Hydraulics Research Station Wallingford Oxon OX1O 8BA Telephone Tef ex

Transcription

1 Measurements f the hydraulic rughness f slimed sewer pipes J A Perkins and I M Gardiner Reprt N. IT 237 July 1982 Crwn Cpyright Hydraulics Research Statin Wallingfrd Oxn OX1O 8BA Telephne Tef ex

2 Abstract A series f experiments n the effect f sliming n the hydraulic rughness f ful sewers has been carried ut n a speciallycnstructed rig at a sewage pumping statin at Littlemre, Oxfrd. Three separate runs f 335,206 and 188 days duratin were cnducted n a cntinuus pipeline made up f vertically-cast cncrete, UPVC, spun cncrete, asbests-cement and unglazed clay pipes f 225mm diameter. The first tw runs were made with the pipes at a gradient f I in 250 and a velcity f 0.75mls: the third run was made at a gradient f I in 100 and a velcity f l.2m/s.in each test the flw was varied cntinuusly n a daily cycle with a peak at 1000 hurs, giving apprximately half-full depths, and a secndary minr peak at 1900 hurs. At weekly intervals measurements were made f hydraulic rughness and sewage temperature and phtgraphs taken f the slime n the pipe walls. Peridically, sectins f pipe were detached and the slime remved and weighed. The main cnclusins frm the tests are:- l. After a rapid, initial build-up f slime, there is a cmplex balance between grwth and lss due t the shearing actin f the flw, which shws sme seasnal variatins but n lng-term trend. 2. The pipe material has sme influence n the amunt f slime present and therefre n the hydraulic rughness. 3. The amunt f slime and the hydraulic rughness decrease with an increase in pipe gradient and velcity. 4. The rughness f a free-surface gravity sewer is much larger when running part-full than full. Values f hydraulic rughness fr ful sewers have been recmmended fr designers, using the Clebrk-White flw equatins r the Wallingfrd Charts and Tables(6'7). These are cnsistent with earlier recmmended values, based n spt field measurements by Ackers et 4(l).

3 Cntents Page I Intrductin 2 Descriptin f experimental rig 3 Experimental prgramme 3 4 Run hydrgraphs 3 5 Experimental measurements 4 6 Experimental results 6.1 Run I Clean water tests Sewage tests Perid frm dy 0 t day 170 Perid frm day 170 t day 230 Perid frm day 230 t day 335 Summary 6.2 Run 2 Clean water tests Sewage tests Perid frm day 15 t day 100 Perid f day 100 t day 206 Summary 6.3 Run 3 Clean water tests Sewage tests Perid frm dy 0 t day 120 Perid frm dqy 120 t dy 188 Summary 6.4 General discussin f results frm Runs1.2 and Intrductin Discussin f results Cmparisns f Runs I and 2 Cmparisns f Runs 2 and 3 J l0 11 1l ll J t2 t2 t2 13 l3 l6 r6 l8

4 Cntents (Cnt'd) Page 7 Rughness f pipes when running part-full 18 8 Measurements f slime weight Weight f accumulated slime Analysis Results E.2 Crrelatin between slime weight and hydraulic rughness Crrelatin between slime weight and velcity 8.4 Appearance f slime in the test pipes l Cmparisn f present research with previus wrk Ackers, Crickmre and Hlmes 24 Bland, Bayley and Thmas 26 Yang and Reid Recmmendatins fr design and analysis Summary and cnclusins Acknwledgements References 30 Tables 1 Rughness f clean pipes ' 2 Disslved xygen measurements

5 Cntents (Cnt'd) Figures I Layut f rig 2 Daily hydrgraphs - Run (1976) 3 Daily hydrgraphs - Run 2 (1977) 4 Daily hydrgraphs - Run 3 (1980) 5 Variatin f rughness with time - Runs 1 and 2 6 Variatin f rughness with time - Run 3 7 Disslved xygen levels in Oxfrd sewers 8 Variatin f rughness in test pipes 9 Variatin f rughness with depth f flw - Run I 10 Variatin f rughness with depth f flw - Run I ll Variatin f rughness with depth f flw - Run 2 12 Variatin f rughness with depth f flw - Run 2 13 Weight f slime scraped frm pipe walls - Run I 14 Weight f slime scraped frm pipe walls - Runs 2 and 3 15 Relatin between slime weight and pipe rughness 16 Variatin f slime weight with time 17 Bquilibrium slime weight as a functin f velcity l8 Variatin f rughness with velcity: Ackers and Littlemre data 19 Rughness as a functin f age in cncrete sewers Plates 1 Test rig 2-7 Bxamples f slime in asbests-cement test pipes

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7 lntrductin There is a limited amunt f data n the effect that sliming has n the hydraulic resistance f ful sewers. In the UK the principal surces f data have been the wrk f Ackers, Crickmre and Hlmes(l) and that f Bland, Bayley and Thmas(2). In 1973, the Department f the Envirnment Wrking Party n Sewers and Water Mains recmmended(3) that further research be carried ut, in rder t extend the range f the wrk previusly undertaken. Bland's experiment5(2) 6a6 been carried ut using l00mm diameter pipes, flwing full under pressure with the sewage being recirculated thrugh the test rig. It was cnsidered that these cnditins did nt truly represent typical cnditins in a nrmal gravity sewer, in which there is usually an air-water interface and a variatin in flw rate thrughut the day. The Wrking Party supprted the Hydraulics Research Statin's (HRS) prpsals t carry ut experiments n 225mm diameter pipes f the materials cmmnly used in the drainage industry, with fresh sewage circulated thrugh the experimental pipe, and with the pipes flwing nly part-full, the flw varying with time in a similar fashin t the flw variatin in a nrmal gravity ful sewer. An experimental rig was cnstructed by HRS at the Littlemre sewage pumping statin f the Thames Water Authrity with the cperatin and assistance f TWA staff. At this site there was a sufficient flw f sewage fr the requirements f the experiments, thus aviding the need t re-circulate thrugh the experimental rig. The pumping statin is lcated at the end f ne f the main Oxfrd sewers, and frm there, sewage is pumped t the main treatment wrks at Sandfrd. The sewage is mainly dmestic, but includes a small amunt f surface water and sme trade and industrial waste amunting t 4 per cent (apprximately) f the dry weather flw (DwF). Descriptin f experimental rig The experimental rig cmprised a pipeline l57m lng, 225mm in diameter, laid abve grund and supprted n a series f screw jack prps. The site did nt permit a cntinuus length f straight pipe, s that it was necessarv t include a 180" bend in the middle f the pipeline. The generalayut f the rig is shwn in Fig I and Plate l. Five test sectins, frm l3 t 20m lng, were incrprated in the pipe line: these cmprised asbests-cement (20m length) spun cncrete (20m) vertically cast cncrete (l3m) unglazed clay (20m) and UPVC (l8m). Upstream frm each f the test lengths, there was a shrter length f pipe f the same material as the test length: this was meant t serve as an entry length and prvided a transitin between test lengths f pipes f different materials. The test length f vertically cast cncrete was incrprated int the rig at a very late stage in design and as a result f space limitatins it was shrter than the test lengths f the ther materials. Vertically cast pipes were included because a significant quantity f this type f pipe is used, and because f the impressin that.the surface rughness was different frm that f spun cncrete pipes.

8 Ideally the pipes shuld have been arranged in parallel but the prblems invlved in prviding a cntrl system that wuld supply identical varying flws t each f the individual pipe lengths and in supplying the large vlume f flw needed, ruled this ut. It was therefre decided t arrange the pipes in series, particularly as it was thught that this wuld nt have any influence n the slime grwth in the varius pipes: ne f the cnclusins frm the wrk f Bland et al(2) was that the rder in which the different pipe materials ccurred in their pipeline was nt a significant factr in their experiments. Dwnstream frm each test length there were shrt pipe sectins f the same material, specially jinted t allw them t be remved easily frm the test rig fr the examinatin and remval f slime grwth. Each test length was fitted with a number f sets f pressure tappings; each set cnsisted f fur brass inserts f 3.l8mm ID, set int the pipe. The fur tappings were at 0" (sffit f pipe), 60", 137" and 240", arund the pipe circumference and were inter-cnnected s that they measured a mean pressure ver the crss-sectin f flw. The varius sets f pressure tappings were linked t manmeter bards fitted with vernier scales that enabled the pressures t be measured t within +0.lmm. In rder t prevent the water in the manmeter tubes frm freezing during the winter,, a ljv slutin (by vlume) f anti-freeze in water, was used in the tubes. The anti-freeze was a mixture f glycl (2 parts by vlume) and methanl (l part). The psitins f the pressure tapping rings alng the test lengths were: Pipe material Vertically cast cncrete UPVC Distance frm dwnstream tapping ring (m) 0, 2.47, 4.95, 7.35, , 5.46,5.98, I1.44, 11.99, Spun cncrete 0, 3.67,7.33, ll.0l, Asbests cement 0, 3.99,7.98, 11.98, 15.97, Clay 0, 3.03, 6.06,9.09, l2.ll, Special access hles were cut int the sffits f the pipes at the upstream ends f the test lengths in rder t allw the insertin f a camera fr taking regular phtgraphs f the interir f the pipes. The whle f the pipeline was insulated with 38mm f expanded plystyrene in rder t prtect it frm extremes f temperature and thus maintain the sewage at the same temperature as the sewage arriving at the pumping statin. The arrangement fr supplying flw t the test rig was t pump frm the wet well f the pumping statin up t a cnstant head tank 8m abve the grund. Here the flw was split, part f it returning t the wet well by means f an verflw in the cnstant head tank, the remainder passing t the test pipeline via a tilting weir, whse angle (and hence the discharge ver it) was cntrlled in a predetermined pattern by a rtating cam mechanism. The sewage in the wet well had already received sme primary treatment t the extent f having grit and mst f the rags remved by grit channels and by carse screens. The reasn fr taking the sewage frm the wet well rather than frm the trunk sewer upstream frm the primary treatment, was because the intentin was t perate the 2

9 experiment cntinuusly ver a lng perid f time. If cmpletely untreated sewage had been used, it was certain that there wuld have been cntinual prblems frm rags blcking the pipeline and interfering with the peratin f the flw cntrl apparatus in the cnstant head tank. As it was, prblems f this type were still experienced, despite the sewage having received sme treatment. A standard flat-vee gauging weir was installed at the dwnstream end f the pipeline fr measuring the discharge during the experiments. An air vent tank was installed at the upstream end f the experimental pipeline in rder t allw the escape f any air that had been entrained in the sewage in its passage frm the cnstant head tank t the pipeline, by way f a vertical drp pipe. In rder t be able t determine the rughness f the pipes in the clean cnditin, befre any sliming had taken place, the rig was designed s that clean water culd be circulated thrugh the experimental pipeline - a separate fresh water pump was installed and a tank was cnstructed t serve as a fresh water reservir. Experimental prgramme Three separate, lng term runs, were carried ut. There were as fllws: Run I - Pipeline at a gradient f I in 250; hydrgraph l. Run started n 22 January 1976 and cncluded n 22 December 1976; ttal run time 335 days. Run 2 - Pipeline at a gradient f I in 250; hydrgraph 2. Run started n 19 April 1977 and cncluded n I I Nvember 1977; ttal run time 206 days. Run 3 - Pipeline at a gradient f I in 100; hydrgraph 3. Run started n l7 April 1980 and finished n22 Octber 1980; ttal run time 188 davs. 4 Run hydrgraphs Three different 24-hur hydrgraphs were used in the experiments. The shape f all three was similar; the difference between them lay in the discharges that ccured at the peaks and trughs. The maximum peak discharge ccurred at hurs (apprximately) with a secndary peak in the evening at hurs: the minimum discharge was at hurs, with a secndary trugh at hurs. The maximum and minimum discharge rates in the hydrgraphs were based n advice received frm a number f drainage engineers. During any run, the hydrgraph was repeated daily, 7 days a week. The characteristics f the different hydrgraphs are given in the fllwing table and are als shwn in Fig 2 (Hydrgraph l), Fig 3 (Hydrgraph 2) and Fig 4 (Hydrgraph 3).

