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CHAPTER 4: WASTEWATER COLLECTION SYSTEMS

DEFINITION OF WASTEWATER COLLECTION SYSTEMS

Wastewater collection systems gather the used water fromour homes, businesses and industries and convey it to awastewater treatment plant. This type of system is alsocalled a sanitary sewer. A similar system known as a stormwater collection system conveys water resulting from runoffof rain and snow from buildings and paved and unpavedareas to a natural watercourse or body of water, usuallywithout treatment. This type of system is also known as astorm sewer. In the past, some sanitary sewers and stormsewers were combined into one system. Unfortunately,during heavy rains the wastewater treatment plants servedby combined sewers often became hydraulically overloadedand washed out into the receiving stream causing a completetreatment system failure. For this reason, combined sewersare now uncommon.

WASTEWATER COLLECTION SYSTEMS OPERATORSThe ancient Romans were one of the first civilizations toemploy wastewater collection through clay pipes andcovered cannel sewers. They understood all too well theimportance of maintaining sanitary conditions. Modernwastewater collection systems are a sophisticatedcombination of components that include; gravity sewerlines, force mains, manholes, and lift stations. Theyrepresent one of the largest financial investments of publicmoney for our municipalities. Why is it then that thepersonnel charged with operating and maintainingwastewater collection systems often feel as though theyare the lowest paid employees in the entire city?

Because these systems are underground where the publicdoes not see them they are all too often “out of sight, out ofmind”. The general public rarely understands that operationand maintenance of wastewater collection systems is criticalto maintaining the modern sanitary conditions we all takefor granted. Only when the system fails, (such as during asewer back-up), does the public take notice of thewastewater collection system. Wastewater collectionsystem operators have a very dangerous, important job andare deserving of respect for their skill and dedication.

WASTEWATER COLLECTION SYSTEM DESIGN

GRAVITY SEWER LAYOUTThe largest component of a wastewater collection systemis usually the gravity sewer. Gravity sewers follow thetopography of the surrounding area, (lay of the land), totake advantage of the natural slope. They are designed toprovide a flow velocity between 2 and 8 feet per second(fps), with 2.5 fps being ideal. If the velocity is too low,settleable solids will deposit in the sewer lines, if the

velocity is too high, erosion and damage of the collectionsystem will occur. Gravity sewers are divided into thefollowing sections:

Building SewersA building sewer connects a building’s internal plumbingto the public wastewater collection system. The buildingsewer may begin at the stub-out, the property line or somedistance (such as 2 to 10 ft.) from the building’s foundation.Where the building sewer ends marks the end of the buildingowner’s responsibility for maintenance and repairs. Beyondthe building sewer, the wastewater collection systemoperators are responsible for maintenance, cleaning andrepairs. This division should be clearly spelled out in localsewer ordinances.

Lateral and Branch SewersLateral and branch sewers are the upper ends of thewastewater collection system. Sometimes they are locatedin easements, although this should be avoided wherepossible due to problems of access and limited work space.

Main SewersMain sewers collect the flow from numerous lateral andbranch sewers and convey it to larger trunk sewers.

Trunk SewersTrunk sewers are the main “arteries” of the wastewatercollection system and convey the wastewater fromnumerous sewer mains to a wastewater treatment plant orto an interceptor sewer.

Intercepting SewerSewer interceptors receive the wastewater from trunksewers and convey it to the wastewater treatment plant.

CHARACTERISTICS OF GRAVITY SEWERSThe following items are considered when determining thecharacteristics of a sewer line:

Slope of SewerAs stated before, the slope of the sewer should follow thelay of the land as closely as practical provided the slope isadequate to produce gravity flow and maintain the minimumvelocity (2 fps). Some areas will be too flat to permitexclusively gravity flow because the sewer lines would haveto be buried excessively deep.

Design FlowWastewater collection systems are designed to convey thepeak flow from a service area when the area has reached itsmaximum population density and has been fully developed

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Figure 4.1 - Schematic of a Typical Gravity Wastewater Collection System

commercially and industrially. Population estimates,growth projections and comparison to similar areas are allused to determine the maximum expected volume to becontributed by a service area. Domestic wastewater flowis often calculated by multiplying the estimated populationin a service area by the per capita flow. In New Mexico theper capita flow ranges from 60 to 125 gallons per day perperson. Businesses and industries will contribute varyingflows and so must be accounted for differently. In additionto the expected wastewater flows, allowances must be madefor infiltration and inflow, (I & I are discussed later in thistext). Also, because the actual flow of wastewater in thecollection system will vary during a 24-hour period,(minimum flow during the early morning hours andmaximum flow around 10:00 AM to 12:00 PM), a peakingfactor must be used in order to ensure the sewer will handlethe maximum instantaneous flow. Peaking factors of 2.5 –3.5 times the total daily flow are often used to sizewastewater collection systems.

