The overflows of the pumping stations are necessary but also a weak point in the system. The overflow water is usually discharged into a nearby water system such as a stream, river or lake. These water systems easily experience high water levels during heavy rains or snowmelt. The pumping stations in Värnamo are no exception. When the water level in Lagan is high, water flows into the sewage system via the overflows of the pumping stations. This leads to stormwater being pumped to the treatment plant and causes significant additional costs for both treatment and pumping. When a larger amount of water that requires treatment is brought to the treatment plant, the costs for operation and maintenance also increase.

The pumping stations need to be able to get rid of overflow water in case the sewage system becomes overloaded, but without protection, the outlets can have the opposite effect and instead lead water into the pumping station. This is a vulnerability for all pumping stations where the overflow water is led to a stream, river, lake or other water system. When backflow occurs, the pumping station becomes overloaded and the pump cannot handle the volume of water in the system. This can lead to extensive basement flooding in the surrounding areas. Installing a check valve on the overflow drain also protects the pumping station from debris and small animals that could otherwise block the drain and render it inoperable.