Abstract: At approximately 3:00 p.m. on November 8, 2000, an employee was injured while he was preparing to clean an aeration tank with a high-pressure water washer. The water washer pressure varied from 300 psi at idle speed, to 2000 psi at maximum engine speed. With the pump switch in the "on" position, and the engine idling, the water flow control valve was open approximately half-way. When water did not flow, the water valve was opened fully. There was still no water flow or indication of gauge pressure. After the pump was "turned-off", the employee who was holding a 15-foot section of hose and who was later injured, placed the hose on the ground, and went to the pumping truck to investigate the cause of the problem. Without warning, water started to flow through the hose, causing the hose to whip violently for several seconds. The whipping hose and nozzle struck the shins of the employee and a coworker, hospitalizing the employee, who was originally holding the hose, with lacerations to his leg.