To prevent the recirculating water in chilled systems freezing as outside temperatures get below 0^oC in the winter it is often necessary to add antifreeze. It is also important to note that when closed circuits contain 25% glycol or above, the glycol has a biostatic effect i.e. preventing the growth of bacteria. As the concentration falls below 25% glycol can act as a food source for bacteria. As bacteria consume the glycol they produce acidic bi-products lowering the pH of the system and accelerating corrosion.