Pressure Sewer Layout
There are now over 100,000 residential grinder systems
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being used in the US today with many more being added each year.
FULFILLING A NEED
The problem of eliminating human wastes has been a major concern of mankind throughout history, with the possible exception of the Dark Middle Ages when the progress of human hygiene and community cleanliness was primarily ignored or not readily understood. Evidence of sewer systems predate this time period with sanitary engineering
historians recording sewers as far back as the 7th Century BC in ancient Babylon.
With the first sewer systems designed specifically for sanitary sewage installed in Hamburg. Germany in 1843. and in Brooklyn, New York in 1857, the use of gravity sewer collection systems became the accepted method to serve the larger cities of the world. This trend expanded to the smaller towns and has continued through the 20th
Century to the point that approximately 70% of the United States population is currently serviced by sanitary sewers.
In more recent history, pressures have developed within our society that have reversed the earlier 20th Century trend of people moving from the rural communities to the major cities. People originally desired to live in the larger cities for improved cultural, spiritual, social, security, and economic conditions. The height of this movement took place during World War II when large masses of the rural populations
moved to the major cities for better paying industrial jobs. Then with the problems of the larger cities developing after World War II such as high density, traffic, crime, air and noise pollution, etc. the flight to the suburbs and rural areas accelerated. As the small developments and individual homes filled in the open spaces between the cities and outer
towns and villages, the metropolitan regions were formed.
Certainly the trend for movement to the suburbs and rural areas will continue the rest of this century, but with it the necessity for providing adequate water and sewage disposal will become a major undertaking.
One of the most difficult problems for today's individual home builder and residential developers is finding available land for building homes that will readily lend itself to sewage disposal either by on-site systems or gravity sewers. The price of good land for development has become so expensive in many areas that it has forced the
individual builder and the residential developers to build on what is referred to as undesirable land. These areas can be normally described as rocky, contain major elevation differences and may have high water tables. In addition, soil structures in many areas of the United States do not lend themselves to on-site treatment. This problem is increasing
due to the growing resistance of society to use up valuable productive farm land that will be needed in the future to feed the world masses.
Another growing problem is the opposition of the rural populations to build gravity sewers due to the fear that it will bring rapid growth to their communities resulting from the flight of the city dwellers to the rural areas.
The growing shortage of water in many parts of the country (i.e. Texas, Arizona) is becoming a major concern of environmentalists and public officials. Methods must be found to protect our water supplies from pollution and discharging so much of our
wastewater into the rivers and oceans rather than back into the ground water tables.
It is projected that the United States population will be over 350,000,000 by the year 2,025. With this expected growth, a major question facing society is:" How can adequate sewage disposal be provided for the future growth in the United States that will be economically feasible and satisfy the regulatory requirements for a
It is universally recognized that there is a major need for a more economical method of collecting, transporting and treating sewage other than gravity sewers. This would be especially true in areas containing rock, rough terrain, sparsely settled and high water table areas.
One of the most viable alternatives to gravity sewers that has gained a great deal of attention over the past several years is the concept of pressure sewer systems. There have been over 200 of these systems (100-1500 connections) installed since 1970 with a large number currently under evaluation, design, or installation.
Although many sanitary consulting engineers have had some experience with pressure sewers, there are of those who have had little exposure to the concept. Therefore, as a start, let's define a pressure sewer system.
A pressure sewer is sometimes described as a potable water system in reverse. By this, we mean a water system will have one inlet pressurization point and a number of end user connections, while a pressure sewer will normally have a pressurized inlet point at each end user connection and one discharge outlet at a treatment plant or
discharge point. The number of end user connections could be anywhere from a few for several thousand. A pressure sewer system is made up of three major segments. They are:
Pump package system (basin, pump, piping, controls, control panel,
Service lines and force mains
Discharge treatment point
There are two primary pumps used in pressure sewers. One is referred to as a grinder pump (GP) and the other an effluent pump (STEP) which is located in or behind a septic tank. The major difference between the two pump types is how the solids are handled.
