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IMPLEMENTATION OF SEWER AND DRAINAGE SYSTEM SOLIDS DEPOSITION

CONTROLS, CAMBRIDGE, MASS.

William Pisano P.E., Ph.D, Owen ORio!"an P.E., F!an# A$o%%e P.E.,Dennis Ca!!, P.E., Ma%%hew T!a&e!s P.E., 'ohn (is)on%i, P.E.

MW* Ame!i)as, In).

+ Fa!nswo!%h S%!ee%, Se)on" Floo!

Bos%on, Massa)h-se%%s +

ABSTRACT/

Deposition of solids within flat drainage and sewerage conveyance pipes can result inproblematic hydraulic restrictions, potential odor and corrosion conditions, and the initial flushof pollutants and solids to receiving waters. This paper reviews planning, design and operationaldetails for managing problematic sedimentation problems within three new major sewerseparation project areas in the City of Cambridge. These projects are elements of the Citys $3million dollar, !"year program of area wide sewer separation and storm water management, andcover a combined area of #! ha of dense residential, commercial and institutional land uses.Two of the three projects described use passive automatic flushing systems to manageproblematic sedimentation problems, while the third project includes a networ% of in"line gritsump manholes, e&tensive catch basin rehabilitation, and isolation structures at river outfalls.

The first project described summari'es the design of passive automatic flushing systems installedin the City of Cambridges storm and sanitary sewer system tributary to the (lewife )roo% aspart of a $*+ million dollar sewer separation program. rit and debris deposition is severe in thee&isting combined sewers, storm drains, and sanitary trun% sewers due to the flat topography ofthe area. This condition is e&acerbated by hydraulic constraints imposed on the systems outletby the (lewife )roo% -shallow stream and downstream sanitary siphons -again because of the(lewife )roo%. The use of pumps to lift flows from sewers and drains to permit self"scouringvelocities is prohibitively e&pensive. To overcome this problem, five automated flushing systemsusing /uic% opening -hydraulic operated gates discharging collected storm water are constructedin conjunction with downstream collector grit pits covering a distance of 0# m for storm drainpipes ranging from 0.# m circular to 0.! m by 0.1 m rectangular. 2ew #+ mm and mmsanitary trun% sewers, +0 m long are flushed daily by two flushing systems using spent filtratewater from Cambridges water treatment plant recently constructed nearby. The flushing systemsare si'ed to achieve wave velocity of 0 ms the end of the flushing segment. The flush vaultvolumes range from 00 to # m3for the storm drain systems and m3for the sanitary system.Construction was completed in 4ay !! and functional testing of the flushing systems occurredin !#. 5artial test results are reported.

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This paper also reviews the Crescent Carver sewer separation and storm water managementproject behind 6arvard 7niversity. This project is an element of a multi"phased programdesigned to reduce sewage and storm water flooding within a 0 ha densely populatedresidential area. 8i& new storm water off"line retention tan%s totaling +9 m3have been

constructed as the e&isting combined sewer system in the area is over a century old and cannotcontain the high rate of storm water and sanitary flow during heavy rainstorms. These conditionsresult in sewage surcharging from the system causing local area public health and floodingproblems. 8everal large diameter sanitary sewers were once used as combined sewers, but in thelast several decades were converted to :over and under; stormsanita ry systems withcommon manholes. (s part of this wor%, these common manholes were separated. The sanitaryconduits are e&tremely flat and sewer solids deposition and odor problems have beenproblematic. (utomated flushing systems using collected storm water from several catch basinswere constructed to flush the 0 mm by 9! mm sanitary sewers. To date all deposition andodor problems have been eliminated.

The third topic reviewed is the Cambrideport improvement program in the southeast portion ofCambridge including the western portion of the 4

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se/uencing plan was developed to ensure that improved and or new drainage systems had bothade/uate hydraulic conveyance capacity and means for stormwater solids to be captured in placeprior to common manhole removals.

INTROD0CTION

The deposition of sewage solids during dry weather in combined sewers has long beenrecogni'ed as a major contributor to >first"flush> phenomena. (nother manifestation of :first"flush;, in addition to the scouring of materials already deposited in the lines, is the first flush ofloose solid particles on the urban ground surface that are transported into the sewerage systemand not trapped by catch basins or inlets. These particulate materials may settle out in the systemand be available for scour and re"suspension during wet periods. 8uch materials also create firstflush loading from storm drainage systems.

Deposition of heavy solids is also a problem in separate sanitary systems and can also result insignificant odor and corrosion problems. 8ewer sediments create odor problems due to septicconditions in the sewer that result from the activity of microorganism and anaerobic conditionsof the sediment layer. The process begins with the biological reduction of sulfate to sulfide bythe anaerobic slime layer residing on pipe and sediment surfaces below the water in wastewatercollection systems. The resulting sulfide ion is transformed into hydrogen sulfide gas afterpic%ing up two hydrogen ions from wastewater. ?nce released to the sewer atmosphere, aerobicbacteria and fungi that reside on sewer walls and surfaces above the water line consume thehydrogen sulfide gas and secrete sulfuric acid.

