Michel Verbanck

International workshop on origin, occurrence and behaviour of sediments in sewer systems: Summary of conclusions

Abstract An international workshop held at the Universite Libre de Bruxelles (4–6 Sept. 1991) on the “origin, occurrence and behaviour of sediments in sewer systems” has shown that much progress has been made in the last 5 years in the general understanding of sewer sediments. Comprehensive research programmes, including field surveys, have indicated that solids found in sewers, either in suspension or in deposition, cannot be considered as having a unique entity. On the basis of sewerage and sources, a categorization can be made between three broad types of solids: sewer grits, sanitary solids and (runoff-generated) fine material. Although some other categorizations are also possible, it is apparent that each of these three types has very specific physico-chemical characteristics and this has important implications both from the environmental and the operational points of view. The diversity of the types of sewer solids is not infinite (from place to place, sediments will differ only within relatively narrow bands), nonetheless the differences are the main reason why the development of a “universal” sewer sediment transport model is a remote prospect. For some elementary processes, such as the movement of non-cohesive grit in drainage conduits, detailed engineering solutions are beginning to emerge. It is also apparent that bed shear stress is a more appropriate parameter than flow velocity for defining erosive or depositional criteria in sewers. There is no doubt that in the future, the various problems of sewer sediments, washout of pollutants during storm overflows, reduction of hydraulic capacity, recurrent costs for cleaning sewerage lines and tanks, will have to be considered intrinsically in all urban drainage applications (design and analysis). It is also likely that wastewater treatment technology (and terminology) will intrude more and more in the apparent unchanging traditions of sewer network operation.

Solids in Sewers: characteristics, effects and control of sewer solids and associated pollutants

This Report presents information on the current state of knowledge of the origins, occurrence, nature and effects of sewer solids for use by engineers, scientists, administrators and water quality planners for the planning, design and operation of sewerage systems. The report addresses both sewer maintenance requirements and environmental protection issues. Increasing environmental standards, coupled with public expectations, have led to stringent water quality standards. In response to this, it has been necessary to develop new methodologies and computer based analytical techniques to model and understand the performance of all aspects of waste water systems. Fundamental to these techniques is the understanding of the way in which sewer solids contribute to the poor performance of wastewater systems and consequential environmental damage. The information presented in this Report about the origins, nature, movement, hydraulic and polluting effects of solids in sewers has enabled strategies and rules to be developed for the management of sewerage systems to minimise the deleterious effects of these solids and associated pollutants. This title belongs to Scientific and Technical Report Series ISBN: 9781900222914 (Print) ISBN: 9781780402727 (eBook)

Mechanics of sewer sediment erosion and transport

Knowledge about the behaviour and effects of solids in sewer systems has advanced considerably in the past decade, with new guidance for the control of sewer solids and associated pollutants emergi...

Seasonal Variations and Resilience of Bacterial Communities in a Sewage Polluted Urban River

The Zenne River in Brussels (Belgium) and effluents of the two wastewater treatment plants (WWTPs) of Brussels were chosen to assess the impact of disturbance on bacterial community composition (BCC) of an urban river. Organic matters, nutrients load and oxygen concentration fluctuated highly along the river and over time because of WWTPs discharge. Tag pyrosequencing of bacterial 16S rRNA genes revealed the significant effect of seasonality on the richness, the bacterial diversity (Shannon index) and BCC. The major grouping: -winter/fall samples versus spring/summer samples- could be associated with fluctuations of in situ bacterial activities (dissolved and particulate organic carbon biodegradation associated with oxygen consumption and N transformation). BCC of the samples collected upstream from the WWTPs discharge were significantly different from BCC of downstream samples and WWTPs effluents, while no significant difference was found between BCC of WWTPs effluents and the downstream samples as revealed by ANOSIM. Analysis per season showed that allochthonous bacteria brought by WWTPs effluents triggered the changes in community composition, eventually followed by rapid post-disturbance return to the original composition as observed in April (resilience), whereas community composition remained altered after the perturbation by WWTPs effluents in the other seasons.

