The transfer of solids in combined sewer networks

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Pergamon 0273-1223(95)00327-4

Wal. Sci. Tech. Vol. 31, No.7. pp. 95-105. 1995. Copyr.ght@ 1995lAWQ Pnnted ID Great Britain. All nghts reserved. 0273-1223195 $9'50 + 00()()

THE TRANSFER OF SOLIDS IN COMBINED SEWER NETWORKS G. Chebbo*, A. Bachoc*, D. LapJace* and B. Le Guennec** * CERGRENE. Ecole Nationale des Ponts et Chauss~es. La Courtine. 93167 Noisy Ie Grand Cedex. France **IMFT, Institut National Polytechnique de Toulouse. Avenue du Professeur Camille Soula. 31400 Toulouse. France

ABSTRACT Suspended solids are the main vectors of pollution in combmed sewer wet weather flows. In spite of being very fine, they decant rather quickly except in the case of light rainfall events rich in organic matter. The part origilmting from sewer sediment deposits provides a large proportion of organic matter mass. By analysing the content and the volume of sewer deposits at different pomts from upstream to downstream. it emerges that the main source of this contribution is probably located in man-entry sewers. In one of these. sewer trunk no. 13 in Marseilles. five hundred metres of which were monilOred continuously for sediment build-up over three years. lIle deposit volume grows mainly dunng certain ram fall events and its surface slope tends 10 an equilibrium with the (millunetric) particle Size. Theoretical bed-load and suspension thresholds have been validated. then analysed for various situations and sections. A typology of the vulnerable areas. in several networks. has been drawn up. as well as a study of shear stress variations during a rainfall event The sediment deposits seem 10 occur in particular hydraulic discontinuity zones where transport capacities can drop sharpl y.

KEYWORDS Bed-load; combined sewer; deposits; particle size; settling velocity; shear stress; solids transport; suspended solids; urban runoff pollution.

INTRODUCTION Solids transported or deposited in combined sewer networks pose two important technical questions at the present time: - the clogging of man-entry sewers leading to sewer cleaning operations which are expensive and difficult to modernize, and - pollution discharged during wet weather into rivers. lakes or the sea, whose main vectors are generally suspended solids. It is for this reason that a research program, carried out over some years {l987-1993}, on solids in sewer networks was set up and carried out under the aegis of the AGHTM (Association Generale des Hygienistes et Techniciens Municipaux: General Association of Municipal Hygienists and Technicians), financed by the 95

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"Plan Urbain" (Urban Plan: A body under the authority of the French Ministry of Public Works - Ministere de I'Equipement). several Water Agencies. many local. concerned administrative authorities and private companies. In this article. we will make a synthesis of the main results obtained.

In the first section. we shall present the research programme carried out on solids in sewer networks. In the second section. we shall present, in a synthetic form. how various complementary procedures based on observation and analysis are used to obtain a better description of the conditions governing the build-up and persistence of deposits in combined sewer networks. The third section deals with the two following themes: - characterization of suspended solids in urban wet weather discharges, - contribution of deposits in combined sewer networks to the pollution of water during wet weather. PRESENTATION OF THE RESEARCH PROGRAMME ON SOLIDS IN SEWER NElWORKS The scjentjfic interests and the objectives of the research pro~ramme The scientific interests of the programme are essentially to be found in the field of improv~d knowledge regarding solid transfer in combined sewer networks.

In 1987, our knowledge of solids in sewer networks was causing a great deal of confusion. Some publications (Sartor et aI. 1974; Desbordes. 1985; Artieres. 1987; CIRlA, 1987; DEA-SSD. 1987; Sanchez, 1987) provide basic elements. which are often fragmentary and sometimes contradictory. Moreover. many ambiguities or uncertainties remain and the confusions that they cause is technically very unproductive. either because they promote solutions which are not very efficient, or because they redirect interest towards non-essential targets. The programme's first objective was therefore to clarify basic notions: - by distinguishing the main phenomena, - by assessing, for key parameters. orders of magnitude and threshold values. The programme's second objective was to improve knowledge likely to clarify technical options or solutions: - aiming at creating a pollution control policy for urban storm water discharges. - seeking to improve the design and the management of sewer networks. Subject" of inyesti~ation

Deposits in man-entry sewers. The problem here is to determine when, how, where and (to a small extent) why deposits form in these sewers (which are the most difficult and costly works to clean).

