and maintenance for all types of pressure sewers, small diameter gravity sewers, and vacuum sewer system components.

    (H) Pumping units, grinder pumps, vacuum sewer appurtenances, interceptor tanks shall be regarded as integral components of the system and not as a part of the home plumbing.

    (I) Provision to ensure collection system integrity during a power outage (two-year event) shall be incorporated into the design. Power outage duration will be determined as described in §317.3(e)(1) of this title (relating to Lift Stations).

  (2) Pressure sewer system design considerations. The following shall be submitted to and approved by the executive director:

    (A) hydraulic calculations for sizing the pressure sewer pumping system shall be based on providing the firm capacity to pump the expected peak flow. These calculations shall include system and pump curves as described in §317.3(c)(4) of this title, wet well capacity calculations based on minimum cycle times as described in §317.3(b)(4)(B) of this title, and emergency and flow equalization storage as necessary. The number of units pumping at any one time may be estimated based on appropriate engineering literature;

    (B) flow velocities in the range of three to five feet per second;

    (C) the installation of air relief valves;

    (D) the provision of means to flush all lines in the system;

    (E) the installation of clean-outs; and


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