Low Pressure Sewer System Replaces
Septic System in Lake Community
A. Lee Head, III, P.E. - Alan Plummer Associates, Inc.
Madelene R. Mayhall, E.I.T. - City of Fort Worth, Texas
Alan R. Tucker, P.E. - Alan Plummer Associates, Inc.
Jeffrey E. Caffey - Alan Plummer Associates, Inc.
Abstract
A uniquely challenging design of a low pressure sewer system to replace outdated
septic systems on approximately 100 lake-front properties was completed as a
demonstration project for the City of Fort Worth Water Department with partial
funding from an Environmental Protection Agency Clean Lakes Program grant. The
lots are owned by the City and leased to the home owner who owns all
improvements to the lots. Many of the houses are over 50 years old and have
multiple existing septic and gray water systems. Also, many of the houses do not
meet current electrical or plumbing codes. These conditions required individualized
analysis and design of the grinder pump system and the electrical connections.
Resident input was important in producing a successful project. The Water
Department cultivated new approaches to fee structures, service agreements and
project development in providing sewer service to Lake Worth.
Introduction
The City of Fort Worth Water Department (Water Department) and Alan Plummer
Associates, Inc. (APAI) faced the challenge of designing an innovative community
sewer system around Lake Worth and demonstrating the viability of such a system.
Replacing the septic tanks around the lake with a reliable sewer system was an
integral part of the Lake Worth Clean Lakes Project, which has been partly funded by
a grant from the Environmental Protection Agency (EPA). One of the goals of the
Clean Lakes Project is to find creative and innovative means of protecting and
improving the quality of the water in reservoirs. The Lake Worth Community Sewer
System, which will provide service to approximately 100 lots along the north shore of
the lower Lake Worth, will serve as a demonstration project for the feasibility of
extending sewer service to the remainder of the lake as a creative solution for
protecting the water quality in Lake Worth.
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Since the City of Fort Worth owns the property around the lake and leases the land to
the residents, the Water Department assumed the role of project developer in
providing sewer service. This new role challenged the project team to think creatively
to overcome the hurdles presented to them. Communicating with the residents was
essential in producing a design that would stimulate their participation in the project.
This paper describes the challenges encountered by the project team in selecting the
type of sewer system used, developing the project and designing the grinder pump
system.
Background
The City of Fort Worth has a unique relationship with the residents around Lake
Worth. The lake, which was constructed circa 1914 as the City's first water supply
lake, is located about 8 miles northwest of downtown Fort Worth. Figure 1 is a map of
Lake Worth. Nearly half of the shoreline is residential. The City owns most of the lots
and leases them to the residents. The area has a wide variety of houses. Many
houses were first constructed in the 1930's as summer residences and have been
added to over time. These houses have multiple septic systems, and in some cases
structures have been constructed on top of existing septic tanks. Other houses have
been constructed recently per modern building codes. Although the area is within the
Fort Worth city limits, the residents have been relatively autonomous, relying on
individual wells for water service, and on-site wastewater disposal. These factors
made individual analysis of each lot a necessity in preparing the system design.
In additional to storing drinking water, the lake is a popular recreation area. Several
shallow aquifers are in the lake vicinity. The majority of individual residents use septic
tanks for on-site wastewater disposal. Many of the septic systems were constructed
prior to the 1980's and do not meet the current Texas Natural Resources
Conservation Commission (TNRCC) standards, and many have experienced recent
operational problems. Elimination of the septic tanks will minimize a potential health
problem resulting from contamination of shallow wells and prevent probable pollution
to the lake from occurring. The overall purpose of the Lake Worth Clean Lakes
Project, which was funded through the TNRCC by the U. S. Environmental Protection
Agency (USEPA), was to restore the quality of drinking water in the lake and control
sources of pollution that could affect the water quality. In February 1990, the Water
Department and APAI completed the first phase of the project, which was a
diagnostic/feasibility study that outlined programs to meet the goals of the project.
The recommendations of the feasibility study include the design and construction of a
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Lake Worth Community Sewer System Demonstration Project. In 1993, the Water
Department submitted an application to the USEPA and received funding for the
second phase of the project, the implementation of the diagnostic/feasibility study
recommendations. The Lake Worth Community Sewer System is one of eight tasks
included in the funded program. The sewer system is scheduled to be completed by
September 1, 1997, in order to receive full funding from the grant.
The Lake Worth Community Sewer System is a representative project to
demonstrate the viability of an innovative sewer system as a cost effective means of
serving the remainder of the lake's residential areas. Three primary candidate areas
around the lake were evaluated for feasibility as a demonstration project. In
determining and ranking the three primary candidate areas, consideration was given
to the density of the area, proximity to the lake, proximity to the City's existing
wastewater collection system and the historical operational problems with on-site
wastewater disposal systems. Cahoba Drive was selected as the demonstration site.
There are 100 lots along Cahoba Drive, and all the lots except for three are located
on the lake shoreline.
System Selection
Three different community sewer systems were evaluated based on advantages,
disadvantages, and construction costs. The system alternatives were as follows:

Low pressure small diameter sewer system with effluent pumps or grinder
pumps

Conventional gravity system with effluent pumps or grinder pumps

Vacuum sewer system
Alternative 1 - Low Pressure Small Diameter Sewer System
A low pressure small diameter sewer system consists of a small diameter (minimum
2 inches) force main that is typically installed within the road right-of-way (ROW). The
system requires installing a septic tank effluent pump or a grinder pump at each
residence to convey wastewater to the low pressure sewer system. The opinion of
probable construction cost for a low pressure, small diameter sewer system was
$1,300,000.
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Advantages

pipe sizes from 2 inches to 6 inches

layout independent of topography

reduced excavation compared to gravity

sewer system infiltration greatly reduced

eliminates manholes

requires less power than vacuum system

lower construction costs than other alternatives
Disadvantages

effluent pump or grinder pump needed at each residence

electrical power required

air release valves needed

flushing connections required

periodic pumping of septic tank for effluent pump system

power outage disturbs service
Alternative 2 - Conventional Gravity System With Effluent Pumps Or Grinder
Pumps
This alternative combines a conventional gravity system with an effluent pump or
grinder pump system as discussed above. An effluent pump or grinder pump would
be installed at each residence and discharge through a one and a half inch diameter
force main into a gravity sewer system located in the road ROW. The opinion of
probable construction cost for a gravity sewer system was $2,200,000.
Advantages

lower operational cost for gravity line compared to low pressure system

flushing of system not required
Disadvantages

minimum pipe size of 8 inches

deep manholes

additional lift station required

higher construction costs

effluent or grinder pump required at each residence
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
electrical power required
Alternative 3 - Vacuum Sewer System
A vacuum sewer system utilizes a partial vacuum to transport sewage through the
collection system. Vacuum sewer systems include a central vacuum station, which
maintains the system at a sub-atmospheric pressure, with a small diameter vacuum
pipeline located in the road ROW. Service connections are made to each residence
and a holding tank replaces the septic tank (much like the grinder pump system). A
vacuum valve located in the holding tank allows rushing air from the service
connection to transport the sewage to the central vacuum station. The central
vacuum station operates 24-hours a day. The opinion of probable construction cost
for a vacuum sewer system was $2,000,000.
Advantages