In the case of GP units, the solids are ground into a slurry and pumped to a discharge point. With a STEP pump, the settleable solids and most of the grease accumulation remain in the septic tank. Consequently, the STEP pump conveys a fairly clear effluent to the discharge.
This newsletter will basically address grinder pump pressure sewers. The following newsletter will address STEP pressure systems.
Service lines from the pump basin to the street are usually 1-1/4" I.D. Type I Schedule 40 PVC solvent welded pipe. Force mains will normally be SDR26 and SDR21 PVC pipe with elastomeric sealed joints running anywhere from 1-1/2" to 6" I.D, in size, depending on the number of house connections. Occasionally, polyethylene
pipe is used for the force mains, especially in extremely cold climates.
Wastewater slurry conveyed from grinder pumps can be discharged into a gravity manhole, treatment plant or lagoon. The slurry is easily treated, although the BOD concentration is higher than gravity flow wastewater. This is due to the fact that the domestic sewage in pressure sewers has not been diluted with infiltration and inflow
HISTORY-GRINDER PRESSURE SEWERS
The concept of grinder pressure sewers was first proposed by Dr Gordon Fair, Professor of Sanitary Engineering, Harvard University, in 1954. His idea of pressure sewers was part of a combined sewer system that would entail hanging a pressure line pipe within a gravity sewer pipe. The objective was to pump the domestic sewage through the pressure pipe and convey storm water by gravity.
Based on the merits of Dr Fair's concept, the American Society of Civil Engineers initiated a study in 1966, supported by government funding. Three years later, a report was released indicating that Dr. Fair's idea was not economical, but the concept of pressure sewers showed promise as a cost effective alternative to conventional sewers
As part of the ASCE study General Electric Company developed a grinder pump in 1967-68 that was to be used as part of a field demonstration. Due to budget considerations, this was never accomplished. However, the New York Department of Environmental Conservation's Research & Development unit decided to sponsor a demonstration with US-EPA grant funding to determine the feasibility of a grinder pressure
sewer system (Albany, NY).
In late 1971, a combination grinder and effluent pressure sewer demonstration project was installed in Grandview Lake, Columbus, Indiana, consisting of 58 house connections in the first phase. The majority of the pumps were grinders along with a few STEP pumps. A USEPA report on this project was published in 1975 (EPA-60012-75-072)
Since then well over 200 systems have been installed in the US, some with over 1000 pumps.
APPLICATIONS FOR GRINDER PUMPS
There are basically two markets for grinder pumps. One is referred to as onesy-twosy (OT) applications Here, we mean that only one or a few grinder pumps will be installed in a given application. The other market area is pressure sewer applications. Specific grinder applications in the OT market would be as follows:
Originally, there were only 1,1 1/2, and 2 HP grinder pumps available which were, in some cases, over applied in the early years with unsatisfactory results. In the middle and late seventies, the 3, 5, and 7 1/2 HP grinder pumps were introduced.
The larger grinder pumps are recommended for most commercial and industrial applications while 2 HP grinders are normally recommended for individual home applications and residential pressure sewers.
Since the subject of this newsletter is related to residential grinder pressure sewers, we will restrict our comments to the 2 HP grinder pump. If more information is needed on other applications or the larger grinder pumps, contact us for assistance.
In pressure sewer applications we normally define a pressure sewer as a system containing at least 25 pumps discharging into a common force main. Most residential pressure sewers are designed with one grinder pump per home. In some pressure systems there will be multiple residential connections to one simplex or duplex grinder pump.
Due to the administrative problems of who pays for power, where power is connected, obtaining right-of ways, etc., the majority of grinder pressure sewers being installed are using one grinder pump per home. The decision on single or multiple pump connections will be up to the design engineer and his client based on their specific
In the early seventies, pressure sewers were being installed primarily in recreational lake developments. Due to tougher septic tank restrictions and the slow buildup rate of homes in this type of development, pressure sewers proved to be cost-effective.
