Larsen, Torben

Sørensen, Morten Steen

English

This paper describes a laboratory investigation of the separation of suspended solids in a circular weir overflow and a vortex separator. The basic idea is to evaluate the efficiency of a vortical flow in the overflow chamber, and to compare these results with other overflow structures

    Aalborg UniversitetComparison of Suspended Solid Separation in Advanced Storm Overflow StructuresLarsen, Torben; Sørensen, Morten SteenPublished in:Proceedings of the Fifth International Conference on Urban Storm Drainage, Suita, Osaka, Japan, July 23-27,1990Publication date:1990Document VersionAccepted author manuscript, peer reviewed versionLink to publication from Aalborg UniversityCitation for published version (APA):Larsen, T., & Sørensen, M. S. (1990). Comparison of Suspended Solid Separation in Advanced Storm OverflowStructures. In Y. Iwasa, & T. Sueishi (Eds.), Proceedings of the Fifth International Conference on Urban StormDrainage, Suita, Osaka, Japan, July 23-27, 1990 Local Organizing Committee of the 5th InternationalConference on Urban Storm Drainage, 1990.General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            ? Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            ? You may not further distribute the material or use it for any profit-making activity or commercial gain            ? You may freely distribute the URL identifying the publication in the public portal ?Take down policyIf you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access tothe work immediately and investigate your claim.Downloaded from vbn.aau.dk on: November 30, 2020• t"resented at the Fifth lnternattc>nal Confe~ce on Urban Storm Draina«e Ou.b., Japan, April1990 COMPARISON OF SUSPENDED SOLID SEPARATION IN ADVANCED STORM OVERFLOW STRUCTURES Morten Steen S0rensen* and Torben Larsen** Nellemann Consulting, Digtervejen 11, DK~9200 Aalborg, Denmark ** Aalborg Univ . , Sohngaardsholmsvej 57, DK~9000 Aalborg, Denmark ABSTRACT: This paper describes a laboratory investigation of the separation of suspended solids in a circular weir overflow and a vortex separator. The basic idea is to evaluate the efficiency of a vortical flow in the overflow chamber, and to compare these results with other overflow structures. KEYWORDS: Hydraulic Scale Model, Solid Separation, Storm Sewage Overflows, Use of Vortical Flow. 1. INTRODUCTION In Denmark Storm Sewage Overflows (SSOs) has for decades been designed to avoid f~oodings in sewer systems rather than to avoid undesired impacts on receiving waters. In 1987 the Danish Parliament passed the Water Enviromental Act . The overall aim of the act is to improve water quality in Danish coastal waters . This means that requirements to the function of and discharges from SSOs will be made more rigorous in the years to come. A SSO, which is found to be a considerable source of pollution for a river , stream or lake, is very often being provided with additional storages volumens . This reduces the overflow frequency and thereby retains a certain amount of pollutants. But it also increases the attention to maintenance f or these storm tanks and the duration of maximal inflow on the treatment plant. In view of avoiding this, advanced storm overflow structures with separation of suspended solids are an alternative (see figure 1). Figure 1 . Principle of suspended solid separation. 1 Especially in England (Halliwell & Saul, 1980), (Saul & Delo, 1982), (Balmforth, 1986), projects have been running in order to assess SSOs performance, hydrau-lically and qualitatively. In these studies the high side weir overflow and the vortex overflow have been prefered. A design guide for storm overflow structures (Balmforth & Henderson, 1988) has recently been published, using results from the abovementioned studies. These results indicate, that the use of a vortical flow in the overflow chamber has a better separating effect on suspended solids than conventional overflows. The work outlined in this paper presents results of suspended solid separation in two SSOs, which have not earlier been systematically examined, namely the circular weir overflow and the vortex separator. A comparison between the re-sults has been made to examine the efficiency of the vortical flow qualitative-ly under identical conditions . Furthermore, a comparison with the results from the abovementioned English investigations has been made, to see whether these two overflow structures appears to exert a good solid separating effect. 