Modelling suspended solids transport is a key issue for predicting the pollution load discharged by CSOs. Nonetheless, there is still much debate on the main drivers for suspended solids transport and on the modelling approach to be adopted. Current sewer models provide suspended solids transport models. These models, however, rely upon erosion-deposition criteria developed in fluvial environments, therewith oversimplifying the sewer sediment characteristics. Consequently, the performance of these models is poor from a theoretical point of view. To get an improved understanding of the temporal and spatial variations in suspended solids transport, a measuring network was installed in the sewer system of Loenen in conjunction with a hydraulic measuring network from June through December 2001. During the measuring period, 15 storm events rendered high-quality data on both the hydraulics and the turbidity. For each storm event, a hydrodynamic model was calibrated using the Clemens' method. The conclusion of the paper is that modelling of suspended solids transport has been and will be one of the challenges in the field of urban drainage modelling. A direct relation of either shear stress or flow velocity with turbidity could not be found, likely because of the time varying characteristics of the suspended solids.

In the practice of integrated urban water management it is difficult to choose the best combination of available techniques. This paper gives the outline of a decision network to make it easier. It consists of six steps: problem definition, technologies, selection procedure, combination, ranking by sustainability and costs. It is used in an iterative way. By looking at soil characteristics, surface water, groundwater and pollution aspects and by giving priority to sustainable techniques, foundations for a decision can be achieved. It is stated that techniques are sustainable, when they handle stormwater near the source and are visible (not hidden under the ground). From this point of view rainwater utilisation and infiltration are more sustainable than separate and combined sewer systems.

A greater understanding of the erosion behaviour of sewer sediments is necessary in order to reliably estimate the amount and nature of the sewer sediments released from deposits in sewers and transported either to waste water treatment plants or discharged into the environment. Research has indicated that microbial activity in sediment can influence the physical release of sediment from in-pipe deposits. This paper reports on a series of erosion tests in which sewer sediments from different sewer networks are kept under different environmental conditions and their resistance to erosion is examined. The erosion tests are carried out under aerobic and anaerobic conditions and two temperatures, one representing ambient sewer temperatures and a lower temperature that significantly suppresses bacterial activity.

This research is based on CSO emissions from Dutch sewer systems. During the years 1982 to 1989 research was done on several sewer systems, all of them equiped with a single overflow weir. Pollutant emissions were calculated from the measurements, whereby each storm was considered as a single event. Extreme emissions have a detrimental, sometimes even desastrous effect on water quality. Such extreme emissions are the result of heavy storms, giving it a low frequency of occurrence. From the measurements a statistical model was developed enabling the user to forecast extreme waste emissions with a certain return period in a range of 2 to 10 years. Five pollutants are put in the model: BOD, COD, Kjeldahl nitrogen, total phosphate and suspended solids. The model operates with standardized emission values in kg per ha of impervious area. When the model is used in practice the runoff area to the specific overflow under consideration has to be known.

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