This paper presents the issues concerning the impact of horizontal forces in the ground slab zones on the safety risk of RC rectangular tanks in sewage treatment plants on the example of a bioreactor and a sewage pre-treatment tank. In the first case, as a result of an in situ testing and analysis of a RCmulti-chamber tank, a number of unsealed leaks in the shrinkage cracks and expansion joints were found in the external walls. The horizontal stability verification of the external wall confirmed the threat to safety (design calculations contained an error). It was recommended to reinforce the ground slab by a rim embracing the tank. In the second instance of a tank in the sewage pre- -treatment plant, a shear phenomenon in the newlymade wall was found at the ground slab joint. The failure took place during the summer of 2015, when the tank was subjected to significant thermal loads, and then charged twice with water during the leaking test. Keywords: RC tank; rectangular tank; structural research; computational analysis.