Food processing equipment, food contact surfaces, and non-food contact surfaces are sanitized or disinfected using an acid-anionic surfactant solution that has efficacy in hard water, is non-corrosive, and is stable at elevated temperatures. The sanitizing/disinfecting solution is prepared as a highly concentrated liquid and diluted to end use concentrations prior to application to food processing equipment or food or non-food contact surfaces. Efficacy in hard water is important for sanitizers and disinfectants that are diluted by the end user using potentially hard water. Stability at elevated temperatures is important for long-term storage and transportation. The diluted sanitizing/disinfecting solution exhibits strong antimicrobial activity against gram positive and gram negative bacteria even when diluted in hard water. The sanitizing/disinfecting solution exhibits low toxicity to humans and the environment since it is prepared from low toxicity ingredients that are readily biodegradable.