10 Hydrgraph N. I 2 3 m3ls m/s l.l8 It m3ls m/s t.07 tf m3,/s m/s 0.00, s9 0.01l50.95 It m3/s m./s Prp'l depth t& Discharge Peaks Ivlaxrmum Intermediate Maximum Intermed te Velcity depth charge city depth charge city depth charge Prp'l Dis- Vel- Prp'l Dis- Vel- Prp'l Dis- Velcity The prprtinal depths*and the mean velcities are calculated assuming that the pipe surface is slimed, with an equivalent sand rughness f k = l.smm. 5 Experimental measurements All the pipes used in the experiments were taken frm the manufacturer's standard stcks and befre being installed in the rig, their bres were all measured. A vlumetric methd was used t determine the mean diameter fr the vertically-cast cncrete and the clay. The diameters f the ther pipes were btained by means f calipers; tw rthgnal diameters were measured at varius psitins alng each pipe. The results are: Material Length f Mean diameter Standard individual pipe deviatin (m) (mm) (mm) Vertically cast cncrete upvc Spun cncrete Asbests cement Clay The general prcedure that was fllwed in each f the three runs was first t determine the hydraulic rughness f the pipes in a clean cnditin, using clean water; these tests were carried ut at Reynlds numbers ranging frm I x 105 t 3 x 105. Fllwing the clean water tests sewage was passed thrugh the rig, allwing slime t build up; the rughness was determined at regular intervals thrughut this sliming phase. The methd fr determining the rughness was t stp the discharge hydrgraph, supply a knwn steady discharge t the experimental pipeline (which was made t flw full by thrttling a valve at the dwnstream end) and measure the pressure at each f the tapping psitins. A best-fit hydraulic gradient was then calculated fr each f the test lengths, frm which it was then pssible t c-mpute the hydraulic rughness, using the Clebrk-White equatin(6'7); v--vcpstgr#+ffir where V : mean velcity (m/s) g = gravitatinal acceleratin (m/sz) 4

11 D S k y : pipe diameter (m) : hydraulic gradient - rughness value (m) = kinematic viscsity f fluid (mzls) In carrying ut this calculatin it was assumed that the pipe diameter was the riginal clean diameter; n allwance was made fr any effect that the sliming wuld have n reducing the pipe diameter. This wuld have been difficult t d because the sliming was nt unifrmly distributed arund the pipe periphery and presented a very uneven surface. The rughness calculated frm the Clebrk-White equatin is fr a pipe with a perimeter that is partially slimed (up t the maximum depth f flw during the slime-building hydrgraph) and partially clean. On the days when the rughness was being determined, the practice was t carry ut three separate tests at Reynlds numbers ranging frm 85 x 103 t 150 x 103 fr Runs 1 and} and frm 130 x 103 t 180 x 103 fr Run 3. The Reynlds number was restricted t this range in rder t ensure that the shear stress during these tests was nt greater than the shear stress that was being generateduring the slime building prcess. The maximum shear stress in Runs I and 2 during the slime building prcess was apprximately 2.5N/m2, whereas the maximum during the rughness determinatin tests, was 2.2N/m2. As well as determining the pipe rughness, a cmplete phtgraphic recrd was made f the changing sliming pattern in each f the test lengths. Peridically the remvable sectins were taken frm the pipe, the interirs were phtgraphed and the slime was then scraped ff, dried and weighed. Frm time t time, disslved xygen levels were measured in the sewage in the rig and in the sewage arriving at the pumping statin. Occasinally, disslved xygen levels were als measured in varius f the sewers in Oxfrd City that cntributed flw t the trunk sewer feeding the pumping statin. A cntinuus recrd f the temperature f the sewage in the rig was maintained. Between each f the three main runs, the slime was remved frm the pipes by means f the Dyn-Rd pressure-jetting system. This ensured that all the pipes were prperly clean befre the experimental runs began. 6 Experimental results Run I 6.1 Clean water tests Befre any sewage was passed thrugh the rig, clean water was circulated and the 'as-new' rughness f the pipes flwing full, was determined. The values btained frm these tests are verall figures which take int accunt surface texture as well as jint discntinuities and pipe misalignments; in this case the latter tw factrs are nt cnsidered t have a great influence because fthe care that was taken in assembling the pipeline. The results are given in Table I and discussed in Sectin 6.3. \

12 Sewage tests The general experimental prcedure has been described in Sectin 5: details f the flw hydrgraph were als given there. Rughness values were determined at weekly intervals. The general pattern f variatin f rughness with time is shwn in Fig 5A tgether with the variatin in sewage temperature ver the perid; the pltted rughness is the mean value frm the three tests carried ut n any ne day. In Figs 5 and 6, the rigins fr the x-axis have been staggered s that the mnths f the year are aligned in the vertical directin. It was nt pssible t start the hydrgraph immediately the fresh water tests were cmpleted, because f varius teething trubles with the rig. Fr abut a week, the rig was perated intermittently, using sewage, and during this time the rughness increased t the values shwn at day zer f the hydrgraph. When the hydrgraph began t perate cntinuusly, the rughness increased very rapidly fr a perid f 30 days, withut any clear difference between the rughness f the different materials becming evident. Fr the next 120 days the rughness varied in different ways in the five pipes, and there was a marked difference between them. At day I53 a pump breakdwn ccurred, which put the rig ut f cmmissin fr tw days at a time when the ambient air temperatures were high - Britain was experiencing an unusually ht summer. Althugh the dwnstream part f the experimental pipeline was filled with sewage whilst the pump was being repaired, s that the air in the upstream part f the pipeline shuld have had a high relative humidity, these precautins may nt have been sufficient t prevent the slime frm drying ut. When the experiment was restarted, the rughness did nt shw any marked drp, but decreased steadily fr a further twenty days. This cntinued the reductin in rughness that was becming apparent befre the pump breakdwn had ccurred. The interruptin t the experiment may have caused the slime t die and subsequently t slugh ff, but it des nt explain why the slime was slw t build up again, nce the experiment had been re-started; the pump breakdwn may merely have accelerated a prcess that was already in train. In rder t determine the effect that the high summer temperatures were having n the sewage, measurements f disslved xygen cncentratins were made at varius pints in the Oxfrd sewerage system n 7 July 1976 (day 167) using an EIL prtable disslved xygen meter (mdel 1520). Further sets f measurements were made n 20 December (day 333) and n l0 August 1977 (day ll2 f Run 2). The results frm these measurements are given in Table 2 and are discussed in Sectin 6.4. The sharp decline in rughness came t an end by abut day 170: fr the fllwing 50 days the rughness f all the pipes fluctuated arund a lw steady mean value. Fr the remaining perid f the run, the rughnesstarted t increase fr the tw types f cncrete pipe and fr the asbests-cement pipe, whereas fr upvc and clay the increase was nt nearly s marked. The decisin t end the first run was taken when a further pump failure ccurred and it was apparent that the rig was ging t be ut f cmmissin fr sme cnsiderable time. Perid frm day 0 t day 170 Mre detailed cmments n the results are as fllws: In the first 20 days, when the slime was establishing itself, there was n significant difference between the different pipe materials. After this, 6

13 differences did begin t emerge. The variatin f rughness f the spun cncrete and f the vertically cast cncrete, althugh nt identical fllwed a very similar pattern; after peaking at day 28, the rughness fell ver a shrt perid f time, but thereafter rse steadily, reaching maximum values f 4.7 and 5.4nim respectively arund day 120. There then fllwed a perid f 50 days during which the rughnessteadily decreased t values f 0.3 and 0.8mm. The clse similarity f the respnse f the tw types f cncrete pipe is encuraging in view f the different behaviur f the ther three types f pipe that were expsed t the same experimental cnditins. The rughness f the clay pipe peaked at day 20, and then fell. Frm day 35 t 70 the rughness remained fairly steady, fluctuating between 0.6 and l.5mm, but it then increased ver a perid f rughly 20 days, reaching a value f apprximately 2.5mm, where it remained fr sme 30 days. Fllwing this, the rughnessteadily reduced, but at a much slwer rate than fr the tw types f cncrete pipe. The rughness f the asbests-cement pipe varied in a similar manner t that f the clay; it peaked at day 35, fell sharply and then increased equally sharply, rising t a value f 3.0mm at day 49 and remaining reasnably cnstant arund this value until day 146. The rughness then rapidly decreased t a value f 0.5mm at day 167. The upvc displayed smewhat different characteristics. The rughness peaked at day 28, but then fell steadily t day 63: it then rse again ver the fllwing 35 days t reach a peak value slightly higher than the initial peak value (l.6mm cmpared with l.3mmm). Immediately after this the rughnes steadily decreased. The main differences between the behaviur f the different pipes during the first 120 days f Run I can be summarised as fllws: The spun and vertically cast cncrete pipes were the rughest and were the nly nes that shwed a fairly cntinuus increase in rughness with time. The next rughest pipe was asbests-cement: this reached its stable value at abut day 44 and maintained this value fr abut 100 days (apprximately). Althugh the peak rughness f the clay pipe was slightly lwer than that fr asbests-cement, the pattern f variatin was different. The clay pipe reached its maximum value very much mre slwly than the asbests-cement: there was an initial perid (up t day 70) during which the rughness f the clay was steady at arund I t I '5mm, but this was fllwed by a perid in which the rughness further increased befre reaching its final stable value. The upvc pipe was different frm all the thers: the rughness variatin shwed tw quite distinct peaks, at days 38 and 98. Hwever day 98 appears as a minr peak n all the ther pipes. If this measurement is ignred it is evident that upvc reaches its peak value at abut day 28 and remains clse t this value fr a further 80 days. Anther interesting feature f this part f the run, is the difference in the times at which the decline in rughness became evident. In the tw cncrete pipes, the rughness decreased after days 125 and 132: asbests-cement after day 125; clay after day I I I and upvc after day 98, r day I I I if the day 98 figure is ignred. There is a general pattern 7

