| (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.
(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
(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.
(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
(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.
(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
(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
(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
(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
(C) the installation of air relief valves;
(D) the provision of means to flush all lines in the
(E) the installation of clean-outs; and