Richard Ashley

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.

The management of sediment in combined sewers

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 effective 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 effectively 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-effective or wholelife solutions. Nonetheless significant advances have been made in enhancing the traditional approaches to sewer solids management which have been in use for more than a century.

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...

The influence of near bed solids transport on first foul flush in combined sewers

The problems associated with deposited sediments in sewers, and their transport through sewer systems have been the subject of detailed fieldwork programmes in the UK, and elsewhere in Europe. Existing laboratory, and some field based research exercises have focused on the relatively small, discrete particles. It is clear, however, that combined sewer systems have inputs which comprise of a significant proportion of large organic solids (faecal and food wastes), as well as the finer range of particle sizes. The increased concern regarding CSO spills into the environment has fuelled the recent development of sewer flow quality models, such as HYDROWORKS QM and MOUSETRAP, some of which make no attempt to represent the transport of these larger organic particles. Herein, the results of a collaborative research programme undertaken between three UK universities and a water authority are discussed. Transport at the bed in sewers, as “near bed solids”, is defined. Based on a comprehensive data collection program undertaken in the Dundee combined sewerage system, a method is presented which may be used to estimate the rate of sediment transport near the bed in sewers. The influence that solids in transport near the bed have on first foul flush in combined sewers is discussed. A methodology is proposed which may be used to estimate the extent to which sediment in transport near the bed in sewers contributes to first foul flush phenomena, by describing the movement of a storm wave along a conceptual sewer length.

Modelling input of fine granular sediment into drainage systems via gully-pots

An existing model of sediment wash-off from paved surfaces has been extended to predict fine granular sediment inputs into drainage systems via roadside gully-pots. The model comprises three separate components: (1) a model of sediment accumulation on paved surfaces; (2) a model of sediment wash-off from these surfaces; and (3) a model of sediment deposition and erosion in gully-pots. The first two model components have been previously successfully tested, whereas the third component of the model is presented in this paper for the first time. The model was applied to two small catchments located in Dundee, Scotland, where data on flow rates and sediment concentrations were collected at the inlets into the main drainage systems from the road gullies. It was found necessary to calibrate only the build-up component of the model to achieve good agreement between the simulated and measured data. It was concluded that only two calibration parameters are required to model the input of fine granular sediment into storm drainage systems via gully-pots.

SWARD: decision support processes for the UK water industry

In industrialised countries water service providers (WSPs) must provide an appropriate level of service with an acceptable performance at an acceptable cost to customers. In the UK a move towards sustainable development is now also a major goal for WSPs. However, the imposition of institutional systems and regulatory targets still encourage the adoption of less sustainable technologies or solutions by the water industry. It is within this context, that the Sustainable Water industry Asset Resource Decisions (SWARD) project has developed a set of decision support processes that allow WSPs to assess the relative sustainability of water/wastewater system asset development decisions. A Guidebook has been produced that takes the WSP and its stakeholders through the processes essential to incorporating sustainability in asset investment decision‐making. Several case studies that demonstrate the SWARD principles in application are included within the Guidebook, the experience of which is described in this paper.

Near bed solids transport rate prediction in a combined sewer network

The physical and biochemical nature of the material transported near the bed (‘bed-load’) in combined sewers is established. The merits of the recent evolution of the terminology used in this field are discussed. The physical characteristics of material found in transport at three separate combined sewer field sites are described. The importance of this mode of transport, in terms of the mass transported and pollutant potential, is demonstrated based on data collected from Dundee combined sewerage system sites. The development of a novel near bed solids transport prediction equation is described. Comparisons are made between measured near bed solids transport rates obtained in the field, at sites with and without deposited beds, with predictions obtained using the new methodology. The need for future research in this subject area is emphasised.

Development and application of new sensors and analysers for on-line controlUsing an array of ultrasonic velocity transducers to improve the accuracy of large sewer mean velocity measurements

Portable in-sewer Doppler velocity measuring equipment has been available for a number of years. However in large sewers (typically greater than one metre diameter) the accuracy of these devices for large flow depths can be very poor. This paper describes a system, developed by the authors, that enables the determination of a velocity profile across the section of the flow in a large sewer. The velocity profile information obtained can then be used to determine a more accurate estimation of the mean velocity of flow in the large sewer. In the paper the hardware and software developed to form the Doppler Transducer Velocity Array System is discussed. Installation of the system in a large sewer and subsequent field testing of the system are also discussed, with results of the field testing included. The paper also shows how the results obtained were used to produce 3-dimensional surface plot graphs that show the variation of velocity against time across a section of the sewer flow.

Determination of sewer roughness and sediment properties using acoustic techniques.

Measurements have been carried out to determine the effect of porous sediments in a pipe on the attenuation and spectrum of the airborne acoustic field. The results show that the presence of even a relatively thin sandy layer results in a considerable increase in the acoustic attenuation over the broad frequency range. The measured value of the relative attenuation is in the range of 0.8 dB/m. The effect of the sediment on the acoustic spectrum is the reduction in energy of the propagating modes, which is an easily detectable phenomenon. These results pave the way for the development of the instrumentation for non-invasive characterisation of the parameters of sediments in wastewater systems.

Inlet and Sewer Traps for Sediment Control in Stormwater Drainage - A Malaysian Case Study

The city of Alor Setar in Kedah, is an important developing centre of business and communications for the Northwest of Malaysia. However, the capacities of the existing storm drains in Alor Setar are severely restricted as a result of sediment deposits washed-in during high intensity rainfall events. These deposits result in stagnant, polluted areas of water within the catchment and a loss of hydraulic capacity needed to deal with large storms. Methods of sediment deposition prediction have been applied to UK catchments with limited degrees of success. The application of these methods has been used to propose an improved sediment management strategy for Alor Setar. The methods involve the development of a simplified hydraulic model of the test catchment and the prediction of areas within the system where deposits are most likely to occur. Using this formation options for invert trap placement can be developed in order to determine their most effective locations. Further analyses are also proposed in order to determine the fill rates of these traps throughout an average year. A proactive maintenance plan can therefore be developed in preference to the reactive maintenance methods currently employed This planned programme of maintenance should ensure that best use of traps is made at all times in order to protect the system from flooding and pollution problems.