The first link was not glamorous. In 1961 a pipeline was laid from Lake Tissø to a new oil refinery, because Kalundborg does not have much fresh water and a refinery needs a great deal of it. Scarcity, not idealism, set the pattern. In 1972 Gyproc, a plasterboard manufacturer, ran a pipeline to take surplus gas off the Tidewater Oil Company rather than see it flared — the first genuine by-product exchange on the site. Each subsequent connection followed the same logic: two neighbours with a mismatch, one with a surplus, one with a need, and a short enough distance between them to make the pipe pay for itself. By the late 1980s the participants noticed they had built something with a shape, and gave it a name. The order of events matters. The name came last.

More than thirty exchanges run between the actors in Kalundborg today. The Asnæs power station sits at the centre of the web, sending steam residuals to the refinery — enough to meet about 40% of its steam requirement — and on to pharmaceutical production and a fish farm. The refinery sends 700,000 cubic metres of cooling water back the other way every year, which Asnæs purifies and uses as boiler feed-water. Flue-gas desulfurization at the power station yields calcium sulphate, which goes to Gyproc as raw gypsum for wallboard, displacing mined mineral. Fly ash and clinker leave for cement producers and road building. Sludge from the fish farm and from the pharmaceutical plants goes out as fertilizer to surrounding fields. Farmers, recycling operators and fish processors sit at the edges of the network, taking the flows the big plants cannot use. None of it is closed-loop: plenty of material still enters and leaves the region entirely.

Nine private and public enterprises make up the park, several of them among the largest firms in Denmark. Novo Nordisk, the world's biggest producer of insulin, anchors the pharmaceutical end. Novozymes — renamed Novonesis in 2024 — is the world's largest enzyme producer. Ørsted owns Asnæs, once Denmark's largest power plant, which spent decades burning coal, began converting in 2017, and now runs a single, far smaller woodchip-fired unit commissioned in 2020 after the last coal units were retired. Gyproc makes the gypsum board. Denmark's largest oil refinery supplies the steam demand and the cooling water. RGS 90 handles soil remediation and recovery, Kara/Novoren treats waste, and Kalundborg Forsyning supplies the town's water and heat and takes away its refuse. Kalundborg Municipality itself is a partner, not a regulator standing outside the arrangement — which is unusual, and probably part of why it works.

The savings are real but slippery, because measuring an avoided input is harder than measuring a sale. Figures from around 2004 put the annual water savings at 2.9 million cubic metres of ground water and a million cubic metres of surface water. Gypsum savings run to roughly 170,000 tons. Taken together, the exchanges have been credited with annual savings of up to $15 million against investments of around $78.5 million, with accumulated savings estimated near $310 million. Those numbers are estimates, and everyone involved says so. The park's boundaries shift, participants come and go, and deciding which flows belong inside the accounting is a judgement call rather than a measurement. What is not in dispute is the direction: the arrangement pays, which is why it survived thirty years of changing owners and energy prices.

Kalundborg's success set off a wave of imitation. Governments in developed countries began writing incentives for corporate materials exchange, and in 1996 the U.S. President's Council on Sustainable Development proposed fifteen eco-industrial parks, defining five characteristics a good one should have: material exchange between separate entities, physical proximity, cooperation between plant managements, infrastructure that does not need expensive retooling, and an anchor tenant with the resources to carry the early years. Devens Regional Enterprise Zone in Massachusetts became one of the better American examples. But the research that followed was blunt. Planned parks failed systematically; the symbioses that lasted were the ones that grew out of ordinary industrial life, the way Kalundborg's had. That finding deflated much of the enthusiasm the Danish example had generated. The uncomfortable conclusion is that the thing worth copying may be the least copyable part — decades of neighbours getting to know each other's balance sheets.