The management of sediment in combined sewers

Urban Water 2 (2000) 263±275

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The management of sediment in combined sewers Richard M. Ashley a,*, Alasdair Fraser b, Richard Burrows c, John Blanksby a a

Department of Civil and Environmental Engineering, University of Bradford, Bradford, West Yorkshire BD7 1DP, UK b Urban Water Technology Centre, University of Abertay Dundee, Bell Street, Dundee DD1 1HG, UK c Department of Civil Engineering, University of Liverpool, Liverpool L69 3GQ, UK Received 8 May 2000; received in revised form 19 February 2001; accepted 19 February 2001

Abstract Sediments in sewers are ubiquitous because of the diverse nature of the inputs. Over the past decade or so, new understanding of the provenance, behaviour and nature of sewer solids is now allowing more e€ective means for solids management. Whilst current computer models are good at representing the hydraulic performance of sewer systems, their handling of sewer solids and associated processes is still embryonic. Hence any attempts to manage in-sewer solids more e€ectively require a diversity of approaches, both for any modelling studies and for the selection of the most appropriate option. Little information currently exists on which to draw to determine cost-e€ective or wholelife solutions. Nonetheless signi®cant advances have been made in enhancing the traditional approaches to sewer solids management which have been in use for more than a century. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Deposition; Management; Modelling; Sediment; Sewers

1. Introduction Solids accumulated in sewer systems constitute major problems in terms of reduction of sewer capacity and as a source of pollution during wet weather over¯ow events to watercourses (e.g. Rees & White, 1993). Since the early development of sewerage management between 1850 and 1900, only modest advances have been made in sewerage design and methods for managing solids (Ashley, Hvitved-Jacobsen, & Bertrand-Krajewski, 1999a). System performance understanding has advanced mostly in terms of the hydrology and hydraulics of the inputs, and the hydraulics of the ¯ows in the system. The problems caused by in-sewer solids are often unacknowledged, even today, and di€erentiated investment policies, which favour capital rather than operational expenditure, often discriminate against an honest look at these problems. This leads to many controlling authorities and utilities preferring not to be aware of in-sewer solids problems, although they are often confronted with process problems due to odours, gases and related phenomena. A recent UK sewerage

*

Corresponding author. Tel.: +44-1274-233-011; fax: +44-1274233-003. E-mail address: [email protected] (R.M. Ashley).

system management review (Osborne & Bottomley, 1998) listed sediment and other `blockage' issues such as fats and greases to be in the highest priority category in terms of operational performance and requiring further study as part of a move to proactive maintenance. In the review, it was reported that proactive sewer cleaning was now no longer the norm in the UK, despite the recommendations in the guidelines (WSA/FWR, 1991). In a parallel UK study, May, Martin, and Price (1998) identi®ed sewer cleaning as ``a very cost-e€ective way of dealing with sewer ¯ooding problems''. In the developed world, sewerage system operators are becoming more aware of the costs of maintaining their sewerage infrastructure and are beginning to try to incorporate sustainability concepts into the way in which maintenance and operation is undertaken. In England and Wales, the preferential investment policies of the Water Companies over the past decade towards capital rather than operational expenditure are now changing under the third round of Asset Management Planning (AMP3) and there is a greater awareness of sewer maintenance costs, as part of the need to maintain assets and improve levels of service, (e.g. Sharman, 1999; Wildebore, 1999). Improvement to some 4000 unsatisfactory Combined Sewer Over¯ows in AMP3 (Chubb, 1998) and the utilisation of technologies such as RealTime Control (RTC) increase the potential for greater

1462-0758/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 4 6 2 - 0 7 5 8 ( 0 1 ) 0 0 0 1 0 - 3

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solids retention in systems and problems of sediment accumulation in storage pipes, chambers, pumping station sumps and sewage treatment works' inlets. The effect of sediment on CSO controls causes failure to meet discharge standards through partial blockage, leading to reduced settings, premature operation and complete blockage, often with operation in dry weather. The process of clearing blockages at storage over¯ows may also cause overloading of treatment plant inlets. In addition to the e€ect on CSO controls, the presence of sediments may also a€ect the performance of CSO screens and impact the treatment process where large storage tanks are constructed. Increasing reliance on pumping in Europe, with consequent impeller abrasion by sediment, also makes sediment exclusion and/or removal upstream of pumping stations very important. As yet, little concerted e€ort has been directed in the UK toward dealing with in-sewer problems. However, an increasing interest in integrated system operation will necessitate greater interest being paid to sediment issues. For example, in England and Wales, the water industry regulator (OFWAT) requires continued improvements in the performance of sewerage assets through more accurately focused maintenance work using better information systems and better utilisation of existing data (OFWAT, 1999a). The operational expenditure (OPEX) savings estimated to be achievable in the determination of price limits in 1999 (OFWAT, 1999b) resulted in signi®cant sta€ reductions by the English and Welsh Water Companies within days of the announcement. The e€ect of this is to create even more imperative demands for e€ective decision support tools which will enable service providers to maintain and improve services whilst ensuring the welfare of employees. Internationally there are a number of initiatives underway relevant to sewer solids deposition and e€ects. There are also advocates of an integrated perspective on wastewater systems as a whole to ensure that these are operated taking due account of sustainability concepts, which would include the e€ects and fate of solids in, and removed from, sewer systems (e.g. Balkema, Weijers, & Lambert, 1998). In the USA, the recently established Task Committee on `Movement of Cohesive and Noncohesive Sediments in Drainage Systems' has concluded that in America there has been very little research undertaken related to sewer sediment problems for more than a decade (Delleur, 2000). Ironically, the earlier investment by the USEPA which resulted in the most universally applied sewer ¯ow hydraulic model: Storm Water Management Model (SWMM), has not been complemented by equivalent sewer ¯ow quality modules. It has now been identi®ed that the management of sediments is vital if future systems are to operate more sustainably (Heaney, Wright, & Sample, 1998), and the USEPA has recently revived its sewer ¯ushing research studies (Pisano, Barsanti, Joyce, & Sorensen, 1998).

