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.

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An easy method is proposed to determine the necessary volume of a vortex separator in comparison to that of a first-flush CSO tank. The method is based on a simplified hydrological approach as well as on steady-flow efficiency data of the vortex separator which are derived from model tests. Transfer to prototype scale is done using efficiency measurements at a real separator. The method works with a balance of spilled COD loads and yields typically separator volumes of 50-70% of that of a conventional CSO tank.

Lamella settlers have been used in the past few years for the sedimentation of particles in wastewater and stormwater applications. A new and very innovative approach for the treatment of stormwater flows is proposed which extends the portfolio of solutions beyond traditional settling tanks. Surface runoff is stored in a sewer or a basin and finally treated in a small but continuously operated lamella clarifier. The low throughput flow will yield good treatment efficiency at a small footprint. The possibilities of using existing storage volume in a storm sewer, as well as the structural flexibility of the arrangement are decisive benefits. As a large operational advantage, the lamellae may be cleaned mechanically, e.g. by pivoting under water. Finally, the flow and the sludge which will be sent to the downstream treatment plant will be minimized. A new comparative simulation method is proposed in order to assess an equivalent degree of stormwater treatment, either by achieving an equal annual volume of treated stormwater or, more directly, an equal amount of spilled pollutant load. The new solution is compared with a traditional settling tank according to current German design rules. Additionally, a case study from a real installation will be presented.

A six-year study of CSO problems showed that in most cases the settleable sewer solids are responsible for poor water quality. This is the reason why the settling behaviour of settleable solids should be taken into account at stormwater treatment to a greater degree than it is done up to now. New efficiency measurements at stormwater tanks with clarifier overflows for combined sewage showed two important results: 1) The annual mean surface loading qA of a clarifier-type CSO tank is only about 25 % of the design surface loading of 10 m3/(m·h) recommended by the German guideline A 128. 2) The settling process in stormwater tanks is effective: - The median settling velocity vs of sewer solids is reduced from about 0,21 cm/s to vs = 0,035 cm/s. - The concentration of settleable solids is reduced about 80 %, that of total solids about 65% and COD concentration about 50 %. With this efficiency, well designed stormwater tanks will be a good measure in many cases to reduce significantly CSO impacts on receiving water quality.

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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.