Sewer Sediment Research Articles

Modelling sewer sediment deposition, erosion, and transport processes to predict acute influent and reduce combined sewer overflows and CO2 emissions

Understanding of solids deposition, erosion, and transport processes in sewer systems has improved considerably in the past decade. This has provided guidance for controlling sewer solids and associated acute pollutants to protect the environment and improve the operation of wastewater systems. Although measures to decrease combined sewer overflow (CSO) events have reduced the amount of discharged pollution, overflows continue to occur during rainy weather in combined sewer systems. The solution lies in the amount of water allotted to various processes in an effluent treatment system, in impact evaluation of water quality and prediction technology, and in stressing the importance of developing a control technology. Extremely contaminated inflow has been a serious research subject, especially in connection with the influence of rainy weather on nitrogen and organic matter removal efficiency in wastewater treatment plants (WWTP). An intensive investigation of an extremely polluted inflow load to WWTP during rainy weather was conducted in the city of Matsuyama, the region used for the present research on total suspended solid (TSS) concentration. Since the inflow during rainy weather can be as much as 400 times that in dry weather, almost all sewers are unsettled and overflowing when a rain event is more than moderate. Another concern is the energy consumed by wastewater treatment; this problem has become important from the viewpoint of reducing CO(2) emissions and overall costs. Therefore, while establishing a prediction technology for the inflow water quality characteristics of a sewage disposal plant is an important priority, the development of a management/control method for an effluent treatment system that minimises energy consumption and CO(2) emissions due to water disposal is also a pressing research topic with regards to the quality of treated water. The procedure to improve water quality must make use of not only water quality and biotic criteria, but also modelling systems to enable the user to link the effect of changes in urban sewage systems with specific quality, energy consumption, CO(2) emission, and ecological improvements of the receiving water.

Impact of an intense combined sewer overflow event on the microbiological water quality of the Seine River

For a better understanding of the short and mid-term impacts of a combined sewer overflow (CSO) on the microbiological quality of the receiving river, we studied the composition of a CSO discharge and monitored during several hours the changes in the concentration of fecal indicator bacteria (FIB) in the impacted river water mass. The CSO occurred at the Clichy outfall (Paris agglomeration, France) in summer 2008 as a result of the most intense rainfall of the year. In 6h, 578, 705 m3 of sewage and 124 t of suspended matter (SM) were discharged into the Seine River. The CSO contained 1.5 × 106E. coli and 4.0 × 105 intestinal enterococci per 100 mL on average, and 77% of the E. coli were attached to SM. It was estimated that 89% of the CSO discharge was contributed by surface water runoff, and that resuspension of sewer sediment contributed to ∼75% of the SM, 10–70% of the E. coli and 40–80% of the intestinal enterococci. Directly downstream from the CSO outfall, FIB concentrations in the impacted water mass of the Seine River (2.9 × 105E. coli and 7.6 × 104 intestinal enterococci per 100 mL) exceeded by two orders of magnitude the usual dry weather concentrations. After 13–14 h of transit, these concentrations had decreased by 66% for E. coli and 79% for intestinal enterococci. This decline was well accounted for by our estimations of dilution, decay resulting from mortality or loss of culturability and sedimentation of the attached fraction of FIB.

Heavy Metals Speciation of Size-fractionated Sediments Collected from Combined Sewer System Using Sequential Extraction Method

Sediments deposited in sewer pipes have been concerned as one of the main pollution loads from combined sewer overflow (CSO). Since the toxicity, mobility and bioavailability of heavy metals depend on their speciation, it is necessary to consider not only their amounts but also their speciation to evaluate the adverse effects on aquatic organisms in receiving waters. Sequential extraction method was applied to the size-fractionated sediments to evaluate heavy metal speciation. Although the speciation of Cr, Ni, Cu, Zn and Pb differed among sampling points as well as their contents, the content of easily exchangeable fraction was strongly correlated with total content for all the metals except for Cu. The result of heavy metals speciation revealed that Ni, Zn and Pb in combined sewer sediments could have high mobility potential. The result of size-fractionated sediments indicated that the contents and mobility potential of the heavy metals were larger as particle size was finer. Thus, it is important to control fine sediments in combined sewer to reduce the impact of CSO pollution.