In France, the problems of sewer sediment management have always been taken seriously, and there is a tradition ensuring that technologies there are in the vanguard of new ideas (e.g. Techniques Sciences Methodes, 1993). In Belgium, `Hydroplan' procedures have been developed for sewer asset management which closely mirror Sewerage Rehabilitation Methodologies, and which analyse the sewer network in terms of its historical performance (Cobbaert, Huberlant, Provost, & Swartenbroeckx, 1998). In France, INSA, linked with Lyonnaise des Eaux have developed a GIS database for the Lyons sewer network. A stochastic semi-empirical deposition model is being used to devise a maintenance strategy based on the links between a network's physical structure and the risk of sediment build-up (Gerard and Chocat, 1999). In Paris, the last four years of studying in detail a small catchment (Chebbo, Gromaire-Metz, & Deutsch, 1998) has resulted in the most precise mass budgets yet produced, accounting for sediment input, deposition and discharge, which are now being scaled up to the whole of Paris. As part of the French studies, the opportunities for recycling of the grit recovered are also being considered in terms of a cost-e€ective framework (Delattre, Bertrand-Krajewski, Picard, Berga, & Balades, 1998). Limited studies have so far been carried out in developing countries, notably in India and Malaysia (Kolsky & Butler, 2001; Ab Ghani, Zakaria, Kassim, & Nasir, 2001), where the solids sizes are typically much larger in the former, but similar to European solids in the latter. 2. Sedimentation in sewers 2.1. Occurrence The primary causes of sedimentation in sewers are known to be discontinuities, both hydraulic and structural (Chebbo, Bachoc, Laplace, & LeGuennec, 1995; Ashley & Verbanck, 1996). In order to achieve more e€ective management of these systems a better understanding is required of the processes which lead to a deterioration in the operational performance. In particular, the ability to predict sediment build-up due to the hydraulic conditions within the pipes is important, because this will enable operators to distinguish between blockages caused by hydraulic conditions and those associated with structural and service conditions. A major problem heretofore is that of predicting where sediments will deposit in systems, given the lack of credibility in this regard of current sewer ¯ow quality models (Ashley et al., 1999a). Whilst system characteristics such as pipe slope and structural condition give an initial indication of the predisposition of pipe sections to su€er sediment accumulation, the other principal factor is the local ¯ow regime to which the pipe is subjected.

R.M. Ashley et al. / Urban Water 2 (2000) 263±275

This is neither adequately characterised by normally available dry weather (foul) and selected `design storm' ¯ow information nor by sewer ¯ow simulation models for which there is no coupling between the hydraulics and sediment bed changes (as in Hydroworks). Furthermore, in many event databases the majority of operational incidents are recorded simply as `blockage'. This is misleading as many of these blockages often reoccur, indicating that current maintenance practices are tackling the symptoms of the problem and not the cause (Fenner & Sweeting, 1999). 2.2. Prediction Methods for analytically predicting sedimentation are still embryonic. Deposition in sewers generally occurs during periods of dry weather and during decelerating ¯ows when storm run-o€ is receding. It is also known that the propensity for sediment deposition will be different depending upon the location of a sewer in a network (i.e. the relative type of ¯ow input, and predominant solids type) and the physical characteristics of the conduit such as size, shape, gradient, etc. (Ashley, Wotherspoon, Goodison, McGregor, & Coghlan, 1992). Settlement and deposition will occur at a rate depending upon the ¯ow ®eld, the nature of the particles and the concentration in suspension and/or near the bed. The importance of rapidly varied ¯ow e€ects for transport and deposition in large sewers has been observed in many studies. In most sewers, the DWF patterns provide `fresh' supplies of solids in private drains and sewers where ¯ow is discontinuous, and these are often laid down in larger sewers during low night-time ¯ows. Storms provide a more random source of material and disturbance to deposits. The deposits therefore become layered and mixed due to these interacting processes and the ongoing biochemical reactions. Hence deposited beds in sewers are heterogeneous and can exhibit thixotropic characteristics. Current methods of predicting sediment deposition include: · Pipe surveys and records ± Locations historically known to exhibit depositional problems are periodically inspected, and if necessary cleaned. · Use of characteristic ¯ows ± Values of shear stress or other indicators are calculated and mapped for a particular system, and compared with a `self-cleansing' criterion. · Conceptual and risk modelling ± The various contributory factors of sedimentation (e.g. pipe slope, discontinuities, etc.) are evaluated and combined with risk assessment and logical systems to produce a ranking for propensity for sediment deposition at various locations. · The use of sewer ¯ow quality models, which include some or all of the above.

