Some manufacturers of plate heat exchangers use partitions made of a vapour-permeable plastic membrane within the heat exchanger, through which some of the moisture is transferred to the cold outside air. Due to the temperature gradient in the heat exchanger, the moisture from the exhaust air condenses on the partition walls and then diffuses through the membrane to the supply air side. There is no direct mixing of the air types. According to manufacturers, impurities and bacteria cannot penetrate the membrane but remain on the exhaust air side.
Source: bauwion