VelocityThe wastewater in a sewer line should move at a speed thatwill prevent the deposition and buildup of solids in thesewer; this is called a “scouring velocity”. A minimumvelocity of 2 fps has been shown by experience to providethis scouring or self-cleaning velocity. Not all lines willmaintain a scouring velocity at all times throughout theday. However, the sewer should be designed to provide ascouring velocity at average flows or, at the very least,during peak flows.

Pipe SizeA sewer line should be at leastlarge enough to allow the useof the cleaning equipmentavailable. When sizedproperly, a sewer line shouldflow one half full duringaverage daily flows. The airspace above the half fullsewer line helps to maintainaerobic conditions in thewastewater and providessome room for error indetermining design flow.

Location and AlignmentLateral, main and trunksewers are generallyconstructed near the center ofpublic roadways so that thelength of building sewers isequalized and access isconvenient. Pipes are

generally laid as straight as possible to facilitate cleaningand for ease of installation. In order to avoid contamination,sewer lines must be located at least 2 ft. vertically belowand 4 ft. horizontally away from potable water distributionlines.

DepthSewer lines are typically placed at a depth of 4 to 8 ft. Thedepth and width of a trench, the backfill materials and themethod of compaction determine the load placed on thesewer line and therefore influence which piping materialsare appropriate.

WASTEWATER COLLECTION SYSTEM

CONSTRUCTION

PIPING AND JOINT MATERIALSThe materials used to construct sewers are selected for theirresistance to deterioration by the wastewater they convey,strength to withstand surface loads, resistance to rootintrusion, their ability to minimize leakage and their cost.Great care should go in to the selection of materials andtheir installation because they may be in service for many,many years. The following is a list of common wastewatercollection system piping materials and their attributes:

Asbestos Cement Pipe, (AC)Asbestos cement is a watertight pipe that is resistant todeterioration by most types of wastewaters. AC pipe issubject to corrosion by hydrogen sulfide that combines with

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moisture to form sulfuric acid near the top of the pipe,(known as “crown rot”). For this reason, AC pipe shouldnot be used where the release of hydrogen sulfide can occur.Joints are made of sleeve and rubber gasket couplings.OSHA restricts the use of AC pipe due to its asbestoscontent.

Cast and Ductile Iron, (CI and DI) PipeCast and Ductile Iron pipe is very rigid and can resist heavysurface and earth loads. Although costly, CI or DI pipeshould be used for bridge crossings and where lines areshallow and subject to heavy traffic loads. Joints aretypically rubber or caulk gasketed mechanical push-on typeor leaded (in old installations).

Reinforced or Non-reinforced Concrete, (RC and C)PipeReinforced and non-reinforced concrete pipe is very rigidand can withstand significant surface loads. Similar to ACpipe, it is subject to crown rot. Coal tar epoxy or plasticlinings are sometimes used to retard the corrosive action.Rubber-gasketed bell and spigot joints are common butproper installation is important or they may leak. Mortaror bituminous filled bell and spigots are also used.

Vitrified Clay, (VC) PipeVitrified clay pipe is rigid but is subject to cracking causedby root intrusion. VC pipe is resistant to acid, caustics,solvents, gases and other material that are sometimes presentin wastewater. Joints usually consist of bells and spigotswith factory formed resilient compression joints made ofpolyvinyl chloride/ polyurethane or consist of rubbercouplings held in place with stainless steel compressionbands (band seal couplings). Older installations used bellsand spigots sealed with mortar.

Fiberglass Reinforced, (FR) PipeFiberglass reinforced pipe issemi-flexible and resistantto corrosion by mostcompounds found inwastewaters. There is someevidence that FR pipe issubject to damage byhydrogen sulfide. Joints aremade with rubber-gasketedbell and spigot. Installationshould be done with greatcare because experience hasshown this pipe to besubject to failure withinseveral years of installationdue to unanticipated surfaceloadings.

Acrylonitrile Butadiene Styrene, (ABS) PipeABS pipe is flexible and resistant to most substances foundin wastewater. However, petroleum products will softenand erode ABS, (these products are normally prohibitedfrom being discharged into sanitary sewers). Joints aresolvent (chemical) weld or gasketed bell and spigot. ABSpipe will deflect into an oval when improperly installed soconstruction inspection should ensure that cleaning toolscan pass through new lines.