Bone histology and 25-OH vitamin D plasma levels in alcoholics without cirrhosis.

A systematic study of electrolytes in patients with chronic alcoholism at the time of admission to the hospital may reveal low levels of serum calcium, magnesium [1, 6] or phosphate [10]. Bone changes are not obvious, but Saville [9] has found that a decrease in bone mass (measured on samples taken at the iliac crest) occurs in alcoholics. Roginsky et al. [8] by measurement of total body calcium found no consistent loss of skeletal mass in alcoholics without cirrhosis (only men were included in their group of patients). In the study of Dal6n et al. [2], the mean bone mineral content was only 3.3% lower in alcoholics than in controls (patients suffering from nutritional deprivation were excluded). It was more obviously reduced in observations made by Beck and Garcia-Pascual [1], who considered that this correlated with a reduction in the plasma concentration of 2S-OH-vitamin D.

Capturing and releasing settleable solids – the significance of dense undercurrents in combined sewer flows

Particulate organic matter concentrations determined at the outlet of a low-sloped combined sewer are observed to increase significantly in the very early stage of storm runoff events. It is found that this is not a result from scouring of actual deposited sediments in the sewer (which are granular), but from the dynamic behaviour of a distinct component of the sewage flow itself. During low flows in dry weather, a part of the coarsest material present in normal sanitary sewage (primarily organic debris such as human and food wastes, of about millimetric size and larger) settles and leaves the main body of the flow, but does not reach the sewer bottom to deposit there. The lowest part of the flowing sewage acts in this case as a temporary storage of highly polluting matter, as its longitudinal velocity is lower than the one of the remaining flow. An explanation for the building of these ‘dense undercurrents’ is formulated, based on a difference between free settling in the main body of flow and hindered settling near the bottom. Although this still merits further confirmation, it is likely that the process which maintains coarse solids in suspension in the undercurrent is a lift force induced by shear in the near-bed zone. The collapse of the undercurrent and the subsequent release of polluting material into the whole water column has been observed in the considered sewer to occur when bed shear stress exceeds about 1.1-1.2 N/m2. This low value can explain flushing effects occurring frequently during rainy period, or even at peak dry weather flows. On the basis of properties illustrated in this study, a number of management options can be considered for revised day-to-day operation of low-sloped interceptor sewers where dense undercurrents are likely to develop.

Whole body and regional retention of99mTc-labeled pyrophosphate at 24 hours: Physiological basis of the method for assessing the metabolism of bone in disease

SummaryThe retention of99mTc-labeled pyrophosphate (PPi) at 24 h was measured in 235 patients, 119 of whom had a normal bone metabolism. The mean retention in the group of normal subjects is 52% of the injected dose. Reproducibility of the measurement in a given person is 5.5% coefficient of variation (CV). The value depends strongly on sex (higher in males) and age (higher with increasing age, especially in cortical bone). Retention increases slowly with the decrease in glomerular filtration rate (GFR) between 50 and 120 ml/min; it rises very rapidly with values below 50 ml/min. The slowing down of the GFR with age does not account for the increase in PPi retention with age. When expressed as a percentage of the expected value for sex and age, retention is frequently low in osteoporosis (P<.001), more so when urinary hydroxyproline is low; it is normal or high in osteomalacia, and in some cases rises after vitamin D treatment is started; it is high in hyperparathyroidism (P<.01). The PPi retention is correlated with bone calcium accretion rate, alkaline phosphatase level, and above all, the urinary hydroxyproline level. The lower the bone mineralization (Ca/hydroxyproline ratio in biopsy), the higher the retention value. We conclude that the PPi retention is an index of bone metabolism when GFR is higher than 50 ml/min. It allows for classification of metabolic bone diseases according to the bone turnover rate. It has the advantage over the usual biologic examinations in that it affords better observation of highly localized bone disorders and can be used in combination with a morphologic record, the bone scintigraphy.