The analysis focused flfstly on an area where deposits build-up. The central question here is to discover when the deposits occur (sedimentation during rain and erosion during dry weather) and how. This leads. in tum, to the postulation of hypotheses concerning how deposits accumulate (where do the solids come from? what conditions govern fluctuations in the quantities deposited?) and why they occur (is there an equilibrium

Transfer of solids in combined sewer networks

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between the slope of the deposit surface in dry weather hydraulic conditions and the granulometry of the deposit's components?). Secondly. these hypotheses have been used as a basis for other investigations. which aim more particularly at: - verifying and identifying more closely why the processes can be different at the same moment in time at different points in the network; this has been carried out in a simplified manner (use of threshold values for simplified typical situations) on a rather large scale (these typical situations are distributed over a whole network); - analysing the conditions which govern the build-up and persistence of deposits in man-entry combined sewers; this analysis is based on an investigation carried out on several groups of man-entry sewers and on a theoretical study of variations through time of the shear stress exerted by the flow over the deposits.

Solids in urban wet weather discharges. The objective here is to reach a better understanding of the main characteristics of urban storm water discharge pollution. by providing information on: - particulate pollution and its importance in relation to dissolved pollution. - the grain size. the specific mass and the settling velocity of these solids. In the first place. the analysis covers the verification and the quantification of tendencies revealed through previous observation (solids are the main vectors of pollution. they are fine but decant quickly). a~ well as covering the variation in amplitude of the values measured. in storm and combined networks.

Secondly. research is oriented towards the assessment of the importance and characteristics of the contribution of deposits in combined sewer networks to the pollution of urban wet weather discharges. Chosen sites

Observation of clogging development in man-entry sewers. They were carried out over a period of 3 years in trunk no. 13 in Marseilles. Located in the centre of Marseilles. sewer trunk no. 13 drains a 134 hectare watershed. which is 85% impermeabilised. Its mean daily flow in dry weather is 0.220 m 3/s at the outlet. Along its path. it meets non• selective gullies and wastewater pipes. A third of this throughput is provided by a lateral flow which slows down the flow of trunk no. 13 upstream of the junction. In the gritty section (480 metres long) the mean invert slope is of 0.001 mlm, and 0.018 mlm immediately upstream. The sewer has an ovoid cross-section (2.75 m high and 1.80 m wide) with lateral shelf.

Location of deposits in man-entry combined sewers. This was carried out in 4 groups of sewer trunks:

In Marseilles, on watershed no. 13, the network of man-entry sewers which is located between the collecting pipes (diameter between 300 and 500 mm) and trunk no. 13 itself. The length of sewer clogged (deposit of more than 2 cm thick) is 1.3 km for a total man-entry section of 12.7 km (10% of the total length of sewer trunks). The de parte mental sewer network of Seine-Saint-Denis under Montreuil-Sous-Bois. It includes 29.7 km of man-entry sewers. The length of sewer where there is more than 10 cm of deposit is 5 km long (18% of the total length). The whole of the man-entry sewer system in Marseilles (height greater than or equal to 1.25 m), except for the main trunk sewer, over the total length where the deposits is of a thickness greater than 30 cm, i.e. over a total of 350 km of man-entry sewers, 8 km studied here.

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In Marseilles (main trunk sewer) and in Paris. about thirty kilometres of larger sewer trunks (greater than 2 m high and 1.5 m wide. with central canal over 0.80 m deep and over 1m wide). These sewer trunks are usually located downstream from the same type as previously mentioned sewers. Characterization of suspended solids in urban wet weather discharges. Table I specifies the characteristics of the drainage areas selected. On each site we have studied 3 to 4 rainfall events of different characteristics.