High Density Polyethylene, (HPDE) PipeAlthough relatively new, HDPE pipe is gaining in use forsanitary sewers. HPDE pipe is flexible, durable andresistant to most substances found in wastewater. HPDEis installed in long sections and thermally butt-welded in

Joint Joint Joint Used With:# description AC CI/DI RC C VC FR ABS HDPE PVC1 Caulked Bell

and Spigot2 Band Seal Coupling3 Mortar or Bituminous Filled

Bell and Spigot4 Polyvinyl Chloride/

PolyurethanePre-formed Gasketsin Bell and Spigot

5 Rubber Gasketin Bell and Spigot

6 Rubber Gasketed Coupling7 Solvent Cemented Coupling

or Solvent Bell and Spigot8 Butt Welded (not shown)

Table 4.1 - Common Types and Joints for Sewer Pipe

Figure 4.2 - Types of Joints

Reprinted, with permission, fromOperation & Maintenance of

Wastewater Collection Systems, Vol.I, 3rd ed., Office of Water Programs,

California State University,Sacramento Foundation

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the field. It is often used for force mains and small diameterlow pressure sewer lines.

Polyvinylchloride, (PVC) PipePVC is flexible and resistant to most substances found inwastewater. Joints are commonly made with a rubber gasketand a factory-formed bell and spigot or a solvent cementwelded coupling joint can be used in diameters up to 8inches. Various grades are available for specificapplications. Because it is inexpensive and easy to workwith, PVC pipe is currently the most commonly installedpipe. However, PVC pipe is subject to damage by heavyexternal loads and therefore care should be given to theselection of the grade used for specific applications.

PIPELINE INSTALLATION

EXCAVATIONMost sanitary and storm sewers are constructed withinexcavated trenches. Excavation is an inherently dangerousactivity. Many laws exist which pertain to excavation safety.These laws are in place to protect you! Refer to Chapter 2Safety Practices and Regulations for an overview ofexcavation safety related subjects. Before beginning anyexcavation the nearby utility lines (natural gas, water,electric and telephone) must be identified in order to avoiddamaging them. In New Mexico a free service known asNew Mexico One Call is available for utility line locationprior to excavation work. In Albuquerque, the phonenumber is 260-1990. For all other cities, towns, villagesand outlying areas call 1-800-321-2537. 48 hours notice isrequired, (not including weekends or holidays).

If you damage utilities during an excavation you or youremployer will be liable for all the associated costs and theheadaches can multiply very rapidly. Most excavationsare done with a backhoe, although trenching machines arebecoming more popular. Situations also exist when theuse of “trench-less technologies” is more attractive that anopen excavation. Trench-less technologies include; in-situpipe liners, pipe bursting, and boring and micro-tunnelingmachines. Some examples of their applications would bethe rehabilitation of sewers with infiltration problems, sewerreplacement in areas with limited access and installationswhere the surface over the sewer cannot be disturbed.

Controlling Line and GradeLine and grade controls for the sewer are used first todetermine the location and depth of the sewer trench andsecond for laying the sewer pipe to the proper line andgrade. There are several ways to establish and maintainthe line and grade of the trench and pipe including; thestring line method and the fixed beam laser method. Initialreference points are established using surveying equipment

and then the construction crew will use one of the methodsto maintain line and grade.

Pipe BeddingPipe placed into a trench must be properly bedded in orderto properly support the pipe. This is one area that requiresgreat care and yet is often done in a slip-shod manner. Theideal bedding material is crushed rock aggregate. Sandand pea gravel can be used but must be compacted carefully.Native material can be used but extreme care must be takento excavate the trench bottom to true grade in order toproduce an undisturbed and compacted trench bottom.Regardless of what type of bedding material is used, it isplaced in the trench bottom and compacted and thencarefully compacted around the sides of the pipe after thepipe has been laid. The depth of the bedding, and thebedding’s compaction directly affect the load that can beplaced on the pipe.

Figure 4.3 - Methods of Pipe JoiningReprinted, with permission, from Operation & Maintenance of Wastewater Collection Systems, Vol. I,

3rd ed., Office of Water Programs, California State University, Sacramento Foundation

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Laying PipeMost manufacturers of sewer pipe will furnish instructionsfor laying and joining their pipe. Read and understand themanufacturers instructions before attempting to lay pipe.Pipe should be handled with careto avoid damaging the joints. Onlysmall diameter pipe (< 10 inches)should be moved by hand. Largepipe should be moved with a forkliftor a backhoe equipped with a pipesling. When installing bell andspigot pipe a small amount ofbedding material should be removedto accommodate the bell so that thepipe is properly supported.

Joining of PipeDifferot joints of small diameter, ablock and bar can be use to force uchas a “come-a-long” and sling is used.Pipe should be placed so that the bellend faces upstream. For other typesof joints, refer to the manufacturersinstructions for joining the pipe.