2 . DESCRIPTION OF THE OVERFLOW MODELS The circular weir overflow has a circular chamber and exerts good hydraulic control, due to the length of the weir. The circular weir overflow causes surcharging of the upstream sewer, which may make it inapplicable under certain circumstance. The scumboard is placed centrally and has its bottom placed under the weir level. In order to obtain optimal efficiency concerning retain-ing of floating solids, ~he inlet is extended into the scumboard. A screen is placed horizontally between the scumboard and the weir, slightly under the weir level. The vortex separator is developed by Hydro Research & Development (HRD) in 1983, and is a further development of the vortex overflow. The water enters the chamber tangentially in a vortex separator. This leads to a spin in the water and a vortex is formed . The secondary currents in this vortex are em-ploye d for dynamically sepa r a t ion of the solids, leading these to the middle of the chamber, where the outlet is placed. In the middle of the chamber a cone is placed. This secures, that the cross section decreases to keep the velocity high. Hereby a greather volumen in the active zone of separation is used. The HRD-separator is known for having a fine separation performance . The separator used in this investigation was not a copy of the HRD-separator . Our separator had significant larger inlet pipe and higher hydraulic load than recommended by HRD . To determine the efficiency of these SSOs, it is necessary to measure the discharges under a large variety of different natural conditions. Due to the fact, that this is timeconsuming as well as rather inconvenient·, it has been necessary to develop and test the SSOs in a laboratory, where reproducible test conditions are ensured. The two overflow structures were established with identical chamber volurnens and physical conditions and was investigated under uniform flow conditions. Both overflow structures were made in model in Perspex (60 cm in diameter). 3. METHOD OF DATA COLLECTION AND ANALYSIS The tests were performed under steady flow conditions . It was hereby possible to eliminate the scale effects due to time. This made it possible to simulate the suspended solids by single plastic beads with different terminal velocity . Instead of introducing a large number of identical particles into the inlet pipe at one time, one particle could be introduced several times showing uni-form results. During the tests the terminal velocity of each particle was 2 determined frequently in a high Perspex tube . If it changed, the results for this particle were discarded . The maximum size of the beads allowed without getting scale effects is 0.07 ·0, where D is the diameter of the inlet pipe (Halliwell & Saul, 1980). In these tests the maximum was 10.08 mm, so to make it clearly visible, beads with a diameter of 10 mm were used. For the test 10 particles were chosen with termi-nal velocities in the range of -0 . 16 m/s to 0 . 16 m/s. The particles was intro-duced in the inlet pipe more than lO · D upstream the chamber in order to get the right distibution as the particles enters the chamber (Halliwell & Saul, 1980) . Two particles with the same terminal velocity were tested to check the steady conditions. There were no significant differences in the test results of these two particles . The conclusions hereof was that a definite efficiency exists for each single particle introduced into the system. The necessary number of introductions of each particle in each test series were examined by using t-test on a large number of introductions. It was hereby found that the minimum number for each particle was 60 introductions (95 % confidense interval) . 22 . 200 introductions of particles on the two models were carried out during this study. 