Optimized operation plan for sewer sediment control

Severe operational problems of sediment deposition have frequently occurred in stormwater sewer systems in Shanghai city due to the flat topography of the area and serious illicit connections. To control sewer sediment and its subsequential problems, optimized operation plans were proposed and an innovative performance assessment method was developed. Simulation results demonstrated that, through changing the way of pump operation and installing necessary actuators in the system, the optimized operations, especially batch intermittent intercept plan, effectively improved the flow velocity in the entire system in dry-weather condition. In conclusion, the optimized operation is an innovative idea for improving the performance and solving the problem of sediment deposition in the sewer system in Shanghai, China.

Laboratory Experiments and Numerical Modelling of the Scouring Effects of Flushing Waves on Sediment Beds

This paper presents an investigation on the scouring effects of flushing waves on sewer sediment deposits in order to derive guidelines on the design and set-up of flushing devices in sewer systems and in stormwater storage tanks. The investigation comprises both laboratory experiments and numerical modelling. The experimental campaign was carried out on a channel at the laboratory of the Department of Hydraulic and Environmental Engineering of the University of Pavia adopting a simple flushing device. The first set of experiments is carried out without sediment and the water levels during the flushing processes are monitored by a digital video-camera. The second set of tests is carried out with uniform layers of sediment at the bottom of the channel and the effects on sediment beds produced by flushes are analysed. The numerical model solves the De Saint Venant equations describing unsteady flow in open channel and the continuity equation of Exner for the conservation of the sediment mass. For this purpose, the MacCormack explicit finite difference scheme is introduced. A sediment erosion relationship linking the rate of erosion to an excess of shear stress at the bottom is included in the model. The comparison between the experimental and computed water depths shows good agreement for different flushing conditions and longitudinal slopes of the channel. The tests with different sediment beds allow the calibration of the parameters that globally describe the erosion process produced by flushes.

Estimation of Uncertainty in Long-Term Sewer Sediment Predictions Using a Response Database

Regulations require U.K. water companies to reduce the number of properties at risk of sewer flooding. One of the potential causes of sewer flooding is the presence of persistent sediment deposits in sewers, such deposits are a common problem in many combined sewers. Although the regulations are risk based, there is a gap in the current knowledge on how the risk assessment is affected by the uncertainty in sewer sediment transport prediction. This paper describes the development of a methodology for estimating uncertainty in sewer sediment deposit depth predictions using existing empirically calibrated sediment load equations and Monte Carlo simulations combined with a response database. This methodology has been used to estimate the range of uncertainty of in-pipe deposit build-up predictions for a U.K. combined sewer system that suffered persistent deposition problems.

An urban drainage stormwater quality model: Model development and uncertainty quantification

Summary The evaluation of urban stormwater quality is of relevant importance for urban drainage, and mathematical models may be of great interest in this respect. To date, several detailed mathematical models are available to predict stormwater quantity–quality characteristics in urban drainage systems. However, only a few models take sewer sediments into account, considering their cohesive-like properties that influence the build-up process of the pollutant load. Furthermore, the model data requirements, especially for the quality aspects, are extensive, which limit their applicability and affect model results with large uncertainty. Uncertainty analysis provides a measure or index regarding the significance and the accuracy of the results obtained by mathematical modelling and is therefore of high interest. Nevertheless, only few studies have been carried out in the urban drainage field, and very few deal with water quality issues. One of the main reasons for this lack of research is the computational burden required by detailed models that preserve this analysis and generally require several Monte Carlo simulation runs. A possible to this problem may be the adoption of simplified parsimonious models that generally require shorter computational times. In this context, this paper presents a parsimonious conceptual model for the evaluation of the pollutant load in-sewers. The model contains two modules: a hydrological and hydraulic module that calculates the hydrographs at the inlet and at the outlet of the sewer system, and a solid transfer module that calculates the pollutographs. The cohesive properties of sewer sediments were carefully considered. Further, the effectiveness of the innovative sewer sediment modelling approach has been verified by taking into account the uncertainty assessed according to the GLUE methodology. The model has been tested using experimental quantity–quality data gathered in two Italian catchments, Fossolo (Bologna) and Parco d’Orleans (Palermo).