Trench BackfillingThe placing of backfill material over thepipe has three essential elements. (1)The pipe must be protected frommovement, breakage and crushingcaused by the backfill material, (2) thebackfill material should be compactedin layers until the excavation has beenfilled completely, and (3) the groundsurface should be restored.

Service Connections (Taps)A service connection is the point ofcontact between the building sewerand the wastewater collection system.For new installations, taps, (known asstub-outs), are installed along with thesewer lines at places close to wherefuture buildings are anticipated to belocated. In existing systems, thecollections system operators may beresponsible for making new servicetaps to the sewer system. There areseveral methods of tapping intoexisting lines and providing stub-outsfor new lines. They include; Clamp-on Saddle Tees, Insertion Wyes andTees, Epoxy Bonded Saddle Tees, andSynthetic Rubber Wedged InsertTees.

However service taps are made, they should provide a tightseal to prevent root intrusion and infiltration. Also, the tapshould be made in such a way that the building sewer doesnot protrude into the sewer lateral or main. This is because

Figure 4.4 - Building Sewer Connections (taps)Reprinted, with permission, from Operation & Maintenance of Wastewater Collection Systems, Vol. I,

3rd ed., Office of Water Programs, California State University, Sacramento Foundation

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protrusions can cause blockages and interfere with cleaningequipment.

MANHOLESManholes are structures installed in lateral, main, trunkand interceptor sewers for the purpose of allowing accessfor maintenance and cleaning operations. Manholes arealso placed at changes in sewer direction, elevation, slope,pipe size and at junctions. Drop manholes are used whenthe difference in elevation of an incoming and outgoingsewer line cannot be accommodated by a drop in themanhole channel without creating excessive turbulence andsplashing. Manholes in straight runs of sewer lines shouldbe spaced no farther apart than the distance that can becleaned by the available equipment, (usually 300 – 500 ft).

Manholes are sometimes equipped with steps andsometimes entered using ladders. The corrosive gasses inthe collection system can cause steps to deteriorate so usecare if they are used for entry.

Be aware that manholes are considered confined spacesand their entry is therefore subject to OSHA regulations.See Chapter 2 - Operator Safety, for an overview of confinedspace entry regulations and procedures.

Manholes can be constructed from materials such as; brick,pre-cast concrete barrels and fiberglass. The most commonmanhole installation in New Mexico is the pre-cast concretemanhole with a poured in place base. Manholes of thistype have six (6) constituent parts. They are:

1. A poured in placeconcrete base withchannels and a slopedbench.

2. The inlet/ outletpiping, sealed where itpenetrates the barrelsection.

3. Pre-cast concretebarrels that fittogether and aresealed with mortar orbituminous material.

4. A concentric oreccentric conesection.

5. Level adjustmentrings for raising thegrade of the manholelid.

6. Standard tight fittingmanhole cast ironring and cover.

Figure 4.6 shows the componentsof a pre-cast concretemanhole.

GRAVITY SEWER MAINTENANCESEWER CLEANING METHODSFrom the time that sanitary sewers were first used by earlycivilizations, methods of cleaning them have beenemployed. Because wastewater carries solids, scum andgrease, collection system lines require regular cleaning toclear blockages. Collection lines also provide and idealplace for roots to grow and therefore blockages caused byroot intrusions are very common. There are many ways toclean sewer lines. Some are more effective on grease, someon roots and others on sand and sediment blockages. Themethods employed around New Mexico often have as muchto do with tradition as with economics and effectiveness.The following sewer cleaning methods are common to NewMexico:

Hand RoddersHand rodders represent the oldest style of cleaningequipment. Hand rodders are typically made of a coil ofspring steel or attachable segments of spring steel that canbe forced into a sewer line to dislodge a blockage. Becausethey are non-mechanical they are reliable, however someblockages, such as roots, are difficult to clear with handrodders. Also, hand rodders are typically limited in lengthto around 100 ft and therefore their effectiveness is limitedto their length and the eagerness of the operator. Becausethey can be used in areas where the access to the sewer islimited hand rodders should be a part of every collection

Figure 4.5 - Power Rodder OperationReprinted, with permission, from Operation & Maintenance of Wastewa-

ter Collection Systems, Vol. I, 3rd ed., Office of Water Programs,California State University, Sacramento Foundation

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Figure 4.6 - Pre-Cast Concrete ManholeReprinted, with permission, from Operation & Maintenance of Wastewater Collection Systems, Vol. I, 3rd ed., Office of Water Programs, California

State University, Sacramento Foundation

4-8Figure 4.7a - Sewer Rodding Tools and Uses

4-9Figure 4.7b - Sewer Rodding Tools and Uses

Reprinted, with permission, from Operation & Maintenance ofWastewater Collection Systems, Vol. I, 3rd ed., Office of Water Programs,

California State University, Sacramento Foundation

4-10

system operator’s toolbox, no matter how large or smallthe system is.

