Hi Paul, In order to design ...

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Hi Paul, In order to design an urban drainage network you can use the open source: SWMM. I think the greatest difficulty is to know the amount of pollution that the runoff can yield and the data are not always easy to find. About this software you can check the rate of pollution in each node. If the network is already present you can make a model it with that reproduces it and calibrate the parameters. Since you have got a satisfactory solution of the discharge, the wastewater are under the laws of the Country. You must build a spillway when the concentration of the black flow exceeds a certain value (the dilution ratio depends on the setting region) the discharge goes to water treatment, other goes to a water course. Once you know what you have to treat you can go to a specialising company. For the second question. The amount of BOD or COD depends on the laws of the State and to the number of inhabitants in to the basin drainage. For example in my country (Italy) for urban agglomerations > 100,000 inhabitants there is the lower limits of 125 [mg / l O2] for COD and 25 [mg / l O2] for BOD. So I think you have to see the legislation and the number of inhabitants of the municipality. But to create a network of the urban drainage SWMM is reliable and not difficult to use, and it all comes down to the reliability of the data. You can do various simulations and to change the scenario and you should always checking the results with other similar basins . The oxygen present in the fluid is the carrier for microorganisms which decompose organic substances, so I would say that a high value is in favor of security. I am a hydraulic engineer, and I do not know the specifications to built to a treatment plant, I know the necessary components to its management. But according my knowledges there are not a open source software to design a treatment water plant. This is the link to download the Open Source SWMM: http://www.epa.gov/athens/wwqtsc/html/swmm.html I hope I have been helpful, a little bit. Regars, Giampietro Martini