Table I. Characteristics of the sites studied Site Retention basin. BEQUIGNEAUX. BORDEAUX

TYQe of network Storm sewer

S {ha = 1()4 m2} Cimll{%} 50% 606ha

Retention basin, LES BROUILLARDS, SSD

Storm sewer

8310 ha

14%

Retention basin, LA MOLETTE, SSD

CSO

8000ha

30%

Trunk sewer nO 13, MARSEILLES

Combined sewer

134ha

87%

Notations: S = surface of the drainage area, qmp = impermeabilisation coefficient, SSD Denis, CSO = Combined Sewer Overflows.

=Seine-Saint•

Contribution of the deposits in combined sewer networks to the pollution of water during wet weather. These investigations were centred on the sewer network in Marseilles and particularly on drainage area no. 13 (drainage area of trunk no. 13).

DEPOSITS IN MAN-ENTRY COMBINED SEWERS Observation of cJo~~jn~ deyelQpment jn Marsejl!es' Sewer Trunk no. 13· Main results and Questions (Laplace. 1991: Laplace et at 1922) The observations carried out on the dynamics of deposits in trunk no 13 in Marseilles have shown the following. _The deposit~' grain size is of a millimetric order of magnitude. The median diameters are about 2 or 3 mm. - The volumes deposited increase slowly and regularly in dry weather. _ The most pronounced increases occur after a series of rather strong and long rainfall events which follow rather long periods of dry weather. These rainfall events disturb the longitudinal profile. They generate new deposit~. which are very localised. in areas of discontinuous hydraulic conditions. as well as causing more punctual erosions on bends or confluence areas. The mass balance obtained is positive. _ These relatively sharp increases are followed by a stabilisation or even erosion periods for a few days. then the increase of deposited volume is steady and slow. During these periods the longitudinal deposit profile gradually gains a more regular appearance. reconstituting a rather homogeneous slope. because of the filling up of troughs and. above all, because of the erosions of peaks. This dry weather erosion process appears as the main factor for a new equilibrium, expressed by the longitudinal slope of the deposit surface. - The temporary equilibrium slope of deposit's surface corresponds. under dry weather hydraulic conditions. to the median of the average surface deposit granulometry. according to the Shields criteria. To explain this behaviour. the following hypotheses have been advanced.

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- In dry weather. during street cleaning. solids enter sewer systems through gullies. Low sewage. or even parasitic flows. which are present at that moment in the network's upstream sewers pipes and trunks. have difficulty in transporting them. Deposits accumulate there and the solid flow received in downstream trunks is relatively reduced as regards transport capacities which generally become. like the flows. considerably increased. The solids which have arrived thus far are nonetheless likely to stop there. in various situations causing discontinuity of hydraulic conditions. where the transport capacities can sharply decrease and they contribute to a regular but moderate increase in the volume of deposits. - In wet weather. the intrusion of solids into the network is greater but also. and without a doubt. the flows become capable of mobilising the particles stored in the upstream sewers and of transporting them to the downstream trunks where they stop in an even more localised manner than in dry weather. This general description was corroborated by an overall modelling which confirms its coherence. Three complementary approaches were developed to reinforce or complete it. They shall be described in the following paragraphs. Study of deposition and erosion possibilities in a number of types of sewer trunks I mm), which represent the essential part of deposits in downstream man-entry sewers, the following points have been demonstrated: In dry weather, particles can be deposited and stocks accumulated: - in pipes of a slope of less than 10-2 mlm, - in sewer heads (upstream ends of the man-entry sewers). - in oversized sewers which drain watersheds with a relatively small surface. In wet weather. transport capacities are, in the first analysis and when the flow is strongest, considerably improved and likely to put into and maintain in movement almost everywhere, particles of a size smaller than 3 mm. even fQr relatively restrained rainfall event~ and even in man-entry combined sewers. Questions remain about what happens in wet weather: - how can solids stop in a trunk like trunk no. 13 when general cQnditions, for strong flows. seem to indicate large transport capacities? - how can stores of deposit in upstream man-entry sewers not be completely exhausted and only seem to be partially worn down, according to in-field observations. To answer these questions it was necessary to refme, in space and time, knowledge of how these deposits accumulate and persist.

