Combined sewage-rain urban drainage system and active sludge WWTP design

In order to regulate water traps existing in many combined city drainage system, a few Italian regional laws recently have classed each ones into two categories (3 or 5) with reference to the constrained calibration of mouth. So, in case of combined piping system collecting such sewage as rain water, becomes important to receive, in primary treatment phases, in rainy weather conditions, a large amount of collected water volumes. Mathematic description and numerical simulations about active sludge WWTP performances are carried out in function of incoming and bypass fluxes. Results of calculation present, as expected, a substantially respect of national law limits for BOD and SS, in case of medium sewage, receiving to primary settling a peak flow included between 4 to 5 times dry weather discharge. Result not far from what just suggested in old UIDA design guideline. Experimental data disposable from Bologna WWTP (500000 AE) made possible a comparison between efficiency foreseen for each treatment phases and organic matter (BOD) removal tested, during full scale functioning, in different rainy and dilution conditions.