Power RoddersPower rodding machines use a rotating steel rod or flexiblecable to turn a rodding tool in the sewer line to clearblockages.Steel rods are either continuous or sectional, flexible cablesare continuous but can have sections added to increase theirlength. Both are stored in a reel-type cage that allows themto be fed out while rotating. The rotating reel may be drivenby a motor, a small engine or a power take off (PTO). Avariety of power rodder tool heads exist for clearingdifferent types of blockages.

Power rodders are typically mounted on a specialized truckor trailer that includes storage for tools, various cutters,etc. Power rodders are capable of clearing the worstblockages caused by roots, grease and sediment. After aline has been cleared with a power rodder it should beflushed or hydraulically cleaned to restore full flowcapacity.

High Velocity Hydraulic Cleaning Machines,(Jet Rodders)High velocity hydraulic cleaning machines, (also knownas jet rodders), use water pressure to propel a nozzle andits attached hose through the sewer line. The nozzle isequipped with jets that scour debris loose and move it towarda manhole where it can be removed. Jet rodders consist of

a water supply tank, a high-pressure pump and an auxiliaryengine for driving the pump. These units have a powereddrum reel capable of holding at least 500 ft of hose, (usuallyone-inch I.D. for large machines). Many different nozzlesare available for accomplishing different cleaning chores.These machines can be mounted on a small trailer or apurpose built truck. They are commonly mounted on autility vehicle that combines a jet rodder and a vacuumdevice for removing grit and debris from the sewer. Jetrodders are effective at opening blockages and removinggrease or sediment. With a root cutting attachment theycan remove roots from sewer lines, although they are notusually as effective as a power rodder at this task. Oneadded benefit of jet rodding machines is that with a washdown gun attachment they can be used to clean manholesand lift stations and for other cleaning jobs where pressurizedwater is not easily available.

PERFORMING CLEANING AND MAINTENANCE

OPERATIONSPipeline cleaning and maintenance operations fall into oneof three categories, these are preventive maintenance,emergency clearing of line blockages (sewer back-ups) andemergency repairs to the system. The old adage “an ounceof prevention is worth a pound of cure” is certainly truewhen it comes to collection systems preventivemaintenance.

Figure 4.8 - Jet Rodder - High Velocity Cleaning Operations

Reprinted, with permission, fromOperation & Maintenance of

Wastewater Collection Systems, Vol. I,3rd ed., Office of Water Programs,

California State University,Sacramento Foundation

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Pipeline cleaning preventive maintenance programsgenerally have three aims:

1. Minimize the number of stoppages per mile ofsewer.

2. Minimize the number of odor complaints.3. Minimize the number of lift station failures.

Preventive maintenance can include activities such as;cleaning problem areas to avoid line blockages, chemicaltreatments to prevent root growth and removal of sedimentfrom lines and manholes. If the preventive maintenanceprogram is well thought out and conducted on a regularbasis, problems in the collection system will be minimized.This will result in fewer sewer back-ups, a reduction ofodors (and the resulting complaints) and lift station callouts will be reduced. All individuals served by thecollection system benefit from these reduced problems butit is the operator that has to deal with a lift station failure ora sewer back-up (always on Super-Bowl Sunday) thatbenefits the most from preventive maintenance.

Unlike preventive maintenance activities that are performedeven when the system is working well, dealing with sewerback-ups and making emergency repairs involves returningthe system to operation. When an operator encounters asewer back-up the aim should be to correct the problem asquickly as possible. Finding the best method of correctingthe problem comes first. Things to consider include:

Does this line have a history of previous stoppages?(A root or grease problem for example).Are trees growing near the line?Has a new connection been installed in the arearecently?Have repairs been made recently either to the seweror to other utilities or the street?Are there any ground or surface indications, suchas settlement or a sinkhole?

With these things considered the best approach to correctingthe problem can be chosen. Usually rodding operationsare conducted from the manhole immediately downstreamof the blockage rodding upstream toward it. This allowseasy access to the line (it is not submerged underwastewater) and provides the best position to remove debristhat is released by the cleaning operation. This debris (roots,grease, sand and sediment) should not be allowed to movefurther down the collection system because it will likelycause another blockage. If a large blockage has been clearedand it is obvious that a large amount of septic wastewaterwas trapped behind the blockage the operators of thewastewater treatment plant should be notified so that theycan take actions that will help the treatment plantaccommodate the septage.

Because of traffic control and safety issues collectionsystem work such as clearing sewer back-ups andemergency repairs should be performed by a minimum oftwo operators.