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Location of deposits in four

~roups

of sewer trunks
The location and the main characteristics of deposits were identified in 4 groups of upstream and downstream man-entry combined sewers (see "chosen sites" section). The result is that: - the areas of deposit are clearly defmed (they are rarely longer than several tens of meters). - they correspond most frequently to a combination of unfavourable basic contexts which are: • upstream ends of the man-entry sewers have already identified weak spots. • conditions likely to have a local effect on the gradient of the energy line (downstream confluence. other downstream influenced areas. - local obstacles. pumping stations - •... ). • a sharp decrease in solid transport capacities along a hydraulic course (changes of incline. the arrival of a small or inclined trunk into a wider or flatter one•.. ). • defluences where the flow loses a part of its volume (and its energy available for solid transport) whilst preserving the bed-loading flow to a lesser degree. Analysis of.!o
This study demonstrates the following points. - The apparent low level of erosion during strong flows can be explained. only partially however. by the large-scale sedimentation occurring at the end of a flood. which would cover up. therefore. immediately preceding erosion. Paving and masking can also interfere. - At certain moments during a flood and especially at the end of a flood. rather long passages of 'tOo beneath the movement thresholds of millimetre particles. can really occur whilst at other moments 'to can reach very high values. - These depressions of 'to are very localised in space. generating sharp decreases along the flow and therefore a massive halt in particles which could be transported thus far. - Much finer analyses of hydraulic conditions which may reign in some locations (overtlows having downstream influence. defluence. confluence) are necessary to describe more deterministically the causes of clogging. A proposed description of deposit dynamics in combined sewers To sum up these investigations. it seems that various observations and analyses converge towards the following description of the dynamics of deposits in man-entry combined sewers. I) Deposit build-up in dry weather (Figure I): pipes. very localised and heterogeneous deposits will build up in less steep sections. where the flow passes over discontinuities (confluences) or obstacles.

- [0

- In upstream sewers. in Marseilles man-entry sewers. deposits build up especially upstream and downstream from confluences. in oversized sewers and in sewer heads at the opening of gullies or pipes.

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- In downstream sewers, deposits increase at a fairly moderate rate (nearly everything stopped before reaching this point), when in contact with breaks and decreases in slope, upstream and downstream from confluences and in other areas under downstream influence.

DRY WEATHER

PIPES

UPSTREAM MAN ENTRY SEWERS

DOWNSTREAM MAN ENTRY SEWERS

Figure 1. Summary description of dry weather deposit build-up in combined sewers.

2) Wet weather erosions and deposits (Figure 2): - in pipes, erosion occurs. - in upstream sewers: * downstream from defluences, deposition builds up, * upstream from confluences, erosion dominates. whereas downstream deposition occurs. * in sewer heads. the two processes occur. - In downstream sewers. deposit build-up dominates. with. however. probable superimposition of very contrasted sequences. the result of which is a succession of very localized deposits wherever there are changes and decreases in slope or confluences. or downstream from the outlets. 3) Reconstruction and transformation of deposits in dry weather following wet weather (Figure 3):

- In pipes and upstream sewers. deposits may build up over solids left by wet weather, in accordance with the laws of progradation logics deposition developing downstream. separation occurring according to grain size (the finest downstream) or density (likewise). Sometimes. these deposits develop upstream (e.g. from an obstacle). - in man-entry sewers. the long profile will be regularized by a localised erosion process. in which organic particles probably play an important role (Un. 1993). Mostly organic particles tend to be swept from peaks in order to fill troughs.

G. CHEBBO et al.

\02

DURING CERTAIN STORMS

r"\

deposit- current situation

/ .•.. \ deposit" prC:\1ous situation

!

I

~o ... ~

UPSTREAM MAN ENTRY SEWERS

PIPES

DOWNSTREAM MAN ENTRY SEWERS

Figure 2. Summary description of wet weather erosions and deposits in combined sewers.