265

Various degrees of success have been achieved using these methods. However, the methods have been characterised by site speci®city or a comparative analysis only. The most common technique is the use of historical data, but this is clearly dependent upon the diligence of previous work forces (in record keeping) and inspecting sta€. Deterministic computational models have been used to predict sedimentation in sewers. These use full-solution hydraulic models linked with complex or more simpli®ed transport equations (e.g. Mark, Cerar, & Perrusquia, 1996). Most of the commercial packages neither couple the sediment bed temporal changes with the ¯ow ®eld and thus utilise a `static' bed for hydraulic computations nor do they account for the wide diversity of the phenomena associated with sewer solids and processes (Ashley et al., 1999a; Fraser & Ashley, 1999; Margetts, 2000). For example, none of the models includes the e€ects of the near-bed organic solids believed to be principally responsible for foul ¯ushes (Arthur & Ashley, 1998; Ahyerre, Chebbo, & Saad, 2001; Skipworth, Tait, & Saul, 2001). As yet, the prolonged computational times required to model annual and longer time series rainfall events make it dicult to use detailed models for the prediction of longer term (even equilibrium) sedimentation in existing sewer systems. As a consequence, a number of simpler alternative approaches have been developed to predict sedimentation and produce control strategies. Detailed localised sedimentation prediction has been achieved successfully in the studies of the Marseilles No. 13 trunk sewer, where the deposits are very coarse, and the bedload transport amenable to modelling using more traditional formulae developed from river studies (e.g. Laplace, Bachoc, Sanchez, & Dartus, 1992; Lin & LeGuennec, 1996). Recently an expert system developed by Gerard and Chocat (1998) has successfully been applied to the entire catchment of Lyon, France. In this approach, a risk of deposition (as a percentage) is assigned to pipes and structures throughout the system based upon key structural characteristics. Although reasonable results were obtained, the approach is neither dynamic enough to allow design options to be tried nor does it include the in¯uence of varying types of sediment inputs, nor determine (with con®dence) actual quantities of sediment deposits. 2.3. Simpli®ed approaches The value of attempting to utilise full solution deterministic models to predict in-sewer deposition is questionable (Ahyerre, Chebbo, Tassin, & Gaume, 1998; Ashley et al., 1999a; Margetts, 2000) and, given the current state of knowledge of the phenomena, is likely to result in misleading supposedly `accurate' results. Hence alternative types of model are likely to be more generally useful. Early approaches to the

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simpli®ed modelling of in-sewer deposition (amount, quality and by pipe length) from USEPA funded studies developed alternative empirical relationships for smaller-sized sewers in dry weather (Pisano, Connick, Queiroz, & Aronson, 1979). The simplest relationship applicable to sequential analysis of individual pipes is   1:2 s Z ˆ 40 for s > sc ; …1† sc Z ˆ 40 for s 6 sc ;

…2†

where Z is the percentage of suspended solids in dry weather ¯ow deposited if the bed shear stress s is less than sc (assumed originally to be 0.004 pounds per square foot). These were based on earlier classical studies relating bed shear stress to particle size to ensure movement, and the ®tting of a single term power function to the data obtained from the small pipe (<500 mm) USEPA ¯ushing studies. Subsequently these relationships were used more widely and applied to larger sewers, to develop a German model THALIA (Iossi®des, 1987). Concurrently (Sonnen & Field, 1977) used a simpli®ed version of riverine relationships to develop a prototype sediment movement model within the SWMM software environment, which in theory could predict sedimentation. Application of the Pisano et al. (1977) relationships to the sewerage systems in Dundee indicated that the approach merited further development (Ashley et al., 1992) notwithstanding the larger sewers investigated. The determination of sewers which were likely to be susceptible to sedimentation was the subject of an unpublished study by the UK Water Research Centre (Gent & Orman, 1991). It considered both dry and wet weather ¯ows and included sequential pipe analyses. The method requires a veri®ed sewer hydraulic simulation model, and evaluates whether a sewer is susceptible to deposition during dry weather, based on a peak of 2  dry weather ¯ow, and if so, whether any deposition will be removed during storms, as illustrated in Fig. 1. The storms used were selected from time series, such as generated by STORMPAC (WRc, 1994), and may be considered to be an `average' yearly maximum, and ± a `typical' storm likely to occur perhaps once per week. The latter must achieve a bed shear stress of at least 2.5 N/m2 to ensure erosion of deposits and the former 9 N/m2 . The approach uses the transport capacity of the ¯ow in a comparative way, from pipe length to pipe length, and consequently the precise choice as to which total sediment transport load equations to use is largely irrelevant. This method was also applied to a number of catchments including Dundee and compared with observation, indicating reasonable correspondence (Goodison & Ashley, 1992).

