In the dairy industry, cleaning and disinfection are essential operations. Fouling occurs because milk residues remain on the surfaces of equipment. Residues of milk that have dried up are difficult to remove. Excessive fouling is costly because milk is lost, increased concentrations of detergents are required, and consequently more wastewater is produced. Fouling of membranes by the formation of a gel layer considerably reduces the flux (see Section 9.4). Heating of milk results in the formation of a deposit on metal surfaces that is difficult to remove. Deposit formation reduces the rate of heat transfer and the flow rate of milk in the equipment. Eventually, the equipment will stop operating. In a multiple-effect evaporator with, e.g., six effects, the costs due to fouling (milk losses + cleaning) can account for more than half of the total running costs (including machinery, energy, and so forth). Cleaning of equipment is necessary to reduce all of these problems and to prevent the growth of microorganisms in milk residues, which is highly undesirable. Several microbes can readily grow at surfaces containing a thin film of milk deposit.