14 Perid frm day 170 t dy 230 Perid frm day 230 t day 335 t the extent that the pipes with the lwest rughness were the sne. t shw a decline. In all cases the decline began when the sewage temperature was between l5c and ltc and was well advanced befre the pump failure ccurred n day 153. Prir t this perid the rughness in all the pipes had very greatly decreased and fr a significant part f this perid it was less than lmm. There were n marked trends evident; the rughnesstayed fairly cnstant, spun and vertically case cncrete pipes being slightly rugher than the grup cmprising clay, asbests cement and upvc. Thrughut this perid, cvering mst f July and August, the sewage temperature exceeded 20C. In this perid, during which sewage temperatures declined frm 20C t belw l5c, marked differences in pipe characteristics were evident again, althugh these were nt identical with thse displayed during the first perid f this run. Again, althugh the tw types f cncrete pipe did nt shw a cmplete identity in the manner in which the rughness varied, nevertheless they were very similar. The rughness f bth started t increase at day 230; vertically cast reached its peak value (3.3mm) n day 266, spun n day 272 (2.3mm). Fllwing the peaks, the vertically cast rughness decreased and then remained fairly cnstant at arund 2 t 2.4mm until day 297, when it again shwed a further decrease in rughness. The rughness f the spun cncrete varied in a slightly different fashin: after the peak, it then fell t a minimum f lmm at day 287, befre rising again t 2.9mm at day 304. It then decreased, similarly t the vertically cast. During this perid f the run the maximum rughness f the tw cncrete pipes was significantly lwer than it had been in the early part f the run (2.9 and 3.3mm cmpared with 5.3 and 4.7mm previusly). The rughness f the asbests-cement pipe during this perid varied in a manner similar t that f the cncrete pipes in the early perid f this run: frm day 210 t day 303 it shwed a steady increase, rising t a maximum value f 3.8mm. Several features are wrthy f nte. The peak rughness value was greater than it had been fr the same pipe in the earlier part f the run; it was als rugher than the cncrete pipes, reversing the situatin that btained in the earlier perid, when the cncrete pipes had been appreciably rugher than the asbestscement. The clay and upvc bth shwed a similar pattern, althugh the abslute values f rughness were nt the same. Bth shwed a peak early n (day 244 fr clay, day 258 fr upvc) fllwed by a trugh, with the rughness f bth pipes rising t a secndary peak at arund day 300. All pipes reduced in rughness ver the perid frm day 304 t day 335, but this may be a misleading cnclusin because n measurements were made between these days. Hwever the lw values n day 335 may be related t the water temperature f ll.3c, the lwest reached during any f the 3 runs. Summary The mst ntewrthy features f this run are: i) The variatin f the rughness in the tw types f cncrete pipe was similar even thugh they were nt laid in sequence in the experimental pipeline.

15 Run Clean water tests Sewage tests Peridfrm day 15 t day 100 ii) In the perid up t day 125 the cncrete pipes cntinued t increase in rughness thrughut; the asbests-cement and upvc pipe reached a stable value fairly quickly, whereas althugh clay sn reached a stable value it then shwed a sudden increase f rughness late in this perid. Asbests-cement was rugher than clay, and its rughness increased rapidly in the early days f the experiment. iii) All the pipes became cnsiderably smther mid-way thrugh the run; hwever the time at which the rughness first started t decrease, varied fr the different pipes; in all cases the significant rate f decrease began between day 100 and day 130; ie clse t the time when the temperature rse rapidly frm l5 YzCt 17"C. iv) The rughness fr all the pipes was lw fr a perid f rughly 60 days frm mid July t early September, the time when the sewage temperatures were at their highest. v) As the sewage temperatures fell, late in the experiment, the pipes became rugher, but they presented a different picture frm that in the early part f the run, viz asbests-cement was nw rugher than cncrete pipes and rugher than in the early part f the run, cncrete and clay were smther than they were in the early part f the run; upvc was much as it was in the early perid. Fllwing the cmpletin f Run l, the pipeline was cleaned by pressure jetting. Apart frm a black discluratin f the lwer part f the cncrete and asbests-cement pipes - pssibly the result f sulphide attack - n visible traces f slime remained in the accessible parts f the pipeline. Clean water was then circulated thrugh the rig nce mre, and the rughness f the clean pipe determined, see Table I and Sectin 6.3 fr discussin. The prcedure adpted during the run was the same as fr the previus ne: the variatin with time f rughness and f temperature, are shwn in Fig 5B. The hydrgraph characteristics fr this run, were very similar t thse fr the previus ne, as can be seen frm the data given in Sectin 4. The build-up f rughness in the first ten days was rapid, but thereafter the rate f increase reduced. During this initial perid a thin (2mm) cntinuus film f slime was bserved t have develped ver the lwer parts f the pipes. In the perid frm day 15 t day 100 (apprximately), there was little systematichange in the rughness f all the pipes: any variatin was in the frm f variatins abut a steady mean value. During this perid, lumps f slime, 5 t l0mm high started t appear in the regin f the mean water line. On l0 August 1977 (day I l2), temperature and disslved xygen levels were measured in the Oxfrd sewers, see Table 2. Mre detailed cmments abut the experimental results are as fllws. Fllwing the initial rapid increase in rughness in the first fifteen days, bth types f cncrete pipes shwed further small increases, with maxima abut lmm greater, althugh the rughness f bth was fluctuating quite rapidly thrughuthis perid. The pattern f rughness variatin was similar in bth pipes and they bth achieved a similar 9

16 Perid f day 100 t dv 206 maximum rughness - 2mm fr spun and 2.3mm fr vertically cast, althugh the maxima did nt ccur simultaneusly in the tw pipes. The rughness f the asbests-cement pipe increased t lmm at day 20, suffered a sharp fall immediately after, and then slwly increased t day 50, when it then settled dwn t a value f 0.8mm (apprximately), maintaining this value until day 83. At this pint there was a sharp increase in rughness. Once the clay pipe had undergne its initial slime building perid, the rughness did nt underg any sensible change until day 51, when it rse frm a steady value f arund lmm t l.4mm. This increase was nly shrt-lived, hwever, the rughness falling t 0.5mm at day 78 and nt changing significantly thereafter. The rughness f the upvc pipe increased until day 29, suffered a shrt-lived decrease, fllwed by a further increase t a value f l.5mm. The rughness then decreased until day 55, thereafteremaining fairly steady, fluctuating between 0.5 and 0.9mm. Cnsidering this part f Run 2 in islatin, there is little f majr interest t cmment n. The rughness f all the pipes remained reasnably cnstant despite Sme ccasinal rather large fluctuatins; mst f the time the range fr all pipes was frm 0.5 t 2.0mm. The sewage temperature ver the perid was between l4c and l9c, and had exceeded 10"C by day 20. During this perid sme f the pipes shwed a marked change in the way that the rughness varied with time. Bth the cncrete pipes started t increase in rughness, and this trend cntinued until the end fthe run, apart frm a shrt perid arund day l50t day 170, when the rughness suffered a decrease. Again, the tw cncrete pipes behaved in a very similar fashin t ne anther. Bth pipes reached their maximum rughness at the end f the run (4.2mm fr vertically cast and 4.5mm fr spun), and were appreciably rugher than they had been in the earlier perid. Asbests-cement behaved in a similar fashin t the cncrete pipes. Initially the rughness increased nly slwly, but frm day 166 the increase in rughness was very rapid, reaching a value f 4.9mm at the end f the run, again appreciably greater than it was during the earlier part f the run. The rughness f clay and upvc pipes was very similar t that ccurring in the early part f the run. Frm day 100 t day 140 the rughness f the clay pipe fluctuated between 0.5 and 0.8mm; but frm day 140 t the end, the range f the fluctuatins was greater, viz frm 0.6 t l.4mm, and the mean rughness was slightly greater. The maximum rughness during this perid is the same as the maximum value that ccurred between day 0 t day 100. The rughness f the upvc pipe during this perid did nt change t any great extent. Frm day 100 t day l14 it was cnstant at 0.3mm, but althugh it increased slightly thereafter, it still fluctuated between 0.3 and 0.8mm, apart frm a single value f l.lmm that ccurred at the end f the run. There was a strng similarity between the variatin f the rughness during this perid and the variatin that ccurred in the first 100 days f the run. Summary There are several features f the results frm this run that are wrthy f cmment. The cncrete and asbests-cement pipes shwed a l0

17 Run Clean water tests different characteristic frm the ther tw pipe materials, in that twards the end f the run when the sewage temperature began t fall, the rughness increased very markedly, whereas the rughness f the clay and upvc was very similar thrughut the duratin f the run. The results frm the asbests-cement pipe are particularly interesting. During the early part f the run the rughnes stayed virtually cnstant fr 80 days and was f a similar rder f magnitude t the UPVC pipe. But after this time there was a prnunced difference between them, the asbests-cement pipe becming slightly rugher whilst the rughness f the upvc pipe remained practically unchanged. Als the asbests-cement pipe was significantly smther than the cncrete pipes during the first 80 days, but ended by being slightly rugher than them, nce the sharp increase in rughness had taken place. The pattern f variatin f rughness in the tw cncrete pipes was similar, as it was in Run l. The prcedure fllwing the cmpletin f Run 2 was similar t that at the end f Run l: the pipes were cleaned by high pressure water jets and the rughness f the clean pipes then determined, having first relaid the pipeline t a gradient f I in 100. The results are given in Table l. Sewage tests There are n cnsistentrends evident frm the measurements n the clean pipe. The rughness f the spun cncrete and the clay pipes after this secnd cleaning is similar t the rughness after the first cleaning. Fr the vertically cast cncrete, upvc and asbests-cement pipes, the rughness is f a similar rder t the rughness in the 'as-new' cnditin, ie smther than they were after the first cleaning. Clearly the data are insufficient t enable any general cnclusins t be drawn abut the effect f sliming n the rughness f the material frming the riginal pipe surface. The differences in rughness between this run and the previus ne culd be due in part t the pipeline having been relaid t a steeper gradient: this had invlved re-assembling the pipeline frm the individual pipe lengths s that the alignment f all the jints wuld be changed. Anther reasn culd be that there was a variability in the quality f the cleaning between the runs. When the rig was brken dwn between Runs 2 and 3 there was evidence f slime still being attached t the pipes; althugh it was pssible t carry ut sme additinal cleaning between Runs 2 and 3 it had nt been pssible t d this between Runs I and 2, which culd explain the higher clean pipe rughnesses at the beginning f Run 2. Althugh the range f rughness is quite large, the crrespnding range f capacities is nt nearly s great. Fr instancein a225mm diameter pipe at a gradient f I in 250, an increase rughness frm 0.09 t 0.25mm (as in the vertically cast cncrete) represents a reductin in capacity frm 0.042m3/s t 0.037m3/s, ie a three-fld change in rughness prduces a reductin f apprximately 12 per cent in the capacity. The prcedure during this run was the same as that fllwed in Runs I and 2: the variatin f rughness with time and with temperature is shwn in Fig 6. Althugh the depth f flw f the peak sliming discharge was similar t that in the previus tw runs, the mean velcities in this run were apprximately 50 per cent greater. Fr the first 100 days f this run, the rughness did nt vary t any significant degree nce the initial sliming had taken place in the first twenty days. During this time the rughness fell between limits f 0.1 and 0.6mm, apart frm a shrt perid f time arund day 70 when the rughness f all the pipes shwed a sudden, albeit temprary, increase ll