Fig. 1. Determination of sewer lengths which are likely to be susceptible to sedimentation.

2.4. A new approach As part of a UK Engineering and Physical Sciences funded research study to develop more e€ective in-sewer sediment management systems, a new simpli®ed model for sediment prediction has been developed (Fraser & Ashley, 1999). The use of invert sediment traps to localise the sediment deposits is also being investigated (Fig. 2). The approach used to predict sedimentation necessitates simulations over lengthy periods (at least 1 year) of time series rainfall with interspersed dry weather periods. Where sedimentation is identi®ed as a potential problem, management options are considered. It is possible to use either recorded storm data or synthetic time series rainfall to apply storm ¯ows from an entire year to the sediment transport and deposition models. Various options to act as a hydraulic driver for the sediment models were assessed. Although the ®rst stages of the sediment prediction procedure used the sewer ¯ow model Hydroworks to provide hydraulic inputs, the computational time taken to simulate up to a year of continuous ¯ows in this way was found to be excessive. As a result of this, a simpli®ed model was

R.M. Ashley et al. / Urban Water 2 (2000) 263±275

Fig. 2. Sedimentation prediction and bed trap application for control.

created drawing from Unit Hydrograph theory applied to sub-catchments of the pipe network to synthesise the rainfall run-o€ process, and downstream ¯ow routing by simple (pipe-full) velocity-based advection. This follows the earlier work of Burrows and Mehmood (1995) which was directed to the prediction of storm spills from branched sewerage networks. The modelling procedure developed (Naja®an & Burrows, 1999) derives the unit hydrograph for the given sub-catchments for rainfall increments, equal in duration to the chosen time-step of the desired simulation. It then determines the out¯ow from the sub-catchment during rainfall as the convolution of these unit hydrographs and the rainfall series used (Mehmood, 1995). Flow is accumulated in the downstream direction by application of time lagging and hydrograph superposition at branch junctions. The approach has been applied to the Dundee Central Area Sewer Model (DCASM), to test applicability and level of performance. This sewerage system has characteristics including: ¯ow bifurcations and loops, created by numerous control gates, and tidal backwater e€ects, which could not be rigorously accounted for in the original Unit Hydrograph approach.

267

Consequently, some minor alterations were made to the way in which ¯ows were routed through the system so as to create a dendritic (branched) network. This largely involved treating the bifurcations as `over¯ow structures' elements so that ¯ow division could be better represented. The Unit Hydrograph procedure was programmed using MATLAB programming platform in combination with the SIMULINK toolbox such that the network could be conveniently represented by distinct elements, as shown in Fig. 3. Each of the elements represents one of the four main out¯ow functions: · sub-catchment run-o€; · advection (lagging); · over¯ow/bifurcation; · system outfall. Each of the sub-catchments was initially calibrated by using unit hydrographs obtained from a rigorously tested, fully detailed Hydroworks model. The modelling outputs were then compared with the Hydroworks model for ®ve rainfall events of varying severities (annual event, six-monthly event, quarterly event, monthly event and weekly event). Figs. 4 give examples of the comparison of the Hydroworks and MATLAB models for ¯ows synthesised at the most signi®cant outfalls in the Dundee and Forfar (Scotland) sewerage systems. The peak magnitude, time span and total volume of the events are considered to be in acceptable agreement for the intended application to sediment modelling, where there is signi®cant uncertainty in understanding in the processes. Further calibration of the Unit Hydrograph approach would be possible, both through re®nement of allowance for initial losses in the rainfall-run-o€ transformation or by adjustment to speeds of advection through the pipe system (Mehmood, 1995). This would bring the model to a closer level of agreement but this was not pursued here, in view of the other hydraulic complexities of the Dundee system. The synthesised ¯ow regime from the default application was considered to be acceptable and the simpli®ed modelling approach is now being used within the overall strategy. The resulting ¯ow simulations may then be used as time ecient hydraulic drivers to determine depths of erosion and deposition during storm events for an average year at predetermined calculation points within the pipe network. The sedimentation part of the model utilises a twostage procedure. Initially the propensity for deposition is evaluated using a similar approach to that shown in Fig. 1, except that the full sequential time series is analysed. Amounts of deposition are then determined using a modi®cation of the Pisano model (Eqs. (1) and (2)):   1:2   s0 Wb Z ˆ 0:889   ; …3† sc Wmax

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Fig. 3. Simulink representation of Dundee sewer ¯ow model.

