| (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;
Cont'd... |
