Urban Water最新文献

Several EU Directives have been adopted which have an influence on the control of urban pollution in particular the EU Urban Wastewater Treatment and the Integrated Pollution Control and Prevention Directive. In addition, the recently adopted Water Framework Directive (WFD) will have an additional impact on the control of urban pollution in particular related to storm overflows. The present paper provides a brief discussion of the EU legislation relevant to the control of urban pollution with special emphasis on the Urban Wastewater Treatment Directive (UWWTD) and its implementation in the Member States.

The recently adopted WFD requires the achievement of good ecological and chemical status in all waters. The implementation of the UWWTD by the Member States, which should be achieved by 2005, will make a significant contribution towards achieving good ecological and chemical status. However, besides the discharge of sewage effluents, storm overflows can have a significant impact on the quality of surface waters and they will therefore require adequate control in order to achieve the final goal of good ecological and chemical status. The paper, therefore, also provides a brief analysis of the permitting requirements in the different countries and of the criteria currently applied by the EU Member States for the design of combined sewer overflow systems. Not all countries currently require a permit for storm overflows and the design criteria for combined sewer overflows are generally based on the spill frequency or treatment capacity in terms of dry weather flow, which do not take into account the effect on the receiving water. However, in the UK a method has been developed to assess the impact of combined sewer overflows on the quality of the receiving water, which is being applied to ensure the design of the storm overflow is adequate for the protection of the receiving water. Member States will need to adapt their methodologies used for the design of storm overflow to ensure the requirements of the WFD, the achievement of good ecological and chemical water quality, is being met.

It is well known that urbanization increases the hydrograph peak and overland flow volume from drainage basins, and that the concentration time of flood flows is diminished. To predict the effects of future urban development on the flood regime, the design hydrograph must be estimated. Nevertheless, the information from which to estimate a design hydrograph is limited where planning procedures (such as the `Urban Master Plan' of some Brazilian cities) specify only the type of urban land use (residential, industrial, etc.) and the recommended population density in areas where development is planned. This paper presents the relationship between parameters of a hydrologic model and urban development characteristics as set out in the Urban Master Plan. A hydrologic model IPH IV [C. E. M. Tucci, B. Braga, A. L. L. Silveira, RBE Caderno de Recursos Hı́dricos 7 (1) (1989)] was used together with a GIS to predict the hydrograph corresponding to alternative urbanization scenarios. Model parameters were fitted and verified using recorded data from the Dilúvio Creek in Porto Alegre, Brazil. The model was applied to predict the design hydrograph for different risks in accordance with the city's Urban Master Plan. Flooded areas corresponding to a storm rainfall with return period 25 years were calculated by simulation, the flooding being mainly due to flow obstruction by bridges.

This paper describes an investigation into stored untreated grey water quality processes and the development of a computer simulation for those processes. A laboratory study was carried out to investigate the changes in water quality with increasing residence time, and the results were used to calibrate and verify the model. Model results gave a good fit for dissolved oxygen (DO) concentrations, but only a reasonable fit for chemical oxygen demand (COD). Still, the main trends of model and laboratory COD data were broadly represented. Measurement and model results tend to confirm the initial hypothesis of four major processes in operation: settlement of suspended solids, aerobic microbial growth, anaerobic release of soluble COD from settled organic matter and atmospheric reaeration. Storing grey water for 24 h may significantly improve water quality through rapid settlement of organic particles, however, storage beyond 48 h leads to depleted DO levels and potential aesthetic problems. A more detailed model of COD fractions within grey water (with the relevant measured data) in conjunction with a characterisation of particulate settling velocities should lead to improvements in model predictions.

Water reuse in Germany has gained in significance in the last 10 years. Several greywater systems, built according to guidelines introduced in 1995, operate today with no public health risk. Two greywater treatment systems are described in this paper: a rotary biological contactor (RBC) built in 1989 for 70 persons, and a fluidized-bed reactor for a one-family household built in 1995 as the biological stage for the treatment of household greywater for use in toilet flushing. Both systems were optimized in the following years with consideration of a minimal energy and maintenance demand. As numerous investigations have shown, biological treatment of the greywater is indispensable in order to guarantee a risk-free service water for reuse applications other than potable water.

Sewer maintenance and rehabilitation strategies developed in a number of countries are reviewed. Comparisons are made between those approaches that focus on a predefined subset of strategic sewers and those that consider proactive maintenance of the whole system to address the wider consequences of failure such as customer satisfaction, social disruption and environmental damage. A number of diverse methods are described which can be used to optimise and prioritise proactive maintenance by analysing sewer performance, and lessons are drawn from maintenance strategies developed for other buried infrastructure assets. Limitations in existing sewer databases are discussed and new methods of obtaining sewer condition information are described. The paper concludes that to be cost effective, proactive maintenance involving inspection and repair must be focussed on those pipes which can be shown to have an early predisposition to failure.

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.

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.