RECORDSWhenever a blockage is cleared or a repair or maintenanceactivity has been performed a complete record should bemade for future reference. The record should include allthe important information including where and when theincident occurred, distance and cause of blockages, linesize, manhole number and a note on the kind and amountof material removed.

LIFT STATIONS

PURPOSE OF LIFT STATIONSLift stations are used to raise wastewater from a lowerelevation to a higher elevation. Pumps are used to movewastewater through a discharge pipe known as a force main.After discharge from the force main, wastewater resumesgravity flow. The location and design of lift stations isdecided by economics and practicality. Some of the reasonsthat a lift station may be required include;

Excavation costs to maintain gravity flow andscouring velocity become excessive.Soil stability is unsuitable for trenching.Ground water table is too high for installing deepsewers, andPresent wastewater flows are insufficient to justifyextension of sewer main and lift station offerseconomical short-term solution.

Lift stations should be designed to move the wastewaterwith maximum efficiency. The pumps size and type shouldbe selected to provided the most constant flow rate possibleto minimize surges of flow at the downstream gravity seweror the wastewater treatment plant. The appearance shouldblend in with the surrounding area and odors, noise andrubbish should be dealt with immediately.

TYPES OF LIFT STATIONSLift stations can be described with two broad categories:Wet Well type and Dry Well type.

Dry Well Lift StationsDry well lift stations contain two chambers. One forcollecting the wastewater before it is pumped and the otherto contain the pumps, motors, valves, electrical controlsand auxiliary equipment in a dry well where access is easyfor service. Dry well lift stations range in size from justlarge enough for a man to enter to large installations thatmay even be constantly manned.

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Wet Well Lift StationsWet well lift stations contain only one chamber, the wetwell where wastewater is collected before it is pumped.The pumps may be located above the wet well, (which isknown as a suction lift pumping arrangement), or the pumps

may be located inside the wet well itself, (submersiblepumps). Both locations have their advantages anddisadvantages. Suction lift pumps can be above ground orin a shallow pit where they can be easily serviced andrepaired, but these types of pumps are prone to losing their

Figure 4.9 - Sewage Lift Stations

4-13Figure 4.10 - Submersible Pump in Wet Well

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prime, particularly when they age. If a pump loses prime,the motor will run and the impeller will turn but the pumpwill not in a shallow pit where they can be easily servicedand repaired, but these types of pumps are prone to losingtheir prime,pump anything. Submersible pump installationsnever lose prime because the pump intakes always havestanding water over them (suction head condition).However, because the pumps are located in the wet wellthey can be hard to access if not designed properly.Equipment located in the wet well should be kept to aminimum. Discharge valves, check valves and otherequipment that requires periodic maintenance should belocated in a concrete vault or small room adjacent to thewet well. When submersible pumps are used in wet welllift stations they should be designed to be removed fromthe surface without entry into the wet well, (which isconsidered a confined space).

LIFT STATION COMPONENTSPumpsLift stations generally employ centrifugal pumps,pneumatic ejectors or screw type pumps, (centrifugalpumps are the most common in New Mexico). ReviewChapter XXX, Pumps and Motors for information of pumpconfiguration, operation and maintenance.

Wet WellThe wet well can be constructed of pre-cast concrete rings(similar to manhole rings), poured in place concrete,fiberglass or metal. It should be designed to withstandcharacteristics common in the wastewater it will pump. Thewet well should be sized so that the pumps will not have tocycle too often, which can cause unnecessary wear, yet notso large that the wastewater becomes septic due to excessiveholding times. (This can also be altered to some extend bychanging the operating levels by adjusting the pumpcontrols).

HardwareThe hardware uses in lift stations should be either high-grade aluminum or stainless steel to prevent corrosion.

Bar RacksIt is often desirable to include a bar rack at the inlet to thewet well of a lift station to catch large objects and ragsbefore they enter and damage the pumps. If bar racks areincluded they must be made of the appropriate material(aluminum or SS) and they must be easily cleaned withoutentry into the wet well.

Dry WellDry well structures commonly have two or more floorlevels. Pumps and valving are located on the lowest levelwhile electrical controls and motors are often located on

an upper level. A sump pump is provided to removed sealwater or any water that leaks into the dry well. Ventilationand atmospheric monitoring are provided to preventdangerous conditions from developing.

ValvesValves are of critical importance to lift station O & M butthey are frequently neglected, abused, misused and installedat improper locations. The major valves found inwastewater lift stations are:

Pump suction and discharge isolation valves (gatevalve, plug valve or knife valve). Isolation valves areused to section off the pump when service is required.These valves should not be used to throttle (control)the flow into or out of the pump because the valve andpump could be damaged.Discharge check valves, (swing check or ball checkvalves). Check valves prevent water from flowingbackward through the pump when the pump shuts off.Hard closing or noisy check valves indicate thatsomething is wrong, such as air trapped in the forcemain.Cross connection control valves. These are discussedlater in this text.