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are used to clear bloc%ages once they have formed, and also as preventative maintenance tools tominimi'e future problems. Bith the e&ception of flushing, these methods are generally used in a>reactive> mode to prevent or clear up hydraulic restrictions.

5ower rodding includes an engine and drive unit, steel rods and a variety of cleaning and drivingunits. The power e/uipment applies tor/ue to the rod as it is pushed through the line, rotating thecleaning device attached to the lead end. 5ower rodders can be used for routine preventativemaintenance, cutting roots and brea%ing up grease deposits. 5ower rodders are efficient for pipesup to 3 mm.

)alling is a hydraulic cleaning method in which the pressure of a water head creates highvelocity water flow around an inflated rubber"cleaning ball. The ball has an outside spiral threadand swivel connection that causes it to spin, resulting in a scrubbing action of the water along thepipe. )alls can remove settled grit and grease buildup inside the line. This techni/ue is useful for

sewers up to mm. 5oly pigs, %ites, and bags are used in a similar manner as balls. The rigidrims of bags and %ites cause the scouring action. Bater pressure moves these devices against thetension of restraining lines. The shape of the devices creates a bac%ward jet of water. The polypig is used for large sanitary sewers and is not restrained by a line, but moves through the pipesegment with water pressure buildup behind it.

etting is a hydraulic cleaning method that directs high velocities of water against the pipe wallsat various angles. The basic jetting machine e/uipment is usually mounted on a truc% or trailerand consists of water supply tan% of at least # , a high pressure water pump, an au&iliaryengine, a powered drum reel holding at least 0+ m of !+ mm hose on a reel having speed anddirection controls and a variety of no''les. etting is efficient for routine cleaning of smalldiameter, low flow sewers.

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Elushing of sewers either by manual or by automated means is generally used to reducehydraulic restriction problems and infre/uently as pollution prevention approach. The concept ofsewer flushing is to induce an unsteady waveform by either rapidly adding e&ternal water orcreating a >dam brea%> effect by the /uic% opening of a restraining gate. This aim is to re"

suspend and transport deposited pollutants to the sewage treatment facility during dry weatherandor to displace solids deposited in the upper reaches of large collection systems closer to thesystem outlet. During wet weather events these accumulated loads may then be more /uic%lydisplaced to the treatment headwor%s before overflows occur or be more efficiently captured bywet weather first flush storage facilities.

4anual methods usually involve discharge from a fire hydrant or /uic% opening valve from atan% truc% to introduce a heavy flow of water into the line at a manhole. Elushing readilyremoves organic deposits@ saturated, water logged floatable solids and fine sand and grit, but isnot very effective for removing heavy debris.

A0TOMATED FL0S*ING E10IPMENT

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*YDROSELF

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VOLUME=35m3

2000mm

300mm

Fi2-!e +3 Fl-shin2 Ga%e Ins%alla%ion Whi%%en, Ge!man$

PUMPFEED FROM RIVER

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BIOGEST (AC00M FL0S*ING SYSTEM

( variation of the 6GD=?8HE is the )

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?nce sediment has been entrained, it may travel down the sewer in one of two general ways.Einer, lighter material tends to travel in suspension, while heavier material travels in a rolling,sliding mode as bed load.

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To meet these criteria, a modified design approach has been developed by the 7nited FingdomConstruction

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FRES* POND PAR4WAY SEWER SEPARATION PRO'ECT/ STORM DRAIN AND

SANITARY SEWER FL0S*ING SYSTEMS

?ver the last twenty years, the City of Cambridge has aggressively separated old combined and

over -storm and under -sanitary sewerage systems throughout the city to enhance drainageservice and to improve the water /uality in the (lewife )roo% and the Charles =iver. 5resently,the City is in the construction phase of separating a 0 ha catchment %nown as C(4 # northwest of 6arvard 8/uare and within a highly urbani'ed and heavily traveled area. Eigure ! depictsthis catchment.

Fi2-!e Th!ee Ci%$ o5 Cam6!i"2e S%-"$ A!eas

rit deposition within both e&isting sewerage and storm drainage systems is a major problembecause of general flatness of the area, the use of siphons in the collection system, and the highbac%water levels that occur in the receiving water bodies during wet weather conditions, whichfre/uently bac%waters into the storm systems. The e&isting and recently constructed storm drainson Eresh 5ond 5ar%way and Concord (venue have invert slopes of appro&imately .3 to.+. Deposition of the stormwater solids that are not captured by the surface best

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management practices -)45s and discharge into these conduits would be severe. 8ince nochemical salting during winter conditions can be tolerated in the low, flat Eresh 5ond=eservation watershed, heavy winter sanding only e&acerbates potential deposition problems.To overcome this problem, automated flushing systems using /uic% opening -hydraulic operatedflushing gates to discharge collected stormwater will flush grit and debris to downstream

enlarged manholes with open grit pits. rit pits are not provided on the sanitary systems beingflushed.