18 Perid frm day 0 t day 120 Perid frm day 120 t day 188 in rughness, crrespnding with a dip in the temperature curve. At day 120, differencestarted t appear, with the cncrete pipes shwing alarge and sudden increase, the asbests-cement shwing a mre gentle increase, and the clay and upvc remaining much as they had been previusly. Mre detailed cmments abut the different pipe materials are as fllws. The pattern f variatin f rughness in the cncrete pipes was similar. They bth reached an initial peak value after 20 days (0.4mm fr the spun and 0.3mm fr the vertically cast); the rughness then stayed cnstant fr a further 30 days, when it decreased temprarily befre rising t a peak value (lmm at day 6l fr the vertically cast, 0.8mm at day 60 fr the spun). Fllwing this peak, the rughness fell just as sharply as it had risen, t reach a rughness similar t that which had ccurred befre the peak. The rughness f the asbests-cement pipe was lw fr the first 60 days (0.2mm r less) and althugh it increased t 0.6mm at day 69, it rapidly decreased again t its frmer lw value. The rughness f the upvc pipe was very similar t that f the asbests-cement pipe and the pattern f the variatin was als very similar. The rughness f the clay pipe varied in a very similar fashin t that f the cncrete pipes, and the magnitude f the rughness was als very similar. During mst f the perid the rughness ranged between 0.2 and 0.4mm; fr a brief perid arund day 69, the rughness increased sharply but this was nly an islated instance. Frm day 120 nwards, the cncrete pipes were much rugher. Over a perid f ll days, the rughness f bth pipes increased sharply, the spun rising t l.2mm at day 126 and the vertically cast t l.7mm at day l3l. Frm this pint until the end f the run, the spun pipe rughness fluctuated in the range 0.6 t l.3mm, the vertically case in the range 1.2 t l.7mm. There was n systematic increase in the rughness during this perid, but the pattern f the variatin was similar in bth pipes ie, they bth shwed a sudden increase in rughness, fllwed by a perid f relative stability. The rughness f the asbests-cement pipe shwed a slw but steady increase during this perid, being rughest at the end f the run, having reached a value f 0.9mm, cnsiderably greater than it had been fr the whle f the rest f the run (apart frm the lcal peak at day 6e). The rughness f the clay pipe was nt significantly different in this perid frm what it had been in the previus perid. Fr the whle f the perid the rughness fluctuated between 0.2 and 0.5mm, with n lng-term trend in evidence. The upvc pipe was like the clay pipe, in that the rughness did nt change significantly ver this perid, ranging frm 0' I t 0.3mm, with n lng-term trend in evidence. Summary The main features f interest in this run were that:- i) The cncrete pipes had very similar rughness characteristics and were rugher than the ther pipe materials. ii) upvc and clay were the smthest and their rughness did nt change t any marked degree thrughuthe test. 12

19 iii) Asbests-cement had a very similar rughness t upvc fr the first 100 days but thereafter the characteristics f the tw pipes diverged, the asbests-cement pipe displaying a slw increase f rughness with time and ending up nly slightly smther than the cncrete pipes. iv) As in Runs I and2 the increase in rughness f the cncrete pipes and the asbests-cement pipe ccurr,bd twards the end f August when sewage temperatures were beginning t fall. v) The peak rughness values fr all pipes were less than ne third f the peak values in Runs I and 2. General discussin f results frm Runs 1. 2 and Intrductin The hydrgraphs fr Runs I and 2, althugh nt identical, were sufficiently similar fr it t be reasnable t cmpare the results frm the tw runs. The velcities in Run 3 were higher s that it is nt pssible t cmpare the results directly with thse frm the tw previus runs, but it is still pssible t cmpare the trends in all three runs with each ther. The purpse f making such a cmparisn is t discver if there is a cmmn pattern in the way that the different pipe materials behaved during the experiments. If, during sme perid f ne f the runs' a pattern f behaviur can be identified (even thugh all the pipes might be behaving differently frm ne anther), which is repeated at ther times in the varius runs, then it wuld be reasnable t expect that there was sme deterministic mechanism gverning the slime grwth in the pipes, bearing in mind that all the pipes are being expsed t the same sewage. On the ther hand, if there was n repeating pattern f behaviur the assumptin must be that there is sme randm prcess having a significant effect n the slime in the pipes. In lking fr an explanatin f the bserved behaviur it is necessary t cnsider the micr-bilgy f the slime itself, and this is cmplex. Slime is frmed by bacteria, prtza and fungi in the sewage: the ppulatins f these varius rganisms are influenced by the sewage - its temperature, the fd that it cntains, the amunt f disslved xygen, and its chemical cmpsitin. The sewage slime will be influenced by the fungi present, eg during winter, the fungi tend t be dminant and they prduce a slime with a tugh skin, whereas in summer the fungi have less influence and the slime is affected by the ther rganisms in the sewage. Slime has a life cycle f its wn: experiments by Heukelekian and Crsby(a) shwed that when sewage slime was cultivated under static cnditins, the amunt f slime varied in a nn-systematic way thrughut the curse f the experiments. Thus in additin t all the ther external factrs that cme int play, there is the grwth and decay cycle f the slime itself t be taken int accunt. In the early stages f planning the experiments, discussins were held with staff frm Stevenage Labratry f the Water Research Centre in rder t btain sme backgrund infrmatin n slime and thus avid any majr shrtcmings in the experimental arrangements. The influence f disslved xygen was f particular interest. Nrmal dnrestic sewage cntains disslved xygen at 30 per cent saturatin level (rughly equivalent t a cncentratin f 3mg/1, the precise figure depending n temperature and barmetric pressure). Sewage bacteria will grw at disslved xygen levels dwn t l0 per cent l3

20 (rughly lmg/l), maybe even dwn t a cncentratin f 0.5mgl1. The experimental arrangement wuld inevitably lead t the additin f further disslved xygen as the sewage passed arund the rig, but WRC were cnfident that the additin f xygen t raise the level abve 30 per cent, wuld nt lead t an artificially high rate f bacterial activity: the xygen wuld simply pass thrugh the rig. When the experiments were in prgress, measurements f disslved xygen levels were made at varius times and at varius pints n the site and in the experimental rig; the results are as fllws: Lcatin f measurement Trunk sewer Disslved xygen cncentratins (mg,/l) (Day 41) (Day 82) (Day 168) (Day 14) Run 1 Run 1 Run I Run < Dwnstream frm primary sedimentatin channels Pumping statin wetwell Cnstant head tank The reasns fr extracting the sewage frm the pumping statin wetwell have been discussed in Sectin 2 f this repri. The measurements f disslved xygen shw that althugh the sewage arriving at Littlemre was anerbic (r nearly s) n and , it was n lnger s by the time it reached the wet-well. Even if it had been pssible t avid adding xygen t the sewage in the experimental rig, sewage taken frm the wet-well wuld still have been cnsiderably mre aerated than the sewage in the trunk sewer. The explanatin fr the high xygen levels in the wet-well is that the flw first passes thrugh a critical depth flume, and the actin f the hydraulic jump is sufficient t add a significant amunt f xygen t the sewage. The cnclusin frm these measurements is that the sewage circulated arund the test rig was cnsiderably mre aerated than the sewage arriving at the site in the trunk sewer, but that when this incming sewage cntained a disslved xygen cncentratin f mre than lmg/l (r even O.5mgll), the additin f further xygen wuld nt increase the rganic life r the slime grwth rate in the experimental pipeline. The psitin is less clear when the sewage in the trunk sewer has a very lw disslved xygen encentratin. Heukelekian and Crsby carried ut a micrscpic analysis f aerbic and anaerbic sewage(a) and fund that the same micrfauna were present in each, but that the predminant types were dependent n the type f sewage. Thus it is pssible that the aeratin f anaerbic sewage in the trunk sewer wuld change the characteristics f the micrfauna ppulatin. Hwever there must be a time element invlved in this prcess; the micrfauna are nt likely t respnd instantaneusly t a change in their envirnment. Althugh death can be virtually instantaneus, an increase in the ppulatin (which is what the additin f xygen t an anaerbic sewage will prduce) must take time. Bacteria multiply by cell divisin and the time taken fr this t ccur is measured in perids f frm minutes t days, depending n the bacterial type. The maximum time taken fr sewage t pass thrugh the test rig was apprximately l3 mins, s that as far as the sewage in circulatin is cncerned, t4

21 the micrfauna ppulatin is unlikely t have had sufficient time t respnd t any significant degree t a change in its envirnment. An additinal factr t be taken int accunt in this argument is that as lng as there is slime in the experimental pipeline, the bacteria ppulatin apprpriate t the aerbic cnditins will als be present: the additin f xygen t anaerbic sewage arriving at the site means that the bacteria in the experimental pipeline will cntinue t have an xygen supply adequate fr them t cntinue t thrive. In ther wrds, as far as the bacteria in the pipeline are cncerned, they are nt affected by the xygen cntent f the sewage arriving at the site. Hwever it is pssible that the anerbic sewage has ther characteristics that are unchanged by the additin f xygen and that are inimical t the survival f bacteria. Measurements made at varius pints in Oxfrd sewerage system shwed that when the sewage in the trunk sewer at Littlemre was anaerbic, elsewhere the sewage was aerbic. The disslved xygen cncentratins given in Table 2have been pltted againstemperature fr sites I t 6 and 8 (Fig 7). Althugh the mean cncentratins fr the three sets f measurements fall n a straight line, there is a cnsiderable scatter in the results, suggesting that the relatinship between temperature and disslved xygen cncentratin varies frm place t place ver the netwrk. Fig 7 als shws the xygen cncentratins fr the 30 per cent and l0 per cent saturatin levels, frm which it is clear that cnsiderable stretches f the Oxfrd sewerage system were carrying aerbic sewageven thugh the sewage temperature was high. The same sewage had becme anerbic by the time it had arrived at the pumping statin. The nly measurement f sewage cnditin that was taken cnsistently in the experimental rig thrughut all three runs was the temperature. Table 2 shws that this was clse t the temperature in the trunk sewer. The cnclusin frm Fig 7 is therefre that this sewage temperature is a measure f the disslved xygen in the sewage within the sewer netwrk. It des nt hwever give any infrmatin abut the cmpsitin f the sewage. Temperature by itself is a significant parameter: accrding t infrmatin supplied by WRC, bacterial grwth rate is a direct functin f temperature and dubles when the temperature is increased frm l0"c t 20"C. Thus if the temperature f the sewage were t be increased withut changing any f its ther characteristics, it wuld be reasnable t expect that the slime grwth wuld als increase. The Thames Water Authrity d nt make regular analyses f the sewage arriving at the Littlemre pumping statin, s n data is available. Regular analyses are carried ut at the N. I sewage wrks, where the sewage cmprises the discharge frm Littlemre plus sewage frm ther pumping statins and sme industrial effluent. The fllwing table summarises the data that have been prvided by the TWA fr sewase arrivine at the N. I wrks. l5