When the US-EPA grants program came into being in the late seventies, there was a shift of interest to utilizing pressure sewers in small rural towns and residential developments in the outer suburban areas. Since a large percentage of the recreational developments are not eligible for government grant and loan money (secondary versus primary residences), we now see a present trend of about 70% of pressure sewers
being installed in rural and suburban areas.
GRINDER PUMP EQUIPMENT FOR PRESSURE SEWERS
As already indicated, the workhorse system for grinder pressure sewers is the simplex rail 2 HP grinder package.
In the early seventies, it was predicted by one industry source that the majority of residential grinder pumps would be installed inside the home. This has notproven to be the case and today over 95% of residential grinder pumps are being installed
outside. The concern of noise, early backup of sewage on failure mode, and problems of entering the house for service (both parents working), has limited the interest in inside installations.
For outside applications, the simplex rail package system has to be the most popular. A prepackaged system arrives at the job site with all piping, valves, etc. already installed. All the contractor has to do is dig the hole and hook up the inlet, outlet and control panel and alarm, and set the pump in the station.
The inlet flange is shipped separate to make it easier to line up the basin inlet with the gravity line from the residence.
Fiberglass basins have proven to be the most popular due to their light weight, corrosion resistance, and long term life Over 90% of the basins used in pressure sewers are furnished in fiberglass.
With the quick disconnect discharge flange design, the pump can be pulled for service without the need to enter the wetwell. The pump is light enough (85 lbs) for one man to service if needed.
Variations of an audio and visual alarm are used, although the most popular and cost-effective is a red flashing alarm light mounted on control the panel. Almost all control panels are located outside so they will be easily accessible to the serviceman. The preferable location of the panel and alarm is on the side of the home or
garage to minimize aesthetic concerns of the home owner. If desired, the panel can be located at the pump basin and mounted on a pedestal.
For maximum corrosion resistance, the discharge piping, junction box and shutoff valve are usually made of plastic materials. Mercury ball level controls are standard. A spring loaded flapper check valve is installed in the discharge piping within the wetwell.
A 2 HP grinder pump system used in pressure sewers is expected to handle the common solids found in wastewater, such as disposable diapers, tissue paper, etc. In addition, they should be able to handle an occasional item such as wash rags, panty hose, mop strings, etc.
There will be occasions when a simplex pump may plug up due to power fluctuations, heavy solids (large bath towel) or a misadjusted cutter mechanism. Assuming the grinder pump is a well designed unit and installed properly, plugging on simplex residential units should be a rare occurrence.
The grinder mechanism design is similar to a garbage disposal except the cutter mechanism is in an upside down position. The cutters will masticate the solids to an approximate size of 1/4" to 1/2" which can be easily pumped through the 1 1/4" discharge lines to the street.
The 2 HP grinder pump has a shutoff head of 105 feet and can pump capacities to 40 GPM. Since the maximum peak flow from an average size home would be no more than 10 GPM, the pump is adequate to handle the peak flow.
A standard size basin for one home is 24" x 84". This would provide about 75 gallons of storage capacity in case of power outages or pump failure and provide 6-8 hours of usage while the serviceman is in route.
SYSTEM DESIGN CONSIDERATIONS
Once the design engineer has made a decision to design a pressure sewer system, he must then decide on design parameters that would effect the operation of the system. The most important considerations would be design flow, friction loss, pump type, scouring velocity and venting.
There are two methods available for the hydraulic design of pressure sewers. One would be the manual (hand calculating) method design procedure and the other would be by use of a computer program. There are several manual methods that have been
developed by pump manufacturers and consulting engineers. Several manufacturers along with a number of consulting engineers, has developed a computer design program.
Friction losses are determined by the Hazin-Williams method with appropriate C factor. A C factor of 140 or 150 is used most frequently in grinder pressure sewer designs. A scouring velocity of 2 to 5 Ft/sec is recommended for the service lines and force mains. Air venting is important to prevent binding at the high points of the
There are several formulas used to determine design flow. In the early years of pressure sewers, many engineers used the standard 100 GPD per capita as per gravity flow designs. Now, recognizing the elimination of infiltration and inflow, 50 to 60 GPD per capita have become the more accepted design figures for pressure sewers.
