The authors aim to highlight the fact that wastewater treatment systems, as critical infrastructure facilities, can be the target of terrorist attacks by collecting and documenting physical and virtual attacks that have occurred and presenting the most serious cases. With the increasing growth of appliance of online process controlling systems, the number of attacks in cyberspace is growing in proportion, and there is an increasing number of attacks targeting water, water management and wastewater systems. This presentation is divided into two parts. First, an overview is provided of physical attacks and cyber incidents targeting wastewater treatment systems. Second, we presenting a recent wastewater treatment plant failure in Poland, and use the lessons learned to portray a possible outcome of a successful terrorist attack on wastewater treatment plant.