22 ph Suspended slids mg/l BOD mg/l Ammniacal nitrgen mg/l Chlride mg/l nd quarter 3rd quarter 4th quarter t.) lst quarter 2nd quarter 3rd quarter 4th quarter l lll l12 r lst quarter The pinin f the Grup Chemist at the sewage wrks was that the sewage was significantly strnger than nrmal frm December 1975 t Nvember 1976, with the maximum values ccurring arund August ln a typical summer the average suspended slids wuld be abut 350mg/1, BOD wuld be abut 250mg/1, ammniacal nitrgen abut 30mg/1. The sewage wrks recrds d nt indicate that any industrial txic discharge in the middle f 1976, but the pssibility f an undetected discharge cannt be ruled ut because dilutin and dispersin befre reaching the wrks might have made it impssible t detect. Discussin f results The hydraulic cnditins in Runs I and 2 were sufficiently similar (maximum prprtinal depths during sliming hydrgraph f 0.6 and 0.5 respectively) fr a quantitative cmparisn f the tw runs t be a reasnable apprach. The results frm Run 3 can nly be cmpared with thse frm the previus runs n a qualitative basis, because the hydraulic cnditins are significantly different. In rder t cmpare the pattern f variatin f rughness in a particular pipe material during the three different runs, the rughness data have been pltted separately fr each material (Fig 8); the data used are the same as were used t plt Figs 5 and 6. Cmparisns f Runs I and 2 The interpretatin f the results frm these runs wuld have been mre straightfrward if the pattern f variatin in the tw runs had been the same. It is clear frm Figs 5, 6 and 8 that this was nt the case and that, althugh the hydraulic cnditins fr the tw runs were virtually the same, ther experimental cnditins were different. Run I was carried ut in 1976, a year when summer temperatures were abnrmally high and there was a drught. Run 2 was carried ut in 1977, when cnditins were similar t thse f a nrmal British summer. Furthermre Run 2 was started 3 mnths later in the year than Run I and finished apprximately I mnth earlier. Neither f the runs was carried ut fr a full year. During the early part f Run I the rughness f mst f the pipes cntinued t increase fr the first 100 t 120 days: at their peaks, the rughnesses ranged frm 1.0 t 5.5mm. In cntrast, the rughness during the first 100 days f Run 2 increased very much mre slwly than in Run I and nly ranged frm 0.5 t 2.0mm. In the middle f Run l, the rughness f all pipes decreased. In Run 2, this ccurred nly in the case f the clay and upvc pipes. Bth runs l6

23 shwed lw rughness values in July and August but Run I (with higher temperatures) generally gave lwer values in this perid. There was a similarity between the tw runs in their later stages, with all f the pipe rnaterials shwing a steady increase in rughness frm July-August nwards. After the initial rapid grwth perid f abut 20 days the fllwing general cnclusins can be drawn: (a) In the Spring (March t June) rughness increases at a steady rate until the water temperature reaches a limiting value when the rughnesstps increasing. Fr cncrete pipes, based n Run l, this temperature seems t be abut 16"C. The threshld temperatures are lwer fr the ther materials: prbably abut l5"c fr clay, l4c fr asbests-cement and l3"c fr upvc. (b) In the Summer (July and August) rughnesses are lw relative t ther times f the year. The exact mechanism which triggers ff the reductins in the Spring rughness is nt clear. (c) In the Autumn (September t Nvember) the rughness increases. (d) In the Winter (December t February), when water temperatures are lwer the rughness falls t a lw value, thugh prbably nt as lw as in mid-summer. Hwever because nne f the runs perated thrughut a winter there is nly a little evidence t supprt this deductin. N precise explanatin f these findings can be given but it is pssible t g smeway t relating these findings t the characteristics f the slime grwth described in Sectin 6.4. lt must be brne in mind that the slime thickness, and rughness, arises frm a balance between the rate f grwth, the rate f slughing due t the shearing actin f the flw and t the natural life-death cycle f the slime itself (see Sectin 6.4.1). Thus if the rate f grwth is reduced the slime thickness and hence the rughness will reduce. It is suggested that the lw rughnesses ccurring in Summer are due t grwth being inhibited by the factrs related t the incming sewage. In Winter grwth is inhibited by the Iw temperatures. Between these tw cnditins there is a range f temperatures where grwth is mre rapid and rughness increases. Varius hyptheses were examined in an attempt t explain the relatinship between rughness and sewage temperature fr each pipe material. Sme limited success was achieved but n hypthesis was able t explain satisfactrily, all the bserved results. t7

24 Cmparisn f Runs 2 snd 3 The hydraulic cnditins fr these tw runs were different, s that quantitative cmparisns are nt pssible: hwever bth runs were started at apprximately the same time f year and extended fr similar lengths f time, s that a qualitative cmparisn is reasnable. After the initial rapid increase in rughness at the start f the runs, there was a lng pericl in bth runs, extending t August, during which the rughness f all the pipes remained fairly steady. Arund August, the rughness f the cncrete and asbests-cement pipes started t increase slwly, this cntinuing until the end f the run, whereas the rughness f the clay and upvc did nt change t any marked degree. In general high and lw rughness values were abut ne third f the crrespnding values in Run 2. 7 Rughness f pipes when running part-full During Runs I and2 a few tests were carried ut under part-full cnditins in rder t get sme indicatin f the variatin f rughness with depth f flw. The prcedure during these tests was t set a steady discharge, using the valve at the dwnstream end f the pipeline t establish unifrm flw cnditins. In practice this was difficult t achieve, partly because the valve did nt allw fine adjustments t be made and partly because the water surface was very undulatry, making it well-nigh impssible t determine when the flw was unifrm. The water depths in the pipe were measured by means f the pressure tappings, having first determined the pipe invert levels at each f the tapping pints. The prcedure fr calculating the rughness f the part-full pipes was first t estimate the mean water surface prfile by fitting a straight line, by eye, thrugh the water surface measurements fr a particular test length. The depth f flw at each end f the test sectin was then determined frm the mean water surface prfile; this enabled the mean velcity and kinetic energy at each end t be calculated and hence the energy gradient. The mean gemetric parameters ver the reach were als cmputed, which then allwed the rughness t be calculated frm the Clebrk-White equatin, assuming that the flw was in the rugh-turbulent regin. This methd f calculatin is nly an apprximatin but it was cnsidered t be sufficiently accurate fr the main purpse f the tests, which was t examine, in a l8

25 qualitative way, hw the rughness varied with depth f flw. As a check n the methd, an alternative methd f calculatin was used; in this, the measure depths at either end f the test sectin were used t determine the mean velcity and the ttal energy at either end f the reach. The mean f the measuredepths f flw was used t calculate the mean gemetric parameters fr the reach. The results frm this methd was very little different frm the mre apprximate methd that was used fr the bulk f the calculatins. The part-full tests were carried ut in Octber 1976, twatds the end f Run I and in Nvember 1977, twards the end f Run 2. The results frm these tests are shwn in Figs The data shw quite a large amunt f scatter; there appear t be significant changes in rughness fr nly small changes in prprtinal depth. Smth lines have been drawn thrugh the data pints because it is pssible that there is a cnsiderable band f errr n the data pints, eg in Run I tw tests were carried ut under virtually identical cnditins and they gave rise t part-full rughnesses f 2.0 and 3.4mm fr upvc pipe, with n apparent reasn t explain the discrepancy. Despite such incnsistencies, the general cnclusin is that there is a marked increase in rughness when the sewer is flwing nly part-full. The fllwing table summarises the principal results frm these tests. Run I Run 2 {aterial Maximum part-full rughness k (mm) Pipe-full rughness k (mm) Prp'l depth fr maximum rughness Maximum part-full rughness k (mm) Pipe-full rughness k (mm) Prp'l depth fr maximum rughness rpvc ipun-cncrete \sbests+ement )lay In general, it appears that the maximum rughness ccurs when the depth f flw crrespnds t that f the maximum depth f sewage, and that the part-full rughness is significantly greater than the pipefull rughness. The variatin f rughness with depth f flw will prduce a stagedischarge characteristic different frm that f a pipe with unifrm rughness arund the periphery. Instead f the maximum capacity ccurring at a prprtinal depth f 0.95, a slimed sewer with nnunifrm rughness will have its maximum capacity when it is flwing full. The discharge will increase cntinuusly with depth and there will be nly ne depth crrespnding t a particular discharge, unlike the pipe with unifrm rughness, which has tw values crrespnding t apar' ticular discharge when the prprtinal depth is greater than 0.8. t9

26 8 Measurements f slime weight Weight f accumulated slime 8.1 In additin t the weekly measurements f head-lss in the test lengths f pipe, separate shrt sectins f each type f pipe, except the vertically-cast cncrete sectin, were peridically remved frm the rig in rder t examine the pattern f slime grwth and weigh the accumulated slime. Fur lm lengths f butt-jinted pipe had been added t the rig at the dwnstream end f each test sectin and these culd be remved and replaced withut disturbing the test lengths. The purpse in making the measurements was t determine whether there was any appreciable difference in the amunt f slime accumulated by different pipe materials and whether any significant crrelatin existed between the amunt f slime and the hydraulic rughness f the pipe. Prir t testing it was felt that the amunt f slime wuld increase slwly with time and the plan was t remve the first three shrt lengths f pipe in sequence at mnthly intervals. Depending n the length f the experiment, this sequence wuld be repeated and a nte made f the time elapsed, since each pipe was last remved and scraped. The furth, and final, pipe in each series was t be left undisturbed until the end f the run. In the event, it became clear that the initial grwth perid f the slime was very shrt (3-4 weeks) and that, after that time, the quantity f slime present n the walls appeared t depend n ther factrs, which were nt necessarily a functin f perid in use. Pipes remved late in the sequence appeared n inspectin t have the same pattern and degree f sliming as adjacent pipes scraped nly ne mnth earlier. The prpsed sequence and interval f pipe remval was, therefre, nt strictly adhered t. The fllwing table summarises, fr each f the three runs, the number f days frm the start f the run at which each pipe was remved and examined. Days frm start f run Pipe I Pipe 2 Pipe 3 PiPe (Run l) r M 1977 (Run 2) 28 )t (Run 3) l9l Each time a pipe was remved frm the rig it was phtgraphed. The height f the slime layer abve the invert was recrded and the material scraped ff fr analysis. Analysis The fllwing table summarises fr all pipe materials the main " physical characteristics f the slime remved frm the pipe walls. 20