Wb and Wmax are the sediment bed width and maximum sediment bed width, respectively. The original relationship predicts a linear deposit build-up rate in contradiction to observation which indicates that deposits tend to steepen the bed and that the particle characteristics become `sorted' longitudinally with time (Coghlan, 1997; Laplace et al., 1992). An empirical relationship based upon more than a year's data has been used for the estimation representation of bed gradient development. A signi®cant limitation of current sediment prediction models is their inability to account for the temporal change in bed gradient as a consequence of the deposits. Without this, accurate bed depth prediction is impossible. Eqs. (4) were used to determine the bed gradient in the main interceptor sewer based on the average sediment depth throughout the pipe, y, from these earlier regression analyses. Sˆ

1320y 2 ‡ 23:533y ‡ 1:0013

for y 6 0:033 m; S ˆ 0:8924 Ln…y† ‡ 3:348

…4† for y > 0:033 m;

where S ˆ bedgradient=pipegradient. The USEPA method was modi®ed to make it more compatible with standard hydraulic model outputs and recent advances in sewer sediment transport theory. Various studies have concentrated on sediment phe-

nomena related to transport over deposited beds (e.g. Perrusquia & Nalluri, 1994; May, 1994; Ab Ghani, 1993; Ackers, 1991), and these have shown that the sediment bed increases boundary roughness signi®cantly. For the model, the maximum in¯uence of the sediment bed in terms of deposition is assumed to occur when the bed width equals the maximum width of the sewer. At this point, the factor Wb =Wmax equals 1 (i.e. maximum deposition). Clearly as the bed width approaches zero, the above calculation becomes invalid. For this reason it is suggested that a minimum bed width of 10 times the particle d50 should be used for clean pipe conditions (as originally recommended in the application of the Ackers and White equations, Ackers, 1991). In order to account for the level of storage provided by each individual pipe, the empirical factor of 40 in the original USEPA equations was also modi®ed. The original relationship was developed from ¯ushing studies in the United States, and calculated the deposition in each length of pipe. The factor was therefore modi®ed using the average length of pipe, and the ¯ushing eciency of the original tests. The resulting factor gives the total percentage of incoming sediment that will deposit, per metre run of pipe (Eq. (3)). The model for sediment deposit location prediction has been applied to the Dundee sewerage system. Some underestimates of the locations of potential sediment

R.M. Ashley et al. / Urban Water 2 (2000) 263±275

269

Fig. 5. Predicted depths of sediment deposits (Dundee interceptor sewer).

Fig. 4. Comparison of MATLAB simpli®ed model with HYDROWORKS for the inlet to the wastewater treatment plant in Forfar and Dundee main outfall sewer.

deposition were found due to the omission of the tidal e€ects in¯uencing the outfalls, and also the increase in pipe roughness as deposits build up (Fraser & Ashley, 1999). It is also probable that the variations in sediment characteristics throughout the sewerage network (not initially accounted for) have in¯uenced the results. The deposition build-up model was tested against a long-term data set taken from a previous study (Coghlan, 1997) from a 200 m length of Dundee's main interceptor sewer over a period of approximately 2.5 years. It can be seen in Fig. 5 that the modelled deposition is generally satisfactory. However, although these results do not include the rainfall e€ects, at the time of data collection signi®cant downstream control existed to restrict storm velocities to approximately dry weather values. This results in a depositional pattern similar to that of a foul sewer with only very limited erosion. If this deposition model is projected beyond the limits of the collected data, into the long term, deposition can be observed tending towards a state of equilibrium.

The inclusion of an erosion module (Wotherspoon, 1994), applied during both dry and wet weather ¯ows, allows a more complete analysis to be carried out. The erosion model used relates the yield strength of the (cohesive like) bed via the hydraulic applied shear stress to an eroded depth of sediment. The removal of downstream controls from the test section of sewer (through pipe cleaning and hydraulic improvement) allowed testing of the complete procedure for estimating sediment bed changes using the same sewer length that had been used previously. The improved system hydraulics and a series of intense storms have resulted in a gradual reduction of sediment levels. These have been monitored through a series of detailed surveys to provide veri®cation of the combination of the deposition and erosion modules. The results of these detailed surveys revealed a tendency for the (e€ective) sediment bed gradient to increase over time under both conditions of erosion and deposition, with the bed tending towards a steady-state condition in the absence of high intensity storm events. The resulting model utilises hydraulic inputs from the simpli®ed hydraulic model to provide detailed modelling (2-min time-steps) of sediment behaviour of prolonged time series. Fig. 9 shows a good correlation between the predicted and observed data sets over a period of six months. The use of this model allows operational estimates to be made of sediment budgets and the prediction of seasonal deposition patterns. The detailed analysis carried out has also indicated that the `critical' value of shear stress should not be purely determined from on the characteristics of the deposits, but also on the rate of incoming solids and depositional properties of the pipe in question. This extended analysis gives the critical shear stress required for a net erosion at a given point in the network. Sensitivity testing was carried out for each of the component sediment prediction models. For the loca-