The storm drain and sanitary sewer systems to be flushed are located along the eastern side of theEresh 5ond =eservoir. They start near the Cambridge Bater Treatment 5lant -CBT5, continueeast to Concord Circle and then northeast to the Eresh 5ond Circle. )oth systems then proceeddown Bheeler 8treet. 8ee Eigure 3 of the general locus plan showing locations for the twosanitary sewer and two storm drain flushing vaults.

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Fi2-!e 7 8 Fl-shin2 (a-l%s o5 %he F!esh Pon" Pa!#wa$ Sewe! Se9a!a%ion P!o:e)%

The piping systems consist of appro&imately +0 m of sanitary trun% sewers, ranging from #mm to mm, and appro&imately 0# m of storm drains with pipe si'es ranging from * mmto 0.! m by 0.1 m. 5ertinent design information for the flushing systems is provided in Table 0.

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Ta6le + 3 Desi2n In5o!ma%ion S-mma!$ F!esh Pon" Pa!#wa$ Fl-shin2 P!o2!am

Si%e

Downs%!eam Fl-she" Pi9e

Diame%e!;m 8 Sa)!amen%o S%!ee% Fl-sh (a-l% Dimensions an" Ca9a)i%ies

8ite ength-m

Bidth-m

Depth-m

8torage Iolume-

Elush Iault 2o. 0 +. !.+ !.9+ +,+#Elush Iault 2o. ! .9+ 3. !.9+ 00,!

The operational logic for each flush vault is identical. There are two level indicating transmittersassociated with the structures that transmit level readings to an on"site control panel. ?ne islocated in the flush water storage area upstream of the flush gate and another just downstream ofthe flush gate in the vault outlet structure. The system is designed to activate when there is a highlevel reading in the storage vault and a low level reading in the outlet structure. Bhen thissituation occurs, the hydraulically activated flush gates open and release the flush wave into thedownstream sanitary sewers. 8ince Elush Iault 2o.0 is located at the upper end of the sanitary

sewer networ% it releases first and flushes sediments downstream of Elush Iault 2o. ! whichthen flushes the combined sediments further down the system to the 4B=( trun% sewers.

Iisual observations during performance testing in !3 indicated that the flush waves near theend of each flushing segment were well in e&cess of 0 ms. 4echanical performance andreadiness statuses of these facilities are inspected on a /uarterly basis. To date the operation hasbeen satisfactory.

CAMBRIDGEPORT COMMON MAN*OLE @ STORMWATER MANAGEMENT

PROGRAM

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The 6ydroBor%s model was used to analy'e to identify problematic :solids depositors;. 4orethan 1+ percent of the entire modeled drainage system -310 mm and larger would not generatepea% velocities of 0 ms for the 3"month event. 4ore than 9 percent of the entire modeleddrainage system -310mm and larger would not generate pea% velocities 0 ms for the +"year, !#"

hour storm event. 8ee Table +. )ed load movement would be the only way of transporting solidsnot captured by catchbasins to the river. (n elaborate system of in"line grit pits -oversi'edmanholes with three foot sumps were designed into the new improvements.

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The overall proposed number of catch basin and areal densities are also noted in Table . Theaverage acreage per catch basin in the future proposed condition for the Cambridgeport area is.! ha basin. This density is commensurate with other areas in Cambridge that are presently

undergoing sewer separation with greatly enhanced level of stormwater service.

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To date nine such pits have been constructed as part of the 8idney 8treet surface improvementproject. These devices will also be incorporated into future design projects in the Cambridgeportarea.

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REFERENCES

(c%ers, .C.@ )utler, D.@ 4ay, =.B.5. -0**Design of Sewers to Control Sediment Problems,=eport 0#0, Construction 7, +#"19.

ager, .@ 8mith B..@Tchobanoglous . -0*99 Catchbasin Technology ?verview and(ssessment, H5("!"99"+0, Cincinnati, ?6.

aplace D.@ )achoc (.@ 8anche' G.-0**3 8olutions techni/ues pour gerer les depots encollecteurs visitables -Technical solutions to manage sewer solids in man entry sewers, 584,

-0, +0*"+!3 -in Erench.

aplace D.@ )ertrand"Frajews%i .@ Chebbo .@ Eelou'is . -0**1 es pieges a charriageAde latheorie a la prati/ue -)ed load trapsA from theory to practice, 5roceedings of 2?I(THC6 *1,yon Erance !, 3!*"33 -in Erench.

4ay, =.B.5. -0**3. Sediment Trans"ort in Pi"es and Sewers with De"osited Beds, =eport8=3!, 6= Ballingford td., Ballingford, Hngland.

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2alluri, C@ (lvare', H.4. -0**! The