27 Invert Water-line Mixture f invert and water-line N. f samples t2 t2 56 Density f wet slime (g,/cm3) mean s t I t Dry weight as per cent f wet weight mean s t Organics as per cent f wet weight mean s Sand (>0.06mm) as per cent f wet weight mean s r.29 s : standard deviatin in the ppulatin sample There was little apparent difference in the physical characteristics f the slime that grew n different pipe materials and in different runs, but samples taken frm the pipe inverts shwed significantly higher bulk density and cntained mre sand than samples taken frm the waterline. There was als a difference in appearance. In the early stages f each run the bttm f the pipes was cvered with a thin (l-2mm) unifrm layer f grey slime, while at the water-line white, gelatinus lumps up t l0mm high were present. This may reflect a difference either in the physical cnditins fr grwth r in the rganisms present. Results The dry weight f slime per unit area f slimed surface (e/mz) has been used as a measure f the quantity f slime present n the pipe walls. This was determined fr each sample frm the measurements f wet weight, percentage f dry slids and the prprtin f the pipe perimeter that had slime attached. Values f dry weight fr Runs 1,2 and 3 have been pltted in Figs 13 and 14 fr each pipe material represented (except vertically-cast cncrete). The graphs shw the fllwing interesting features: l. The increase in slime weight with time shws a similar pattern t the increase in hydraulic rughness. In particular, the drp in hydraulic rughness which was in evidence befre the pump failure n day 156 f Run I is als reflected in a drp in slime weight (Fig 13) n day 154. This seems t cnfirm that sme factr ther than the pump failure was respnsible fr lss f slime. In bth Run I and Run 2 the increase in rughness at the end f the test is reflected in an increase in the amunt f slime present. 2. After the initial perid f grwth n clean pipes, the spun cncrete surface shwed cnsistently mre slime than clay r upvc' Althugh clean asbests-cement pipe has a very lw rughness factr (k = 0.02mm), twards the end f each run bth the rughness and the amunt f slime present were higher than in either the UPVC r clay pipes. 3. In Run 3 (Fig 14) the higher velcity resulted in much less slime grwth in all pipe materials,, althugh the general pattern f a unifrm, smth layer n the invert with larger slime lumps at 2l

28 the water-line was still in evidence. The difference between the pipe materials was als much less marked, althugh spun cncrete and asbests-cement surfaces had slightly mre slime than upvc r clay. Crrelatin between slime weight and hydraulic rughness 8.2 As bth the slime weight and hydraulic rughness shwed bradly similar trends in the curse f all these runs, there exists the pssibility f a crrelatin between k values and slime weight. If the crrelatin cefficient is large, it might then be pssible fr a drainage engineer t btain an apprximate estimate f k value in a mature ful sewer by remving and weighing the slime frm a given area f pipe surface. All k values described s far in the reprt are the cmpsite values fr bth slimed and clean prtins f the perimeter when the pipe is running full. Befre trying t crrelate the data frm the test rig, it was necessary t cmpute k values fr the slimed prtins f the perimeter, s that the influence f the relatively clean pipe crwns (which wuld have very much lwer k values) culd be eliminated. \":\"F"*\,P, where \ : frictin factr p = prprtin f ttal perimeter ccupied by a surface rughness k values fr the slimed prtin f the perimeter were calculated by the methd f prprtinal frictin factrs:- Subscripts:- c : cmpsite surface s = slimed surface n = new (r clean) surface 20<!"<r k. The prcedure is recmmended in the HRS Tables and Charl5(6'7);s1 surfaces where:- The crwn f each pipe was assumed t be cmpletely unaffected by slime and t have clean pipe rughness values ie:- spun cncrete clay UPVC asbests-cement k" (mm) Values f k" fr each type f pipe were btained frm measurements f pipefull headlss immediately befre r after the time f slime remval. Fig l5 shws the calculated values f k" pltted againsthe dry weight f slime per unit area f pipe wall. Data frm all pipe materials (except vertically-cast cncrete) and frm all three runs is included. Althugh hydraulic rughness tends t increase with the amunt f slime present, the crrelatin is nt gd enirgh fr a reliable predictin f k, frm slime weight. The textural "rughness" f slime lumps at the water-line, in part-full sewers clearly has a much greater influence n hydraulic rughness than the ttal vlume f slime present. 22

29 Crrelatin between slime weight and velcity 8.3 When a new sewer pipe cmes int use, there is usually a fairly rapid increase in slime, which levels ut when grwth is balanced by slughing due t the shearing actin f the flw. Subsequently the amunt f slime present stays rughly cnstant in the shrt term but, in the lng term, the pint f balance may be altered by physical' chemical r bilgical changes in the slime r sewage and the amunt f slime present may increase r decrease befre a new balance pint is reached. The pint f initial balance is difficult t establish frm infrequent spt measurements f slime weight but an examinatin f data frm Yang and pg16(s) and Bland et al?) as well as the present HRS data indicates that the higher the velcity in the pipe the quicker equilibrium is established. In Fig 16 the dry weight f slime has been pltted against number f days in cntact with sewage. All the data is frm part-full pipes and cvers a range f velcities frm 0.23 t 2.40m/s ver a perid f 100 days frm new. Befre pltting the Yang and Reid data, factrs were applied t the reprted slime weights t allw fr a smaller sample area. Neither the actual pipe diameter nr the area frm which each sample was scraped are stated in the reprt but have been calculated frm estimates f pipe diameter and depth f flw. Bland et al analysed slime fr'n abve and belw the water-line separately. In their tests significant amunts f slime were attached t the crwns f the pipes because f faming assciated with entry cnditins t the test lengths; this is nt nrmally a feature f lng sewers s nly the data frm the basal part f the pipe has been pltted in Fig 16. The times taken t establish an initial equilibrium between slime grwth and slughing, as estimated by the authrs cncerned r frm an examinatin f their data are as fllws:- Mean velcity (m/s) l. t Days frm new Surce 70 Yang and Ps14(5) 70 Yang and Reid 7 Bland et al(2) 35 HRS, Littlemre experiments 28 HRS, Littlemre experiments 7 Bland et al(2) The table shws that (with the exceptin f Bland's data at 0.55mls), the lwer the velcity the lnger the perid f grwth befre initial equilibrium is established. The average dry weight f samples taken frm all types f pipe within the perid f equilibrium fr each test have been pltted against mean velcity in Fig 17. In the case f the Yang and Reid and the Bland tests, the equilibrium perid is assumed t ccur between the times given in the abve table and the end f each test. Hwever, the HRS tests cntinued fr much lnger perids, during which time, changes in the bilgical and chemical cnditins in the sewage and the slime 23

30 are thught t have ccurred. Slime weights in the HRS tests after lt, days are, therefre, nt included in the plt. The best fit line t all the data pints fr all pipe materials has the equatin:- W : 48.92y-2'r0 with acrrelatin cefficient r2 : 0.9L (W : dry weight f slime per unit area f slimed surface in g/mz: V : rn n velcity in m/s). The expnent suggests that, when there is a state f balance between grwth and slughing, the amunt f slime present is inversely prprtinal t the square f the velcity and hence t the shear stress exerted by the flw. Appearance f slime in the test pipes 8.4 Fig 15 shws that slime weight is a pr predictr f hydraulic rughness; a better estimate f k value in an existing ful sewer may be pssible by cmparing the slime depsits with phtgraphs taken in the test rig at a time when the cmpsite pipe-full rughness (k.) was measured. The selectin f phtgraphs in Plates 2 t 7 are all f the same length f 225mm diameter asbests-cement pipe, which includes a sleeved butt-jint with slight displacement. Similar depsits in ther pipe materials and pipe sizes wuld give rughness values f the same rder. In the pipes the pattern f sliming is the result f running a hydrgraph giving a maximum flw depth f Yz pipe diameter and a maximum velcity f 0.76m/s. Values f k" fr each phtgraph were btained directly by measuring the head-lss with pipe-full flw. Values f k, have been calculated frm k" and l:", using the methd described in Sectin Cmparisn f present research with previus wrk The main UK surces f data n slimed sewers are Ackers et al(l) and Bland et alq). In the USA sme wrk has been dne by Yang and Reid(5); in additin Ackers et al analysed sme early American data n sewer rughnesses. Ackers, Crickmre and Hlmes 9.1 This research cmprised field measurements in twenty sewers at five sites in England. The sewers ranged in size frm l5in t 66in, in age frm 2 t 100 years, and at gradients frm I in22t I in The sewers were mainly cncrete (12 in all); f the remainder, 4 were brick, 2 were salt-glazed clay, I was bitumenjined steel and I was steel-lined brick. The rughnesses f all the sewers were determined under the part-full cnditins that happened t btain at the time that the measurements were being made and they relate t the cnditin f the sewer as it was at that particular time; cntinuus measurements ver a lng perid f time were nt made. The sewerughnesses determined by Ackers et al have been pltted in Fig 18 as a functin f the velcity that was measured during the experiments. Thse sewers (which included all the brick sewers) in which sediment depsits were fund n the invert have been mitted, in rder t make it easier fr a cmparisn with the Littlemre data. The range f rughness measured in the three Littlemre runs have als been shwn n the same diagram. 24