Sources, nature, and fate of heavy metal-bearing particles in the sewer system

A preliminary insight into metal cycling within the urban sewer was obtained by determining both the heavy metal concentrations (Cu, Zn, Pb, Cd, Ni, Cr) in sewage and sediments, and the nature of metal-bearing particles using TEM–EDX, SEM–EDX and XRD. Particles collected from tap water, sump-pit deposits, and washbasin siphons, were also examined to trace back the origin of some mineral species. The results show that the total levels in Cu, Pb, Zn, Ni, and Cr in sewage are similar to that reported in the literature, thus suggesting that a time-averaged heavy metal fingerprint of domestic sewage can be defined for most developed cities at the urban catchment scale. Household activities represent the main source of Zn and Pb, the water supply system is a significant source of Cu, and in our case, groundwater infiltration in the sewer system provides a supplementary source of Ni and Cd. Concentrations in heavy metals were much higher in sewer sediments than in sewage suspended solids, the enrichment being due to the preferential settling of metal-bearing particles of high density and/or the precipitation of neoformed mineral phases. TEM and SEM–EDX analyses indicated that suspended solids, biofilms, and sewer sediments contained similar heavy metal-bearing particles including alloys and metal fragments, oxidized metals and sulfides. Copper fragments, metal carbonates (Cu, Zn, Pb), and oxidized soldering materials are released from the erosion of domestic plumbing, whereas the precipitation of sulfides and the sulfurization of metal phases occur primarily within the household connections to the sewer trunk. Close examination of sulfide phases also revealed in most cases a complex growth history recorded in the texture of particles, which likely reflects changes in physicochemical conditions associated with successive resuspension and settling of particles within the sewer system.

Numerical simulation of flushing effect on sewer sediments and comparison of four sediment transport formulas

Results of numerical modeling of sediment flushing in a combined sewer reach are herein presented. Simulations were performed by using a numerical model based on the solution of the De SaintVenant–Exner equations by the TVD MacCormack scheme. The model, previously validated by means of data derived from laboratory experiments, was applied to the Lacassagne trunk sewer in Lyon, France. A Hydrass flushing gate was put into operation at this site in 2003 to remove sediments accumulated during 3 years. A 5-month long experimental campaign was carried out to measure the sediment profile evolution during flushing operations. Four sediment transport formulas were used for the numerical simulations and compared. The model proved to be able to reproduce correctly the global evolution of the sediment profiles as a function of the number of flushing operations.

Biodegradability of organic matter associated with sewer sediments during first flush

The high pollution load in wastewater at the beginning of a rain event is commonly known to originate from the erosion of sewer sediments due to the increased flow rate under storm weather conditions. It is essential to characterize the biodegradability of organic matter during a storm event in order to quantify the effect it can have further downstream to the receiving water via discharges from Combined Sewer Overflow (CSO). The approach is to characterize the pollutograph during first flush. The pollutograph shows the variation in COD and TSS during a first flush event. These parameters measure the quantity of organic matter present. However these parameters do not indicate detailed information on the biodegradability of the organic matter. Such detailed knowledge can be obtained by dividing the total COD into fractions with different microbial properties. To do so oxygen uptake rate (OUR) measurements on batches of wastewater have shown itself to be a versatile technique. Together with a conceptual understanding of the microbial transformation taking place, OUR measurements lead to the desired fractionation of the COD. OUR results indicated that the highest biodegradability is associated with the initial part of a storm event. The information on physical and biological processes in the sewer can be used to better manage sediment in sewers which can otherwise result in depletion of dissolved oxygen in receiving waters via discharges from CSOs.

Model-based assessment of sediment sources in sewers

Transport behaviour of a particle under certain flow conditions is mainly defined by its geometric and physical properties. Besides, the pollution potential of sewer sediments depends strongly on size and origin. This paper presents an approach for keeping track of freely defined particle classes. A total load transport model was modified to facilitate fraction wise transport and distinguish between sedimentation and erosion as two different processes. Compared to approaches using only one characteristic diameter modelled transport rates are in the same order of magnitude but with additional information of size distribution and origin. A virtual case study shows the general applicability. Main constraint is the increase of model parameters and data needs.

