EI 11C201
CEMP-E
1 March 1997
pumps, valves and piping materials. Design of these systems will be in accordance with special
criteria developed for the particular situation. Selection of materials for pumps, piping, valves
and controls, etc., will be based on manufacturers' recommendations, product specifications, and
any other appropriate design manuals or applicable criteria.
1-6. LIMITATIONS. To protect sewers, pumping stations and treatment facilities from unwanted
pollutants and extraneous flows that result in excessive operation and maintenance, fire and
explosion hazards, or reduced wastewater treatment efficiencies, limitations must be placed on
the use of the sewer system. Wastewaters from fuel loading and dispensing systems, grease
and oil from vehicle wash racks, aircraft washing and garage or shop floor drains, must be
directed through oil/water separators to prevent such wastes from entering the sewers.
Combined sewers will not be permitted, and collection of storm drainage of any kind must be
avoided. The general guidelines cited in Water Environment Federation (WEF) Manual of
Practice FD 3 for identifying wastes not admissible to sewers will be followed closely. Chapters 3
and 8 of TM 5-814-8 provide a description of the various types of industrial wastes generated at
military installations, and give criteria governing discharge to sanitary or industrial waste sewers.
1-7. ALTERNATIVES TO GRAVITY SYSTEMS. There may be areas in which the topography is
not well suited for construction of a gravity sewer system. In such areas, the installation of a
gravity system would require deep and expensive trench excavation, jacking, boring, tunneling, or
construction of long sewer lines to avoid unfavorable terrain. In cases like these, the existing
topography and subsurface conditions at the site will be studied to determine if an alternative
system would be more feasible. Depths of gravity sewers greater than 15 to 20 feet are usually
uneconomical.
a. Wastewater pumping. The operation and maintenance costs of a pumping station with a
forcemain, when capitalized, may offset or exceed the construction costs of a deep gravity sewer
system. When it is not readily apparent which solution would be more economical, the decision
to use one or the other will be based on a life cycle cost analysis. Initial capital and construction
costs for pumps, ejectors, structures, force mains, etc., plus operation and maintenance costs,
will be compared with the costs of deep trench excavation, or other special construction methods
required for a gravity system. Generally, a gravity sewer system will be justified until its cost
exceeds the cost of a pumped system by 10 percent. TM 5-814-8 contains criteria for economic
evaluation of wastewater pumping.
b. Low Pressure Systems. Some areas under consideration may be further limited by high
groundwater, unstable soil, shallow rock, or extremely adverse topography, and neither gravity
sewers nor pump or ejector stations will be suitable. To overcome these difficulties, low
pressure systems using grinder pumps with small diameter (less than 100 mm (4-inch)) pressure
sewers may be utilized. Low pressure systems are also used with flat topography where low
flows are anticipated. In a typical installation, wastewater from individual buildings will be
discharged to a holding tank, and then periodically transferred by a grinder pump station through
small diameter pipe, into either a central pressure main, conventional gravity sewer, pumping
station, or wastewater treatment facility. Grinder pump stations, which are most common in
housing units are package units, consist of single or duplex pumps in small underground
fiberglass reinforced polyester or concrete tanks with controls mounted above the tank or on the
side of the building served. The pumps cut and shred the waste before pumping it and create
only enough head to empty the tank - not the forcemain. Normally a system must consist of
several grinder pump stations before sufficient pressure is generated to pump waste to its
1-2
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