DRY WEATHER AFTER STORMS

i

~~~

deposit: current SItuation / ..... \

deposit: previous situation

~"T

il~' \ ~~ ,'"

i

--L -¢<:)A£k~_ ~~

\

/'

"'-"--:r._...--....\} ....... I

PIPES

UPSTREAM MAN ENTRY SEWERS

,'I

......

".f·····:'>" ": I' .... ,

f

\

"'.".,~

DOWNSTREAM MAN ENTRY SEWERS

Figure 3. Summary description of deposit reconstruction and rransformation during dry weather following wet weather m combined sewers.

Transfer of solids in comblDed sewer networks

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SOLIDS IN URBAN WET WEATHER DISCHARGES CharacterizatiQn Qf suspended SQlids in urban wet weather discharges (ChebbQ 1922: ChebbQ and BachQc .

.lml The main results are the following. - Suspended solids are the main vectQr of pollutiQn in urban wet weather pollution. They fix 83 to 92% of the tQtaI pQllution Qf the COD, 77 tQ 95% Qf the BOD5, 48 to 82% Qf the Kjeldahl nitrogen, 82 to 99% Qf the hydrocarbQns and 80 to 99% of the lead. - These SQlids are fme. The median diameter is generally between 25 and 44 of the particles less than 100 ~m is Qn the Qrder Qf 66 to 85%.

~m

and the percentage in mass

- However they tend to settle quickly (4 m/h < V 50 < I I m/h), V 50 being the median settling velocity in mass. Values vary, depending Qn whether these are combined sewer discharges (lower velocities), especially from lighter rainfalls or, on the cQntrary. separate sewer discharges. This is particularly striking fQr the slowest particles: the V20 indicator which represents a threshold where 80% of the mass of particles fall faster. varies from 0.7 mlh (for the lighter rainfall) tQ 2 m/h (heavy rainfall) in a separate sewer network, and from 0.06 m/h (light rainfall) to 1.3 m/h (heavier rainfall) in combined sewers. The relatively low settling velocity Qf the portiQns which take longer tQ settle deserves special attentiQn, as it will influence the size Qf decanters which will be considered for treatment The identification of the origins and cQnditions of movement of solids which make up the least settleable fraction will indicate preventive solutions. It will help to adjust strategies for distributing treatment capacities in a sewer network and also to provide the principles for flow management. CQntributiQn Qf depQsits in cQmbined sewer netwQrks tQ wet weather PQllution (ChebbQ. 1992: ChebbQ and BachQc. 1993) StQrmwater in cQmbined sewer netwQrks. comparable with water in strictly stQrmwater trunks fQr a certain number Qf PQllutiQn parameters, seems, in rather general cQmparisQns. to be mQre heavily lQaded with organic matter. This difference is Qnly very partially explained by the dilutiQn of these waters by industrial or dQmestic sewage. The other notable source which can make a difference is the resuspension of particles deposited in combined sewer networks. The assessment of each of these sources started with the analysis of measurements taken at the outlet of drainage area no. 13 (the drainage area of trunk nQ. 13) in Marseilles - a combined network - which were carried out during three rainfall events. A very strong one (60 mm in 2 hours). a rather strong one (20 mm in I hour 30 minutes) and a small one (5 mm in 35 minutes). The context was defmed as accurately as possible using the available data. In spite of residual uncertainty. the solids in sewage seem only tQ playa significant role for small rainfall events. The deposits seems to provide. for an average or strong rainfall event. from 30 to 45% of the total suspended solids (slightly coarser than solids in separate sewer networks). The calculations carried out produce a very astonishing proportion (40 to 80%) of volatile (organic) matter in this contribution. Following these results, research was started so as to distinguish the possible locations of the main deposits of this contribution and to achieve progress in the knowledge of the formation and the transfer of solids in combined sewer networks.

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CONCLUSIONS Deposits in man-entry combined sewers The observations carried out on the dynamics of deposits in man-entry combined sewers when they are situated downstream of drainage areas of several hundred hectares have shown that: - essentially millimetre-sized particles are involved. - the most pronounced increases in volumes deposited occur during certain rainfall events. - in dry weather these increases are reduced. the flows reconstituting an equilibrium profile of the new bed (the surface of tit~ deposits) because of their erosion capacities which appear to correspond to the classic Shields threshold.