31 The trend in the Ackers data is fr the rughness t decrease as the velcity increases. Hwever there is a cnsiderable scatter in the rughness with an rder f magnitude in range fr any particular velcity. The Littlemre data frm Runs I and 2 verlap the Ackers data t a cnsiderablextent whereas the data frm Run 3 tend t be belw the Ackers data. On the whle, thugh, there is a satisfactry agreement between the tw sets f data. When making the cmparisns a number f factrs shuld be brne in mind. The ranges f Littlemre values are btained frm repeated measurements thrughut a number f lng-duratin experiments n a few pipes, during which the rughness varied appreciably. The Ackers data were btained frm measurements made ver a very shrt perid - a few days - n a number f different sites thrughut England. Each f Ackers' data pints is the equivalent f taking a spt measurement duqing ne f the Littlemre runs, and s is unlikely t be representative f the lng-term variatin f sewer rughness. A further difference between the tw sets f experiments is that the values btained by Ackers are fr the part-full cnditin (the prprtinal depths ranged frm 0.07 t 0.72, with the bulk f the sewers flwing at prprtinal depths less than 0.4). If pltted in Fig l8 n the pipe-full basis the data wuld be lwer by a factr f up t 2. The results frm the experiments at Littlemre have been criticised because they were nly carried ut ver a limited perid f time, which was relatively shrt in relatin t the life expectancy cf a nrmal sewer. The critics felt that rughness wuld g n increasing with time s that eventually it wuld be fund that there was n difference between the rughnesses f the varius pipe materials. The cntinuus measurements that were made at Littlemre did nt shw any steady cntinuus increase f rughness with time, nr des ther experimental wrk (see Sectin 9.2). On the cntrary, they suggest that nce the initial rapid sliming has taken place, the rughness then fluctuates quite significantly. This is cnsidered t be the result f a cntinual prcess in which slime is building up and being sheared frm the pipe surface. At any ne time, slime will be building up in sme places peeling ff at thers. The phtgraphs f the interir f the experimental pipeline shw that the rughness is very uneven in size and nnunifrmly distributed. In rder t examine the effect f age n sewer rughness, the Littlemre results have been cmpared with thse in Ackers' paper. The data given is fr sewers ranging frm2 t 33 years ld; thse relating t the 19 cncrete sewers have been pltted in Fig 19 tgether with sme data frm American cncrete sewers, als given in the same paper. One sewer with sediment n the invert has been eliminated. Only fur f these sewers had rughnesses greater than 6mm; three f 6.lmm and ne f 9.lmm. The average rughness f sewers less than l0 years ld was 2.2mm. The average rughness f sewers between 20 and 35 years ld was 2.7mm. It is nt cnsidered that there is any significant difference between these tw mean values, bearing in mind the wide variatins in cnditins that the varius sewers will represent. It is significant that the rughnesses measured in the Littlemre experiments are very much in line with the values pltted in Fig 18, adding supprt t the view that the age f the sewer is nt a significant factr as far as the effect f sliming n the rughness is cncerned. It must be acknwledged hwever that in lder sewers, factrs ther than sliming can have a significant influence n the rughness eg structural cnditin f sewer, cnditin f the jints. 25

32 The general cnclusin frm the cmparisn f the Littlemre data with that given by Ackers et al, is that there is very gd agreement between them. Bland, Bayley and Thmas 9.2 The experiments f Bland et al were carried ut n a specially cnstructed rig at the Water Pllutin Research Labratry (WPRL) at Stevenage (nw part f the Water Research Centre). Sewage was passed thrugh a test rig cnsisting f lengths f unglazed clay and upvc pipe, l00mm in diameter. Because there was insufficient flw at the site t prvide a cntinuus supply f fresh sewage, it was necessary t re-circulate mst f the sewage, with apprximately 5 per cent f fresh sewage added t it. In rder t simplify the experimental arrangement, the pipes flwed full at all times, and during any particular test the discharge remained cnstant. The duratin f each test was frm 50 t 100 days and the velcities during the tests ranged frm 0.76 t 2.Im/s. The WPRL test at 0.76m/s can be cmpared with Runs I and2 f the Littlemre experiments and there are sme similarities in the results. The rughness in the WPRL experiments fluctuated in much the same way as in the Littlemre experiments; the initial slime build up was rapid, in 12 days reaching a peak rughness f 15.7 and l4.5mm fr the clay and the upvc respectively. The mean values ver the perid f the test were 3.52mm fr clay and 3.97mm fr upvc. These are rugher than the mean values f the cmbined HRS Runs I and 2 (taking all values int accunt), viz l.2mm fr clay and 0.7mm fr UPVC. The extent f sliming in the Littlemre experiments was less than in the WPRL experiments because f the different flw cnditins under which the slime was frmed, but even when this factr is taken int accunt by calculating the rughness f the slimed prtin f the pipes in the Littlemre experiment, the values cme ut at 2.lmm fr clay and l.6mm fr upvc, still appreciably lwer than the WPRL experimental values. Fur separate tests were carried ut by Bland et al at a velcity f l.lm,/s and despite the difference in velcities, the variatin in rughness bth during a test, and frm test t test, was similar t that which was apparent in Runs I and 2 f the Littlemre experiments. Thus in the test f 15 days duratin, the peak rughness was 2.7mm fr clay and 1.6mm fr upvc. In the 45 day test the peak rughness was l.7mm fr clay and 2.lmm fr upvc: hwever these peak values were nly achieved twards the end f the test and fr apprximately 40 days the rughness fluctuated between 0.5 and l.omm. In the 90 day test, the peak values were 0.8mm fr clay and l.5mm fr upvc, but these were reached n nly ne ccasin; fr mst f the test (bth befre and after the peak value was reached) the rughness ranged between 0.2 and 0.5mm fr clay and 0.5 and l.omm fr UPVC. The test, lasting 68 days, gave results in sharp cntrast t thse frm the ther tests. The rughness gradually increased fr the first 35 days and then it remained fairly steady fr the rest f the test, fluctuating between 2.9 and 5.0mm fr the clay and between 5.0 and 6.3mm fr the upvc. These values are much higher than either the peak r the mean values that were reached in any f the ther tests at this velcity. A similar pattern was nticed in the Littlemre experiments, where the rughnesses in the first half f Run I were significantly higher than the rughnesses either in the secnd part f that run r in Run 2. The test that was carried ut at 1.5mls shwed an rder f magnitude variatin in the rughness - rughly between 0.05 and 0.5mm - but the values were f a similar magnitude t Run 3 f Littlemre experiment, in which the maximum mean velcity during the slime build up was l.l5m/s. 26

33 It is clear frm the cmparisn between the WPRL and Littlemre experiments that even after making allwances fr the greater surface area f slimed pipe in the WPRL experiments, their values are higher than thse that were btained during the Littlemre experiments. One reasn fr the difference culd be that the amunt f slime grwth depends n the characteristics f the sewage: recirculated sewage was used in the WPRL experiments, fresh sewage at Littlemre. The absence f an air-water interface in the WPRL experiments culd als have affected the slime grwth. Unfrtunately, there is nt sufficient infrmatin t be able t thrw any light n the mst likely explanatin fr the differences. Yang and Reid 9.3 Yang and ps16(5) carried ut a series f experiments with the aim f determining the relatinship between the thickness f a slime layer, the prductin f dur and the reductin f the carrying capacity f pipes. This wrk fllwed sme earlier experiments f Reid and Keeley n the relatin between slime grwth and rughness. The Reid and Keeley experiments were nly f shrt duratin - 27 days - and little quantitative data were btained. Their main cnclusins were that: a) the amunt f slime grwth reduced as the flw velcity increased, b) cncrete, asbests-cement and unglazed clay pipes sustained a heavier slime grwth than a glazed clay pipe. The subsequent experiments f Yang and Reid prduced cnclusins that were similar t thse f Reid and Keeley. Althugh sme measurements were taken during the experiments, there is n reference t rughness in the Yang and Reid paper. a) Slime grwth decreases with velcity. b) Rugh pipes maintain mre slime grwth than smth nes, eg slime weight n asbests-cement was twice than n glazed pipe. 10 Recmmendatins fr design and analysis Their cnclusins were that:- The main purpse f carrying ut the research was t prvide data fr the drainage engineer in design r analysis f sewerage systems. During the experimental runs, see Figs 5, 6 and 8, the rughness varied ver a cnsiderable range and it is difficult t chse just ne value that reflects this wide range. The discussin in Sectin went sme way twards explaining the variatin in rughness with time during the experiments but nt far enugh t enable a predictin fr any sewer t be made. Therefre, t arrive at recmmendatins it is necessary t cnsider Runs I and 2 simply as typical f sewers flwing at abut 0.76m/s. It is clear that there will be lng perids, when relatively high rughness ccurs cntinuusly. The designer f a new sewer may therefre need t use such values since it is inevitable that high rughness will cincide with high values f flw at sme perids during the year. On the ther hand if an analysis is being made f a sewer during a particular event, in the absence f site infrmatin n the rughness, a best estimate f the rughness wuld be the median value btained during the tw runs. In drawing up the fllwing table the high value is taken as the value which the measured rughness exceeded fr ne mnth in 1976 (Run l, which started very early in the year). In mst cases this value was als exceeded fr a shrt perid twards the end f Run 2. The lw values are thse which exceeded 27

34 the measured values fr ne mnth in 1977 (Run 2 when the summer temperature was typical f mst years). The median values refer t all the data in bth runs and are seen t lie rughly mid-way between the high and lw values. Material High k (mm) Lw k (mm) Median k (mm) Vertically cast cncrete Spun cncrete Asbests-cement Clay l.l upvc l.l The abve values are fr pipe-full flw and apply t pipes with velcities f abut 0.75m/s, carrying nly sewage and slimed t apprximately half-depth. Fr steeper pipes with velcities arund l.zm/s, crrespnding t Run 3, high, lw and median values are rughly ne third f thse given in the table. It is recmmended that, wherever pssible, the designer shuld use the abve values directly in the Clebrk-White flw equatins, using his wn discretin t select a value within the apprpriate range. His chice will depend n specific site cnditins and an assessment f the benefits and risks invlved. Fr designers using the Wallingfrd Charts and Tables(6'7) the median and high rughness values have been runded t the nearest "standard" k values as fllws:- Velcities abut 0.75m/s Material Values f rughness, k (mm) Mid High Cncrete, spun and vertically cast Asbests-cement Clay UPVC J Vefcities abut l.2m/s Material Values f rughness, k (mm) Mid High Cncrete, spun and vertically cast 0.6 Asbests-cement 0.3 Clay upvc lnterplatin shuld be used fr velcities between 0.75 and l.zm/s. 28

35 11 Summary and cnclusins A series f experiments n the effect f sliming n the rughness f ful sewers has been carried ut n a specially cnstructed rig at the sewage pumping statin at Littlemre, Oxfrd, belnging t the Thames Water Authrity. The purpse f the study was t extend the scpe f wrk carried ut by ther investigatrs n hw the rughness varies with time, with velcity and with pipe material. Three separate, lng perid runs were carried ut using a cnstantly varying sewage flw t frm the slime. In the first tw runs, the maximum mean velcity prduced by the slime frming flw, was apprximately 0.75m/s, and the runs lasted fr 335 and206 days, respectively. ln the third run, the maximum mean velcity prduced by the slime building flw was apprximately l.2m/s and the run lasted fr 188 days. Measurements were made at weekly intervals thrughut all three runs, frm which the rughness f the sewers was calculated. The principal cnclusins frm the experiments were as fllws:- a) Slime builds up n the sewer very quickly: nce this initial sliming has taken place, there is n evidence f a subsequent cntinuus increase; the rughness varies, suggesting that there is a cntinuus prcess f slime building and slime remval iaking place. b) Althugh it was pssible t identify similar trends in the three runs, there were als instances when sme f the pipes displayed incnsistencies their behaviur, suggestihg that deterministic and randm factrs bth have a significant influence n the amunt f sliming. c) The pipe material des have an effect n its rughness; the results frm all three runs pinted t the same cnclusin. d) The runs frm the Littlemre experiments are in general agreement with thse btained frm the field experiments f Ackers et al. There is nt such gd agreement with the results frm the experiments carried ut by Bland et al at WPRL. e) Sme limited experiments were carried ut t determine the rughness f sewers flwing part-full. These shwed that under these cnditins the rughness was much greater than when the sewer is flwing full. On the basis f the Littlemre experiments rughness values fr slimed ful sewers have been recmmended fr use in design. 12 Acknwledgements The research wrk described in this reprt frmed part f the research prgramme f the Hydraulics Research Statin and is published with the permissin f the Managing Directr. The wrk was carried ut in Mr J A Perkins' sectin in the Rivers and Drainage Divisin headed by Mr A J M Harrisn. Mr I M Gardiner was in cntrl f the site measurements. These were undertaken under what were, at times, very trying cnditins and recgnitin must be made f the significant part played by Mr D Lrd, Mr M Evers, Mr G Wldridge, and Mr I Fish in bringing the wrk t a satisfactry cnclusin. 29