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tion model, the most important parameters were those of the sewer sediment characteristics. These clearly have a profound e€ect on the areas of predicted deposition. Although it is dicult to represent an entire catchment with one value of d50 and speci®c gravity, it is suggested that material in the grit range (d50  3 mm) be used. This is in the range suggested by the original unpublished WRc study (Gent & Orman, 1991) and will include some of the coarsest sediments usually deposited in the upstream sections of sewerage systems. 3. Sediment management Ideally sediments and other solids should be kept out of wastewater systems. Failing this option, it is presumed that the solids transferred into the system are most economically managed by their conveyance to a downstream facility (Butler & Clark, 1995). However, this assumption has not been fully explored and it is probable that local circumstances may determine which option is the best for sediment management. Much of the solids and associated pollutants arise from human activity, with domestic, construction and highway sources typically comprising the largest amounts. Controls thus relate to: · Education and behaviour at the level of the individual as to the type and amounts of solids introduced to wastewater systems. · Solids control via street sweeping, and e.g. runo€ management from construction sites. · Design and operation of devices and structures at inlets to sewer systems. A recent review of the sources of sediment during wet weather (Heaney et al., 1998) estimated that directly connected impervious areas typically contribute the most pollutants in run-o€, whereas for combined sewers, the largest solids and pollutant loads are likely to originate from domestic sewage inputs. Some of these solids are large organics, which can become mixed with the main bed deposits, although large faecal particles tend to degrade as they move down the network. In the UK, in addition to the inputs of sediments, some 2.5 million tampons, 1.4 million sanitary towels and 700,000 panty

liners are ¯ushed into sewers via the water Closet (WC) every day (UK Absorbent Hygiene Product Manufacturers Association). It is not only in the UK that the WC is being used as a rubbish bin. A limited questionnaire survey was undertaken of the items disposed via the WC in 72 countries. Some 33% of respondents claimed that sanitary items, other than faeces and toilet paper, were regularly ¯ushed, and in some countries `disposable' nappies are also put into the WC (Ashley et al., 1999b). Of concern is the increase in the amount of plastic being used in these items and changes in public usage patterns, shifting to using more of the plastic-based products. Whilst pressure can be applied to manufacturers to develop fully biodegradable products using less plastic, the numbers and weights of the items being disposed of are unlikely to reduce in the foreseeable future. However, evidence from the study investigating public attitudes to ¯ushing these and other items such as condoms and `cue tips', suggests that given the right information, the public may be willing to change behaviour. For the foreseeable future, however, it is unlikely that there will be any signi®cant reduction in these items found in sewers, necessitating expensive screens and transport systems for their control and disposal. The replacement of small unsatisfactory CSOs during the next investment period in the UK with larger chambers will increase the quantity of sediment in the downstream system. This will a€ect the performance of the treatment plant inlet processes, where the current trend is to install small aperture screens. The presence of sediment in screenings will a€ect the performance of handling plant, especially where maceration is part of the process, and increase the mass of treated screenings. In the USA and certain other countries, kitchen sink grinders allow the introduction of more organic solids into the sewer system than is usual in countries like Denmark, where these are not used. An example of the e€ects of garbage grinders is given in Table 1, as summarised in USEPA (1992). This table illustrates the signi®cance of kitchen grinders, as these generate typically the highest solids loads per capita and the greatest concentration of solids in the ¯ow due to the relatively low water volume discharged concurrently (8 l/cap/day). Garbage grinders

Table 1 Comparative pollutant loads from residential sources discharging to sewer Parameter

BOD5 Suspended solids Nitrogen Phosphorus

Garbage grinders

Toilets

Basins, sinks, appliances

(g/cap/day)

(mg/l)

(g/cap/day)

(mg/l)

(g/cap/day)

(mg/l)

11±31 16±44 0.2±0.9 0.1

2380 3500 79 13

7±24 13±37 4.1±16.8 0.6±1.6

260 450 140 20

25±39 11±23 1.1±2.0 2.2±3.4

260 160 17 26

R.M. Ashley et al. / Urban Water 2 (2000) 263±275

are now being installed in the UK, and supposedly will take a range of wastes, including plastics. Where sediments are not excluded from sewer systems, the approaches to maintenance are twofold: · Extract, transport and dispose of solids from locations where they collect (either by design or by default). · Re-entrain any deposits and encourage them to move `downstream' to some main collection point where they can be collected and dealt with. Another option, usually impractical, for in-sewer sediments, is to alter the ambient hydraulic regimes to minimise deposition. However, this usually necessitates large-scale infrastructural investment which is generally presumed to be uneconomic. As yet, no de®nitive studies have looked at the relative merits of the options for managing sediments. Solids (and attached pollutants) which remain in systems, even when being transported, have the potential to become modi®ed as part of the ongoing sewer processes. Thus if the sewer is considered to be a `reactor', these solids will experience `treatment' as they pass through the system. One disadvantage of this is the potential for anaerobic conditions to occur, with the generation of hydrogen sulphide, and attendant problems. A comparison of sewer ¯ushing compared with sediment removal or treatment has recently been made (Pisano et al., 1998) and has concluded that ¯ushing is a viable economic solution for in-sewer sediment management, notwithstanding the potential need to continue in some situations, to provide some in-sewer treatment to avoid anaerobic conditions developing where temperatures are high. Tighter regulations and restricted options for ultimate disposal of arisings from extracted sediments are making handling increasingly more dicult. Ultimately it is probable that in developed countries arisings will have to receive some form of treatment prior to reuse or land®ll. Maintenance of larger man-entry sewers has not altered signi®cantly from the methods developed at the end of the 19th century, although the equipment in use is more sophisticated. A recent survey of UK sewerage operators indicted that jetting is the method most used to clean all sizes of sewer (Fraser, Ashley, Vollertsen, & Sutherland, 1998). Frequent maintenance of the larger systems requires signi®cant manual labour and is hence very expensive, consequently many operators undertake only reactive rather than pro-active maintenance, when blockages or ¯ooding necessitate. A recent study in Bordeaux concluded that some 30 kg per head or 440 kg/ha of sediments had to be collected from the larger sewers and disposed per year (Delattre et al., 1998). A number of devices have been developed to deal with sediments in sewers. These include erosive devices,

