The basis of cheese making is separating the solids (curd) from the liquid (whey) in milk.
As milk is about 87% water, whey is by far the biggest by-product of the business.
It takes about 10 litres of milk to make 1kg of cheese.
What to do with all that whey is a major challenge for small producers. It cannot be poured down the drain as it kills the oxygen in the water. For similar reasons only a certain amount can be sprayed onto farmland.
Because it contains a certain amount of protein it can be fed to pigs, but many producers have to pay to have it properly disposed of.
The big dairies have much better options. With enough whey it makes economic sense to distil it into ethanol. In New Zealand that is used to run buses. In Ireland it can be used as the base for some coffee liqueurs. In France a power station in the Alps is powered solely on the whey from the big makers of Beaufort, a Gruyere style cheese.
In powdered form it is the basis for an ever-growing industry making protein bars and those huge tubs of protein powder found in health food shops.