4. RESULTS In literature the ability of retaining suspended solids in the chamber is cal-led the efficiency of the SSO. The definition of efficiency is the ratio be-tween suspended solids in the outlet and suspended s'olids in the inlet . The efficiency can be described as the ratio between outflow and inflow , if the particles has no terminal velocity . It is hereby obv ious that the efficiency depends on the inflow as well as the outflow. To examine whether the vortical flow gives a better efficiency the two models were investigated for retainment of each particle under a number of identical steady flow conditions. In figure 2 the efficiency for particles with an ave-rage terminal velocity of 2.72 cm/ s is illustrated. With low ratio of outflow versus inflow, responding to low retainment , the efficiency for the vortex se-parator appears to be better than for the circular we i r overflow . The diffe-rence is not significant, and on basis of these results it is subject to uncer-tainty to deside whether the vortical flow has a positive efficiency or not. VO~TEX SE, . Ert . lll COMPARISON OF CIRCULAR WE I R OVERFLOW AND VORTEX SEP ARATOR 100 110 10 70 10 50 • :10 20 10 . .. ···• ~>· · · ····· .·· . •• . . . ~-··· ·· . . · . ••• •••• • .. ··· . ' .' Figure 2 . Comparison of efficiency of the circular weir overflow and the vortex separator for particles with a terminal velocity of 2 . 72 cm/s . 3 One of the reasons why the vortex separator was not functioning as expected, was that the model has a very little overflow orifice, which gives higher velocities in the chamber and thus more turbulence. The conclusion must be, that the expected positive efficiency from the vortical flow can easily be spoiled by a poor design of the overflow construction. The efficiency of both models are found to depend on the inflow as well as the outflow. In figure 3 the efficiency for the circular weir overflow is given for two different outflows and a steady inflow (left), and for two different inflows and a steady outflow (right). The efficiency is more dependent on variation in the inflow than in the outflow. C.,ICU:NCT 110 lOO • ~ 00 70 .., .. .., )0 .. 10 -· ··· •··-i I I O'nCifloCCY Ill 100 - - ~-.. 00 ,. .. •• -.;!-=--'. ;···. ' .. '" ,·/ ' ', \. ! :/ ' : •. ' ' +I ' ' ' . ' . ' 0 .\...... !/ ""'. . ._., 1/1 ' I Q I" ' ' y I PAIIT it~.o[ I Y[L.O(flT ~ AAl t CL! ' Y£. CC I' t ! 0 -· '-----::-----:- .7. --::_,---,----:---::------:-----:-----:.9 !C~/5) I '----_,:------:-_ ,--.• =-----~, ---:---:------=------=-----::-------=- ·c ... . ~~ I L-----------------------~1 '-----------------------------------~ Figure 3 . The efficiency of the circular weir overflow for constant inflow, Qin- 5.63 1/s, (left) and for constant outflow, Qout - 0.97 1/s, (right) . This means that a design criteria for these kinds of overflow structures , e.g . which physical condition they ought to have to retain an expected amount of suspended solids, must depend on the inflow as well as the outflow . To upgrade the results from scale models to full scale constructions, the effi-ciency shall be described using a dimensionsless parameter WfV0 , where W is the particles terminal velocity and V0 is defined by where Qin Qout F V -0 F is the inflow to the chamber is the outflow from the chamber is the cross section area in the chamber By plotting the efficiency, E(W), for each test against the dimensions less parameter , WjV0 it is found, that all results can be described as an inverse normal distribution curve M W - J.L E(W) ~ l · Exp ( . (--) 2 ) j21fa j2 a where J.L, a and M are normal distribution parameters. For each of t he tests appears one value for J.L, a and M. By using regression 4 analysis on these values, one description of the three parameters for all the tests, depending only of the inflow and the outflow. ciency curve can be described for any flowconditions. appears valid Now the effi-where K Qo Circular Weir Overflow K #-' a -140. 697 • ~ut/Qin -2.308 ·Q1n/~ 1 . 617 ·Qin/~ Vortex Separator K #-' a - 79 . 782 · ~utfQin • 5 . 015 • Qin/~ 2 . 659 · Q1n/~ + 133.86 + 3.17 0 . 27 + 133.54 + 3. 92 0 . 22 (R (R (R (R (R (R -0 . 969) -0.978) 0. 997) -0.882) -0.997) 0.995) By using e . g . Simpsons rule and a well-known distribution of suspended solids in the sewage as a function of the terminal velocity, the expected total efficiency for the actual overflow structure can simply be calculated. EF'"ICifNCY (I) soo ~-~----~ ~--'--.......... ......," ., CtACtA.£R ~IR ~ / 90 \ ' ' \ \ OVERI'LON I \ ·~., '\ / I eo \ \ '\ I \ ' \ I \ ' 'IOATEX I \ ~ OVERFLON \ ',· ~\/ i \ /\ • \\ I \ HIGH SlOE WEIR ' ~ ~ I ' OVERFLOW ' \ \ I \ ' . \ '-• •\. \ I " \ \ \ I \ \\\ I \ '.• ~ I ~, /\ ', _.