Based on several years experience, the following formula is used to determine peak design flows in pressure sewers:
|Peak Design Flow = Total Number Dwellings x
gal/day usage/dwel x 4
||24 x 60
To determine the number of pumps operating at a peak flow period, the design flow would be divided by 12-14 GPM (average pump capacity).
There is a limit to how many homes can be paralleled together in one system depending on pump head capabilities and the TDH requirements of the system. About 500 homes would be the upper limit with relatively flat topography.
ADVANTAGES OF GRINDER PRESSURE SEWERS VERSUS GRAVITY
Small Diameter Plastic Pipe-0ne of the advantages of pressure sewers over gravity systems is the ability to use small diameter plastic pipe for the service lines and force mains. In pressure systems, the service line is normally 1-1/4" I.D and l-1/2" to 6" I.D for force mains. This is in comparison to gravity service
lines of 4" and 6" and gravity mains up to 14" and 16" for a comparable number of house connections. Consequently, there is a major cost advantage in the piping collection system for pressure sewers versus gravity.
Pipe Alignment - Based on the pumping capability of the pump, the service lines and force mains can run up and down hill following the contour (within shutoff head limitations) of the topography whereas gravity lines must be aligned in a continuous downward slope. Deep cuts for gravity lines become quite expensive, especially where rock and high water tables are present
Narrow and Shallow Trenches- Pressure sewer service lines and force mains can be easily installed in narrow trenches just below the frost line. In the case of the Sunbelt states, the pipe has to be laid deep enough for mechanical protection.
Minimum Equipment & Labor- Pressure sewers can be installed with a trencher and backhoe. The force mains usually are installed in the right-of-ways along the road. In comparison, gravity sewer installations require large power shovels and it may be necessary to install the mains in the streets. Labor and equipment requirements to handle large diameter gravity sewer pipe in comparison to small PVC pipe can be substantially higher.
Road and Bridge Crossings -Small diameter pressure sewer service lines and pressure mains can be bored under roads with minimum difficulty (assuming no rock) since precise slope and alignment are not critical. Small force mains can be installed across bridges without the use of pump stations. If freezing is of concern, the small pipe can be insulated within limitations.
Reduction or Elimination of Pump Stations and Manholes-Another advantage of pressure sewers is the ability to eliminate manholes and pumping stations within limitations of the grinder pumps head capabilities. Manholes are not required with pressure sewers. However, a valve box is recommended about every 1000 feet or major change in direction. If intermediate pump are required in a pressure sewer system, they are usually much smaller pumps and wetwells due to lower requirements.
Infiltration & Inflow - One of the major problems with gravity sewers is the problem of heavy infiltration due to broken pipe and leaky joints. In the flooding seasons this can cause problems at the treatment plant by overloading the treatment system due to the heavy inflows. In the case of pressure sewers, infiltration is virtually eliminated due to the design of a tight system throughout.
Service Connections - In older communities, the septic tanks are usually located in back of homes with the house plumbing oriented to the back. This can create a problem of installing a gravity sewer line to the street without changing the house plumbing. On the other hand, the grinder system can be installed between the house and septic tank and the service line laid around the house to the street at a minimum cost using small diameter PVC pipe.
Treatment Plant Size- If the treatment plant is being designed to handle flow from a pressure sewer system only, the size can be reduced over normal design requirements due to the elimination of infiltration and inflow. Rather than sizing the plant for 100 GPD per capita, it can be sized at 50-60 GPD per capita. Since the wastewater is ground into a slurry, bar screens and commuters wouldn't be necessary .
Water Conservation - Since water conservation is gaining a great deal of attention in many parts of the country, the value of pressure sewers in this regard becomes an important consideration. By eliminating infiltration and inflow, the amount of water polluted is reduced considerably. Since no manholes are used in pressure sewers, the
drain wastewater from washing cars and lawn watering doesn't end up being polluted by the domestic sewage and is returned in part to the water table.