Electrical SystemsAll but the smallest lift stations are provided 3-phaseelectrical service so that the system will work efficientlyand for practicality. Many lift stations have emergencyback-up generators to maintain pumping during poweroutages. If a generator is not provided a transfer switchshould be included so that a portable generator could berapidly hooked up to maintain pumping during times ofloss of power.

AlarmsBecause the failure of a lift station could result in damageto homes, property and the environment all lift stationsshould be equipped with some type of audible and visualalarm at a minimum. For large installations a computerwill automatically contact operators in the event of a failureor the system will be monitored through telemetry. A “highwater alarm” should be included for every installation.

Motor Control Center (MCC)The pump motor controls are located in a motor control center.This is typically an electrical box with switches for operatingthe pumps in either hand (manual) mode, in automatic mode orfor turning them off. For this reason these are known a H-O-A switches (Hand-Off-Auto). MCC panels usually includepump starter coils, which allow the 110 V switch to engage thehigher voltage motor circuit and a system for alternating leadand lag pumps. Alarm lights and reset buttons will also belocated on the MCC panel.

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Figure 4.11 - Pneumatic Controllers

Hours RecordersThe pumps in a lift station should be equipped withequipment hours recorders so that the total run time of eachpump can be compared. If one pump in an automaticalternating duplex lift station runs much more than the otherit is a indication that something is wrong, such as a leakingcheck valve. Also, flow through the lift station can beestablished if the pumping rate and operating hours areknown.

Pump ControlsAll lift station pump controls are similar in that they controlthe pumps based on the level of water in the wet well.Controls vary greatly in how they accomplish the task.Common lift station control systems used in NewMexico include:

Float controllers. Floats are one of the oldestpump control devices. As the water in thewet well rises, the float (a ball attached to arod) rises with it until a switch is triggeredwhich turns the pump on. When the pumplowers the level to a pre-set point an actuatorshuts the pump off.Electrode Controllers (probes). Electrodecontrollers utilize electrode probes or leadsthat are an open circuit until the water levelrises, wetting both electrodes and allowingcurrent to flow which enacts the controlcircuit on the pump to turn it on. When thewater level drops and the upper probebecomes exposed, the circuit opens and thepump shuts off. This type of control issubject to problems caused by grease andrags.Pneumatic Controllers (bubblers).Pneumatic controllers work by sensing thepressure required to force air bubbles out ofa tube located near the bottom of the wetwell. When the wet well is full, it takes morepressure to force bubbles out of the tube(due to the higher head created by thestanding water). The on off set points forthe pumps on this type of controller areactually pressure set points. Bubblers relyon an air pump, similar to a fish aquariumpump, to provide air so that the pressure canbe measured as bubbles are forced out thetube near the bottom of the wet well. If thetube becomes clogged by a rag or grease,or the air pump fails the system will stopworking. If the length of the bubbler linechanges, (accidental damage whileremoving a pump for example), the systemwill function incorrectly, if at all.

Mercury Float Switches. Mercury float switches areone of the most common types of lift station controls.They consist of a float that contains a sealed chamberwith two electrodes and a small amount of mercury(quicksilver). When the float is in one position (vertical,while hanging for example) the mercury is pooled awayfrom the electrodes and the circuit is open. When thefloat is tilted to the other position (horizontal, whilefloating) the mercury flows down to the electrodes andthe circuit is completed which triggers the pump controlto turn the pump on. Mercury float switches can bepurchased as either normally open (on switch) ornormally closed (off switch) when in the vertical position.The floats must be kept clean so that they will tilt when

Figure 4.12 - Mercury Float Switch

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floating and they must be lowered carefully back downinto the wet well after they have been cleaned andchecked.

Force MainsA force main is the discharge line for the lift station. Thedischarge lines of the pumps come together in a manifoldand then enter the force main. Air accumulation in forcemains can create a problem know a water hammer, whichis a high pressure shock wave that travels up and down aforce main. This problem is associated with pump checkvalves slamming, sometimes repeatedly, which causesdamage to valves and piping. Air release valves arenormally installed at the high points in force mains toautomatically blow off accumulated air.

TROUBLE SHOOTING LIFT STATION PROBLEMSLift station problems can be summarized in the followingfour categories:

1.Power [power outage, electrical circuit failure, motorburned out].

2.Control System [pump control failure, telemetry systemfailure].

3.Pumping System [pump failure].4.Structural [grit deposits, plugged force main or check

valve].Experience is often the best tool in the operator’s toolboxwhen it comes to trouble shooting. All but the very simpleelectrical problems (changing fuses) should be left to alicensed electrician unless proper training has been givento operators. Most lift station failures can be preventedthrough a well though out preventive maintenance program.