271

such as gates, balls and ¯ushing systems (Pisano et al., 1998). The use of traps located in combined sewer inverts is being investigated in both the UK and France and is advocated for the open drains in Malaysia (Department of Irrigation & Drainage, 2000) is being studied. These are still in use in the North of Scotland Water region, and have also been identi®ed in several other UK locations (Fraser et al., 1998). Traps are also used extensively in France (Bertrand-Krajewski, Madiec, & Moine, 1996). Fig. 6 shows a French trap from Laplace et al. (1998), which has been adapted by constructing hinged doors, allowing only a narrow slot for sediment to drop into the trap. In combined sewers, traps should operate in such a way as to collect as much of the granular bed-load as possible and minimise the collection of the near-bed solids (mostly organic), which should remain transported in the ¯ow. The slot should be designed to ensure that the jump length of the saltating granular solids is such that these solids fall into the trap, whilst the lighter organics are conveyed over the slot. This minimises the problems of disposal of the removed solids, provided the organic content is minimal. Ideally trapped solids should resemble those collected by detritors at wastewater treatment plant inlets. An example of the solids trapped in a traditional invert trap is given in Fig. 7, where it can be seen that a lot of organics are present. The trap is situated on a 900 mm sewer and is 900 mm  3000 mm  925 mm. The core

Fig. 6. Example of in-sewer sediment trap with easy access hinged doors, as in use in Marseilles.

272

R.M. Ashley et al. / Urban Water 2 (2000) 263±275

Fig. 7. Deposits in a combined sewer invert trap (Scotland).

was obtained by cryogenic extraction and illustrates the important strati®cation that occurs in the deposits in such traps. During dry weather the material intercepted is predominantly organic near-bed solids (although some suspended settlement also occurs), whereas during storms, the material is mostly granular. The change in characteristics from layer B to layer A was due to a sudden alteration in the ambient ¯ow conditions caused by pumping downstream, with ambient velocities increasing, respectively, from <0.1 m=s to 0.25±0.3 m/s. In normal circumstances these traps operate continuously intercepting near-bed solids, so that during dry weather they accumulate organic solids, and inorganic solids during wet weather periods. As yet there are no general rules for the design of such traps, and these are being developed as part of the ongoing UK and French studies (Fraser et al., 1998). Relationships to predict near-bed solids transport rates have been used to represent the masses moving in the lower part of the ¯ow for both dry and wet weather, respectively. Under dry weather conditions, an approach based upon ®eld data considering a mixture of solids types moving near the bed was used. However, during storm ¯ows, a large proportion of the organic content of the near-bed material is re-entrained, thus leaving predominantly larger inorganics at the near-bed region. As no actual ®eld measurements of the near-bed solids during wet weather have been achieved by any of the ongoing research programmes, other than speculatively (Ristenpart, Ashley, & Uhl, 1995; Ashley, Watson, & McGregor, 1995; Verbanck, 2001), a modi®ed laboratory-based rela-

tionship was used for this phase, which was derived from tests using granular material. No attempt was made to account for the `settlement' of suspended solids in the trap, sediment wash-out or consolidation. The model predictions for rate of trap ®lling have been compared with a limited set of measured data taken from a previous study (Fig. 8, adapted from Fraser et al., 1998). Unfortunately only a few measurements were available for the early stages whilst the trap was ®lling. Actual rainfall data were available for this period and individual events were ranked in a hierarchy of signi®cance to be used in conjunction with a sewer ¯ow model (see Fig. 9).

Fig. 8. Example of modelled and measured trap ®ll rates (Scotland).

R.M. Ashley et al. / Urban Water 2 (2000) 263±275

Fig. 9. Predicted average sediment depths in main Dundee interceptor sewer using simpli®ed combined erosion/deposition model.