electrical power not needed at each residence

reduced excavation compared to gravity sewer system

grade and depth not critical

exfiltration virtually impossible

manholes not needed
Disadvantages

electrical power required at central vacuum station

vacuum valves have a shorter life than pumps

higher energy consumption than other alternatives

major odor problem at central vacuum station

vacuum loss requires immediate repair
Alternative No. 1 Selected
Based on the projected construction costs and the associated advantages, the low
pressure small diameter sewer system was selected.
Pump Selection
Once the system was selected, the next step was to evaluate the type of pumps
available to optimize the function of the low pressure system. Septic tank effluent
pumps and grinder pumps were compared for use in the system.
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Septic Tank Effluent Pump — The septic tank effluent pump is installed in the septic
tank. The concept is that approximately 80 per cent of the solids fall out in the septic
tank and partially treated wastewater effluent is pumped to the low-pressure sewer
system through a one and a half inch diameter force main. Keeping a majority of the
solids in the septic tank helps prevent line blockage in the low pressure sewer
system. With the septic tank effluent pump system, the septic tanks will require
periodic pumping of solids. Effluent pumps require less power to operate, they have
approximately a $300 initial cost savings compared to a grinder pump, and they
pump less solids. However, many septic tanks do not meet current state and local
design requirements, and, because a septic tank requires periodic pumping of solids,
which requires a removable lid, the system can not be water tight.
Grinder Pump — The concept of a grinder pump system consists of replacing the
septic tank with a holding tank. All solids introduced into the sewage holding tank are
ground and then pumped to the low-pressure sewer system. Each time the grinder
pump is activated, the contents of the holding tank are removed. Grinder pumps
eliminate the septic tank, so that there is no longer any need to pump solids from
septic tank, and since they are watertight, they greatly reduce infiltration. The grinder
pump would make bidding the project easier for the Contractors, because, unlike the
effluent pump where each septic tank installation is different, each grinder pump
station would be identical. Also, with the smaller tank capacity, grinder pumps pump
fresher sewage, reducing odor problems. The Water Department elected to abandon
the use of all septic systems along the project and opted to install as uniform a
system as possible along the "one-of-a-kind" street. The grinder pump system was
selected because it would simplify bidding and construction, reduce inflow and
infiltration into the holding tank and, since each grinder pump station is identical,
provide a uniform system.
Project Development
Coordination of this project required communicating with the residents. Several
meetings were held with the Lake Worth Civic Club and residents along Cahoba
Drive to present the project, encourage resident participation, receive input and
discuss alternatives.
The Water Department agreed to a 50 percent matching grant under the EPA Clean
Lakes Program. Initially, it was proposed that residents pay for any construction on
their lot to connect to the innovative system. Over several years, this position evolved
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to a point where all costs associated with the construction of the force main, the
installation of the grinder pump stations, the household connection to the station, and
the service connection from the grinder pump station to the force main were paid for
with the grant. This expanded the scope of the Water Department's participation from
the original intent of the task, and tested the bounds of the budget.
Resident Participation and Fees
The Water Department could not require the residents to participate in the project.
However, the City decided that connection to available water or wastewater service
would be a stipulation in the future sale of the land. Grant funding made it quite
advantageous to the residents to participate. The system as designed is operational
at 50 percent resident participation. The Water Department was willing to install the
system if only 20 percent of the residents participated. At the time of the start of
construction, over 90 percent of the residents had committed to participate in the
project.
The Water Department required the homeowners to pay Wastewater Tap and Impact
Fees as a commitment to participate in the project. The City ordinance Wastewater
Tap Fees were $850.00 and Impact Fees were $580.20, with the total cost to the
resident totaling $1,430.20. No construction costs associated with the project were
charged to the homeowner. The Grant fund and the Water Department paid
approximately $14,800.00 per lot in construction costs for the installation of the
collection system, grinder pump, and for the plumbing and electrical connections.
Once the Water Department had established commitment from the residents to
participate in the project, the issue of how to charge for wastewater service had to be
resolved. The customary wastewater service charge is based on the customer's
metered water volume. In this case, the residents were using private water sources.
Several alternative methods were considered. These alternatives included metering
pump flow rates, pump run time meters, a volume per capita fee, and a dwelling
volume fee. The Water Department decided to bill based on a flat 900 cubic feet per
month volume charge until metered City water service is available.
Resident Ownership of Pumps
Long range planned improvements of the Lake Worth area will likely result in the
installation of several hundred grinder pump stations. TNRCC guidelines for
alternative wastewater collection systems require a responsible management
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structure for operation and maintenance. An acceptable vehicle for this structure
includes a local ordinance allocating responsibility for the maintenance of the grinder
pump to the homeowner. The Lake Worth leases include a clause that the City may
not pass an ordinance that is directed to the lessee. The opinion of the City's legal
department was that passing a city ordinance to address the ownership of the grinder
pump station violated the conditions of the lease. However, the City already had in
place an ordinance which addressed the responsibility of homeowners regarding the
wastewater collection system, and the legal department considered the Lake Worth
Area residents to be included under the existing ordinance.
The existing ordinance stated that the responsibility of the homeowner included their
service connection up to the tap at the City sewer main. Since the City plans to sell
the lots eventually, the Water Department preferred to transfer the obligation of
grinder pump operation and maintenance to the property lessee, who would likely
become the property owner at the time of the land sale. The Water Department
representatives working with the TNRCC created a service agreement providing for