36 The Hydraulics Research Statin is grateful fr the cntributin f f5000 frm the Clay Pipe Develpment Assciatin t the cst f the research: this mney was used in analysing the results frm the experiments. Finally grateful thanks are due t the staff f the Thames Water Authrity fr the tlerance they shwed thrughut ur lng presence at the pumping statin and fr the valuable assistance that they prvided whilst the experimental rig was being cnstructed and whilst the experiments were in prgress. 13 References 1. ACKERS P, CRICKMORE M J and HOLMES D W. Effects f use n the hydraulic resistance f drainage cnduits. Prc. Instn. Civil Engineers, Vl 28, pp , July BLAND C E G, BAYLEY R W and THOMAS E V. Sme bservatins n the accumulatin f slime in drainage pipes and the effect f these accumulatins n the resistance t flw. The Public Health Engineer, January DEPARTMENT OF THE ENVIRONMENT. Wrking Party n Sewers and Water Mains. First reprt. HMSO, Lndn, HEUKELEKIAN H and CROSBY E S. Slime frmatin in plluted waters. Sewage and Industrial Wastes. Yl27, N. 12, December 1955, Vl 28, N. l, January 1956, Vl 28, N.2, February YANG T S and REID G W. Sewer durs and pipe materials. Paper delivered at Texas Annual Water and Sewage Shrt Schl. Bureau f Water Resurces Research, University f Oklahma, March HYDRAULICS RESEARCH STATION. Charts fr the hydraulic design f channels and pipes. Furth editin (metric), HMSO, Lndn HYDRAULICS RESEARCH STATION. Tables fr the hydraulic design f pipes. Metric editin, HMSO, Lndn DDB Dd /42

37 Tables

38 Table I Rughness f clean pipes Material Befre Run I Rughness, k (mm) Befre Run 2 Befre Run 3 Vertically cast cncrete upvc Spun cncrete l Asbests-cement Clay l Table 2 Disslved xygen measurements Site N. Lcatin 7 July 1916 Tempera- Disslved ture xygen OC mg/l 20 December August 1977 Tempera- Disslved Tempera- Disslved ture c xygen mg/l ture c xygen mg/l Five mile Drive/ Rthafield Rad 22.r Hamiltn Rad/ Kings Crss Bradmre Rad/ Nrham Cardens 23.0 J.J t4.r 5. l5 20.s 4.25 Maltfield Rad/ Stckleys Rad r9. t Gypsy Lane tr Bttm f Edgeway Rad 19.3 < t7.l 0.3 Hlywell Street/ Mansfield Rad Radcliffe Rad/ Iffley Rad r r l8.l Mill Lane 22.6 < <0.1 l0 Littlemre trunk sewer 22.6 < r Experimental rig 21.5 I 1.8 l9.l Mean f sites I t 6 & tr J.J

39 Figures

40 c.9 U t,/ )O ru ^(U rly: c tr -9.s r c g :Ee = lrjc= E'l il c O.= ll.jcr b J c gl.s f c, E I 3 I -i3 ;t g erg cn F -v, '= O.y :rc 0, E fr6 de.- i (Ltr lc, >L _tj ;c ;.: ul > J (f, F lj - f,, L I c (, c f. (n.. -s trj= -l :l l (l,l -l 1- I I v G\t lij qy UI F ci is 1r) i-- I I I L.f c E (, UI c, c! J F c! 0, F 0, E =, E E L (L Fig 1 Lyut f rig

41 0.020 := 0.6 D V = 0'78m/s (') E, gl E u '0r 0 I = 0't2 V = 0'67 m/s V = 0 52m/s T i m e ( hu rs : 2l+ hur clck ) Fig 2 Hydrgrph : Run I (1976) U' (n E (, gl.c. ().9 0'0r0 D V = 0'75 m/s I I = 0'11 D V = 0'66 m/s Fig 3 Dily hydrgrphs.40 mls Time (hurs:2t hur ctck) Hvdrqrqph : Run 2 (1977) 20

42 0'025 V = '1"18 m/s (vt E c, l L _c u i5 0.0r 5 0.0r 0 a $=.ga $ =.as V = 1'07m/s 0'005 d D V = 0'18 = 0'64 m/s Time (hurs :24hur clck ) Hydrgrph : Run 3 (1980) Fig t, Dily hydrgrph

43

44 :r 20, t c' L &r E c, ql ;0 v, \.2 -t/, -._y'.-l Dys a. \Fa- *._\ r40 r60 r80 a E E - J c t t c ' 0 0 April My I 60 June tl I -v- Vert cst cncrete -x- Spun cncrete -.- cly r,l V--. SL Jury Dys I f-"...'.>", 120 -n'-t' '- Augusl rl I September I ll Octber 3.0 Asbests cement?, tr : 3 r'0 g. l J E 0 July Dys I I August rl I September I Octber tl Run 3, 1980 Fig 6 Vritin f rughness with time - Run 3

45 vb \b *b Site 1 x Site 2 Site 3 a Slte 4 v Site 5 Site 6 * Site 8 -+ Men f sites lt6 & 8 = 7 July 1976 b=20 December 1976 c= l0 August 1977, ).q gl E 5 a b ltt.. b c c, r4 x p 93 v, i5 +C q\ * I xc +G \ al \ x '\ sturlin Temperture ('C ) Frg 7 Disslved xygen levels in Oxfrd sewers

46

47 E E t c 6.0 lr'0 -j-qlqy Asbests-cement g 2'0 (r Fig Prprtinl depth d D 0'8 1.0? E ; c 4'0 9 z (r.-a-. -' Spun cncrete upvc Prprtinl depth (l D Fig 10 Vritin f rughness with depth f flw - Run 1

48 r0.0 E.0 E E 6'0 d! --x-- -- Asbesls-cement Cly J rt c E 0't l E '1 0.6 Fig 11 Prprlinl depth d D 10.0 c L g J Ul c E r J a.- Spun cncrete UPVC b Prpriinl depth d D Fig 12 Vritin f rughness with depth f f lw - Run 2

49 -+- Spun cncrete Asbests-cement _..ax.._ cly --q- u PVC N E l ; 500 U J rf /r00,.e r,! _ 300 0, :- 5 z c q, E ; 100 -c..9^ ; r00 Pump filure Dys f sliming 1976 : Run 1 Fig 13 Weight f slime scrped f rm pipe wlls - Run 1

50 'il c gp E; u9 F,i) U9 (J i# aa (n< (J J ili OxO iri 6E crl.e E = OE, (lt - \ [t\ N/ gl c OE =61 Ut Ec f lj,.c (V) Gl (2tu/6) a3dtjns Peuills l DaJD llun u aultls l lq6ram Ilg Fig 14 Weight f slime scrqped f rm Pipe wlls- Runs 2 nd 3

51 500 a GT F (tl \ tj ct t 1f E a t = ct c._e t' c l '6 3 e fr I r )9 ).9 t K,' "g l" if v il, bf t, I a t lb _e r I l ) Rughness f slimed prtin f pipe k (mm) Run l O Spun cncrete Asbests-cement c Cly 6 upyc Run 2 I Spun cncrete tr Asbests-cement El Cly f, UPYC Run 3 Y Spun cncrete V Asbests-cement 9 Cly il IPYC 15 Relqtin between slime weight nd pipe rughness

52 6l E l (J ) Men vlues fr ll types f pipe Blnd et l HRS (1976,1977 ) HRS ( 1980) Yng & Reid (Test I) Yng & Reid (Test ll ) tr f J. '23 mts -'' a x 0'36 m/s E q, E q, = c:) L c, q, E Ẹ 9c, 3 L A- x --a-- =:j --r tr a.' le x a / I x tr'/,/? ' 0'55 m/s tr 72.1mls ';""" :::':g'.:/_i"" I i' lt,i, / I i./ Oi i/' I,1.2 mls,t 1I x a A J x x l Cncrete Asbests-cement Cly upvc 0'76 m/s Dys in cntct Fig 16 Vqriqtin f slime weight with time

53 E l W = 48'92 v-z'l = (, + L ) E E,.= c J r00 _ l0 tr = Dt surces Velcities (m/s) 0.23 Yng & Reid ( Test I ) 0.35 Yng & Reid ( Test II ) 0.55 Blnd et l 0'76 HRS ( 1976,1977, 1.2A HRS ( r98 ) 2'40 Blnd et l x Cncrete Asbests- cement Cty A UPVC.9 3 r'0 0.1 Vetcity (V) m/s Fig 17 Equilibrium slime weight s functin f velcity

54 380 O l0 5 a 460 a 460 a E S.r 'J 450 Ackers dt (l )? I a Rnge f Littlemre dqt Pipe dimeler (mm) shwn lngside pltted pints J c E l f E ' a 990 a.001 r Velcity ( m/s ) Fig 18 Vriqtin f rughness with velcity : Ackers nd Littlemre dt

55 r UK dt USA dt 'r0 I E 5 r1 c 6 a x c ctl e2 (r ro a t a a x a Age (yers ) Fig 19 Rughness cls cl f unctin f ge in cncrete sewers

56

57 Plates

58

59 Plate I Testrig

60 ,a'!:!il;i'!l l i, k : 0.02mm Plate 2 Clean.Rughness Plate 3 Rughnessf cmpsitesurface(pipe-full)k" : 0.4mm Rughnessf slimedsurface(half'full) k" = l.lmm Slime in asbests-cement test pipes

61 Plate 4 Rughnessf cmpsitesurface(pipe-full)k" = 0'8mm Rughnessdf slimedsurface(half'full) k" = 2.4mm Plate 5 Rughness f cmpsite surface (pipe-full) k. = 1.2mm Rughness f slimed surface (half-full) k" = 3.7mm test pipes Slime in asbests-cement