4. Handling and disposal of arisings Disposal of arisings from sewer sediment removal is becoming more of a problem. In the UK this waste with a variety of constituents is dicult to classify, in terms of the legislation it might seem appropriate to consider it a `special waste'. However, it is not as yet so classi®ed, as this designation is presently reserved for waste that is ¯ammable, toxic or corrosive. Since 1992, any carrier of sediment arisings (other than the undertaker) must be registered with the local waste disposal authority. New initiatives by the UK Government may, in due course, rede®ne special wastes to include those which could be infectious to man and other organisms, and which include substances with ecotoxic e€ects on animals, plants, soil, water and via the food chain. The EU Directive for Land®ll (2000) also has signi®cance. This is based on the leachate concentration from the waste, which should be assessed. It is clear that in view of the high variability in sewer sediment composition, both temporally and spatially, general classi®cation of arisings will be impossible. This may necessitate the routine taking of samples and analysis prior to disposal of any material removed.

273

Alternatives to land®ll disposal include incineration (with subsequent land®ll), co-disposal with normal municipal wastes and/or with treatment plant screenings and grit, or cleaning the solids using hydro-cyclones (prevalent in the Netherlands for dredged wastes). A novel study in France has considered the recovery of sewer and storm inlet sediments for subsequent reuse (Delattre et al., 1998). The term `Sewerage Solids ByProducts' (SSBPs) has been used to promote the view that these solids are potential resources. The study considered speci®cally the nature and associated quality of the SSBPs at a range of locations from cities in Southern France, and thence the appropriate degree of treatment for cleaning to an adequate condition for reuse. Table 2 shows some of the key characteristics of the sediments and attached pollutants. It is concluded that these solids have low organic and nutrient contents, but high levels of metals and hydrocarbons, with wide di€erences between sites. Overall some 9000±13000 ton/yr of solids are expected to be removed from sewers and associated infrastructure, including traps. More than 90% of the particles were found to be > 0:2 mm in size. Solids treatment using 4-stage sieving/washing/classi®cation/dewatering was found to provide e€ectively clean sand suitable for road construction. Alternatively less rigorous treatment makes the sediment more amenable for land®ll disposal. The wash water is highly contaminated and requires careful treatment. The overall costs are claimed to be more e€ective than current disposal options. Other studies have considered the reuse of sand from highway deicing as part of an enhanced management strategy to minimise non-point pollution and improve trac safety (Guo, 1999) and it is clear that conclusions as to the bene®t of doing this are very location-speci®c. Any attempt to reuse materials which are potentially contaminated with metals and other toxic substances, requires careful risk and cost assessments, within a multi-criteria decision framework, as described above. The inherent uncertainties involved in these processes make it imperative that these are accounted for if the best option is to be selected (e.g. Stansbury, Bogardi, & Stakhiv, 1999).

Table 2 Characteristics of sediments from a range of sites from Southern France (adapted from Delattre et al., 1998) Location

Mass of solids available/year

Organic content (%)

TOC (g/100 g dry solids)

Zinc (mg/kg dry solids)

Catchbasins Detention tanks (surface) Streets Combined sewers Sanitary sewers Storm sewers

60 kg/basin Small <27 000 ton ± 0.7 ton/km ±

2±35 4±22 5±7 1±42 3±4 <1

2±23 3±13 2±3 1±15 1±3 <1

10±1480 100±970 410±600 400±600 10±380 <30

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R.M. Ashley et al. / Urban Water 2 (2000) 263±275

5. Conclusions Sedimentation in combined sewers has been largely ignored for most of the last century. Sewerage owners and operators consider that as the customer cannot see what is happening underground, the customers are not particularly concerned about in-sewer processes and deposition. Di€erentiated incentives to invest in capital rather than operations also militates against interest in better sewer operational management (Ashley & Hopkinson, 2000). With new approaches to integrated operation of wastewater systems, moves to greater eciency and sustainability, and future changes in water use in the home as part of these new approaches, a more ecient and e€ective approach to proactively manage sediments is required. Existing semi-empirical methods for sewer sediment deposition prediction have been shown to be useful in speci®c localised studies. However, as yet there are no universally applicable ways of predicting where and by how much sediments will build up in sewers. Promising results for sediment prediction have been obtained using simple models developed from USEPA research in the 1970s and relatively limited data. Further veri®cation work of the use of these models is currently being undertaken, collecting data in new catchments. Recent development of the approach has included the use of a rainfall sediment erosion/deposition model, the routing of sediment particle size distributions through sewer networks, and the sizing and shaping of sediment collection traps based on CFD and laboratory studies (e.g. Stovin & Saul, 2000). Ultimately, a methodology is required which will allow engineers to assess the sediment deposition within systems under variable operational regimes, and to select the most appropriate control strategy. This should couple cost-bene®t models with predictive models to allow design and operational decisions to be made more objectively and cost-e€ectively than is possible at present. It is clear that there is still a long way to go in understanding the behaviour of solids in sewers, and that options for management need to be developed which are robust and deal with whole-life and sustainable perspectives. For the time being the management of these solids still requires the application of a variety of techniques which need to be applied at di€erent points and scales within a sewer catchment (Chebbo, Laplace, Bachoc, Sanchez, & LeGuennec, 1996). Acknowledgements The research described is part of a series of continuing studies funded by the UK Engineering and Physical Sciences Research Council, and includes access to sewers in

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