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1 month without rain: "The big flush"? Impact of long dry periods on sewers and overflows

Yesterday, the 15th of June, it has been a month since we last saw any rain in our region. The forecasts predict some rain this Sunday...

Prolonged dry periods not only affect the groundwater levels but also have an impact on our water treatment facilities and sewage systems.

During droughts, sewage flow rate is low, causing solids to accumulate in the sewage system. As soon as the first summer rain shower arrives, a sudden surge of highly polluted water enters the purification plant, possibly resulting in a sudden and temporary overload of the treatment system. These peaks of contamination luckily disappear quickly.

We observed this phenomenon quite clearly with our Olpas TSS sensor installed in an influent stream of a Dutch wastewater treatment plant. After a heavy rain shower the suspended solid content increase twofold (0.2 g/l to 0.4 g/l) whereas the flow increased five times! By measuring suspended solids real-time with our sensors, operators can ensure that the first batch of heavily polluted water is treated as good as possible - minimizing sewage overflow during the beginning of the ‘big flush’.

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