the operation and maintenance of the grinder pump stations to be the responsibility
of the homeowner.
Service Agreement and Warranty
As part of the agreement for the homeowner to assume the responsibility of the
grinder pump operation and maintenance, the Water Department included a 5-year
warranty and service agreement as part of the pump supplier's responsibilities.
Naturally, this was an important issue to the homeowner. They had no experience
with grinder pumps, and therefore had some apprehension about assuming the
responsibility without some support from the Water Department. The warranty
provided by the manufacturer included a 100 percent unconditional parts and labor
guarantee. The 5-year service agreement included annual maintenance of the pump
station, emergency service, a 24-hour service telephone number, an O&M manual for
the grinder pump station with a quick checklist for self-help and two training classes
for the residents.
System Design
The main concerns that had to be addressed in the design of the low pressure sewer
system were system hydraulics, force main routing, force main maintenance, and
plumbing and electrical connections on individual lots.
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System Hydraulics
Grinder pumps can be categorized as one of two types: the centrifugal pump and the
semi-positive displacement pump. One of the main differences between the two
types of pumps is the operating curves. The semi-positive displacement pump has a
much steeper operating curve and will maintain a fairly constant flow from 9 gpm to
13 gpm over the range of pressure heads. The flow from a centrifugal pump will vary
from about 10 gpm up to 30 to 40 gpm depending on the system pressures. Also, for
a single phase motor, the highest pumping head on centrifugal pumps is about 95
feet, while the semi-positive displacement pump can continuously pump at 138 feet.
The advantage of the centrifugal is that several manufacturers have extensive
experience with existing systems; whereas, there is only one semi-positive
displacement pump manufacturer with extensive experience. Therefore, the design
specifications were written to allow both centrifugal pumps and semi-positive
displacement pumps.
In a low-pressure sewer system, grinder pumps with a minimum 60-gallon holding
tank, do not all operate at the same time. Published data shows that only a
percentage of the grinder pumps operate at one time, under normal operating
conditions. The larger the system, the less percentage of grinder pumps operate
simultaneously. Table 1 (developed by Environment/One) is a list of the maximum
number of grinder pumps that operate at one time on a daily basis in a given size
system.
Table 1
Maximum Number Of Grinder Pumps Operating Daily
Number of Grinder Pumps
Maximum Number of Grinder Pumps
Operating Simultaneously
1
1
2-3
2
4-9
3
10-18
4
19-30
5
31-50
6
51-80
7
81-113
8
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114-146
9
147-179
10
180-212
11
213-245
12
246-278
13
279-311
14
312-344
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The project area included approximately 100 existing residences that could connect
to the community sewer system. Since most of the homes are lake-front properties
located on the south side of Cahoba Drive, additional lots could be added almost
anywhere along the length of the force main on the north side of the drive. The
system was designed based on 9 pumps operating at one time, which gives a future
build-out capacity of 146 residences.
Careful consideration was given to the hydraulics of the low-pressure, small-diameter
sewer system. Flow velocities of greater than three feet per second are
recommended to flush solids from the system. The sewer system should be selfflushing or the solids will settle in the pipeline and cause blockage. If a low pressure
sewer system is designed with excessive future growth, initial flows will not be
capable of flushing solids from the system. Some systems exist that were designed
for excessive future growth. These systems require the small diameter pipelines to
be flushed once a month and, in some systems, once a week. For a low-pressure,
small-diameter sewer system, a large future growth can be handled by a parallel
pipeline in the future.
Force Main Routing
The force main was routed along the north side of Cahoba Drive, the side opposite
the majority of house, for ease of construction. This route avoids conflict with the
main gas line, which runs on the south side of the road, driveways and landscaping
and allows space for the future water line to be installed. However, since the service
connections are located on the opposite side of the road as the residents, this
required using a long pipe under the road pavement between the isolation valve and
force main connection. The sewer line was located a minimum of 4 feet from the
roadway and at least 3 feet below the roadway to allow for future expansion of the
road.
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Force Main Maintenance
An important part of maintaining a low pressure sewer system is providing adequate
facilities to flush the system. A flushing connection was located every 500 feet based
on the length of the City's rooting equipment, at every branch connection and at the
end of lines. To save costs, the flushing connections were combined with the air
release valves. Manual air release valves were preferable to automatic valves due to
concerns that automatic valves would clog. The flushing connections/air release
valves were located at high points in the system.
The flushing connection/air release valve consisted of a hose connection for pressure
water flushing, an isolation valve that also serves as the manual air release valve,
and a connection that can be opened to pig the system.
Individual Lot Design
In order to provide adequate information for the contractor's bidding and construction
of the project, individual lot drawings were prepared locating the electrical meter, the
existing septic tanks, the sewer connections, grease traps and gray water discharges.
The drawings also contained the proposed grinder pump location and discharge line
route. Individual inspection of each lot was required to develop the drawings.
One of the main concerns with the septic tank tie-ins was the electrical connection.
Four connection alternatives were developed. Since many of the houses were
constructed under different electrical codes, one of the options was to provide a
second electric meter on a utility pole to service the grinder pump separate from the
house. The contractor was given the flexibility to utilize this option if he considered
upgrading the existing house wiring to current codes too expensive.
Conclusions
The Lake Worth Community Sewer System is a demonstration project to determine
the feasibility of providing community sewer systems to the remainder of Lake Worth.
Near the lake, septic tanks are no longer a solution for wastewater disposal. Of the
three systems considered for the community sewer system, a low pressure grinder
pump sewer system was the most advantageous and cost-effective solution.
Communicating with the residents was important in developing an effective project
since the residents will own and operate the grinder pumps.
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