Resumen
Several forms of supra-municipal cooperation between water and wastewater utilities have evolved in Finland since the 1950s: bilateral contract-based, municipal federations or authorities operating on a wholesale basis as well as supra-municipal companies. These may take care of community water supply or sewerage, or both. This paper explores and analyzes the most common form of cooperation: contracts for water and wastewater services between neighboring municipalities. The extent of contractual cooperation until 2006 was explored by an inquiry sent to bigger water and wastewater utilities (n = 233 replies; 88 %). Compared to a study performed in 1975, the number of bilateral contracts had tripled. The study also included interviews targeting nine selected cases. We examined also the centralization of wastewater treatment when small plants are closed down and wastewater is directed to larger plants. Centralization has led to the construction of transfer sewers; the paper also explores their evolution from 1995 to 2015. Contrary to common arguments, contractual cooperation proved rather easy to manage and should be seen as a serious option for other tighter forms of supra-municipal cooperation that are also increasing.