Sterols: a tracer of organic matter in combined sewers

The important organic pollution of combined wet weather flows (WWF), its acute impact on receiving waters have been widely demonstrated. The main three possibly origins for this organic pollution are: runoff water (streets and roofs), wastewater and erosion of sewer sediments in combined sewer system. This work, for tracing the origin of organic particles bound in combined sewer system, has been focused on the innovative use of sterols. So, eight sterols have been selected and analysed for each kind of sample. Results are represented in contents of sterols (microg g(-1)) and in sterol profiles (%). The comparison of contents and profiles leads the separation between two groups: runoff water, characterized by the total absence of coprostanol, epicoprostanol and coprostanone, and the group of sewer deposits (gross bed sediment (GBS), organic layer (OL), biofilms) and wastewater. Moreover, sewer deposits and wastewater can be distinguished by their sterol contents and profiles. To evaluate their contribution to WWF a comparison between sterol signatures is done which shows that these effluents have a strong similarity in profiles and in contents of sterols to the organic layer.

Estimation of uncertainty in long term combined sewer sediment behaviour predictions, a UK case study

There are regulatory driven requirements for UK water companies to reduce the number of properties at risk of sewer flooding. One of the potential causes of sewer flooding is the presence of persistent sediment deposits in sewers. This is a common problem in many combined sewers. Although the regulation is risk based, there is a gap in current knowledge on how risk assessment is affected by the uncertainty in sewer solids behaviour prediction. This paper describes a UK case study exploring the possibility of estimating uncertainty in sewer sediment deposit level predictions, using Monte Carlo simulations combined with a response database.

Uncertainty assessment of sewer sediment erosion modelling

Urban stormwater quality modelling has become a fundamental issue in the evaluation of the receiving water bodies' quality state. Laboratory study as well as field campaigns have widely demonstrated that combined sewer sediments present cohesive-like properties which increase their resistance to erosion. However, only few models take sewer sediments into account considering their rheological properties. In the present paper different sewer sediment erosion models have been tested and their uncertainties have been assessed. The main goal was to discriminate the algorithms with respect to their robustness and the reduction of uncertainty. In order to accomplish such objective the GLUE methodology has been used. The model has been tested using the quantity-quality data gathered for the Fossolo catchment (Bologna, Italy). Results show a general tendency of sewer sediment erosion models to become overparameterised when trying to fit the complexity of the physical processes thus increasing modelling uncertainty.

Laboratory investigation on the effects of flushes on cohesive sediment beds

Hydraulic flushing gates have recently been successfully adopted for the removal of sewer sediments. A large number of researches are aimed at the evaluation of the performances of these devices in the case of granular sediment deposits, but little information exists about the effectiveness of flushes on cohesive sediments as yet in literature. The current paper reports the results of an experimental investigation on the erosive performances of flushing waves on cohesive sediment beds. Experiments were performed in a laboratory flume adopting two flushing hydraulic conditions and comparing the erosive effects on cohesive and granular sediment beds. Different behaviours of cohesive sediments were observed during flushing operations: in particular, erosive effects in cohesive sediment beds were observed to be smaller than in granular sediments during initial flushes whereas erosion in cohesive sediments proved to be higher during subsequent flushes.

Long term monitoring of sewer sediment accumulation and flushing experiments in a man-entry sewer

This paper presents continuous field experiments carried out during 4 years in a man-entry egg-shaped combined sewer in Lyon, France in order to contribute to the knowledge and the modelling of sediment accumulation and sediment removal by means of a Hydrass flushing gate. The 250 microm sediments are mainly mineral, and their physical and chemical characteristics appear as rather stable in time and space. Long-term sediment monitoring reveals: (i) a regular asymptotic increase of both the sediment mass and the slope of its longitudinal profile, (ii) a clear correlation between local sediment profile irregularities and sewer ancillaries but without significant influence on the global and long term accumulation. Simple sediment accumulation modelling shows: (i) a good suitability of a three parameters conceptual model to reproduce asymptotic sediment volume accumulation, and (ii) a good suitability of the Velikanov model to reproduce sediment profiles. Both models reproduce observations with an acceptable margin of uncertainty for operational management purposes but are very sensitive to input data and parameter values. The Hydrass flushing gate is efficient and it appeared that the mass of sediments moves downstream linearly with the number of flushes.

