For a long time people have questioned what the "best" sewer system is for limiting the pollution load released into the receiving waters. In this paper the traditional separate and combined sewer systems are compared using a pollution load balance. The investigation is based on measured concentration data for a range of pollutant parameters in the sewer from the new database "ATV-DVWK Datenpool 2001". The approach also accounted for the wastewater treatment plant outflow which contributes to the total pollutant load considerably. In spite of a number of neglected effects, the results show that the separate system is superior to the combined for some parameters only, such as nutrients, whereas for other parameters, e.g. heavy metals and COD, the combined system yields less total loads. Any uncritical preference of the separate system as a particularly advantageous solution is thus questionable. Individual investigations case by case are recommended.
A study was conducted on 34 German combined sewer systems including combined sewer overflow (CSO) tanks and treatment plant to show up actual hidden reserves and bottlenecks in stormwater treatment. The study gave also a general insight into the water pathways in urban hydrology. A special focus was given to undesired non-polluted water infiltrating into the sewer, labelled infiltration and inflow (I/I) or infiltration inflows, which is widely underestimated. It leads to a bad performance of the drainage system although the parasite waters are themselves non-polluted. In existing combined systems, pollution control can be considerably improved by reducing I/I. It is equivalent to the reduction of surface runoff e.g. by a separate drainage as frequently proposed alternative. Artificial infiltration of surface runoff may even increase infiltration inflows.
Sedimentation of sewer solids in tanks, ponds and similar devices is the most relevant process for the treatment of stormwater and combined sewer overflows in urban collecting systems. In the past a lot of research work was done to develop deterministic models for the description of this separation process. But these modern models are not commonly accepted in Germany until today. Water Authorities are sceptical with regard to model validation and transferability. Within this paper it is checked whether this scepticism is reasonable. A framework-proposal for the validation of mathematical models with zero or one dimensional spatial resolution for particle separation processes for stormwater and combined sewer overflow treatment is presented. This proposal was applied to publications of repute on sewer solids separation by sedimentation. The result was that none of the investigated models described in literature passed the validation entirely. There is an urgent need for future research in sewer solids sedimentation and remobilization!
The small town of Waldenburg in South Germany planned to continue the implementation of stormwater treatment. The CSO-Masterplan dated from the year 1976. Meanwhile, new guidelines were issued and new technologies became available, like mathematical sewer models. Since the ecological impact of stormwater treatment is today much more accounted for, it was decided to revise the CSO-Masterplan. The goal was to find a solution with minimized ecological impact at low costs. This urban hydrological study featured some unusual approaches. First, the present state of city, sewer system, treatment plant and receiving waters was analyzed in an interdisciplinary field study. Using the quantity-quality sewer model ASMI, the annual pollutant loads for various planning alternatives were calculated. This allowed statements about the environmental impact of each alternative, taking into account the sensitivity of the receiving waters known from the biological field study. The proposed final alternative saved nearly 1 million DM at a much lower impact on the environment compared with the original planning, showing that it is possible to cope equally with ecological and economical requirements.
In Germany, today about 24,000 combined sewer overflow tanks (CSO tanks) are in operation. For their dimensioning and design, German standards are available and respected. However, efficiency and performance of these costly structures are usually not known by the operator nor by the local water authority. Thus, a trend towards monitoring the overflow activity is observed. This paper points out the state-of-the-art in CSO overflow monitoring. Basic features of water level measurement as well as some plausibility checks for the data are shown. Evaluation of overflow data is rather difficult. Assessment of the overflow activity versus hydrological data, e.g. by comparison with simulation results, is costly. A much simpler way of evaluation is the ranking and rating of measured overflow activity. On the 8th ICUD 1999, already a paper on this subject was presented. Now, 6 years later, the database of the ranking has grown to more than 500 years of measurement.
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