Computing near-bed solids transport in sewers and similar sediment-carrying open-channel flows

Abstract Near-bed solids (NBS) in sewers are known to convey a large part of the particulate polluting material and to introduce serious bias in sewer flow quality modelling when their existence is not properly acknowledged. It is proposed that a suspended-load approach should be used to characterize this material and the related flow pattern. A two-layer suspension model (proposed as an alternative to the classical Rouse equation) is applied to combined sewer flows displaying a NBS pattern, but also to capacity suspension flows carefully controlled under laboratory conditions. Both applications are conclusive and stress the importance of a good knowledge of particle settling velocity in ambient (turbulent) conditions. In combined sewers, the marked increase of effective shear velocity associated with rain runoff events is understood to lift the coarse settleable material which ordinarily remains in a concentrated NBS flow zone, into the upper part of the flow.

How fast can a river flow over alluvium

A new “vortex–drag” approach is proposed to compute the rate of dissipation of mean–motion energy in alluvial streams flowing over mobile bed forms. 1D–routing exercises strongly suggest that, for a given depth and energy–slope, there exists a certain flow velocity limit an alluvial stream is unlikely to exceed. This remarkable flow condition would actually correspond to a fundamental mode in terms of vortex Strouhal frequency (to which only higher harmonics can eventually be associated). Interestingly, the maximum possible velocity is not attained over a plane bed, but occurs when the wavy shape of the water–surface remains in phase with the sand wave. The model treats flow over sandy alluvium as a combination of boundary–attached and –detached flow features, both necessary to explain how well–marked protrusions can develop spontaneously in the bed profile and be maintained in response to extreme water–sediment discharge and stream power conditions. Through the vortex–drag concept and its mathematical development, a lot is learned about the nature of the mutual interactions existing between the various alluvial processes: bedform development, alluvial channel resistance, sediment transport capacity and turbulence–damping. Bedform types occurring in the upper alluvial regime are at the center of the analysis. Based on the model and on confrontation with available experimental evidence, it is suggested that the inphase wave [IPW] flow configuration is associated with minimum possible drag, creating low–turbulence high–velocity conditions ideal to evacuate extreme river discharges. Occurrence of IPW conditions is therefore important in terms of flooding alleviation, but will develop only if the right type of sediment is maintained available on the stream bed. The boundary–attached component of the model introduces a generalized Froude number formulation in alluvial hydraulics, to represent topographically–forced gravity waves occurring in non–shallow environments such as ripple fields. For the boundary–detached component of the model, we introduce a “control factor m” (m ≥ 1) which reflects a complex feedback–control loop process active in the separation cell immediately downstream of the bedform crest. Lift–up of bed particles (under the action of the corresponding vortices shedding out of the reattachment region) implies that control factor m should also play an important role in the further development of bursting–based conceptual models of non–cohesive sediment transport.

Assessment of sediment behaviour in a cunette-shaped sewer section

The accumulation of deposits in sewers causes widespread concerns of either operational or environmental nature. It is believed that a number of sediment-related nuisances can substantially be controlled in adapting the characteristics of sewer pipes as a function of local constraints and circumstances. In particular, key design parameters such as cross-section shape or hydraulic roughness of inner walls are currently selected basing more on empiricism and intuition than on full knowledge of the sediment transport driving processes. A valid track for optimization of these parameters is to run mathematical simulations of the sediment transport behaviour under varying design scenarios. This option, however, supposes that a robust mathematical procedure to compute sediment transport capacity in sewers is available, embracing all primary physical factors of influence. Starting from a theoretical description of shear turbulence suggested by Bagnold (1966), a suspension formula is developed dedicated to the specific sewer flow properties. Applying this formula to the case of a main sewer presenting a composite cross-section allows to illustrate how geometrical discontinuities influence sediment transport characteristics in real conduits.