An examination of the distribution of deposits in combined sewer networks led to the following observations: - in pipes, very localised and slightly random deposits are formed at points of discontinuity (confluences) or around obstacles, - in downstream trunks, deposits increase moderately where there is a change in incline, upstream and downstream of confluences and in other areas under downstream influence. During rainfall periods, erosion (through increase in transport capacity) and downstream deposits occur. Solids in urban wet weather discharees The different measures taken lead to the following main results: - The pollution of urban wet weather discharges is essentially particular even for hydrocarbons, with the exception of Kjeldahl nitrogen. - Suspended solids are fine and decant rather quicldy, except in the case of light rainfall events in combined sewer networks, which are richer in organic matter than those in separate sewer networks. - The deposits seem to provide, for an average or strong rainfall event, 30 to 45% of the total suspended solids which incorporates 40 to 80% of organic matter. REFERENCES Artieres, O. (1987). Les dtpilts en r~aux d'assainissement unitaires. These de Doctorat de l'Universite Louis Pasteur de

Strasbourg, 214 p. Bacboc, A. (I 992a). Le transfer! des solides en reseaux d'assaimssement unitaires. These de Doctorat de I'lnstitut National Polytechnique de Toulouse, 281 p. + appendices. Bacboc, A. (I992b). Location and general cbaracteristics of sediment deposits into man-entry combined sewers. Wat. Sci. Tech., 25(8),47-55. Bacboc, A., Laplace, D. and Dartus, D. (1993). Build-up and erosion of sediment deposits in combined sewers networks. Proceedings Of the SixJh International Conference on Urban Storm Drainage, Niagara Falls, Ontario, Canada. September 12-17, 863-868. Chebbo, G. (1992). Solides des rejets urbains par temps de pluie: caracterisation et traitabilite. These de Doctorat de l'Ecole Nationale des Ponts et Chaussees, 410 p. + appendices. Chebbo, G. and Bachoc. A. (1992). Characterization of suspended solids in urban wet weather discharges. Wat. Sci. Tech., 25(8), 171-179. Chebbo. G. and Bachoe. A. (1993). Suspended wet weather solids in combined sewers: wbere does organiC matter come from? Proceedings Of the SixJh International Conference on Urban Storm Drainage, Niagara Falls, Ontario. Canada, September 12-17, 869-874.

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CIRIA (Construction Industry Research and Information Association) (\987). Sediment movement in combmed sewerage and storm water drainage systems. Project Report, Construction Industry Research and In/onnation Association. 1. 98 p. + appendices. DEA-SSD (1987). D6canlabilit6 des eaux de ruissellement - Les surverses des rtseaux unitalres: La Molette. Rapport d'Etudes de la Direction de I'Eau et de l'Assainissement du Departement de La Seme Saint Denis. 64 p. + appendices. Desbordes. M. (1985). BHan des 6tudes et recherches sur la pollution du ruissellement pluvial urbaID dans les pays d'Europe de l'Ouest et d'Amerique du Nord. Rapport d'E/udes, LHM (USTL). 89 p. Laplace. D. (1991). Dynamique du dtpOt en collecteur d'assainissement. These de Doctorat de l'Institur National Polytechnique de Toulouse. 202 p. + appendIces. Laplace. D.• Bachoc. A.• Sanchez. Y. and Dartus. D. (1992). Trunk sewer clogging development: description and solutions. Wat. Sci, Teck, 25(8).91-100.

Lin. H. (1993). Le transport solide en collecteurs uDltaires d'assainissement et sa modelisation. These de Doctorat de l'Ecole Nationale des Pants et Chaussees. 199 p. SlUlchez. Y. (\987). Curage des ouvrages visilables des reseaux d'assainissement. SyntMse des principaJes methodes appliquees en France. TSM-Eau. 2. 91-97. Sartor J. D.• Boyd. G. B, and Agardy. F. J, (1974). Water polluuon aspects of street surface contaminants. J. Water Poll. Control Fed., 49(3). 458-467.

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