Land Planning - Public officials in rural areas can limit the amount of population expansion in a community by the amount of potential growth that is sized into the pressure sewer mains. This alleviates some of the fear of rapid growth if gravity sewers are installed. Since pressure sewers are more adaptable to hilly, rocky and high water table areas, the developer has more flexibility in laying out his lots for maximum lot sales dollars in comparison to some restrictions.
DISADVANTAGES OF GRINDER PRESSURE SEWERS VERSUS GRAVITY
Mechanical Equipment - Probably the major concern of installing a pressure sewer system is the addition of a large amount of mechanical equipment which will have to be serviced in perpetuity. This is especially true when an individual pump system is installed at each home. However, a grinder pump can be looked upon as another appliance and is subject to the same ongoing service requirements as a home furnace, refrigerator, hot water heater, etc. Obviously, with the amount of equipment involved, it is important to have a service capability established to handle it. This can be done by a service department within the sewer authority, or local service center established by the
manufacturer or local pump representative.
Power Outages - The extra holding capacity available for a standard simplex grinder system is 60-100 gallons. If there is a power outage, the homeowner has about 8 hours excess capacity to continue using the sewage system. Hopefully, the homeowner would refrain from washing clothes and taking showers during this time frame to
minimize any problems. With a gravity collections system, the homeowner can continue to use the sewage system assuming standby power is available to operate the pumping stations and potable water is still available.
Power Costs - With the ongoing increase in power costs, a homeowner is concerned about any equipment that will increase his electric bill by any substantial amount. Since a simplex grinder pump only operates on the average of 10-15 minutes a day,the annual power cost will be under $20.00 a year, based on an average kilowatt per hour
cost of twelve cents.
Aesthetics of System-Some homeowners are concerned about the appearance of the equipment in their yard. Since the basin cover is buried flush with the ground, it is difficult to notice from the road. Control panels with alarms are attached to the sides of the home, making them inconspicuous except if visual or audio alarm is
activated. The homeowner may also be concerned about the damage to the premises when installing the basin and service line. Actually, restoration is less of a problem than with installing a 4" or 6" gravity service line due to the narrow and shallow trenches required.
Operating Problems - Once a pressure sewer system is started up and any initial problems are corrected, the system will require ongoing service as with any other sewer system. Occasionally, a pump will plug up due to a large, solid object getting into the basin which proves to be more than the pump can handle Grease can build up on the
controls over a period of time in homes where the family uses a lot of grease to cook with (less than 5% of homes). This can be overcome by periodic preventive maintenance involving hosing off the the interior of the station twice a year with house tap water.
Easements - Since pressure sewers are, in most cases, owned and maintained and serviced by a sewer authority, it is necessary to obtain a permanent easement to insure access to the pumping equipment. This can take time due to the necessity of obtaining easements from each homeowner before the pressure system is installed.
Unfamiliarity with Pressure Sewers - Most homeowners will be familiar with septic tanks and/or gravity sewers. However, very few will have any working knowledge of pressure sewers and how they work. This fact will require the engineer to educate the
community authorities and homeowners to insure a smooth transition when pressure sewers are installed.
One of the key questions asked about pressure sewers is their cost. This is a rather difficult question to answer since there are so many factors involved. However, we will endeavor to provide some budget figures for consideration.
The first prerequisite to keep in mind is the cost of a grinder pump package system when comparing the price of one unit versus a purchase of several hundred units at one time. Obviously, the price for several hundred units will be considerably less than buying just one unit. This comes about due to the equipment manufacturer being able to buy large quantities of raw materials and parts at volume discounts and passing a
major portion of the savings on to the customer. There would also be a considerable savings in assembly costs due to the ability of the manufacturer to set up and assemble a large number of units at one time.
Based on volume pricing, we can get an idea for the equipment cost based on the bids that have been submitted over the past few years on large project jobs. A standard simplex 2 HP grinder package system including a 24" x 7' fiberglass basin, rail components, controls, piping, control panel and alarm is being bid in the range of
$1,800-2,800 per unit (depending on the quantity and configuration of the pump system). Installation costs to include installing the basin, power hook up and running service line to the street will, on the average, be about half the equipment cost. Thus, the totalinstalled cost of equipment costing $2,200 would be approximately $3,300. This price could vary plus or minus 20% based on weather conditions, local labor rates, etc.