INFILTRATION AND INFLOW (I & I)Ground water that enters into the sewer system throughbroken joints and leaking manhole barrels is referred to asinfiltration. Storm water that enters manhole covers andillegal connections like gutter drains routed into house cleanouts is known as inflow. Both infiltration and inflow cancontribute significant amounts of water to the wastewatercollection system. Once in the collection system the I & Ibecomes wastewater that must be treated by the treatmentplant. Some treatment plants become hydraulicallyoverloaded during storm events from inflow or during thespring run-off from infiltration. Because of this theidentification and control of infiltration is important to thewastewater collection system operator.

IDENTIFICATION OF I & IStudies conducted to identify I & I can be very elaborateinvolving engineering consultants and costing largeamounts of money or they can be basic and conducted bysystem operators. The size of the system and the extent ofthe problem will dictate what measures are necessary.

Methods that are commonly used for identifying the locationand extent of infiltration and inflow are:

Late Night Survey. Very little flow should be occurringin the collection system in the early morning hours(2:00 AM – 4:00 AM). By surveying manholes forclear water that is near ground temperature ageneralized idea of the extent of infiltration can bemade. (This may not work in all areas of largersystems).Closed Circuit Television (CCTV) inspection. CCTVinspection is often used to identify infiltrationproblems. A purpose built camera is inserted into thecollections lines and the line is video taped so problemareas can be analyzed. A thorough line cleaning priorto video tapping is very important.Smoke Testing. Smoke testing is used to identifybroken joints and leaking manhole barrels that couldallow infiltration to enter the system and to identifyillegal taps (customer not paying sewer fee) orconnections to the sewer system that would allow stormwater to enter. For this test smoke is forced into thecollection system by an engine driven fan located overa manhole opening.Flow Records. Wastewater treatment flow recordsoften are used to identify storm events that introduceinflow into the collection system. Also, flow recordsthat show a constant early morning flow during periodsof run-off can be used to identify infiltration. Chartrecording flow measurement devices allow the volumeand duration of I & I to be characterized.

CONTROL OF I & INumerous methods are available to designers and engineersto correct infiltration and inflow problems once they havebeen identified. These include sewer replacement, sliplining (in-situ form), pipe bursting, chemical grouting andimprovement of storm sewers.Not many of these options are within the capabilities of theaverage collections department, so contractors most oftenperform them. Collections crews do have the ability tominimize some infiltration and inflow problems, such asrepairing or replacing individual deteriorated manholes,raising manhole rings and covers in area that flood, andrepairing broken joints when they are discovered.Whenever possible, collection system operators shouldwork to limit I & I so that the treatment plant can performits job of treating wastewater.

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Figure 4.13 -Air Gap for Cross-Connection Control

CROSS CONNECTION CONTROLA cross connection is connection between a potable(drinking) water system and water from an unsafe orunknown source. A common place in wastewater collectionsystems where this can occur is on the seal water line for alift station pump. When potable water is used as the sealwater supply and a no protection is in place, a direct crossconnection exists. Without protection, wastewater frominside the pump could enter and contaminate the potablewater system. Several devices for controlling crossconnections exist.

The use of these devices is related to the “degree of hazard”that exists to the potable water system. An air gap deviceoffers the highest degree of protection and is commonlyused in wastewater applications.

Several excellent programs are available to train operatorsabout the dangers of cross connections and the devices usedfor control cross connections.

AS-BUILT PLANSAt the start of a sewer construction project, the inspectorshould obtain two sets of the plans for the project. Onecopy is the working plan that will be used to guide theconstruction project. The other copy is known as the “as-built” plans. Any daily construction that deviates from theworking plan should be recorded on the as-built plan bythe inspector. This plan then becomes the true record ofwhere stub-outs and taps are placed, where lines andmanholes are located, etc.

The as-built plans reflect what actually exists underground.Because of this fact, they are invaluable to the collectionsystem operators. When a project is completed, the as-built plans are submitted to the project engineer who either

files them directly or has a revised drawing made. Theseplans should always be kept available to collections systemworkers.

All too often few or no maps exist of the wastewatercollection system. Although some collections workers mayfeel that this offers “job security”, the headaches broughtabout by not having proper plans to work with probablyoffsets this feeling.

ReferencesOffice of Water Programs, California State University,Sacramento, Operation & Maintenance of WastewaterCollection Systems, Vol. I, 3rd ed., Chapters 1 – 7Office of Water Programs, California State University,Sacramento, Operation & Maintenance of WastewaterCollection Systems, Vol. 2, 5th ed., Chapter 8