Combined sewer overflow and dilution in urban drainage

Abstract

It is a well established practice in Serbia within the design of sewer systems to adopt the so called required dilution as a criterion for design of combined sewer overflows (CSOs). This required dilution is defined as the fixed ratio of storm water and wastewater quantities that has been recommended about 30 years ago in the development phase of the Belgrade sewer system. Having in mind that the approach to surface water quality control has significantly changed since then, and that the operation and effectiveness of the future wastewater treatment plant and the operation of CSOs are strongly interrelated, it is necessary to reconsider these design criteria. Using the Belgrade sewer system as an example, it is shown that the dilution is certainly not a fixed value but depends on many factors. In order to define overflow frequencies, overflow volumes and the overflow quality, it is necessary to perform detailed analysis of storm runoff and detailed hydraulic analysis of the sewer system.

U radu se razmatra dugogodišnja domaća praksa u projektovanju kanalizacionih sistema da se rasteretni prelivi dimenzionišu na "merodavno" razblaženje, pod kojim se podrazumeva mešanje kisnih i upotrebljenih voda u odnosima koji su preporučeni pre oko 30 godina u fazi razvoja Beogradskog vodovoda i kanalizacije. S obzirom da se od tada primenio pristup kontroli zagađenja vodotoka otpadnim vodama, kao i da rad i efikasnost budućeg postrojenja za prečišćavanje otpadnih voda zavisi u velikoj meri od funkcionisanja rasteretnih preliva, neophodno je da se do sada primenjivani kriterijumi ponovo razmotre. Na primeru beogradskpg kanalizacionog sistema pokazano je da razblaženje u sistemu sigurno nije fiksirana vrednost već zavisi od velikog broja faktora. kao i da je neophodno sprovesti detaljne hidrološke analize oticaja kišnih voda i hidrauličke proračune sistema da bi se analizirala učestalost prelivanja, količine prelivene vode i njihov kvalitet pri direktno ispuštanju u vodoprijemnike.