~ ) ,-· \ '•)ij-' 60 70 •o JO 20 .: ' ' ' ' : ' ' , . •. , 10 VORTEX SEPARATOR\\. // 0 L;---;:-~-;:-:;--'\,_::,...._?'::-4.,7/--:;;-:;----;;-:---;;-:;---;;-;---;;--:----;:-N/V, (H -o .s -o .• -o .J -0 . 2 ~.s -o .o 0 . 1 o.2 o J o • o 5 Figure 4. Comparison of the efficiency of the high s ide weir overflow (Saul & Delo , 1982), the vortex overflow (Balmforth , 1986) and the circular weir overflow and the vortex separator (this investigation) for iden-tical flow conditions (Qout/Qin ~ 0 . 16) . To compare the results from these t es ts with results from well known success-fully tests on other storm ove rflow structures, one has to plot the efficiency as a function of the dimensionsless parameter, WfVi , where Vi is t he average velocity in the inflow . This technique was first introduced by (Halliwell & Saul, 1980). In (Balmforth, 1986) there is a comparison of the vortex overflow and the high side weir overflow desc r i bed for a ratio between outflow and in-flow of 0.16. In figure 4 these results are compared t o results of the circu-lar weir overflow and the vortex separator, founded at the same flow ratio. It is obvious that the vortex separator cannot retain floating particles as well as the other overflow structures. This is due to the poor design of the used model. There is no doubt, that the most efficient overflow structure in 5 retaining suspended solids is the vortex overflow as described by (Balmforth, 1984). The circular weir overflow seems to give a better efficiency than the high side weir overflow, just like the vortex separator seems to do for falling particles. In the future new tests must be required to decide whether the vortex separator can be advantageous used in Danish sewers. New tests on scale-models and perhaps a full-scale test are being planned at this moment of writing. 5. CONCLUSIONS The resul~ obtained in this study yields no indication of a vortical flow giving a better efficiency concerning retaining of suspended solids. This is tested by comparing a circular weir overflow and a vortex separator under identical physical conditions and flow conditions. It is found that the efficiency of both models depends of the inflow as well as the outflow. The design criteria used for full-scale overflow structures has to involve both flows. Compared to other modeltests, the circular weir overflow prove to be as good or even better than the high side weir overflow . Due to a poor design of the model, the efficiency of the vortex separator in this study is not as good as the other overflow structures . But the best efficiency is connected to the vortex overflow developed by (Balmforth, 1984) . It is obvious that advanced storm overflow structures are an alternative to storm tanks. One way of supporting this is by developing prototype structures and set up field monitoring programmes. 6. REFERENCES Ba1mforth, D.J. et.a1. (1984) Development of a Vortex Storm Sewage Overflow with Peripheral Spill, Pro-ceedings of Third International Conference on Urban Storm Drainage, Goteborg . Ba1mforth, D.J. (1986) Effectiveness of Storm Sewage Overflow Structures In Handling Gross Polluting Solids, Proceedings of International Conference on Combined Sewage Overflows and Effects on Receiving Streams, Wageningen, Holland. Ba1mforth, D.J . & Henderson, R.J. (1988) A Guide to the Design of Storm Overflow Structures, ER 304E, WRc, Swindon. Halliwell, A.R . & Saul, A.J. (1980) The Use of Hydraulic Models to Examine the Performance of Storm Sewage Overflows, Proceedings of the Institution of Civil Engineers, Part 2. Hydro Research and Development Ltd . (1983) Hydro-Dynamic Separator. Publication No. TP2. Hydro International Ltd. Hydro Research and Development Ltd . (1987) The Development of Dynamic Separation, personal notes. R.P.M.Smisson, Clevedon Avon, England . Saul, A.J. & Delo , E .A. (1982) Performance of a High Side Weir Storm-Sewage Urban Stormwater Hydraulics and Hydrology, Colorado, USA . S0rensen, M. S . (l989) Chamber Incorporating Storage, Water Resources Publications, Solid Separation in Storm Sewage Overflows, Research thesis carried out under the auspices of the Academy for Technical Sciences (In Danish). 6 

Comparison of Suspended Solid Separation in Advanced Storm Overflow Structures

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Comparison of Suspended Solid Separation in Advanced Storm Overflow Structures

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