Kalundenborg in Denmark, is one of the best-known (and one of the oldest) examples of an industrial ecology in action. Different companies at Kalundenberg use each others waste.

For instance Kalundborg’s power plant produces various materials and energy flows; gypsum which then is used to produce plasterboard, and steam, which warms fish farming ponds. In turn sludge from the fish-ponds and factories fertilise local farms.