합류식 관거 내 고형물 퇴적량 산정기법 분석

건조기 때의 합류식 관거 내 고형물의 퇴적으로 인해 통수능이 감소하여 여름철 장마시 국지적인 침수가 발생하며 이로 인해 관거 내 퇴적을 더욱 초래할 수가 있다. 또한 관거 시스템에 퇴적된 고형물로 인해 우기시 'first-flush' 현상을 초래하기도 한다. 이와 같은 문제를 해결하기 위해서는 장기간에 걸친 관거 내 고형물 퇴적량을 관측할 필요가 있으나 많은 비용과 노력이 수반되어야 한다. 따라서 본 연구에서는 미국환경보호청에서 개발한 건조기의 합류관거 내 고형물 퇴적량 산정기법을 우리나라의 배수유역에 적용하여 분석하였다. 분석결과 적용 배수유역의 가용 자료에 따라 모형을 선택하여 실무에서 간편하게 적용할 수가 있지만 모형별 변수산정 방법별로 많은 차이를 보이고 있다. 초기에 유도된 Eqs. (1)<TEX>$\sim$</TEX>(4)가 후에 유도된 Eqs. (5)<TEX>$\sim$</TEX>(9)보다 산정치가 모두 크게 나타나고 있으며 전반적으로 중간모형이 정밀모형이나 단순모형보다 크게 나타나고 있다. 건조기 동안의 관거 내 퇴적고형물의 관측자료가 구비되어 있지 않아 실질적으로 비교하기가 곤란하지만 대상 배수유역 관거에서의 장기적인 관측이 이루어지고 이에 따라 국내실정에 맞도록 산정공식의 수정이 이루어지면 실무에서 많은 비용과 노력을 줄이고 관거 관리를 할 수 있다고 판단된다. The deposition of sewer solids during dry weather in combined sewer systems results in a loss of flow capacity that may restrict flow and cause a local flooding and enhanced solids deposition. Sewer solid accumulations in drainage systems also create the 'first-flush' phenomena during wet weather runoff periods. In order to solve these problems, measurement of these loadings for a given sewer system for extended period is needed but this task is very difficult and extremely expensive. In this study, generalized procedures for estimating sewer sediment solid during dry weather in combined sewer systems developed by the U. S. Environmental Protection Agency were applied in a drainage system in Korea. As result, the appropriate equation can be selected and applied according to the available data. However, the estimated solid sediment shows considerable difference between methods which classified by model and estimation methods of variable. The estimated values using equations (1) <TEX>$\sim$</TEX> (4) are greater than that of equations (5) <TEX>$\sim$</TEX> (9) and intermediate models show greater values than elaborate or simplest models. The comparison between simulated and measured solid deposition is difficult due to the absent of measurement data, but this estimation method can be used usefully for the management of sewer solid with reduction of cost and effort if the measurement is carried out and the equation is adjusted according to the actual drainage systems in Korea.

Discussion of “Gate and Vacuum Flushing of Sewer Sediment: Laboratory Testing” by Qizhong Guo, Chi-Yuan Fan, Ramjee Raghaven, and Richard Field

Discussion of “Gate and Vacuum Flushing of Sewer Sediment: Laboratory Testing” by Qizhong Guo, Chi-Yuan Fan, Ramjee Raghaven, and Richard Field

Closure to “Gate and Vacuum Flushing of Sewer Sediment: Laboratory Testing” by Qizhong Guo, Chi-Yuan Fan, Ramjee Raghaven, and Richard Field

Closure to “Gate and Vacuum Flushing of Sewer Sediment: Laboratory Testing” by Qizhong Guo, Chi-Yuan Fan, Ramjee Raghaven, and Richard Field

The erosion behaviour of biologically active sewer sediment deposits: Observations from a laboratory study

The erosion behaviour of various fine-grained sediment deposits has been investigated in laboratory experiments. This work mainly focused on tests using sewer sediment in which strong biochemical reactions were observed during the deposit formation period. A small number of initial tests were conducted in which the deposits were made from mixtures of “clean” mineral and organic sediments. The erosion behaviour observed in these tests was compared with the erosion characteristics for sediments taken from deposits in a sewer. The impact of the biological processes on physical properties such as bulk density, water content, deposit structure and the erosive behaviour as a function of bed shear stress are quantified and discussed. Based on these observations it is believed that bio-processes weaken the strength of the in-pipe sediment deposits. A significantly weaker sediment surface layer was observed during deposition under quiescent oxygen-rich conditions. This resulted in a deposit with low shear strength which may be a cause of a first foul flush of suspended sediment when flow rates were increased. Comparison between tests with sewer sediments and the artificial representative surrogates suggested that the deposits of the later did not correctly simulate the depositional development and the resultant erosion patterns observed with the more bio-active sewer sediment.