This pricing does not include the homeowner's proportionate cost of the force mains and treatment plant.
Pricing is based on a standard package with a simplified control panel. Expensive control panel options or larger basins will increase the price accordingly. Installation costs are based on the assumption that there is a minimum of rock in the area. Heavy rock or high water tables will results in increased installation costs. An
inside simplex system installed in the basement will be less costly on the equipment side, but the savings would normally be offset by the higher installation cost. This is due to the cost of breaking into the basement floor and the resulting restoration work required.
OPERATION & MAINTENANCE
The most important consideration in installing a grinder pressure sewer system, aside from capital costs, is the effect and cost of ongoing maintenance. It is generally recognized that pressure sewers will require more maintenance than a conventional sewer system due to the large amount of mechanical equipment on hand. This is
especially true if you are installing a pump system at each individual home, which would be the case in over 80% of pressure sewers installed to date.
Since the industry has over 20 years of field experience with grinder pressure sewer operation, it is possible to offer some observations on their operation and sewer requirements.
If a pressure sewer is designed properly, installed correctly and quality pump equipment is utilized, the system will perform satisfactorily. The submersible centrifugal grinder pump is the most reliable component within the pump system. Once any bugs are worked out of system on start up, the grinder pump should operate up to 10-15 years before major overhaul will be required (replace seals, bearings,
motor). During the interim some pumps will require service but this should be in the range of 1% of units in operation per year. This means that if you have 1,000 units in a given project you could expect approximately 10 service calls per year on the pump. This percentage would increase accordingly as you approached 10 years of operation.
In addition to the pump, there will be service requirements for other problems, such as capacitor and relay failures, grease build-up on level controls, occasional pump plugging, pump air lock, sand and grit infiltration, etc. These types of nuisance problems can usually be corrected at the installation site and will account for
another 3% of service calls per year. This, in conjunction with pump failure service calls, would add up to 4% service calls per year for pump systems in operation.
It is a general consensus of opinion that a once a year preventive maintenance program is helpful to reducing potential service problems. This would include removing cover, operate pump manually, check for grease, exercise shutoff valve and check alarm system. A small percentage of homes (approximately 5%) will require more frequent
preventative maintenance due to family living habits, such as heavy use of cooking grease in these cases, the pump vault would be checked every 3 or 6 months. If there is a heavy grease build-up, it can be broken down with the garden hose
One important step to reduce potential service calls is the education of the homeowner's. It is very helpful to distribute a pamphlet to the public describing how a pressure sewer works and what the homeowner should do in case of a failure.
A list of "Do's and Don'ts" will reduce the possibility of the user discharging unwanted items into the drain system, such as towels, panty hose and clothing articles. Although the grinder pump is designed to handle these items within reasonable limits, it is better to keep such articles out of either conventional gravity
sewers or pressure sewers whenever possible.
Experience has indicated that the maintenance cost to maintain the service lines and force mains is negligible. Occasionally a pipe is frozen or a pipe broken due to excavation work in the area.
The overall important question is what should the monthly sewer charge be to cover the operation and maintenance cost of the pump equipment and to build up equity for the overhaul of the pump at the end of the projected 10 to 15 year life cycle. This monthly sewer charge would exclude debt service and administrative costs.
Based on our years of experience with pressure sewers and ongoing communication with maintenance personnel, we recommend an annual sewer charge of $40-50 per year. This figure would be based on the assumption that the pressure system was designed and installed properly and the pump equipment met standards of quality and performance.
The growing use of pressure sewers as a viable alternative to conventional gravity sewers should continue to expand in the future.. This will be due to pressure sewers lower costs, particularly in small rural towns.
The decision to use gravity sewers or alternative systems is dependent on a number of factors. There are many aspects of design, construction, equipment and economics that must be considered. However, it is now generally recognized that pressure sewers offer an effective and versatile tool to solve wastewater problems when terrain and
other unusual conditions exist.